CD4 and autoimmune disease: IL-2 is mainly secreted by CD4+ T cells and plays crucial roles in the expansion and differentiation of CD4+ T, CD8+ T, NK, and other cells through the IL-2R–JAK–STAT5 signaling pathway.169,170 IL-2 can fine-tune immune responses and maintain self-tolerance,171 and its deficiency accounts for the occurrence of autoimmune diseases.172