Kinetics of conversion of Neisseria gonorrhoeae to resistance to complement by cytidine 5'-monophospho-N-acetyl neuraminic acid

Anti-Virulence Therapeutic Approaches for Neisseria gonorrhoeae

While antimicrobial resistance (AMR) is seen in both Neisseria gonorrhoeae and Neisseria meningitidis, the former has become resistant to commonly available over-the-counter antibiotic treatments. It is imperative then to develop new therapies that combat current AMR isolates whilst also circumventing the pathways leading to the development of AMR. This review highlights the growing research interest in developing anti-virulence therapies (AVTs) which are directed towards inhibiting virulence factors to prevent infection. By targeting virulence factors that are not essential for gonococcal survival, it is hypothesized that this will impart a smaller selective pressure for the emergence of resistance in the pathogen and in the microbiome, thus avoiding AMR development to the anti-infective. This review summates the current basis of numerous anti-virulence strategies being explored for N. gonorrhoeae.

Gonococcal lipooligosaccharide sialylation: virulence factor and target for novel immunotherapeutics

Gonorrhea has become resistant to most conventional antimicrobials used in clinical practice. The global spread of multidrug-resistant isolates of Neisseria gonorrhoeae could lead to an era of untreatable gonorrhea. New therapeutic modalities with novel mechanisms of action that do not lend themselves to the development of resistance are urgently needed. Gonococcal lipooligosaccharide (LOS) sialylation is critical for complement resistance and for establishing infection in humans and experimental mouse models. Here we describe two immunotherapeutic approaches that target LOS sialic acid: (i) a fusion protein that comprises the region in the complement inhibitor factor H (FH) that binds to sialylated gonococci and IgG Fc (FH/Fc fusion protein) and (ii) analogs of sialic acid that are incorporated into LOS but fail to protect the bacterium against killing. Both molecules showed efficacy in the mouse vaginal colonization model of gonorrhea and may represent promising immunotherapeutic approaches to target multidrug-resistant isolates. Disabling key gonococcal virulence mechanisms is an effective therapeutic strategy because the reduction of virulence is likely to be accompanied by a loss of fitness, rapid elimination by host immunity and consequently, decreased transmission.

Utilizing complement evasion strategies to design complement-based antibacterial immunotherapeutics: Lessons from the pathogenic Neisseriae

Novel therapies are urgently needed to combat the global threat of multidrug-resistant pathogens. Complement forms an important arm of innate defenses against infections. In physiological conditions, complement activation is tightly controlled by soluble and membrane-associated complement inhibitors, but must be selectively activated on invading pathogens to facilitate microbial clearance. Many pathogens, including Neisseria gonorrhoeae and N. meningitidis, express glycans, including N-acetylneuraminic acid (Neu5Ac), that mimic host structures to evade host immunity. Neu5Ac is a negatively charged 9-cabon sugar that inhibits complement, in part by enhancing binding of the complement inhibitor factor H (FH) through C-terminal domains (19 and 20) on FH. Other microbes also bind FH, in most instances through FH domains 6 and 7 or 18-20. Here we describe two strategies to target complement activation on Neisseriae. First, microbial binding domains of FH were fused to IgG Fc to create FH18-20/Fc (binds gonococci) and FH6,7/Fc (binds meningococci). A point mutation in FH domain 19 eliminated hemolysis caused by unmodified FH18-20, but retained binding to gonococci. FH18-20/Fc and FH6,7/Fc mediated complement-dependent killing in vitro and showed efficacy in animal models of gonorrhea and meningococcal bacteremia, respectively. The second strategy utilized CMP-nonulosonate (CMP-NulO) analogs of sialic acid that were incorporated into LOS and prevented complement inhibition by physiologic CMP-Neu5Ac and resulted in attenuated gonococcal infection in mice. While studies to establish the safety of these agents are needed, enhancing complement activation on microbes may represent a promising strategy to treat antimicrobial resistant organisms.

Factor H Binding and Function in Sialylated Pathogenic Neisseriae is Influenced by Gonococcal, but Not Meningococcal, Porin

Neisseria gonorrhoeae and Neisseria meningitidis both express the lacto-N-neotetraose (LNT) lipooligosaccharide (LOS) molecule that can be sialylated. Although gonococcal LNT LOS sialylation enhances binding of the alternative pathway complement inhibitor factor H and renders otherwise serum-sensitive bacteria resistant to complement-dependent killing, the role of LOS sialylation in meningococcal serum resistance is less clear. We show that only gonococcal, but not meningococcal, LNT LOS sialylation enhanced factor H binding. Replacing the porin (Por) B molecule of a meningococcal strain (LOS sialylated) that did not bind factor H with gonococcal Por1B augmented factor H binding. Capsule expression did not alter factor H binding to meningococci that express gonococcal Por. Conversely, replacing gonococcal Por1B with meningococcal PorB abrogated factor H binding despite LNT LOS sialylation. Gonococcal Por1B introduced in the background of an unsialylated meningococcus itself bound small amounts of factor H, suggesting a direct factor H-Por1B interaction. Factor H binding to unsialylated meningococci transfected with gonococcal Por1B was similar to the sialylated counterpart only in the presence of higher (20 microg/ml) concentrations of factor H and decreased in a dose-responsive manner by approximately 80% at 1.25 microg/ml. Factor H binding to the sialylated strain remained unchanged over this factor H concentration range however, suggesting that LOS sialylation facilitated optimal factor H-Por1B interactions. The functional counterpart of factor H binding showed that sialylated meningococcal mutants that possessed gonococcal Por1B were resistant to complement-mediated killing by normal human serum. Our data highlight the different mechanisms used by these two related species to evade complement.

Enhanced factor H binding to sialylated Gonococci is restricted to the sialylated lacto-N-neotetraose lipooligosaccharide species: implications for serum resistance and evidence for a bifunctional lipooligosaccharide sialyltransferase in Gonococci

We isolated serologically identical (by serovar determination and porin variable region [VR] typing) strains of Neisseria gonorrhoeae from an infected male and two of his monogamous female sex partners. One strain (termed 398078) expressed the L1 (Galalpha1 --> 4 [corrected] Galbeta1 --> 4Glcbeta1 --> 4HepI) lipooligosaccharide (LOS) structure exclusively; the other (termed 398079) expressed the lacto-N-neotetraose (LNT; Galbeta1 --> 4GlcNAcbeta1 --> 3Galbeta1 --> 4Glcbeta1 --> 4HepI) LOS structure. The strain from the male index case expressed both glycoforms and exhibited both immunotypes. Nuclear magnetic resonance analysis revealed that sialic acid linked to the terminal Gal of L1 LOS via an alpha2 --> 6 linkage and, as expected, to the terminal Gal of LNT LOS via an alpha2--> 3 linkage. Insertional inactivation of the sialyltransferase gene (known to sialylate LNT LOS) abrogated both L1 LOS sialylation and LNT LOS sialylation, suggesting a bifunctional nature of this enzyme in gonococci. Akin to our previous observations, sialylation of the LNT LOS of strain 398079 enhanced the binding of the complement regulatory molecule, factor H. Rather surprisingly, factor H did not bind to sialylated strain 398078. LOS sialylation conferred the LNT LOS-bearing strain complete (100%) resistance to killing by even 50% nonimmune normal human serum (NHS), whereas sialylation of L1 LOS conferred resistance only to 10% NHS. The ability of gonococcal sialylated LNT to bind factor H confers high-level serum resistance, which is not seen with sialylated L1 LOS. Thus, serum resistance mediated by sialylation of gonococcal L1 and LNT LOS occurs by different mechanisms, and specificity of factor H binding to sialylated gonococci is restricted to the LNT LOS species.

Regulation of gonococcal sialyltransferase, lipooligosaccharide, and serum resistance by glucose, pyruvate, and lactate

Strain F62 of Neisseria gonorrhoeae gonococci (GC) is sensitive to normal human serum unless CMP-N-acetylneuraminic acid (CMP-NANA) is present. NANA is transferred primarily to a 4.5-kDa lipooligosaccharide (LOS) structure by a GC sialyltransferase (Stase). We investigated LOS and Stase expression and serum resistance in strain F62 grown in different carbon sources and growth conditions. Pyruvate-grown GC expressed 1.9- to 5.6-fold more Stase activity than did glucose-grown GC, whereas lactate-grown GC generally expressed intermediate Stase activities. Broth-grown GC expressed two- to fourfold more Stase activity than did plate-grown GC in all carbon sources. Pyruvate- or lactate-grown GC expressed significantly more of the sialylateable 4.5-kDa LOS species than did glucose-grown GC. Anaerobically, the 4.5-kDa LOS species was expressed in greater quantity than the 4.9-kDa N-acetyl galactosamine-terminating species in all carbon sources. Pyruvate-grown GC also incorporated up to threefold more radiolabelled CMP-NANA onto the 4.5-kDa LOS species than did glucose-grown GC. In serum resistance studies, pyruvate-grown GC were 6.5- to 16.1-fold more serum resistant than glucose-grown GC at limiting CMP-NANA concentrations (1.56 to 12.50 microg/ml). Taken together, these results indicate that gonococcal expression of Stase activity is up-regulated by growth in pyruvate or lactate, which correlates with enhanced expression of the sialylateable 4.5-kDa LOS and, for growth in pyruvate, correlates with enhanced sialylation of gonococcal LOS and greater serum resistance. In different in vivo niches, gonococcal LOS sialylation, serum resistance, and interaction with host cells can be highly regulated.