T Cell Ontogeny and Effector Functions in Galphai2-deficient Colitis

Abstract

Inflammatory bowel disease (IBD) comprising Crohn s disease and ulcerative colitis is a disease that affects millions of people worldwide, especially in northern countries such as Sweden, and is characterised by chronic uncontrolled inflammation of the intestinal mucosa. It is the general belief that multiple factors such as genetic and environmental aspects are involved in disease pathogenesis. Alterations in T cell subsets, an important cell type in cell-mediated immune responses in the adaptive immune system, are certainly one element contributing to disease development. The present work was aimed at elucidating the role of the T cells in the pathogenesis of a mouse model for inflammatory bowel disease, Galphai2-deficient (Galphai2-/-) mice. These mice spontaneously develop a lethal colitis strikingly similar to ulcerative colitis in humans. This study demonstrates that one or several secondary genetic factor(s) in addition to the Galphai2 deficiency, possibly involving regulation of IL-12p40 production, has to be fulfilled for colitis to develop. Galphai2-deficient mice were shown to have a seriously impaired T cell development associated with accelerated thymic involution, decreased recruitment of T cell precursors to the thymus due to diminished responsiveness to CXCL12, reduced frequency of apoptotic CD4+CD8- and CD4-CD8+ thymocytes suggesting impaired negative selection, and insufficient down regulation of CD69 on mature CD4+ T cells, indicating impaired thymic egression. The impaired T cell ontogeny in Galphai2-/- mice was demonstrated to have great implications on the ability of the peripheral T cell repertoire to regulate the normal state of "physiologic inflammation" in the intestinal mucosa: Splenic, but not mesenteric lymph node Galphai2-/- T cells, were shown to be highly colitogenic when adoptively transferred into immunodeficient hosts. The effector function of peripheral as well as mucosal T cells from Galphai2-/- mice were shown to be T helper 1 (Th1) dominated following stimulation with superantigens, bacterial products or dietary proteins. Th1 associated and pro-inflammatory mediators such as IFN-gamma and IL-12 are likely to instigate the immune response initiating what will result in established chronic colitis in Galphai2-/- mice. Furthermore, we show that the lifespan of Galphai2-/- mice could be substantially prolonged following reconstitution with wild type bone marrow cells. Finally and surprisingly, by inhibiting the recruitment of T cells to the inflamed tissue by administration of antibodies specific for integrin alpha4, the colonic inflammation was aggravated. The aggravated colitis was accompanied by increased production of pro-inflammatory cytokines such as IL-1beta, TNF-alpha and IFN-gamma and was believed to be, at least in part, explained by in situ proliferation of activated mucosal resident T cells. Collectively these results conclude that T cells are highly important for disease development in Galphai2-/- mice.