The Absence of NLRP6 in Donor T Cells Exacerbates Gvhd

The Nlrp6 (NOD-like receptor family pyrin domain containing 6) inflammasome is important for intestinal epithelial cell innate immune responses and for maintaining gut homeostasis by preventing microbial dysbiosis. Contrary to its role in epithelial cell inflammasome-mediated responses, we recently showed that Nlrp6 in gut epithelial cells exacerbates GVHD in a manner independent of the inflammasome or gut microbiota. However, donor allogeneic T cells are also critical for GVHD development, yet, the function of Nlrp6 in allogeneic T cells is unknown. We hypothesized that Nlrp6 deficient donor T cells would ameliorate experimental GVHD.

To test our hypothesis, WT-BALB/crecipients were lethally irradiated and transplanted on day 0 with 5x10 ⁶ bone marrow and 1.0x10 ⁶ splenic CD90 ⁺T cells from either syngeneic WT-BALB/c, allogeneic MHC-mismatched WT-B6 or Nlrp6 ^-/- donors. Contrary to our hypothesis, the survival of allogeneic recipients of Nlrp6 ^-/- donor T cells was significantly worse than those receiving WT-B6 T cells (p<0.05). Nlrp6 ^-/- donor T cells also caused greater GVHD mortality and morbidity in an MHC mismatched haploidentical B6 into B6D2F1 model (p<0.05) and an MHC mismatched B10.BR into B6 model. Similar results were obtained using B6 into BALB/c and B6 into B6D2F1 models performed at the University of Michigan, suggesting our results were not unique to local environmental factors. By contrast, GVHD severity and mortality were similar in an MHC matched multiple minor antigen mismatched B6 into C3H.sw model. Because the B6 into C3H.sw model is largely driven by CD8+ T cells whereas the previous models are mediated by both CD4+ and CD8+ T cells, we examined whether Nlrp6 separately regulates CD4+ and CD8+ T cell-mediated GVHD. In order to test this, we transplanted C3H.sw recipients as above except we infused either 1x10 ⁶ CD4+ or CD8+ T cells from B6-WT or Nlrp6 ^-/- animals. GVHD severity and mortality (P<0.05) were enhanced only when Nlrp6 ^-/- CD4+ T cells were transplanted. These data suggested that Nlrp6 regulates allogeneic T cell responses in a subset-specific manner.

To explore how Nlrp6 regulates intrinsic responses in donor T cell subsets, we tested naïve T cell proliferation in vitro after allogeneic or non-specific TCR stimulation. Consistent with the lack of increased GVHD induced by CD8+ Nlrp6 ^-/- donor T cells in the B6 into C3H.sw model, Nlrp6 ^-/- CD4+ but not CD8+ T cells proliferated more than WT-B6 CD4+ or CD8+ T cells, respectively, when stimulated with either anti-CD3/CD28 antibodies or lethally irradiated allogeneic antigen presenting cells in a mixed lymphocyte reaction. In addition, activation-induced apoptosis was decreased in Nlrp6 ^-/- CD4+ T cells compared to WT T cells. Importantly, Treg suppressive function was not altered in Nlrp6 ^-/- T cells. Therefore, increased proliferative responses and resistance to activation-induced apoptosis may have contributed to the enhanced GVHD caused by Nlrp6 ^-/- donor T cells.

Increased Th1 and Th17 polarization is associated with worse GVHD. Because only CD4+ Nlrp6 ^-/- T cells enhanced GVHD, we tested whether Nlrp6 influenced T helper cell differentiation into Th1, Th17, and Th2 subsets. Consistent with our in vivo data, Th1 in vitro differentiation was enhanced in Nlrp6 ^-/- CD4+ T cells. To determine the molecular signaling events altered by Nlrp6 deficiency, we tested various T cell activation signaling pathways and found that phosphorylation of ZAP-70 was increased in Nlrp6 ^-/- T cells. These data suggested that Nlrp6 in donor T cells may regulate allo-immune responses via ZAP-70 pathway.

GVH and graft-versus-tumor (GVT) responses are intricately linked. Because CD8+ responses were not affected by Nlrp6 deficiency, we hypothesized that GVT responses would be unaltered in Nlrp6 ^-/- donor T cells. Indeed, Nlrp6 ^-/- T cells showed equivalent in vivo GVL responses to MLL-AF4 leukemia cells as WT-T cells. Hence Nlrp6in donor T cells is not required for GVT responses.

Altogether our data suggested that Nlrp6 negatively-regulates allogeneic donor CD4+ T cell responses, possibly via negative regulation of ZAP-70 signaling, resulting in mitigation of GVHD and maintenance of robust GVT responses.