BriefDefinitive Report FIBRONECTIN AND SERUM AMYLOID

Receptors for C3 exist in two states, an active state in which the receptors promote phagocytosis and an inactive state in which they do not . For example, cultured human monocytes (MO) bear separate receptors that bind C3band C3bi-coated erythrocytes, but neither receptor promotes phagocytosis of such C3-coated erythrocytes (1) . Brief stimulation ofthe MO with the tumor promoter phorbol myristate acetate (PMA), however, renders both C3 receptors competent to generate a phagocytec response . Similarly, C3 receptors of resident murine peritoneal macrophages do not promote phagocytosis but are activated when these cells are incubated with a specific lymphokine (2) . We have been unable to identify a comparable human lymphokine, and therefore sought other molecules that might represent physiological regulators of C3 receptor activity in human mononuclear phagocytes . Recenty, Pommier et al . (3) reported that soluble fibronectin (FN) activates C3b receptors of freshly isolated human monocytes . Other investigators, however, failed to observe this effect (4, 5) . While soluble FN may cause modest activation of C3 receptors on freshly explanted blood monocytes (3), we report here that substrate-bound FN as well as substrate-bound serum amyloid P component (SAP) cause a marked activation of the C3b and the C3bi receptors of MO and that this activation of C3 receptor activity occurs by a novel mechanism : interaction of substrate-bound FN or SAP with the basal portion of the MO plasma membrane activates C3 receptors on the apical portion of the MO plasma membrane .