The contrasting mechanisms of serum resistance of Neisseria gonorrhoeae and group B Neisseria meningitidis

Neisseria gonorrhoeae and Neisseria meningitidis have evolved intricate mec hanisms to evade complement-mediated killing. Sialylation of gonococcal lip ooligosaccharide (LOS) results in conversion of previously serum sensitive strains to unstable serum resistance, which is mediated by factor H binding . Porin (Por) is also instrumental in mediating stable serum resistance in gonococci. The 5th loop of certain gonococcal Por1As binds factor H, which efficiently inactivates C3b to iC3b. Factor H glycan residues may be essent ial for factor H binding to certain Por1A strains. Por1A strains can also r egulate the classical pathway by binding to C4b-binding protein (C4bp) prob ably via the Ist loop of the For molecule. Certain serum resistant Por1B st rains can also regulate complement by binding C4bp through a loop other tha n loop 1. Purified C4b can inhibit binding of C4bp to For 1B, but not Por1A , suggesting different binding sites on C4bp for the two For types. Unlike serum resistant gonococci, resistant meningococci have abundant C3b on thei r surface, which is only partially processed to iC3b. The main mechanism of complement evasion by group B meningococci is inhibition of membrane attac k complex (MAC) insertion by their polysaccharide capsule. LOS structure ma y act in concert with capsule to prevent MAC insertion. Meningococcal strai ns with Class 3 For preferentially bind factor H, suggesting Class 3 For ac ts as a receptor for factor H. (C) 1999 Elsevier Science Ltd, All rights re served.