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Our understanding of tissue-resident memory T (T RM ) cell biology has been largely developed from acute infection models in which antigen is cleared and sterilizing immunity is achieved. Less is known about T RM cells in the context of chronic antigen persistence and inflammation. We investigated factors that underlie T RM maintenance in a kidney transplantation model in which T RM cells drive rejection. In contrast to acute infection, we found that T RM cells declined markedly in the absence of cognate antigen, antigen presentation, or antigen sensing by the T cells. Depletion of graft-infiltrating dendritic cells or interruption of antigen presentation after T RM cells were established was sufficient to disrupt T RM maintenance and reduce allograft pathology. Likewise, removal of IL-15 transpresentation or of the IL-15 receptor on T cells during T RM maintenance led to a decline in T RM cells, and IL-15 receptor blockade prevented chronic rejection. Therefore, antigen and IL-15 presented by dendritic cells play nonredundant key roles in CD8 T RM cell maintenance in settings of antigen persistence and inflammation. These findings provide insights that could lead to improved treatment of chronic transplant rejection and autoimmunity.

PURPOSE Graft-versus-host disease (GVHD) causes morbidity and mortality following allogeneic hematopoietic cell transplantation. Naive T cells (TN) cause severe GVHD in murine models. We evaluated chronic GVHD (cGVHD) and other outcomes in three phase II clinical trials of TN-depletion of peripheral blood stem-cell (PBSC) grafts. METHODS One hundred thirty-eight patients with acute leukemia received TN-depleted PBSC from HLA-matched related or unrelated donors following conditioning with high- or intermediate-dose total-body irradiation and chemotherapy. GVHD prophylaxis was with tacrolimus, with or without methotrexate or mycophenolate mofetil. Subjects received CD34-selected PBSC and a defined dose of memory T cells depleted of TN. Median follow-up was 4 years. The primary outcome of the analysis of cumulative data from the three trials was cGVHD. RESULTS cGVHD was very infrequent and mild (3-year cumulative incidence total, 7% [95% CI, 2 to 11]; moderate, 1% [95% CI, 0 to 2]; severe, 0%). Grade III and IV acute GVHD (aGVHD) occurred in 4% (95% CI, 1 to 8) and 0%, respectively. The cumulative incidence of grade II aGVHD, which was mostly stage 1 upper gastrointestinal GVHD, was 71% (95% CI, 64 to 79). Recipients of matched related donor and matched unrelated donor grafts had similar rates of grade III aGVHD (5% [95% CI, 0 to 9] and 4% [95% CI, 0 to 9]) and cGVHD (7% [95% CI, 2 to 13] and 6% [95% CI, 0 to 12]). Overall survival, cGVHD-free, relapse-free survival, relapse, and nonrelapse mortality were, respectively, 77% (95% CI, 71 to 85), 68% (95% CI, 61 to 76), 23% (95% CI, 16 to 30), and 8% (95% CI, 3 to 13) at 3 years. CONCLUSION Depletion of TN from PBSC allografts results in very low incidences of severe acute and any cGVHD, without apparent excess risks of relapse or nonrelapse mortality, distinguishing this novel graft engineering strategy from other hematopoietic cell transplantation approaches.

Activation of host T cells that mediate allograft rejection is a 2-step process. The first occurs in secondary lymphoid organs where T cells encounter alloantigens presented by host DCs and differentiate to effectors. Antigen presentation at these sites occurs principally via transfer of intact, donor MHC-peptide complexes from graft cells to host DCs (cross-dressing) or by uptake and processing of donor antigens into allopeptides bound to self-MHC molecules (indirect presentation). The second step takes place in the graft, where effector T cells reengage with host DCs before causing rejection. How host DCs present alloantigens to T cells in the graft is not known. Using mouse islet and kidney transplantation models, imaging cytometry, and 2-photon intravital microscopy, we demonstrate extensive cross-dressing of intragraft host DCs with donor MHC-peptide complexes that occurred early after transplantation, whereas host DCs presenting donor antigen via the indirect pathway were rare. Cross-dressed DCs stably engaged TCR-transgenic effector CD8(+) T cells that recognized donor antigen and were sufficient for sustaining acute rejection. In the chronic kidney rejection model, cross-dressing declined over time but was still conspicuous 8 weeks after transplantation. We conclude that cross-dressing of host DCs with donor MHC molecules is a major antigen presentation pathway driving effector T cell responses within allografts.

Hematopoietically-restricted minor histocompatibility antigens (miHAs) specific T cells can mediate graft-versus-leukemia and promote engraftment with a low risk of graft-vs-host disease in allogeneic stem cell transplantation (alloSCT). Thus the miHA are ideal targets for adoptive T cell immunotherapy for the recipient of alloSCT. We developed a novel single-cell based high-throughput technology (TCXpress) for cloning T cell receptors (TCRs) and successfully cloned multiple TCRs reactive against the miHA, HA-1 from a parous woman who was naturally immunized to HA-1 through pregnancy. We identified an HLA-A*02:01 woman homozygous for the non-immunogenic HA-1 alleles (R/R) who had pregnancies from an HA-1(H/R) father. TCRs were cloned from single-cell-sorted HA-1 dextramer+ (dexHA-1+) T cells from unstimulated peripheral blood mononuclear cells (PBMCs) and subsequently from CD8 cells cultured with HA-1 peptide-pulsed antigen-presenting cells (APCs). TCRs were re-expressed in reporter cells and analyzed for dextramer binding and CD69 upregulation after culture with peptide-pulsed APCs. TCR sequencing was performed to define CDR3 diversity. 48 dexHA-1+CD8+ T cells were single-cell sorted from 3.9 x10e7 PBMCs of a parous woman. Using TCXpress technology, we cloned 38 TCRs from 48 single-cell dexHA-1+CD8+ T cells. Of these 38, 16 unique TCRs, when expressed in Jurkat cells, were functionally reactive against HA-1(H) peptide by ELISpot. These TCRs had a broad range of EC50s as measured by CD69 upregulation when cultured with HA-1(H) peptide-pulsed T2 cells. CD8+ T cells from the same donor were expanded with autologous HA-1(H)-peptide pulsed APCs for one week. 704 additional TCRs were cloned from dexHA-1+ cells from these cultures. 440 clones were confirmed to bind HA-1 dextramer when expressed in CD8-expressing 293 cells. TCR sequencing of these 440 TCRs identified six additional unique anti-HA-1 TCRs. Several TCRs, when re-expressed in primary CD8+ T cells, killed HA-1+ target cells. TCR sequencing revealed that almost all dexHA-1+ CD8+ T cells used TRBV7-9, consistent with other anti-HA-1 TCR clones in previous reports. In summary, TCXpress technology has yielded 22 unique anti-HA-1 TCRs with a broad functional affinity from a single donor in only two experiments. Our data also highlight the wide range of TCR affinities that can arise from a natural immune response against a single allopeptide/HLA complex. We aim to apply this technology to clone and characterize TCRs against other miHAs, particularly those with expression relatively restricted to hematopoietic cells. Citation Format: Sawa Ito, Constantinos G. Panousis, Alexander M. Rowe, Kedwin Ventura, Jennifer D. Roy, Stephanie Stras, Egidio Brocca-Cofano, Josh Kim, Mark J. Shlomchik, Warren D. Shlomchik. High throughput single-cell based cloning reveals functional diversity of T cell receptors targeting minor histocompatibility antigen [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5189.

Tissue resident memory T cells (T(RM)) contained at sites of previous infection provide local protection against re-infection. Whether they form and function in organ transplants where cognate antigen persists is unclear. This is a key question in transplantation as T cells are detected long-term in allografts, but it is not known whether they are exhausted or are functional memory T cells. Using a mouse model of kidney transplantation, we showed that antigen-specific and polyclonal effector T cells differentiated in the graft into T(RM) and subsequently caused allograft rejection. T(RM) identity was established by surface phenotype, transcriptional profile, and inability to recirculate in parabiosis and re-transplantation experiments. Graft T(RM) proliferated locally, produced IFNγ upon re-stimulation, and their in vivo depletion attenuated rejection. Importantly, the vast majority of antigen-specific and polyclonal T(RM) lacked phenotypic and transcriptional exhaustion markers. Single cell analysis of graft T cells early and late after transplantation identified a transcriptional program associated with transition to the tissue resident state that could serve as a platform for the discovery of therapeutic targets. Thus, recipient effector T cells differentiate into functional graft T(RM) that maintain rejection locally. Targeting these T(RM) could improve renal transplant outcomes.

In hematopoietic cell transplants, alloreactive T cells mediate the graft-versus-leukemia (GVL) effect. However, leukemia relapse accounts for nearly half of deaths. Understanding GVL failure requires a system in which GVL-inducing T cells can be tracked. We used such a model wherein GVL is exclusively mediated by T cells that recognize the minor histocompatibility antigen H60. Here we report that GVL fails due to insufficient H60 presentation and T cell exhaustion. Leukemia-derived H60 is inefficiently cross-presented whereas direct T cell recognition of leukemia cells intensifies exhaustion. The anti-H60 response is augmented by H60-vaccination, an agonist αCD40 antibody (FGK45), and leukemia apoptosis. T cell exhaustion is marked by inhibitory molecule upregulation and the development of TOX+ and CD39−TCF-1+ cells. PD-1 blockade diminishes exhaustion and improves GVL, while blockade of Tim-3, TIGIT or LAG3 is ineffective. Of all interventions, FGK45 administration at the time of transplant is the most effective at improving memory and naïve T cell anti-H60 responses and GVL. Our studies define important causes of GVL failure and suggest strategies to overcome them., In hematopoietic stem cell transplants, T cells mediate graft-versus-leukemia (GVL), but GVL can fail leading to leukemia relapse. Here the authors use a mouse model in which T cells target the minor histocompatibility antigen H60 to show how this can occur, characterize the CD8+ T cell response and demonstrate how anti-CD40 antibody therapy improves GVL.

CD8+ Foxp3+ T cells (Tregs) are a potent regulatory population whose functional and ontological similarities to CD4+ Fox3+ T cells have not been well delineated. Using an experimental model of graft-versus-host disease (GVHD), we observed that CD8+ Tregs were significantly less potent than CD4+ Tregs for the suppression of GVHD. To define the mechanistic basis for this observation, we examined the T-cell repertoire and the transcriptional profile of in vivo-derived CD4+ and CD8+ Tregs that emerged early during this disease. Polyclonal and alloantigen-induced CD8+ Tregs had repertoire diversity that was similar to that of conventional CD8+ T cells, indicating that a restricted repertoire was not the proximate cause of decreased suppression. Transcriptional profiling revealed that CD8+ Tregs possessed a canonical Treg transcriptional signature that was similar to that observed in CD4+ Tregs, yet distinct from conventional CD8+ T cells. Pathway analysis, however, demonstrated that CD8+ Tregs had differential gene expression in pathways involved in cell death and survival. This was further confirmed by detailed mRNA sequence analysis and protein expression studies, which demonstrated that CD8+ Tregs had increased expression of Bim and reduced expression of Mcl-1. Transplantation with CD8+ Foxp3+ Bim-/- Tregs resulted in prolonged Treg survival and reduced GVHD lethality compared with wild-type CD8+ Tregs, providing functional confirmation that increased expression of Bim was responsible for reduced in vivo efficacy. Thus, Bim regulates the survival and suppressive capability of CD8+ Tregs, which may have implications for their use in regulatory T-cell therapy.

Background: Graft vs. host disease (GVHD) remains a major complication of allogeneic stem cell transplantation (alloSCT). To create space for donor stem cells and prevent their rejection, alloSCT protocols rely on conditioning regimens involving chemotherapy, frequently combined with radiation. Interleukin (IL)-33 is a nuclear protein that is found in the secondary lymphoid organs and barrier tissues. IL-33 is augmented in recipient barrier tissues by conditioning. We have established that IL-33 is a clinically-relevant signal. While we have further elucidated that IL-33 is acting directly on donor T cells to augment GVHD, neither the critical place nor timing of IL-33 stimulation of donor T cells during the alloimmune response mediating GVHD have been defined. Methods: We used both IL-33 knockout mice as recipients and ST2 (IL33 receptor) knockout mice as T cell donors in MHC mismatch alloSCT models to define the spatial and temporal importance of IL-33 in GVHD tissue damage. More specifically, to establish the role of IL-33 in T cell effector function in the small intestine during GVHD, irradiated Il33-/- and Il33+/+ C57BL/6 (B6) recipients were given BALB/c bone marrow (BM) and T cells. Additionally, irradiated BALB/c mice were given St2+/+ B6 BM and Cd4 Cre x St2fl/fl or St2-/- and St2+/+ T cells. To determine the role of IL-33 on T cell persistence in GVHD, we used an MHCII-disparate model. Irradiated Il33-/- and Il33+/+ bm12 recipients were reconstituted with B6 BM, with or without CD90.1+ B6 T cells. Results: When donor T cell responses were characterized by flow cytometry at days 7, 14, and 28 post alloSCT, the bulk of these data suggest that IL-33 sustains CD4+ T helper Type 1 (Th1) cells in the barrier tissues at late points following GVHD initiation. Specifically, B6 Il33-/- recipients had fewer BALB/c CD4+Tbet+ cells in the small intestine lamina propria (SI LP; WT 15.9 ± 1.31, KO 9.53 ± 1.32; p=0.03; n=3/group), but not in the spleen (WT 54.8 ± 7.37, KO 57.68 ± 4.90; p=0.75; n=4/group) at day 7. Additionally, we found that when we used therapeutic doses of anti-IL-12p40 to neutralize IL-12 there was no impact on the CD4+Tbet+ cells in the SI of B6 il33-/- recipients (KO control IgG 7.29 ± 0.63, KO anti-IL-12p40 6.61 ± 0.75; p=0.52; n=4/group) suggesting that the decrease in the frequency of CD4+Tbet+ T cells in Il33-/- in the SI is independent of IL-12. In the CD4-dependent B6 to bm12 GVHD model, we find that IL-33-deficient recipients have reduced overall CD4+ donor T cells in the SI LP at both day 14 and day 28 (d14: WT 16.15 ± 2.55, KO 0.35 ± 0.07; p=0.003; n=3/group; d28: WT 33.07 ± 7.17, KO 6.27 ± 1.59; p=0.022; n=3/group). Consistent with our B6 recipients of BALB/c T cells above, this decrease was not observed in the spleen or lymph nodes. Reduced clusters of CD3+ cells were evident in immunohistochemistry of bm12 Il33-/- relative to bm12 Il33+/+ at day 28 post-alloSCT. In corroboration with our IL-33-deficient recipient results, B6 CD4+ T cells lacking the ST2 receptor when co-transferred with ST2 competent T cells were not sustained in the SI LP at day 7 (ST2 WT 18.55 ± 0.77, ST2 KO 6.35 ± 1.42; p=0.0003; n=4/group). ST2 deficient CD4+ donor cells were of equal frequency to the ST2 competent donor cells in the spleen of BALB/c recipients (ST2 WT 54.58 ± 1.914, ST2 KO 50 ± 2.53; p=0.19; n=4/group). Conclusions: Our data reveal that IL-33 is not required for donor Th1 responses in the spleen and LN, but IL-33 is critical for Th1 cells to persist in barrier tissues like the small intestine after alloSCT in the absence of IL-12 stimulation. These data suggest that targeting IL-33 signaling may be an effective therapy to diminish GVHD in target tissues. Disclosures Blazar: Fate Therapeutics Inc.: Research Funding; Childrens' Cancer Research Fund: Research Funding; BlueRock Therapeutics: Research Funding; BlueRock Therapeuetic: Consultancy; Magenta Therapeutics: Consultancy; KidsFirst Fund: Research Funding; Tmunity: Other: Co-founder. Shlomchik:Bluesphere Bio: Consultancy, Other: shareholder.

Graft-versus-host disease (GVHD) is a major cause of morbidity and mortality in allogeneic hematopoietic stem cell transplantation (alloSCT). By static microscopy, cutaneous GVHD lesions contain a mix of T cells and myeloid cells. We used 2-photon intravital microscopy to investigate the dynamics of CD4+ and CD8+ T cells and donor dendritic cells (DCs) in cutaneous GVHD lesions in an MHC-matched, multiple minor histocompatibility antigen-mismatched (miHA) model. The majority of CD4 and CD8 cells were stationary, and few cells entered and stopped or were stopped and left the imaged volumes. CD8 cells made TCR:MHCI-dependent interactions with CD11c+ cells, as measured by the durations that CD8 cells contacted MHCI+ vs MHCI− DCs. The acute deletion of Langerin+CD103+ DCs, which were relatively rare, did not affect CD8 cell motility and DC contact times, indicating that Langerin−CD103− DCs provide stop signals to CD8 cells. CD4 cells, in contrast, had similar contact durations with MHCII+ and MHCII− DCs. However, CD4 motility rapidly increased after the infusion of an MHCII-blocking antibody, indicating that TCR signaling actively suppressed CD4 movements. Many CD4 cells still were stationary after anti-MHCII antibody infusion, suggesting CD4 cell heterogeneity within the lesion. These data support a model of local GVHD maintenance within target tissues.

International audience; Cells of the immune system that reside in barrier epithelia provide a first line of defense against pathogens. Langerhans cells (LCs) and CD8+ tissue-resident memory T cells (TRM cells) require active transforming growth factor-beta1 (TGF-beta) for epidermal residence. Here we found that integrins alphavbeta6 and alphavbeta8 were expressed in non-overlapping patterns by keratinocytes (KCs) and maintained the epidermal residence of LCs and TRM cells by activating latent TGF-beta. Similarly, the residence of dendritic cells and TRM cells in the small intestine epithelium also required alphavbeta6. Treatment of the skin with ultraviolet irradiation decreased integrin expression on KCs and reduced the availability of active TGF-beta, which resulted in LC migration. Our data demonstrated that regulated activation of TGF-beta by stromal cells was able to directly control epithelial residence of cells of the immune system through a novel mechanism of intercellular communication

Introduction: Multiple myeloma (MM) is an incurable malignancy of B-cell lineage. Introduction of immunomodulatory drugs such as lenalidomide has significantly improved overall survival of patients with MM. Lenalidomide maintenance is currently the standard of care for maintaining remission in MM. However, second primary malignancies are known to arise in patients on lenalidomide though their pathogenesis is not known. We report a patient who developed B-lymphoblastic leukemia (B-ALL) while on lenalidomide, which went into spontaneous remission after stopping lenalidomide. Whole exome sequencing (WES) was performed to examine the mutational landscape and clonal evolution of the different malignant clones. We also aimed to postulate a mechanism for lenalidomide-induced ALL. Case history and methods: A 59-year-old female with history of rheumatoid arthritis was diagnosed with IgG-κ MM and treated with 8 cycles of bortezomib, Lenalidomide and dexamethasone followed by lenalidomide-maintenance (10 mg/day). At 36 months after initiation of treatment for MM, she developed lymphopenia and her bone marrow (BM) biopsy showed 38% leukemic B lymphoblasts. Lenalidomide was discontinued and a follow-up BM biopsy done 2 months later showed spontaneous complete remission of ALL, which was confirmed at 8 months. She remains in complete remission of ALL and MM without any specific treatment at 12 months after the diagnosis of ALL. BM aspirate samples at diagnosis of MM, on lenalidomide-maintenance with MM in remission, at diagnosis of ALL, and after stopping lenalidomide with MM and ALL in remission were used to perform WES with 2x150 bp reads and 100x coverage utilizing the Illumina Hiseq. The raw reads were mapped to reference genome (hg19) and compared with peripheral blood T-lymphocytes as germ line control. Variants were annotated using dbSNP, CLINVAR and COSMIC through Mutect. Clonal evolution was analyzed by SciClone. The study was approved by Institutional Review Board. Results: The burden of somatic mutations was significantly higher at diagnosis of MM when compared to the three other time points. Analysis of clonal architecture revealed the distinct clustering of mutations at specific time points (Figure). Most mutations detected at diagnosis of MM (e.g. NOTCH2, BTG1, BCLAF1) disappeared after treatment for MM. Similarly, most mutations detected only at diagnosis of ALL (e.g.PIK3CD, CDK16) became undetectable at spontaneous remission. Interestingly, clones with mutations of IGSF3 (immunoglobulin superfamily) and CXXC4 (Wnt signaling pathway) were detectable while the patient was on lenalidomide and at diagnosis of ALL but disappeared after stopping lenalidomide, which suggests that these clones gained pro-survival advantage from lenalidomide. Only a few mutations (GSDMC, NBPF20, ANAPC1) persisted in both MM and ALL stages. GSDMC is a gasdermin family member which may modulate function of MYC. NBPF20 has been described in relapsed pediatric ALL. ANAPC1 is a cell cycle gene whose transcription is regulated by Ikaros. Loss of repressor function of Ikaros was recently reported to deregulate ANAPC1 expression and cause mitotic progression of ALL in vitro. As lenalidomide is known to induce degradation of Ikaros, we hypothesize that lenalidomide may create a favorable selection pressure for B-cell clones harboring mutations in Ikaros-dependent genes. Conclusions: Clonal evolution analysis suggests that MM and ALL arose from different B-cell sub-clones, which was consistent with previous observation. However, there are a few shared mutations between MM and pre-B ALL, which may be responsible for leukemogenesis in our case. Lenalidomide may affect intracellular protein interactions to induce selection of rare B-cell clones evolving into secondary ALL. Also, our case demonstrated that simply stopping lenalidomide may lead to spontaneous and durable regression of ALL. Transcriptomic and proteomic analysis including Ikaros expression is required to further understand the mechanism of appearance of ALL and its regression after stopping lenalidomide. Figure Disclosures Shlomchik: BlueSphere Bio: Other: Founder and Equity Interest.

Effector memory T cells (TEM) are less capable of inducing graft-versus-host disease (GVHD) compared with naive T cells (TN). Previously, in the TS1 TCR transgenic model of GVHD, wherein TS1 CD4 cells specific for a model minor histocompatibility Ag (miHA) induce GVHD in miHA-positive recipients, we found that cell-intrinsic properties of TS1 TEM reduced their GVHD potency relative to TS1 TN. Posttransplant, TS1 TEM progeny expressed higher levels of PD-1 than did TS1 TN progeny, leading us to test the hypothesis that TEM induce less GVHD because of increased sensitivity to PD-ligands. In this study, we tested this hypothesis and found that indeed TS1 TEM induced more severe skin and liver GVHD in the absence of PD-ligands. However, lack of PD-ligands did not result in early weight loss and colon GVHD comparable to that induced by TS1 TN, indicating that additional pathways restrain alloreactive TEM. TS1 TN also caused more severe GVHD without PD-ligands. The absence of PD-ligands on donor bone marrow was sufficient to augment GVHD caused by either TEM or TN, indicating that donor PD-ligand–expressing APCs critically regulate GVHD. In the absence of PD-ligands, both TS1 TEM and TN induced late-onset myocarditis. Surprisingly, this was an autoimmune manifestation, because its development required non-TS1 polyclonal CD8+ T cells. Myocarditis development also required donor bone marrow to be PD-ligand deficient, demonstrating the importance of donor APC regulatory function. In summary, PD-ligands suppress both miHA-directed GVHD and the development of alloimmunity-induced autoimmunity after allogeneic hematopoietic transplantation.

Acute graft-versus-host disease (aGVHD) is the most common complication for patients undergoing allogeneic stem cell transplantation. Despite extremely aggressive therapy targeting donor T cells, patients with grade III or greater aGVHD of the lower GI tract, who do not respond to therapy with corticosteroids, have a dismal prognosis. Thus, efforts to improve understanding of the function of local immune and non-immune cells in regulating the inflammatory process in the GI tract during aGVHD are needed. Here, we demonstrate, using murine models of allogeneic BMT, that type 2 innate lymphoid cells (ILC2s) in the lower GI tract are sensitive to conditioning therapy and show very limited ability to repopulate from donor bone marrow. Infusion of donor ILC2s was effective in reducing the lethality of aGVHD and in treating lower GI tract disease. ILC2 infusion was associated with reduced donor proinflammatory Th1 and Th17 cells, accumulation of donor myeloid-derived suppressor cells (MDSCs) mediated by ILC2 production of IL-13, improved GI tract barrier function, and a preserved graft-versus-leukemia (GVL) response. Collectively, these findings suggest that infusion of donor ILC2s to restore gastrointestinal tract homeostasis may improve treatment of severe lower GI tract aGVHD.

Mice devoid of T, B, and natural killer (NK) cells distinguish between self and allogeneic nonself despite the absence of an adaptive immune system. When challenged with an allograft, they mount an innate response characterized by accumulation of mature, monocyte-derived dendritic cells (DCs) that produce interleukin-12 and present antigen to T cells. However, the molecular mechanisms by which the innate immune system detects allogeneic nonself to generate these DCs are not known. To address this question, we studied the innate response of Rag2-/- γc-/- mice, which lack T, B, and NK cells, to grafts from allogeneic donors. By positional cloning, we identified that donor polymorphism in the gene encoding signal regulatory protein α (SIRPα) is a key modulator of the recipient's innate allorecognition response. Donors that differed from the recipient in one or both Sirpa alleles elicited an innate alloresponse. The response was mediated by binding of donor SIRPα to recipient CD47 and was modulated by the strength of the SIRPα-CD47 interaction. Therefore, sensing SIRPα polymorphism by CD47 provides a molecular mechanism by which the innate immune system distinguishes between self and allogeneic nonself independently of T, B, and NK cells.

The graft-versus-leukemia (GVL) effect, mediated by alloreactive T cells, reduces the risk of leukemia relapse after allogeneic hematopoietic stem cell transplantation. While much work has focused on how T cells kill leukemia cells, less attention has been paid to the process whereby activated alloreactive T cells find leukemic targets. We initially focused on how chemokine receptor signaling impacts effector T cells egress from blood, entry into the marrow space and subsequent scanning of leukemia cells. We further tested how this process is influenced by transplantation and the consequences of alloreactive T cells. We tracked the migration of CD8+ RAG1-/- OT-1 TCR transgenic effectors (generated by in vivo vaccination of B6 mice seeded with naïve OT-1 cells) in recipients. OT-1 cells recognize the SIINFEKL peptide from ovalbumin and are not reactive in our models. OT-1 effectors were treated (or not) with pertussis toxin (PTX) to block chemokine receptor signaling, and labeled with fluorescent dyes so as they could be identified by flow cytometry and two photon intravital microscopy (2PIM). PTX+ and PTX- OT-1 cells were transferred together into the following recipients: unmanipulated B6 mice, irradiated (B6xBALB/c) CB6F1mice transplanted with B6 bone marrow (BM) cell, with or without B6 T cells. In some experiments on day 12 post-transplant, F1 mice also received WT or IFN-γR-/- B6 blast crisis CML cells (BC-CML) generated by transduction of BM with retrovirus encoding bcr-abl and the NUP98/HOXA9 fusion as we have described (J Clin Invest. 2017;127(7):2765-2776). OT-1 effectors were infused on day 14 and 3 hours later the calvarium was analyzed by 2PIM to enumerate PTX treated or untreated OT-1 effectors that were inside or outside the blood vessels. Blood vessels were identified using labeled dextran or Qdot 655. PTX treatment was highly effective as PTX-treated OT-1 were nearly completely excluded from lymph node (LN). PTX treatment reduced OT1 migration into the BM of unmanipulated B6 mice and similarly in F1 recipients without T cells. However, the presence of a GVH response reduced the recruitment of PTX- OT-1 cells into BM. The ratio of OT-1 outside/inside vessels was still higher in the PTX- group; however, this was mostly driven by an increase of PTX+ OT-1 in vessels in GVH mice. These data suggest that GVH may reduce effector T cell recruitment to the BM, though some of this effect could be due to effectors trafficking instead to other locations, perhaps GVHD target tissues. The presence of WT BC-CML leukemia cells in F1 GVH recipients increased recruitment of OT-1 PTX- effectors into the BM, both relative to PTX+ OT-1 and in total numbers compared to experiments wherein no BC-CML cells were infused. We considered the possibility that IFN-g-stimulation of BC-CML cells may have released chemokines that recruited PTX- OT-1. However, IFNγR-/- BC-CML also increased PTX- OT-1 recruitment in GVHD mice to a similar degree. Experiments are ongoing to better understand how BC-CML cells promote PTX-inhibitable effector T cell recruitment. Disclosures Shlomchik: NapaJen: Consultancy.

Graft-versus-host disease (GVHD) is a major cause of morbidity and mortality in allogeneic hematopoietic stem cell transplantation (alloSCT). In GVHD, donor T cells recognize recipient tissues as non-self and mount a broad attack that results in multi-organ damage. While there has been some success in diminishing the incidence of GVHD, less progress has been made in treating established or steroid-refractory disease without severe global immunosuppression. This is in part due to a lack of understanding of the underlining mechanisms that sustain GVHD despite chronic T cell antigen exposure. To address this, we developed a mouse GVHD model that allows us to track the progeny of single alloreactive T cell clones. We used this model to test hypotheses on GVHD maintenance: 1) GVHD is driven by the continuous output and trafficking of alloreactive T cells from secondary lymphoid tissues (SLT) into GVHD target organs; and 2) once tissues are seeded with alloreactive T cells from SLT, GVHD is maintained locally within affected tissues. We reasoned that if GVHD is maintained by the continuous SLT output of T cells, the progeny of single alloreactive clones in a target tissue would come into equilibrium with those in SLT and other target tissues. Alternatively, if there is a degree of local tissue GVHD maintenance, our model predicts that clonal progeny should be unequally distributed and not in equilibrium with SLT. We first tested these possibilities using a GVHD model wherein BALB/c RAG2-/- TCR transgenic (Tg) CD4 T cells (TS1) that recognize the S1 peptide derived from influenza hemagglutinin (HA) induce GVHD in BALB/c RAG2-/- mice that ubiquitously express HA (HA104 mice). We generated TS1 TCR Tg mice on 9 congenic backgrounds based on the expression of CD45.1/2, Thy1.1/2 and GFP. We transferred 500 naïve TS1 cells of 1 clonotype (to induce GVHD) along with single naïve TS1 cells from the remaining 8 clonotypes and BALB/c RAG2-/- bone marrow (BM) into lethally irradiated HA104 mice. We recovered a total of 432 single-cell derived TS1 clones (72% of input clones) from tissues of 79 mice, analyzed 7-35 days after transfer. We enumerated the TS1 clonal composition of each tissue (expressed as the % of all TS1 of that tissue) and found disparate clonal distribution across tissues within individual mice. For example, in a representative mouse analyzed at day 33 (Figure 1), the fractions of a single TS1 clone were relatively high in the colon (1.4%) and small intestine intraepithelial lymphocyte (IEL) (4.6%) when compared to the spleen (0.07%), BM (0.02%) and liver (0.03%). These data support that TS1 clones are not equally distributed among tissues and are not in equilibrium with SLT, suggesting that GVHD is at least in part maintained locally. We also analyzed TS1 clonal frequency distribution in a second model. BALB/c RAG2-/- BM and polyclonal T cells were transplanted into F1 (BALB/c HA104xB10.D2) recipients along with 8 single distinct TS1 cells. In this system, GVHD is induced by polyclonal BALB/c cells and TS1 cells are trackers of reactivity to HA. Preliminary experiments also indicate unequal distributions of TS1 clones across tissues in individual mice. In a second approach to test whether GVHD is maintained locally, irradiated HA104 mice were reconstituted with either 500 Thy1.1 or 500 Thy1.2 TS1 cells and CD45.1 or CD45.2 BALB/c RAG2-/- BM. One partner also received congenic TS1 single cells. We performed parabiosis of mice from one group to the other 21-28 days later. We analyzed 4 pairs 4 weeks post-joining, looking first at the blood to establish a baseline for TS1 crossover from the parabiotic partner. In blood, 18.9% ±1.9 of all TS1 were derived from the partner. Importantly, relative to equilibration in blood, there were far fewer partner-derived TS1 cells in all other tissues (Figure 2). Only a few single cell-derived TS1 clones were detected in the partner mouse at very low frequencies, even when they were dominant in tissues of the corresponding partner. Together, these data indicate that once GVHD is established, local maintenance dominates over new TS1 entry. Consistent with this, TS1 cells incorporate BrdU in vivo even at late time points. We are combining proliferation and clonality data at multiple timepoints to develop a mathematical model of GVHD establishment and maintenance. We are also extending our observations in a polyclonal GVHD model wherein the progeny from single alloreactive CD8 cells can be enumerated. Disclosures Shlomchik: NapaJen: Consultancy.

Allogeneic hematopoietic stem cell transplantation (HSCT) is a potentially curative therapy for many malignant and non-malignant hematological diseases. The curative effect of allo-HSCT results from both the chemotherapy and/or radiation in the conditioning regimen and the donor T mediated graft-versus-host (GVH) response against the malignancy known as a graft-versus-leukemia (GVL) effect. In most cases, the GVH response is not limited against the malignant cells, but also mediated against host epithelial cells causing graft-versus-host disease (GVHD). The safety and effectiveness of allogeneic transplants has increased substantially over the last several years. Despite the progress, GVHD and relapse remain the biggest hurdles for a more widespread and effective use of this potent therapy. Experimental data from recent years have provided deeper and better insights into the process of GVH responses. Emerging data have refined our understanding of the role of antigen presenting cells (APC) in GVHD. Novel molecular targets have been identified in donor T cell subsets and host non-hematopoietic cells that can potentially be exploited for mitigating GVHD and to better harness GVL. During recent years the role of various cytokines in GVHD continues to be explored. This work has led to improved understanding of potential causes for tissue specificity of GVHD. The role of host and donor microbiome in regulating GVH responses has been a subject of intense focus as well. Notable advances have been made in the defining various other aspects of GVH responses , but these go beyond the scope of this brief review. Herein we will focus on three specific aspects of GVH responses, namely 1) the evolving role of hematopoietic APCs as the regulators of GVHD, 2) novel strategies for enhancing GVL responses and 3) the recently identified molecular targets for mitigating GVH responses.

Successful engraftment of organ transplants has traditionally relied on preventing the activation of recipient (host) T cells. Once T-cell activation has occurred, however, stalling the rejection process becomes increasingly difficult, leading to graft failure. Here we demonstrate that graft-infiltrating, recipient (host) dendritic cells (DCs) play a key role in driving the rejection of transplanted organs by activated (effector) T cells. We show that donor DCs that accompany heart or kidney grafts are rapidly replaced by recipient DCs. The DCs originate from non-classical monocytes and form stable, cognate interactions with effector T cells in the graft. Eliminating recipient DCs reduces the proliferation and survival of graft-infiltrating T cells and abrogates ongoing rejection or rejection mediated by transferred effector T cells. Therefore, host DCs that infiltrate transplanted organs sustain the alloimmune response after T-cell activation has already occurred. Targeting these cells provides a means for preventing or treating rejection., Blocking T cell activation in organ transplantation is important to prevent rejection. Here the authors show that unconventional monocyte-derived host dendritic cells enter allogeneic grafts to amplify the T cell response outside lymph nodes.

Background Acute allograft rejection is dependent on adaptive immunity, but it is unclear whether the same is true for chronic rejection. Here we asked whether innate immunity alone is sufficient for causing chronic rejection of mouse cardiac allografts.

The graft-versus-leukemia (GVL) effect in allogeneic hematopoietic stem cell transplantation (alloSCT) is potent against chronic phase chronic myelogenous leukemia (CP-CML), but blast crisis CML (BC-CML) and acute myeloid leukemias (AML) are GVL resistant. To understand GVL resistance, we studied GVL against mouse models of CP-CML, BC-CML, and AML generated by the transduction of mouse BM with fusion cDNAs derived from human leukemias. Prior work has shown that CD4+ T cell-mediated GVL against CP-CML and BC-CML required intact leukemia MHCII; however, stem cells from both leukemias were MHCII negative. Here, we show that CP-CML, BC-CML, and AML stem cells upregulate MHCII in alloSCT recipients. Using gene-deficient leukemias, we determined that BC-CML and AML MHC upregulation required IFN-γ stimulation, whereas CP-CML MHC upregulation was independent of both the IFN-γ receptor (IFN-γR) and the IFN-α/β receptor IFNAR1. Importantly, IFN-γR-deficient BC-CML and AML were completely resistant to CD4- and CD8-mediated GVL, whereas IFN-γR/IFNAR1 double-deficient CP-CML was fully GVL sensitive. Mouse AML and BC-CML stem cells were MHCI+ without IFN-γ stimulation, suggesting that IFN-γ sensitizes these leukemias to T cell killing by mechanisms other than MHC upregulation. Our studies identify the requirement of IFN-γ stimulation as a mechanism for BC-CML and AML GVL resistance, whereas independence from IFN-γ renders CP-CML more GVL sensitive, even with a lower-level alloimmune response.

Donor T cells contribute to the success of allogeneic hematopoietic stem cell transplantation (alloSCT). Alloreactive donor T cells attack leukemia cells, mediating the GVL effect. Donor T cells, including the memory T cells (TM) that are generated after infection, also promote immune reconstitution. Nonetheless, leukemia relapse and infection are major sources of treatment failure. Efforts to augment GVL and immune reconstitution have been limited by GVHD, the attack by donor T cells on host tissues. One approach to augmenting GVL has been to infuse ex vivo–generated T cells with defined specificities; however, this requires expertise that is not widely available. In the present study, we tested an alternative approach, adoptive immunotherapy with CD8+ TM from donors vaccinated against a single minor histocompatibility antigen (miHA) expressed by leukemia cells. Vaccination against the miHA H60 greatly augmented TM-mediated GVL against mouse chronic-phase (CP-CML) and blast crisis chronic myeloid leukemia (BC-CML). TM-mediated GVL was antigen specific and was optimal when H60 expression was hematopoietically restricted. Even when H60 was ubiquitous, donor H60 vaccination had a minimal impact on GVHD. TM from lymphocytic choriomeningitis virus (LCMV)–immune and H60-vaccinated donors augmented GVL and protected recipients from LCMV. These data establish a strategy for augmenting GVL and immune reconstitution without elaborate T-cell manipulation.

Rapamycin (Rapa), an immunosuppressive drug that acts through mammalian target of Rapa inhibition, broadly synergizes with tolerogenic agents in animal models of transplantation and autoimmunity. Rapa preferentially inhibits conventional CD4+ Foxp3− T cells (Tconv) and promotes outgrowth of CD4+Foxp3+ regulatory T cells (Treg) during in vitro expansion. Moreover, Rapa is widely perceived as augmenting both expansion and conversion of Treg in vivo. However, most quantitative studies were performed in lymphopenic hosts or in graft-versus-host disease models. We show in this study that in replete wild-type mice, Rapa significantly inhibits both homeostatic and alloantigen-induced proliferation of Treg, and promotes their apoptosis. Together, these lead to significant Treg depletion. Tconv undergo depletion to a similar degree, resulting in no change in the percent of Treg among CD4 cells. Moreover, in this setting, there was no evidence of conversion of Tconv into Treg. However, after withdrawal of Rapa, Treg recover Ag-induced proliferation more quickly than Tconv, leading to recovery to baseline numbers and an increase in the percent of Treg compared with Tconv. These findings suggest that the effects of Rapa on Treg survival, homeostasis, and induction, depend heavily on the cellular milieu and degree of activation. In vivo, the resistance of Treg to mammalian target of Rapa inhibition is relative and results from lymphopenic and graft-versus-host disease models cannot be directly extrapolated to settings more typical of solid organ transplantation or autoimmunity. Moreover, these results have important implications for the timing of Rapa therapy with tolerogenic agents designed to increase the number of Treg in vivo.

Recipient antigen presenting cells (APCs) are required for CD8-mediated graft-versus-host disease (GVHD), and have an important and nonredundant role in CD4-mediated GVHD in mouse major histocompatibility complex-matched allogeneic bone marrow transplantation (alloBMT). However, the precise roles of specific recipient APCs-dendritic cells, macrophages, and B cells-are not well defined. If recipient B cells are important APCs they could be depleted with rituximab, an anti-CD20 monoclonal antibody. On the other hand, B cells can downregulate T cell responses, and consequently, B cell depletion could exacerbate GVHD. Patients with B cell lymphomas undergo allogeneic hematopoietic stem cell transplantation (alloSCT) and many are B-cell-deficient because of prior rituximab. We therefore studied the role of recipient B cells in major histocompatibility complex-matched murine models of CD8- and CD4-mediated GVHD by using recipients genetically deficient in B cells and with antibody-mediated depletion of host B cells. In both CD4- and CD8-dependent models, B cell-deficient recipients developed clinical and pathologic GVHD. However, although CD8-mediated GVHD was clinically less severe in hosts genetically deficient in B cells, it was unaffected in anti-CD20-treated recipients. These data indicate that recipient B cells are not important initiators of GVHD, and that efforts to prevent GVHD by APC depletion should focus on other APC subsets.

Type I interferons (IFN-I) link innate to adaptive immunity in microbial infection, autoimmune disease and tumor immunity. It is not known whether IFN-I have an equally central role in alloimmunity. Here we tested this possibility by studying skin allograft survival and donor-specific CD8+ T-cell responses in mice that lack the IFN-I receptor (IFN-IR-/-). We found that IFN-IR-/- mice reject fully allogeneic wild-type skin grafts at the same rate as wild-type recipients. Similarly, allograft rejection was not delayed if IFN-IR-/- male skin was transplanted to syngeneic IFN-IR-/- female mice. Quantitation of the male (H-Y)-specific CD8+ T-cell response in these mice revealed normal generation of donor-specific CD8+ effector T cells but fourfold reduction in CD8+ memory T cells. Memory CD8+ T cells generated in the absence of IFN-IR had normal phenotype and recall function, assessed by ex vivo cytokine production and the ability of IFN-IR-/- mice to mount second set rejection. Finally, these memory T cells were maintained at a constant number despite their inability to respond to IFN-1. Our findings indicate that IFN-I cytokines are not critical for acute allograft rejection or for the expansion and differentiation of donor-specific CD8+ T cells into long-lived, functional memory T cells.

In allogeneic hemopoietic stem cell transplantation, mature donor αβ T cells in the allograft promote T cell reconstitution in the recipient and mediate the graft-vs-leukemia (GVL) effect. Unfortunately, donor T cells can attack nonmalignant host tissues and cause graft-vs-host disease (GVHD). It has previously been shown that effector memory T cells not primed to alloantigen do not cause GVHD yet transfer functional T cell memory and mediate GVL. Recently, central memory T cells (TCM) have also been reported to not cause GVHD. In contrast, in this study, we demonstrate that purified CD8+ TCM not specifically primed to alloantigens mediate GVHD in the MHC-mismatched C57BL/6 (B6)→BALB/c and the MHC-matched, multiple minor histocompatibility Ag-mismatched C3H.SW→B6 strain pairings. CD8+ TCM and naive T cells (TN) caused similar histological disease in liver, skin, and bowel. B6 CD8+ TCM and TN similarly expanded in BALB/c recipients, and the majority of their progeny produced IFN-γ upon restimulation. However, in both models, CD8+ TCM induced milder clinical GVHD than did CD8+ TN. Nonetheless, CD8+ TCM and TN were similarly potent mediators of GVL against a mouse model of chronic-phase chronic myelogenous leukemia. Thus, in contrast to what was previously thought, CD8+ TCM are capable of inducing GVHD and are substantially different from TEM but only subtly so from TN.

Glucocorticoids (GCs) exert profound influences on many physiologic functions by virtue of their diverse roles in growth, development, and maintenance of homeostasis. We previously created a novel gain of function in the human glucocorticoid receptor (hGR), hGRM604L, which is active at GC concentrations 5–10-fold lower than wild-type GR. To gain a greater insight into GC physiology in vivo, we inserted this mutant GR (GRM610L in mice) into mice via homologous recombination. Mice expressing the allele are phenotypically normal with respect to GC function. However, corticosterone levels, ACTH levels, and adrenocortical size are markedly reduced, suggesting they are phenotypically normal because the mutant GR alters the basal regulation of the hypothalamic-pituitary-adrenal axis. We demonstrate via physiologic and immunologic studies that GRM610L mice have increased sensitivity to GCs in vivo. Sensitivity to the actions of endogenous GCs may be an important factor underlying the development of many human diseases including hypertension, obesity, and diabetes. Our model may provide a new and powerful tool for the study of GC physiological and pathological processes in vivo.

Much of the efficacy of allogeneic hematopoietic stem cell transplantation (alloSCT) in curing hematologic malignancies is due to a graft-versus-leukemia (GVL) effect mediated by donor T cells that recognize recipient alloantigens on leukemic cells. Donor T cells are also important for reconstituting immunity in the recipient. Unfortunately, donor T cells can attack nonmalignant host tissues and cause graft-versus-host disease (GVHD). We previously reported that donor CD4(+) effector memory T cells (T(EMs)) do not cause GVHD but transfer functional T-cell memory. In the present work, we demonstrate in an MHC-mismatched model that CD4(+) T(EMs) (unprimed to recipient antigens) mediate GVL against clinically relevant mouse models of chronic phase and blast crisis chronic myelogenous leukemia, without causing GVHD. By creating gene-deficient leukemias and using perforin-deficient T cells, we demonstrate that direct cytolytic function is essential for T(EM)-mediated GVL, but that GVL is retained when killing via FasL, TNF-alpha, TRAIL, and perforin is individually impaired. However, T(EM)-mediated GVL was diminished when both FasL and perforin pathways were blocked. Taken together, our studies identify T(EMs) as a clinically applicable cell therapy for promoting GVL and immune reconstitution, particularly in MHC-mismatched haploidentical alloSCTs in which T cell-depleted allografts are commonly used to minimize GVHD.

Langerhans cells (LCs) are bone marrow (BM)-derived epidermal dendritic cells (DCs) that develop from precursors found in the dermis. Epidermal LCs are absent in transforming growth factor (TGF) beta1-deficient mice. It is not clear whether TGFbeta1 acts directly on LC precursors to promote maturation or whether it acts on accessory cells, which in turn affect LC precursors. In addition, the physiologic source of TGFbeta1 is uncertain because BM chimera experiments showed that neither hematopoietic nor nonhematopoietic-derived TGFbeta1 is required for LC development. To address these issues, we created mice transgenic for a bacterial artificial chromosome (BAC) containing the gene for human Langerin into which Cre recombinase had been inserted by homologous recombination (Langerin-Cre). These mice express Cre selectively in LCs, and they were bred to floxed TGFbetaRII and TGFbeta1 mice, thereby generating mice with LCs that either cannot respond to or generate TGFbeta1, respectively. Langerin-Cre TGFbetaRII mice had substantially reduced numbers of epidermal LCs, demonstrating that TGFbeta1 acts directly on LCs in vivo. Interestingly, Langerin-Cre TGFbeta1 mice also had very few LCs both in the steady state and after BM transplantation. Thus, TGFbeta1 derived from LCs acts directly on LCs through an autocrine/paracrine loop, and it is required for LC development and/or survival.

BACKGROUND. Graft-versus-host disease (GVHD) is a major cause of morbidity and mortality following allogeneic hematopoietic stem cell transplantation (HCT). In mice, naive T cells (TN) cause more severe GVHD than memory T cells (TM). We hypothesized that selective depletion of TN from human allogeneic peripheral blood stem cell (PBSC) grafts would reduce GVHD and provide sufficient numbers of hematopoietic stem cells and TM to permit hematopoietic engraftment and the transfer of pathogen-specific T cells from donor to recipient, respectively. METHODS. In a single-arm clinical trial, we transplanted 35 patients with high-risk leukemia with TN-depleted PBSC grafts following conditioning with total body irradiation, thiotepa, and fludarabine. GVHD prophylactic management was with tacrolimus immunosuppression alone. Subjects received CD34-selected PBSCs and a defined dose of TM purged of CD45RA+ TN. Primary and secondary objectives included engraftment, acute and chronic GVHD, and immune reconstitution. RESULTS. All recipients of TN-depleted PBSCs engrafted. The incidence of acute GVHD was not reduced; however, GVHD in these patients was universally corticosteroid responsive. Chronic GVHD was remarkably infrequent (9%; median follow-up 932 days) compared with historical rates of approximately 50% with T cell–replete grafts. TM in the graft resulted in rapid T cell recovery and transfer of protective virus-specific immunity. Excessive rates of infection or relapse did not occur and overall survival was 78% at 2 years. CONCLUSION. Depletion of TN from stem cell allografts reduces the incidence of chronic GVHD, while preserving the transfer of functional T cell memory. TRIAL REGISTRATION. ClinicalTrials.gov ({"type":"clinical-trial","attrs":{"text":"NCT 00914940","term_id":"NCT00914940"}}NCT 00914940). FUNDING. NIH, Burroughs Wellcome Fund, Leukemia and Lymphoma Society, Damon Runyon Cancer Research Foundation, and Richard Lumsden Foundation.

SummaryEpidermal Langerhans cells (LCs), a distinct skin-resident dendritic cell population, acquire antigen in the skin and migrate to draining lymph nodes where they are thought to initiate adaptive immune responses. To examine the functional requirement of LCs in skin immunity, we generated BAC transgenic mice in which the regulatory elements from human Langerin were used to drive expression of diphtheria toxin. The resulting mice have a constitutive and durable absence of epidermal LCs but are otherwise intact. Unexpectedly, we found that contact hypersensitivity (CHS) was amplified rather than abrogated in the absence of LCs. Moreover, we showed that LCs act during the priming and not the effector phase. Thus, LCs not only were dispensable for CHS, but they served to regulate the response, a previously unappreciated function.

Chronic graft-versus-host disease (cGVHD) is an increasingly common cause of morbidity and mortality in allogeneic stem cell transplantation (alloSCT). Relative to acute GVHD (aGVHD), much less is understood about cGVHD. Using the B10.D2 → BALB/c murine cGVHD model, which shares critical pathologic features with human cGVHD, we find that radiation-resistant host T cells regulate cGVHD. We initially observed that recipients lacking all lymphocytes developed accelerated and more severe cGVHD. Using genetically deficient recipients, we determined that αβ+CD4+ T cells were required to regulate cGVHD. Increased cGVHD severity was not due to the absence of T cells per se. Rather, the potency of regulation was proportional to host T-cell receptor (TCR) diversity. Only CD4+CD25+, and not CD4+CD25-, host T cells ameliorated cGVHD when added back, indicating that host T cells acted not via host-versus-graft activity or by reducing homeostatic proliferation but by an undefined regulatory mechanism. Thus, preparative regimens that spare host CD4+CD25+ T cells may reduce cGVHD. Donor CD4+CD25+ T cells also reduced cGVHD. Depletion of CD4+CD25+ cells from the inoculum exacerbated disease, whereas transplantation of additional CD4+CD25+ cells protected against severe cGVHD. Additional CD4+CD25+ cells also promoted healing of established lesions, suggesting that their effects persist during the evolution of cGVHD.

Graft-versus-host disease (GVHD) is a major source of morbidity in allogenic stem cell transplantation. We previously showed that recipient antigen-presenting cells (APCs) are required for CD8-dependent GVHD in a mouse model across only minor histocompatibility antigens (minor H antigens). However, these studies did not address the function of donor-derived APCs after GVHD is initiated. Here we show that GVHD develops in recipients of donor major histocompatibility complex class I-deficient (MHC I(-)) bone marrow. Thus, after initial priming, CD8 cells caused GVHD without a further requirement for hematopoietic APCs, indicating that host APCs are necessary and sufficient for GHVD. Nonetheless, GVHD was less severe in recipients of MHC I(-) bone marrow. Therefore, once initiated, GVHD is intensified by donor-derived cells, most probably donor APCs cross-priming alloreactive CD8 cells. Nevertheless, donor APCs were not required for CD8-mediated graft-versus-leukemia (GVL) against a mouse model of chronic-phase chronic myelogenous leukemia. These studies identify donor APCs as a new target for treating GVHD, which may preserve GVL.

The graft-versus-leukemia (GVL) effect, mediated by donor T cells, has revolutionized the treatment of leukemia. However, effective GVL remains difficult to separate from graft-versus-host disease (GVHD), and many neoplasms are GVL resistant. Murine studies aimed at solving these problems have been limited by the use of leukemia cell lines with limited homology to human leukemias and by the absence of loss-of-function leukemia variants. To address these concerns, we developed a GVL model against murine chronic-phase chronic myelogenous leukemia (mCP-CML) induced with retrovirus expressing the bcr-abl fusion cDNA, the defining genetic abnormality of chronic-phase CML (CP-CML). By generating mCP-CML in gene-deficient mice, we have studied GVL T-cell effector mechanisms. mCP-CML expression of Fas or tumor necrosis factor (TNF) receptors is not required for CD8-mediated GVL. Strikingly, maximal CD4-mediated GVL requires cognate interactions between CD4 cells and mCP-CML cells as major histocompatibility complex-negative (MHC II-/-) mCP-CML is relatively GVL resistant. Nevertheless, a minority of CD4 recipients cleared MHC II-/- mCP-CML; thus, CD4 cells can also kill indirectly. CD4 GVL did not require target Fas expression. These results suggest that CPCML's GVL sensitivity may in part be explained by the minimal requirements for T-cell killing, and GVL-resistance may be related to MHC II expression. (Blood. 2004;103:4353-4361)

Graft-versus-host disease (GVHD) remains a major cause of morbidity and mortality in allogeneic stem cell transplantation (alloSCT). Donor T cells that accompany stem cell grafts cause GVHD by attacking recipient tissues; therefore, all patients receive GVHD prophylaxis by depletion of T cells from the allograft or through immunosuppressant drugs. In addition to providing a graft-versus- leukemia effect, donor T cells are critical for reconstituting T cell-mediated immunity. Ideally, immu- nity to infectious agents would be transferred from donor to host without GVHD. Most donors have been exposed to common pathogens and have an increased precursor frequency of memory T cells against pathogenic antigens. We therefore asked whether memory CD62L - CD44 + CD4 + T cells would induce less GVHD than unfractionated or naive CD4+ T cells. Strikingly, we found that memory CD4 cells induced neither clinical nor histologic GVHD. This effect was not due to the increased number of CD4+CD25+ regulatory T cells found in the CD62L-CD44+ fraction because memory T cells deple- tion of these cells did not cause GVHD. Memory CD4 cells engrafted and responded to antigen both in vivo and in vitro. If these murine results are applicable to human alloSCT, selective administra- tion of memory T cells could greatly improve post-transplant immune reconstitution.

Maturation of T cell-activating APCs directly links innate and adaptive immunity and is typically triggered by microbial infection. Transplantation of allografts, which are sterile, generates strong T cell responses; however, it is unclear how grafts induce APC maturation in the absence of microbial-derived signals. A widely accepted hypothesis is that dying cells in the graft release "danger" molecules that induce APC maturation and initiate the adaptive alloimmune response. Here, we demonstrated that danger signals associated with dying cells are not sufficient to initiate alloimmunity, but that recognition of allogeneic non-self by the innate immune system is required. In WT as well as in T cell-, B cell-, and innate lymphoid cell-deficient mice, allogeneic grafts elicited persistent differentiation of monocytes into mature DCs that expressed IL-12 and stimulated T cell proliferation and IFN-γ production. In contrast, syngeneic grafts in the same mice elicited transient and less pronounced differentiation of monocytes into DCs, which neither expressed IL-12 nor stimulated IFN-γ production. In a model in which T cell recognition is restricted to a single foreign antigen on the graft, rejection occurred only if the allogeneic non-self signal was also sensed by the host's innate immune system. These findings underscore the importance of innate recognition of allogeneic non-self by monocytes in initiating graft rejection.

Graft-versus-host disease (GVHD) is a major cause of morbidity and mortality of allogeneic stem cell transplantation. Strategies to control GVHD while maintaining graft versus leukemia (GVL) include herpes simplex virus thymidine kinase (HSV-tk) gene transduction of donor T cells followed by treatment with ganciclovir (GCV). Alternatively, GVHD and GVL may be mediated by distinct processes. In this regard, whether cytokine polarization occurs and to what degrees various subsets of cytokine-producing T cells mediate GVHD or GVL has been an active area of research using cytokine or cytokine antibody infusion or genetically deficient mice. This study takes a different approach that allows simultaneous investigation into both the mechanisms underlying GVHD reactions and the efficacy of HSV-tk suicide gene-based T-cell deletion. A source of donor T cells, splenocytes from mice transgenic for HSV-tk controlled by elements of either the interleukin-2 (IL-2) or IL-4 promoters (IL-2-tk and IL-4-tk, respectively) was used, thus allowing investigation into the roles of T1 and T2 cells in ongoing GVHD reactions. To assess treatment rather than prevention of GVHD, GCV was started at peak disease. Remarkably, treatment at this late time point rescued mice from the clinical effects of GVHD caused by T cells expressing either transgene. Thus, both T1 and T2 cells play an important role in clinical GVHD in a minor histocompatibility antigen-mismatched setting. In addition, because clinical disease was reversible even at its maximum, these observations provide controlled evidence that this strategy of treating ongoing GVHD could be effective clinically.

Graft-versus-host disease (GVHD) accounts for substantial morbidity and mortality after allogeneic bone marrow transplantation. Allograft T cells are stimulated by both host and donor-derived antigen presenting cells (APCs). After differentiating into T effector cells, donor T cells migrate to GVHD target organs where they mediate damage directly and indirectly. However, very little is known about the dynamics of T cells in GVHD target tissues and how their behaviors are affected by the local environments, including by tissue-resident APCs. We utilized two-photon intravital microscopy (2PIM) to analyze T cell locations, motility and interactions with APCs. 129S1/SVLmJ (129) (H-2b) hosts were irradiated and reconstituted with C57Bl/6 (B6) (H-2b) CD11-YFP transgenic mice bone marrow together with B6 dsRed (RFP) CD8 cells and cyan fluorescent protein (CFP) CD4 cells. Mice were imaged from days 25-42 post-transplant. We focused on the colon which is strongly targeted in this model and in humans and which previously has been difficult to image by 2PIM. In syngeneic B6 into B6 recipients, donor CD11c+, CD8+ and CD4+ cells populate the colonic mucosa. However, a greater number of cells accumulated in the allogenic as compared to syngeneic recipients. Based on the analysis of 7-8 mice per group, T cells in allogenic mice were less motile. The mean CD8 speeds in syngeneic and allogeneic mice were 3.38 ± 0.07 µm/min and 2.21 ± 0.03 µm/min, respectively. Mean CD4 speeds in syngeneic and allogeneic mice were 3.11 ± 0.08 µm/min and 2.38 ± 0.04 µm/min, respectively. The majority of T cells were stationary, with few entering or leaving the imaged volume. Perfusion was confirmed by i.v. rhodamine-dextran; therefore, stationary behaviors was not due to tissue viability. Using optical clearing we imaged whole-mount colonic tissue from mucosa to serosa. T cells were aggregated in the sub-mucosa and infiltrated crypts and surprisingly also the muscular layer. Ki67+ T cells were found throughout, especially in the submucosa. Given the 2PIM data which show few new T cells entering the imaged volumes, these cells were likely stimulated to divide locally. Preliminary data with EdU pulsing suggest this to be the case. To determine whether T cell stability is microenvironment- or antigen-driven, we injected OT-1 TCR transgenic effectors which do not recognize any antigen in the recipient and imaged the next day. Most colonic OT-1 cells showed the same stationary behavior as nearby donor-derived CD4 and CD8 cells, suggesting that factors in the GVHD colon microenvironment drive T cell stability. Nonetheless there was also an antigen-driven component as injection of an anti-MHCII antibody (but not isotype control Ab) increased CD4+ T cell motility presumably by disrupting TCR:MHCII interaction. These results show the motility of T cells in the GVHD colon is influence by both TCR:MHC cognate interactions and by the microenvironment. That T cells are dividing and may be activated in situ, suggesting GVHD may be maintained locally. Current studies are focusing on what antigen-independent factors affect T cell motility and on defining the roles played by tissue APCs. Disclosures No relevant conflicts of interest to declare.

The reconstitution of specific immune niches after bone marrow transplant is mediated by donor immune cells repopulating specific microenvironments. Here, we demonstrate that type 2 innate lymphoid cells (ILC2) in the GI tract are rapidly eliminated by irradiation and/or chemotherapy and are not repopulated in the first month post-transplant by donor bone marrow cells. In contrast, ILC2 cells in the lung are not sensitive to conditioning therapy and quickly recover quantitatively post-transplant. Donor ILC2 cell infusion with donor T cells and TCD bone marrow has been shown by us to markedly diminish lethality and clinical score associated with acute GvHD. However, the mechanism for that finding was unclear. Here, we demonstrate that donor ILC2 infusion enhances the persistence of GI tract, but not splenic or pulmonary MDSCs (Fig 1a). This persistence required the expression of IL-13 by the ILC2 cells (Fig 1b). Absence of IL-13 production by ILC2 cells greatly diminished the activity of those cells to prevent GvHD (Fig 1c). ILC2 cell infusion decreased the number of T cells generating IFN-γ and IL-17A in the GI tract. However, we found no difference in the number of IFN-γ or IL-17A generating T cells in the GI tract after the infusion of IL-13-/- ILC2 cells. To evaluate the function of ILC2 cells in MDSC biology we performed in vitro quantitation of MDSCs in co-culture with ILC2 cells. MDSCs cultured with WT ILC2s, survived significantly better than MDSCs cultured alone, MDSCs cultured with IL-13 and MDSC cultured in the presence of IL-7 and IL-33. In addition, MDSCs co-cultured with WT ILC2s survived significantly better than those cultured with IL-13-/- ILC2s. Surprisingly, enhancement of MDSC survival requires cell-to-cell contact with ILC2s, as seen when either WT or IL-13-/- ILC2s and MDSCs are cultured in plates across semi-permeable membranes. ILC2s have been shown to have a role in intestinal barrier repair via amphiregulin (Areg). When compared to untreated recipients, ILC2 treated recipients showed significantly enhance barrier function which can be observed as early as 12 days post-transplant and as late as day 20. Interestingly, recipients of Areg deficient ILC2s show improved survival compared to control treatment but diminished survival compared to recipients of WT ILC2 cells. Several cell types have been shown to suppress aGvHD, including MDSCs and regulatory T cells (Treg). To evaluate the function of ILC2 cells as treatment, we compared them to MDSC and Treg infusion. When used to treat aGvHD on day 7 post-transplant, IL-13 derived MDSCs have no effect on recipient survival. In contrast ILC2 infusion results in a significant prolongation of survival compared to MDSC infusion with approximately 35% of recipient mice surviving past day 50. When compared to Treg infusion (1:1 ratio with Tcons) both ILC2 and Treg infusions block aGvHD. However, unlike ILC2 infusion in which no mice develop P815 tumors post-transplant, 80% of mice given Tregs developed tumors. At a lower Treg dose (1:4) the GvL response was restored but without the beneficial effect on GvHD. In summary, ILC2s represent a novel cell population that are required for recovery of GI tract homeostasis following allo-SCT. ILC2s play a pivotal role in the development of donor MDSCs and intestinal barrier repair. Additional investigation should be focused on the recovery of specific immunosuppressive cell populations from donor bone marrow cells after allo-SCT. IL-13 dependent ILC2 induced expansion of donor MDSCs. (a) Frequencies CD11b+/GR-1+/Ly-6C+/Ly-6G+ MDSCs in the LP of the colon of allo-SCT recipients 12 days post-transplant with or without WT ILC2s. These represent 2 independent experiments, bar graphs are average ± SEM, student's t test using GraphPad Prism, * p Disclosures No relevant conflicts of interest to declare.