IL-15 is highly expressed in inflammatory bowel disease and regulates local T cell-dependent cytokine production.

IL-15 shares biological activities but no significant sequence homology with IL-2. It induces T cell recruitment to sites of inflammation, T cell proliferation, and cytokine production and rescue from apoptosis. The aim of this study was to investigate expression of IL-15 and its effects on proinflammatory cytokine production in inflammatory bowel disease (IBD). Immunohistochemistry demonstrated local IL-15 production by macrophages in inflamed mucosa from IBD patients. Isolated lamina propria mononuclear cells from these patients but not from controls produced IL-15 when stimulated with LPS or IFN-gamma. Moreover, lamina propria T cells (LP-T) from IBD patients were more responsive to IL-15 as compared with controls, and IL-15 alone without a primary T cell stimulus induced IFN-gamma and TNF production by isolated IBD LP-T cells, especially by LP-T cells from patients with Crohn's disease. LP-T cells from IBD patients could induce CD40-CD40 ligand (CD40L) interaction-dependent TNF and IL-12 production by monocytes in a coculture system. This capacity of LP-T cells was strongly enhanced by preincubation in IL-15 and was the result of higher CD40L expression after culture in IL-15. These data indicate that IL-15 is overexpressed in the inflamed mucosa in IBD and that IL-15 enhances local T cell activation, proliferation, and proinflammatory cytokine production by both T cells and macrophages, the latter via a CD40-CD40L interaction-dependent mechanism. Treatment directed against IL-15 may have therapeutic potential in IBD.