Bystander activation of effector CD4+ T cells exacerbates inflammatory bowel disease and rheumatoid arthritis via TL1A-mediated production of IFN-γ and IL-17 synergistically with pro-inflammatory cytokines such as IL-12, IL-18 and IL-23 (35, 44). The gene discussed is TNFSF15; the disease is rheumatoid arthritis.