Relative opsonic and protective activities of antibodies against K1, O and lipid A antigens of Escherichia coli.

The K1 Escherichia coli capsular antigen has been implicated as a virulence factor because of the frequency of isolation of K1 containing strains from certain invasive human infections. In the study of the interaction between K1 strains, normal human polymorphonuclear cells (PMNs) and fresh human serum, we have found varying susceptibility to phagocytosis and killing; thus, the in vitro opsonophagocytic and in vivo protective role of K1, somatic O and core glycolipid antibodies remain unclear. We have therefore examined strains of E. coli with defined susceptibility to phagocytosis by normal PMNs and sera and compared the effect of K1, somatic O and lipid A antibodies in opsonophagocytic tests and mouse protection experiments. K1 E. coli strains demonstrating relative resistance to phagocytosis and killing were effectively opsonized only with specific K1 capsular antisera. Similarly, K1 capsular antisera, but not anti-O or lipid A antisera, also provided protection in mice challenged with a LD100 of K1 E. coli that were "resistant" to phagocytosis. The ability of purified capsular antigens from Neisseria meningitidis group B and K1 "resistant" E. coli to inhibit the phagocytosis of a "sensitive" non-K1 and a K1 E. coli strain of "intermediate" susceptibility to opsonophagocytosis was also investigated. Purified K1 and group B capsular antigens were able to block specific capsular-antibody mediated opsonophagocytosis, yet these capsular antigens failed to inhibit the phagocytosis of non-K1 "sensitive" or K1 "intermediate" E. coli. These studies suggest that K1 antibodies are obligatory for the in vitro and in vivo opsonophagocytosis of "resistant" K1 E. coli and that the K1 antigen must remain in situ on the bacterial surface to exert an anti-phagocytic effect.