CD40L acts as an allosteric activator of integrins for signal transduction independent of inside-out signaling

Abstract

Abstract CD40L plays a major role in immune response and is a target for inflammatory disease therapy. Besides CD40, CD40L binds to several integrins but their role in signaling is unclear. We showed that integrins αvβ3 and α5β1 bind to the CD40L trimeric interface through classical ligand binding site of integrins (site 1). Several CD40L mutants from HIGM1 (hyper-IgM syndrome type 1) patients were clustered in trimeric interface and defective in integrin binding, and in NF-κB and B cell activation, but still bound CD40 and acted as antagonists of CD40L signaling. Thus, integrins play a critical role in CD40L signaling. Previous studies showed that CX3CL1 and sPLA2-IIA can activate integrins by binding to a recently identified allosteric site (site 2) of integrins. Using cell-free assays, we found that soluble (s)-CD40L activated soluble integrins, α4β1, αvβ3 and α5β1, and induced their binding to known specific ligands independent of inside-out signaling. Also, sCD40L activated cell-surface integrins in CHO cells lacking CD40. Finally, sCD40L bound to a cyclic peptide derived from site 2. These findings indicate that CD40L acts as an allosteric activator of integrins by binding to site 2. Docking simulation predicted that the integrin site 2-binding site of CD40L is located outside of CD40L trimer. Importantly, four HIGM1 mutations are clustered in the predicted site 2-binding site of CD40L. The K143T and G144E mutants were the most defective in integrin activation, indicating that integrin binding occurs around these residues of the predicted site 2-binding site. We propose that allosteric integrin activation by CD40L plays a role in CD40L signaling and that defective integrin site 2-binding may be causal for defective CD40L signaling in HIGM1.