Regulation of Integrin Function by CD47 Ligands

We examined the regulation of α 4 β 1 integrin function in melanoma cells and T cells by ligands of CD47. A CD47 antibody (B6H12) that inhibited α v β 3 -mediated adhesion of melanoma cells induced by CD47-binding peptides from thrombospondin-1 directly stimulated α 4 β 1 -mediated adhesion of the same cells to vascular cell adhesion molecule-1 and N-terminal regions of thrombospondin-1 or thrombospondin-2. B6H12 also stimulated α 4 β 1 - as well as α 2 β 1 - and α 5 β 1 -mediated adhesion of CD47-expressing T cells but not of CD47-deficient T cells. α 4 β 1 and CD47 co-purified as a detergent-stable complex on a CD47 antibody affinity column. CD47-binding peptides based on C-terminal sequences of thrombospondin-1 also specifically enhanced adhesion of melanoma cells and T cells to α 4 β 1 ligands. Unexpectedly, activation of α 4 β 1 function by the thrombospondin-1 CD47-binding peptides also occurred in CD47-deficient T cells. CD47-independent activation of α 4 β 1 required the Val-Val-Met (VVM) motif of the peptides and was sensitive to inhibition by pertussis toxin. These results indicate that activation of α 4 β 1 by the CD47 antibody B6H12 and by VVM peptides occurs by different mechanisms. The antibody directly activates a CD47-α 4 β 1 complex, whereas VVM peptides may target an unidentified G i -linked receptor that regulates α 4 β 1 .