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 (<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 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-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.