Major Histocompatibility Complex Class II (MHCII) is dynamically expressed on intestinal epithelial cells (IECs) throughout the intestine, but its regulation remains poorly understood. We observed that spontaneous upregulation of IEC MHCII in locally-bred Rag1 −/− mice correlated with serum Interleukin (IL)-18, was transferrable via cohousing to commercially-bred immunodeficient mice and could be inhibited by both IL-12 and IL-18 blockade. Overproduction of intestinal IL-18 due to an activating Nlrc4 mutation upregulated IEC MHCII via classical inflammasome machinery independently of immunodeficiency or dysbiosis. Immunodeficient dysbiosis increased Il18 transcription, which synergized with NLRC4 inflammasome activity to drive elevations in serum IL-18. This IL-18-MHCII axis was confirmed in several other models of intestinal and systemic inflammation. Elevated IL-18 reliably preceded MHCII upregulation, suggesting an indirect effect on IECs, and mice with IL-18 overproduction showed activation or expansion of type 1 lymphocytes. Interferon Gamma (IFNg) was uniquely able to upregulate IEC MHCII in enteroid cultures and was required for MHCII upregulation in several in vivo systems. Thus, we have linked intestinal dysbiosis, systemic inflammation, and inflammasome activity to IEC MHCII upregulation via an intestinal IL-18-IFNg axis. Understanding this process may be crucial for determining the contribution of IEC MHCII to intestinal homeostasis, host defense, and tolerance.

Intraepithelial lymphocytes (IELs) are T cells defined by direct contact with epithelial surfaces. While important for barrier integrity, the mechanisms of recruitment and functional maturation remain unknown. We identified sphingosine-1-phosphate (S1P) receptor 1 (S1PR1) as highly enriched in the IEL population compared to lamina propria T cells by microarray, suggesting importance in IEL recruitment. Blockade of S1P signaling using FTY720 significantly reduced recruitment of TCRab+ IELs, but not TCRgd+ IELs, following bacterial recolonization of antibiotic-treated mice, which is known to recruit IELs. Since mice lacking either Myd88 or an intact microbiome lack IELs, we hypothesized that either the epithelium or mononuclear phagocytes responded to microbial TLR ligands and produced S1P. Myeloid lineage cells were deemed the cells responsible for Myd88/microbe dependent IEL recruitment. In vitro, cecal contents stimulated LysM-Cre+ Myd88fl/fl bone-marrow derived macrophages do not upregulate sphk1, the enzyme responsible for most S1P production, and fail to induce S1P dependent T cell migration in a transwell assay. To understand how the IEL-IEC synapse affects IEL function, we utilized Villin-Cre+ MHCIIfl/fl mice. We observed altered maturation in both CD4+ and CD4− CD8− IEL compartments, associated with increased CD44− CD62L+ naive and CD44+ CD62L+ central memory IELs, and decreased CD44+ CD62L− effector, and CD44− CD62L− “revertant” IELs, suggesting a block in T cell maturation. Together these data show a coordinated, multi-system process by which IELs are first recruited to, and then functionally matured by the IECs. Further work will investigate how inflammation of the mucosal barrier alters these processes.