Antibodies against bacterial membrane proteins

Abstract

Prior studies have shown that antiserum raised to the Escherichia coli J5 vaccine binds to three bacterial outer membrane proteins (OMPs) that are present on serum-exposed smooth Enterobacteriaceae and in LPS-containing fragments that are released by bacteria incubated in human serum. The present studies were performed to further analyze release of the three OMPs into human serum. Normal human serum was incubated with E. coli O18K+ bacteria and then filtered to remove intact bacteria. Lipopolysaccharide (LPS) was affinity-purified from sterile filtrates using murine monoclonal IgG directed against the O-polysaccharide chain of E. coli O18 LPS (monoclonal anti-O IgG). Components of sterile filtrates were also separated by ultracentrifugation on sequential potassium bromide and cesium chloride gradients. Samples were analyzed by immunoblotting, using anti-J5 IgG and polyclonal anti-O IgG as primary antibodies. Three OMPs, of MWs 5—9, 18, and 35 kDa were detected in samples that were affinity-purified using monoclonal anti-O IgG, indicating that complexes containing at least these three OMPs and LPS (OMP/LPS complexes) are released into human serum. The 18 kDa OMP was detected in low density fractions of the density gradients, indicating that this OMP is also released separately from the OMP/LPS complexes in a form that floats at the same density as serum lipoproteins.