Localization of macrophage agglutination factor activity to the gelatin-binding domain of fibronectin

We previously proposed that macrophage agglutination factor (MAggF, a T cell-derived guinea pig lymphokine) is a fibronectin (FN). We now show MAggF binding to gelatin and to peritoneal macrophages is mediated by domains similar to corresponding domains of plasma FN. MAggF activity in lymphokine concentrates prepared by two different methods differed nearly 10-fold in m.w. on gel filtration chromatography. Despite this difference, MAggF dose-activity curves of both preparations were parallel, and MAggF in both preparations bound reversibly to gelatin and to monoclonal anti-guinea pig FN immunoadsorbents. MAggF activity in one preparation was inhibited by the addition of soluble monoclonal antibody specific for the gelatin-binding domain of human FN; inhibitory activity of this antibody was blocked by purified guinea pig plasma FN or partially purified MAggF from the other preparation. Measured MAggF activity of both preparations was reduced in a dose-dependent manner by pretreatment of indicator macrophages with monoclonal anti-human monocyte FN receptor antibody or F(ab')2 fragments or with guinea pig plasma FN. Neither anti-FN receptor antibody nor plasma FN interacted directly with MAggF. Indirect immunofluorescence studies confirmed the presence of uncomplexed plasma membrane receptors for FN on indicator macrophages in MAggF-responsive populations that were able to bind added FN. Our identification of MAggF as lymphokine FN provides a basis for future biochemical analysis of delayed hypersensitivity inflammatory reactions.