| Chemokines are a superfamily of structurally related, small, chemotactic cytokines that can be classified into four families based on the location of the first two cysteines in a four cysteine motif. Originally thought of as only chemotactic molecules, chemokines are now considered as important immunomodulators. CCL5 (RANTES) is a member of the CC or β chemokine subfamily that has been implicated in regulating T cell function.; To understand the molecular mechanisms associated with the biological activities of CCL5, we investigated the signal transduction pathways that are activated by CCL5. Using the human T cell lines Jurkat and MOLT-4, we demonstrate that nM concentrations of CCL5 can activate Stat1 and Stat3 homo- and hetero-dimers with DNA binding activity. Moreover, CCL5 can lead to the increased gene expression of a putative Stat-inducible gene, c-fos , indicating that CCL5 activated Stat complexes can regulate gene expression in T cells. CCL5-inducible c-fos gene expression correlated with the gene expression of CCR5, a receptor for CCL5. To investigate the role of CCR5 in CCL5-inducible signal transduction, a CCR5 expressing human T cell line, PM1, was used. CCL5 treatment leads to phosphorylation and activation of Jak2 and Jak3 in a Gi-protein independent manner. CCL5-inducible phosphorylated Jak2 associates with phosphorylated CCR5, indicating that Jaks are recruited to chemokine receptors upon activation. In addition, phosphorylated Jak3 associates with phosphorylated lck, suggesting that CCR5 signaling can crosstalk with components of the TCR signaling machinery. CCL5 also activates the p38 MAP kinase pathway in PM1 cells, indicating that additional signal events are invoked by CCL5-CCR5 interactions.; To understand the functional outcome of CCL5-CCR5 interactions in T cells, we investigated the effects of varying doses of CCL5 on the viability of CCR5 expressing T cells. CCL5 at μM, but not nM, doses leads to a decrease in T cell viability, by inducing apoptosis in a manner similar to activation induced cell death. This increase in cell death is dependent on CCL5-CCR5 interactions and CCL5-GAG binding. The binding of CCL5 to cell surface GAGs sequesters CCL5 and induces CCL5 aggregation. Using aggregation mutants of CCL5, we demonstrate that tetramers of CCL5 are the minimal higher ordered aggregates required for CCL5-induced cell death. |
