CD40

CD40 (TNFRSF5) is a co-stimulatory receptor of the TNF receptor superfamily and the central orchestrator of humoral and cellular immunity. CD40 is a 45-50 kDa type I transmembrane protein with four extracellular cysteine-rich domains, expressed on B cells, dendritic cells, macrophages, monocytes, epithelial and endothelial cells, fibroblasts and various carcinomas. Its ligand CD40L (CD154), transiently expressed on activated CD4+ T cells and platelets, triggers receptor trimerization and recruitment of TRAF2/3/5/6, activating canonical and non-canonical NF-κB, MAPK, JAK3-STAT3 and PI3K pathways. In B cells, CD40 signaling drives germinal center formation, immunoglobulin class switching, somatic hypermutation, memory B-cell and plasma-cell differentiation; in dendritic cells, it licenses antigen presentation and interleukin-12 production. CD40-CD40L defects cause X-linked hyper-IgM immunodeficiency, whereas sustained CD40 signaling contributes to atherosclerosis, autoimmune inflammation and B-cell lymphomas; agonistic CD40 antibodies and CD40L blockers are both therapeutic development tracks in cancer and autoimmunity.