Leukocyte engagement of fibrin(ogen) via the integrin receptor αMβ2/Mac-1 is critical for host inflammatory response in vivo

Abstract

The leukocyte integrin α M β 2 /Mac-1 appears to support the inflammatory response through multiple ligands, but local engagement of fibrin(ogen) may be particularly important for leukocyte function. To define the biological significance of fibrin(ogen)-α M β 2 interaction in vivo, gene-targeted mice were generated in which the α M β 2 -binding motif within the fibrinogen γ chain (N 390 RLSIGE 396 ) was converted to a series of alanine residues. Mice carrying the Fibγ 390-396A allele maintained normal levels of fibrinogen, retained normal clotting function, supported platelet aggregation, and never developed spontaneous hemorrhagic events. However, the mutant fibrinogen failed to support α M β 2 -mediated adhesion of primary neutrophils, macrophages, and α M β 2 -expressing cell lines. The elimination of the α M β 2 -binding motif on fibrin(ogen) severely compromised the inflammatory response in vivo as evidenced by a dramatic impediment in leukocyte clearance of Staphylococcus aureus inoculated into the peritoneal cavity. This defect in bacterial clearance was due not to diminished leukocyte trafficking but rather to a failure to fully implement antimicrobial functions. These studies definitively demonstrate that fibrin(ogen) is a physiologically relevant ligand for α M β 2 , integrin engagement of fibrin(ogen) is critical to leukocyte function and innate immunity in vivo, and the biological importance of fibrinogen in regulating the inflammatory response can be appreciated outside of any alteration in clotting function.

A T cell-binding fragment of fibrinogen can prevent autoimmunity

“…Alanine substitutions within the Mac-1 binding site have been shown to abolish Mac-1 binding [8]. To determine if the protective effect of γC399tr is dependent on this Mac-1-binding site we mutated this site within γC399tr to alanine.…”

“…Individual amino acid residues in the NRLSIGE sequence (residues 390-396 of γC) were simultaneously substituted to alanine as previously described [8]. …”

“…The ability of γC to bind this integrin was of interest because osteopontin also binds to β3 integrins and this interaction is important in inducing Th1 immune responses [7]. In addition to the αVβ3-binding site, an αMβ2-binding site has also been described within γC [8]. This integrin is expressed on the surface of microglia and macrophages and it plays a role in their activation and migration.…”

Fibrinogen and factor XIII A-subunit genotypes interactively influence C-reactive protein levels during inflammation

“…Several aspects of inflammatory processes including chemotaxis,2 3 cell adhesion and migration4–7 as well as the antimicrobial host defence1 8–10 are dependent on or at least significantly influenced by fibrin(ogen) and its derivatives. In this context fibrin(ogen) functions as a modulator of different types of cell–cell interactions 7 11 12.…”

“…Several aspects of inflammatory processes including chemotaxis,2 3 cell adhesion and migration4–7 as well as the antimicrobial host defence1 8–10 are dependent on or at least significantly influenced by fibrin(ogen) and its derivatives. In this context fibrin(ogen) functions as a modulator of different types of cell–cell interactions 7 11 12. Finally, fibrinogen is involved in the pathogenesis of rheumatoid arthritis (RA),13–15 and it is a target of anticitrullinated protein antibodies (ACPA), which characterise a clinically more severe subset of RA 16…”