Functional Interaction of CD154 Protein with α5β1 Integrin Is Totally Independent from Its Binding to αIIbβ3 Integrin and CD40 Molecules

In addition to its classical CD40 receptor, CD154 also binds to alpha IIb beta 3, alpha 5 beta 1, and alpha M beta 2 integrins. Binding of CD154 to these receptors seems to play a key role in the pathogenic processes of chronic inflammation. This investigation was aimed at analyzing the functional interaction of CD154 with CD40, alpha IIb beta 3, and alpha 5 beta 1 receptors. We found that the binding affinity of CD154 for alpha IIb beta 3 is similar to 4-fold higher than for alpha 5 beta 1. We also describe the generation of sCD154 mutants that lost their ability to bind CD40 or alpha IIb beta 3 and show that CD154 residues involved in its binding to CD40 or alpha IIb beta 3 are distinct from those implicated in its interaction to alpha 5 beta 1, suggesting that sCD154 may bind simultaneously to different receptors. Indeed, sCD154 can bind simultaneously to CD40 and alpha 5 beta 1 and biologically activate human monocytic U937 cells expressing both receptors. The simultaneous engagement of CD40 and alpha 5 beta 1 activates the mitogen-activated protein kinases, p38, and extracellular signal-related kinases 1/2 and synergizes in the release of inflammatory mediators MMP-2 and -9, suggesting a cross-talk between these receptors.