Antibodies to O-antigen of lipopolysaccharide are protective against neonatal infection with Escherichia coli K1.

Monoclonal IgM specific for the O18 antigen conferred passive protection to 1-week-old rats against bacteremia and killing after oral challenge with O18:K1 Escherichia coli. Specific protection of the pups was also achieved by immunizing the pregnant rats with purified O18 lipopolysaccharide. We suppose that most human newborns that are colonized by potentially invasive K1 E. coli are protected by the transplacental transfer of anti-lipopolysaccharide immunoglobulin G, and we suggest that treatment with such antibodies might in the future be considered a therapeutic option. Rat serum from 1-week-old animals had only about one-third of the complement hemolytic activity of adult rat serum. This low level of hemolytic activity correlated with a relatively poor bactericidal activity in antibody-dependent and antibody-independent bactericidal in vitro assays. Monoclonal anti-O18 immunoglobulin M, although protective in vivo and bactericidal when added to adult rat serum, only poorly inhibited the multiplication of O18:K1 cells in serum from 1-week-old rats. This suggests that other elements of host defense besides complement participate in antibody-mediated in vivo protection.