TAMI-12. CANCER-IMMUNE CELL INTERACTIONS DRIVE TRANSITIONS TO MESENCHYMAL-LIKE STATES IN GLIOBLASTOMA

Communication between cancer cells and immune cells is a key determinant of the glioblastoma ecosystem and its response to therapies, but remains poorly understood. Here we leveraged single-cell RNA-sequencing (scRNA-seq) of human samples and mouse models, deconvolution analysis of bulk specimens from The Cancer Genome Atlas (TCGA) and functional approaches to dissect cellular states and cross-talk in glioblastoma. We demonstrate that macrophages induce a transition of glioblastoma cells into mesenchymal-like (MES-like) states. This effect is mediated, both in vitro and in vivo, by macrophage-derived Oncostatin M (OSM) and its cognate receptor OSMR on glioblastoma cells. We show that MES-like glioblastoma states are associated with increased T cells cytotoxicity and potentially with better clinical response to immunotherapies. Overall, our work dissects the cellular interactions within the glioblastoma microenvironment, with potential implications for therapies.

395 Detection of viral antigen and immune activation after intra-tumor injection of CAN-3110 (ICP-34.5 expressing HSV-1 oncolytic virus) in patients with recurrent high-grade glioma

Background

Recurrent high-grade glioma (HGG) represents a significant clinical unmet need with expected survival between 6 to 9 months. Oncolytic viruses are a new therapeutic approach for solid tumors that deploy oncolytic activity combined with local and systemic immune activation. CAN-3110 (rQNestin34.5v2) is an oncolytic herpes simplex virus (HSV), modified to encode the HSV1 ICP34.5 protein under control of the nestin promoter. Selective expression of nestin in brain tumors confers tumor-restricted replication of CAN-3110. We conducted an open-label dose-escalation phase 1 clinical trial in patients with recurrent HGG to evaluate safety, tolerability, and immunological changes after CAN-3110 treatment.

Methods

Thirty patients with biopsy-confirmed recurrent HGG were enrolled from September 2017 to February 2020. CAN-3110 was injected intratumorally starting at 1x106 plaque forming units (pfu) and dose-escalated by half log to 1x1010 pfu. Patients also received standard of care. Peripheral blood mononuclear cells (PBMCs), plasma and tumor samples were collected for analysis at different time-points post treatment. We evaluated HSV antigen expression in tumor tissue. RNA sequencing and T cell receptor (TCR) rearrangement analysis was performed in matched tissue and PBMCs. Cytokine profiling was completed in 29 patients at baseline, day 2, and day 28 post treatment.

Results

Eighteen patients were recruited at their first recurrence and 12 at the second recurrence. Three patients presented with multifocal disease. Tumor volume ranged from 357.4 to and 54,036.1mm3 (median 7,733.9mm3, SDV 15,610.2). CAN-3110 was well-tolerated with no dose-limiting toxicity. Median overall survival was 11.7 months. We demonstrated persistence of HSV antigen and CD8+ T cell infiltrates at the site of injected tumor. Preliminary analysis revealed expansion of shared TCR clonotypes and upregulation of pro-inflammatory genes in post-treatment tumors and peripheral blood samples. Longitudinal modeling of cytokine profiling demonstrated increased levels of IL-6, VEGF alpha, CCL2 and IL1-RA and a decrease in GCP-2 levels at day 2 post-treatment (p <0.05). Significant correlations were observed between CXCL2 and CXCL6 (r=0.89 and r=0.95, respectively, at day 2 and day 28 post treatment; p<0.05), CCL2 and CXCL6 (r=0.73 and r=0.61 at days 2 and 28 post treatment; p<0.05) and between CCL2 and CXCL2 (r=0.68, p<0.05 at day 2 post treatment) in patients surviving more than 12 months.

Conclusions

Intratumoral administration of CAN-3110 appears well-tolerated in recurrent HGG. Histologic, molecular, and cytokine analyses demonstrate persistence of viral antigen as well as local and systemic immune activation after treatment.

Ethics Approval

The study was approved by the Office for Human Research Studies at Dana-Farber Cancer Institute, Protocol Number 16–557.

OTME-8. Inhibitory CD161 receptor identified in glioma-infiltrating T cells by single-cell analysis

T-cells are critical effector cells of cancer immunotherapies, but little is known about T-cell gene expression programs in diffuse gliomas. We leveraged single-cell RNA-seq to chart the gene expression and clonal landscape of tumor-infiltrating T-cells across 31 patients with isocitrate dehydrogenase (IDH) wild-type glioblastoma and IDH mutant glioma. Our analysis revealed subsets of T-cells that expressed several NK-cell receptors, in particular the inhibitory CD161 receptor (KLRB1 gene). KLRB1 was overexpressed by clonally expanded CD8 T-cells, and larger populations of T-cells expressed CD161 than PD-1. The CLEC2D ligand of CD161 was expressed by malignant cells and myeloid cells, and inactivation of KLRB1 enhanced anti-tumor T-cell function. KLRB1 was also expressed by substantial T-cell populations in multiple other human cancers. CD161 and other NK-cell receptors expressed by T-cells represent opportunities for immunotherapy of diffuse gliomas and other human cancers.

OTME-7. Cancer - immune cell interactions drive transitions to mesenchymal-like state in glioblastoma

Communication between cancer cells and immune cells is a key determinant of the glioblastoma ecosystem and its response to therapies, but remains poorly understood. Here we leveraged single-cell RNA-sequencing (scRNA-seq) of human samples and mouse models, deconvolution analysis of bulk specimen from The Cancer Genome Atlas (TCGA) and functional approaches, to dissect cellular cross-talks in glioblastoma. We demonstrate that macrophages induce a transition of glioblastoma cells into mesenchymal-like (MES-like) states. This effect is mediated, both in vitro and in vivo, by macrophage-derived Oncostatin M (OSM) and its cognate receptor OSMR on glioblastoma cells. We show that MES-like glioblastoma states are also associated with increased expression of a mesenchymal program in macrophages and with increased cytotoxicity of T cells, highlighting extensive alterations of the immune microenvironment with potential therapeutic implications.

EXTH-37. TARGETING EPIGENETIC VULNERABILITIES IDENTIFIED FROM A CRISPR SCREEN IN H3.3K27M DIPG

Children diagnosed with diffuse intrinsic pontine glioma (DIPG), a type of high grade glioma in the brainstem, currently have a dismal 5-year overall survival of only 2%. The majority of DIPG patients harbor a K27M mutation in histone 3.3 encoding genes (H3.3K27M). To understand if the aberrant epigenetic landscape induced by H3.3K27M provides an opportunity for novel targeted therapies, we conducted the first CRISPR/Cas9 screen using a focused library of 1,350 epigenetic regulatory and cancer related genes in six H3.3K27M DIPG patient-derived primary neurosphere cell lines. We identified gene dependencies in chromatin regulators, polycomb repressive complexes 1 and 2 (PRC1 and PRC2), histone demethylases, acetyltransferases and deacetylators as novel tumor cell dependencies in DIPG. We hypothesized that targeting dysregulated functions of chromatin regulators by genetically deleting and chemically targeting these epigenetically induced vulnerabilities, we could ameliorate, or even reverse the downstream oncogenic effects of the aberrant epigenetic landscape of DIPG. In our secondary CRISPR nanoscreen, we first used six single guide RNAs (sgRNA) to knockout each gene using CRISPR/Cas9 ribonucleoprotein nucleofections, followed by use of three best sgRNAs combined with homology directed repair templates. Compared to lentiviral delivery, nucleofection is a rapid method, with reduced off-target toxicity, suitable for single gene knockouts in DIPG neurospheres. Secondary CRISPR validations confirmed dependencies in BMI1, CBX4, KDM1A, EZH2, EED, SUZ12, HDAC2, and EP300. Next, we conducted a chemical screen using 20 inhibitors and degraders to target the aberrant activity of HDAC, KDM1A, P300/CBP, PRC1 and PRC2. We identified eight chemical compounds that were effective in H3.3K27M DIPG neurosphere cell lines at low drug concentrations. Among these, an inhibitor and degrader targeting P300/CBP activity indicates a novel strategy of epigenetic therapy in DIPG. Through our combinatorial testing, we will identify a synergistic combination of epigenetic therapy for treating children diagnosed with H3.3K27M DIPG.

EPCO-35. SINGLE-CELL RNA-SEQ OF PEDIATRIC EPENDYMOMA REVEALS PROGNOSTIC IMPACT OF IMPAIRED NEURONAL-GLIAL FATE SPECIFICATION

Ependymoma represents a heterogeneous disease affecting the entire neuraxis. Extensive molecular profiling efforts have identified molecular ependymoma subgroups based on DNA methylation. However, the intratumoral heterogeneity and developmental origins of these groups are only partially understood, and effective treatments are still lacking for about 50% of patients with high-risk tumors. We interrogated the cellular architecture of ependymoma using single cell/single nucleus RNA-sequencing to analyze 24 tumor specimens across major molecular subgroups and anatomic locations. We additionally analyzed ten patient-derived ependymoma cell models and two patient-derived xenografts (PDXs). Interestingly, we identified an analogous cellular hierarchy across all ependymoma groups, originating from undifferentiated neural stem cell-like populations towards different degrees of impaired differentiation states comprising neuronal precursor-like, astro-glial-like, and ependymal-like tumor cells. While prognostically favorable ependymoma groups predominantly harbored differentiated cell populations, aggressive groups were enriched for undifferentiated subpopulations. Projection of transcriptomic signatures onto an independent bulk RNA-seq cohort stratified patient survival even within known molecular groups, thus refining the prognostic power of DNA methylation-based profiling. Furthermore, we identified novel potentially druggable targets such as IGF- and FGF-signaling within poorly prognostic transcriptional programs. Ependymoma-derived cell models/PDXs widely recapitulated the transcriptional programs identified within fresh tumors and are leveraged to validate identified target genes in functional follow-up analyses. Taken together, our analyses reveal a developmental hierarchy and transcriptomic context underlying the biologically and clinically distinct behavior of ependymoma groups. The newly characterized cellular states and underlying regulatory networks could serve as basis for future therapeutic target identification and reveal biomarkers for clinical trials.

Harvesting wind energy to detect weak signals using mechanical stochastic resonance

Wind is free and ubiquitous and can be harnessed in multiple ways. We demonstrate mechanical stochastic resonance in a tabletop experiment in which wind energy is harvested to amplify weak periodic signals detected via the movement of an inverted pendulum. Unlike earlier mechanical stochastic resonance experiments, where noise was added via electrically driven vibrations, our broad-spectrum noise source is a single flapping flag. The regime of the experiment is readily accessible, with wind speeds ∼20 m/s and signal frequencies ∼1 Hz. We readily obtain signal-to-noise ratios on the order of 10 dB.

Genome-Wide STAT3 Binding Analysis after Histone Deacetylase Inhibition Reveals Novel Target Genes in Dendritic Cells

STAT3 is a master transcriptional regulator that plays an important role in the induction of both immune activation and immune tolerance in dendritic cells (DCs). The transcriptional targets of STAT3 in promoting DC activation are becoming increasingly understood; however, the mechanisms underpinning its role in causing DC suppression remain largely unknown. To determine the functional gene targets of STAT3, we compared the genome-wide binding of STAT3 using ChIP sequencing coupled with gene expression microarrays to determine STAT3-dependent gene regulation in DCs after histone deacetylase (HDAC) inhibition. HDAC inhibition boosted the ability of STAT3 to bind to distinct DNA targets and regulate gene expression. Among the top 500 STAT3 binding sites, the frequency of canonical motifs was significantly higher than that of noncanonical motifs. Functional analysis revealed that after treatment with an HDAC inhibitor, the upregulated STAT3 target genes were those that were primarily the negative regulators of proinflammatory cytokines and those in the IL-10 signaling pathway. The downregulated STAT3-dependent targets were those involved in immune effector processes and antigen processing/presentation. The expression and functional relevance of these genes were validated. Specifically, functional studies confirmed that the upregulation of IL-10Ra by STAT3 contributed to the suppressive function of DCs following HDAC inhibition.

A Sec22b knockout mouse offers novel insights into the molecular mechanisms of crosspresentation

Crosspresentation critically contributes to the adaptive immune response to tumors and viral infection by presenting extracellular antigen on MHC I to activate CD8 T cells. However, its molecular regulation is poorly understood. Using in vitro shRNA-mediated knockdown (KD) methods, another group had shown that Sec22b, an ER-SNARE, specifically regulates crosspresentation. Thus, we tested the hypothesis that Sec22b would mediate crosspresentation in vivo. We generated DC-specific Sec22b knockout (KO) mice, which have an intact, functional immune system. After adoptively transferring OT-I T cells into these mice, then injecting with soluble OVA i.p., we analyzed OT-I T cell proliferation to measure crosspresentation. Surprisingly, we saw no difference in OVA crosspresentation by KO versus FL mice. This observation was verified in vitro using KO DCs from BM and spleen crosspresenting soluble and insoluble OVA. We concluded Sec22b is not necessary for crosspresentation, invalidating our hypothesis. However, we were able to reproduce the reduced crosspresentation by shRNA-mediated KD of Sec22b. Intriguingly, when we treated KO BMDCs with the same Sec22b-targeting shRNA, we again observed reduced crosspresentation comparable to that seen in shRNA-treated WT BMDCs. This suggested the shRNA mediates crosspresentation through its effects on an off-target gene. RNAseq data on shRNA-treated BMDCs provided a candidate list of potential crosspresentation mediators. Our data demonstrates that (a) Sec22b is not necessary for crosspresentation, (b) there may be a novel critical mediator of crosspresentation targeted by the shRNA, and (c) extrapolating mechanisms or phenotypes based on KD studies alone must be done with caution.

Mature T cell responses are controlled by microRNA-142

T cell proliferation is critical for immune responses; however, the molecular mechanisms that mediate the proliferative response are poorly understood. MicroRNAs (miRs) regulate various molecular processes, including development and function of the immune system. Here, utilizing multiple complementary genetic and molecular approaches, we investigated the contribution of a hematopoietic-specific miR, miR-142, in regulating T cell responses. T cell development was not affected in animals with a targeted deletion of Mir142; however, T cell proliferation was markedly reduced following stimulation both in vitro and in multiple murine models of graft-versus-host disease (GVHD). miR-142-deficient T cells demonstrated substantial cell-cycling defects, and microarray and bioinformatics analyses revealed upregulation of genes involved in cell cycling. Moreover, 2 predicted miR-142 target genes, the atypical E2F transcription factors E2f7 and E2f8, were most highly upregulated in miR-142-deficient cells. Clustered regularly interspaced short palindromic repeat interference-mediated (CRISPRi-mediated) silencing of E2F7 and E2F8 in miR-142-deficient T cells ameliorated cell-cycling defects and reduced GVHD, and overexpression of these factors in WT T cells inhibited the proliferative response. Together, these results identify a link between hematopoietic-specific miR-142 and atypical E2F transcription factors in the regulation of mature T cell cycling and suggest that targeting this interaction may be relevant for mitigating GVHD.

Ikaros deficiency in host hematopoietic cells separates GVL from GVHD after experimental allogeneic hematopoietic cell transplantation

The graft-versus-leukemia (GVL) effect following allogeneic hematopoietic stem cell transplantation (allo-HCT) is critical for its curative potential. Hwever, GVL is tightly linked to graft-versus-host disease (GVHD). Among hematological malignancies, acute lymphoblastic leukemia (ALL) is the most resistant to GVL, although the reasons for this remain poorly understood. Clinical studies have identified alterations in Ikaros (Ik) transcription factor as the major marker associated with poor outcomes in ALL. We have shown that the absence of Ik in professional host-derived hematopoietic antigen-presenting cells (APCs) exacerbates GVHD. However, whether Ik expression plays a role in resistance to GVL is not known. In this study we used multiple clinically relevant murine models of allo-HCT to explore whether Ik expression in hematopoietic APCs and/or leukemic cells is critical for increasing resistance to GVL and thus inducing relapse. We found that Ik deficiency in host APCs failed to enhance GVL despite increased GVHD severity. Mechanistic studies with bone marrow (BM) chimeras and tetramer analyses demonstrated reduced tumor-specific immunodominant (gag+) antigen responses in the [B6Ik^-/-→B6] group. Loss of GVL was observed when both the leukemia cells and the host APCs were deficient in Ik. We found that calreticulin (CRT) expression in host antigen-presenting dendritic cells (DCs) of Ik^-/- animals was significantly lower than in wild-type animals. Rescuing CRT expression in Ik^-/- DCs improved leukemic-specific cytotoxic T cell function. Together, our data demonstrate that the absence of Ikaros in host hematopoietic cells promotes resistance to GVL despite increasing GVHD and thus provides a potential mechanism for the poor outcome of Ik^-/- ALL patients.

Microbial metabolites modulate GI mucosal damage from graft versus host disease (GHVD). (MUC4P.850)

The microbiome of the GI tract is known to affect GI mucosal responses and regulate disease processes. GI tract damage is the major cause for graft versus host disease (GVHD) mortality. Studies have shown the microbiome affects the course of allogeneic bone marrow transplant and modulates GVHD. However, little is known about the microbial metabolites crucial for achieving the effects of the microbiome on immune responses, specifically GVHD. We examined the metabolome of naïve mice and murine recipients of syngeneic (B6→B6) and allogeneic (B6→BALB/c) BMT, in an unbiased way. We made a surprising observation that short chain fatty acids (SCFA), including butyrate, were altered in the intestinal epithelium but not in the luminal contents of allo-BMT recipients. Butyrate, a known metabolite of the microbiome was previously shown to have HDAC inhibitor (HDACi) and salutary effects on the intestinal epithelium. Additionally, recent data showed systemic chemical inhibition of HDAC protects GI epithelium from GVHD severity. The decrease of butyrate in the epithelial cells of the small intestine (P&lt;0.03) suggested poor uptake of butyrate following allo-BMT. Consistent with this notion, we found that treatment of epithelial cells with butyrate confers protection from irradiation and decreases allo-induced cell death. Our results suggest that the microbiome regulates the damage from allogeneic GVHD due to the reduction in the tissue level of the SCFA-metabolite and HDACi, butyrate.

Neddylation regulates T cell responses. (TRAN3P.914)

Post-translational modifications (PTM) of proteins regulate cellular processes by proteasomal degradation. PTM of Cullin-RING ligase (CRL) protein via neddylation and the presence of its critical adapter element, sensitive to apoptosis gene (SAG) protein, modulate the process of neddylation-dependent ubiquitination. The role of neddylated proteins in regulating T cells is not known. We explored the role of neddylation in T cells by utilizing two different, but complementary approaches. We generated a T-cell specific knockout (SLCK-KO) of SAG (critical for the neddylation process) and utilized a small molecule inhibitor specific for neddylation (MLN4924). Phenotypic analysis of thymic and splenic T cells from SLCK-KO showed no differences when compared to WT. In vitro analysis of SLCK-KO T cells, with either allo-splenocytes or non-specific CD3/CD28 antibodies demonstrated reduced activation (CD69, CD44), proliferation, and secretion of T helper signature cytokines (IFNγ, IL-4, IL-17) (P&lt;0.003). In vivo studies utilizing multiple MHC disparate models of BMT (B6→BALB/c, B6→B6D2F1), demonstrated that SLCK-KO T cells in the recipient animals caused reduced severity of GVHD and increased survival (P&lt;0.01). Similar results were obtained in vitro and in vivo with small molecule, MLN4924. Our results demonstrate a novel biological role for neddylation in regulating T cell functions in vitro and in vivo.

The role of dendritic cells in graft-versus-tumor effect

Dendritic cells (DCs) are the most potent antigen presenting cells. DCs play a pivotal role in determining the character and magnitude of immune responses to tumors. Host and donor hematopoietic-derived DCs play a critical role in the development of graft-versus-host disease (GVHD) following allogeneic hematopoietic cell transplantation. GVHD is tightly linked with the graft-versus-tumor (GVT) effect. Although both host and donor DCs are important regulators of GVHD, the role of DCs in GVT is poorly understood. GVT is caused by donor T cells that attack recipient tumor cells. The donor T cells recognize alloantigens, and tumor specific antigens (TSAs) are mediating GVHD. The process of presentation of these antigens, especially TSAs remains unknown. Recent data suggested that DC may be essential role for inducing GVT. The mechanisms that DCs possess may include direct presentation, cross-presentation, cross-dressing. The role they play in GVT will be reviewed.

Allogeneic T cell responses are regulated by a specific miRNA-mRNA network

Donor T cells that respond to host alloantigens following allogeneic bone marrow transplantation (BMT) induce graft-versus-host (GVH) responses, but their molecular landscape is not well understood. MicroRNAs (miRNAs) regulate gene (mRNA) expression and fine-tune the molecular responses of T cells. We stimulated naive T cells with either allogeneic or nonspecific stimuli and used argonaute cross-linked immunoprecipitation (CLIP) with subsequent ChIP microarray analyses to profile miR responses and their direct mRNA targets. We identified a unique expression pattern of miRs and mRNAs following the allostimulation of T cells and a high correlation between the expression of the identified miRs and a reduction of their mRNA targets. miRs and mRNAs that were predicted to be differentially regulated in allogeneic T cells compared with nonspecifically stimulated T cells were validated in vitro. These analyses identified wings apart-like homolog (Wapal) and synaptojanin 1 (Synj1) as potential regulators of allogeneic T cell responses. The expression of these molecular targets in vivo was confirmed in MHC-mismatched experimental BMT. Targeted silencing of either Wapal or Synj1 prevented the development of GVH response, confirming a role for these regulators in allogeneic T cell responses. Thus, this genome-wide analysis of miRNA-mRNA interactions identifies previously unrecognized molecular regulators of T cell responses.

A critical role for neddylation in regulating dendritic cell function. (P1369)

Post-translational modifications (PTMs) of proteins modulate cellular processes. However, the role, specifically that of neddylation (a type of PTM), in regulating immune cells is not known. Neddylation of Cullin-RING ligase (CRL) protein modulates the process of ubiquitination. Thus, CRL activity is critical for the quotidian functions of cells. We explored the role of neddylation in dendritic cells (DCs) utilizing the small molecule inhibitor of neddylation (MLN4924) and the knockdown of SAG, a critical adapter element of CRL. Inhibition of neddylation in the DC significantly reduced the expression and release of several proinflammatory cytokines (TNFα, IL-6) and significantly reduced the ability of DCs to stimulate T cells (P&lt;0.001) following stimulation by several TLR and NLR ligands. This suppression of DC functions was more than that caused by the known anti-inflammatory drug dexamethasone. Mechanistic studies showed that reduction of IκB degradation was critical for the suppression of DCs by the inhibitors of neddylation. This was further confirmed by the absence of NF-κB translocation and function. However, MAPK/ERK pathway was unperturbed by the inhibition of neddylation. Our results demonstrate a novel biological role of inhibition of the PTM, neddylation, in regulating DC functions.

PU.1-dependent transcriptional regulation of miR-142 contributes to its hematopoietic cell-specific expression and modulation of IL-6

MicroRNAs (miRs) have emerged as critical modulators of immune responses, but little is known about their transcriptional regulation and tissue specificity. miR-142 is specifically expressed in hematopoietic tissues and plays an important role in regulating immunity. In this study we identified the key transcriptional elements for regulation of miR-142 and its impact on TLR4-mediated expression of IL-6. The PU.1, C/EBPβ, and Runx1 transcription factor binding sites are conserved and constitutively occupied by the respective transcription factors in the miR-142 gene promoter only in the hematopoietic cells. Specific knockdown experiments in hematopoietic cells and rescue experiments in nonhematopoietic cells show that PU.1 is critical for miR-142 gene expression and that it synergizes with Runx1, C/EBPβ, and CBFβ. Furthermore, TLR4 stimulation enhanced miR-155 whereas experiments with knockdown and mimic expression of miR-155 demonstrated that miR-155 negatively regulates miR-142-3p expression by targeting PU.1. Thus, TLR4 stimulation represses PU.1, resulting in downregulation of miR-142 and increased expression of IL-6. These results collectively reveal the direct cis-acting sequences of miR-142 specific promoter and that transcription factor PU.1 is necessary for its exclusive expression in hematopoietic cells and regulation of IL-6.

Influence of donor microbiota on the severity of experimental graft-versus-host-disease

The link between microbial flora and the shaping of immune responses is being increasingly appreciated, and recent data have uncovered a role for recipient microbiota in the severity of graft-versus-host disease (GVHD). The impact of donor microbiota on T cell-mediated alloresponses and GVHD is not known, however. Using multiple clinically relevant murine models, we analyzed the effect of donor microbiota on the severity of GVHD induced by T cells from specific pathogen-free and germ-free donors, and found that donor microbiota does not alter the expansion or differentiation of alloreactive T cells or the severity of GVHD.

Donor- but not host-derived interleukin-10 contributes to the regulation of experimental graft-versus-host disease

IL-10 is a key immune-regulatory cytokine, and its gene polymorphisms correlate with severity of clinical GVHD. IL-10 is made by a variety of donor and host cells, but the functional relevance of its source and its role in the biology of acute GVHD are not well understood. We used preclinical models to examine the relevance of IL-10(-/-) in donor and host cellular subsets on the severity of GVHD. IL-10(-/-) in host tissues or in the donor grafts did not alter donor Teff-mediated severity of GVHD. Furthermore, neither host-derived nor donor Teff-derived IL-10 was required for regulation of GVHD by WT CD4(+)CD25(+) donor Tregs. By contrast, Treg-derived IL-10, although not obligatory, was necessary for optimal reduction of GVHD by mature donor Tregs. Importantly, IL-10 from donor BM grafts was also critical for optimal donor Treg-mediated suppression of GVHD. Together, these data suggest that IL-10 does not contribute to the induction of GVHD severity by the Teffs. However, donor BM graft and Treg-derived IL-10 are important for donor Treg-mediated suppression of GVHD.