The alternatively spliced αEC domain of human fibrinogen-420 is a novel ligand for leukocyte integrins αMβ2 and αXβ2

The interaction of human plasma fibrinogen with leukocyte integrins alpha (M)beta (2) (CD11b/ CD18, Mac-1) and alpha (X)beta (2) (CD11c/CD18, p150,95) is an important component of the inflammatory response. Previously, it was demonstrated that binding of fibrinogen to these integrins is mediated by gammaC, the globular C-terminal domain of the gamma chain. In this study, evidence was found of another fibrinogen domain that can serve as a ligand for the 2 leukocyte integrins: alpha C-E, a homologous domain that extends the a chains in a recently discovered subclass of fibrinogen known as fibrinogen-420. Recombinant alpha C-E supported strong adhesion and migration of cells expressing alpha (M)beta (2) and alpha (x)beta (2), including nonactivated and activated U937 and THP-1 monocytoid cells, and neutrophils. Cells transfected with complementary DNA for these integrins also bound alpha C-E. The specificity of interaction was substantiated by inhibition of cell adhesion with antibodies against alpha (M), alpha (x), and beta (2) subunits. Also, neutrophil inhibitory factor, a specific inhibitor of alpha (M)beta (2) and alpha (x)beta (2) function, efficiently blocked cell adhesion to alpha C-E. In alpha (M)beta (2) and alpha (x)beta (2), the I domain is the binding site for alpha C-E, since alpha C-E bound to recombinant alpha (M) I and alpha I-x domains in a dose-dependent and saturable manner. Synthetic peptides that duplicated sequences gamma 190 to 202 and gamma 377 to 395, previously considered putative binding sites in gammaC, effectively inhibited alpha (M)beta (2)- and alpha (x)beta (2)-mediated adhesion to alpha C-E, suggesting that recognition of alpha C-E by the I domain involves structural features in common with those of gammaC. These findings identify alpha C-E as a second domain in fibrinogen-420 that binds alpha (M)beta (2) and alpha (x)beta (2) and can mediate leukocyte adhesion and migration. (C) 2001 by The American Society of Hematology.