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

Abstract

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 (<D14), but not late (>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 (STING^HAQ/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 <i>Sci. Transl. Med.</i>) 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-Nur77^GFP T cells were used to explicitly monitor T cell receptor signaling. Indeed, STING^−/− spleens had greater numbers of donor Nur77^GFP 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.