Graft-versus-host Disease Target Tissues Research Articles

Separation of GVL from GVHD -location, location, location.

Allogeneic hematopoietic cell transplantation (HCT) is a curative therapy for various hematologic malignancies. However, alloimmune response is a double-edged sword that mediates both beneficial graft-versus-leukemia (GVL) effects and harmful graft-versus-host disease (GVHD). Separation of GVL effects from GVHD has been a topic of intense research to improve transplant outcomes, but reliable clinical strategies have not yet been established. Target tissues of acute GVHD are the skin, liver, and intestine, while leukemic stem cells reside in the bone marrow. Tissue specific effector T-cell migration is determined by a combination of inflammatory and chemotactic signals that interact with specific receptors on T cells. Specific inhibition of donor T cell migration to GVHD target tissues while preserving migration to the bone marrow may represent a novel strategy to separate GVL from GVHD. Furthermore, tissue specific GVHD therapy, promoting tissue tolerance, and targeting of the tumor immune microenvironment may also help to separate GVHD and GVL.

Blockade of PD-L1/CD80 Interactions Augments Function of Activated Donor T Cells Results in Augmenting Gvhd Induced By Donor Naïve CD8 + t Cells and GVL Effect Mediated By Donor Memory CD8 + T Cells

Preventing graft-versus-host disease (GVHD) while preserving graft-versus-leukemia/lymphoma (GVL) activity remains an elusive goal for allogeneic hematopoietic cell transplantation (HCT). It was reported by others that while donor naïve CD8 + T cells mediate both GVHD and GVL activity, donor memory CD8 + T cells could mediate GVL activity without GVHD. We recently reported that in vivo specific blockade of PD-L1/CD80 interactions augment tumor immunity mediated by memory CD8 + T cells (Zhang et al: PNAS 2023). In the current studies, we evaluated the impact of blockade of PD-L1/CD80 on GVHD induced by donor naïve CD8 + T cells and GVL activity-mediated by donor memory CD8 + T cells. Sorted naïve CD8 + T or memory CD8 + T cells and TCD-BM cells from C57BL/6 donors were transplanted into lethal TBI-conditioned BALB/c recipients with or without bearing ALL cancer cells. 4 days after HCT, the recipients were injected I.P. with anti-PD-L1 mAb (43H12) that specifically blocks PD-L1/CD80 interactions without inferring PD-L1/PD-1 interactions or control IgG. We observed that naïve but not memory CD8 + T cells severely infiltrated GVHD target tissues, and the memory CD8 + T cells expanded mainly in the lymphoid tissues. In vivo blockade of PD-L1/CD80 interactions augmented GVHD induced by the naïve T cells and GVL activity mediated by the memory CD8 + T cells. In addition, blockade of PD-L1/CD80 interactions increased production of granzyme B and inflammatory cytokines (IFN-g and TNF-α) of activated donor CD8 + T cells. Since cytolytic activity of effector CD8 + T cell is reported to be associated with its metabolic fitness, we evaluated the effect of blockade PD-L1/CD80 interactions on metabolism of alloreactive donor CD8 + T cells. And blockade of the interactions enhanced the fitness of mitochondria, as indicated by electronic microscopy analysis. The blockade also increased ECAR, OCR, and ATP production of the activated CD8 + T cells, with increased influx of glucose and fatty acids; and the blockade augmented mitophagy of the activated CD8 + T cells, as indicated by reducing mitochondria mass and dysfunctional mitochondria in the CD8 + T cells. The results indicate that 1) PD-L1/CD80 interactions play an important role in regulating the mitochondria function of activated alloreactive CD8 + T cells; 2) Blockade of PD-L1/CD80 interactions can augment GVL activity mediated by donor memory T cells without augmenting GVHD. This is different from blockade of PD-L1/PD-1 interactions that resulted in lethal GVHD. *Zhang and Song are co-first authors of this abstract

Tissue-infiltrating alloreactive T cells require Id3 to deflect PD-1–mediated immune suppression during GVHD

AbstractPersisting alloreactive donor T cells in target tissues are a determinant of graft-versus-host disease (GVHD), but the transcriptional regulators that control the persistence and function of tissue-infiltrating T cells remain elusive. We demonstrate here that Id3, a DNA-binding inhibitor, is critical for sustaining T-cell responses in GVHD target tissues in mice, including the liver and intestine. Id3 loss results in aberrantly expressed PD-1 in polyfunctional T helper 1 (Th1) cells, decreased tissue-infiltrating PD-1+ polyfunctional Th1 cell numbers, impaired maintenance of liver TCF-1+ progenitor-like T cells, and inhibition of GVHD. PD-1 blockade restores the capacity of Id3-ablated donor T cells to mediate GVHD. Single-cell RNA-sequencing analysis revealed that Id3 loss leads to significantly decreased CD28- and PI3K/AKT-signaling activity in tissue-infiltrating polyfunctional Th1 cells, an indicator of active PD-1/PD-L1 effects. Id3 is also required for protecting CD8+ T cells from the PD-1 pathway–mediated suppression during GVHD. Genome-wide RNA-sequencing analysis reveals that Id3 represses transcription factors (e.g., Nfatc2, Fos, Jun, Ets1, and Prdm1) that are critical for PD-1 transcription, exuberant effector differentiation, and interferon responses and dysfunction of activated T cells. Id3 achieves these effects by restraining the chromatin accessibility for these transcription factors. Id3 ablation in donor T cells preserved their graft vs tumor effects in mice undergoing allogeneic hematopoietic stem cell transplantation. Furthermore, CRISPR/Cas9 knockout of ID3 in human CD19–directed chimeric antigen receptor T cells retained their antitumor activity in NOD/SCID/IL2Rg−/− mice early after administration. These findings identify that ID3 is an important target to reduce GVHD, and the gene-editing program of ID3 may have broad implications in T-cell–based immunotherapy.

Donor T cell STAT3 deficiency enables tissue PD-L1-dependent prevention of graft-versus-host disease while preserving graft-versus-leukemia activity.

STAT3 deficiency (STAT3-/-) in donor T cells prevents graft-versus-host disease (GVHD), but the impact on graft-versus-leukemia (GVL) activity and mechanisms of GVHD prevention remains unclear. Here, using murine models of GVHD, we show that STAT3-/- donor T cells induced only mild reversible acute GVHD while preserving GVL effects against nonsusceptible acute lymphoblastic leukemia (ALL) cells in a donor T cell dose-dependent manner. GVHD prevention depended on programmed death ligand 1/programmed cell death protein 1 (PD-L1/PD-1) signaling. In GVHD target tissues, STAT3 deficiency amplified PD-L1/PD-1 inhibition of glutathione (GSH)/Myc pathways that regulate metabolic reprogramming in activated T cells, with decreased glycolytic and mitochondrial ATP production and increased mitochondrial ROS production and dysfunction, leading to tissue-specific deletion of host-reactive T cells and prevention of GVHD. Mitochondrial STAT3 deficiency alone did not reduce GSH expression or prevent GVHD. In lymphoid tissues, the lack of host-tissue PD-L1 interaction with PD-1 reduced the inhibition of the GSH/Myc pathway despite reduced GSH production caused by STAT3 deficiency and allowed donor T cell functions that mediate GVL activity. Therefore, STAT3 deficiency in donor T cells augments PD-1 signaling-mediated inhibition of GSH/Myc pathways and augments dysfunction of T cells in GVHD target tissues while sparing T cells in lymphoid tissues, leading to prevention of GVHD while preserving GVL effects.

IFN-γR/STAT1 signaling in recipient hematopoietic antigen-presenting cells suppresses graft-versus-host disease.

The absence of IFN-γ receptor (IFN-γR) or STAT1 signaling in donor cells has been shown to result in reduced induction of acute graft-versus-host disease (GVHD). In this study, we unexpectedly observed increased activation and expansion of donor lymphocytes in both lymphohematopoietic organs and GVHD target tissues of IFN-γR/STAT1-deficient recipient mice, leading to rapid mortality following the induction of GVHD. LPS-matured, BM-derived Ifngr1-/- Stat1-/- DCs (BMDCs) were more potent allogeneic stimulators and expressed increased levels of MHC II and costimulatory molecules. Similar effects were observed in human antigen-presenting cells (APCs) with knockdown of Stat1 by CRISPR/Cas9 and treatment with a JAK1/2 inhibitor. Furthermore, we demonstrated that the absence of IFN-γR/STAT1 signaling in hematopoietic APCs impaired the presentation of exogenous antigens, while promoting the presentation of endogenous antigens. Thus, the indirect presentation of host antigens to donor lymphocytes was defective in IFN-γR/STAT1-deficient, donor-derived APCs in fully donor chimeric mice. The differential effects of IFN-γR/STAT1 signaling on endogenous and exogenous antigen presentation could provide further insight into the roles of the IFN-γ/STAT1 signaling pathway in the pathogenesis of GVHD, organ rejection, and autoimmune diseases.

Associations of Minor Histocompatibility Antigens with Clinical Outcomes Following Allogeneic Hematopoietic Cell Transplantation

Introduction Donor T-cell recognition of tumor-presented minor histocompatibility antigens (mHAs) is thought to be a central mediator of the therapeutic graft versus leukemia (GvL) effect following allogeneic hematopoietic cell transplantation (alloHCT). In contrast, graft versus host disease (GvHD) develops when donor T-cells react to mHAs presented by normal recipient cells. While the role of mHAs in outcomes following alloHCT is recognized, it has not been well-characterized. It remains unclear whether GvL and GvHD are mediated by the cumulative effect of total mHAs in a donor-recipient pair (DRP) or individual immunodominant mHAs. We hypothesized that an increased mHA count and individual mHAs would be associated with lower relapse and disease-related mortality (DRM) and/or increased GvHD mortality. To this end, we predicted genome-wide mHA peptides in a large patient population and analyzed the associations between clinical outcomes and 1) the total number of predicted class I and class II mHAs, and (2) individual and groups of mHAs. Methods The study population consisted of 2249 European American DRPs in the DISCOVeRY-BMT dataset treated for Acute Myeloid Leukemia (AML) and Myelodysplastic Syndrome (MDS) with alloHCT. To predict mHAs in a DRP, we translated nonsynonymous SNP alleles present in alloHCT recipients, but absent in their donor, into all possible peptides of varying lengths that would encompass the altered amino acid. To identify mHAs most likely to be immunogenic, peptides were filtered based on (1) source protein gene expression data in GvL and GvHD target tissues, including AML, liver, skin, and colon, from UCSC's Xena Dataset repository and (2) recipient HLA binding affinity from netMHCpan. To test the association of mHA count and individual mHAs with all-cause mortality (OS) and leukemia-free survival (LFS) we used cox proportional hazard models and competing risk models for DRM, treatment-related mortality (TRM), and GvHD mortality. Individual mHA analyses were stratified by HLA type to recognize that T-cell responses depend on HLA-mHA complexes. Models were adjusted for clinical covariates. Bonferroni correction for multiple testing was based on the number of independent SNPs. Results Patients with a class I mHA count greater than the population median had a 39% (HR=1.39, 95%CI 1.005,1.772, P=0.046) increase in hazard of GvHD mortality compared to those with a count less than the median. Patients with a class II mHA count greater than the population median had a decreased overall survival compared to those with a count less than the median (Fig. 1, P=0.029). In recipients with HLA-B*08:01, the class I mHA DLRCKYISL was associated with increased risk of GVHD mortality (HR=2.84, 95%CI 1.52,5.31, P=0.01). Within HLA-B*40:01, WEHGPTSLL was associated with decreased LFS (HR=1.94, 95%CI 1.27,2.95, P=0.044). Within HLA-C*03:04, STSPTTNVL was associated with increased risk of DRM (HR=2.32, 95%CI 1.5,3.6, P=0.008). In recipients with HLA-DRB1*15:01, the class II mHA YQEIAAIPSAGRERQ was associated with increased risk of TRM (HR=3.05, 95%CI 1.75,5.31, P=0.02). Two mHAs, WEHGPTSLL and STSPTTNVL, were found within the HLA haplotype B*40:01-C*03:04. Recipients with both mHAs (versus neither) had a greater hazard of 1-year all-cause mortality (HR=3.58, P=7.9×10-5) and DRM (HR=3.65, P=0.003) (Fig. 2). Patients with either mHA (versus neither) had greater 1-year all-cause mortality (HR=1.69, P=0.008) and increased DRM (HR=2.05, P=0.008) Conclusions The number of class I mHAs in a DRP was associated with increased GvHD mortality, which is consistent with our hypothesis. We also found two mHAs correlated with GvHD mortality and DRM which may represent increased GvH effects driven by these peptides. However, contrary to our hypothesis, an increased number of class II mHAs was correlated with greater risk of all-cause mortality that may be due to decreased LFS. Similarly, we found two mHAs correlated with increased DRM and decreased LFS. These two mHAs are found in an HLA haplotype where they exhibit a dose effect relationship with the risk of DRM and decreased OS. These findings may reflect an unexpectedly attenuated GvL effect that requires further investigation. Our study reports the first large scale investigation of the role of predicted mHA peptides in clinical outcomes following alloHCT. If validated, these results may be used to help guide clinical decision making. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Trafficking between clonally related peripheral T-helper cells and tissue-resident T-helper cells in chronic GVHD

AbstractChronic graft-versus-host disease (cGVHD) is an autoimmune-like syndrome. CXCR5-PD-1hi peripheral T-helper (Tph) cells have an important pathogenic role in autoimmune diseases, but the role of Tph cells in cGVHD remains unknown. We show that in patients with cGVHD, expansion of Tph cells among blood CD4+ T cells was associated with cGVHD severity. These cells augmented memory B-cell differentiation and production of immunoglobulin G via interleukin 21 (IL-21). Tph cell expansion was also observed in a murine model of cGVHD. This Tph cell expansion in the blood is associated with the expansion of pathogenic tissue-resident T-helper (Trh) cells that form lymphoid aggregates surrounded by collagen in graft-versus-host disease (GVHD) target tissues. Adoptive transfer experiments showed that Trh cells from GVHD target tissues give rise to Tph cells in the blood, and conversely, Tph cells from the blood give rise to Trh cells in GVHD target tissues. Tph cells in the blood and Trh cells in GVHD target tissues had highly overlapping T-cell receptor α and β repertoires. Deficiency of IL-21R, B-cell lymphoma 6 (BCL6), or T-bet in donor T cells markedly reduced the proportions of Tph cells in the blood and Trh cells in GVHD target tissues and reduced T-B interaction in the lymphoid aggregates. These results indicate that clonally related pathogenic Tph cells and Trh cells traffic between the blood and cGVHD target tissues, and that IL-21R-BCL6 signaling and T-bet are required for the development and expansion of Tph and Trh cells in the pathogenesis of cGVHD.

IL-33 acts as a costimulatory signal to generate alloreactive Th1 cells in graft-versus-host disease.

Antigen-presenting cells (APCs) integrate signals emanating from local pathology and program appropriate T cell responses. In allogeneic hematopoietic stem cell transplantation (alloHCT), recipient conditioning releases damage-associated molecular patterns (DAMPs) that generate proinflammatory APCs that secrete IL-12, which is a driver of donor Th1 responses, causing graft-versus-host disease (GVHD). Nevertheless, other mechanisms exist to initiate alloreactive T cell responses, as recipients with disrupted DAMP signaling or lacking IL-12 develop GVHD. We established that tissue damage signals are perceived directly by donor CD4+ T cells and promoted T cell expansion and differentiation. Specifically, the fibroblastic reticular cell–derived DAMP IL-33 is increased by recipient conditioning and is critical for the initial activation, proliferation, and differentiation of alloreactive Th1 cells. IL-33 stimulation of CD4+ T cells was not required for lymphopenia-induced expansion, however. IL-33 promoted IL-12–independent expression of Tbet and generation of Th1 cells that infiltrated GVHD target tissues. Mechanistically, IL-33 augmented CD4+ T cell TCR-associated signaling pathways in response to alloantigen. This enhanced T cell expansion and Th1 polarization, but inhibited the expression of regulatory molecules such as IL-10 and Foxp3. These data establish an unappreciated role for IL-33 as a costimulatory signal for donor Th1 generation after alloHCT.

Retention of Donor T Cells in Lymphohematopoietic Tissue and Augmentation of Tissue PD-L1 Protection for Prevention of GVHD While Preserving GVL Activity.

Allogeneic hematopoietic cell transplantation (Allo-HCT) is a curative therapy for hematological malignancies (i.e., leukemia and lymphoma) due to the graft-versus-leukemia (GVL) activity mediated by alloreactive T cells that can eliminate residual malignant cells and prevent relapse. However, the same alloreactive T cells can cause a serious side effect, known as graft-versus-host disease (GVHD). GVHD and GVL occur in distinct organ and tissues, with GVHD occurring in target organs (e.g., the gut, liver, lung, skin, etc.) and GVL in lympho-hematopoietic tissues where hematological cancer cells primarily reside. Currently used immunosuppressive drugs for the treatment of GVHD inhibit donor T cell activation and expansion, resulting in a decrease in both GVHD and GVL activity that is associated with cancer relapse. To prevent GVHD, it is important to allow full activation and expansion of alloreactive T cells in the lympho-hematopoietic tissues, as well as prevent donor T cells from migrating into the GVHD target tissues, and tolerize infiltrating T cells via protective mechanisms, such as PD-L1 interacting with PD-1, in the target tissues. In this review, we will summarize major approaches that prevent donor T cell migration into GVHD target tissues and approaches that augment tolerization of the infiltrating T cells in the GVHD target tissues while preserving strong GVL activity in the lympho-hematopoietic tissues.

Umbilical cord-derived mesenchymal stem cells promote myeloid-derived suppressor cell enrichment by secreting CXCL1 to prevent graft-versus-host disease after hematopoietic stem cell transplantation

Background aimsHuman mesenchymal stem cells (MSCs) from various tissues have emerged as attractive candidates for the prevention and treatment of graft-versus-host disease (GVHD). However, the molecular machinery that defines and channels the behavior of these cells remains poorly understood. MethodsIn this study, the authors compared the efficacy of four tissue-derived MSC types in controlling GVHD in a murine model and investigated their immunomodulatory effects. ResultsHuman umbilical cord-derived mesenchymal stem cells (hUCMSCs) effectively decreased the incidence and severity of GVHD, which was mediated by the enrichment of myeloid-derived suppressor cells in GVHD target tissues. RNA sequencing results showed that hUCMSCs highly expressed CXCL1. ConclusionsThese results suggest a novel prophylactic application of hUCMSCs for controlling GVHD after allogeneic hematopoietic stem cell transplantation.

Tolerogenic anti–IL-2 mAb prevents graft-versus-host disease while preserving strong graft-versus-leukemia activity

AbstractDonor T cells mediate both graft-versus-leukemia (GVL) activity and graft-versus-host disease (GVHD) after allogeneic hematopoietic cell transplantation (allo-HCT). Development of methods that preserve GVL activity while preventing GVHD remains a long-sought goal. Tolerogenic anti–interleukin-2 (IL-2) monoclonal antibody (JES6-1) forms anti–IL-2/IL-2 complexes that block IL-2 binding to IL-2Rβ and IL-2Rγ on conventional T cells that have low expression of IL-2Rα. Here, we show that administration of JES6 early after allo-HCT in mice markedly attenuates acute GVHD while preserving GVL activity that is dramatically stronger than observed with tacrolimus (TAC) treatment. The anti–IL-2 treatment downregulated activation of the IL-2-Stat5 pathway and reduced production of granulocyte-macrophage colony-stimulating factor (GM-CSF). In GVHD target tissues, enhanced T-cell programmed cell death protein 1 (PD-1) interaction with tissue–programmed cell death-ligand 1 (PD-L1) led to reduced activation of protein kinase–mammalian target of rapamycin pathway and increased expression of eomesodermin and B-lymphocyte-induced maturation protein-1, increased T-cell anergy/exhaustion, expansion of Foxp3–IL-10–producing type 1 regulatory (Tr1) cells, and depletion of GM-CSF–producing T helper type 1 (Th1)/cytotoxic T cell type 1 (Tc1) cells. In recipient lymphoid tissues, lack of donor T-cell PD-1 interaction with tissue PD-L1 preserved donor PD-1+TCF-1+Ly108+CD8+ T memory progenitors and functional effectors that have strong GVL activity. Anti–IL-2 and TAC treatments have qualitatively distinct effects on donor T cells in the lymphoid tissues, and CD8+ T memory progenitor cells are enriched with anti–IL-2 treatment compared with TAC treatment. We conclude that administration of tolerogenic anti–IL-2 monoclonal antibody early after allo-HCT represents a novel approach for preventing acute GVHD while preserving GVL activity.

From Graft-Versus-Host Disease to Lymphoma Pathogenesis: Emerging Roles for Stromal Notch Ligands in Hematology

Notch signaling is a highly conserved signaling pathway with multiple functions in health and disease. Notch ligands of the Delta-like and Jagged families interact with Notch1-4 on adjacent cells to trigger proteolytic activation of the Notch receptors, followed by context-dependent transcriptional activation of Notch target genes. In the hematopoietic system, Notch was first identified for its oncogenic role in T cell acute lymphoblastic leukemia (T-ALL). More recently, Notch has been recognized as a recurrent oncogenic pathway in multiple B cell lymphoproliferative disorders, including CLL/SLL, mantle cell lymphoma and marginal zone lymphoma. In parallel, essential functions of Notch signaling have been identified in early T cell development and B cell homeostasis, as well as in regulating T and B cell immune responses in secondary lymphoid organs. We have discovered a conserved pathogenic function of Notch signaling in the control of graft-versus-host disease (GVHD), both in mouse and non-human primate models of hematopoietic cell transplantation. Within days after allogeneic hematopoietic cell transplantation, donor-derived T cells interact with Delta-like Notch ligands expressed by specialized subsets of non-hematopoietic fibroblastic stromal cells in spleen and lymph nodes. In turn, Notch signaling programs alloreactive T cells to become pathogenic effector cells that mediate long-term damage in GVHD target tissues, while suppressing the expansion of regulatory T cells. Emerging evidence suggests the existence of an active crosstalk between T cells, B cells and well-defined specialized immunological stromal niches expressing Notch ligands in secondary lymphoid organs, both in GVHD and in other types of immune responses. Moreover, many B cell lymphomas demonstrate oncogenic Notch activation as a result of gain-of-function mutations that increase the persistence of intracellular Notch after proteolytic cleavage, thus requiring the presence of Notch ligands in the microenvironment to trigger oncogenic Notch signals. New evidence suggests that key Notch ligands in human lymphomas are present in the lymph node microenvironment, and that oncogenic Notch activation can happen even in the absence of mutations in a substantial fraction of lymphoma patients, including the majority of patients with CLL/SLL. In these cases, lymphoma cells appear to interact with microenvironmental Notch ligands in secondary lymphoid organs through unmutated Notch receptors, borrowing from the playbook of normal lymphocytes. Furthermore, the expression of Notch ligands by defined subsets of fibroblastic stromal cells illustrates a new paradigm through which microenvironmental signals can profoundly influence the differentiation and function of adjacent lymphoid cells. We speculate that better understanding the regulation of Notch signaling and fibroblastic stromal cells in secondary lymphoid organs will provide valuable new information with translational potential both in immunobiology and in malignant hematology. Disclosures Maillard: Genentech: Consultancy; Regeneron: Consultancy.

The Innate Immune Sensor Sting Promotes Donor CD8+ T Cell Activation and Recipient APC Death Early after Preclinical Allogeneic Hematopoietic Stem Cell Transplantation

Graft-versus-host disease (GVHD) remains a significant cause of morbidity and mortality in patients receiving allogeneic hematopoietic stem cell transplants (aHSCTs). Pre-HSCT chemoradiation results in the death of dividing cells and release of endogenous danger signals. These molecules drive the activation of antigen presenting cells (APCs) and the differentiation of allo-reactive donor T cells. We identified a role for Stimulator of Interferon Genes (STING), an innate immune sensor, in GVHD using pre-clinical MHC-matched unrelated donor (MUD) HSCT models. Here we show that STING rapidly promotes donor CD8+ T cell activation and recipient APC death early after aHSCT. Post MUD HSCT, we previously found a >2x reduction in IFNβ, TNFα and IL-6 mRNA in STING−/− vs WT recipient colonic mRNA expression (48 hrs) as well as decreased weight loss, GVHD scores and skin pathology (6 wks) vs WT. Chimeric studies showed that STING loss in non-hematopoietic cells induced this effect. Conversely, we recently found a single early ( D100), dose of the STING agonist DMXAA increased GVHD scores and lethality in WT, but not STING−/−, recipients. Thus, the activation of this pathway can promote GVHD following MUD HSCT. Furthermore, mice homozygous for a murine homologue of a human allele associated with diminished STING activity (STINGHAQ/HAQ) also exhibited reduced GVHD after MUD HSCT, suggesting potential clinical importance. Interestingly, our findings that STING deficiency ameliorates GVHD in MUD HSCT (Bader CS, et al, in revision Sci. Transl. Med.) contrasts reported observations that STING deficiency can exacerbate GVHD after MHC-mismatched (MMUD) HSCT. Since CD4+ and CD8+ T cells are central in MMUD and MUD GVHD, respectively, we transplanted MMUD BALB/c BM + CD8+ T cells into B6-WT and STING−/− mice and notably, found that STING−/- recipients now developed reduced GVHD clinical scores, skin pathology and frequencies of activated T cells 8 wks post-HSCT vs WT. We also found that STING−/− mice had greater numbers of recipient splenic CD11b+CD11c+ APCs and these cells expressed reduced MHC I protein 1 day after MMUD B6 into BALB/c aHSCT vs WT (Fig. A, B). Moreover, STING−/− recipient spleens contained lower numbers of donor CD8+ T cells producing IFNγ and TNFα (Fig. C), supporting the hypothesis that STING contributes to early activation of donor CD8+ T cells and loss of recipient APCs. Next, to identify if reduced host MHC II+ APCs affected donor CD4+ T cell activation, B6-Nur77GFP T cells were used to explicitly monitor T cell receptor signaling. Indeed, STING−/− spleens had greater numbers of donor Nur77GFP CD4+ T cells expressing GFP, CD69 and IFNγ 6 days post-HSCT (Fig. D). Overall, these data provide a mechanism by which STING could promote CD8+ T cell-mediated GVHD yet diminish CD4+-mediated GVHD. Ongoing studies are exploring the use of STING agonists to augment GVL following aHSCT.

The Innate Immune Sensor Sting Promotes CD8+ T Cell-Mediated Gvhd after Preclinical Allogeneic Hematopoietic Stem Cell Transplantation

Graft-versus-host disease (GVHD) remains a significant cause of morbidity and mortality in patients receiving allogeneic hematopoietic stem cell transplants (aHSCTs). Pre-HSCT conditioning typically consists of irradiation and drug administration resulting in the death of rapidly dividing cells. Damage to host tissues initiates a cytokine storm, promoting activation and expansion of donor anti-host alloreactive T cells. Cell death following conditioning has promoted the hypothesis that sensors of cytoplasmic DNA damage in GVHD target tissues contribute to cytokine production. We identified a role for Stimulator of Interferon Genes (STING), an innate immune sensor, in GVHD using pre-clinical MHC-matched (MUD) aHSCT models. Here, we show that the STING pathway promotes CD8+ T cell-mediated GVHD after MHC matched and MHC-mismatched aHSCT. To assess a role for STING after MHC-matched aHSCT, we transplanted C3H.SW BM + unfractionated T cells into irradiated B6-WT or STING−/− recipients. STING−/− recipients developed reduced weight loss, GVHD scores and skin pathology 6 wks post-HSCT. Chimeric studies demonstrated that the absence of STING in non-hematopoietic cells was responsible for the amelioration of GVHD. Therefore, to test STING signaling in non-hematopoietic intestinal cells, we generated intestinal organoid cultures which upregulated IFNβ, TNFα, IL-6, CXCL10 and MHC class I mRNA 6 hrs after stimulation with the highly specific STING agonist DMXAA, supporting the notion that STING can contribute to GI inflammation in vivo (Fig. 1A). To evaluate the potential impact of STING in the clinical setting, we examined MHC-matched recipients homozygous for a human allele associated with diminished STING activity and found STINGHAQ/HAQ mice exhibited diminished GVHD vs WT. Since MHC-matched GVHD is primarily mediated by CD8+ T cells, we next asked if CD4+ T cells were dispensable for the reduction of GVHD in STING−/− recipients. Indeed, GVHD was reduced in STING−/− recipients after transplant of C3H.SW BM and CD8+ T cells (Fig. 1B,C). Notably, STING deficiency was recently reported to worsen GVHD after MHC-mismatched aHSCT (Fischer J, et al, Sci. Transl. Med. 2017). Since CD4+ T cells are dominant during MHC-mismatched GVHD, we transplanted BALB/c BM + CD8+ T cells into B6-WT and STING−/− mice. Again, STING−/- recipients developed lower GVHD clinical scores, reduced skin pathology and had lower frequencies of activated T cells 8 wks post-HSCT vs WT, supporting a role for STING in the promotion of CD8+ T cell-mediated GVHD – consistent with this pathways upregulation of MHC class I (Fig. 1D,E). In total, these studies demonstrate that STING plays an important role in regulating aHSCT and promotes CD8+ T cell-mediated GVHD regardless of the genetic disparity between donor and recipient. The findings support the notion that this pathway may provide a target for new strategies to ameliorate GVHD.

Stat3 Regulates Alloreactive T Cell Susceptibility Towards PD-L1-Mediated Tolerance Signaling, Gvhd and GVL Activity

Prevention of graft versus host disease (GVHD) while preserving graft-versus-leukemia/lymphoma (GVL) activity of alloreactive donor T cells remains an elusive and long-sought goal. We and others reported that Stat3 deficiency in donor T cells results in expansion of Foxp3+ Treg cells and reduction of pathogenic Th17 cells, leading to prevention of chronic GVHD. We have now tested whether Stat3 deficiency in donor T cells can prevent acute GVHD while preserving GVL activity.We transplanted wild-type (WT) or Stat3-/- T cells from C57BL/6 (H-2b) donors into irradiated WT BALB/c recipients bearing B cell lymphoma line BCL1 or aggressive acute lymphocytic leukemia (ALL) cells. WT donor T cells caused acute GVHD, as expected. Stat3-/- donor T cells did not cause acute GVHD but still had strong GVL activity. This separation of GVL activity from GVHD was associated with augmented expansion and impaired tolerance by Stat3-/- T cells in lymphoid tissues such as the spleen. In contrast, tolerance by Stat3-/- T cells was enhanced in GVHD target tissues by different mechanisms. The yield of Stat3-/- T cells in gut tissues was lower than the yield of WT T cells due to the markedly higher apoptosis of Stat3-/- T cells compared to WT T cells. The yield of Stat3-/- T cells in the lung and liver was not lower than the yield of WT T cells, but expression of anergy/exhaustion markers such as PD-1 and KLRG1 was markedly higher in Stat3-/- T cells than in WT T cells.We recently reported that GVHD tissue expression of PD-L1 can tolerize donor CD8+ T cells in the absence of donor CD4+ T cells, but not when CD4+ and CD8+ T cells were given together. In contrast, PD-L1 expression in recipient GVHD target tissues tolerized Stat3-/- donor T cells when both CD4+ T cells and CD8+ T cells were given together. The induction of tolerance in Stat3-/- T cells by GVHD target tissue expression of PD-L1 depended on PD-L1/PD-1 interaction, which led to marked reduction of glycolysis in Stat3-/- T cells but not in WT T cells. These observations fit with previous findings that enhanced glycolysis was associated with enhanced GVHD activity of alloreactive T cells, as reported by Xue-zhong Yu's group. Our results indicate that Stat3 deficiency in donor T cells can augment their susceptibility to tissue PD-L1-mediated tolerance mechanisms such as down-regulation of glycolysis. Current experiments are testing whether Stat3 knockdown in mature T cells in the graft can prevent GVHD while preserving GVL activity. (Grant support: R01AI066008 to Zeng) DisclosuresNo relevant conflicts of interest to declare.

A Bhlhe40/GM-CSF Axis Potentiates Gastrointestinal Tract Inflammation during Acute Graft Versus Host Disease

Damage to the gastrointestinal tract is the major cause of morbidity during acute graft-versus-host disease (GVHD). While T cells are the proximate drivers of GVHD, disease induction and amplification rely on crosstalk between the innate and adaptive arms of the immune system. The cellular and cytokine networks which mediate this interplay, however, are not well understood. We previously identified a colitogenic CD4+ IL-23R+ CD11c+ T cell population that possesses an innate-like gene expression signature, indicating that these cells appear to be positioned at the interface of the innate and adaptive arms of the immune system. Notably, we also observed that these cells had increased expression of bhlhe40 which is a transcription factor that has been shown to regulate the production of GM-CSF. Given the well characterized ability of GM-CSF to activate myeloid cell populations in autoimmunity, we sought to define the role of this transcription factor and cytokine as a potential bridge between innate and adaptive immunity in GVHD. Using a well-defined murine GVHD model [C57BL/6 (H-2b)→Balb/c (H-2d)], we observed that mice transplanted with Rag-1-/- bone marrow (BM) and CD4+ bhlhe40-/- T cells were completely protected from GVHD, whereas animals transplanted with Rag-1-/- BM and wild type (WT) CD4+ T cells uniformly developed lethal disease. Further analysis revealed that CD4+ bhlhe40-/- T cells produced less GM-CSF and more IL-10 than their WT counterparts, and had preferentially less pathological damage in the colon. To examine the specific role of GM-CSF, we employed the same GVHD model along with a corresponding syngeneic control (B6→B6.PL). We observed robust GM-CSF production in allogeneic, but not syngeneic, recipients in all GVHD target tissues, but most prominently in the colon. This was largely attributable to donor-derived CD4+ T cells, as there was little GM-CSF produced by CD8+ T cells. Notably, whereas the vast majority (~80%) of these cells in the lung and liver also produced IFN-γ, ~50% of GM-CSF-expressing CD4+ T cells in the colon only produced GM-CSF, suggesting that these cells might represent a separate CD4+ T cell lineage. In that regard, antibody blockade of IL-6, IL-23 and IL-27 had no effect on the frequency of CD4+ GM-CSF+ T cells, indicating that the development of these cells was not regulated by cytokines affecting TH1 and TH17 differentiation. To define the functional significance of donor T cell-derived GM-CSF, recipients were transplanted with BM or BM plus splenocytes from WT or GM-CSF-/- animals. Recipients of GM-CSF-/- grafts had significantly increased survival when compared to WT controls. Furthermore, histological analysis demonstrated a significant reduction in pathology in the colon of animals that received GM-CSF-/- grafts, as well as a decrease in infiltrating TH1 cells, whereas there was no difference in pathological damage in the lung or liver. A similar outcome was observed in complementary experiments in which recipient animals that were treated with an anti-GM-CSF antibody had significantly increased survival compared to mice treated with an isotype control antibody. To confirm a role for GM-CSF signaling in CD4+ T cells, Balb/c recipients were transplanted with Rag-1-/- BM alone or together with purified CD4+ T cells from WT or GM-CSF-/- mice. Mice that received CD4+ GM-CSF-/- T cells had a significant increase in survival compared to those that received WT CD4+ T cells, confirming a proinflammatory role for GM-CSF production by donor CD4+ T cells. Given that GM-CSF acts on a diverse subset of innate immune cells, we then examined which myeloid cell subsets were responsive to GM-CSF two weeks post-transplantation when donor APCs have repopulated the APC compartment. Using established markers for macrophages, neutrophils, and dendritic cells, we observed no difference in the number of donor macrophages or neutrophils between groups. However, there was a significant reduction in dendritic cells (DCs) in the colon of mice receiving CD4+ GM-CSF-/- T cells, and donor-derived DCs were virtually absent from the mesenteric lymph nodes, indicating that GM-CSF facilitates the accumulation of DCs in the GI tract and associated lymphoid tissue during GVHD. Collectively, these studies demonstrate that a CD4+ T cell-intrinsic bhlhe40/GM-CSF axis potentiates gastrointestinal inflammation during GVHD by promoting inflammatory cytokine production and DC recruitment. DisclosuresNo relevant conflicts of interest to declare.

The Innate Immune Sensor Sting Regulates Intestinal Inflammation and GVHD after Allogeneic Hematopoietic Stem Cell Transplantation in Knock-out and Human Allele Knock-in Recipient Mice

Graft-versus-host disease (GVHD) remains a significant cause of morbidity and mortality in patients receiving allogeneic hematopoietic stem cell transplants (allo-HSCTs). Pre-HSCT conditioning typically consists of irradiation and drug administration resulting in the death of rapidly dividing cells. Damage to host tissues initiates a cytokine storm, promoting activation and expansion of donor anti-host alloreactive T cells. Cell death following conditioning has promoted the hypothesis that sensors of cytoplasmic DNA damage in GVHD target tissues contribute to cytokine production. We identified a role for Stimulator of Interferon Genes (STING), an innate immune sensor, in GVHD using pre-clinical MHC-matched (MUD) allo-HSCT models. Our studies show that the STING pathway rapidly regulates cytokine production in the intestinal tract and non-hematopoietic cells can contribute to these responses. Using mice expressing a human STING allele associated with decreased STING activity (Patel S, et al, J Immunol. 2016), we demonstrate its potential clinical importance.To assess STING involvement immediately post-HSCT, cytokine mRNA expression was examined 48 hrs after transplant of C3H.SW bone marrow (BM) + T cells into irradiated B6-WT or STING-/- recipients. Colonic tissue from STING-/- recipients had >2x reduction in IFNβ, TNFα and IL-6 mRNA vs. WT. On day 10 post-transplant, colons from STING-/- recipients exhibited reduced inflammation and overall pathology scores than WT. MHC-matched STING-/- HSCT recipients also experienced decreased weight loss, GVHD scores and skin pathology 6 weeks post-HSCT vs. WT. Chimeric studies demonstrated that the absence of STING in non-hematopoietic cells was responsible for the amelioration of GVHD. Therefore, to test STING signaling in non-hematopoietic intestinal cells, we generated intestinal organoid cultures. Intestinal organoids upregulated IFNβ, TNFα, IL-6 and CXCL10 mRNA 6hrs after stimulation with the highly specific STING agonist DMXAA, supporting the notion that STING in intestinal tissues can contribute to inflammation in vivo. Interestingly, expression of these cytokines returned to baseline levels 24 hrs after stimulation (Fig. 1A). Next, we posited that if the absence of the STING pathway in recipients ameliorated GVHD after MHC-matched HSCT, pathway stimulation would exacerbate GVHD. B6-WT mice were injected with DMXAA immediately prior to HSCT with donor C3H.SW BM + T cells. Administration of a single dose of DMXAA increased expression of IFNβ, TNFα and IL-6 mRNA in colon tissue 48 hrs after transplant (Fig. 1B). Importantly, DMXAA treatment of WT - but not STING-/- - recipients significantly increased GVHD scores and lethality post-HSCT. To evaluate the potential impact of STING in the clinical setting, we evaluated recipients after transplant of C3H.SW BM + T cells into mice homozygous for a human allele associated with diminished STING activity (HAQ-MPYS knock-in mice, termed B6N-STINGHAQ/HAQ here) and found that STINGHAQ/HAQ mice contained a lower frequency of donor T cells expressing an activated phenotype (CD44hiCD62Llo) vs. WT recipients and the former also exhibited diminished GVHD (Fig. 1C,D). In contrast to STING knock-out recipients completely lacking protein, these results indicate that reduced STING activity can also affect GVHD.Interestingly, our findings that STING deficiency ameliorates GVHD in MHC-matched allo-HSCT contrast reported observations that STING activation can exacerbate GVHD after MHC-mismatched HSCT (Fischer J, et al, Sci. Transl. Med. 2017). We are currently investigating how the STING pathway regulates CD4+ and CD8+ T cell mediated GVHD and initial findings may provide insight into understanding the pathway's involvement in MHC-matched vs. mismatched allo-HSCT. In total, our studies demonstrate that STING plays an important role in regulating allo-HSCT and suggest this pathway can provide a target for new therapeutic strategies to ameliorate GVHD. DisclosuresLevy:Allergan: Consultancy; Capricor Therapeutics: Consultancy; HEAT Biologics: Consultancy, Membership on an entity's Board of Directors or advisory committees; Pelican Therapeutics: Consultancy; OccuRx: Research Funding; Shire: Research Funding.

Photoconversion of Alloreactive T Cells in Murine Peyer's Patches During Acute Graft-Versus-Host Disease: Tracking the Homing Route of Highly Proliferative Cells In Vivo.

The regulation of immune cell migration throughout the body is essential to warrant immunosurveillance and to maintain immune homeostasis. Marking and tracking of these cells has proven important to study mechanisms of immune cell trafficking and cell interaction in vivo. Photoconversion is a well-suited technique for intravital application because it enables contactless time- and location-specific marking of cells in the tissue without surgically manipulating the microenvironment of the cells in question. However, in dividing cells the converted fluorescent protein may decline quickly. Here, we provide a detailed description of the photoconversion technique and its applicability to tracking highly proliferating T cells from the priming site of T cell activation to peripheral target organs of effector function in a preclinical model. Dendra2+ T cells were photoconverted in the Peyer’s patches during the initiation phase of acute graft-versus-host disease (GvHD) and tracked through the mesenteric lymph nodes and the peripheral blood to the small intestine with flow cytometry and intravital two-photon microscopy. Photoconverted alloreactive T cells preserved the full proliferative capacity, homing, and migration of alloreactive T cells in the intestinal lamina propria. We conclusively proved that photoconversion of highly proliferative alloreactive T cells in the Peyer’s patches is an effective tool to study trafficking of alloreactive T cells under physiologic conditions and to GvHD target tissues. This technique can also be applied to the study of immune cell tracking under inflammatory and non-inflammatory conditions.

230 - The Innate Immune Sensor Sting Can Augment or Ameliorate Graft-Versus-Host Disease Dependent on the Genetic Disparity between Donors and Recipients

Abstract Graft-versus-host disease (GVHD) remains a significant cause of morbidity and mortality in patients receiving allogeneic hematopoietic stem cell transplants (allo-HSCTs). Pre-HSCT conditioning typically consists of irradiation and drug administration which results in the death of rapidly dividing cells and the initiation of a cytokine storm, promoting activation and expansion of donor anti-host alloreactive T cells in HSCT recipients. However, the precise mechanism of cytokine production remains unclear. Cell death following pre-transplant conditioning has promoted the hypothesis that sensors of cytoplasmic DNA damage in GVHD target tissues contribute to cytokine production. One such sensor is Stimulator of Interferon Genes (STING) which following activation induces phosphorylation of IRF3 and IκBα. Although STING activation was recently reported to worsen GVHD after MHC-mismatched allo-HSCT (Fischer J, et al, Sci. Transl. Med. 2017), STING involvement in MHC-matched allo-HSCT has not yet been thoroughly evaluated. Here, using B6-STING knock-out recipients and either MHC-mismatched or matched (“MUD”) donor strains we corroborate that STING deficiency worsens - but in contrast - ameliorates GVHD in the former and latter pre-clinical mouse models, respectively. To evaluate the effect of STING deficiency on GVHD after MHC-mismatched allo-HSCT, BALB/c BM + T cells were transplanted into 11.5Gy irradiated B6-WT and B6-STING-/- recipients. Consistent with previously reported results using B6-STINGgt/gt mice, STING-/- recipients experienced increased weight loss and GVHD clinical scores post-HSCT as well as decreased survival relative to WT recipients (Fig. 1A). Notably, when irradiation was given on day 0 instead of day -1 using the same donor/recipient combination no significant difference in GVHD was observed between WT and STING-/- recipients (Fig. 1B & 1E). We next investigated the role of STING after clinically relevant models of MHC-matched allo-HSCT. To assess for a role by STING immediately after HSCT, we examined cytokine mRNA expression 48 hrs. after transplant of LP/J BM + T cells → 8.5Gy irradiated B6-WT and B6-STING-/- recipients. Colonic tissue from STING-/- recipients had reproducibly >2x reduction in IFNβ, TNFα and IL-6 mRNA compared to WT (Fig. 1C). Importantly, IFNβ is one of the major downstream effector molecules produced as a result of IRF3 phosphorylation, consistent with the notion that STING activation occurs shortly after conditioning and/or allo-HSCT. Notably, LN examination at this time demonstrated fewer donor CD8+ T cells and CD8+CD44hiCD62Llo cells in STING-/- vs WT recipients. In contrast to STING-/- recipients of MHC-mismatched HSCT, MHC-matched STING-/- HSCT recipients experienced decreased weight loss and GVHD clinical scores vs. WT mice. STING-/- recipients contained 20-50% fewer donor lymphocytes and a significantly reduced frequency of activated donor CD4+ and CD8+ T cells in lymphoid tissues on D7. Analysis of peripheral LN and spleen 1-3 mo. post-HSCT revealed a similar profile and a higher frequency of naive T cells - consistent with their decreased clinical signs of disease. Histological examination of recipient skin also showed higher pathology scores in WT recipients relative to STING-/- recipients. These findings were corroborated using a second MUD model, i.e. C3H.SW → B6 (Fig. 1D). Furthermore, measurement of serum cytokines 6 wks. post-transplant showed that B6-STING-/- recipients had higher levels of the immunosuppressive cytokine IL-10. The effect of donor T cell dose was also examined. Increased donor T cell numbers resulted in enhanced GVHD clinical scores in STING-/- recipients, however, the GVHD remained less severe than that observed in WT. Notably, HSCT studies using fully reconstituted chimeric B6-CD45.1↔B6-STING-/- indicated that STING expression in non-hematopoietic tissues is important for the development of GVHD. In total, this is the first report we are aware of in which the same pathway appears to differentially impact the outcome of allogeneic HSCT based upon the genetic disparity across the transplant. These findings reveal that STING's contribution to the development of GVHD apparently differs depending on the presence or absence of an MHC-disparity between donors and recipients as well as the timing of the pathway's activation and HSCT. Download : Download high-res image (232KB) Download : Download full-size image Disclosures Levy: Capricor Therapeutics: Consultancy; OccuRx: Research Funding; Shire: Research Funding; Allergan: Consultancy.

The Innate Immune Sensor Sting Can Augment or Ameliorate Graft-Versus-Host Disease Dependent on the Genetic Disparity between Donors and Recipients

Graft-versus-host disease (GVHD) remains a significant cause of morbidity and mortality in patients receiving allogeneic hematopoietic stem cell transplants (allo-HSCTs). Pre-HSCT conditioning typically consists of irradiation and drug administration which results in the death of rapidly dividing cells and the initiation of a cytokine storm, promoting activation and expansion of donor anti-host alloreactive T cells in HSCT recipients. However, the precise mechanism of cytokine production remains unclear. Cell death following pre-transplant conditioning has promoted the hypothesis that sensors of cytoplasmic DNA damage in GVHD target tissues contribute to cytokine production. One such sensor is Stimulator of Interferon Genes (STING) which following activation induces phosphorylation of IRF3 and IκBα. Although STING activation was recently reported to worsen GVHD after MHC-mismatched allo-HSCT (Fischer J, et al, Sci. Transl. Med. 2017), STING involvement in MHC-matched allo-HSCT has not yet been thoroughly evaluated. Here, using B6-STING knock-out recipients and either MHC-mismatched or matched (“MUD”) donor strains we corroborate that STING deficiency worsens - but in contrast - ameliorates GVHD in the former and latter pre-clinical mouse models, respectively.To evaluate the effect of STING deficiency on GVHD after MHC-mismatched allo-HSCT, BALB/c BM + T cells were transplanted into 11.5Gy irradiated B6-WT and B6-STING-/- recipients. Consistent with previously reported results using B6-STINGgt/gt mice, STING-/- recipients experienced increased weight loss and GVHD clinical scores post-HSCT as well as decreased survival relative to WT recipients (Fig. 1A). Notably, when irradiation was given on day 0 instead of day -1 using the same donor/recipient combination no significant difference in GVHD was observed between WT and STING-/- recipients (Fig. 1B & 1E).We next investigated the role of STING after clinically relevant models of MHC-matched allo-HSCT. To assess for a role by STING immediately after HSCT, we examined cytokine mRNA expression 48 hrs. after transplant of LP/J BM + T cells → 8.5Gy irradiated B6-WT and B6-STING-/- recipients. Colonic tissue from STING-/- recipients had reproducibly >2x reduction in IFNβ, TNFα and IL-6 mRNA compared to WT (Fig. 1C). Importantly, IFNβ is one of the major downstream effector molecules produced as a result of IRF3 phosphorylation, consistent with the notion that STING activation occurs shortly after conditioning and/or allo-HSCT. Notably, LN examination at this time demonstrated fewer donor CD8+ T cells and CD8+CD44hiCD62Llo cells in STING-/- vs WT recipients. In contrast to STING-/- recipients of MHC-mismatched HSCT, MHC-matched STING-/- HSCT recipients experienced decreased weight loss and GVHD clinical scores vs. WT mice. STING-/- recipients contained 20-50% fewer donor lymphocytes and a significantly reduced frequency of activated donor CD4+ and CD8+ T cells in lymphoid tissues on D7. Analysis of peripheral LN and spleen 1-3 mo. post-HSCT revealed a similar profile and a higher frequency of naïve T cells - consistent with their decreased clinical signs of disease. Histological examination of recipient skin also showed higher pathology scores in WT recipients relative to STING-/- recipients. These findings were corroborated using a second MUD model, i.e. C3H.SW → B6 (Fig. 1D). Furthermore, measurement of serum cytokines 6 wks. post-transplant showed that B6-STING-/- recipients had higher levels of the immunosuppressive cytokine IL-10. The effect of donor T cell dose was also examined. Increased donor T cell numbers resulted in enhanced GVHD clinical scores in STING-/- recipients, however, the GVHD remained less severe than that observed in WT. Notably, HSCT studies using fully reconstituted chimeric B6-CD45.1↔B6-STING-/- indicated that STING expression in non-hematopoietic tissues is important for the development of GVHD.In total, this is the first report we are aware of in which the same pathway appears to differentially impact the outcome of allogeneic HSCT based upon the genetic disparity across the transplant. These findings reveal that STING's contribution to the development of GVHD apparently differs depending on the presence or absence of an MHC-disparity between donors and recipients as well as the timing of the pathway's activation and HSCT. [Display omitted] DisclosuresLevy:Capricor Therapeutics: Consultancy; OccuRx: Research Funding; Shire: Research Funding; Allergan: Consultancy.