The (alpha 2 -> 8)-linked polysialic acid capsule and lipooligosaccharide structure both contribute to the ability of serogroup B Neisseria meningitidis to resist the bactericidal activity of normal human serum

The molecular basis for the resistance of serogroup B Neisseria meningitidi s to the bactericidal activity of normal human sera (NHS) was examined with a NHS-resistant, invasive serogroup B meningococcal isolate and geneticall y and structurally defined capsule-, lipooligosaccharide (LOS)-, and sialyl ation-altered mutants of the wild-type strain. Expression of the (alpha 2-- >8)-linked polysialic acid serogroup B capsule aas essential for meningococ cal resistance to NHS. The very NHS-sensitive phenotype of acapsular mutant s (99.9 to 100% killed in 10, 25, and 50% NHS) was not rescued by complete LOS sialylation or changes in LOS structure. However, expression of the cap sule was necessary but not sufficient for a fully NHS-resistant phenotype. In an encapsulated background, loss of LOS sialylation by interrupting the alpha 2,3 sialyltransferase gene, ist, increased sensitivity to 50% NHS. In contrast, replacement of the lacto-N-neotetraose alpha-chain (Gal beta 1-4 GlcNAc beta 1-3Gal beta 1-4Glc) with glucose extensions (Glc(N)) in a galE mutant resulted in a strain resistant to killing by 50% NHS at all time poi nts. Encapsulated meningococci expressing a Hep(2)(GlcNAc)-->KDO2 --> lipid A LOS without an alpha-chain demonstrated enhanced sensitivity to 50% NHS (98% killed at 30 min) mediated through the antibody-dependent classical co mplement pathway. Encapsulated LOS mutants expressing truncated Hep(2) --> KDO2 --> lipid A and KDO2 --> lipid A structures were also sensitive to 50% NHS (98 to 100% killed at 30 min) but, unlike the wild-type strain and mut ants with larger oligosaccharide structures, they were killed by hypogammag lobulinemic sera. These data indicate that encapsulation is essential but t hat the LOS structure contributes to the ability of serogroup B N. meningit idis to resist the bactericidal activity of NHS.