The CC chemokines MDC and TARC induce platelet activation via CCR4.

While chemokines have received considerable attention for their role in leukocyte chemotaxis, their effects on platelets have not been well described. We found that two CC chemokine receptor 4 (CCR4) ligands, macrophage-derived chemokine (MDC) and thymus and activation-regulated chemokine (TARC) induce concentration-dependent platelet aggregation and calcium flux. Flow cytometric analysis revealed the expression of CCR4 on platelets and a monoclonal antibody (mAb) to CCR4 inhibited MDC- and TARC-induced platelet aggregation, confirming that this effect is mediated through their common receptor CCR4. MDC fully desensitized TARC-induced calcium mobilization in platelets, while TARC was unable to completely desensitize a subsequent MDC response, which is similar to observations made in Th2 CD4(+) lymphocytes and CCR4-transfected cells. Aspirin (ASA) treatment of platelets allowed reversible primary aggregation but inhibited irreversible complete aggregation, suggesting that MDC- and TARC-induced full platelet aggregation is dependent on cyclooxygenase metabolites of arachidonic acid. MDC and TARC were unable to induce platelet aggregation and platelet secretion in washed human platelets, even though they induced a calcium flux, suggesting that plasma components are required for MDC- and TARC-induced platelet aggregation. Since Th2-type cytokines induce the release of MDC and TARC from cells and the expression of these chemokines is increased in Th2-type inflammation, we hypothesize that MDC and TARC may play a role in platelet activation seen in Th2 diseases, such as asthma and atopic dermatitis.