MACROPHAGE-DERIVED CHEMOKINE IS A FUNCTIONAL LIGAND FOR THE CC-CHEMOKINE RECEPTOR-4

Macrophage-derived chemokine (MDC) is a recently identified member of the CC chemokine family. MDC is not closely related to other chemokine s, sharing most similarity with thymus- and activation-regulated chemo kine (TARC), which contains 37% identical amino acids. Both chemokines are highly expressed in the thymus, with little expression seen in ot her tissues, in addition, the genes for MDC and TARC are encoded by hu man chromosome 16. To explore this relationship in greater detail, we have more precisely localized the MDC gene to chromosome 16q13, the sa me position reported for the TARC gene, We have also examined the inte raction of MDC with CC chemokine receptor 4 (CCR4), recently shown to be a receptor for TARC. Using a fusion protein of MDC with secreted al kaline phosphatase, we observed high affinity binding of MDC-secreted alkaline phosphatase to CCR4-transfected L1.2 cells (K-d = 0.18 nM). M DC and TARC competed for binding to CCR4, while no binding competition was observed for six other chemokines (MCP-1, MCP-3, MCP-4, RANTES (r egulated on activation normal T cell expressed and secreted), macropha ge inflammatory protein-1 alpha, macrophage inflammatory protein-1 bet a). MDC was tested for calcium mobilization in L1.2 cells tranfected w ith seven different CC chemokine receptors. MDC induced a calcium flux in CCR4-transfected cells, but other receptors did not respond to MDC . TARC, which also induced calcium mobilization in CCR4 transfectants, was unable to desensitize the response to MDC. In contrast, MDC fully desensitized a subsequent response to TARC. Both MDC and TARC functio ned as chemoattractants for CCR4 transfectants, confirming that MDC is also a functional ligand for CCR4, Since MDC and TARC are both expres sed in the thymus, one role for these chemokines may be to attract CCR 4-bearing thymocytes in the process of T cell education and differenti ation.