The Regulatory Mechanisms of PIR-B and STARD7 in Inflammatory Bowel Disease

Abstract

Rationale: The inflammatory bowel diseases (IBD), Crohn’s Disease (CD) and Ulcerative Colitis (UC) are chronic relapsing gastrointestinal (GI) inflammatory diseases that are driven by a breakdown of the intestinal epithelial barrier which permits an aberrant intestinal inflammatory response against microbial antigens. We have previously identified 1) the inhibitory receptor, paired immunoglobulin-like receptor (PIR-B) and 2) the lipid transfer protein, steroidogenic acute regulatory (StAR) protein-related lipid transfer (START) domain-containing protein 7, STARD7, to play critical roles in innate inflammatory signals and integrity of epithelial barrier respectively. The aim of this dissertation is to define the involvement of PIR-B in CD4+ T-cell functionality and STARD7 in integrity of intestinal epithelial barrier and the role of these functions in susceptibility to colitis.

Results: We demonstrate that Pirb-/-Il10-/- mice were protected from development of Il10-/- spontaneous colitis phenotype and αCD3-mediated intestinal enteropathy. The reduced disease phenotype was associated with fewer CD4+ Th17 cells in the mesenteric lymph nodes and diminished systemic IL-17a levels. Employing a CD4+ CD45RBhi T-cell transfer model of colitis we show that Rag-/- which received Pirb-/- naïve CD4+ T-cells were protected from T-cell mediated colitis. In vitro polarization of naïve CD4+ T-cells revealed an intrinsic deficiency of Pirb-/- Th17 cell survival and cell cycle progression due to hyperactivation of mTORC1 signaling. In silico analyses established upregulation of Th17 and tissue resident memory (TRM) transcriptional signatures in PIR-B+ murine CD4+ T cells and LILRB3+ human CD4+ T cells. Severe CD patients marked with high LILRB3 expression were strongly associated with mucosal injury and a proinflammatory Th17-signature. Additional In silico analyses identified significantly reduced expression of Stard7 in UC patients which was strongly associated with impaired metabolic function and detrimental inflammatory outcomes. STARD7 knockdown in intestinal epithelial cells resulted in compromised mitochondrial architecture which severely impaired oxidative phosphorylation. Notably, loss of STARD7 directly impacted the formation of cristae and expression of respiratory Complex I mitochondrial energy generation which compromised intestinal epithelial barrier function. Activation of the energy sensor AMPK reconstituted the mitochondrial morphology, upregulated expression of tight junction proteins, and enhanced integrity of the barrier in STARD7 deficient epithelial cells. In vivo we demonstrate that Stard7TgIl10-/- mice were more susceptible to the development of DSS-induced and the Il10-/- spontaneous colitis phenotypes due to exaggerated activation of macrophage and CD4+ T cell compartments.

Conclusions: These data support the concept that 1) PIR-B regulates CD4+ Th17 differentiation and development of mucosal T cell memory responses and that 2) STARD7 is a crucial regulator for mitochondrial function and maintenance of the intestinal epithelial barrier. Collectively, this dissertation provides new insight into functions for inhibitory receptors and lipid transport proteins in regulating inflammatory responses and maintenance of intestinal homeostasis and identifies that loss of either molecule can impact susceptibility to IBD.