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Rabbit alveolar macrophages contain two highly cationic microbicidal proteins. These were shown to be distinct from histones and not to arise from granulocyte contamination. The macrophage proteins were especially active against Candida albicans and Candida parapsilosis. Gram-positive bacteria (Bacillus subtilis, Listeria monocytogenes, and Streptococcus faecalis) were also susceptible, whereas Escherichia coli and Salmonella typhimurium appeared more resistant. The proteins may be present in lysosomes, based on their solubilization by dilute acids and their distribution with lysosomal markers on sucrose density gradients. Such microbicidal proteins have not previously been demonstrated in any mammalian macrophage. They may play a significant role in the host-defense functions of the rabbit lung. The function of alveolar macrophages in host defense is well documented, and numerous reports describe their microbicidal proficiency (recently reviewed by Hocking and Golde [21]). Relatively little is known concerning the microbicidal mechanisms used by alveolar macrophages (4, 23). Recent studies in our laboratory (26) demonstrated that the mechanism(s) whereby rabbit alveolar macrophages killed Candida albicans differed substantially from the myeloperoxidase-H202-mediated antifungal systems predominantly operative in human monocytes (24) and neutrophils (25). Rabbit granulocytes have been shown to contain microbicidal lysosomal cationic proteins (41-43), certain of which kill C. albicans (27). As macrophages and granulocytes are thought to arise from a common stem cell (21), and as these cells contain certain constituents in common (e.g., lysozyme), we sought the presence of microbicidal cationic proteins with candidacidal activity in rabbit alveolar macrophages. This communication describes our findings. MATERIALS AND METHODS Cell preparations and fractionation. (i) Isolation of pulmonary macrophages. Macrophages were obtained by ravaging the airways of 2-to 3-kg New Zealand white rabbits according to the procedure of Myrvik (34) with the following modifications: (i) the lungs were ravaged in situ after the animal had been sacrificed by intravenous injection of 180 mg of sodium

We purified two microbicidal cationic proteins, MCP-1 and MCP-2, from rabbit alveolar macrophages. MCP-1 was remarkably rich in arginine (25.5 mol%) and half cystine (18.7 mol%) residues and constituted approximately 1.5% of the total protein content of Freund adjuvant-elicited alveolar macrophages. MCP-2 was approximately half as abundant as MCP-1 and contained relatively less arginine (14.9 mol%) and half cystine (9.8 mol%). The amino acid compositions of MCP-1 and MCP-2 resembled those reported for the lysosomal cationic proteins of rabbit granulocytes, but were distinct from those of any known histone. MCP-i (i Lsg/ ml) killed 99.6% of Candida albicans in 20 min, whereas MCP-2 killed approximately 80% under similar conditions. Both proteins rapidly suppressed 02 consumption by C. albicans and induced a rapid loss of intracellular Rb+. Although more information is needed about the biological origin, distribution, and roles of macrophage microbicidal proteins, it seems likely that MCP-1 and MCP-2 contribute to the microbicidal efficacy of rabbit alveolar macrophages.

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