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Background CD103 is an integrin specifically expressed on the surface of cancer-reactive T cells. CD103 has been linked with better disease-specific survival in patients with ovarian, cervical, and endometrial cancer. The number of CD103+ T cells significantly increases during successful immunotherapy across human malignancies and therefore might be an attractive biomarker for non-invasive immune PET imaging of T cell infiltration. Indeed, we previously demonstrated that zirconium-89 (89Zr) radiolabeled anti-CD103 antibodies could be used for PET imaging of CD103+ T cells at relevant cell densities. However, the long half-life of antibodies precluded repeat imaging of CD103+ T cell dynamics early in therapy, and is associated with a significant radiation burden.Methods Two different anti-human CD103 Fab fragments radiolabeled with 89Zr or 68Ga were developed, namely 89Zr- hCD103 Fab and 68Ga-hCD103 Fab respectively. In vivo evaluation of these tracers was performed in nude mice (BALB/cOlaHsd-Foxn1nu) with established CD103-expressing CHO (CHO.CD103) or CHO wildtype (CHO.K1) xenografts, followed by serial PET imaging and ex vivo bio-distribution.Results In vivo, both 89Zr- and 68Ga- hCD103 Fab tracers showed high target-to-background ratios, high target site selectivity and high sensitivity in human CD103 positive xenografts.Conclusion We conclude that the two novel human CD103 immuno-PET tracers may be used for future non-invasive assessment of cancer reactive T cell infiltration. Consequently, both 89Zr and 68Ga- hCD103 Fab PET tracers should be explored in the clinical setting for stratification of patients who could benefit from immune checkpoint inhibition therapy.

Supplementary figures containing vaccine design, FACS plots and supporting data on other costimulatory molecules addressed in the manuscript.

While showing promise, vaccination strategies to treat cancer require further optimization. Likely barriers to efficacy involve cancer-associated immunosuppression and peripheral tolerance, which limit the generation of effective vaccine-specific cytotoxic T lymphocytes (CTL). Because CD4+ T cells improve CTL responsiveness, next-generation vaccines include helper epitopes. Here, we demonstrate in mice how CD4+ T-cell help optimizes the CTL response to a clinically relevant DNA vaccine engineered to combat human papillomavirus–expressing tumors. Inclusion of tumor-unrelated helper epitopes greatly increased CTL priming, effector, and memory T-cell programming. CD4+ T-cell help optimized the CTL response in all these aspects via CD27/CD70 costimulation. Notably, administration of an agonistic CD27 antibody could largely replace helper epitopes in promoting primary and memory CTL responses, acting directly on CD8+ T cells. CD27 agonism improved efficacy of the vaccine without helper epitopes, more so than combined PD-1 and CTLA-4 blockade. Combining CD27 agonism with CTLA-4 blockade improved vaccine-induced CTL priming and tumor infiltration, but only combination with PD-1 blockade was effective at eradicating tumors, thereby fully recapitulating the effect of CD4+ T-cell help on vaccine efficacy. PD-1 blockade alone did not affect CTL priming or tumor infiltration, so these results implied that it cooperated with CD4+ T-cell help by alleviating immune suppression against CTL in the tumor. Helper epitope inclusion or CD27 agonism did not stimulate regulatory T cells, and vaccine efficacy was also improved by CD27 agonism in the presence of CD4+ T-cell help. Our findings provide a preclinical rationale to apply CD27 agonist antibodies, either alone or combined with PD-1 blockade, to improve the therapeutic efficacy of cancer vaccines and immunotherapy generally. Cancer Res; 76(10); 2921–31. ©2016 AACR.

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PurposeCD103, an integrin specifically expressed on the surface of cancer-reactive T cells, is significantly increased during successful immunotherapy across human malignancies. In this study, we describe the generation and zirconium-89 (89Zr) radiolabeling of monoclonal antibody (mAb) clones that specifically recognize human CD103 for non-invasive immune positron-emission tomography (PET) imaging of T cell infiltration as potential biomarker for effective anticancer immune responses.Experimental designFirst, to determine the feasibility of anti-CD103 immuno-PET to visualize CD103-positive cells at physiologically and clinically relevant target densities, we developed an89Zr-anti-murine CD103 PET tracer. Healthy, non-tumor bearing C57BL/6 mice underwent serial PET imaging after intravenous injection, followed by ex vivo biodistribution. Tracer specificity and macroscopic tissue distribution were studied using autoradiography combined with CD103 immunohistochemistry. Next, we generated and screened six unique mAbs that specifically target human CD103 positive cells. Optimal candidates were selected for89Zr-anti-human CD103 PET development. Nude mice (BALB/cOlaHsd-Foxn1nu) with established CD103 expressing Chinese hamster ovary (CHO) or CHO wild-type xenografts were injected with89Zr-anti-human CD103 mAbs and underwent serial PET imaging, followed by ex vivo biodistribution.Results89Zr-anti-murine CD103 PET imaging identified CD103-positive tissues at clinically relevant target densities. For human anti-human CD103 PET development two clones were selected based on strong binding to the CD103+CD8+T cell subpopulation in ovarian cancer tumor digests, non-overlapping binding epitopes and differential CD103 blocking properties. In vivo, both89Zr-anti-human CD103 tracers showed high target-to-background ratios, high target site selectivity and a high sensitivity in human CD103 positive xenografts.ConclusionCD103 immuno-PET tracers visualize CD103 T cells at relevant densities and are suitable for future non-invasive assessment of cancer reactive T cell infiltration.

CD103, an integrin specifically expressed on the surface of cancer-reactive T cells, is significantly increased during successful immunotherapy across human malignancies. In this study, we describe the generation and zirconium-89 (First, to determine the feasibility of anti-CD103 immuno-PET to visualize CD103-positive cells at physiologically and clinically relevant target densities, we developed anCD103 immuno-PET tracers visualize CD103 T cells at relevant densities and are suitable for future non-invasive assessment of cancer reactive T cell infiltration.

Antibody-secreting cells (ASCs) secrete IgM, IgA, or IgG antibodies and are key components of humoral immunity; however, little is known about unique characteristics of the Ig-classes due to limited availability of material and challenges to quantify many intracellular molecular modalities at a single-cell resolution. We combined a method to in vitro differentiate peripheral B-cells into ASCs with integrated multi-omic single-cell sequencing technologies to quantify subclass-specific hallmark surface markers, transcriptional profiles and signaling transduction pathway components. Our approach detected differential expression of plasmablast and plasma cell markers, homing receptors and IL-2, IL-6, JAK/STAT and mTOR signaling activity across Ig-subclasses. Taken together, our integrated multi-omics approach allowed high-resolution phenotypic characterization of single cells in a complex sample of in vitro differentiated human ASCs. Our strategy is expected to further our understanding of human ASCs in healthy and diseased samples and provide a valuable tool to identify novel biomarkers and potential drug targets.TeaserIntegrated single-cell analysis allows tri-modal phenotypic analysis of in-vitro generated human antibody-secreting cells.

The programmed cell death 1 (PD-1) pathway represents a major immune checkpoint, which may be engaged by cells in the tumor microenvironment to overcome active T-cell immune surveillance. Pembrolizumab (Keytruda®, MK-3475) is a potent and highly selective humanized mAb of the IgG4/kappa isotype designed to directly block the interaction between PD-1 and its ligands, PD-L1 and PD-L2. This blockade enhances the functional activity of T cells to facilitate tumor regression and ultimately immune rejection. Pembrolizumab binds to human and cynomolgus monkey PD-1 with picomolar affinity and blocks the binding of human and cynomolgus monkey PD-1 to PD-L1 and PD-L2 with comparable potency. Pembrolizumab binds both the C′D and FG loops of PD-1. Pembrolizumab overcomes human and cynomolgus monkey PD-L1–mediated immune suppression in T-cell cultures by enhancing IL2 production following staphylococcal enterotoxin B stimulation of healthy donor and cancer patient cells, and IFNγ production in human primary tumor histoculture. Ex vivo and in vitro studies with human and primate T cells show that pembrolizumab enhances antigen-specific T-cell IFNγ and IL2 production. Pembrolizumab does not mediate FcR or complement-driven effector function against PD-1–expressing cells. Pembrolizumab displays dose-dependent clearance and half-life in cynomolgus monkey pharmacokinetic and toxicokinetic studies typical for human IgG4 antibodies. In nonhuman primate toxicology studies, no findings of toxicologic significance were observed. The preclinical data for pembrolizumab are consistent with the clinical anticancer activity and safety that has been demonstrated in human clinical trials.

BackgroundImmune checkpoint inhibitors (ICI) have radically changed cancer therapy, but most patients with cancer are unresponsive or relapse after treatment. MK-5890 is a CD27 agonist antibody intended to complement ICI therapy. CD27 is a member of the tumor necrosis factor receptor superfamily that plays a critical role in promoting responses of T cells, B cells and NK cells.MethodsAnti-CD27 antibodies were generated and selected for agonist activity using NF-кB luciferase reporter assays. Antibodies were humanized and characterized for agonism using in vitro T-cell proliferation assays. The epitope recognized on CD27 by MK-5890 was established by X-ray crystallography. Anti-tumor activity was evaluated in a human CD27 knock-in mouse. Preclinical safety was tested in rhesus monkeys. Pharmacodynamic properties were examined in mouse, rhesus monkeys and a phase 1 dose escalation clinical study in patients with cancer.ResultsHumanized anti-CD27 antibody MK-5890 (hIgG1) was shown to bind human CD27 on the cell surface with sub-nanomolar potency and to partially block binding to its ligand, CD70. Crystallization studies revealed that MK-5890 binds to a unique epitope in the cysteine-rich domain 1 (CRD1). MK-5890 activated CD27 expressed on 293T NF-κB luciferase reporter cells and, conditional on CD3 stimulation, in purified CD8+ T cells without the requirement of crosslinking. Functional Fc-receptor interaction was required to activate CD8+ T cells in an ex vivo tumor explant system and to induce antitumor efficacy in syngeneic murine subcutaneous tumor models. MK-5890 had monotherapy efficacy in these models and enhanced efficacy of PD-1 blockade. MK-5890 reduced in an isotype-dependent and dose-dependent manner circulating, but not tumor-infiltrating T-cell numbers in these mouse models. In rhesus monkey and human patients, reduction in circulating T cells was transient and less pronounced than in mouse. MK-5890 induced transient elevation of chemokines MCP-1, MIP-1α, and MIP-1β in the serum of mice, rhesus monkeys and patients with cancer. MK-5890 was well tolerated in rhesus monkeys and systemic exposure to MK-5890 was associated with CD27 occupancy at all doses.ConclusionsMK-5890 is a novel CD27 agonistic antibody with the potential to complement the activity of PD-1 checkpoint inhibition in cancer immunotherapy and is currently undergoing clinical evaluation.

Synthesis of ligand-functionalized nanomaterials with control over size, shape and ligand orientation, facilitates the design of tailored nanomedicines for therapeutic purposes. DNA nanotechnology has emerged as a powerful tool to rationally construct two- and three-dimensional nanostructures, enabling site-specific incorporation of protein ligands with control over stoichiometry and orientation. To efficiently target cell surface receptors, exploration of the parameters that modulate cellular accessibility of these nanostructures is essential. In this study we systematically investigate tunable design parameters of antibody-functionalized DNA nanostructures binding to therapeutically relevant receptors. We show that, although the native affinity of antibody-functionalized DNA nanostructures remains unaltered, the absolute number of bound surface receptors is lower compared to soluble antibodies and is mainly governed by nanostructure size and DNA handle location. The obtained results provide key insights in the ability of ligand-functionalized DNA nanostructures to bind surface receptors and yields design rules for optimal cellular targeting.

Summary To further our understanding of how biochemical information flows through cells upon external stimulation, we require single-cell multi-omics methods that concurrently map changes in (phospho)protein levels across signaling networks and the associated gene expression profiles. Here, we present quantification of RNA and intracellular epitopes by sequencing (QuRIE-seq), a droplet-based platform for single-cell RNA and intra- and extracellular (phospho)protein quantification through sequencing. We applied QuRIE-seq to quantify cell-state changes at both the signaling and the transcriptome level after 2-, 4-, 6-, 60-, and 180-min stimulation of the B cell receptor pathway in Burkitt lymphoma cells. Using the multi-omics factor analysis (MOFA+) framework, we delineated changes in single-cell (phospho)protein and gene expression patterns over multiple timescales and revealed the effect of an inhibitory drug (ibrutinib) on signaling and gene expression landscapes.

Current high-throughput single-cell multi-omics methods cannot concurrently map changes in (phospho)protein levels and the associated gene expression profiles. We present QuRIE-seq (Quantification of RNA and Intracellular Epitopes by sequencing) and use multi-factor omics analysis (MOFA+) to map signal transduction over multiple timescales. We demonstrate that QuRIE-seq can trace the activation of the B-cell receptor pathway at the minute and hour time-scale and provide insight into the mechanism of action of an inhibitory drug, Ibrutinib.

Background Colorectal cancer (CRC) associated with Lynch syndrome is characterized by an abundance of infiltrating lymphocytes. To study whether tumor-specific antibodies with therapeutic potential can be isolated from these patients, the B-cell repertoire from a patient with Lynch syndrome who recovered from a stage IV colon carcinoma was screened. Here we describe an antibody, AT1636 that recognizes a previously unidentified O-mannosylated 70kDa form of E-cadherin. The intercellular interactions by E-cadherin on tumor cells have for long been recognized as protective in cancer metastasis, and deregulation of E-cadherin is a hallmark for epithelial-mesenchymal transition (EMT). Methods The study protocol was approved by the Medical Ethical Committee of the Academic Medical Centre, Amsterdam, The Netherlands (NL42718.018.12). AIMM’s BCL6 and Bcl-xL immortalization method1 was used to interrogate the human antibody repertoire. From a carrier of a pathogenic gene variant in the MSH6 gene diagnosed with stage IV CRC and liver metastasis that had been treated with avastin, capecitabine and oxaliplatin, peripheral-blood memory B cells were obtained 9 years after last treatment. Antibodies-containing supernatant of cultured B-cells were screened for binding to 3 different CRC cell lines (DLD1, LS174T and COLO205) and absence of binding to fibroblast by flow cytometry. A high-affinity variant of AT1636 (AT1636IYN) was sorted from the original AT1636, AID-expressing B-cell clone.2 Results Antibodies that demonstrated differential binding to CRC cells were characterized and targets recognized by such antibodies were identified using immunoprecipitation and mass-spectrometry. One of the antibodies, AT1636, recognized a previously unidentified O-mannosylated 70kDa E-cadherin variant (ECV). Although the 70kDa ECV is found in all full-length E-cadherin expressing cells, tumor-specific binding of AT1636 is dependent on the O-mannosylation pattern in the antibody epitope on ECV. Using shRNA knock-down AT1636 binding was shown to depend on the transmembrane O-mannosyltransferase targeting cadherins 3 (TMTC3). 3 In accordance, coexpression of TMTC3 and E-cadherin in tumor cells is predictive for AT1636 binding. In addition, we observed that (over)expression of ECV results in a strong de-adhesive, EMT-like phenotype. Although AT1636 by itself is not able to induce ADCC, the CD3-bispecific antibody (single-chain UCHT1) AT1636 format specifically killed CRC cell lines. Conclusions The AT1636 antibody retrieved from a patient with Lynch syndrome binds a previous unidentified cancer-specific O-mannosylated 70kDa form of E-cadherin. This variant might play a role in tumor-cell invasion and metastasis. More importantly, we provide a rationale to advance AT1636 based therapeutics for treatment of CRC. Ethics Approval The study protocol was approved by the Medical Ethical Committee of the Academic Medical Centre, Amsterdam, The Netherlands (NL42718.018.12) References M.J. Kwakkenbos, et al. Generation of stable monoclonal antibody-producing B cell receptor-positive human memory B cells by genetic programming. Nature Medicine 2010;16:123–128. K. Wagner, et al. Bispecific antibody generated with sortase and click chemistry has broad antiinfluenza virus activity. Proc Natl Acad Sci USA 2014; 111: 16820–16825. I.S.B. Larsen, et al. Discovery of an O-mannosylation pathway selectively serving cadherins and protocadherins. Proc Natl Acad Sci USA 2017;114:11163–11168.

Background Adaptive immunity to cancer cells forms a crucial part of cancer immunotherapy. Recently, the importance of tumor B-cell signatures were shown to correlate with melanoma survival. We investigated whether tumor-targeting antibodies could be isolated from a patient that cured (now 13 years tumor-free) metastatic melanoma following adoptive transfer of ex vivo expanded autologous T cells. Methods Patient‘s peripheral blood B cells were isolated and tested for the presence of tumor-reactive B cells using AIMM’s immmortalisation technology. Antibody AT1412 was identified by virtue of its differential binding to melanoma cells as compared to healthy melanocytes. AT1412 binds the tetraspanin CD9, a broadly expressed protein involved in multiple cellular activities in cancer and induces ADCC and ADCP by effector cells. Results Spontaneous immune rejection of tumors was observed in human immune system (HIS) mouse models implanted with CD9 genetically-disrupted A375 melanoma (A375-CD9KO) tumor cells, while A375wt cells were not cleared. Most notably, no tumor rejection of A375-CD9KO tumors was observed in NSG mice, indicating that blockade of CD9 makes tumor cells susceptible to immune rejection.CD9 has been described to regulate integrin signaling, e.g. LFA-1, VLA-4, VCAM-1 and ICAM-1. AT1412 was shown to modulate CD9 function by enhancing adhesion and transmigration of T cells to endothelial (HUVEC) cells. AT1412 was most potently enhancing transendothelial T-cell migration, in contrast to a high affinity version of AT1412 or other high affinity anti-CD9 reference antibodies (e.g. ALB6). Enhanced immune cell infiltration is also observed in immunodeficient mice harbouring a human immune system (HIS). AT1412 strongly enhanced CD8 T-cell and macrophage infiltration resulting in tumor rejection (A375 melanoma). PD-1 checkpoint blockade is further sustaining this effect. In a second melanoma model carrying a PD-1 resistant and highly aggressive tumor (SK-MEL5) AT1412 together with nivolumab was inducing full tumor rejection, while either one of the antibodies alone did not. Conclusions The safety of AT1412 has been assessed in preclinical development and is well tolerated up to 10 mg/kg (highest dose tested) by non human primates. AT1412 demonstrated a half-life of 8.5 days, supporting 2–3 weekly administration in humans. Besides transient thrombocytopenia no other pathological deviations were observed. No effect on coagulation parameters, bruising or bleeding were observed macro- or microscopically. The thrombocytopenia is reversible, and its recovery accelerated in those animals developing anti-drug antibodies. First in Human clinical study is planned to start early 2021. Ethics Approval Study protocols were approved by the Medical Ethical Committee of the Leiden University Medical Center (Leiden, Netherlands). Consent Blood was obtained after written informed consent by the patient.

Persistent antigen exposure in chronic infection and cancer has been proposed to lead to cytotoxic T lymphocyte (CTL) "exhaustion", i.e., loss of effector function and disease control. Recent work identifies a population of poorly differentiated TCF-1(+)PD-1(+)CD8(+)T cells as precursors of the terminally exhausted CTL pool. These "predysfunctional" CTLs are suggested to respond to PD-1 targeted therapy by giving rise to a pool of functional CTLs. Supported by gene expression analyses, we present a model in which lack of CD4(+)T cell help during CD8(+)T cell priming results in the formation of predysfunctional CTLs. Our model implies that predysfunctional CTLs are formed during priming and that the remedy for CTL dysfunction is to provide "help" signals for generation of optimal CTL effectors. We substantiate that this may be achieved by engaging CD4(+)T cells in new CD8(+)T cell priming, or by combined PD-1 blocking and CD27 agonism with available immunotherapeutic antibodies.

Despite recent advances in treatment of B-acute lymphoblastic leukemia (B-ALL) there is still a need for novel targeted therapies. The tetraspanin CD9 is expressed in 60-80% of B-ALL and correlates with adverse prognosis. Recently, the mouse CD9 antibody ALB6 was shown to induce leukemia rejection in NOD/SCID mice. However, clinical development of ALB6 and other CD9-targeting antibodies was hampered by their CD9 mediated induction of platelet aggregation. It is known that CD9 is still expressed on tumor cells after treatment with chemotherapy or blinatumomab (Leung, 2019; Linder, 2016). AT1412 is a fully human antibody isolated from B cells of a patient that was cured from stage IV metastatic melanoma (Verdegaal, 2011). AT1412 targets CD9, without inducing platelet aggregation in vitro or thrombosis in cynomolgus monkeys after intravenous administration at therapeutic dose levels. By crystallography AT1412 was shown to bind a unique epitope preventing homodimerization of CD9, distinctly different from other CD9 antibodies. AT1412 binds a majority of patient B-ALL samples, but not T-ALL and induces ADCC and ADCP of CD9 positive B-ALL primary cells and the level of cytotoxicity significantly correlated with that of AT1412 binding. In both NSG and immunodeficient mice harboring a human immune system (HIS mice) AT1412 demonstrated a strong, dose-dependent tumor rejection of B-ALL, most pronounced in the extramedullary sites. In HIS mice AT1412 treatment led to an accumulation of T cells and CD14+ myeloid cells at the tumor sites. To support clinical development, pre-clinical safety of AT1412 was evaluated in cynomolgus monkeys. AT1412 demonstrated a half-life of 8.5 days, supporting 2-3 weekly administration in humans. Besides transient thrombocytopenia no other pathological deviations were observed. No effect on coagulation parameters, bruising or bleeding were observed macro- or microscopically. The thrombocytopenia is reversible, and its recovery accelerated in those animals developing anti-drug antibodies. Taken together, we demonstrate that CD9 on B-ALL cells can be successfully targeted by AT1412. AT1412 targets a unique epitope and does not induce thrombosis. Pre-clinical safety assessment is supporting that AT1412 can be safely administered. A First in Human clinical study is scheduled to start early 2021 in human solid tumors to determine safety and efficacy. AT1412 efficacy will be evaluated in B-ALL in expansion cohorts. Leung, K.T., Zhang, C., Chan, K.Y.Y., Li, K., Cheung, J.T.K., Ng, M.H.L., Zhang, X.-B., Sit, T., Lee, W.Y.W., Kang, W., et al. (2019). CD9 blockade suppresses disease progression of high-risk pediatric B-cell precursor acute lymphoblastic leukemia and enhances chemosensitivity. Leukemia. Linder, K., Gandhiraj, D., Hanmantgad, M., Seiter, K., and Liu, D. (2016). Complete remission after single agent blinatumomab in a patient with pre-B acute lymphoid leukemia relapsed and refractory to three prior regimens: HyperCVAD, high dose cytarabine mitoxantrone and CLAG. Exp. Hematol. Oncol. 5, 5-8. Verdegaal, E.M.E., Visser, M., Ramwadhdoebé, T.H., van der Minne, C.E., van Steijn, J. a Q.M.J., Kapiteijn, E., Haanen, J.B. a G., van der Burg, S.H., Nortier, J.W.R., and Osanto, S. (2011). Successful treatment of metastatic melanoma by adoptive transfer of blood-derived polyclonal tumor-specific CD4+ and CD8+ T cells in combination with low-dose interferon-alpha. Cancer Immunol. Immunother. 60, 953-963. Disclosures Schotte: AIMM Therapeutics: Current Employment, Current equity holder in private company. Villaudy:AIMM Therapeutics: Current Employment, Current equity holder in private company. Levie:AIMM Therapeutics: Current Employment, Current equity holder in private company. Go:AIMM Therapeutics: Current Employment, Current equity holder in private company. Yasuda:AIMM Therapeutics: Current Employment, Current equity holder in private company. Frankin:AIMM Therapeutics: Current Employment, Current equity holder in private company. Cercel:AIMM Therapeutics: Current Employment, Current equity holder in private company. van Hal-van Veen:AIMM Therapeutics: Current Employment, Current equity holder in private company. van de Berg:AIMM Therapeutics: Current Employment, Current equity holder in private company. Fatmawati:AIMM Therapeutics: Current Employment, Current equity holder in private company. Kedde:AIMM Therapeutics: Current Employment, Current equity holder in private company. Claassen:AIMM Therapeutics: Current Employment, Current equity holder in private company. Rijneveld:Amgen: Research Funding; Servier: Research Funding. Spits:AIMM Therapeutics: Current equity holder in private company, Ended employment in the past 24 months, Membership on an entity's Board of Directors or advisory committees. van Eenennaam:AIMM Therapeutics: Current Employment, Current equity holder in private company.

Background Accumulating preclinical data indicate that targeting the SIRPα/CD47 axis alone or in combination with existing targeted therapies or immune checkpoint inhibitors enhances tumor rejection. Although several CD47-targeting agents are currently in phase I clinical trials and demonstrate activity in combination therapy, high and frequent dosing was required and safety signals (acute anemia, thrombocytopenia) were recorded frequently as adverse events. Based on the restricted expression pattern of SIRPα we hypothesized that antibodies targeting SIRPα might avoid some of the concerns noted for CD47-targeting agents. Methods SIRPα-targeting antibodies were generated and characterized for binding to human SIRPα alleles and blockade of the interaction with CD47. Functional activity was established in vitro using human macrophages or neutrophils co-cultured with human Burkitt’s lymphoma cell lines. The effect of SIRPα versus CD47 targeting on human T-cell activation was studied using an allogeneic mixed lymphocyte reaction and a Staphylococcus enterotoxin B-induced T-cell proliferation assay. Potential safety concerns of the selected SIRPα-targeting antibody were addressed in vitro using a hemagglutination assay and a whole blood cytokine release assay, and in vivo in a single-dose toxicity study in cynomolgus monkeys. Results The humanized monoclonal IgG2 antibody ADU-1805 binds to all known human SIRPα alleles, showing minimal binding to SIRPβ1, while cross-reacting with SIRPγ, and potently blocking the interaction of SIRPα with CD47. Reduced FcγR binding proved critical to retaining its function towards phagocyte activation. In vitro characterization demonstrated that ADU-1805 promotes macrophage phagocytosis, with similar potency to anti-CD47 antibodies, and enhances neutrophil trogocytosis. Unlike CD47-targeting agents, ADU-1805 does not interfere with T-cell activation and is not expected to require frequent and extensive dosing due to the restricted expression of SIRPα to cells of the myeloid lineage. ADU-1805 is cross-reactive to cynomolgus monkey SIRPα and upon single-dose intravenous administration in these non-human primates (NHPs) did not show any signs of anemia, thrombocytopenia or other toxicities. Conclusions Blocking the SIRPα-CD47 interaction via SIRPα, while similarly efficacious in vitro, differentiates ADU-1805 from CD47-targeting agents with respect to safety and absence of inhibition of T-cell activation. The data presented herein support further advancement of ADU-1805 towards clinical development.

The programmed cell death 1 (PD-1) pathway represents a major immune checkpoint, which may be engaged by cells in the tumor microenvironment to overcome active T-cell immune surveillance. Pembrolizumab (Keytruda®, MK-3475) is a potent and highly selective humanized mAb of the IgG4/kappa isotype designed to directly block the interaction between PD-1 and its ligands, PD-L1 and PD-L2. This blockade enhances the functional activity of T cells to facilitate tumor regression and ultimately immune rejection. Pembrolizumab binds to human and cynomolgus monkey PD-1 with picomolar affinity and blocks the binding of human and cynomolgus monkey PD-1 to PD-L1 and PD-L2 with comparable potency. Pembrolizumab binds both the C'D and FG loops of PD-1. Pembrolizumab overcomes human and cynomolgus monkey PD-L1-mediated immune suppression in T-cell cultures by enhancing IL2 production following staphylococcal enterotoxin B stimulation of healthy donor and cancer patient cells, and IFNγ production in human primary tumor histoculture.

A significant proportion of human epidermal growth factor receptor 2 (Her2/ErbB2)-positive metastatic breast cancer patients are refractory to Her2-targeted trastuzumab-like therapy. Some of this resistance has been attributed to the upregulation of immune checkpoints such as programmed cell death-1 (PD-1) and its ligand, PD-L1 in Her2-positive breast cancer patients. Therefore, therapies targeting both the PD-1/PD-L1 interaction and oncogenic Her2 signaling are of significant clinical interest. Here, we constructed a mouse bispecific antibody targeting PD-L1 and rat Her2 (referred to as BsPD-L1xrErbB2) aiming to redirect the anti-PD-L1 response toward Her2-expressing tumor cells. BsPD-L1xrErbB2 demonstrated additive binding to interferon (IFN)-γ treated Her2(+) TUBO tumor cells, but it did not affect the proliferation of tumor cells in-vitro. BsPD-L1xrErbB2 also blocked the PD-1/PD-L1 interaction. This bispecific antibody was constructed with a mouse IgG2a Fc backbone and interacted with Fcγ receptors and resulted in complement deposition (C3). ADCC and complement action could be potential mechanisms of action of this molecule. BsPD-L1xrErbB2 successfully reduced TUBO tumor growth and increased tumor rejection rate compared to the monovalent anti-PD-L1, monovalent anti-ErbB2 or the combination of anti-PD-L1 and anti-ErbB2 monotherapies. The enhanced anti-tumor effect of BsPD-L1xrErbB2 was dependent on CD8(+) T lymphocytes and IFN-γ, as depletion of CD8(+) T lymphocytes and neutralization of IFN-γ completely abolished the antitumor activity of the bispecific antibody. Consistently, BsPD-L1xrErbB2 treatment also increased the frequency of intratumor CD8(+) T lymphocytes. Taken together, our data support a bispecific antibody approach to enhance the anti-tumor efficacy of PD-1/PD-L1 checkpoint blockade in Her2-positive metastatic breast cancers.

While showing promise, vaccination strategies to treat cancer require further optimization. Likely barriers to efficacy involve cancer-associated immunosuppression and peripheral tolerance, which limit the generation of effective vaccine-specific cytotoxic T lymphocytes (CTL). Because CD4+ T cells improve CTL responsiveness, next-generation vaccines include helper epitopes. Here, we demonstrate in mice how CD4+ T-cell help optimizes the CTL response to a clinically relevant DNA vaccine engineered to combat human papillomavirus–expressing tumors. Inclusion of tumor-unrelated helper epitopes greatly increased CTL priming, effector, and memory T-cell programming. CD4+ T-cell help optimized the CTL response in all these aspects via CD27/CD70 costimulation. Notably, administration of an agonistic CD27 antibody could largely replace helper epitopes in promoting primary and memory CTL responses, acting directly on CD8+ T cells. CD27 agonism improved efficacy of the vaccine without helper epitopes, more so than combined PD-1 and CTLA-4 blockade. Combining CD27 agonism with CTLA-4 blockade improved vaccine-induced CTL priming and tumor infiltration, but only combination with PD-1 blockade was effective at eradicating tumors, thereby fully recapitulating the effect of CD4+ T-cell help on vaccine efficacy. PD-1 blockade alone did not affect CTL priming or tumor infiltration, so these results implied that it cooperated with CD4+ T-cell help by alleviating immune suppression against CTL in the tumor. Helper epitope inclusion or CD27 agonism did not stimulate regulatory T cells, and vaccine efficacy was also improved by CD27 agonism in the presence of CD4+ T-cell help. Our findings provide a preclinical rationale to apply CD27 agonist antibodies, either alone or combined with PD-1 blockade, to improve the therapeutic efficacy of cancer vaccines and immunotherapy generally. Cancer Res; 76(10); 2921–31. ©2016 AACR.

INTRODUCTION Colorectal cancer (CRC) associated with Lynch syndrome is characterized by an abundance of infiltrating lymphocytes. To study whether tumor-specific antibodies with therapeutic potential can be isolated from these patients, the B-cell repertoire from a patient with Lynch syndrome who recovered from a stage IV colon carcinoma was screened. Here we describe a novel human antibody, AT1636 that recognizes a previously unidentified O-mannosylated 70kDa form of E-cadherin. The intercellular interactions by E-cadherin on tumor cells have for long been recognized as protective in cancer metastasis, and deregulation of E-cadherin is a hallmark for epithelial-mesenchymal transition (EMT). METHODS AIMM's BCL6 and Bcl-xL immortalization method[1] was used to interrogate the human antibody repertoire against targets on colon cancer cells. From a carrier of a pathogenic gene variant in the MSH6 gene diagnosed with stage IV CRC and liver metastasis that had been treated with avastin, capecitabine and oxaliplatin, peripheral-blood memory B cells were obtained 9 years after last treatment. Antibodies-containing supernatant of cultured B-cells were screened for binding to 3 different CRC cell lines (DLD1, LS174T and COLO205) and absence of binding to fibroblast by flow cytometry. High-affinity variants of AT1636 (AT1636IYN) were sorted from the AID-expressing immortalized B-cells clone[2]. RESULTS Patient derived antibodies that demonstrated differential binding to CRC cells were further characterized. Targets recognized by such antibodies were identified using immunoprecipitation and mass-spectrometry. AT1636 binds to a previously unidentified single O-mannosylated 70kDa E-cadherin variant (ECV). Although the 70 kDa ECV is found in all cells that express full length E-cadherin, tumor-specific binding of AT1636 is dependent on the single O-mannosylation pattern in the antibody epitope on ECV. Using shRNA knock-down AT1636 binding was shown to depend on the transmembrane O-mannosyltransferase targeting cadherins 3 (TMTC3)[3]. In accordance, coexpression of TMTC3 and E-cadherin in tumor cells is predictive for AT1636 binding. In addition, we observed that (over)expression of ECV results in a strong de-adhesive, EMT-like phenotype. Although AT1636 by itself is not able to induce ADCC, the CD3-bispecific antibody (single-chain UCHT1) AT1636 format specifically killed CRC cell lines. CONCLUSION The AT1636 antibody retrieved from a patient with Lynch syndrome binds a previous unidentified cancer-specific O-mannosylated 70kDa form of E-cadherin. This variant might play a role in tumor-cell invasion and metastasis. More importantly, we provide a rationale to advance AT1636 based therapeutics for treatment of CRC. references 1) Kwakkenbos et al. Generation of stable monoclonal antibody-producing B cell receptor-positive human memory B cells by genetic programming. Nature Medicine 2010 2) Wagner et al. Bispecific antibody generated with sortase and click chemistry has broad antiinfluenza virus activity. PNAS 2014 3) Larsen et al. Discovery of an O-mannosylation pathway selectively serving cadherins and protocadherins. PNAS 2017 Citation Format: Martijn Kedde, Tim Beaumont, Sabrina J. Merat, Mark J. Kwakkenbos, Lina Bartels, Dorien van de Berg, Koen Wagner, Arjen Q. Bakker, Kelly Maijoor, Martino Böhne, Camille Bru, Veronika Kattler, Hans van Eenennaam, Victorine H. Roos, Frank G.J. Kallenberg, Jan Paul Medema, Paul J. Hensbergen, Pauline van Helden, Evelien Dekker, Hergen Spits. A colon cancer survivor-derived antibody recognizes a previously unidentified truncated, O-mannosylated 70kDa variant of E-cadherin [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5163.

Background Adaptive immunity to cancer cells has been shown to form a crucial part of cancer immunotherapy. Recently, the importance of tumor B-cell signatures were shown to correlate with melanoma survival. We investigated whether anti-tumor antibodies could be isolated from a patient with metastatic melanoma that was tumor-free for 6 years following adoptive transfer of ex vivo expanded autologous T cells (Verdegaal 2011). Methods Peripheral blood memory B cells were immortalized using AIMM's immmortalisation technology (ectopic Bcl-6 and Bcl-xL; Kwakkenbos 2010) expression and analyzed for the presence of tumor-reactive B cells. Results AT1412 antibody was identified by virtue of its differential binding to melanoma cells as compared to healthy melanocytes. AT1412 was found to bind the tetraspanin CD9, a broadly expressed protein involved in multiple cellular activities and disease progression. The crystal structure of the CD9 large extracellular loop in complex with an AT1412 Fab fragment revealed that AT1412 binds the CD9 epitope in an extended and unfolded conformation. In addition to melanoma, AT1412 binds other tumor types including gastric, colon- and pancreatic cancer. AT1412 was shown to induce antibody dependent cellular cytotoxicity (ADCC) and antibody dependent cellular phagocytosis (ADCP) of cancer cells. In addition, AT1412 demonstrated activation of monocytes, most likely via activation of CD9 on platelets-bound monocytes. In mice carrying a human immune system (HIS-mice; van Lent 2010) AT1412 strongly enhanced tumor rejection of A375 and SKMEL-5 tumor cells. AT1412 treatment was shown to enhance tumor infiltration of CD8 T cells and macrophages. AT1412 efficacy was further synergistically enhanced when combined with an anti-PD1 antibody (nivolumab). Other groups have shown that anti-CD9 antibodies can induce tumor rejection in mice. However, these antibodies could not be advanced due to induction of platelet aggregation. We show that AT1412 does not induce aggregation of both human and cynomolgus platelets. Also affinity matured versions of AT1412 do not induce platelet aggregation, supporting that AT1412 binds a unique epitope on CD9. In addition, we studied the safety of AT1412 during a one month exposure in a weekly administration in cynomolgus monkeys. Besides a transient thrombocytopenia AT1412 was well tolerated up to 10 mg/kg antibody (highest dose tested) and did not lead to other adverse events nor were any coagulation factors affected Conclusions Taken together, applying AIMM's proprietary B-cell immortalization technology we isolated antibody AT1412 that targets a unique epitope on CD9. AT1412 was shown to induce tumor rejection as a single agent and enhances the activity of anti-PD-1 antibodies, while not inducing platelet aggregation. One month exposure of AT1412 in monkeys indicated that AT1412 can be safely administered. Preclinical development of AT1412 is ongoing to initiate clinical evaluation in 2020. Funding Dutch Cancer Society, grant UVA 2010-4822 Citation Format: Remko Schotte, Julien Villaudy, Martijn Kedde, Wouter Pos, Koen Wagner, Viviana Neviani, Daniel Go, Etsuko Yasuda, Christien Fatmawati, Els Verdegaal, Susan van Hal, Yvonne Claasen, Hans van Eenennaam, Pauline van Helden, Piet Gros, Sjoerd van der Burg, Hergen Spits. A patient-derived anti-CD9 antibody induces tumor rejection and synergistically enhances anti-PD1 activity [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 532.

Introduction Despite rapid advances in immunotherapeutic options for precursor B-acute lymphoblastic leukemia (BCP-ALL), outcomes remain poor especially for adult ALL and relapsed pediatric ALL. With conventional chemotherapy remission percentages in adult ALL range from 75 to 90%, but relapse rates are high and long-term leukemia-free survival ranges between 35-70% depending on age and risk group. The introduction of CD19-targeting immunotherapy has significantly improved patient outcomes in (relapsed) BCP-ALL. However, tumor escape via downregulation of CD19 occurs in a significant number of patients. An ongoing medical need remains for the identification of alternative immunotherapeutic targets for treatment of ALL. Methods CD9 is expressed in 60-80% of BCP-ALL, is correlated with adverse prognosis and has been proposed as therapeutic target for BCP-ALL. AT1412 is a fully human CD9-targeting antibody, that was identified from a patient cured of metastatic melanoma after adoptive T-cell therapy, using our B-cell immortalization technology (AIMSelect) [Kwakkenbos et al. Nat. Med. 2010]. The antibody was selected based on differential binding to melanoma cells as compared to healthy melanocytes and was shown to be successful in killing melanoma cells in vitro and in vivo. Results Binding of AT1412 to BCP-ALL cell lines SUP-B15, MHH-CALL-2 and CCRF-SB varied as expected based on CD9 levels that we detected using a commercial CD9 antibody. AT1412-induced antibody dependent cellular cytotoxicity (ADCC) and antibody dependent cellular phagocytosis (ADCP) correlated with the level of AT1412 binding. No binding was seen to the T-ALL cell line Jurkat. These findings were confirmed in primary ALL samples, obtained prospectively at diagnosis from a cohort of patients with T- or B-ALL (n=38). AT1412 showed binding to the majority of B-ALL samples but not to T-ALL samples. AT1412 induced ADCC of nearly all B-ALL samples it bound to (10/11) and of none of the non-binding B-ALL or T-ALL samples. Cytotoxicity significantly correlated with the level of AT1412 binding. These findings were corroborated by the observation that AT1412 induced specific B-ALL cell death when a bone marrow sample from a newly diagnosed BCP-ALL patient was incubated with AT1412. Remarkably, AT1412 induced cell death in the absence of added effector cells or other (chemo)therapeutic agents, while the sample contained over 80% blasts and as little as 3% lymphocytes. We are currently investigating the in vivo efficacy of the antibody in a humanized immune system mouse model with human BCP-ALL. Conclusion Taken together, the majority of BCP-ALL express CD9 and this is associated with poor prognosis. Our data demonstrate that CD9 can be successfully targeted by the human CD9 antibody AT1412, suggesting that AT1412 has the potential to be developed as a therapeutic antibody for B-ALL. AT1412 is currently being advanced through preclinical development. Citation Format: Greta de Jong, Sophie E. Levie, Remko Schotte, Wouter Pos, Daniel Go, Etsuko Yasuda, Madalina Cercel, Susan E. van Hal, Esmay Frankin, Aniko Szabo, Martijn Kedde, Els Verdegaal, Julien Villaudy, Sjoerd van der Burg, Pauline M. van Helden, Hans van Eenennaam, Hergen Spits, Anita van Rijneveld, Mette D. Hazenberg. AT1412, a patient-derived antibody in development for the treatment of CD9-positive precursor B-acute lymphoblastic leukemia [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 531.

Despite rapid advances in immunotherapeutic options for precursor B-acute lymphoblastic leukemia (ALL), outcomes remain poor especially for adult ALL and relapsed pediatric ALL. With conventional chemotherapy, remission percentages in adult ALL range from 75 to 90%, but relapse rates are high and long-term leukemia-free survival ranges between 35-70% depending on age and risk group. The introduction of CD19 targeting immunotherapy has significantly improved patient outcomes in (relapsed) B-ALL. However, tumor escape via downregulation of CD19 occurs in a significant number of patients. Therefore an ongoing urgency remains for the identification of additional or alternative immunotherapeutic targets for the treatment of ALL. AT1412 is an antibody that was identified from the peripheral blood memory B cell pool of a patient cured of metastatic melanoma after adoptive T-cell therapy, using a B cell immortalization technology (AIMSelect) with ectopic Bcl-6 and Bcl-xL expression as described previously [Kwakkenbos et al. Nat. Med. 2010]. The antibody was selected based on differential binding to melanoma cells as compared to healthy melanocytes and was shown to be successful in killing melanoma cells in vitro and in vivo [manuscript submitted]. In addition to melanoma, AT1412 binds other tumor types including B-ALL, gastric, colon- and pancreatic cancer. The target of AT1412 is the tetraspanin CD9, which is expressed by more than half of all B-ALL. Expression of CD9 has been correlated with adverse prognosis [Liang et al. Cancer Biomark. 2018]. We assessed binding of this human CD9 antibody to a panel of ALL cell lines using flow cytometry. Binding of AT1412 to the B-ALL cell lines SUP-B15, MHH-CALL-2 and CCRF-SB varied as expected based on the CD9 levels that we detected using a commercial CD9 antibody. AT1412 induced antibody dependent cellular cytotoxicity (ADCC) on these cells, in line with the level of AT1412 binding. No binding was seen to the T-ALL cell line Jurkat. Importantly, these findings were confirmed in primary ALL samples, obtained prospectively at diagnosis from a cohort of patients with T- or B-ALL (n=30). AT1412 showed binding to 61% of B-ALL samples but not to T-ALL samples. The potential of AT1412 to induce ADCC was tested on patient samples from the same panel. Remarkably, AT1412 induced ADCC of all B-ALL samples it bound to (8 out of 14) and of none of the T-ALL samples. Cytotoxicity significantly correlated with the level of AT1412 binding. These findings were supported by the observation that AT1412 induced B-ALL cell death when a freshly drawn whole bone marrow sample from a patient with newly diagnosed B-ALL was cocultured with AT1412. AT1412-induced cell death of B-ALL blasts occurred without affecting the monocytic, granulocytic and lymphocytic populations. This cell death was not observed when this patient's ALL blasts were incubated with AML-targeting antibodies. Remarkably, AT1412 induced cell death in the absence of added effector cells or other (chemo)therapeutic agents, while the bone marrow sample contained over 80% blasts and as little as 3% lymphocytes. We are currently investigating the in vivo efficacy of the antibody in a humanized immune system mouse model with human B-ALL. Taken together, the majority of precursor B-ALL blasts express CD9 and expression of CD9 is associated with a dismal outcome. Our data demonstrate that CD9 can be successfully targeted by the human CD9 antibody AT1412, suggesting that AT1412 has the potential to be developed as a therapeutic antibody for B-ALL. AT1412 is currently being advanced through preclinical development. Disclosures De Jong: AIMM Therapeutics: Employment. Levie:AIMM Therapeutics: Employment. Schotte:AIMM Therapeutics: Employment, Equity Ownership, Patents & Royalties: Patent WO2017119811A1. Pos:AIMM Therapeutics: Patents & Royalties: Patent WO2017119811A1. Go:AIMM Therapeutics: Employment, Patents & Royalties: Patent WO2017119811A1. Yasuda:AIMM Therapeutics: Employment, Equity Ownership. Cercel:AIMM Therapeutics: Employment. van Hal-van Veen:AIMM Therapeutics: Employment. Frankin:AIMM Therapeutics: Employment. Villaudy:AIMM Therapeutics: Employment, Equity Ownership, Patents & Royalties: Patent WO2017119811A1. van Helden:AIMM Therapeutics: Employment, Equity Ownership, Patents & Royalties: Patent WO2017119811A1. van Eenennaam:AIMM Therapeutics: Employment. Spits:AIMM Therapeutics: Employment, Equity Ownership, Patents & Royalties: Patent WO2017119811A1. Hazenberg:AIMM Therapeutics: Other: Employment/equity of partner/spouse.

Background: SIRPα signaling dependent immunoregulatory activity on myeloid cells is activated by binding of its ligand CD47, and blockade of the pathway may enhance anti-tumor immunity. Hence the pathway is thought to represent a novel immune checkpoint. CD47 has been extensively studied in the context of “don’t-eat-me” signaling. Since CD47 is ubiquitously expressed on normal cells, upregulated on many cancer cells, and also has SIRPα-independent functions, the safety of a CD47-targeted antibody has been a concern. Alternative strategies therefore are focusing on directly targeting SIRPα because of its more restricted expression to cells of the myeloid lineage. Methods: Using Aduro Biotech’s B-select platform, we have identified and characterized ADU-1805: a highly selective pan-allele anti-SIRPα antibody (EC50 SIRPαV1/SIRPαV2 ≤ 3nM) that lacks appreciable SIRPβ binding (EC50 > 120nM) but cross-reacts with SIRPγ (EC50 ≤ 5nM). We also demonstrate that ADU-1805 is cross-reactive with SIRP family members in non-human primates (NHPs) including cynomolgus monkeys. Results: ADU-1805 potently blocks CD47 binding (IC50 ≤ 1.5nM) to SIRPα in all known human SIRPA genotypes (including homozygous and heterozygous genotypes) and antagonizes SIRPα-CD47 interactions on primary SIRPα+ myeloid cells (IC50 ≤ 4nM). In line with its antagonistic properties, ADU-1805 enhances tumor cell clearance by human granulocytes and macrophages. Interestingly, anti-CD47 antibodies but not ADU-1805 inhibit T cell activation in an allogeneic mixed lymphocyte reaction. In addition, anti-SIRPα combines in a synergistic manner with PD-1 blockade to reduce tumor burden in several syngeneic mouse tumor models. Unlike CD47-targeting antibodies, ADU-1805 does not trigger hemagglutination or platelet binding in vitro, suggesting a reduced risk of red blood cell and platelet depletion in vivo. ADU-1805 is tested in a NHP study in cynomolgus monkeys at increasing dose levels to assess safety of the antibody. Conclusions: We have identified ADU-1805 as a potentially best-in-class antagonistic anti-SIRPα antibody with a unique binding profile as it binds all reported human SIRPα alleles but does not appreciably bind to the activating SIRPβ receptor. Blocking the SIRPα-CD47 innate immune checkpoint with ADU-1805 modulates myeloid cells in the tumor microenvironment and promotes antigen presentation and cross-priming of dendritic cells. We are currently advancing ADU-1805 through preclinical studies to address the safety, pharmacokinetics, and pharmacodynamics profile of this anti-human SIRPα antibody in vivo. Citation Format: Erik Voets, Joost Kreijtz, Paul Vink, David Lutje Hulsik, Mark Parade, Sanne Spijkers, Inge Reinieren-Beeren, Joost Rens, Wout Janssen, Peter van Zandvoort, Brian Francica, Meredith Leong, Andrea van Elsas, Hans van Eenennaam. Preclinical development of ADU-1805, a highly selective pan-allele anti-SIRPα antibody that blocks the SIRPα-CD47 innate immune checkpoint [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1203.

T-cell checkpoint inhibitors treat the cancer patient's immune system potentially inducing significant long-term survival. Pembrolizumab demonstrates clinical activity in patients diagnosed with melanoma and other cancers. Its mode of action suggests a rationale for combination with other treatment modalities, urging oncologists to brush up their knowledge of immunology. Clin Cancer Res; 21(19); 4251–3. ©2015 AACR. See related article by Patnaik et al., p. 4286

Here we show that overexpression or activation of B-cell maturation antigen (BCMA) by its ligand, a proliferation-inducing ligand (APRIL), promotes human multiple myeloma (MM) progression in vivo. BCMA downregulation strongly decreases viability and MM colony formation; conversely, BCMA overexpression augments MM cell growth and survival via induction of protein kinase B (AKT), MAPK, and nuclear factor (NF)-κB signaling cascades. Importantly, BCMA promotes in vivo growth of xenografted MM cells harboring p53 mutation in mice. BCMA-overexpressing tumors exhibit significantly increased CD31/microvessel density and vascular endothelial growth factor compared with paired control tumors. These tumors also express increased transcripts crucial for osteoclast activation, adhesion, and angiogenesis/metastasis, as well as genes mediating immune inhibition including programmed death ligand 1, transforming growth factor β, and interleukin 10. These target genes are consistently induced by paracrine APRIL binding to BCMA on MM cells, which is blocked by an antagonistic anti-APRIL monoclonal antibody hAPRIL01A (01A). 01A is cytotoxic against MM cells even in the presence of protective bone marrow (BM) myeloid cells including osteoclasts, macrophages, and plasmacytoid dendritic cells. 01A further decreases APRIL-induced adhesion and migration of MM cells via blockade of canonical and noncanonical NF-κB pathways. Moreover, 01A prevents in vivo MM cell growth within implanted human bone chips in SCID mice. Finally, the effect of 01A on MM cell viability is enhanced by lenalidomide and bortezomib. Taken together, these data delineate new molecular mechanisms of in vivo MM growth and immunosuppression critically dependent on BCMA and APRIL in the BM microenvironment, further supporting targeting this prominent pathway in MM.

Objective: The cannabinoid receptor 2 (CB2) has been implicated to play a role in various inflammatory proc- esses. Since atherosclerosis is currently considered a chronic inflammatory disease, we studied the effect of haema- topoietic CB2 deficiency on atherosclerosis development. Methods and results: To investigate the effect of CB2 deficiency in immune cells on atherogenesis in vivo, a bone marrow transplantation was performed in irradiated LDL receptor deficient mice (LDLr -/- ), using CB2 deficient (CB2 -/- ) or wild- type (WT) donor mice. After 12 weeks on a high fat-high cholesterol diet, en face analysis showed that atherosclerosis in the aortic arch was significantly increased in CB2 -/- transplanted animals (6.40 ± 3.21%) as compared to WT transplanted mice (3.85 ± 1.61%). Although the total lesion area in the aortic root was not significantly different between WT and CB2 -/- transplanted mice (0.45 ± 0.13 mm 2 and 0.51 ± 0.17 mm 2 , respectively), CB2 -/- transplanted mice showed a signifi- cantly larger plaque area (0.13 ± 0.07 mm 2 ) than WT transplanted mice (0.08 ± 0.05 mm 2 ) in the aortic valve in which atherogenesis is in an earlier stage than in the other aortic valves. Conclusions: Lack of endocannabinoid signaling via the CB2 receptor aggravates early atherosclerosis development in LDLr -/- mice, suggesting that CB2 specific activation may prevent the development of atherosclerosis.

A proliferation inducing ligand (APRIL) is a natural ligand with higher affinity than BAFF for both B cell maturation antigen (BCMA) and transmembrane activator and CAML interactor (TACI), which are overexpressed on multiple myeloma (MM) cells. APRIL, which is abundantly secreted by myeloma-supporting osteoclasts and macrophages, promotes MM cell progression in vivo and further induces regulatory T cells (Treg) via TACI, but not BCMA, to promote an immunosuppressive MM bone marrow (BM) microenvironment (Blood 2017;130:3066). In preclinical studies, an antagonistic APRIL monoclonal antibody (mAb) significantly inhibited human MM cell growth in SCID-hu mice and abrogated APRIL-induced immunosuppression mediated by Tregs. Here we characterized the ability of BION-1301, a neutralizing APRIL mAb, currently under clinical development in MM (NCT03340883) to overcome protection conferred by APRIL against MM cell lysis induced by the anti-BCMA J6M0 mAb (Blood 2017 130:499). Our studies show that APRIL protects against MM cell lysis induced by J6M0 in the presence of FcR-expressing effector cells (PBMC, monocytes or NK), an effect which is blocked by neutralizing anti-APRIL mAb. APRIL also downregulates J6M0-induced cell membrane CD107a expression on NK cells co-cultured with BCMA-expressing MM cells, which is similarly abrogated by anti-APRIL mAb. Importantly, anti-APRIL mAb also significantly decreases protection conferred by osteoclasts against MM cell lysis induced by J6M0. These data indicate that blocking APRIL with anti-APRIL mAb may enhance BCMA mAb targeted (J6M0)-induced MM cell lysis. Next, using anti-human IgG1 to detect J6M0 binding to MM cell surface BCMA, we found that APRIL in a dose-dependent manner directly competes with J6M0 for binding to BCMA, which was confirmed by ELISA. In addition, APRIL effectively inhibits J6M0 binding to BCMA at 4°C, which argues against APRIL-induced BCMA receptor shedding and/or internalization. In contrast, BAFF affects J6M0 binding to BCMA only at higher concentrations (>1 µg/mL), consistent with > 2-log higher affinity of APRIL vs. BAFF for BCMA. We further assessed APRIL, BAFF, BCMA, and TACI levels in the serum of patients with MM at various stages of disease. Specifically, we used ELISA to measure free APRIL and the ELLA automated immunoassay platform to determine the levels of soluble BAFF, soluble BCMA, and soluble TACI in serum samples from patients with MM (n=193) as well as serum samples from healthy volunteers (HV, n=100). Patient samples included monoclonal gammopathy of undetermined significance (MGUS, n=12), smoldering MM (SMM, n=20), newly diagnosed MM (ND, n=39), post induction pre-autologous stem cell transplant (ASCT, n=55), post-ASCT (n=6), and relapsed refractory MM (RR, n=61). We found that free APRIL levels are significantly increased in serum samples from patients with MM at all stages of disease, when change from baseline levels were compared to those from HVs (post-ASCT: p=0.0003; other groups (MGUS, SMM, ND, pre-ASCT, and RR): p Our studies therefore indicate that therapies directed at the APRIL/BCMA and APRIL/TACI axes may simultaneously target MM cells and counteract APRIL-induced immunosuppression, and that combination strategies targeting APRIL with BCMA directed therapy may augment anti-MM activity. Moreover, elevated free APRIL serum levels in MGUS and all stages of MM suggest a role for APRIL in mediating immunosuppression during the development and in the pathogenesis of MM. Disclosures Dulos: Aduro Biotech: Employment. Guelen:Aduro Biotech Europe: Employment. van Zandvoort:Aduro Biotech Europe: Employment. van de Wiel:Aduro Biotech Europe: Employment. van de Crommert:Aduro Biotech Europe: Employment. Lejeune:Aduro Biotech Europe: Employment. Namini:Aduro Biotech: Employment. Eenennaam:Aduro Biotech Europe: Employment, Equity Ownership. van Elsas:Aduro Biotech Europe: Employment. Richardson:Oncopeptides: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Karyopharm: Membership on an entity's Board of Directors or advisory committees; BMS: Research Funding; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Membership on an entity's Board of Directors or advisory committees. Munshi:OncoPep: Other: Board of director. Anderson:Takeda Millennium: Consultancy; C4 Therapeutics: Equity Ownership; Bristol Myers Squibb: Consultancy; Gilead: Membership on an entity's Board of Directors or advisory committees; Celgene: Consultancy; Oncopep: Equity Ownership.

A proliferation-inducing ligand (APRIL) is produced by multiple accessory and myeloid cells in the bone marrow niche. Through binding to its receptors B-cell maturation antigen (BCMA) and transmembrane activator and CAML interactor (TACI), APRIL plays an important role in the development and maintenance of cells derived from the B cell lineage. Both APRIL and its receptors have been identified as promotors of multiple myeloma (MM), a disease that is characterized by malignant proliferation of plasma cells in the bone marrow. We have shown that BION-1301, a first-in-class APRIL-targeting humanized antibody, blocks APRIL-induced effects on MM cell survival and the tumor microenvironment (TME). Despite recent approval of new therapies for MM, this type of cancer remains incurable as patients relapse and eventually become refractory to the current therapeutic regimens. We hypothesize that the presence of high levels of APRIL in the bone marrow of patients with MM establishes a protected environment favoring tumor cell survival and proliferation. The effects of APRIL were studied on tumor cell lines alone or in co-culture with osteoclasts, which are thought to produce APRIL in the bone marrow niche. We further hypothesize that engaging APRIL with BION-1301 sensitizes MM cells for other MM-targeting therapeutic agents. First, stimulation of MM cells with APRIL led to rapid phosphorylation of proteins in the NFκB pathway, that was completely blocked by the BION-1301 parental antibody hAPRIL.01A. In addition, by validating expression of known NFκB target genes, APRIL appears to protect MM cells from drug-induced toxicity by upregulation of anti-apoptotic genes (e.g. MCL-1), since APRIL enhanced survival of MM cells that were treated with anti-MM drugs, such as dexamethasone. Importantly, addition of hAPRIL.01A blocked this pro-survival effect, highlighting the potential of BION-1301 to enhance MM sensitivity towards MM-targeting agents. We confirmed the immune suppressive potential of APRIL by showing that APRIL upregulates known immunosuppressive proteins such as IL-10 and PD-L1, indicating that BION-1301 could increase sensitivity to therapies such as daratumumab that depend on the immune system. Finally, in vitro APRIL increased secretion of the chemokines MIP-1α and MIP-1β by MM cell lines, which are known to stimulate osteoclast differentiation and activation. This suggests a positive feedback loop in which APRIL, produced mainly by osteoclasts in the bone marrow, induces chemokine production by MM cells, and these chemokines in turn stimulate osteoclasts. Importantly, when hAPRIL.01A is added to block APRIL, chemokine production drops, indicating that in addition to sensitizing MM tumor cells for drug treatment, BION-1301 also suppresses osteoclast activity. Altogether, we demonstrate in vitro that APRIL induces anti-apoptotic signaling, triggers expression of an immune suppressive phenotype, and could potentially modulate the tumor microenvironment through the production of key chemokines. Blocking APRIL with BION-1301 may dampen these effects, illustrating the potential of BION-1301, as a therapeutic agent for MM, particularly in combination therapies. BION-1301 is currently in a phase 1 clinical trial for MM. Disclosures Bakker: Aduro Biotech: Employment. Habraken:Aduro Biotech: Employment. Dulos:Aduro Biotech: Employment. Van Elsas:Aduro Biotech Europe: Employment. Vink:Aduro Biotech Europe: Employment. Eenennaam:Aduro Biotech Europe: Employment, Equity Ownership.

The immune checkpoint CTLA-4 can be targeted with antibodies that are applied as single agent therapy or in combination with other therapies including surgery, radiation and chemotherapy. Anti-CTLA-4 antibodies may also augment other immunotherapies. Indeed, in syngeneic mouse tumor models anti-CTLA-4 strongly enhanced anti-tumor efficacy of live, attenuated double-deleted Listeria monocytogenes (LADD) and of the STING pathway activator ADU-S100. Anti-CTLA-4 therapy has shown clinical activity and durable responses in advanced cancers, but is also associated with severe immune-related adverse events (irAEs), although manageable and reversible. Local anti-CTLA4 application could limit the induction of irAEs while retaining its capacity to induce and enhance tumor-specific T cell responses. Here we present the results from the preclinical development of ADU-1604, a novel hIgG1 anti-CTLA-4 antibody. ADU-1604 binds a unique epitope on human CTLA-4 and was extensively characterized in vitro demonstrating potent CTLA-4 binding and blockade of CD80 and CD86 and its capability to enhance human T cell responses. As the antibody cross-recognizes CTLA-4 of cynomolgus monkeys this species was selected for the preclinical studies. Male animals received a single intravenous dose of the antibody to assess its pharmacokinetics and pharmacodynamics. ADU-1604 was well tolerated and showed an acceptable half-life. The antibody also enhanced the T cell-dependent antibody response in hepatitis B surface antigen immunized animals. After the single dose PK study a GLP 4-week repeat-dose toxicity study was conducted in cynomolgus monkeys administered 5 doses of ADU-1604 intravenously once a week followed by an 8-week recovery period. In general, ADU-1604 was well tolerated at up to 30 mg/kg/dose. Histopathology data is being processed and recovery data is pending. Proof of concept for the local administration of anti-CTLA-4 antibodies was conducted in syngeneic mouse models with the mouse CTLA-4 antibody 9D9 and in NSCLC humanized PDX models using ADU-1604 in the context of CD45+ human immune cells. These models demonstrated anti-tumor activity of systemically applied ADU-1604, comparable to a reference anti-CTLA-4 antibody. Intratumoral application of ADU-1604 in these models also induced tumor growth inhibition In conclusion, the preclinical development package warrants clinical investigation of ADU-1604 as monotherapy and for future combinations with other Aduro proprietary platforms including STING agonists and (p)LADD. Currently ADU-1604 is progressing through regulatory filing in preparation of clinical development. Citation Format: Maaike Hendriks, Joost Kreijtz, Paul Vink, David Lutje Hulsik, Imke Lodewijks, Astrid Bertens, Jos van de Crommert, Maurice Habraken, Wout Janssen, Judith Stammen-Vogelzang, Olivia Gardner, Weiwen Deng, Meredith Leong, Sarah McWhirter, Hans van Eenennaam, Andrea van Elsas. Assessment of pharmacology and toxicology of anti-CTLA-4 antibody (ADU-1604) in non-human primates and evaluation of local anti-CTLA-4 application [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1702.

APRIL critically triggers BCMA to drive proliferation and survival of human multiple myeloma (MM) cells. Importantly, APRIL induces resistance to lenalidomide, bortezomib and other standard-of-care drugs. Furthermore, APRIL drives expression of PD-L1, IL-10, VEGF and TGFβ forcing an immunosuppressive phenotype on BCMA+ cells. MM survival, resistance to treatment and the immunosuppressive phenotype can be blocked by BION-1301, the first-in class humanized antibody neutralizing APRIL. BION-1301 blocks APRIL binding to the MM relevant receptors BCMA and TACI in contrast with anti-BCMA bispecifics, ADC and CAR T-cells which use only BCMA for MM cell killing. In addition, BION-1301 inhibits immune suppressive effects of regulatory T-cells via TACI which differentiates it from BCMA targeting cytotoxic approaches. In vivo, BION-1301 was shown to suppress T cell-independent B cell responses to NP-Ficoll. Furthermore, APRIL blockade demonstrated single agent anti-MM activity in a humanized SCID model confirming its activity in vivo targeting MM cells in the tumor-protective bone marrow microenvironment. Here, we report on the preclinical pharmacokinetics (PK)/ pharmacodynamics (PD) relationship and safety analysis of BION-1301. A single-dose non-human primate (NHP) study administering intravenous BION-1301 at 0.3, 3 and 30 mg/kg, yielded PK parameters typical for IgG4 class antibodies and absence of tolerability issues. PD analysis showed a statistically significant reduction in total IgA and IgM in a dose-dependent fashion. Another measure of target modulation by BION-1301 was provided by co-injection of the hapten TNP-Ficoll (T-cell independent antigen) in different dose groups. Consistent with previous observations in hAPRIL transgenic mice, BION-1301 reduced in a dose-dependent fashion, TNP-specific IgA and IgM. Using a weekly dosing regimen, a 4-week repeat-dose toxicity study was conducted at doses of 10, 30 and 100 mg/kg, demonstrating that BION-1301 had no effect on vital organs and other toxicology related factors examined, at any dose level up to 100 mg/kg. Chronic exposure to BION-1301 led to significantly reduced levels of IgA, IgG and IgM observed at all dose levels. To determine target engagement of BION-1301, quantitative assays were developed to detect free APRIL and BION-1301. BION-1301 reduced free APRIL levels in serum in a dose-dependent manner following single and multiple dose administrations. Using a MABEL approach, PK/PD modeling informed a proposed human starting dose of 50 mg. In summary, BION-1301 showed no toxicity in NHP and binding of APRIL resulted in decreased IgA, IgG and IgM levels. PK and target engagement biomarkers predict the first in human dose using PK/PD pharmacometric modeling. A first-in-human study is ongoing to characterize safety and PK/PD of BION-1301 in heavily pretreated MM patients. Citation Format: John Dulos, Lilian Driessen, Peter van Zandvoort, Jos van de Crommert, Nitya Nair, Britta Randlev, Eduard de Cock, Jeroen Elassaiss Schaap, Hans van Eenennaam, Andrea van Elsas. Preclinical pharmacokinetics, pharmacodynamics and safety of BION-1301, a first-in-class antibody targeting APRIL for the treatment of multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3780.

RORγt is critical for the differentiation and proliferation of Th17 cells associated with several chronic autoimmune diseases. We report the discovery of a novel allosteric binding site on the nuclear receptor RORγt. Co-crystallization of the ligand binding domain (LBD) of RORγt with a series of small-molecule antagonists demonstrates occupancy of a previously unreported allosteric binding pocket. Binding at this non-canonical site induces an unprecedented conformational reorientation of helix 12 in the RORγt LBD, which blocks cofactor binding. The functional consequence of this allosteric ligand-mediated conformation is inhibition of function as evidenced by both biochemical and cellular studies. RORγt function is thus antagonized in a manner molecularly distinct from that of previously described orthosteric RORγt ligands. This brings forward an approach to target RORγt for the treatment of Th17-mediated autoimmune diseases. The elucidation of an unprecedented modality of pharmacological antagonism establishes a mechanism for modulation of nuclear receptors., Upon the binding of small ligands, nuclear receptors regulate the transcription of genes that are associated with a number of disease mechanisms. Here, the authors report on a novel allosteric ligand binding site on the nuclear receptor RORγt.

APRIL, or tumor necrosis factor super family member 13 (TNFSF13), is a ligand for the receptors B-cell maturation antigen (BCMA) and transmembrane activator calcium modulator and cyclophilin ligand (CAML) interactor (TACI). APRIL serum levels are enhanced in patients diagnosed with multiple myeloma (MM), chronic lymphocytic leukemia (CLL) and colorectal carcinoma and are correlated with poor prognosis. As a paracrine factor produced by osteoclasts, macrophages and other cells in the bone marrow niche, APRIL binds to BCMA to drive proliferation and survival of human Multiple Myeloma (MM) cells and induces resistance to several standard of care agents. Using a mouse anti-human APRIL blocking antibody 01A1 originally discovered using Aduro’s B-Select platform, we demonstrated that osteoclast-induced MM cell in vitro proliferation and survival is dependent on APRIL. In these co-cultures, cytolytic activity of lenalidomide and bortezomib is significantly enhanced by 01A in a dose-dependent fashion2. Importantly, APRIL drives and 01A inhibits expression of an immunosuppressive gene set including the immune checkpoint programmed death ligand-1 (PD-L1), interleukin-10, vascular endothelial growth factor, and transforming growth factor beta. The human APRIL antagonist antibody 01A was humanized and designated BION-1301. The antibody binds an epitope overlapping the BCMA and TACI binding sites potently and fully blocks BCMA and TACI binding (IC50 To our knowledge, BION-1301 is a first-in-class humanized APRIL antagonist demonstrated in to inhibit multiple myeloma survival, drug resistance and an immune suppressive phenotype preclinical. These data suggest a rationale to develop BION-1301 as a single agent, and in combination with lenalidomide, bortezomib, or possibly checkpoint inhibitors such as anti-PD-1. BION-1301 is expected to enter Phase 1 in 2017. 1) Guadagnoli at al. Blood. 2011 Jun 23;117(25):6856-65 2) Yu-Tzu et al. Blood. 2016 Jun 23;127(25):3225-36 Citation Format: John Dulos, Lilian Driessen, Marc Snippert, Marco Guadagnoli, Astrid Bertens, David Lutje Hulsik, Tai Yu Tzu, Kenneth Anderson, Jan Paul Medema, Kate Cameron, Hans Eenennaam, Andrea Elsas. Development of a first in class APRIL fully blocking antibody BION-1301 for the treatment of multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2645. doi:10.1158/1538-7445.AM2017-2645

Item does not contain fulltext Toll-like receptor (TLR) 10 is the only pattern-recognition receptor without known ligand specificity and biological function. We demonstrate that TLR10 is a modulatory receptor with mainly inhibitory effects. Blocking TLR10 by antagonistic antibodies enhanced proinflammatory cytokine production, including IL-1 beta, specifically after exposure to TLR2 ligands. Blocking TLR10 after stimulation of peripheral blood mononuclear cells with pam3CSK4 (Pam3Cys) led to production of 2,065 +/- 106 pg/mL IL-1 beta (mean +/- SEM) in comparison with 1,043 +/- 51 pg/mL IL-1 beta after addition of nonspecific IgG antibodies. Several mechanisms mediate the modulatory effects of TLR10: on the one hand, cotransfection in human cell lines showed that TLR10 acts as an inhibitory receptor when forming heterodimers with TLR2; on the other hand, cross-linking experiments showed specific induction of the anti-inflammatory cytokine IL-1 receptor antagonist (IL-1Ra, 16 +/- 1.7 ng/mL, mean +/- SEM). After cross-linking anti-TLR10 antibody, no production of IL-1 beta and other proinflammatory cytokines could be found. Furthermore, individuals bearing TLR10 polymorphisms displayed an increased capacity to produce IL-1 beta, TNF-alpha, and IL-6 upon ligation of TLR2, in a gene-dose-dependent manner. The modulatory effects of TLR10 are complex, involving at least several mechanisms: there is competition for ligands or for the formation of heterodimer receptors with TLR2, as well as PI3K/Akt-mediated induction of the anti-inflammatory cytokine IL-1Ra. Finally, transgenic mice expressing human TLR10 produced fewer cytokines when challenged with a TLR2 agonist. In conclusion, to our knowledge we demonstrate for the first time that TLR10 is a modulatory pattern-recognition receptor with mainly inhibitory properties.

Item does not contain fulltext The recessively inherited developmental disorder, cartilage-hair hypoplasia (CHH) is highly pleiotropic with manifestations including short stature, defective cellular immunity, and predisposition to several cancers. The endoribonuclease RNase MRP consists of an RNA molecule bound to several proteins. It has at least two functions, namely, cleavage of RNA in mitochondrial DNA synthesis and nucleolar cleaving of pre-rRNA. We describe numerous mutations in the untranslated RMRP gene that cosegregate with the CHH phenotype. Insertion mutations immediately upstream of the coding sequence silence transcription while mutations in the transcribed region do not. The association of protein subunits with RNA appears unaltered. We conclude that mutations in RMRP cause CHH by disrupting a function of RNase MRP RNA that affects multiple organ systems.

Contains fulltext : 272864.pdf (Publisher’s version ) (Closed access) RNase MRP is a ribonucleoprotein particle involved in the processing of pre-rRNA. The RNase MRP particle is structurally highly related to the RNase P particle, which is involved in pre-tRNA processing. Their RNA components fold into a similar secondary structure and they share several protein subunits. We have identified and characterised human and mouse cDNAs that encode proteins homologous to yPop4p, a protein subunit of both the yeast RNase MRP and RNase P complexes. The human Pop4 cDNA encodes a highly basic protein of 220 amino acids. Transfection experiments with epitope-tagged hPop4 protein indicated that hPop4 is localised in the nucleus and accumulates in the nucleolus. Immunoprecipitation assays using extracts from transfected cells expressing epitope-tagged hPop4 revealed that this protein is associated with both the human RNase MRP and RNase P particles. Polyclonal rabbit antibodies raised against recombinant hPop4 recognised a 30 kDa protein in total HeLa cell extracts and specifically co-immunoprecipitated the RNA components of the RNase MRP and RNase P complexes. Finally we showed that anti-hPop4 immunoprecipitates possess RNase P enzymatic activity. Taken together, these data show that we have identified a protein that represents the human counterpart of the yeast Pop4p protein.

TAK1 (TGF-β Activated Kinase 1) is a MAPK kinase kinase, which activates the p38- and JNK-MAPK and NF-κB pathways downstream of receptors such as Toll-Like-, cytokine- and T-cell and B-cell receptors. Representing such an important node in the pro-inflammatory signal-transduction network, the function of TAK1 has been studied extensively. TAK1 knock-out mice are embryonic lethal, while conditional knock-out mice demonstrated either a pro- or anti-inflammatory function. To study the function of TAK1 protein in the adult immune system, we generated and characterized a transgenic mouse expressing TAK1 shRNA under the control of a doxycycline-inducible promoter. Following treatment of TAK-1 shRNA transgenic mice with doxycycline an effective knockdown of TAK1 protein levels was observed in lymphoid organs and cells in the peritoneal cavity (>50% down regulation). TAK1 knockdown resulted in significant changes in leukocyte populations in blood, bone marrow, spleen and peritoneal cavity. Upon TAK1 knockdown mice demonstrated splenomegaly, signs of systemic inflammation (increased levels of circulating cytokines and increase in cellularity of the B-cell areas and in germinal center development in the follicles) and degenerative changes in heart, kidneys and liver. Not surprisingly, TAK1-Tg mice treated with LPS or anti-CD3 antibodies showed enhanced cytokine/chemokine secretion. Finally, analysis of progenitor cells in the bone marrow upon doxycycline treatment showed increased proliferation and differentiation of myeloid progenitor cells. Given the similarity of the phenotype with TGF-β genetic models, our data suggest that in our model the function of TAK1 in TGF-β signal-transduction is overruling its function in pro-inflammatory signaling.