THE T-CELL-DIRECTED CC CHEMOKINE TARC IS A HIGHLY SPECIFIC BIOLOGICALLIGAND FOR CC CHEMOKINE RECEPTOR-4

Thymus and activation-regulated chemokine (TARC) is a recently identif ied CC chemokine that is expressed constitutively in thymus and transi ently in stimulated peripheral blood mononuclear cells. TARC functions as a selective chemoattractant for T cells that express a class of re ceptors binding TARC with high affinity and specificity. To identify t he receptor for TARC, we produced TARC as a fusion protein with secret ed alkaline phosphatase (SEAP) and used it for specific binding. By st ably transfecting five orphan receptors and five known CC chemokine re ceptors (CCR1 to -5) into K562 cells, we found that TARC-SEAP bound se lectively to cells expressing CCR4. TARC-SEAP also bound to K562 cells stably expressing CCR4 with a high affinity (K-d = 0.5 nM). Only TARC and not five other CC chemokines (MCP-1 (monocyte chemoattractant pro tein-1), RANTES (regulated upon activation, normal T cells expressed a nd secreted), MIP-1 alpha (macrophage inflammatory protein-1 alpha), M IP-1 beta, and LARC (liver and activation-regulated chemokine)) compet ed with TARC-SEAP for binding to CCR4. TARC but not RANTES or MIP-1 al pha induced migration and calcium mobilization in 293/EBNA-1 cells sta bly expressing CCR4. K562 cells stably expressing CCR4 also responded to TARC in a calcium mobilization assay. Northern blot analysis reveal ed that CCR4 mRNA was expressed strongly in human T cell lines and per ipheral blood T cells but not in B cells, natural killer cells, monocy tes, or granulocytes. Taken together, TARC is a specific functional li gand for CCR4, and CCR4 is the specific receptor for TARC selectively expressed on T cells.