Cross-reactivity of rabbit antibodies to lipopolysaccharides of Escherichia coli J5 and other gram-negative bacteria.

Antiserum to rough gram-negative mutants such as Escherichia coli J5 and Salmonella minnesota Re595 is thought to neutralize the toxic effects of lipopolysaccharides (LPSs). To verify that such antisera are capable of binding heterologous endotoxins, we examined IgG and IgM class antibodies induced in rabbits to a variety of LPSs. Immunization with rough mutants or lipid A induced high IgG antibody responses to the homologous purified LPS and relatively low but significant responses to heterologous LPSs. Increases in IgM antibodies were also primarily to homologous LPS. Immunization with smooth organisms induced little or no antibody to heterologous LPSs. Soluble LPS, outer membrane vesicles, and whole bacteria produced strong homologous inhibition but little or no heterologous inhibition in enzyme-linked immunosorbent assays. Cross-adsorption of antisera to rough mutants suggested that the IgG and IgM antibodies induced to heterologous LPS were adsorbed by the heterologous LPS and not by the core LPS used to immunize the animals. Rabbit antibody directed to J5 or Re595 LPS fails to bind to any substantial degree to heterologous LPS. Immunization with whole bacterial vaccines, particularly the rough mutants and lipid A, does increase antibody to a wide variety of antigens. The possibilities that the protective effects of antisera to rough mutants are due to a polyclonal antibody response or to the induction of as yet unidentified factor(s) deserve further investigation.