Role of resident macrophages, peripheral neutrophils, and translymphatic absorption in bacterial clearance from the peritoneal cavity

Microbial pathogens within the peritoneal cavity are thought to encounter three categories of host defense mechanisms: (1) removal mechanisms, which occur via diaphragmatic lymphatic absorption; (ii) killing mechanisms, in which host phagocytes act as effector cells; and (iii) sequestration mechanisms due to fibrin trapping and the formation of adhesions between visceral surfaces. We sought to define and quantitate the relative role of the first two components in an experimental rat model of Escherichia coli peritonitis in which fibrinous adhesions do not form. Intraperitoneal challenge with ≥ 2 x 10⁸ CFU of viable E. coli led to an initial decline in bacterial numbers followed by ongoing proliferation and > 50% mortality. With inocula of ≤ 5 x 10⁷ CFU, elimination of bacteria occurred after moderate initial proliferation, and no mortality ensued. Nonviable, radiolabeled E. coli organisms were utilized to examine bacterial clearance via translymphatic absorption and phagocytosis. Both processes were extremely rapid, serving to eliminate free bacteria rapidly within the peritoneal cavity. Although macrophages and polymorphonuclear leukocytes within the peritoneal cavity demonstrated similar phagocytic capacities, the predominance of macrophages at the time of the initial bacterial insult led to the conclusion that these cells, in addition to translymphatic absorption, represent the first line of host defenses, acting to eliminate bacteria in the incipient stages of infection.