Role for C5 and Neutrophils in the Pulmonary Intravascular Clearance of Circulating Cryptococcus neoformans

Although C5a-induced intravascular pulmonary sequestration of neutrophils has been investigated with regards to lung injury, relatively few studies have addressed the possible role for this mechanism in the intravascular clearance of circulating microorganisms. A murine model was used in which the complement-fixing, encapsulated yeast Cryptococcus neoformans (Cne) was inoculated intravenously (IV), and lung clearance of the organism was measured 24 h later. In normal mice, clearance was remarkably effective, but in leukocyte-depleted or C5-deficient (C5-) animals, clearance was significantly decreased. In vitro assays indicated that C5 was necessary for neutrophils to kill encapsulated Cne and evidence was obtained that C5a was involved. In vivo studies using light and electron microscopy demonstrated that 30 min after an IV inoculation of encapsulated yeast into C5-sufficient (C5+) mice, neutrophils accumulated in pulmonary vessels and engulfed Cne. However, in C5- mice, neutrophils failed to accumulate in pulmonary vessels and there was no endocytosis of encapsulated yeasts. These studies suggested that following Cne interaction with complement in the blood, release of C5a activated circulating neutrophils to adhere to Cne, and perhaps to adjacent endothelium, which facilitated rapid phagocytosis and killing of the organism. In contrast to the IV infection model, when Cne was inoculated into the tracheas of C5+ and C5- mice, no evidence was obtained for an early PMN-C5-dependent clearance mechanism. C5a-dependent neutrophil killing in the lung vasculature may provide important host protection against Cne during vascular dissemination and perhaps against other disseminating microorganisms that activate complement.