GRK2 transactivation of CXCR4 is required for TCR-mediated TCR-CXCR4 complex formation

Abstract T cell cytokine production was recently shown to depend not only on the T cell antigen receptor (TCR), but also on the chemokine receptor CXCR4. Our lab recently published that stimulation of the TCR results in the TCR associating with and transactivating CXCR4, which, in turn, activates a PREX-1/Rac-1 pathway that stabilizes cytokine mRNA. This pathway therefore normally significantly increases IL-2, IL-4, and IL-10 production by activated T cells. Here, we further characterize the molecular mechanisms responsible for the first step of this pathway: TCR-mediated TCR-CXCR4 complex formation. First, we used FRET and proximity ligation assay (PLA) to define the required structural motifs of CXCR4, showing that this pathway requires the CXCR4 cytoplasmic tail domain and phosphorylation of CXCR4-S339. Second, we showed that TCR-induced TCR-CXCR4 complex formation requires TCR-induced activity of Src family and ZAP-70 tyrosine kinases, as well as one or more Ser/Thre kinases. Third, we found that the G protein-coupled receptor kinase-2 (GRK2) is required both for TCR-induced phosphorylation of CXCR4-S339 and for the formation of TCR-CXCR4 complexes. Finally, we found that GRK2 is required for robust IL-2, IL-4, and IL-10 cytokine secretion by normal human T cells. Together these results identify a novel role for TCR-induced GRK-2 in inducing the TCR-CXCR4 complex formation that subsequently signals to increase TCR-mediated cytokine production. Targeting GRK2 or other mechanisms required for TCR-induced TCR-CXCR4 formation may therefore be therapeutically useful for limiting production of T cell cytokines in humans, for example, during graft versus host disease or immunotherapy.