Genetic basis of Neisseria gonorrhoeae lipooligosaccharide antigenic variation

Gonococcal PorB: a multifaceted modulator of host immune responses

Neisseria gonorrhoeae is a human-specific pathogen responsible for the sexually transmitted infection, gonorrhoea. N. gonorrhoeae promotes its survival by manipulating both innate and adaptive immune responses. The most abundant gonococcal outer-membrane protein is PorB, an essential porin that facilitates ion exchange. Importantly, gonococcal PorB has several immunomodulatory properties. To subvert the innate immune response, PorB suppresses killing mechanisms of macrophages and neutrophils, and recruits negative regulators of complement to the gonococcal cell surface. For manipulation of adaptive immune responses, gonococcal PorB suppresses the capability of dendritic cells to stimulate proliferation of T cells. As gonococcal PorB is highly abundant in outer-membrane vesicles, consideration of the immunomodulatory properties of this porin is critical when designing gonococcal vaccines.

Microencapsulated IL-12 Drives Genital Tract Immune Responses to Intranasal Gonococcal Outer Membrane Vesicle Vaccine and Induces Resistance to Vaginal Infection with Diverse Strains of Neisseria gonorrhoeae

An experimental gonococcal vaccine consisting of outer membrane vesicles (OMVs) and microsphere (ms)-encapsulated interleukin-12 (IL-12 ms) induces Th1-driven immunity, with circulating and genital antibodies to Neisseria gonorrhoeae, after intravaginal (i.vag.) administration in female mice, and generates resistance to vaginal challenge infection. Because i.vag. administration is inapplicable to males and may not be acceptable to women, we determined whether intranasal (i.n.) administration would generate protective immunity against N. gonorrhoeae. Female and male mice were immunized i.n. with gonococcal OMVs plus IL-12 ms or blank microspheres (blank ms). Responses to i.n. immunization were similar to those with i.vag. immunization, with serum IgG, salivary IgA, and vaginal IgG and IgA antigonococcal antibodies induced when OMVs were administered with IL-12 ms. Male mice responded with serum IgG and salivary IgA antibodies similarly to female mice. Gamma interferon (IFN-γ) production by CD4⁺ T cells from iliac lymph nodes was elevated after i.n. or i.vag. immunization with OMVs plus IL-12 ms. Female mice immunized with OMVs plus IL-12 ms by either route resisted challenge with N. gonorrhoeae to an equal extent, and resistance generated by i.n. immunization extended to heterologous strains of N. gonorrhoeae. Detergent-extracted OMVs, which have diminished lipooligosaccharide, generated protective immunity to challenge similar to native OMVs. OMVs from mutant N. gonorrhoeae, in which genes for Rmp and LpxL1 were deleted to eliminate the induction of blocking antibodies against Rmp and diminish lipooligosaccharide endotoxicity, also generated resistance to challenge infection similar to wild-type OMVs when administered i.n. with IL-12 ms. IMPORTANCE We previously demonstrated that female mice can be immunized intravaginally with gonococcal outer membrane vesicles (OMVs) plus microsphere (ms)-encapsulated interleukin-12 (IL-12 ms) to induce antigonococcal antibodies and resistance to genital tract challenge with live Neisseria gonorrhoeae. However, this route of vaccination may be impractical for human vaccine development and is inapplicable to males. Because intranasal immunization has previously been shown to induce antibody responses in both male and female genital tracts, we have evaluated this route of immunization with gonococcal OMVs plus IL-12 ms. In addition, we have refined the composition of gonococcal OMVs to reduce the endotoxicity of lipooligosaccharide and to eliminate the membrane protein Rmp, which induces countereffective blocking antibodies. The resulting vaccine may be more suitable for ultimate translation to human application against the sexually transmitted infection gonorrhea, which is becoming increasingly resistant to treatment with antibiotics.

Mechanisms of host manipulation by Neisseria gonorrhoeae

Neisseria gonorrhoeae (also known as gonococcus) has been causing gonorrhoea in humans since ancient Egyptian times. Today, global gonorrhoea infections are rising at an alarming rate, in concert with an increasing number of antimicrobial-resistant strains. The gonococcus has concurrently evolved several intricate mechanisms that promote pathogenesis by evading both host immunity and defeating common therapeutic interventions. Central to these adaptations is the ability of the gonococcus to manipulate various host microenvironments upon infection. For example, the gonococcus can survive within neutrophils through direct regulation of both the oxidative burst response and maturation of the phagosome; a concerning trait given the important role neutrophils have in defending against invading pathogens. Hence, a detailed understanding of how N. gonorrhoeae exploits the human host to establish and maintain infection is crucial for combating this pathogen. This review summarizes the mechanisms behind host manipulation, with a central focus on the exploitation of host epithelial cell signaling to promote colonization and invasion of the epithelial lining, the modulation of the host immune response to evade both innate and adaptive defenses, and the manipulation of host cell death pathways to both assist colonization and combat antimicrobial activities of innate immune cells. Collectively, these pathways act in concert to enable N. gonorrhoeae to colonize and invade a wide array of host tissues, both establishing and disseminating gonococcal infection.

Neisseria gonorrhoeae physiology and pathogenesis

Neisseria gonorrhoeae is an obligate human pathogen that is the cause of the sexually transmitted disease gonorrhoea. Recently, there has been a surge in gonorrhoea cases that has been exacerbated by the rapid rise in gonococcal multidrug resistance to all useful antimicrobials resulting in this organism becoming a significant public health burden. Therefore, there is a clear and present need to understand the organism's biology through its physiology and pathogenesis to help develop new intervention strategies. The gonococcus initially colonises and adheres to host mucosal surfaces utilising a type IV pilus that helps with microcolony formation. Other adhesion strategies include the porin, PorB, and the phase variable outer membrane protein Opa. The gonococcus is able to subvert complement mediated killing and opsonisation by sialylation of its lipooligosaccharide and deploys a series of anti-phagocytic mechanisms. N. gonorrhoeae is a fastidious organism that is able to grow on a limited number of primary carbon sources such as glucose and lactate. The utilization of lactate by the gonococcus has been implicated in a number of pathogenicity mechanisms. The bacterium lives mainly in microaerobic environments and can grow both aerobically and anaerobically with the aid of nitrite. The gonococcus does not produce siderophores for scavenging iron but can utilize some produced by other bacteria, and it is able to successful chelate iron from host haem, transferrin and lactoferrin. The gonococcus is an incredibly versatile human pathogen; in the following chapter, we detail the intricate mechanisms used by the bacterium to invade and survive within the host.

The Lst Sialyltransferase of Neisseria gonorrhoeae Can Transfer Keto-Deoxyoctanoate as the Terminal Sugar of Lipooligosaccharide: a Glyco-Achilles Heel That Provides a New Strategy for Vaccines to Prevent Gonorrhea

The lipooligosaccharide (LOS) of Neisseria gonorrhoeae plays key roles in pathogenesis and is composed of multiple possible glycoforms. These glycoforms are generated by the process of phase variation and by differences in the glycosyltransferase gene content of particular strains. LOS glycoforms of N. gonorrhoeae can be terminated with an N-acetylneuraminic acid (Neu5Ac), which imparts resistance to the bactericidal activity of serum. However, N. gonorrhoeae cannot synthesize the CMP-Neu5Ac required for LOS biosynthesis and must acquire it from the host. In contrast, Neisseria meningitidis can synthesize endogenous CMP-Neu5Ac, the donor molecule for Neu5Ac, which is a component of some meningococcal capsule structures. Both species have an almost identical LOS sialyltransferase, Lst, that transfers Neu5Ac from CMP-Neu5Ac to the terminus of LOS. Lst is homologous to the LsgB sialyltransferase of nontypeable Haemophilus influenzae (NTHi). Studies in NTHi have demonstrated that LsgB can transfer keto-deoxyoctanoate (KDO) from CMP-KDO to the terminus of LOS in place of Neu5Ac. Here, we show that Lst can also transfer KDO to LOS in place of Neu5Ac in both N. gonorrhoeae and N. meningitidis Consistent with access to the pool of CMP-KDO in the cytoplasm, we present data indicating that Lst is localized in the cytoplasm. Lst has previously been reported to be localized on the outer membrane. We also demonstrate that KDO is expressed as a terminal LOS structure in vivo in samples from infected women and further show that the anti-KDO monoclonal antibody 6E4 can mediate opsonophagocytic killing of N. gonorrhoeae Taken together, these studies indicate that KDO expressed on gonococcal LOS represents a new antigen for the development of vaccines against gonorrhea.IMPORTANCE The emergence of multidrug-resistant N. gonorrhoeae strains that are resistant to available antimicrobials is a current health emergency, and no vaccine is available to prevent gonococcal infection. Lipooligosaccharide (LOS) is one of the major virulence factors of N. gonorrhoeae The sialic acid N-acetylneuraminic acid (Neu5Ac) is present as the terminal glycan on LOS in N. gonorrhoeae In this study, we made an unexpected discovery that KDO can be incorporated as the terminal glycan on LOS of N. gonorrhoeae by the alpha-2,3-sialyltransferase Lst. We showed that N. gonorrhoeae express KDO on LOS in vivo and that the KDO-specific monoclonal antibody 6E4 can direct opsonophagocytic killing of N. gonorrhoeae These data support further development of KDO-LOS structures as vaccine antigens for the prevention of infection by N. gonorrhoeae.

Preclinical Efficacy of a Lipooligosaccharide Peptide Mimic Candidate Gonococcal Vaccine

The global spread of multidrug-resistant strains of Neisseria gonorrhoeae constitutes a public health emergency. With limited antibiotic treatment options, there is an urgent need for development of a safe and effective vaccine against gonorrhea. Previously, we constructed a prototype vaccine candidate comprising a peptide mimic (mimitope) of a glycan epitope on gonococcal lipooligosaccharide (LOS), recognized by monoclonal antibody 2C7. The 2C7 epitope is (i) broadly expressed as a gonococcal antigenic target in human infection, (ii) a critical requirement for gonococcal colonization in the experimental setting, and (iii) a virulence determinant that is maintained and expressed by gonococci. Here, we have synthesized to >95% purity through a relatively facile and economical process a tetrapeptide derivative of the mimitope that was cyclized through a nonreducible thioether bond, thereby rendering the compound homogeneous and stable. This vaccine candidate, called TMCP2, when administered at 0, 3, and 6 weeks to BALB/c mice at either 50, 100 or 200 μg/dose in combination with glucopyranosyl lipid A-stable oil-in-water nanoemulsion (GLA-SE; a Toll-like receptor 4 and T_H1-promoting adjuvant), elicited bactericidal IgG and reduced colonization levels of gonococci in experimentally infected mice while accelerating clearance by each of two different gonococcal strains. Similarly, a 3-dose biweekly schedule (50 μg TMCP2/dose) was also effective in mice. We have developed a gonococcal vaccine candidate that can be scaled up and produced economically to a high degree of purity. The candidate elicits bactericidal antibodies and is efficacious in a preclinical experimental infection model.IMPORTANCE Neisseria gonorrhoeae has become resistant to most antibiotics. The incidence of gonorrhea is also sharply increasing. A safe and effective antigonococcal vaccine is urgently needed. Lipooligosaccharide (LOS), the most abundant outer membrane molecule, is indispensable for gonococcal pathogenesis. A glycan epitope on LOS that is recognized by monoclonal antibody (MAb) 2C7 (called the 2C7 epitope) is expressed almost universally by gonococci in vivo Previously, we identified a peptide mimic (mimitope) of the 2C7 epitope, which when configured as an octamer and used as an immunogen, attenuated colonization of mice by gonococci. Here, a homogenous, stable tetrameric derivative of the mimitope, when combined with a T_H1-promoting adjuvant and used as an immunogen, also effectively attenuates gonococcal colonization of mice. This candidate peptide vaccine can be produced economically, an important consideration for gonorrhea, which affects socioeconomically underprivileged populations disproportionately, and represents an important advance in the development of a gonorrhea vaccine.

Progress Toward a Gonococcal Vaccine: The Way Forward

The concept of immunizing against gonorrhea has received renewed interest because of the recent emergence of strains of Neisseria gonorrhoeae that are resistant to most currently available antibiotics, an occurrence that threatens to render gonorrhea untreatable. However, despite efforts over many decades, no vaccine has yet been successfully developed for human use, leading to pessimism over whether this goal was actually attainable. Several factors have contributed to this situation, including extensive variation of the expression and specificity of many of the gonococcal surface antigens, and the ability of N. gonorrhoeae to resist destruction by complement and other innate immune defense mechanisms. The natural host restriction of N. gonorrhoeae for humans, coupled with the absence of any definable state of immunity arising from an episode of gonorrhea, have also complicated efforts to study gonococcal pathogenesis and the host's immune responses. However, recent findings have elucidated how the gonococcus exploits and manipulates the host's immune system for its own benefit, utilizing human-specific receptors for attachment to and invasion of tissues, and subverting adaptive immune responses that might otherwise be capable of eliminating it. While no single experimental model is capable of providing all the answers, experiments utilizing human cells and tissues in vitro, various in vivo animal models, including genetically modified strains of mice, and both experimental and observational human clinical studies, have combined to yield important new insight into the immuno-pathogenesis of gonococcal infection. In turn, these have now led to novel approaches for the development of a gonococcal vaccine. Ongoing investigations utilizing all available tools are now poised to make the development of an effective human vaccine against gonorrhea an achievable goal within a foreseeable time-frame.

Complement alone drives efficacy of a chimeric antigonococcal monoclonal antibody

Multidrug-resistant Neisseria gonorrhoeae is a global health problem. Monoclonal antibody (mAb) 2C7 recognizes a gonococcal lipooligosaccharide epitope that is expressed by >95% of clinical isolates and hastens gonococcal vaginal clearance in mice. Chimeric mAb 2C7 (human immunoglobulin G1 [IgG1]) with an E430G Fc modification that enhances Fc:Fc interactions and hexamerization following surface-target binding and increases complement activation (HexaBody technology) showed significantly greater C1q engagement and C4 and C3 deposition compared to mAb 2C7 with wild-type Fc. Greater complement activation by 2C7-E430G Fc translated to increased bactericidal activity in vitro and, consequently, enhanced efficacy in mice, compared with “Fc-unmodified” chimeric 2C7. Gonococci bind the complement inhibitors factor H (FH) and C4b-binding protein (C4BP) in a human-specific manner, which dampens antibody (Ab)-mediated complement-dependent killing. The variant 2C7-E430G Fc overcame the barrier posed by these inhibitors in human FH/C4BP transgenic mice, for which a single 1 μg intravenous dose cleared established infection. Chlamydia frequently coexists with and exacerbates gonorrhea; 2C7-E430G Fc also proved effective against gonorrhea in gonorrhea/chlamydia-coinfected mice. Complement activation alone was necessary and sufficient for 2C7 function, evidenced by the fact that (1) “complement-inactive” Fc modifications that engaged Fc gamma receptor (FcγR) rendered 2C7 ineffective, nonetheless; (2) 2C7 was nonfunctional in C1q^−/− mice, when C5 function was blocked, or in C9^−/− mice; and (3) 2C7 remained effective in neutrophil-depleted mice and in mice treated with PMX205, a C5a receptor (C5aR1) inhibitor. We highlight the importance of complement activation for antigonococcal Ab function in the genital tract. Elucidating the correlates of protection against gonorrhea will inform the development of Ab-based gonococcal vaccines and immunotherapeutics.

A chimeric antibody that contains a "complement-enhancing" mutation in Fc (so-called HexaBody technology) shows increased bactericidal activity compared to antibody bearing wild-type Fc and may represent a promising immunotherapeutic approach against multidrug-resistant gonorrhea.

Targeting Lipooligosaccharide (LOS) for a Gonococcal Vaccine

The increasing incidence of gonorrhea worldwide and the global spread of multidrug-resistant strains of Neisseria gonorrhoeae, constitute a public health emergency. With dwindling antibiotic treatment options, there is an urgent need to develop safe and effective vaccines. Gonococcal lipooligosaccharides (LOSs) are potential vaccine candidates because they are densely represented on the bacterial surface and are readily accessible as targets of adaptive immunity. Less well-understood is whether LOSs evoke protective immune responses. Although gonococcal LOS-derived oligosaccharides (OSs) are major immune targets, often they undergo phase variation, a feature that seemingly makes LOS less desirable as a vaccine candidate. However, the identification of a gonococcal LOS-derived OS epitope, called 2C7, that is: (i) a broadly expressed gonococcal antigenic target in human infection; (ii) a virulence determinant, that is maintained by the gonococcus and (iii) a critical requirement for gonococcal colonization in the experimental setting, circumvents its limitation as a potential vaccine candidate imposed by phase variation. Difficulties in purifying structurally intact OSs from LOSs led to "conversion" of the 2C7 epitope into a peptide mimic that elicited cross-reactive IgG anti-OS antibodies that also possess complement-dependent bactericidal activity against gonococci. Mice immunized with the 2C7 peptide mimic clear vaginal colonization more rapidly and reduce gonococcal burdens. 2C7 vaccine satisfies criteria that are desirable in a gonococcal vaccine candidate: broad representation of the antigenic target, service as a virulence determinant that is also critical for organism survival in vivo and elicitation of broadly cross-reactive IgG bactericidal antibodies when used as an immunogen.

Biology of the Gonococcus: Disease and Pathogenesis

Neisseria gonorrhoeae infection is a major public health problem worldwide. The increasing incidence of gonorrhea coupled with global spread of multidrug-resistant isolates of gonococci has ushered in an era of potentially untreatable infection. Gonococcal disease elicits limited immunity, and individuals are susceptible to repeated infections. In this chapter, we describe gonococcal disease and epidemiology and the structure and function of major surface components involved in pathogenesis. We also discuss the mechanisms that gonococci use to evade host immune responses and the immune responses following immunization with selected bacterial components that may overcome evasion. Understanding the biology of the gonococcus may aid in preventing the spread of gonorrhea and also facilitate the development of gonococcal vaccines and treatments.

Refinement of immunizing antigens to produce functional blocking antibodies against the AniA nitrite reductase of Neisseria gonorrhoeae

The emergence of multi-drug resistant Neisseria gonorrhoeae has generated an urgent need for novel therapies or a vaccine to prevent gonococcal disease. In this study we investigate the potential of targeting the surface exposed nitrite reductase, AniA, to block activity by producing functional blocking antibodies. AniA activity is essential for anaerobic growth and biofilm formation of N. gonorrhoeae and functional blocking antibodies may prevent colonisation and disease. Seven peptides covering regions adjacent to the active site were designed based on the AniA structure. Six of the seven peptide conjugates generated immune responses. Peptide 7, GALGQLKVEGAEN, was able to elicit antibodies capable of blocking AniA activity. Antiserum raised against the peptide 7 conjugate detected AniA in 20 N. gonorrhoeae clinical isolates. Recombinant AniA protein antigens were also assessed in this study and generated high-titre, functional blocking antibody responses. Peptide 7 conjugates or truncated recombinant AniA antigens have potential for inclusion in a vaccine against N. gonorrhoeae.

Functional and structural studies on the Neisseria gonorrhoeae GmhA, the first enzyme in the glycero-manno-heptose biosynthesis pathways, demonstrate a critical role in lipooligosaccharide synthesis and gonococcal viability

Sedoheptulose-7-phosphate isomerase, GmhA, is the first enzyme in the biosynthesis of nucleotide-activated-glycero-manno-heptoses and an attractive, yet underexploited, target for development of broad-spectrum antibiotics. We demonstrated that GmhA homologs in Neisseria gonorrhoeae and N. meningitidis (hereafter called GmhAGC and GmhANM , respectively) were interchangeable proteins essential for lipooligosaccharide (LOS) synthesis, and their depletion had adverse effects on neisserial viability. In contrast, the Escherichia coli ortholog failed to complement GmhAGC depletion. Furthermore, we showed that GmhAGC is a cytoplasmic enzyme with induced expression at mid-logarithmic phase, upon iron deprivation and anaerobiosis, and conserved in contemporary gonococcal clinical isolates including the 2016 WHO reference strains. The untagged GmhAGC crystallized as a tetramer in the closed conformation with four zinc ions in the active site, supporting that this is most likely the catalytically active conformation of the enzyme. Finally, site-directed mutagenesis studies showed that the active site residues E65 and H183 were important for LOS synthesis but not for GmhAGC function in bacterial viability. Our studies bring insights into the importance and mechanism of action of GmhA and may ultimately facilitate targeting the enzyme with small molecule inhibitors.

Antigenic Variation in Bacterial Pathogens

Antigenic variation is a strategy used by a broad diversity of microbial pathogens to persist within the mammalian host. Whereas viruses make use of a minimal proofreading capacity combined with large amounts of progeny to use random mutation for variant generation, antigenically variant bacteria have evolved mechanisms which use a stable genome, which aids in protecting the fitness of the progeny. Here, three well-characterized and highly antigenically variant bacterial pathogens are discussed: Anaplasma, Borrelia, and Neisseria. These three pathogens display a variety of mechanisms used to create the structural and antigenic variation needed for immune escape and long-term persistence. Intrahost antigenic variation is the focus; however, the role of these immune escape mechanisms at the population level is also presented.

Gonorrhea - an evolving disease of the new millennium

Etiology, transmission and protection: Neisseria gonorrhoeae (the gonococcus) is the etiological agent for the strictly human sexually transmitted disease gonorrhea. Infections lead to limited immunity, therefore individuals can become repeatedly infected. Pathology/symptomatology: Gonorrhea is generally a non-complicated mucosal infection with a pustular discharge. More severe sequellae include salpingitis and pelvic inflammatory disease which may lead to sterility and/or ectopic pregnancy. Occasionally, the organism can disseminate as a bloodstream infection. Epidemiology, incidence and prevalence: Gonorrhea is a global disease infecting approximately 60 million people annually. In the United States there are approximately 300, 000 cases each year, with an incidence of approximately 100 cases per 100,000 population. Treatment and curability: Gonorrhea is susceptible to an array of antibiotics. Antibiotic resistance is becoming a major problem and there are fears that the gonococcus will become the next "superbug" as the antibiotic arsenal diminishes. Currently, third generation extended-spectrum cephalosporins are being prescribed. Molecular mechanisms of infection: Gonococci elaborate numerous strategies to thwart the immune system. The organism engages in extensive phase (on/off switching) and antigenic variation of several surface antigens. The organism expresses IgA protease which cleaves mucosal antibody. The organism can become serum resistant due to its ability to sialylate lipooligosaccharide in conjunction with its ability to subvert complement activation. The gonococcus can survive within neutrophils as well as in several other lymphocytic cells. The organism manipulates the immune response such that no immune memory is generated which leads to a lack of protective immunity.

Invited review: Lipopolysaccharide biosynthesis: which steps do bacteria need to survive?

A detailed knowledge of LPS biosynthesis is of the utmost importance in understanding the function of the outer membrane of Gram-negative bacteria. The regulation of LPS biosynthesis affects many more compartments of the bacterial cell than the outer membrane and thus contributes to the understanding of the physiology of Gram-negative bacteria in general, on the basis of which only mechanisms of virulence and antibiotic resistance can be studied to find new targets for antibacterial treatment. The study of LPS biosynthesis is also an excellent example to demonstrate the limitations of `genomics' and `proteomics', since secondary gene products can be studied only by the combined tools of molecular genetics, enzymology and analytical structural biochemistry. Thus, the door to the field of `glycomics' is opened.

Opacity proteins of neisseria gonorrhoeae in lipooligosaccharide mutants lost ability to interact with neutrophil-restricted CEACAM3 (CD66d)

Lipooligosacharide (LOS) of Neisseria gonorrhoeae (gonococci, GC) is involved in the interaction of GC with host cells. Deletion of the alpha-oligosaccharide (alpha-OS) moiety of LOS (lgtF mutant) significantly impairs invasion of GC into epithelial cell lines. GC opacity (Opa) proteins, such as OpaI, mediate phagocytosis and stimulate chemiluminescence responses in neutrophils in part through interaction with members of the carcinoembryonic antigen (CEA) family, which includes CEACAM3 (CD66d), a human neutrophil specific receptor for phagocytosis of bacteria. In the present work, we examined the effects of OpaI-expressing lgtF mutant on phagocytosis by HeLa-CEACAM3 cells and chemiluminescence responses in neutrophils. The results showed that lgtF mutant even expressing OpaI completely lost the ability to promote either phagocytosis mediated by CEACAM3 interaction in HeLa cells or chemiluminescence responses in neutrophils. These data indicated that Opa proteins in the lgtF mutant, which might result from the conformational change, cannot be functional.

Mobile DNA in the Pathogenic Neisseria

The genus Neisseria contains two pathogenic species of prominant public health concern: Neisseria gonorrhoeae and Neisseria meningitidis. These pathogens display a notable ability to undergo frequent programmed recombination events. The recombination-mediated pathways of transformation and pilin antigenic variation in the Neisseria are well-studied systems that are critical for pathogenesis. Here we will detail the conserved and unique aspects of transformation and antigenic variation in the Neisseria. Transformation will be followed from initial DNA binding through recombination into the genome with consideration to the factors necessary at each step. Additional focus is paid to the unique type IV secretion system that mediates donation of transforming DNA in the pathogenic Neisseria. The pilin antigenic variation system uses programmed recombinations to alter a major surface determinant, which allows immune avoidance and promotes infection. We discuss the trans- and cis- acting factors which facilitate pilin antigenic variation and present the current understanding of the mechanisms involved in the process.

Male infertility: a public health issue caused by sexually transmitted pathogens

Sexually transmitted diseases (STDs) are caused by several pathogens, including bacteria, viruses and protozoa, and can induce male infertility through multiple pathophysiological mechanisms. Additionally, horizontal transmission of STD pathogens to sexual partners or vertical transmission to fetuses and neonates is possible. Chlamydia trachomatis, Ureaplasma spp., human papillomavirus, hepatitis B and hepatitis C viruses, HIV-1 and human cytomegalovirus have all been detected in semen from symptomatic and asymptomatic men with testicular, accessory gland and urethral infections. These pathogens are associated with poor sperm quality and decreased sperm concentration and motility. However, the effects of these STD agents on semen quality are unclear, as are the effects of herpes simplex virus type 1 and type 2, Neisseria gonorrhoeae, Mycoplasma spp., Treponema pallidum and Trichomonas vaginalis, because few studies have evaluated the influence of these pathogens on male infertility. Chronic or inadequately treated infections seem to be more relevant to infertility than acute infections are, although in many cases the exact aetiological agents remain unknown.

Meningococcal disease and the complement system

Despite considerable advances in the understanding of the pathogenesis of meningococcal disease, this infection remains a major cause of morbidity and mortality globally. The role of the complement system in innate immune defenses against invasive meningococcal disease is well established. Individuals deficient in components of the alternative and terminal complement pathways are highly predisposed to invasive, often recurrent meningococcal infections. Genome-wide analysis studies also point to a central role for complement in disease pathogenesis. Here we review the pathophysiologic events pertinent to the complement system that accompany meningococcal sepsis in humans. Meningococci use several often redundant mechanisms to evade killing by human complement. Capsular polysaccharide and lipooligosaccharide glycan composition play critical roles in complement evasion. Some of the newly described protein vaccine antigens interact with complement components and have sparked considerable research interest.

Neisseria gonorrhoeae RecQ helicase HRDC domains are essential for efficient binding and unwinding of the pilE guanine quartet structure required for pilin antigenic variation

The strict human pathogen Neisseria gonorrhoeae utilizes homologous recombination to antigenically vary the pilus, thus evading the host immune response. High-frequency gene conversion reactions between many silent pilin loci and the expressed pilin locus (pilE) allow for numerous pilus variants per strain to be produced from a single strain. For pilin antigenic variation (Av) to occur, a guanine quartet (G4) structure must form upstream of pilE. The RecQ helicase is one of several recombination or repair enzymes required for efficient levels of pilin Av, and RecQ family members have been shown to bind to and unwind G4 structures. Additionally, the vast majority of RecQ helicase family members encode one "helicase and RNase D C-terminal" (HRDC) domain, whereas the N. gonorrhoeae RecQ helicase gene encodes three HRDC domains, which are critical for pilin Av. Here, we confirm that deletion of RecQ HRDC domains 2 and 3 causes a decrease in the frequency of pilin Av comparable to that obtained with a functional knockout. We demonstrate that the N. gonorrhoeae RecQ helicase can bind and unwind the pilE G4 structure. Deletion of the RecQ HRDC domains 2 and 3 resulted in a decrease in G4 structure binding and unwinding. These data suggest that the decrease in pilin Av observed in the RecQ HRDC domain 2 and 3 deletion mutant is a result of the enzyme's inability to efficiently bind and unwind the pilE G4 structure.

Transcription of a cis-acting, noncoding, small RNA is required for pilin antigenic variation in Neisseria gonorrhoeae

The strict human pathogen Neisseria gonorrhoeae can utilize homologous recombination to generate antigenic variability in targets of immune surveillance. To evade the host immune response, N. gonorrhoeae promotes high frequency gene conversion events between many silent pilin copies and the expressed pilin locus (pilE), resulting in the production of variant pilin proteins. Previously, we identified a guanine quartet (G4) structure localized near pilE that is required for the homologous recombination reactions leading to pilin antigenic variation (Av). In this work, we demonstrate that inactivating the promoter of a small non-coding RNA (sRNA) that initiates within the G4 forming sequence blocks pilin Av. The sRNA promoter is conserved in all sequenced gonococcal strains, and mutations in the predicted transcript downstream of the G4 forming sequence do not alter pilin Av. A mutation that produces a stronger promoter or substitution of the pilE G4-associated sRNA promoter with a phage promoter (when the phage polymerase was expressed) produced wild-type levels of pilin Av. Altering the direction and orientation of the pilE G4-associated sRNA disrupted pilin Av. In addition, expression of the sRNA at a distal site on the gonococcal chromosome in the context of a promoter mutant did not support pilin Av. We conclude that the DNA containing the G-rich sequence can only form the G4 structure during transcription of this sRNA, thus providing a unique molecular step for the initiation of programmed recombination events.

To evade the host immune response, pathogens have evolved mechanisms to provide genetic diversity in targets of immune surveillance. Organisms that express these diversification systems are under strong evolutionary pressure to provide subpopulations of preexisting variants and often rely on cellular recombination machinery to catalyze dedicated high-frequency reactions without disturbing genome integrity. Previously, we defined a guanine quartet (G4) structure in the strict human pathogen Neisseria gonorrhoeae that is required for initiating the homologous recombination reactions leading to pilin antigenic variation (Av). G4 structures have been implicated in many biological processes, however the mechanisms allowing their formation within a chromosome have not been elucidated. In this work, we show a direct link between transcription of a small RNA (sRNA) that initiates within the G4 structure forming sequence and pilin Av and conclude that the process of transcription is necessary for G4 structure formation. sRNAs have emerged as important regulatory molecules in both eukaryotes and prokaryotes, and this is a novel activity of a sRNA in a bacterium. We anticipate that the reliance of G4 structure formation on transcription is a mechanism used by other biological systems that rely on this alternative DNA structure.

Human lipooligosaccharide IGG that prevents endemic meningococcal disease recognizes an internal lacto-N-neotetraose structure

Antibodies that initiate complement-mediated killing of Neisseria meningitidis as they enter the bloodstream from the oropharynx protect against disseminated disease. Human IgGs that bind the neisserial L7 lipooligosaccharide (LOS) are bactericidal for L3,7 and L2,4 meningococci in the presence of human complement. These strains share a lacto-N-neotetraose (nLc4) LOS α chain. We used a set of mutants that have successive saccharide deletions from the nLc4 α chain to characterize further the binding and bactericidal activity of nLc4 LOS IgG. We found that the nLc4 α chain conforms at least four different antigens. We separately purified IgG that required the nLc4 (non-reducing) terminal galactose (Gal) for binding and IgG that bound the truncated nLc3 α chain that lacks this Gal residue. IgG that bound the internal nLc3 α chain killed both L3,7 and L2,4 strains, whereas IgG that required the nLc4 terminal Gal residue for binding killed L2,4 stains but not L3,7 strains. These results show that the diversity of LOS antibodies in human serum is as much a function of the conformation of multiple antigens by a single glycoform as of the production of multiple glycoforms. Differences in sensitivity to killing by human nLc4 LOS IgG may account for the fact that fully two-thirds of endemic group B meningococcal disease in infants and children is caused by L3,7 strains, but only 20% is caused by L2,4 stains.

Focusing homologous recombination: pilin antigenic variation in the pathogenic Neisseria

Some pathogenic microbes utilize homologous recombination to generate antigenic variability in targets of immune surveillance. These specialized systems rely on the cellular recombination machinery to catalyse dedicated, high-frequency reactions that provide extensive diversity in the genes encoding surface antigens. A description of the specific mechanisms that allow unusually high rates of recombination without deleterious effects on the genome in the well-characterized pilin antigenic variation systems of Neisseria gonorrhoeae and Neisseria meningitidis is presented. We will also draw parallels to selected bacterial and eukaryotic antigenic variation systems, and suggest the most pressing unanswered questions related to understanding these important processes.

Sequence-based predictions of lipooligosaccharide diversity in the Neisseriaceae and their implication in pathogenicity

Endotoxin [Lipopolysaccharide (LPS)/Lipooligosaccharide (LOS)] is an important virulence determinant in gram negative bacteria. While the genetic basis of endotoxin production and its role in disease in the pathogenic Neisseria has been extensively studied, little research has focused on the genetic basis of LOS biosynthesis in commensal Neisseria. We determined the genomic sequences of a variety of commensal Neisseria strains, and compared these sequences, along with other genomic sequences available from various sequencing centers from commensal and pathogenic strains, to identify genes involved in LOS biosynthesis. This allowed us to make structural predictions as to differences in LOS seen between commensal and pathogenic strains. We determined that all neisserial strains possess a conserved set of genes needed to make a common 3-Deoxy-D-manno-octulosonic acid -heptose core structure. However, significant genomic differences in glycosyl transferase genes support the published literature indicating compositional differences in the terminal oligosaccharides. This was most pronounced in commensal strains that were distally related to the gonococcus and meningococcus. These strains possessed a homolog of heptosyltransferase III, suggesting that they differ from the pathogenic strains by the presence a third heptose. Furthermore, most commensal strains possess homologs of genes needed to synthesize lipopolysaccharide (LPS). N. cinerea, a commensal species that is highly related to the gonococcus has lost the ability to make sialyltransferase. Overall genomic comparisons of various neisserial strains indicate that significant recombination/genetic acquisition/loss has occurred within the genus, and this muddles proper speciation.

Use of nfsB, encoding nitroreductase, as a reporter gene to determine the mutational spectrum of spontaneous mutations in Neisseria gonorrhoeae

Background

Organisms that are sensitive to nitrofurantoin express a nitroreductase. Since bacterial resistance to this compound results primarily from mutations in the gene encoding nitroreductase, the resulting loss of function of nitroreductase results in a selectable phenotype; resistance to nitrofurantoin. We exploited this direct selection for mutation to study the frequency at which spontaneous mutations arise (transitions and transversions, insertions and deletions).

Results

A nitroreductase- encoding gene was identified in the N. gonorrhoeae FA1090 genome by using a bioinformatic search with the deduced amino acid sequence derived from the Escherichia coli nitroreductase gene, nfsB. Cell extracts from N. gonorrhoeae were shown to possess nitroreductase activity, and activity was shown to be the result of NfsB. Spontaneous nitrofurantoin-resistant mutants arose at a frequency of ~3 × 10^-6 - 8 × 10^-8 among the various strains tested. The nfsB sequence was amplified from various nitrofurantoin-resistant mutants, and the nature of the mutations determined. Transition, transversion, insertion and deletion mutations were all readily detectable with this reporter gene.

Conclusion

We found that nfsB is a useful reporter gene for measuring spontaneous mutation frequencies. Furthermore, we found that mutations were more likely to arise in homopolymeric runs rather than as base substitutions.

Variation of Neisseria gonorrhoeae lipooligosaccharide directs dendritic cell-induced T helper responses

Gonorrhea is one of the most prevalent sexually transmitted diseases in the world. A naturally occurring variation of the terminal carbohydrates on the lipooligosaccharide (LOS) molecule correlates with altered disease states. Here, we investigated the interaction of different stable gonoccocal LOS phenotypes with human dendritic cells and demonstrate that each variant targets a different set of receptors on the dendritic cell, including the C-type lectins MGL and DC-SIGN. Neisseria gonorrhoeae LOS phenotype C constitutes the first bacterial ligand to be described for the human C-type lectin receptor MGL. Both MGL and DC-SIGN are locally expressed at the male and female genital area, the primary site of N. gonorrhoeae infection. We show that targeting of different C-type lectins with the N. gonorrhoeae LOS variants results in alterations in dendritic cell cytokine secretion profiles and the induction of distinct adaptive CD4⁺ T helper responses. Whereas N. gonorrhoeae variant A with a terminal N-acetylglucosamine on its LOS was recognized by DC-SIGN and induced significantly more IL-10 production, phenotype C, carrying a terminal N-acetylgalactosamine, primarily interacted with MGL and skewed immunity towards the T helper 2 lineage. Together, our results indicate that N. gonorrhoeae LOS variation allows for selective manipulation of dendritic cell function, thereby shifting subsequent immune responses in favor of bacterial survival.

Neisseria gonorrhoeae is a species of Gram-negative bacteria responsible for the sexually transmitted disease gonorrhea. Although effective antibiotic treatments are available, little is known about the host immune response to this pathogen. Here, we analyzed three well-characterized gonococcal variants that only differ in the outer sugar present on the lipooligosaccharide of the bacteria. We found that human dendritic cells use different receptors, including the C-type lectins MGL and DC-SIGN, to detect the three N. gonorrhoeae phenotypes. Dendritic cells carrying the MGL and DC-SIGN receptor were present in the human genital tissues, the site of gonoccocal invasion. DC activation with the gonococcal variants resulted in different cytokine secretion profiles and alterations in the subsequent adaptive T cell response. In particular, LOS containing a terminal N-acetylgalactosamine induced more T helper 2-type responses, which are unfavorable for clearing the bacteria. Our data provide new insights into the pathogenesis of N. gonorrhoeae and suggest that variation of lipooligosaccharide glycosylation enables to bacteria to selectively subvert host immune defense mechanisms.

Effect of gonococcal lipooligosaccharide variation on human monocytic cytokine profile

Background

Neisseria gonorrhoeae is an obligate human pathogen that causes significant worldwide morbidity. N. gonorrhoeae expresses lipooligosaccharide (LOS), a phase variable molecule that plays an important role during pathogenesis of the organism. Alteration in the structure of gonococcal LOS correlates with altered disease presentation. In addition, LOS sialylation occurs readily in vivo, though the role of this sialylation during disease is unknown.

Results

Challenge of human monocytes with purified LOS preparations isolated from strains expressing distinct structurally defined LOSs resulted in identical production of the proinflammatory cytokines tumor necrosis factor alpha (TNFα) and interleukin-12 (IL-12). Similar results were seen when monocytes were challenged with either live or gentamicin-killed whole cell gonococcal variants expressing these LOS structures, although greater cytokine production was observed in comparison with challenge by purified LOS. Challenge of a human primary monocyte model with distinct LOS variants resulted in similar production of TNFα, IL-12, interleukin-10 (IL-10), and interleukin-8 (IL-8). A cytokine array was employed to allow measurement of a broad range of cytokines in samples challenge with gonococcal LOS variants as well as variants expressing sialylated LOS. Challenge of primary monocytes with sialylated gonococci was shown to elicit the production of more MCP-2 (monocyte chemoattractant protein-2) in comparison with challenge by unsialylated gonococci.

Conclusion

We demonstrated that while alterations in the carbohydrate moiety of LOS do not impact the production of most cytokines by human monocytes, whole-cell bacterial challenge is more stimulatory than challenge with purified LOS, implying that other gonococcal cell surface antigens are important for the elicitation of cytokines. Challenge with gonococci expressing sialylated LOS resulted in elicitation of more of the chemokine MCP-2 from challenged cells in comparison with gonococci expressing unsialylated LOS. As MCP-2 is an important chemoattractant, this indicates that in vivo sialylation may play an important role during the pathogenesis of N. gonorrhoeae.

Role of N-acetylglucosamine within core lipopolysaccharide of several species of gram-negative bacteria in targeting the DC-SIGN (CD209)

Our recent studies have shown that the dendritic cell-specific ICAM nonintegrin CD209 (DC-SIGN) specifically binds to the core LPS of Escherichia coli K12 (E. coli), promoting bacterial adherence and phagocytosis. In this current study, we attempted to map the sites within the core LPS that are directly involved in LPS-DC-SIGN interaction. We took advantage of four sets of well-defined core LPS mutants, which are derived from E. coli, Salmonella enterica serovar Typhimurium, Neisseria gonorrhoeae, and Haemophilus ducreyi and determined interaction of each of these four sets with DC-SIGN. Our results demonstrated that N-acetylglucosamine (GlcNAc) sugar residues within the core LPS in these bacteria play an essential role in targeting the DC-SIGN receptor. Our results also imply that DC-SIGN is an innate immune receptor and the interaction of bacterial core LPS and DC-SIGN may represent a primeval interaction between Gram-negative bacteria and host phagocytic cells.

Biochemical analysis of Lpt3, a protein responsible for phosphoethanolamine addition to lipooligosaccharide of pathogenic Neisseria

The inner core of neisserial lipooligosaccharide (LOS) contains heptose residues that can be decorated by phosphoethanolamine (PEA). PEA modification of heptose II (HepII) can occur at the 3, 6, or 7 position(s). We used a genomic DNA sequence of lpt3, derived from Neisseria meningitidis MC58, to search the genomic sequence of N. gonorrhoeae FA1090 and identified a homolog of lpt3 in N. gonorrhoeae. A PCR amplicon containing lpt3 was amplified from F62DeltaLgtA, cloned, mutagenized, and inserted into the chromosome of N. gonorrhoeae strain F62DeltaLgtA, producing strain F62DeltaLgtAlpt3::Tn5. LOS isolated from this strain lost the ability to bind monoclonal antibody (MAb) 2-1-L8. Complementation of this mutation by genetic removal of the transposon insertion restored MAb 2-1-L8 binding. Mass spectrometry analysis of LOS isolated from the F62DeltaLgtA indicated that this strain contained two PEA modifications on its LOS. F62DeltaLgtAlpt3::Tn5 lacked a PEA modification on its LOS, a finding consistent with the hypothesis that lpt3 encodes a protein mediating PEA addition onto gonococcal LOS. The DNA encoding lpt3 was cloned into an expression vector and Lpt3 was purified. Purified Lpt3 was able to mediate the addition of PEA to LOS isolated from F62DeltaLgtAlpt3::Tn5.

Genetic and functional analyses of the lgtH gene, a member of the β-1,4-galactosyltransferase gene family in the genus Neisseria

Lipooligosaccharide (LOS) is a major virulence factor of the pathogenic Neisseria. Three galactosyltransferase genes, lgtB, lgtE and lgtH, responsible for the biosynthesis of LOS oligosaccharide chains, were analysed in five Neisseria species. The function of lgtH in Neisseria meningitidis 6275 was determined by mutagenesis and chemical characterization of the parent and mutant LOS chains. The chemical characterization included SDS-PAGE, immunoblot, hexose and mass spectrometry analyses. Compared with the parent LOS, the mutant LOS lacked galactose, and its oligosaccharide decreased by three or four sugar units in matrix-assisted laser desorption ionization (MALDI)-MS analysis. The results show that lgtH encodes a β-1,4-galactosyltransferase, and that the glucose moiety linked to heptose (Hep) in the α chain is the acceptor site in the biosynthesis of Neisseria LOS. To understand the sequence diversity and relationships of lgtB, lgtE and lgtH, the entire lgt-1 locus was further sequenced in three N. meningitidis strains and three commensal Neisseria strains, and compared with the previously reported lgt genes from Neisseria species. Comparison of the protein sequences of the three enzymes LgtB, LgtE and LgtH showed a conserved N-terminal region, and a highly variable C-terminal region, suggesting functional constraint for substrate and acceptor specificity, respectively. The analyses of allelic variation and evolution of 23 lgtB, 12 lgtE and 14 lgtH sequences revealed a distinct evolutionary history of these genes in Neisseria. For example, the splits graph of lgtE displayed a network evolution, indicating frequent DNA recombination, whereas splits graphs of lgtB and lgtH displayed star-tree-like evolution, indicating the accumulation of point mutations. The data presented here represent examples of the evolution and variation of prokaryotic glycosyltransferase gene families. These imply the existence of multiple enzyme isoforms for biosynthesis of a great diversity of oligosaccharides in nature.

The molecular mechanisms used by Neisseria gonorrhoeae to initiate infection differ between men and women

The molecular mechanisms used by the gonococcus to initiate infection exhibit gender specificity. The clinical presentations of disease are also strikingly different upon comparison of gonococcal urethritis to gonococcal cervicitis. An intimate association occurs between the gonococcus and the urethral epithelium and is mediated by the asialoglycoprotein receptor. Gonococcal interaction with the urethral epithelia cell triggers cytokine release, which promotes neutrophil influx and an inflammatory response. Similarly, gonococcal infection of the upper female genital tract also results in inflammation. Gonococci invade the nonciliated epithelia, and the ciliated cells are subjected to the cytotoxic effects of tumor necrosis factor alpha induced by gonococcal peptidoglycan and lipooligosaccharide. In contrast, gonococcal infection of the lower female genital tract is typically asymptomatic. This is in part the result of the ability of the gonococcus to subvert the alternative pathway of complement present in the lower female genital tract. Gonococcal engagement of complement receptor 3 on the cervical epithelia results in membrane ruffling and does not promote inflammation. A model of gonococcal pathogenesis is presented in the context of the male and female human urogenital tracts.

Phase and antigenic variation in bacteria

Phase and antigenic variation result in a heterogenic phenotype of a clonal bacterial population, in which individual cells either express the phase-variable protein(s) or not, or express one of multiple antigenic forms of the protein, respectively. This form of regulation has been identified mainly, but by no means exclusively, for a wide variety of surface structures in animal pathogens and is implicated as a virulence strategy. This review provides an overview of the many bacterial proteins and structures that are under the control of phase or antigenic variation. The context is mainly within the role of the proteins and variation for pathogenesis, which reflects the main body of literature. The occurrence of phase variation in expression of genes not readily recognizable as virulence factors is highlighted as well, to illustrate that our current knowledge is incomplete. From recent genome sequence analysis, it has become clear that phase variation may be more widespread than is currently recognized, and a brief discussion is included to show how genome sequence analysis can provide novel information, as well as its limitations. The current state of knowledge of the molecular mechanisms leading to phase variation and antigenic variation are reviewed, and the way in which these mechanisms form part of the general regulatory network of the cell is addressed. Arguments both for and against a role of phase and antigenic variation in immune evasion are presented and put into new perspective by distinguishing between a role in bacterial persistence in a host and a role in facilitating evasion of cross-immunity. Finally, examples are presented to illustrate that phase-variable gene expression should be taken into account in the development of diagnostic assays and in the interpretation of experimental results and epidemiological studies.

Transcriptional phase variation at the flhB gene of Pseudomonas putida DOT-T1E is involved in response to environmental changes and suggests the participation of the flagellar export system in solvent tolerance

Frameshift mutations in a poly(G) track at the flhB gene of Pseudomonas putida DOT-T1E are responsible for the diminished swimming of this strain on semisolid medium, which contrasts with the high swimming ability of P. putida KT2440, which does not exhibit a poly(G) track at the flhB gene. We previously showed that a mutant lacking FlhB was more sensitive to solvents than the wild-type strain (Segura et al., J. Bacteriol., 183:4127-4133, 2001). In this study, we show that swimming ability correlates with solvent tolerance in P. putida DOT-T1E, so that growth conditions favoring a functional flhB gene (growth on semisolid medium) resulted in increased innate tolerance to a sudden toluene shock.

The lgtABCDE gene cluster, involved in lipooligosaccharide biosynthesis in Neisseria gonorrhoeae, contains multiple promoter sequences

Biosynthesis of the variable core domain of lipooligosaccharide (LOS) in Neisseria gonorrhoeae is mediated by glycosyl transferases encoded by lgtABCDE. Changes within homopolymeric runs within lgtA, lgtC, and lgtD affect the expression state of these genes, with the nature of the LOS expressed determined by the functionality of these genes. However, the mechanism for modulating the amount of multiple LOS chemotypes expressed in a single cell is not understood. Using mutants containing polar disruptions within the lgtABCDE locus, we determined that the expression of this locus is mediated by multiple promoters and that disruption of transcription from these promoters alters the relative levels of simultaneously expressed LOS chemotypes. Expression of the lgtABCDE locus was quantified by using xylE transcriptional fusions, and the data indicate that this locus is transcribed in trace amounts and that subtle changes in transcription result in phenotypic changes. By using rapid amplification of 5' cDNA ends, transcriptional start sites and promoter sequences were identified within lgtABCDE. Most of these promoters possessed 50 to 67% homology with the consensus gearbox promoter sequence of Escherichia coli.

Biochemical properties of Neisseria gonorrhoeae LgtE

A fragment of chromosomal DNA encoding the lgtE gene of Neisseria gonorrhoeae strain F62 was amplified by PCR and cloned into the expression vector pET15b. Functional LgtE was purified and its biochemical properties were determined. The purified enzyme was maximally active in buffer containing manganese; minimal activity was obtained in buffer containing other divalent cations. LgtE was only able to mediate the addition of UDP-galactose into neisserial lipooligosaccharides (LOSs). We used a variety of genetically defined and chemically verified LOS structures to determine acceptor specificity. LgtE was able to mediate the addition of galactose into a variety of LOS structures, indicating the this enzyme possesses broad acceptor specificity. Furthermore, it was able to add multiple galactose residues onto LOS. We also determined that this enzyme was capable of adding galactose onto both the alpha and beta chains of neisserial LOS.

Immunologic and genetic characterization of lipooligosaccharide variants in a Neisseria meningitidis serogroup C strain

Neisseria meningitidis shows great variation in expression of structurally different lipooligosaccharides (LOS) on its cell surface. To better understand the LOS diversity that may occur within an individual strain, a group C wild-type strain, BB305-Tr4, and two stable isogenic LOS variants, Tr5 and Tr7, were selected for this study. SDS-PAGE analysis showed a size reduction of Tr5 and Tr7 LOS compared to that of Tr4. Immunoblotting showed that parental Tr4 LOS reacted with L1, L2 and L3,7 antibodies, variant Tr5 LOS with L1 and L6 antibodies, while Tr7 LOS was non-typeable. Genetic analysis showed that the gene organization at the lgt-1 locus in the three strains was lgtZ,C,A,B,H4 in Tr4, lgtZ,C,A,H4 in Tr5 and lgtZ,C,A,H9 in Tr7. The genetic differences in the three strains were consistent with their phenotypic changes. Sequence comparison revealed two independent recombination events. The first was the recombination of repeated DNA fragments in the flanking regions to delete lgtB in Tr5. The second was the recombination of a fragment of two genes, lgtB and lgtH4, to create an inactive lgtH9 allele with a mosaic structure in Tr7. These findings suggest that besides phase variation, homologous recombination can contribute to the genetic diversity of the lgt locus and to the generation of LOS variation in N. meningitidis.

Neisseria gonorrhoeae strain PID2 simultaneously expresses six chemically related lipooligosaccharide structures

Neisseria gonorrhoeae strain PID2 was isolated from a woman suffering from pelvic inflammatory disease. When LOS expressed by this strain is analyzed on SDS-PAGE gels, at least six different lipooligosaccharide (LOS) components are visualized. We characterized the LOSs made by this strain by exoglycosidase digestion, sugar composition analysis, mass spectrometry, and analysis of the genes needed for its synthesis. DNA sequence analysis showed that the lgt gene cluster in this strain has undergone a rearrangement and that it possesses two copies of lgtA, one copy of lgtB and lgtC, and a hybrid gene containing sequences from lgtB and lgtE. We determined that the hybrid lgtB/E gene retained the lgtE gene function. DNA sequence analysis of the gene organization suggested that an intramolecular recombination between lgtA and lgtD and lgtB and lgtE had occurred via homologous recombination between similar sequences. Our studies demonstrated that fluorophore-assisted carbohydrate electrophoresis can be utilized to rapidly determine the composition of LOS. By combining exoglycosidase digestion, in combination with mass spectrometry analysis and compositional analysis, the data indicate that all of the LOS components produced by PID2 extend off of the alpha chain. The longest alpha chain oligosaccharide structure is Gal-GlcNAc-Gal-GlcNAc-Gal-Glc-Heptose I, and the six LOS components are built up by sequentially adding sugars onto the first heptose. PID2 LOS is the first Neisserial LOS to be shown to be devoid of phosphoethanolamine modifications. Because PID2 can surface express its LOS, it indicates that the addition of phosphoethanolamine is not required for LOS surface expression.

Genetic diversity of three lgt loci for biosynthesis of lipooligosaccharide (LOS) in Neisseria species

Lipooligosaccharide (LOS) is a major virulence factor of the pathogenic Neisseria. Nine lgt genes at three chromosomal loci (lgt-1, 2, 3) encoding the glycosyltransferases responsible for the biosynthesis of LOS oligosaccharide chains were examined in 26 Neisseria meningitidis, 51 Neisseria gonorrhoeae and 18 commensal Neisseria strains. DNA hybridization, PCR and nucleotide sequence data were compared to previously reported lgt genes. Analysis of the genetic organization of the lgt loci revealed that in N. meningitidis, the lgt-1 and lgt-3 loci were hypervariable genomic regions, whereas the lgt-2 locus was conserved. In N. gonorrhoeae, no variability in the composition or organization of the three lgt loci was observed. lgt genes were detected only in some commensal Neisseria species. The genetic organization of the lgt-1 locus was classified into eight types and the lgt-3 locus was classified into four types. Two types of arrangement at lgt-1 (II and IV) and one type of arrangement at lgt-3 (IV) were novel genetic organizations reported in this study. Based on the three lgt loci, 10 LOS genotypes of N. meningitidis were distinguished. Phylogenetic analysis revealed a gene cluster, lgtH, which separated from the homologous genes lgtB and lgtE. The lgtH and lgtE genes were mutually exclusive and were located at the same position in lgt-1. The data demonstrated that pathogenic and commensal Neisseria share a common lgt gene pool and horizontal gene transfer appears to contribute to the genetic diversity of the lgt loci in Neisseria.

Structural characterization of lipo-oligosaccharide (LOS) from Yersinia pestis: regulation of LOS structure by the PhoPQ system

The two-component regulatory system PhoPQ has been shown to regulate the expression of virulence factors in a number of bacterial species. For one such virulence factor, lipopolysaccharide (LPS), the PhoPQ system has been shown to regulate structural modifications in Salmonella enterica var Typhimurium. In Yersinia pestis, which expresses lipo-oligosaccharide (LOS), a PhoPQ regulatory system has been identified and an isogenic mutant constructed. To investigate potential modifications to LOS from Y. pestis, which to date has not been fully characterized, purified LOS from wild-type plague and the phoP defective mutant were analysed by mass spectrometry. Here we report the structural characterization of LOS from Y. pestis and the direct comparison of LOS from a phoP mutant. Structural modifications to lipid A, the host signalling portion of LOS, were not detected but analysis of the core revealed the expression of two distinct molecular species in wild-type LOS, differing in terminal galactose or heptose. The phoP mutant was restricted to the expression of a single molecular species, containing terminal heptose. The minimum inhibitory concentration of cationic antimicrobial peptides for the two strains was determined and compared with the wild-type: the phoP mutant was highly sensitive to polymyxin. Thus, LOS modification is under the control of the PhoPQ regulatory system and the ability to alter LOS structure may be required for survival of Y. pestis within the mammalian and/or flea host.

Lipopolysaccharide endotoxins

Bacterial lipopolysaccharides (LPS) typically consist of a hydrophobic domain known as lipid A (or endotoxin), a nonrepeating "core" oligosaccharide, and a distal polysaccharide (or O-antigen). Recent genomic data have facilitated study of LPS assembly in diverse Gram-negative bacteria, many of which are human or plant pathogens, and have established the importance of lateral gene transfer in generating structural diversity of O-antigens. Many enzymes of lipid A biosynthesis like LpxC have been validated as targets for development of new antibiotics. Key genes for lipid A biosynthesis have unexpectedly also been found in higher plants, indicating that eukaryotic lipid A-like molecules may exist. Most significant has been the identification of the plasma membrane protein TLR4 as the lipid A signaling receptor of animal cells. TLR4 belongs to a family of innate immunity receptors that possess a large extracellular domain of leucine-rich repeats, a single trans-membrane segment, and a smaller cytoplasmic signaling region that engages the adaptor protein MyD88. The expanding knowledge of TLR4 specificity and its downstream signaling pathways should provide new opportunities for blocking inflammation associated with infection.

Characterization of a Brucella species 25-kilobase DNA fragment deleted from Brucella abortus reveals a large gene cluster related to the synthesis of a polysaccharide

In the present study we completed the nucleotide sequence of a Brucella melitensis 16M DNA fragment deleted from B. abortus that accounts for 25,064 bp and show that the other Brucella spp. contain the entire 25-kb DNA fragment. Two short direct repeats of four nucleotides, detected in the B. melitensis 16M DNA flanking both sides of the fragment deleted from B. abortus, might have been involved in the deletion formation by a strand slippage mechanism during replication. In addition to omp31, coding for an immunogenic protein located in the Brucella outer membrane, 22 hypothetical genes were identified. Most of the proteins that would be encoded by these genes show significant homology with proteins involved in the biosynthesis of polysaccharides from other bacteria, suggesting that they might be involved in the synthesis of a Brucella polysaccharide that would be a heteropolymer synthesized by a Wzy-dependent pathway. This polysaccharide would not be synthesized in B. abortus and would be a polysaccharide not identified until present in the genus Brucella, since all of the known polysaccharides are synthesized in all smooth Brucella species. Discovery of a novel polysaccharide not synthesized in B. abortus might be interesting for a better understanding of the pathogenicity and host preference differences observed between the Brucella species. However, the possibility that the genes detected in the DNA fragment deleted in B. abortus no longer lead to the synthesis of a polysaccharide must not be excluded. They might be a remnant of the common ancestor of the alpha-2 subdivision of the class Proteobacteria, with some of its members synthesizing extracellular polysaccharides and, as Brucella spp., living in association with eukaryotic cells.

Identification and characterization of a second extracellular collagen-like protein made by group A Streptococcus: control of production at the level of translation

A recent study found that group A Streptococcus (GAS) expresses a cell surface protein with similarity to human collagen (S. Lukomski, K. Nakashima, I. Abdi, V. J. Cipriano, R. M. Ireland, S. R. Reid, G. G. Adams, and J. M. Musser, Infect. Immun. 68:6542-6553, 2000). This streptococcal collagen-like protein (Scl) contains a long region of Gly-X-X motifs and was produced by serotype M1 GAS strains. In the present study, a second member of the scl gene family was identified and designated scl2. The Scl2 protein also has a collagen-like region, which in M1 strains is composed of 38 contiguous Gly-X-X triplet motifs. The scl2 gene was present in all 50 genetically diverse GAS strains studied. The Scl2 protein is highly polymorphic, and the number of Gly-X-X motifs in the 50 strains studied ranged from 31 in one serotype M1 strain to 79 in serotype M28 and M77 isolates. The scl1 and scl2 genes were simultaneously transcribed in the exponential phase, and the Scl proteins were also produced. Scl1 and Scl2 were identified in a cell-associated form and free in culture supernatants. Production of Scl1 is regulated by Mga, a positive transcriptional regulator that controls expression of several GAS virulence factors. In contrast, production of Scl2 is controlled at the level of translation by variation in the number of short-sequence pentanucleotide repeats (CAAAA) located immediately downstream of the GTG (Val) start codon. Control of protein production by this molecular mechanism has not been identified previously in GAS. Together, the data indicate that GAS simultaneously produces two extracellular human collagen-like proteins in a regulated fashion.

Analysis of lipooligosaccharide biosynthesis in the Neisseriaceae

Neisserial lipooligosaccharide (LOS) contains three oligosaccharide chains, termed the alpha, beta, and gamma chains. We used Southern hybridization experiments on DNA isolated from various Neisseria spp. to determine if strains considered to be nonpathogenic possessed DNA sequences homologous with genes involved in the biosynthesis of these oligosaccharide chains. The presence or absence of specific genes was compared to the LOS profiles expressed by each strain, as characterized by their mobilities on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gel and their reactivities with various LOS-specific monoclonal antibodies. A great deal of heterogeneity was seen with respect to the presence of genes encoding glycosyltransferases in Neisseria. All pathogenic species were found to possess DNA sequences homologous with the lgt gene cluster, a group of genes needed for the synthesis of the alpha chain. Some of these genes were also found to be present in strains considered to be nonpathogenic, such as Neisseria lactamica, N. subflava, and N. sicca. Some nonpathogenic Neisseria spp. were able to express high-molecular-mass LOS structures, even though they lacked the DNA sequences homologous with rfaF, a gene whose product must act before gonococcal and meningococcal LOS can be elongated. Using a PCR amplification strategy, in combination with DNA sequencing, we demonstrated that N. subflava 44 possessed lgtA, lgtB, and lgtE genes. The predicted amino acid sequence encoded by each of these genes suggested that they encoded functional proteins; however, structural analysis of LOS isolated from this strain indicated that the bulk of its LOS was not modified by these gene products. This suggests the existence of an additional regulatory mechanism that is responsible for the limited expression of these genes in this strain.

Effect of alpha-oligosaccharide phenotype of Neisseria gonorrhoeae strain MS11 on invasion of Chang conjunctival, HEC-1-B endometrial, and ME-180 cervical cells

The genes encoding the glycosyltransferases responsible for the addition of the five sugars in the alpha-oligosaccharide (alpha-OS) moiety of lipooligosaccharide (LOS) have been identified. Disruption of these glycosyltransferase genes singly or in combination results in corresponding truncations in LOS. In the present work we show that sequential deletion of the terminal four sugar residues of gonococcal alpha-OS had no discernible effect on the invasion of human conjunctival, endometrial, and cervical cell lines. However, deletion of the proximal glucose, which resulted in the complete deletion of alpha-OS, significantly impaired invasion of the gonococci into all three cell lines. The effect of deleting alpha-OS on invasion was independent of and additive to the known invasion-promoting factor OpaA. These data suggest that the proximal glucose residue of the alpha-OS chain of LOS is required for efficient invasion of gonococci into host mucosa.

Phase variation of a beta-1,3 galactosyltransferase involved in generation of the ganglioside GM1-like lipo-oligosaccharide of Campylobacter jejuni

Ganglioside mimicry by Campylobacter jejuni lipo-oligosaccharide (LOS) is thought to be a critical factor in the triggering of the Guillain-Barré and Miller-Fisher syndrome neuropathies after C. jejuni infection. The combination of a completed genome sequence and a ganglioside GM1-like LOS structure makes C. jejuni NCTC 11168 a useful model strain for the identification and characterization of the genes involved in the biosynthesis of ganglioside-mimicking LOS. Genome analysis identified a putative LOS biosynthetic cluster and, from this, we describe a putative gene (ORF Cj1139c), which we have termed wlaN, with a significant level of similarity to a number of bacterial glycosyltransferases. Mutation of this gene in C. jejuni NCTC 11168 resulted in a LOS molecule of increased electrophoretic mobility, which also failed to bind cholera toxin. Comparison of LOS structural data from wild type and the mutant strain indicated lack of a terminal beta-1,3-linked galactose residue in the latter. The wlaN gene product was demonstrated unambiguously as a beta-1,3 galactosyltransferase responsible for converting GM2-like LOS structures to GM1-like by in vitro expression. We also show that the presence of an intragenic homopolymeric tract renders the expression of a functional wlaN gene product phase variable, resulting in distinct C. jejuni NCTC 11168 cell populations with alternate GM1 or GM2 ganglioside-mimicking LOS structures. The distribution of wlaN among a number of C. jejuni strains with known LOS structure was determined and, for C. jejuni NCTC 12500, similar wlaN gene phase variation was shown to occur, so that this strain has the potential to synthesize a GM1-like LOS structure as well as the ganglioside GM2-like LOS structure proposed in the literature.

Host cell invasion by pathogenic Neisseriae

As outlined in this review, various experimental techniques have been employed in an attempt to understand neisserial pathogenesis. In vitro genetic analysis has been used to study the genetic basis for the structural variability of cell surface components. Transformed or primary epithelial cell cultures have provided the simplest model to analyze bacterial adherence and invasion, while the infection of polarized epithelial monolayers, fallopian tube and nasopharyngeal organ cultures, and ureteral tissue have each been used to more closely represent the events which occur in vivo. Finally, the in vivo infection of human volunteers with N. gonorrhoeae has provided a powerful means to confirm and expand the results obtained in vitro. By these various approaches, a number of neisserial adhesins (i.e. pilli, Opa, Opc and P36) and additional putative virulence determinants which affect bacterial adherence and invasion into host cells (i.e. LOS, capsule, PorB) have been identified. Clearly, neisserial surface variation serves as an adaptive mechanism which can modulate tissue tropism, immune evasion and survival in the changing host environment. Important progress has been made in recent years with respect to the host cellular receptors and subsequent signal transduction processes which are involved in neisserial adherence, invasion and transcytosis. This has led to the identification of (i) CD46 as a receptor for pilus which allows adherence to epithelial and endothelial cells, (ii) HSPGs, in cooperation with vitronectin and fibronectin, as receptors for a particular subset of Opa proteins and Opc, which may both mediate invasion into most epithelial and endothelial cells, and (iii) CD66 as the receptors for most Opa variants, potentially being involved in cellular interactions including adherence, invasion and transcytosis with epithelial, endothelial and phagocytic cells. As most of these data have been obtained using transformed cell lines growing in vitro, attempts must be made to translate these basic observations into a more natural situation. It can be expected that the successful ongoing integration of laboratory findings from the various infection models with human volunteer studies will further increase our understanding of the biology of neisserial infection. Perhaps the most difficult but also most rewarding challenge for the future will be to use volunteer studies to identify and understand the role of host factors which are important for the infectious process. Hopefully, insights gained from each of these studies will reveal new and useful strategies for the preventive and/or therapeutic intervention into infection and disease by these fascinating microbes.