"Cytoskeletal regulation of differential T cell chemotactic responses"

Franklin Staback Rodríguez, Tanner F. Robertson and Janis K. Burkhardt

In vivo, signaling by the chemokine CCL19 and its receptor CCR7 has been associated with T cell retention in lymph nodes, while signaling by the lipid chemoattractant S1P via S1PR1 has been shown to mediate lymph node egress. Our lab recently discovered that T cells engage in distinct modes of motility in response to these two chemotactic signals. Whereas CCL19 is known to activate actin polymerization at the leading edge of migrating cells, we found that S1P induces myosin-dependent bleb-based motility. To better understand the mechanisms controlling these differential responses, we have explored the relevant signaling cascades downstream of CCR7 and S1PR1. Using G-LISAs and immunoblot analysis, we found that stimulation with CCL19 induced activation of the Rho GTPase Rac1 and phosphorylation of its effector Pak. In contrast, stimulation with S1P led to activation of RhoA and phosphorylation of myosin regulatory light chain (MLC) at a site known to activate myosin contractility. Moreover, treatment of T cells with an inhibitor of the Rho effector ROCK blocked S1P-induced MLC phosphorylation. Additional inhibitor studies also point to a role for phospholipase C, calmodulin, and myosin light chain kinase in the regulation of MLC phosphorylation induced by S1P. Taken together, these studies are consistent with a model in which CCL19 and S1P induce distinct motile behavior by signaling through Rac1 and RhoA/PLC, respectively.