The sialylatable
lacto-N-neotetraose (
LNnT;
Gal-GlcNAc-Gal-Glc) moiety from heptose I (HepI) of the lipooligosaccharide (LOS) of Neisseria gonorrhoeae undergoes positive selection during human
infection.
Lactose (Gal-Glc) from HepII, although phase variable, is commonly expressed in humans; loss of HepII
lactose compromises gonococcal fitness in mice. Anti-LOS
monoclonal antibody (MAb) 2C7, a promising antigonococcal immunotherapeutic that elicits
complement-dependent bactericidal activity and attenuates gonococcal colonization in mice, recognizes an
epitope comprised of lactoses expressed simultaneously from HepI and HepII.
Glycan extensions beyond
lactose on HepI modulate binding and function of MAb 2C7 in vitro Here, four gonococcal LOS mutants, each with
lactose from HepII but fixed (unable to phase-vary) LOS HepI
glycans extended beyond the
lactose substitution of HepI (
lactose alone, Gal-
lactose,
LNnT, or GalNAc-
LNnT), were used to define how HepI
glycan extensions affect (i) mouse vaginal colonization and (ii) efficacy in vitro and in vivo of a human
IgG1 chimeric derivative of MAb 2C7 (2C7-Ximab) with a
complement-enhancing E-to-G Fc mutation at position 430 (2C7-Ximab-E430G). About 10-fold lower 2C7-Ximab-E430G concentrations achieved similar
complement-dependent killing of three gonococcal mutants with
glycan extensions beyond
lactose-substituted HepI (
lactose alone,
LNnT, or GalNAc-
LNnT) as 2C7-Ximab (unmodified Fc). The fourth mutant (Gal-
lactose) resisted direct
complement-dependent killing but was killed approximately 70% by 2C7-Ximab-E430G in the presence of polymorphonuclear leukocytes and
complement. Only mutants with (sialylatable)
LNnT from HepI colonized mice for >3 days, reiterating the importance of
LNnT sialylation for
infection. 2C7-Ximab-E430G significantly attenuated colonization caused by the virulent mutants.