Human IgG but not IgM antibodies can protect mice from the challenge with live O6 Escherichia coli

We evaluated the ability of human anti-lipopolysaccharide (LPS) O6 immunoglobulin G (IgG) and IgM antibodies to protect mice challenged with Escherichia coli serotype O6:K2ac. Purified whole IgG, commercial gammaglobulin, whole IgM-effluent, pool of normal human serum (NHS), agammaglobulinaemic serum (test groups) or phosphate-buffered saline (control group) was injected into adult male 18 h before a challenge with viable O6 E. coli. The mortality rate was assessed over a period of 72 h. To determine the opsonic and phagocytic activity of the antibody isotypes, we incubated peritoneal macrophages from the control and test groups collected at different times after challenge with the live bacteria with acridine orange for fluorescent analysis. Tumour necrosis factor (TNF)-alpha and interleukin (IL)-6 were quantified in serum of both the test and control groups. All mice that received commercial gammaglobulin or NHS survived. Purified whole IgG (containing 1.1 mg/l of anti-LPS O6 IgG antibodies) protected 87.5% of the animals tested in this experiment, while whole IgM-enriched effluent with 1.5 mg/l of anti-LPS O6 IgM antibodies protected only 12.5%. The agamma serum showed no protective capacity compared with PBS (serving as control). The minimal concentration of anti-LPS O6 IgG antibodies able to protect 50% of animals was 0.137 mg/l of purified whole IgG. Whole IgM-enriched effluent showed no protective capacity independently of the concentration tested (0.048-17.0 mg/l of anti-LPS O6 IgM antibodies). Fluorescent analysis of peritoneal macrophages from animals pretreated with purified whole IgG showed no bacteria at 8 h after the challenge. By contrast, whole IgM effluent showed an increasing number of live bacteria at the same time. Mice that had received whole IgM effluent (1.5 mg/l of anti-LPS O6 IgM antibodies) before the challenge with LPS O6 presented 20.5 microg/l of IL-6 and 1.5 microg/l of TNF-alpha. Serum from animals pretreated with purified IgG did not present any detectable pro-inflammatory cytokine. Our findings suggest that IgG but not IgM antibodies protect animals from a challenge with E. coli O6 serotype.