Complement (C3)-Activated Phagocytosis by Lung Macrophages

Abstract

We have investigated the interaction between rabbit lung macrophages and Escherichia coli lipopolysaccharide-coated oil droplets opsonized with a fragment of C3, and of albumin-coated oil droplets opsonized with anti-albumin IgG. We also examined in detail C3-dependent, IgG-activated, and nonspecific phagocytosis by trypsinized rabbit lung macrophages. We found that: 1) there was no basal ingestion of the LPS particles by macrophages, and reaction of particles with heated serum or anti-LPS antibody failed to elicit ingestion. Only fresh serum (deposition of C3) caused LPS particles to be ingested by macrophages. C3 alone, therefore, is the activator of phagocytosis in this system. 2) The ingestion of C3-coated particles occurred at a rapid constant rate for 2 min, followed by a slower constant rate; the ingestion of IgG-coated particles occurred at a single constant rate. 3) The initial rapid phase of C3-mediated ingestion was dependent upon the presence of divalent cations in the incubation medium; the secondary rate of ingestion of C3-coated particles and the ingestion of IgG-coated particles were not dependent on divalent cations. 4) Heated serum, which contains enzymes that destroy opsonically active C3 on particles, diminished the ingestibility of C3-coated particles, but did not alter the biphasic kinetics of the ingestion mechanism. Although heated serum decreased the ingestibility of C3-coated particles by rabbit exudate polymorphonuclear leukocytes more rapidly than by macrophages, the ultimate loss of ingestibility was equivalent for both cell types. 5) Trypsin rapidly diminished, but did not abolish, C3-activated phagocytosis without affecting IgG-activated or nonspecific ingestion. The principal effect of trypsin was to abolish the initial rapid phase of ingestion. 6) After trypsinization, the macrophages could recover trypsin-sensitive functions. Recovery was dependent upon: a) time, 4 hr being required for complete recovery; b) temperature, with recovery occurring at 37°C but not at 0°C; c) exogenous serum protein (serum or albumin); and d) endogenous protein synthesis (cycloheximide inhibited recovery). The role of serum protein was independent of its lipid-carrying capacity. Drugs thought to influence secretion by acting on cytoplasmic microtubules (colchicine) or which inhibit secretion of a macrophage protease (dexamethasone) had no effect on recovery. C3-mediated ingestion and IgG-mediated ingestion by lung macrophages differ in their kinetic patterns. Trypsin treatment of macrophages and removal of divalent cations from the medium affect C3-mediated ingestion primarily by altering its kinetics. They exert their effects during an initial rapid phase of ingestion which is not seen in IgG-mediated ingestion. The effect of trypsin on macrophages is reversible with time under appropriate conditions.