Anti-capsular antibodies activate killing of <i>Escherichia coli </i>O8:K87 by the alternate complement pathway in porcine serum

Enterotoxigenic Escherichia coli (ETEC) strains that produce K88 (F4)+ fi mbria are important causes of diarrhea and post-diarrheal septicemia in swine. ETEC O8:K87, a serotype represented by a number of these strains, is typically serum resistant. Strain-specifi c antibodies are known to activate alternative C pathway-mediated killing of other serum-resistant E. coli [Hill, A.W., Shears, A.L., Hibbitt, K.G., 1978. The requirement of specifi c antibody for the killing of E. coli by the alternate complement pathway in bovine serum. Immunology 34, 131–136], but their antigenic targets have not been determined. We tested the hypothesis that anti-K87 antibodies activate alternative pathway-mediated killing of ETEC O8: K87. Pigs were immunized with ETEC O8:K87 strain 2534-86 cells or purifi ed K87 polysaccharide. Post-, but not pre-immunization sera killed 2534-86 cells, and absorption with 2534-86 cells or by K87 affi nity chromatography eliminated bactericidal activity. Complementation of absorbed serum with anti-K87 antibodies restored bactericidal activity, confi rming the ability of these antibodies to activate C-mediated serum killing. Serum from age-matched, non-vaccinated control pigs also killed 2534-86. This activity was eliminated by absorption with 2534-86 cells, but not K87 affi nity chromatography, indicating that specifi c non-capsular antibodies are also able to activate C-mediated killing. In all cases, Mg-EGTA-treated serum was as effective as non-treated serum in killing, suggesting that bactericidal activity was mediated predominantly if not exclusively via the alternative C pathway.