Chemokines: signal lamps for trafficking of T and B cells for development and effector function.

Chemokines can act as signal lamps for trafficking of lymphocytes at the important crossing points of lymphoid tissues. Lymphoid progenitors at different differentiation stages are differentially localized in primary lymphoid tissues and have differential responsiveness to thymic or bone marrow chemokines: SDF-1, CKbeta-11/MIP-3beta/ELC, SLC/6Ckine/Exodus2, MIP-1beta, and TECK. Naive T cells and B cells circulate to secondary lymphoid tissues for possible activation. Chemokines, SDF-1, SLC/6Ckine/Exodus2, CKbeta-11/MIP-3beta/ELC, BLC/BCA-1, and DC-CK1/PARC, are expressed in the specialized microenvironments of secondary lymphoid tissues and regulate the migration of naive lymphocytes. Effector lymphocytes express a different set of chemokine receptors from naive lymphocytes. T helper (Th) 0 and 1 cells predominantly express CXCR3 and CCR5, whereas Th2 cells express CCR3, CCR4, and CCR8, which, with other factors such as expression patterns of adhesion molecules, likely determine the tissue-specific infiltration of effector lymphocytes.