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Hematopoietic cell transplantation (HCT) is the only readily available cure for many life-threatening pediatric nonmalignant diseases (NMD), but most patients lack a matched related donor and are at higher risk for graft-versus-host disease (GVHD). Use of abatacept (Aba) to target donor T-cell activation has been safe and effective in preventing GVHD after unrelated donor (URD) HCT for malignant diseases (Aba2 trial). Our primary objective was to evaluate the tolerability of Aba added to standard GVHD prophylaxis (cyclosporine and mycophenolate mofetil) in pediatric patients with NMD undergoing URD HCT. In this single-arm, single-center phase 1 trial, 10 patients receiving reduced intensity or nonmyeloablative conditioning underwent URD HCT. Immune reconstitution was assessed longitudinally via flow cytometry and compared to pediatric patients on Aba2. Nine patients successfully engrafted, with 1 primary graft rejection in the setting of inadequate cell dose; secondary graft rejection occurred in 1 patient with concurrent cytomegalovirus viremia. Two deaths occurred, both unrelated to Aba. One patient developed probable posttransplant lymphoproliferative disease, responsive to rituximab and immune suppression withdrawal. No patients developed severe acute or chronic GVHD, and 8 patients were off systemic immunosuppression at 1 year. Immune reconstitution did not appear to be impacted by Aba, and preservation of naïve relative to effector memory CD4+ T cells was seen akin to Aba2. Thus, 4 doses of Aba were deemed tolerable in pediatric patients with NMD following URD HCT, with encouraging preliminary efficacy and supportive immune correlatives in this NMD cohort.

PURPOSE Severe (grade 3-4) acute graft-versus-host disease (AGVHD) is a major cause of death after unrelated-donor (URD) hematopoietic cell transplant (HCT), resulting in particularly high mortality after HLA-mismatched transplantation. There are no approved agents for AGVHD prevention, underscoring the critical unmet need for novel therapeutics. ABA2 was a phase II trial to rigorously assess safety, efficacy, and immunologic effects of adding T-cell costimulation blockade with abatacept to calcineurin inhibitor (CNI)/methotrexate (MTX)-based GVHD prophylaxis, to test whether abatacept could decrease AGVHD. METHODS ABA2 enrolled adults and children with hematologic malignancies under two strata: a randomized, double-blind, placebo-controlled stratum (8/8-HLA-matched URD), comparing CNI/MTX plus abatacept with CNI/MTX plus placebo, and a single-arm stratum (7/8-HLA-mismatched URD) comparing CNI/MTX plus abatacept versus CNI/MTX CIBMTR controls. The primary end point was day +100 grade 3-4 AGVHD, with day +180 severe-AGVHD-free-survival (SGFS) a key secondary end point. Sample sizes were calculated using a higher type-1 error (0.2) as recommended for phase II trials, and were based on predicting that abatacept would reduce grade 3-4 AGVHD from 20% to 10% (8/8s) and 30% to 10% (7/8s). ABA2 enrolled 142 recipients (8/8s, median follow-up = 716 days) and 43 recipients (7/8s, median follow-up = 708 days). RESULTS In 8/8s, grade 3-4 AGVHD was 6.8% (abatacept) versus 14.8% (placebo) ( P = .13, hazard ratio = 0.45). SGFS was 93.2% (CNI/MTX plus abatacept) versus 82% (CNI/MTX plus placebo, P = .05). In the smaller 7/8 cohort, grade 3-4 AGVHD was 2.3% (CNI/MTX plus abatacept, intention-to-treat population), which compared favorably with a nonrandomized matched cohort of CNI/MTX (30.2%, P < .001), and the SGFS was better (97.7% v 58.7%, P < .001). Immunologic analysis revealed control of T-cell activation in abatacept-treated patients. CONCLUSION Adding abatacept to URD HCT was safe, reduced AGVHD, and improved SGFS. These results suggest that abatacept may substantially improve AGVHD-related transplant outcomes, with a particularly beneficial impact on HLA-mismatched HCT.

Organ infiltration by donor T cells is critical to the development of acute graft-versus-host disease (aGVHD) in recipients after allogeneic hematopoietic stem cell transplant (allo-HCT). However, deconvoluting the transcriptional programs of newly recruited donor T cells from those of tissue-resident T cells in aGVHD target organs remains a challenge. Here, we combined the serial intravascular staining technique with single-cell RNA sequencing to dissect the tightly connected processes by which donor T cells initially infiltrate tissues and then establish a pathogenic tissue residency program in a rhesus macaque allo-HCT model that develops aGVHD. Our results enabled creation of a spatiotemporal map of the transcriptional programs controlling donor CD8+ T cell infiltration into the primary aGVHD target organ, the gastrointestinal (GI) tract. We identified the large and small intestines as the only two sites demonstrating allo-specific, rather than lymphodepletion-driven, T cell infiltration. GI-infiltrating donor CD8+ T cells demonstrated a highly activated, cytotoxic phenotype while simultaneously developing a canonical tissue-resident memory T cell (TRM) transcriptional signature driven by interleukin-15 (IL-15)/IL-21 signaling. We found expression of a cluster of genes directly associated with tissue invasiveness, including those encoding adhesion molecules (ITGB2), specific chemokines (CCL3 and CCL4L1) and chemokine receptors (CD74), as well as multiple cytoskeletal proteins. This tissue invasion transcriptional signature was validated by its ability to discriminate the CD8+ T cell transcriptome of patients with GI aGVHD from those of GVHD-free patients. These results provide insights into the mechanisms controlling tissue occupancy of target organs by pathogenic donor CD8+ TRM cells during aGVHD in primate transplant recipients.

One of the central challenges in the field of allo-immunity is deciphering the mechanisms driving T cells to infiltrate and subsequently occupy target organs to cause disease. The act of CD8-dominated T cell infiltration is critical to acute graft-versus-host disease (aGVHD), wherein donor T cells become activated, tissue-infiltrating and highly cytotoxic, causing wide-spread tissue damage after allogeneic hematopoietic stem cell transplant (allo-HCT). However, in human and non-human primate studies, deconvolving the transcriptional programs of newly recruited relative to resident memory T cells in the gastrointestinal (GI) tract has remained a challenge. In this study, we combined the novel technique of Serial Intravascular Staining (SIVS) with single-cell RNA-Seq (scRNA-seq) to enable detailed dissection of the tightly connected processes by which T cells first infiltrate tissues and then establish a pathogenic tissue residency program after allo-HCT in non-human primates. Our results have enabled the creation of a spatiotemporal map of the transcriptional drivers of CD8 T cell infiltration into the primary aGVHD target-organ, the GI tract. We identify the large and small intestines as the only two sites demonstrating allo-specific, rather than lymphdepletion-driven T cell infiltration. The donor CD8 T cells that infiltrate the GI tract demonstrate a highly activated, cytotoxic phenotype while simultaneously rapidly developing canonical tissue-resident memory (TRM) protein expression and transcriptional signatures, driven by IL-15/IL-21 signaling. Moreover, by combining SIVS and transcriptomic analysis, we have been able to work backwards from this pathogenic TRM programing, and, for the first time, identify a cluster of genes directly associated with tissue invasiveness, prominently including specific chemokines and adhesion molecules and their receptors, as well as a central cytoskeletal transcriptional node. The clinical relevance of this new tissue invasion signature was validated by its ability to discriminate the CD8 T cell transcriptome of patients with GI aGVHD. These results provide new insights into the mechanisms controlling tissue infiltration and pathogenic CD8 TRM transcriptional programing, uncovering critical transitions in allo-immune tissue invasion and destruction.One sentence summaryFlow cytometric and transcriptomic analysis reveals coordinated tissue-infiltration and tissue-residency programs driving gastrointestinal aGVHD.

Organ infiltration by donor T cells is critical to the development of acute graft-versus-host disease (aGVHD) in recipients after allogeneic hematopoietic stem cell transplant (allo-HCT). However, deconvoluting the transcriptional programs of newly recruited donor T cells from those of tissue-resident T cells in aGVHD target organs remains a challenge. Here, we combined the Serial Intravascular Staining technique with single-cell RNA-seq to dissect the tightly connected processes by which donor T cells initially infiltrate tissues and then establish a pathogenic tissue-residency program in a rhesus macaque allo-HCT model that develops aGVHD. Our results enabled creation of a spatiotemporal map of the transcriptional programs controlling donor CD8(+) T cell infiltration into the primary aGVHD target organ, the gastrointestinal (GI) tract. We identified the large and small intestines as the only two sites demonstrating allo-specific, rather than lymphodepletion-driven, T cell infiltration. GI-infiltrating donor CD8(+) T cells demonstrated a highly activated, cytotoxic phenotype while simultaneously developing a canonical tissue-resident memory T cell (T(RM)) transcriptional signature driven by IL-15/IL-21 signaling. We discovered expression of a cluster of genes directly associated with tissue invasiveness, including those encoding adhesion molecules (ITGB2), specific chemokines (CCL3, CCL4L1) and chemokine receptors (CD74), as well as multiple cytoskeletal proteins. This tissue invasion transcriptional signature was validated by its ability to discriminate the CD8(+) T cell transcriptome of patients with GI aGVHD from those of GVHD-free patients. These results provide insights into the mechanisms controlling tissue occupancy of target organs by pathogenic donor CD8(+) T(RM) cells during aGVHD in primate transplant recipients.

Background: Chronic graft-versus-host disease (CGVHD) is the leading cause of long-term morbidity and mortality following hematopoietic stem cell transplant (HCT) and occurs in over 50% of patients undergoing unrelated donor HCT. Despite its frequency, the mechanisms driving this disease remain incompletely understood, making its prevention and successful treatment challenging. To address this issue, we have undertaken a transcriptomic analysis of T cell reconstitution after unrelated donor HCT, to dissect mechanisms driving CGVHD. Methods: The patients studied were enrolled on a Phase 2, randomized, placebo-controlled trial of abatacept for GVHD prevention in patients receiving 8/8 unrelated-donor HCT for hematologic malignancies (NCT01743131). All immune analyses in the current study were performed on patients randomized to standard GVHD prophylaxis with calcineurin inhibition + methotrexate alone (placebo cohort, n =69), and thus provide insights into the drivers of CGVHD during standard unrelated donor HCT. On Day +100, CD4+ T cells were purified from the peripheral blood of these patients, and then analyzed by RNASeq. To determine the transcriptomic drivers of CGVHD without the confounder of significant prior acute GVHD (AGVHD) or exposure to steroids, we focused on profiling the CD4+ transcriptome of de novo CGVHD (CGVHD which develops in the absence of prior grade II-IV AGVHD, n = 7) and compared these patients to those who were 'operationally tolerant' and never developed either grade II-IV AGVHD or any CGVHD (n= 4). Gene expression from the resulting transcriptomes was quantified using kallisto. Differentially expressed (DE) genes were identified using DESeq2 (threshold for DE, adjusted (for multiple testing) p Results: DE analysis identified 101 genes that were significantly upregulated in CD4+ T cells from de novo CGVHD group and 54 genes that were significantly upregulated in the 'operationally tolerant' group (Figure 1A). GSEA identified that the mostly highly enriched signatures in patients with de novo CGVHD encompassed naïve CD4+ transcriptional programing (Figure 1B-C), in agreement with flow cytometric analysis, which also demonstrated expansion of CD4+ naïve T cells at Day +100 in patients developing de novo CGVHD compared to those demonstrating operational tolerance (Figure 1D). Importantly, the naïve CD4+ T cell signatures that were identified were distinct from those defining CD4+ stem cell memory T cells (which did not enrich in the de novo CGVHD cohort). In contrast, the gene signature of the operationally tolerant patients were enriched for regulatory gene sets (Figure 1C), consistent with a large body of evidence demonstrating that Treg expansion can be protective against CGVHD. Discussion: This study represents, to our knowledge, the first interrogation of the transcriptomic features of patients developing de novo CGVHD versus those operationally tolerant patients who develop neither significant AGVHD nor CGVHD after HCT. These patients may represent a particularly effective cohort in which to study immunologic drivers of CGVHD, given their freedom from prior treatment with corticosteroids, which can confound downstream transcriptomic analyses. Our data provide compelling evidence for a prominent naïve CD4+ T cell signature in patients who develop moderate-to-severe CGVHD despite their lack of antecedent AGVHD. These results are provocative, as they implicate a cell subset that is often considered more quiescent (naïve T cells) as associated with patients who develop immune pathology associated with CGVHD. These results suggest that naïve CD4+ T cells may represent a potent reservoir for alloreactivity, that, once activated, can cause significant disease. This would be in agreement with the implications of previously reported trials of naïve T cell depletion, which resulted in significant control of CGVHD. These results suggest that strategies to restrain naïve T cell pathogenic activation after Day +100 may improve CGVHD outcomes, and that the CD4+ T cell transcriptomic signature at this timepoint could be developed into a robust immunologic biomarker for the risk of developing CGVHD versus operational tolerance after HCT. Figure 1 Disclosures Watkins: Bristol Myers Squib: Honoraria. Qayed:Novartis: Consultancy; Mesoblast: Consultancy. Blazar:Tmunity: Other: Co-founder; KidsFirst Fund: Research Funding; BlueRock Therapeutics: Research Funding; Childrens' Cancer Research Fund: Research Funding; BlueRock Therapeuetic: Consultancy; Magenta Therapeutics: Consultancy; Fate Therapeutics Inc.: Research Funding. Horan:Bristol Myers Squib: Honoraria, Research Funding. Langston:Kadmon Corporation: Research Funding; Astellas Pharmaceuticals: Research Funding; Jazz Pharmaceuticals: Research Funding; Incyte: Research Funding; Bristol Myers Squib: Research Funding; Chimerix: Research Funding; Takeda: Research Funding. Kean:fortyseven: Consultancy; regeneron: Research Funding; hifibio: Consultancy; kymab: Consultancy; Bristol Meyers Squibb: Research Funding; gilead: Research Funding; novartis: Consultancy; bluebird bio: Research Funding; magenta: Research Funding.

Background/purpose To determine the maximum tolerated dose, toxicities, and response of sirolimus combined with oral metronomic therapy in pediatric patients with recurrent and refractory solid and brain tumors. Procedure Patients younger than 30 years of age with recurrent, refractory, or high-risk solid and brain tumors were eligible. Patients received six-week cycles of sirolimus with twice daily celecoxib, and alternating etoposide and cyclophosphamide every three weeks, with Bayesian dose escalation over four dose levels (NCT01331135). Results Eighteen patients were enrolled: four on dose level (DL) 1, four on DL2, eight on DL3, and two on DL4. Diagnoses included solid tumors (Ewing sarcoma, osteosarcoma, malignant peripheral nerve sheath tumor, rhabdoid tumor, retinoblastoma) and brain tumors (glioblastoma multiforme [GBM], diffuse intrinsic pontine glioma, high-grade glioma [HGG], medulloblastoma, ependymoma, anaplastic astrocytoma, low-grade infiltrative astrocytoma, primitive neuroectodermal tumor, nongerminomatous germ cell tumor]. One dose-limiting toxicity (DLT; grade 4 neutropenia) was observed on DL2, two DLTs (grade 3 abdominal pain and grade 3 mucositis) on DL3, and two DLTs (grade 3 dehydration and grade 3 mucositis) on DL4. The recommended phase II dose of sirolimus was 2 mg/m2 (DL3). Best response was stable disease (SD) in eight patients, and partial response (PR) in one patient with GBM. A patient with HGG was removed from the study with SD and developed PR without further therapy. Western blot analysis showed inhibition of phospho-S6 kinase in all patients during the first cycle of therapy. Conclusion The combination of sirolimus with metronomic chemotherapy is well tolerated in children. A phase II trial of this combination is ongoing.

Allogeneic hematopoietic cell transplantation (HCT) may be curative for patients with marrow and immune disorders, but graft-vs-host-disease (aGVHD) and infections cause significant morbidity and non-relapse mortality. We have conducted a multicenter, double blind, placebo-controlled phase II trial of costimulation blockade with abatacept (Aba) combined with standard GVHD prophylaxis with a calcineurin inhibitor and methotrexate (CNI + MTX) following HLA matched unrelated donor transplant (n=142). In order to assess the effects of Aba on immune reconstitution, and to assess whether this reconstitution is influenced during CMV reactivation, we longitudinally evaluated post-transplant whole blood samples with multiparameter flow cytometry using markers for CD3, CD4, CD8, CD197 and CD45RA to measure reconstitution of CD4 and CD8 T cell populations and their respective memory subsets over time. Results: We observe that post-transplant CMV reactivation induces a marked expansion of CD8 effector memory (EM) cells, which is similar in magnitude for Aba vs placebo patients. We found that development of moderate (gr 2-4) or severe (gr 3-4) GVHD was not associated with an increased frequency of CMV reactivation, but patients with moderate GVHD showed a blunted expansion of CD8 EM cells compared to those without GVHD, and CD8 EM expansion was essentially absent among CMV reactivating patients with severe aGVHD. Clinical correlates will be presented. Conclusions: Our results suggest that adding abatacept to CNI/MTX does not materially affect reconstitution of T cell immunity in the presence or absence of CMV reactivation, but aGVHD remains a major driver of compromised immune recovery after HCT. Disclosures Watkins: Bristol Myers Squibb: Research Funding. Qayed:Mesoblast: Consultancy; Novartis: Consultancy. Horan:Bristol Myers Squib: Honoraria, Research Funding. Kean:gilead: Research Funding; bluebird bio: Research Funding; fortyseven: Consultancy; magenta: Research Funding; regeneron: Research Funding; hifibio: Consultancy; kymab: Consultancy; Bristol Meyers Squibb: Research Funding; novartis: Consultancy. Langston:Kadmon Corporation: Research Funding; Bristol Myers Squib: Research Funding; Incyte: Research Funding; Chimerix: Research Funding; Takeda: Research Funding; Astellas Pharmaceuticals: Research Funding; Jazz Pharmaceuticals: Research Funding.

Allogeneic hematopoietic cell transplantation (HCT) may be curative for patients with marrow and immune disorders, but acute graft-vs-host-disease (aGVHD) and infections cause significant morbidity and non-relapse mortality. We conducted a multicenter, double blind, placebo-controlled phase II trial of costimulation blockade with abatacept (Aba) combined with standard GVHD prophylaxis with a calcineurin inhibitor and methotrexate (CNI + MTX) following HLA matched unrelated donor transplant (n=140). In order to assess whether Aba produces clinically relevant compromise of immune reconstitution, and whether this reconstitution is influenced during CMV reactivation, we longitudinally evaluated post-transplant whole blood samples with multiparameter flow cytometry using markers for CD3, CD4, CD8, CD197 and CD45RA to measure reconstitution of CD4 and CD8 T cell populations and their respective memory subsets over time. We show here that post-transplant CMV reactivation induces a marked expansion of CD8 effector memory (EM) cells, which is similar in Aba vs placebo pts. We found that development of moderate (gr 2–4) or severe (gr 3–4) GVHD does not cause more CMV reactivation in patients, but patients with moderate GVHD show a blunted expansion of CD8 EM cells compared to those without GVHD. Remarkably, CD8 EM expansion was essentially absent among CMV reactivating patients with severe aGVHD. Our results suggest that adding abatacept to CNI/MTX does not materially affect reconstitution of T cell immunity in the presence or absence of CMV reactivation, but aGVHD remains a major driver of compromised immune recovery after HCT.

We performed a Ph2 trial in adults and children to test abatacept for AGVHD prevention (‘ABA2', Clinicaltrials # NCT01743131), based on our promising preclinical and pilot patient data. ABA2 had 2 cohorts: A) HLA-mismatched (‘7/8', n = 43), a single-arm study with pre-specified CIBMTR matched analysis (vs CNI+MTX or CNI+MTX+ATG). B) HLA-matched (‘8/8', n = 142), randomized double-blind, comparing CNI+MTX+placebo vs CNI+MTX+ABA (‘ABA'). For each ABA arm, patients received 4 doses of 10mg/kg on d -1,5,14,28. ABA2 was designed as a screening Ph2 trial, with relaxed Type 1 error (0.2) and standard Type 2 error (0.2). Power analysis assumed ABA would decrease Gr 3-4 AGVHD from 30%–>10% in 7/8s and from 20%–>10% in 8/8s. Here we report top-line results for the 7/8s (median f/u = 708d, 264-1491) and 8/8s (median f/u = 369d, 180-1175). Reduced Grade 3-4 AGVHD: ABA was associated with decreased d180 Gr 3-4 AGVHD. In 7/8s, Gr 3-4 AGVHD = 2.5% in ABA (1 event/43 patients) vs 31% (CNI+MTX) and 22% (+ATG), 1 sided p = 0.001, 0.005,). In 8/8s, Gr 3-4 AGVHD = 6.85 % (5 events/73 patients) in ABA vs 14.6% (10 events) in placebo, 1 sided p = 0.068). Reduced Grade 2-4 AGVHD in 8/8s: ABA was associated with decreased d180 Gr 2-4 AGVHD. In 7/8s, Gr 2-4 AGVHD = 42% (ABA) vs 54% (CNI+MTX) and 45% (+ATG, 1 sided p = 0.098, 0.25). In 8/8s, Gr 2-4 AGVHD = 44.5% in ABA vs 62.3% in placebo (1 sided p = 0.004). Chronic GVHD: For 7/8s, 1 yr CGVHD = 38.8% (ABA) vs 43.5% (CNI+MTX) and 35.5% (+ATG, p = 0.4, 0.99). For 8/8s, follow up is still too short to adequately evaluate, and adjudication is ongoing. No Increase In Relapse: In 7/8s, relapse = 9.37% at 1 y vs 12.9% in CNI+MTX and 13.6% in +ATG (p = 0.115 and 0.085). In 8/8s, relapse = 13.8% at 1y (ABA) vs 20.5% (placebo, p = 0.7). Remarkably, in 7/8s, where median follow up=708d, there have been no further relapses reported in ABA2, with 9.37% relapse at 2 yr vs 20.63% (CNI+MTX) and 23.4% (ATG), despite matched disease risk. Statistically significant survival advantage in 7/8s: TRM: For 7/8s, 1 yr TRM = 10.5% (ABA) vs 32.7% (CNI+MTX) and 26% (+ATG, p = 0.024, 0.365). For 8/8s, TRM = 7.1% vs 14.6% at 1 yr (p = 0.5). Relapse-Free Survival (RFS): For 7/8s, RFS = 73.7% (ABA) vs 38.7% in CNI+MTX and 48.7% in +ATG (p = 0.001 and 0.027). For 8/8s, RFS = 79.1% for ABA vs 64.9% (placebo, p = 0.38). OS: For 7/8s, OS = 71% (ABA) vs 47.5% (CNI+MTX) and 58% (+ATG, p = 0.01 and 0.145). For 8/8s, OS = 83.2% (ABA) vs 76.6 (placebo, p = 0.32). Our results suggest that short-course (4 doses) ABA can safely prevent AGVHD without compromising relapse. While this was a modestly sized study (7/8s: n = 43, 8/8 n = 142), the comparative event size for 7/8s was high enough that the protective effect of ABA was highly statistically significant. For 8/8s, there was a statistically significant improvement for Gr 2-4 GVHD and a trend toward an advantage for ABA in all other parameters. These results are the first to demonstrate efficacy of in vivo T cell costimulation blockade in preventing AGVHD.

Although cytomegalovirus (CMV) reactivation has long been implicated in posttransplant immune dysfunction, the molecular mechanisms that drive this phenomenon remain undetermined. To address this, we combined multiparameter flow cytometric analysis and T-cell subpopulation sorting with high-throughput sequencing of the T-cell repertoire, to produce a thorough evaluation of the impact of CMV reactivation on T-cell reconstitution after unrelated-donor hematopoietic stem cell transplant. We observed that CMV reactivation drove a >50-fold specific expansion of Granzyme B(high)/CD28(low)/CD57(high)/CD8(+) effector memory T cells (Tem) and resulted in a linked contraction of all naive T cells, including CD31(+)/CD4(+) putative thymic emigrants. T-cell receptor β (TCRβ) deep sequencing revealed a striking contraction of CD8(+) Tem diversity due to CMV-specific clonal expansions in reactivating patients. In addition to querying the topography of the expanding CMV-specific T-cell clones, deep sequencing allowed us, for the first time, to exhaustively evaluate the underlying TCR repertoire. Our results reveal new evidence for significant defects in the underlying CD8 Tem TCR repertoire in patients who reactivate CMV, providing the first molecular evidence that, in addition to driving expansion of virus-specific cells, CMV reactivation has a detrimental impact on the integrity and heterogeneity of the rest of the T-cell repertoire. This trial was registered at www.clinicaltrials.gov as #NCT01012492.

Although acute graft-versus-host-disease (AGVHD) is one of the major causes of non-relapse mortality after hematopoietic stem cell transplant (HCT), we are still unable to predict which patients will develop the most severe form of this disease, or which molecular pathways are dysregulated in the T cells that cause disease. Thus, understanding the molecular features of AGVHD is a critical unmet need. To address this, we have performed a companion mechanistic study as a part of our completed Phase 2 trial of abatacept, a CD28:CD80/86 costimulation blockade agent, for severe AGVHD prevention (Clinicaltrials.gov # NCT01743131, 'ABA2'). ABA2 has demonstrated significant improvement in AGVHD in patients prophylaxed with abatacept in addition to calcineurin inhibition (CNI) + Methotrexate (MTX) compared to controls receiving CNI/MTX alone. To begin to uncover mechanisms responsible for the control of AGVHD with abatacept, and given that CD4+ T cells have been consistently documented to be the main therapeutic target of this drug, we interrogated the transcriptome of CD4+ T cells reconstituting in patients prophylaxed with abatacept compared to CNI/MTX. To perform this analysis, we flow cytometrically sorted CD4+ T cells on Days 21-28 post-transplant from all patients on ABA2, as well as a cohort of 12 untransplanted healthy controls, and subsequently performed mRNA-sequencing on these cells. Weighted Gene Correlation Network Analysis (WGCNA) was performed on the top 6000 most variant transcripts from the resulting sequencing data. Hierarchical clustering of the WGCNA co-expression matrix enabled the identification of self-assembling modules (SAMs) that met a threshold of coexpression (Figure 1A). For the ABA2 dataset, we considered the following variables in the WGCNA model: patient cohort (7/8 patients, 8/8 patients, healthy controls), +/- prophylaxis with abatacept, CMV reactivation, EBV reactivation, Grade of GVHD (0-4), relapse, non-relapse mortality, and all-cause mortality. The WGCNA clustering analysis resulted in the identification of 4 discrete SAMs, which were highly correlated with clinical variable metamodules. This analysis revealed a strong positive correlation of a 476-gene SAM (the Turquoise module) in patients prophylaxed with CNI/MTX + placebo and anti-correlation of this module in patients prophylaxed with CNI/MTX + abatacept, as demonstrated in both the WGCNA heatmap and through Gene Set Enrichment Analysis (Figure 1 A-B). These opposing correlations suggested that interrogation of this module would reveal mechanistic correlates with standard prophylaxis that were decoupled by abatacept. Pathway analysis using the Reactome database (Figure 1C) revealed the turquoise SAM to be dominated by four types of pathways: (1) Those that define canonical cell-cycle pathways (2) Those involved in T cell metabolism (3) Those involved in apoptosis and (4) Those involved in T cell activation, consistent with upregulation of these transcripts in placebo versus abatacept patients. In addition to being highly correlated with patients receiving placebo, the expression of a subset of the transcripts in the Turquoise module were also directly correlated with the severity of AGVHD in these patients. Thus, linear regression analysis of the 476 transcripts in this module identified a subset of 93 genes for which transcript expression level was increased both in placebo compared to abatacept, and for which expression level also positively correlated with Grade of AGVHD. As with the Turquoise module as a whole, this subset of genes also formed a highly correlated network, linking transcripts involved in T cell proliferation, apoptosis, activation, metabolism as well as the T cell checkpoint (Figure 1D). This analysis represents the first comprehensive interrogation of the transcriptomic correlates of AGVHD. It identifies a novel set of transcripts which positively associate with the severity of AGVHD, and which costimulation blockade with abatacept down-regulates and de-couples from AGVHD severity. These results suggest a profound reprograming of T cell activation with abatacept that is correlated with the control of AGVHD. Disclosures Qayed: Bristol-Myers Squibb: Honoraria. Langston:Astellas Pharma: Other: Research Support; Incyte: Other: Research Support; Jazz Pharmaceuticals: Other: Research Support; Chimerix: Other: Research Support; Takeda: Other: Research Support; Kadmon Corporation: Other: Research Support; Novartis: Other: Research Support; Bristol Myers Squibb: Other: Research Support. Blazar:Fate Therapeutics, Inc.: Research Funding; RXi Pharmaceuticals: Research Funding; Alpine Immune Sciences, Inc.: Research Funding; Abbvie Inc: Research Funding; Leukemia and Lymphoma Society: Research Funding; Childrens' Cancer Research Fund: Research Funding; KidsFirst Fund: Research Funding; Tmunity: Other: Co-Founder; BlueRock Therapeutics: Membership on an entity's Board of Directors or advisory committees; Kamon Pharmaceuticals, Inc: Membership on an entity's Board of Directors or advisory committees; Five Prime Therapeutics Inc: Co-Founder, Membership on an entity's Board of Directors or advisory committees; Regeneron Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Magenta Therapeutics and BlueRock Therapeuetics: Membership on an entity's Board of Directors or advisory committees. Kean:HiFiBio: Consultancy; BlueBirdBio: Research Funding; Gilead: Research Funding; Regeneron: Research Funding; EMDSerono: Consultancy; FortySeven: Consultancy; Magenta: Research Funding; Kymab: Consultancy; Jazz: Research Funding; Bristol Meyers Squibb: Patents & Royalties, Research Funding. OffLabel Disclosure: Abatacept: Approved for Rheumatoid Arthritis; used in this trial for prevention of GVHD.

We performed a first-in-disease trial of in vivo CD28:CD80/86 costimulation blockade with abatacept for acute graft-versus-host disease (aGVHD) prevention during unrelated-donor hematopoietic cell transplantation (HCT). All patients received cyclosporine/methotrexate plus 4 doses of abatacept (10 mg/kg/dose) on days −1, +5, +14, +28 post-HCT. The feasibility of adding abatacept, its pharmacokinetics, pharmacodynamics, and its impact on aGVHD, infection, relapse, and transplantation-related mortality (TRM) were assessed. All patients received the planned abatacept doses, and no infusion reactions were noted. Compared with a cohort of patients not receiving abatacept (the StdRx cohort), patients enrolled in the study (the ABA cohort) demonstrated significant inhibition of early CD4+ T cell proliferation and activation, affecting predominantly the effector memory (Tem) subpopulation, with 7- and 10-fold fewer proliferating and activated CD4+ Tem cells, respectively, at day+28 in the ABA cohort compared with the StdRx cohort (P

Recently several different JAK2 exon12 mutations have been identified in V617F negative polycythaemia vera (PV) or idiopathic erythrocytosis (IE) patients. The patients present with erythrocytosis, ligand-independent cell growth and low serum erythropoietin (EPO) levels. Within this group, a deletion of amino acids 542-543 (N542-E543del) of JAK2 is most prevalent. We have previously shown that in the presence of JAK2(V617F), suppressor of cytokine signalling 3 (SOCS3) is unable to negatively regulate EPO signalling and proliferation of V617F-expressing cells. Here we report a PV patient heterozygous for the somatic JAK2(N542-E543del) mutation and a previously unreported germline mutation within the SH2 domain of SOCS3 (F136L). Interestingly, the SOCS3(F136L) mutation was detected in a Japanese myeloproliferative disorder patient cohort at double the frequency of healthy controls. Cells expressing SOCS3(F136L) had markedly elevated EPO-induced proliferation and extended EPO-induced JAK2 phosphorylation. Additionally, compared to wild-type SOCS3, mutant SOCS3 had an extended half-life in the presence of JAK2 and JAK2(N542-E543del). Our findings suggest that this loss-of-function SOCS3 mutation may have contributed to disease onset by causing deregulated JAK2 signalling in the presence of a constitutively active JAK2(N542-E543del) mutant.

Respiratory syncytial virus (RSV) infection causes bronchiolitis and pneumonia in infants. RSV has a linear single-stranded RNA genome encoding 11 proteins, 2 of which are nonstructural (NS1 and NS2). RSV specifically downregulates STAT2 protein expression, thus enabling the virus to evade the host type I interferon response. Degradation of STAT2 requires proteasomal activity and is dependent on the expression of RSV NS1 and NS2 (NS1/2). Here we investigate whether RSV NS proteins can assemble ubiquitin ligase (E3) enzymes to target STAT2 to the proteasome. We demonstrate that NS1 contains elongin C and cullin 2 binding consensus sequences and can interact with elongin C and cullin 2 in vitro; therefore, NS1 has the potential to act as an E3 ligase. By knocking down expression of specific endogenous E3 ligase components using small interfering RNA, NS1/2, or RSV-induced STAT2, degradation is prevented. These results indicate that E3 ligase activity is crucial for the ability of RSV to degrade STAT2. These data may provide the basis for therapeutic intervention against RSV and/or logically designed live attenuated RSV vaccines.

s / Biol Blood Marrow Transplant 21 (2015) S54eS78 S69 after RIC (34 vs 29%; p1⁄40.04). There was no statistical difference between MAC vs RIC for OS, LFS, RI and NRM (Figure 1). The analyses were performed separately for pts

JAK2 V617F, identified in the majority of patients with myeloproliferative neoplasms, tyrosine phosphorylates SOCS3 and escapes its inhibition. Here, we demonstrate that the JAK2 exon 12 mutants described in a subset of V617F-negative MPN cases, also stabilize tyrosine phosphorylated SOCS3. SOCS3 tyrosine phosphorylation was also observed in peripheral blood mononuclear cells and granulocytes isolated from patients with JAK2 H538QK539L or JAK2 F537-K539delinsL mutations. JAK kinase inhibitors, which effectively inhibited the proliferation of cells expressing V617F or K539L, also caused a dose-dependent reduction in both mutant JAK2 and SOCS3 tyrosine phosphorylation. We propose, therefore, that SOCS3 tyrosine phosphorylation may be a novel bio-marker of myeloproliferative neoplasms resulting from a JAK2 mutation and a potential reporter of effective JAK2 inhibitor therapy currently in clinical development.

The somatic JAK2 valine-to-phenylalanine (V617F) mutation has been detected in up to 90% of patients with polycythemia and in a sizeable proportion of patients with other myeloproliferative disorders such as essential thrombocythemia and idiopathic myelofibrosis. Suppressor of cytokine signaling 3 (SOCS3) is known to be a strong negative regulator of erythropoietin (EPO) signaling through interaction with both the EPO receptor (EPOR) and JAK2. We report here that JAK2 V617F cannot be regulated and that its activation is actually potentiated in the presence of SOCS3. Instead of acting as a suppressor, SOCS3 enhanced the proliferation of cells expressing both JAK2 V617F and EPOR. Additionally, although SOCS1 and SOCS2 are degraded in the presence of JAK2 V617F, turnover of SOCS3 is inhibited by the JAK2 mutant kinase and this correlated with marked tyrosine phosphorylation of SOCS3 protein. We also observed constitutive tyrosine phosphorylation of SOCS3 in peripheral blood mononuclear cells (PBMCs) derived from patients homozygous for the JAK2 V617F mutant. These findings suggest that the JAK2 V617F has overcome normal SOCS regulation by hyperphosphorylating SOCS3, rendering it unable to inhibit the mutant kinase. Thus, JAK2 V617F may even exploit SOCS3 to potentiate its myeloproliferative capacity.