Macrophage migration is dependent on the fibrinogen α_Mβ₂-binding motif but not on factor XIII–mediated fibrin crosslinking. Mice expressing a form of fibrinogen that is unable to interact with the integrin α_Mβ₂ display increased total leukocyte counts (A) and macrophage (B) migration in sterile peritonitis. This increased migration is not seen in mice that are unable to crosslink fibrin (FXIII^−/−). (C) In vitro migration of macrophages through fibrin was examined in the presence or absence of factor XIIIa. In the absence of plasminogen, macrophages were unable to migrate through a fibrin matrix regardless of the degree of factor XIIIa-mediated crosslinking (lanes A-D). In contrast, in the presence of plasminogen, macrophages are able to migrate through fibrin regardless of the degree of factor XIIIa–mediated crosslinking (lanes E-H).