Antibody response to Escherichia coli O antigens and influence of the protein moiety of the endotoxin.

Exposure of mice to different serotypes of bacteria, either O4 or O6, resulted in an enhanced indirect IgG-PFC response to the alternate bacteria. This effect seemed to be mediated by a protein connected with the endotoxin structure. This protein moiety had some weak adjuvant activities and increased the antibody response against sheep red blood cell about two-fold. This effect was not likely to be due to any contamination with the B-cell mitogen lipid A, a constituent of the endotoxin from which the protein was isolated. In addition, experiments were performed in which irradiated spleen cells (0–400 R) from mice injected with O4 bacteria were transferred to irradiated (800 R) recipients together with O6 bacteria. Decreasing numbers of antibody forming cells with increasing irradiation dose were found. The parallel experiment employing O6 bacteria for both primary and secondary antigen injections revealed an increased immune response for an irradiation dose of 50 R, showing that suppressor cells are more irradiation sensitive than the other cells involved in this immune response, but that the effect of such cells is possibly overcome by the influence of the protein residue isolated from endotoxin. A secondary response to O6 bacteria was also noted in agreement with previous results. It was found that this immune response could be reduced drastically by injecting primed thymocytes, simultaneously with the second injection.