Modulation of effects of lipopolysaccharide on macrophages by a major outer membrane protein of Proteus mirabilis as measured in a chemiluminescence assay.

Our previous studies have shown that a major protein isolated from purified cell walls of Proteus mirabilis (39-kDa protein) is a strong modulator of the specific immune responses to lipopolysaccharide (LPS) from this bacterium. When the protein is mixed with LPS before immunization of mice, the responses of antibody-producing cells specific for LPS are greatly enhanced and converted predominantly to the immunoglobulin G isotype. In the present study, the immunomodulating effects of the 39-kDa protein were tested at the level of interaction of LPS with macrophages. Activation of macrophages was determined by measuring the production of oxygen radicals in a chemiluminescence assay with lucigenin as the amplifier. LPS from P. mirabilis induced strong oxidative metabolism in both peritoneal and bone marrow-derived murine macrophages. These responses were inhibited in a dose-dependent manner by mixing LPS with increasing amounts of the protein. In contrast, bovine serum albumin and methylated bovine serum albumin enhanced the response of macrophages dramatically when complexed with LPS. The inhibiting activity of the 39-kDa protein was also observed with LPS from Escherichia coli K-12.