CD4 and inflammatory bowel disease: Although no study has yet evaluated the action of IL-34 on the differentiation/function of T cells in IBD, it is noteworthy that IL-34-primed macrophages constitutively express membrane-type IL-1α, which stimulates differentiation of memory Th17 cells (54), a subset of CD4+ T cells that infiltrates massively inflamed gut in IBD and contributes to mucosal injury seen in mice with experimental colitis (55, 56).