NLRP6 in Donor T Cells Separately Regulates CD4 and CD8 Mediated Graft-Versus-Host Disease in Experimental Murine BMT

NLRP6 (NOD-like receptor family pyrin domain containing 6) is an important inflammasome component and is highly expressed in intestinal epithelial and in immune cells. NLRP6 mediated inflammasome activation plays a critical role in response to intestinal infection and preventing dysbiosis of gut microbiota through the secretion of IL-18 and mucus. However, we recently found that NLRP6 plays a pathogenic role in GVHD that is independent of microbial dysbiosis, which is in contrast to its well-appreciated microbiome-dependent protective role in intestinal colitis and tumorigenesis. Interestingly, we also found that activated T cells increased NLRP6 expression, but the T cell autonomousrole of NLRP6 in regulating T cell responses is unknown. Because NLRP6 is an important regulator of GVH responses, we tested the hypothesis that NLRP6 deficiency in donor T cells would ameliorate GVHD.

To test our hypothesis, we first performed adetailed phenotypic analysis of various T cell subsets and activation markers in naïve NLRP6^-/-and wild-type (WT) B6 animals and found a similar distribution of naïve, memory, effector and regulatory T cells. In order to examine whether the absence of NLRP6 in donors affects GVHD, WT-BALB/canimals were lethally irradiated (700cGy) and transplanted on day 0 with 5x10⁶bone marrow and 1.0x10⁶ splenic CD90⁺T cells from either syngeneic WT-BALB/c, allogeneic MHC-mismatched WT-B6 or NLRP6^-/-animals. Contrary to our hypothesis, the recipients receiving donor T cells from NLRP6^-/-animals showed a significantly worse survival compared to allogeneic WT-B6 animals (p<0.05). GVHD mortality and severity were also increased in an MHC mismatched B6 into B10.BR model, and in an MHC mismatched haploidentical B6 into F1model (p<0.05). In contrast, GVHD severity and mortality were similar in an MHC matched multiple minor antigen mismatched B6 into C3H.sw model.

We hypothesized that GVHD severity and mortality was similar in the B6 into C3H.sw model because NLRP6 regulates CD4+ and CD8+ T cell responses, differently. In order to test this, we transplanted C3H.sw recipients as above except we infused either 1x10⁶CD4+ 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 transplanted. We confirmed enhanced GVHD mortality and severity mediated by donor NLRP6^-/-CD4+ T cells in a second MHC-mismatched GVHD model, B6 into BALB/c (p<0.05).

To explore how NLRP6 effects T cell responses independent ofinflammasome activation, we tested naïve T cell proliferation in vitro after allogeneic or non-specific TCR stimulation by anti-CD3 and CD28 antibody and found that NLRP6^-/-CD4+ but not CD8+T cells proliferated more than WT-B6 CD4+ or CD8+ T cells, respectively, following either stimulus. Furthermore, allogeneicNLRP6^-/-T cells also caused greater mortality compared to WT allogenic T cells in a non-irradiated B6 into F1 model, which lacks inflammasome activation associated with conditioning induced DAMPs and PAMPs. Microarray analysis of activated T cells from NLRP6^-/-animals showed higher expression of IL-2 and IFN-γ than WT B6 T cells, and we observed no effect of NLRP6 in a Treg suppression assay. These data suggest that NLRP6 regulates CD4+ T cell- mediated immune responses and that NLRP6 in donor T cells is critical for controlling CD4+ T cell mediated GVHD. The effect of NLRP6 on T cell mediated GVL is currently under investigation.