In IBD patients, with change in the intestinal bacteria, the cytotoxicity of CD4CD8αα IELs is activated, which leads to secretion of a large number of proinflammation cytokines such as TNF-α, IL-15, and IFN-γ, and upregulation of CD107a expression, implying the pathogenic potential of cytotoxic CD4CD8αα IELs (23). This evidence concerns the gene IL15 and inflammatory bowel disease.