Interaction of macrophage cationic proteins with the outer membrane of Pseudomonas aeruginosa

The interaction of the polycationic rabbit alveolar macrophage cationic proteins MCP-1 and MCP-2 (or their identical neutrophil equivalents NP-1 and NP-2) with the surface of Pseudomonas aeruginosa was investigated. Both proteins bound avidly to purified lipopolysaccharide, as judged by their ability to competitively displace the probe dansyl polymyxin with 50% inhibition (I50) values of 2 to 3 microM. Similar I50 were measured with dansyl polymyxin as a probe for cell surface binding, suggesting that the initial binding site for MCP-1 and MCP-2 on the surface of cells was lipopolysaccharide. Both MCP-1 and MCP-2 permeabilized outer membranes to the hydrophobic fluorescent probe 1-N-phenylnaphthylamine (NPN). The initial rate of NPN uptake plotted against the concentration of MCP-1 or MCP-2 gave sigmoidal curves, suggesting cooperative permeabilization of the outer membrane. Replotting the data as a Hill plot gave an affinity parameter, S0.5, the concentration of MCP giving a half-maximal increase in the rate of NPN uptake, of 5 and 25 microM for MCP-1 and MCP-2, respectively, and thus subsequent studies concentrated on the more active permeabilizer MCP-1. Permeabilization of outer membranes to NPN was a function of buffer pH, with lower pH considerably favoring the permeabilizing effects of MCP-1. Thin-section electron microscopic visualization of MCP-1-treated cells showed production of extended blebs. Further evidence of an altered cell surface after MCP-1 treatment was obtained by demonstrating that treated unopsonized cells were more efficiently phagocytosed by unelicited rabbit alveolar macrophages. The data overall suggest that macrophage cationic proteins interact with the P. aeruginosa outer membrane in a manner typical of other polycations and suggest that one of their major functions may be to permeabilize the outer membrane.