chemokines

Chemokines are a family of ~8-12 kDa cytokines that direct leukocyte migration (chemotaxis) along concentration gradients, playing a key role in immune surveillance, inflammation, and immune cell trafficking. They are divided into four main structural classes based on the position of conserved N-terminal cysteine residues: CXC (α-chemokines, e.g., IL-8/CXCL8, primarily recruit neutrophils), CC (β-chemokines, e.g., MCP-1/CCL2, RANTES/CCL5, recruit monocytes, eosinophils, lymphocytes), C (lymphotactin), and CX3C (fractalkine, mediates adhesion and chemotaxis of leukocytes). Chemokines signal through seven-transmembrane GPCRs (e.g., CXCR4, CCR5) coupled to Gi proteins, activating pathways that promote cytoskeletal rearrangement, integrin activation, and directed cell movement. Clinically, chemokine receptors are important in HIV pathogenesis—CCR5 and CXCR4 serve as co-receptors for HIV entry into CD4+ T cells, and CCR5-Δ32 mutation confers resistance to HIV infection. Chemokines also play roles in atherosclerosis (MCP-1 recruiting monocytes into vessel walls), tumor metastasis (chemokine receptor expression on cancer cells directing spread to specific organs), and autoimmune diseases. Chemokine gradients are established by binding to glycosaminoglycans on endothelial cell surfaces, creating a haptotactic gradient for leukocyte adhesion and extravasation (following the steps of rolling, tight adhesion via integrins, and transmigration). Understanding chemokines is essential for USMLE questions regarding immune cell recruitment, HIV coreceptor biology, and pharmacologic targets like CCR5 antagonists (e.g., maraviroc).