| CXCL13, also known as B‑cell‑attracting chemokine‑1 (BCA‑1) or B‑lymphocyte chemoattractant (BLC), is a CXC chemokine that drives B‑cell recruitment, positioning, and activation within secondary lymphoid organs and ectopic lymphoid structures, and it functions as the principal ligand for the G‑protein‑coupled receptor CXCR5 to organize lymphoid architecture and coordinate humoral immune responses. The chemokine adopts the conserved CXC fold with an N‑terminal activation region, a three‑stranded β‑sheet, and a C‑terminal α‑helix, and it forms gradients on follicular dendritic cells and stromal organizers that attract CXCR5‑expressing B cells and follicular helper T cells into B‑cell zones where germinal center reactions and antibody affinity maturation occur. CXCL13 binding to CXCR5 triggers Gαi‑dependent signaling, leading to activation of PI3K, MAPK, and small GTPases, with downstream engagement of Akt and Rac that reorganize the actin cytoskeleton and direct polarized migration along CXCL13 gradients, while CXCR5 also recruits β‑arrestin to modulate receptor trafficking and signal duration, integrating migration with survival and activation cues that support B‑cell homeostasis and differentiation. The CXCL13/CXCR5 axis acts as a lymphoid organizer by recruiting CXCR5‑positive B cells and T follicular helper cells to mesenchymal lymphoid tissue organizer cells, where it promotes the formation of tertiary lymphoid structures in non‑lymphoid tissues during chronic inflammation, infection, or autoimmunity, and CXCL13 expression by peripheral helper T cells and macrophages in disease settings drives ectopic lymphoid neogenesis and pathologic B‑cell accumulation. In cancer, the CXCL13/CXCR5 axis shapes the tumor microenvironment by recruiting B cells, regulatory B cells, and T follicular helper cells into tumor‑associated tertiary lymphoid structures, where its activity can promote pro‑neoplastic immune suppression by attracting suppressive lymphocytes while simultaneously enabling anti‑tumor immunity through organized germinal center‑like responses and high‑affinity antibody production, and CXCL13 also drives tumor cell proliferation, invasion, and angiogenesis through PI3K/AKT and NF‑κB signaling in malignant cells. In autoimmune diseases, ectopic CXCL13 expression in inflamed tissues correlates with CXCR5‑dependent B‑cell infiltration, formation of tertiary lymphoid structures, autoantibody production, and chronic inflammation in rheumatoid arthritis, multiple sclerosis, systemic lupus erythematosus, Sjögren's syndrome, myasthenia gravis, and inflammatory bowel disease, positioning CXCL13 as a biomarker and therapeutic target in these conditions. CXCL13 is also induced by environmental carcinogens such as tobacco smoke and benzo(a)pyrene in lung epithelial cells, where it contributes to lung carcinogenesis and colorectal cancer, and CXCL13 knockout in mice reduces these carcinogen‑driven malignancies, linking CXCL13 to both immune‑mediated tumor control and tumor promotion in a context‑dependent manner. |
