CD40L modulates CD4+ T cell polarization and activation through the binding of Receptor for Activated C Kinase 1

Inhibition of the co-stimulatory ligand CD40L has shown beneficial effects in many experimental models of auto-immune disease and inflammation. Here, we show that CD40L deficiency in T-cells in mice causes a reduction of CD4+ T cell activation and specifically a strong reduction in interferon- γ (IFN-γ) producing T helper 1 (Th1) cells. In vitro, we could not reproduce this APC-dependent effects, but found that T-cell CD40L affects cell death and proliferation. We identified receptor of activated C kinase (RACK1), the canonical PKC binding partner and known to drive proliferation and apoptosis, as a mediator of CD40L reverse signaling. Furthermore, we found that CD40L clustering stabilizes IFN-γ mediated Th1 polarization through STAT1, a known binding partner of RACK1. Together this highlights the importance of both CD40L forward and reverse signaling. Comparison of gene expression in murine CD4+ T cells from T cell-specific CD40L-deficient mice and wild-type mice stimulated with αCD40L or isotype control antibodies (n=3 per group)