CD40 ligand binds to α5β1 integrin and triggers cell signaling

It was originally thought that the critical role of the CD40 ligand (CD40L) in normal and inflammatory immune responses was mainly mediated through its interaction with the classic receptor, CD40. However, data from CD40L(-/-) and CD40(-/-) mice suggest that the CD40L-induced inflammatory immune response involves at least one other receptor. This hypothesis is supported by the fact that CD40L stabilizes arterial thrombi through an alpha IIb beta 3-dependent mechanism. Here we provide evidence that soluble CD40L (sCD40L) binds to cells of the undifferentiated human monocytic U937 cell line in a CD40- and alpha IIb beta 3-independent manner. Binding of sCD40L to U937 cells was inhibited by anti-CD40L monoclonal antibody 5C8, anti-alpha 5 beta 1 monoclonal antibody P1D6, and soluble alpha 5 beta 1. The direct binding of sCD40L to purified alpha 5 beta 1 was confirmed in a solid phase binding assay. Binding of sCD40L to alpha 5 beta 1 was modulated by the form of alpha 5 beta 1 expressed on the cell surface as the activation of alpha 5 beta 1 by Mn2+ or dithiothreitol resulted in the loss of sCD40L binding. Moreover, sCD40L induced the translocation of alpha 5 beta 1 to the Triton X-100-insoluble fraction of U937 cells, the rapid activation of the MAPK pathways ERK1/2, and interleukin-8 gene expression. The binding of sCD40L to CD40 on BJAB cells, an alpha 5 beta 1-negative B cell line, and the resulting activation of ERK1/2 was not inhibited by soluble alpha 5 beta 1, suggesting that sCD40L can bind concomitantly to both receptors. These results document the existence of novel CD40L-dependent pathways of physiological relevance for cells expressing multiple receptors (CD40, alpha 5 beta 1, and alpha IIb beta 3) for CD40L.