Immunological characterization of antibody responses to Escherichia coli O antigens in bacteraemic patients

Abstract

Sera from 15 patients with Escherichia coli bacteraemia were analyzed using ELISA. Antibody titers to eight different E. coli O serotype lipopolysaccharide antigens were studied. In 11 15 sera, a significant increase in antibody titer to the infecting O serotype was noted. Five of these sera also showed an antibody response to one or more of the heterologous antigens. In sera from the remaining four patients, two exhibited high, but not increasing, antibody titers to the homologous antigen; however, one was also reactive with antibodies to heterologous antigens. Sera from one patient showed no antibody increase to any of the tested antigens. The remaining patient showed an antibody response to heterologous antigens only. In all sera, the antibody binding to LPS, homologous with the infecting O serotype, could be inhibited by purified homologous O polysaccharide. Using the same inhibitors, decreased antibody binding to heterologous coating antigens was observed with six sera. Four sera were tested using heterologous polysaccharides as inhibitors, and in no case was inhibition of antibody binding noted to the LPS homologous with the O serotype of the infecting E. coli strain. On the other hand, a significant inhibition was observed using these polysaccharides, when assayed against O antigen LPS, homologous as well as heterologous with the polysaccharide inhibitor. The apparent “cross-reactivity” observed in the patient's sera may be due to several factors. Bacteraemic infections may induce a polyclonal B-cell activation, which may trigger an anamnestic response of antibodies against other E. coli O serotypes. Furthermore, the patient might be infected with more than one E. coli strain of different O serotypes. It is also possible that common antigenic epitope(s) exist within the core and the O antigen polysaccharide.