Integrin Binding to the Trimeric Interface of CD40L Plays a Critical Role in CD40/CD40L Signaling

摘要

CD40L plays a major role in immune response and is a major therapeutic target for inflammation. Integrin alpha 5 beta 1 and CD40 simultaneously bind to CD40L. It is unclear if alpha 5 beta 1 and CD40 work together in CD40/CD40L signaling or how alpha 5 beta 1 binds to CD40L. In this article, we describe that the integrin-binding site of human CD40L is predicted to be located in the trimeric interface by docking simulation. Mutations in the predicted integrin-binding site markedly reduced the binding of alpha 5 beta 1 to CD40L. Several CD40L mutants defective in integrin binding were defective in NF-kappa B activation and B cell activation and suppressed CD40L signaling induced by wild-type CD40L; however, they still bound to CD40. These findings suggest that integrin alpha 5 beta 1 binds to monomeric CD40L through the binding site in the trimeric interface of CD40L, and this plays a critical role in CD40/CD40L signaling. Integrin alpha nu beta 3, a widely distributed vascular integrin, bound to CD40L in a KGD-independent manner, suggesting that alpha nu beta 3 is a new CD40L receptor. Several missense mutations in CD40L that induce immunodeficiency with hyper-IgM syndrome type 1 (HIGM1) are clustered in the integrin-binding site of the trimeric interface. These HIGM1 CD40L mutants were defective in binding to alpha 5 beta 1 and alpha nu beta 3 (but not to CD40), suggesting that the defect in integrin binding may be a causal factor of HIGM1. These findings suggest that alpha 5 beta 1 and alpha nu beta 3 bind to the overlapping binding site in the trimeric interface of monomeric CD40L and generate integrin-CD40L-CD40 ternary complex. CD40L mutants defective in integrins have potential as antagonists of CD40/CD40L signaling.