Augmentation of phagocytosis by a specific fibronectin fragment that links particulate activators to the fibronectin adherence receptor of human monocytes.

Intact human plasma fibronectin of 44,000 m.w. and a fibronectin fragment of 180,000 m.w. promote dose-dependent adherence of gelatin-coated particles to human monocytes without phagocytosis. Both of these proteins, however, augment monocyte ingestion of gelatin-coated targets that are particulate activators of the alternative complement pathway or of nonactivators bearing IgG. Unlike intact fibronectin, the 180,000 m.w. fragment also binds directly to particulate activators that lack gelatin to augment their phagocytosis by human monocytes. Prior attachment to monocytes of gelatin-coated sheep erythrocytes bearing increasing concentrations of intact fibronectin decreases in a dose-dependent fashion the capacity of these monocytes to engage in augmented phagocytosis of particulate activators opsonized with the 180,000 m.w. fibronectin. Occupation of the monocyte fibronectin receptors with particle-bound, intact fibronectin does not decrease monocyte phagocytosis of plain particulate activators or of IgG-coated particles. Thus, the 180,000 m.w. fibronectin fragment both directly opsonizes particulate activators and interacts with monocyte fibronectin receptors to promote particle adherence, thereby enhancing phagocytosis through a concerted action with the distinct receptors for particulate activators.