CD4 (cluster of differentiation 4), also known as L3T4 or T4, is a 55 kDa single chain type I transmembrane glycoprotein belonging to the immunoglobin (Ig) superfamily. CD4 is predominantly expressed on most thymocytes, a subset of mature T lymphocytes, and weakly on monocytes, tissue macrophages, dendritic cells, and granulocytes. It is also expressed on neurons and glial cells in the brain (1). CD4 is expressed along with CD8 on double positive T cells during their development in the thymus. Either CD4 or CD8 expression is then lost giving rise to single positive (SP) CD4+ or CD8+ mature T cells. CD4+ SP cells (T helper cells) further differentiate into multiple subsets of CD4+ cells including Th1, Th2, Th17, Tfh, and Treg cells which regulate humoral and cellular immunity (2). The extracellular region of CD4 consists of 372 amino acids (aa) with four immunoglobin-like domains (D1-D4). The structures of D1 and D3 resemble variable (IgV) domains while D2 and D4 resemble constant (IgC) domains (3).
Given its critical role in T cell development, CD4 also has diverse immunology-related functions. CD4 acts as a coreceptor with the T-cell receptor (TCR) during T cell activation and thymic differentiation by binding directly to major histocompatibility complex (MHC) class II antigens and associating with the protein tyrosine kinase, Lck (4). This interaction contributes to the formation of the immunological synapse (5). Defects in antigen presentation cause dysfunction of CD4+ T cells and the almost complete loss of MHC II expression on B cells in peripheral blood, as observed in severe combined immunodeficiency (SCID) (6). CD4 also functions as a receptor for the human immunodeficiency virus (HIV) by binding to gp120, the envelope glycoprotein of HIV‑1. It has been shown that the V-like domains are critical for binding to gp120 (7). In immune mediated and infectious diseases of the central nervous system, CD4 functions as an indirect mediator of neuronal damage (8).
