Mechanism of action of blocking immunoglobulin G for Neisseria gonorrhoeae

Mechanisms of antibody-dependent enhancement of infectious disease

Antibody-dependent enhancement (ADE) of infectious disease is a phenomenon whereby host antibodies increase the severity of an infection. It is well established in viral infections but ADE also has an underappreciated role during bacterial, fungal and parasitic infections. ADE can occur during both primary infections and re-infections with the same or a related pathogen; therefore, understanding the underlying mechanisms of ADE is critical for understanding the pathogenesis and progression of many infectious diseases. Here, we review the four distinct mechanisms by which antibodies increase disease severity during an infection. We discuss the most established mechanistic explanation for ADE, where cross-reactive, disease-enhancing antibodies bound to pathogens interact with Fc receptors, thereby enhancing pathogen entry or replication, ultimately increasing the total pathogen load. Additionally, we explore how some pathogenic antibodies can shield bacteria from complement-dependent killing, thereby enhancing bacterial survival. We interrogate the molecular mechanisms by which antibodies can amplify inflammation to drive severe disease, even in the absence of increased pathogen replication. We also examine emerging roles for autoantibodies in enhancing the pathogenesis of infectious diseases. Finally, we discuss how we can leverage these insights to improve vaccine design and future treatments for infectious diseases.

Preclinical efficacy of a cell division protein candidate gonococcal vaccine identified by artificial intelligence

IMPORTANCE

Vaccines to curb the global spread of multidrug-resistant gonorrhea are urgently needed. Here, 26 vaccine candidates identified by an artificial intelligence-driven platform (Efficacy Discriminative Educated Network[EDEN]) were screened for efficacy in the mouse vaginal colonization model. Complement-dependent bactericidal activity of antisera and the EDEN protective scores both correlated positively with the reduction in overall bacterial colonization burden. NGO1549 (FtsN) and NGO0265, both involved in cell division, displayed the best activity and were selected for further development. Both antigens, when fused to create a chimeric protein, elicited bactericidal antibodies against a wide array of gonococcal isolates and significantly attenuated the duration and burden of gonococcal colonization of mouse vaginas. Protection was abrogated in mice that lacked complement C9, the last step in the formation of the membrane attack complex pore, suggesting complement-dependent bactericidal activity as a mechanistic correlate of protection of the vaccine. FtsN and NGO0265 represent promising vaccine candidates against gonorrhea.

Immune responses to Neisseria gonorrhoeae and implications for vaccine development

Neisseria gonorrheoae is the causative agent of gonorrhea, a sexually transmitted infection responsible for a major burden of disease with a high global prevalence. Protective immunity to infection is often not observed in humans, possible due to high variability of key antigens, induction of blocking antibodies, or a large number of infections being relatively superficial and not inducing a strong immune response. N. gonorrhoeae is a strictly human pathogen, however, studies using mouse models provide useful insights into the immune response to gonorrhea. In mice, N. gonorrhoea appears to avoid a protective Th1 response by inducing a less protective Th17 response. In mouse models, candidate vaccines which provoke a Th1 response can accelerate the clearance of gonococcus from the mouse female genital tract. Human studies indicate that natural infection often induces a limited immune response, with modest antibody responses, which may correlate with the clinical severity of gonococcal disease. Studies of cytokine responses to gonococcal infection in humans provide conflicting evidence as to whether infection induces an IL-17 response. However, there is evidence for limited induction of protective immunity from a study of female sex workers in Kenya. A controlled human infection model (CHIM) has been used to examine the immune response to gonococcal infection in male volunteers, but has not to date demonstrated protection against re-infection. Correlates of protection for gonorrhea are lacking, which has hampered the progress towards developing a successful vaccine. However, the finding that the Neisseria meningitidis serogroup B vaccines, elicit cross-protection against gonorrhea has invigorated the gonococcal vaccine field. More studies of infection in humans, either natural infection or CHIM studies, are needed to understand better gonococcal protective immunity.

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.

Markerless gene editing in Neisseria gonorrhoeae

Neisseria gonorrhoeae, the gonococcus, is a pathogen of major public health concern, but sophisticated approaches to gene manipulation are limited for this species. For example, there are few methods for generating markerless mutations, which allow the generation of precise point mutations and deletions without introducing additional DNA sequence. Markerless mutations are central to studying pathogenesis, the spread of antimicrobial resistance (AMR) and for vaccine development. Here we describe the use of galK as a counter-selectable marker that can be used for markerless mutagenesis in N. gonorrhoeae. galK encodes galactokinase, an enzyme that metabolizes galactose in bacteria that can utilize it as a sole carbon source. GalK can also phosphorylate a galactose analogue, 2-deoxy-galactose (2-DOG), into a toxic, non-metabolisable intermediate, 2-deoxy-galactose-1-phosphate. We utilized this property of GalK to develop a markerless approach for mutagenesis in N. gonorrhoeae. We successfully deleted both chromosomally and plasmid-encoded genes, that are important for gonococcal vaccine development and studies of AMR spread. We designed a positive-negative selection cassette, based on an antibiotic resistance marker and galK, that efficiently rendered N. gonorrhoeae susceptible to growth on 2-DOG. We then adapted the galK-based counter-selection and the use of 2-DOG for markerless mutagenesis, and applied biochemical and phenotypic analyses to confirm the absence of target genes. We show that our markerless mutagenesis method for N. gonorrhoeae has a high success rate, and should be a valuable gene editing tool in the future.

Vaccine Candidates for the Control and Prevention of the Sexually Transmitted Disease Gonorrhea

The World Health Organization (WHO) has placed N. gonorrhoeae on the global priority list of antimicrobial resistant pathogens and is urgently seeking the development of new intervention strategies. N. gonorrhoeae causes 86.9 million cases globally per annum. The effects of gonococcal disease are seen predominantly in women and children and especially in the Australian Indigenous community. While economic modelling suggests that this infection alone may directly cost the USA health care system USD 11.0–20.6 billion, indirect costs associated with adverse disease and pregnancy outcomes, disease prevention, and productivity loss, mean that the overall effect of the disease is far greater still. In this review, we summate the current progress towards the development of a gonorrhea vaccine and describe the clinical trials being undertaken in Australia to assess the efficacy of the current formulation of Bexsero^® in controlling disease.

Antibody-Dependent Enhancement of Bacterial Disease: Prevalence, Mechanisms, and Treatment

Antibody-dependent enhancement (ADE) of viral disease has been demonstrated for infections caused by flaviviruses and influenza viruses; however, antibodies that enhance bacterial disease are relatively unknown. In recent years, a few studies have directly linked antibodies with exacerbation of bacterial disease. This ADE of bacterial disease has been observed in mouse models and human patients with bacterial infections. This antibody-mediated enhancement of bacterial infection is driven by various mechanisms that are disparate from those found in viral ADE. This review aims to highlight and discuss historic evidence, potential molecular mechanisms, and current therapies for ADE of bacterial infection. Based on specific case studies, we report how plasmapheresis has been successfully used in patients to ameliorate infection-related symptomatology associated with bacterial ADE. A greater understanding and appreciation of bacterial ADE of infection and disease could lead to better management of infections and inform current vaccine development efforts.

Complement interactions with the pathogenic Neisseriae: clinical features, deficiency states, and evasion mechanisms

Neisseria gonorrhoeae causes the sexually transmitted infection gonorrhea, while Neisseria meningitidis is an important cause of bacterial meningitis and sepsis. Complement is a central arm of innate immune defenses and plays an important role in combating Neisserial infections. Persons with congenital and acquired defects in complement are at a significantly higher risk for invasive Neisserial infections such as invasive meningococcal disease and disseminated gonococcal infection compared to the general population. Of note, Neisseria gonorrhoeae and Neisseria meningitidis can only infect humans, which in part may be related to their ability to evade only human complement. This review summarizes the epidemiologic and clinical aspects of Neisserial infections in persons with defects in the complement system. Mechanisms used by these pathogens to subvert killing by complement and preclinical studies showing how these complement evasion strategies may be used to counteract the global threat of meningococcal and gonococcal infections are discussed.

Complement-Dependent Serum Bactericidal Assays for Neisseria gonorrhoeae

The global spread of multidrug-resistant gonorrhea has spurred efforts to develop a safe and effective vaccine against gonorrhea. Complement plays an important role in host defenses against Neisseria infections. Complement-dependent bactericidal activity of antibodies (either natural antibodies or those elicited by immunization) is a well-established correlate of protection against meningococcal infections. Although correlates of protection against gonococcal infection have not been defined, there is evidence to suggest that complement-mediated killing may also predict vaccine efficacy against this disease. This chapter describes methods to prepare human complement sources and perform bactericidal assays against Neisseria gonorrhoeae.

Human Immune Responses and the Natural History of Neisseria gonorrhoeae Infection

The intimate relationship between humans and Neisseria gonorrhoeae infections span centuries, which is evidenced in case reports from studies dating back to the late 1700s and historical references that predate medical literature. N. gonorrhoeae is an exclusive human pathogen that infects the genital tract of both men and women as well as other mucosal surfaces including the oropharynx and rectum. In symptomatic infections, N. gonorrhoeae induces a robust inflammatory response at the site of infection. However, infections can also present asymptomatically complicating efforts to reduce transmission. N. gonorrhoeae infections have been effectively treated with antibiotics since their use was introduced in humans. Despite the existence of effective antibiotic treatments, N. gonorrhoeae remains one of the most common sexually transmitted pathogens and antibiotic resistant strains have arisen that limit treatment options. Development of a vaccine to prevent infection is considered a critical element of controlling this pathogen. The efforts to generate an effective gonococcal vaccine is limited by our poor understanding of the natural immunologic responses to infection. It is largely accepted that natural protective immunity to N. gonorrhoeae infections in humans does not occur or is very rare. Previous studies of the natural history of infection as well as some of the humoral and cellular immune responses to infection offer a window into the issues surrounding N. gonorrhoeae vaccine development. In this review, we summarize the current body of knowledge pertaining to human immune responses to gonococcal infections and the role of these responses in mediating protection from N. gonorrhoeae.

Different Bactericidal and Inflammatory Activities of Human and Mouse Blood

We performed side-by-side experiments to compare the behavior of four strains of Escherichia coli and one strain of Pseudomonas aeruginosa in fresh human and mouse blood. Bacteria were multiplied in mouse whole blood and plasma but were killed in human whole blood and plasma. The percentage of granulocytes associated with fluorescence-labeled heat-killed E coli relative to total leukocytes counted was higher in human compared to mouse blood as assessed by flow cytometry analysis. Concentrations of proinflammatory cytokines were high in human blood, but undetectable in mouse blood despite high concentrations of bacteria. We conclude that bacterial killing, phagocytosis, and cytokine induction in blood during human bacteremia with these organisms are probably not mimicked in mouse models of bacterial challenge. Understanding the mechanisms for low cytokine induction with high bacterial loads in mouse blood may be helpful to interpret murine models of bacteremia and develop new approaches for treating sepsis in humans.

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.

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.

Phase-Variable Heptose I Glycan Extensions Modulate Efficacy of 2C7 Vaccine Antibody Directed against Neisseria gonorrhoeae Lipooligosaccharide

Neisseria gonorrhoeae, the causative agent of the sexually transmitted infection, gonorrhea, has developed resistance to most conventional antibiotics. Safe and effective vaccines against gonorrhea are needed urgently. A candidate vaccine that targets a lipooligosaccharide (LOS) epitope recognized mAb 2C7 attenuates gonococcal burden in the mouse vaginal colonization model. Glycan extensions from the LOS core heptoses (HepI and HepII) are controlled by phase-variable LOS glycosyltransferase (lgt) genes; we sought to define how HepI glycan extensions affect mAb 2C7 function. Isogenic gonococcal mutants in which the lgt required for mAb 2C7 reactivity (lgtG) was genetically locked on and the lgt loci required for HepI variation (lgtA, lgtC, and lgtD) were genetically locked on or off in different combinations were created. We observed 100% complement-dependent killing by mAb 2C7 of a mutant that expressed lactose (Gal-Glc) from HepI, whereas a mutant that expressed Gal-Gal-Glc-HepI fully resisted killing (>100% survival). Mutants that elaborated 4- (Gal-GlcNAc-Gal-Glc-HepI) and 5-glycan (GalNAc-Gal-GlcNAc-Gal-Glc-HepI) structures displayed intermediate phenotypes (<50% killing with 2 μg/ml and >95% killing with 4 μg/ml mAb 2C7). The contrasting phenotypes of the lactose-HepI and the Gal-Gal-Glc-HepI LOS structures were recapitulated with phase variants of a recently isolated clinical strain. Despite lack of killing of the Gal-Gal-Glc-HepI mutants, mAb 2C7 deposited sufficient C3 on these bacteria for opsonophagocytic killing by human neutrophils. In conclusion, mAb 2C7 showed functional activity against all gonococcal HepI LOS structures defined by various lgtA/C/D on/off combinations, thereby providing further impetus for use of the 2C7 epitope in a gonococcal vaccine.

Antibody to reduction modifiable protein increases the bacterial burden and the duration of gonococcal infection in a mouse model

Antibodies against reduction modifiable protein (anti-Rmp Abs) can block complement-dependent killing of Neisseria gonorrhoeae by otherwise bactericidal Abs. An anti-lipooligosaccharide bactericidal monoclonal Ab (mAb) 2C7, a gonococcal vaccine candidate Ab, attenuates vaginal colonization by gonococci in BALB/c mice. Here we show that anti-Rmp Abs block the efficacy of mAb 2C7 in mice in a dose-dependent manner. Anti-Rmp Abs also counteract 2C7-mediated enhancement of C3 deposition on gonococci in vivo. The mouse model will prove useful to study how blocking Abs influence the efficacy of gonococcal vaccines. Preexisting anti-Rmp Abs will be an important consideration in evaluating the efficacy of gonococcal vaccine candidates.

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.

Properdin is critical for antibody-dependent bactericidal activity against Neisseria gonorrhoeae that recruit C4b-binding protein

Gonorrhea, a sexually transmitted disease caused by Neisseria gonorrhoeae, is an important cause of morbidity worldwide. A safe and effective vaccine against gonorrhea is needed because of emerging resistance of gonococci to almost every class of antibiotic. A gonococcal lipooligosaccharide epitope defined by the mAb 2C7 is being evaluated as a candidate for development of an Ab-based vaccine. Immune Abs against N. gonorrhoeae need to overcome several subversive mechanisms whereby gonococcus evades complement, including binding to C4b-binding protein (C4BP; classical pathway inhibitor) and factor H (alternative pathway [AP] inhibitor). The role of AP recruitment and, in particular, properdin in assisting killing of gonococci by specific Abs is the subject of this study. We show that only those gonococcal strains that bind C4BP require properdin for killing by 2C7, whereas strains that do not bind C4BP are efficiently killed by 2C7 even when AP function is blocked. C3 deposition on bacteria mirrored killing. Recruitment of the AP by mAb 2C7, as measured by factor B binding, occurred in a properdin-dependent manner. These findings were confirmed using isogenic mutant strains that differed in their ability to bind to C4BP. Immune human serum that contained bactericidal Abs directed against the 2C7 lipooligosaccharide epitope as well as murine antigonococcal antiserum required functional properdin to kill C4BP-binding strains, but not C4BP-nonbinding strains. Collectively, these data point to an important role for properdin in facilitating immune Ab-mediated complement-dependent killing of gonococcal strains that inhibit the classical pathway by recruiting C4BP.

Novel blocking human IgG directed against the pentapeptide repeat motifs of Neisseria meningitidis Lip/H.8 and Laz lipoproteins

Ab-initiated, complement-dependent killing contributes to host defenses against invasive meningococcal disease. Sera from nonimmunized individuals vary widely in their bactericidal activity against group B meningococci. We show that IgG isolated from select individuals can block killing of group B meningococci by human sera that are otherwise bactericidal. This IgG also reduced the bactericidal efficacy of Abs directed against the group B meningococcal protein vaccine candidates factor H-binding protein currently undergoing clinical trials and Neisserial surface protein A. Immunoblots revealed that the blocking IgG was directed against a meningococcal Ag called H.8. Killing of meningococci in reactions containing bactericidal mAbs and human blocking Abs was restored when binding of blocking Ab to meningococci was inhibited using either synthetic peptides corresponding to H.8 or a nonblocking mAb against H.8. Furthermore, genetic deletion of H.8 from target organisms abrogated blocking. The Fc region of the blocking IgG was required for blocking because F(ab')(2) fragments were ineffective. Blocking required IgG glycosylation because deglycosylation with peptide:N-glycanase eliminated blocking. C4b deposition mediated by an anti-factor H-binding protein mAb was reduced by intact blocking IgG, but not by peptide:N-glycanase-treated blocking IgG, suggesting that blocking resulted from inhibition of classical pathway of complement. In conclusion, we have identified H.8 as a meningococcal target for novel blocking Abs in human serum. Such blocking Abs may reduce the efficacy of select antigroup B meningococcal protein vaccines. We also propose that outer membrane vesicle-containing meningococcal vaccines may be more efficacious if purged of subversive immunogens such as H.8.

Pathogenic neisseriae: surface modulation, pathogenesis and infection control

Although renowned as a lethal pathogen, Neisseria meningitidis has adapted to be a commensal of the human nasopharynx. It shares extensive genetic and antigenic similarities with the urogenital pathogen Neisseria gonorrhoeae but displays a distinct lifestyle and niche preference. Together, they pose a considerable challenge for vaccine development as they modulate their surface structures with remarkable speed. Nonetheless, their host-cell attachment and invasion capacity is maintained, a property that could be exploited to combat tissue infiltration. With the primary focus on N. meningitidis, this Review examines the known mechanisms used by these pathogens for niche establishment and the challenges such mechanisms pose for infection control.

Bacterial complement escape

Complement activation is a crucial step in our innate immune defense against invading bacteria. Complement proteins can quickly recognize invading bacteria and subsequently label them for phagocytosis or kill them by direct lysis. In order to survive in the human host, bacterial pathogens have evolved a number of excreted and membrane-bound proteins that interfere with several steps of the complement cascade. In this chapter we summarize the most successful complement-modulating strategies by human bacterial pathogens.

Monoclonal antibodies can affect complement deposition on the capsule of the pathogenic fungus Cryptococcus neoformans by both classical pathway activation and steric hindrance

The capsule of the human pathogenic fungus Cryptococcus neoformans presents the immune system with a formidable problem for phagocytosis. Capsule-mediated activation of the alternative complement (C) pathway results in component 3 (particularly, C3) binding to the capsule near the cell wall surface. Hence, for cells with large capsule, C3 cannot interact with the complement receptor (CR) and is not opsonic. However, C activation in either immune serum or in the presence of monoclonal antibody (mAb) to capsular polysaccharide localizes C3 to the capsular edge. When C. neoformans cells were coated with both C and antibody (Ab) opsonins, Ab bound first and promoted C3 deposition at the edge of the capsule. The mechanism for the Ab-mediated change in C3 localization to the capsule edge involved both classical C pathway activation and steric hindrance preventing C3 penetration into the capsule. The change in C3 localization changed the mode of phagocytosis in macrophages, such that localizing C3 at the edge of the capsule allowed phagocytosis through C3-CR3 and C3-CR4 interactions, which did not occur in serum without Ab. These findings reveal a new mechanism of Ab action whereby Abs affect the location of C3 and its interaction with its receptor in macrophages depending on the immunoglobulin concentration.

Comparison of immune responses to gonococcal PorB delivered as outer membrane vesicles, recombinant protein, or Venezuelan equine encephalitis virus replicon particles

Porin (PorB) is a major outer membrane protein produced by all Neisseria gonorrhoeae strains and has been a focus of intense interest as a vaccine candidate. In this study, the immunogenicity of PorB in mice was investigated after several immunization regimens. Outer membrane vesicles (OMV), recombinant renatured PorB (rrPorB), and PorB-expressing Venezuelan equine encephalitis (VEE) virus replicon particles (PorB VRP) were delivered intranasally (i.n.) or subcutaneously (s.c.) into the dorsal area or the hind footpad in three-dose schedules; the PorB VRP-immunized mice were given a single additional booster dose of rrPorB in Ribi adjuvant. Different delivery systems and administration routes induced different immune responses. Mice immunized s.c. with rrPorB in Ribi had the highest levels of PorB-specific serum immunoglobulin G (IgG) by enzyme-linked immunosorbent assay. Surprisingly, there was an apparent Th1 bias, based on IgG1/IgG2a ratios, after immunization with rrPorB in Ribi in the footpad while the same vaccine given in the dorsal area gave a strongly Th2-biased response. PorB VRP-immunized mice produced a consistent Th1 response with a high gamma interferon response in stimulated splenic lymphocytes and very low IgG1/IgG2a ratios. Immunization by OMV delivered i.n. was the only regimen that resulted in a serum bactericidal response, and it generated an excellent mucosal IgA response. Serum from mice immunized with rrPorB preferentially recognized the surface of whole gonococci expressing a homologous PorB, whereas serum from PorB VRP-immunized mice had relatively low whole-cell binding activity but recognized both heterologous and homologous PorB equally. The data resulting from this direct comparison suggested that important aspects of the immune response can be manipulated by altering the form of the antigen and its delivery. This information coupled with an understanding of protective antigonococcal immune responses will enable the design of the optimal vaccine for N. gonorrhoeae.

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.

Antibodies to keyhole limpet hemocyanin cross-react with an epitope on the polysaccharide capsule of Cryptococcus neoformans and other carbohydrates: implications for vaccine development

Cryptococcus neoformans causes a life-threatening meningoencephalitis in AIDS patients. Mice immunized with a glycoconjugate vaccine composed of the glucuronoxylomannan (GXM) component of the cryptococcal capsular polysaccharide conjugated to tetanus toxoid produce Abs that can be either protective or nonprotective. Because nonprotective Abs block the efficacy of protective Abs, an effective vaccine must focus the Ab response on a protective epitope. Mice immunized with peptide mimetics of GXM conjugated to keyhole limpet hemocyanin (KLH) with glutaraldehyde developed Abs to GXM. However, control peptides P315 and P24 conjugated to KLH also elicited Abs to GXM. GXM-binding Abs from mice immunized with P315-KLH were inhibited by KLH treated with glutaraldehyde (KLH-g), but not by P315. Furthermore, KLH-g inhibited binding of GXM by serum of mice immunized with GXM-TT, indicating that glutaraldehyde treatment of KLH reveals an epitope(s) that cross-reacts with GXM. Vaccination with KLH-g or unmodified KLH elicited Abs to GXM, but did not confer protection against C. neoformans, suggesting the cross-reactive epitope on KLH was not protective. This was supported by the finding that 4H3, a nonprotective mAb, cross-reacted strongly with KLH-g. Sera from mice immunized with either native KLH or KLH-g cross-reacted with several other carbohydrate Ags, many of which have been conjugated to KLH for vaccine development. This study illustrates how mAbs can be used to determine the efficacy of potential vaccines, in addition to describing the complexity of using KLH and glutaraldehyde in the development of vaccines to carbohydrate Ags.

High affinity mimotope of the polysaccharide capsule of Cryptococcus neoformans identified from an evolutionary phage peptide library

Cryptococcus neoformans causes a life-threatening meningoencephalitis in a significant percentage of AIDS patients. Mice immunized with a glycoconjugate vaccine composed of the glucuronoxylomannan (GXM) component of the cryptococcal capsular polysaccharide conjugated to tetanus toxoid (TT) produce Abs that, based on the epitope recognized, can be either protective or nonprotective. Since nonprotective Abs block the efficacy of protective Abs, we are interested in developing a vaccine that would focus the immune response specifically to protective epitopes. Previously, we screened a phage display library with 2H1, a protective anti-GXM mAb, and isolated PA1, a representative peptide that had a K(d) of 295 nM for 2H1. Mice immunized with PA1 conjugated to keyhole limpet hemocyanin developed high anti-peptide (1/13,000), but low anti-GXM (maximum, 1/200) titers. We now report our efforts to improve this vaccine by screening a sublibrary with six random amino acids added to either end of the PA1 motif to identify higher affinity peptides. P206.1, a peptide isolated from this sublibrary, had 80-fold higher affinity for 2H1 (K(d) = 3.7 nM) than PA1. P206.1 bound protective, but not nonprotective, anti-GXM Abs. Mice immunized with P206.1 conjugated to various carriers did not mount an Ab response to GXM despite developing high anti-peptide titers. However, mice primed with GXM-TT and boosted with P206.1-TT developed significant anti-GXM titers (maximum, 1/180,000). This latter immunization scheme focused the immune response on protective epitopes, since only 2-5% of these titers were directed against nonprotective de-O-acetylated GXM epitopes compared with 20-60% in animals primed and boosted with GXM-TT.

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.

Structural and immunochemical characterization of the lipooligosaccharides expressed by Neisseria subflava 44

Neisserial lipooligosaccharides (LOSs) are a family of complex cell surface glycolipids. We used mass spectrometry techniques (electrospray ionization, collision-induced dissociation, and multiple step), combined with fluorophore-assisted carbohydrate electrophoresis monosaccharide composition analysis, to determine the structure of the two low-molecular-mass LOS molecules (LOSI and LOSII) expressed by Neisseria subflava 44. We determined that LOSI contains one glucose on both the alpha and beta chains. LOSII is structurally related to LOSI and differs from it by the addition of a hexose (either glucose or galactose) on the alpha chain. LOSI and LOSII were able to bind monoclonal antibody (MAb) 25-1-LC1 when analyzed by Western blotting experiments. We used a set of genetically defined Neisseria gonorrhoeae mutants that expressed single defined LOS epitopes and a group of Neisseria meningitidis strains that expresses chemically defined LOS components to determine the structures recognized by MAb 25-1-LC1. We found that extensions onto the beta-chain glucose of LOSI block the recognition by this MAb, as does further elongation from the LOSII alpha chain. The LOSI structure was determined to be the minimum structure that is recognized by MAb 25-1-LC1.

Development of natural immunity to Neisseria meningitidis

Although meningococcal disease is rare in industrialized nations, Neisseria meningitidis holds a prominent position amongst pediatric infections because of the dramatic clinical presentation of the disease, high mortality, epidemic potential and the recent disappearance of many other important infectious diseases in developed countries through improvements in public health and vaccination. The precise nature of natural immunity to meningococci remains unknown, although a complex interaction between the organism and nasopharyngeal mucosal barrier, innate immune mechanisms and acquired immunity is involved. Study of the mechanisms of natural immunity may provide the key to development of vaccines that can reduce the burden of disease in early childhood.

Complement-resistance mechanisms of bacteria

Despite more than a century of parallel research on bacteria and the complement system, relatively little is known of the mechanisms whereby pathogenic bacteria can escape complement-related opsonophagocytosis and direct killing. It is likely that pathogenicity in bacteria has arisen more accidentally than in viruses, and on the basis of selection from natural mutants rather than by outright stealing or copying of genetic codes from the host. In this review we will discuss complement resistance as one of the features that makes a bacterium a pathogen.

Functional activities and epitope specificity of human and murine antibodies against the class 4 outer membrane protein (Rmp) of Neisseria meningitidis

Antibodies against the class 4 outer membrane protein (OMP) from Neisseria meningitidis have been purified from sera from vaccinees immunized with the Norwegian meningococcal group B outer membrane vesicle vaccine. The human sera and purified antibodies reacted strongly with the class 4 OMP in immunoblots, whereas experiments with whole bacteria showed only weak reactions, indicating that the antibodies mainly reacted with parts of the class 4 molecule that were not exposed. The purified human anti-class 4 OMP antibodies and the monoclonal antibodies (MAbs) were neither bactericidal nor opsonic against live meningococci. Three new MAbs against the class 4 OMP were generated and compared with other, previously described MAbs. Three linear epitopes in different regions of the class 4 OMP were identified by the reaction of MAbs with synthetic peptides. The MAbs showed no blocking effect on bactericidal activity of MAbs against other OMPs. However, one of the eight purified human anti-class 4 OMP antibody preparations, selected from immunoblot reactions among sera from 27 vaccinees, inhibited at high concentrations the bactericidal effect of a MAb against the class 1 OMP. However, these antibodies were not vaccine induced, as they were present also before vaccination. Therefore, this study gave no evidence that vaccination with a meningococcal outer membrane vesicle vaccine containing the class 4 OMP induces blocking antibodies. Our data indicated that the structure of class 4 OMP does not correspond to standard beta-barrel structures of integral OMPs and that no substantial portion of the OmpA-like C-terminal region of this protein is located at the surface of the outer membrane.

Characterization of human xenoreactive antibodies in liver failure patients exposed to pig hepatocytes after bioartificial liver treatment: an ex vivo model of pig to human xenotransplantation

BACKGROUND

There are limited experimental data on the nature of the humoral response elicited in humans against pig antigens. In this study, we have examined the xenoantibody (XAb) response in eight patients with acute liver failure exposed to pig hepatocytes after treatment with the bioartificial liver (BAL).

METHODS

Patients' plasma samples obtained before and after BAL treatment were tested for IgM and IgG XAbs, IgG subclasses, and XAb cytotoxicity, using enzyme-linked immunosorbent assay and flow-cytometric assays. The characterization of pig aortic endothelial cell (PAEC) surface xenoantigens was analyzed by immunoprecipitation.

RESULTS

We observed by day 10, a strong anti-pig IgG and IgM XAb response in patients undergoing two or more BAL treatments, with a significant increase in all the IgG subclasses; in contrast, XAb titers did not change if the patients received only one BAL treatment. The majority of the XAbs produced to porcine antigens were primarily specific for the alphaGal epitope. Both IgG and IgM XAbs were cytotoxic to PAECs, and the cytotoxic activity of IgG was associated with high levels of IgG1 and IgG3 subclasses, known to be efficient on complement activation. The characterization of porcine surface antigens demonstrated that IgM human XAbs, before and after BAL exposure, recognized xenoantigens on PAECs with similar molecular weights, suggesting that the same population of XAbs were present in the patients before and after exposure to pig antigens.

CONCLUSIONS

Repetitive exposure of humans to porcine antigens after BAL treatment, results in a strong IgG and IgM XAb responses that are primarily directed against the alphaGal epitope. These XAbs are cytotoxic to PAECs and the IgG toxicity correlates with high IgG1 and IgG3 levels. Our data also suggest that no new XAb specificity emerges after porcine exposure.

Binding of complement factor H to loop 5 of porin protein 1A: a molecular mechanism of serum resistance of nonsialylated Neisseria gonorrhoeae

Neisseria gonorrhoeae isolated from patients with disseminated infection are often of the porin (Por1A) serotype and resist killing by nonimmune normal human serum. The molecular basis of this resistance (termed stable serum resistance) in these strains has not been fully defined but is not related to sialylation of lipooligosaccharide. Here we demonstrate that Por1A bearing gonococcal strains bind more factor H, a critical downregulator of the alternative complement pathway, than their Por1B counterparts. This results in a sevenfold reduction in C3b, which is >75% converted to iC3b. Factor H binding to isogenic gonococcal strains that differed only in their porin serotype, confirmed that Por1A was the acceptor molecule for factor H. We identified a surface exposed region on the Por1A molecule that served as the binding site for factor H. We used gonococcal strains with hybrid Por1A/B molecules that differed in their surface exposed domains to localize the factor H binding site to loop 5 of Por1A. This was confirmed by inhibition of factor H binding using synthetic peptides corresponding to the putative exposed regions of the porin loops. The addition of Por1A loop 5 peptide in a serum bactericidal assay, which inhibited binding of factor H to the bacterial surface, permitted 50% killing of an otherwise completely serum resistant gonococcal strain. Collectively, these data provide a molecular basis to explain serum resistance of Por1A strains of N. gonorrhoeae.

Alloantigen-blocking antibodies in sera from highly sensitized uremic patients: antibody class and relationship with lymphocytotoxins

BACKGROUND

Previously, we identified an antimitogenic IgG antibody separated from sera of patients with known kidney transplant chronic rejection. This antibody inhibits individual patients' own unprimed T helper cell responses to alloantigens as well as a third-party mixed lymphocyte response, but does not inhibit autologous unprimed T helper cell proliferation to adherent anti-CD3 antibody. We suggest that the mechanism of inhibitory action is allogeneic-dependent.

METHODS

We used a series of similar experimental designs to test the presence of this antibody in uremic, sensitized patients and have studied its relationship to sensitization as defined by the presence of lymphocytotoxins in four uremic groups: highly sensitized with or without previous graft loss, moderately sensitized with or without graft loss, nonsensitized without previous graft loss, and nonsensitized with graft loss.

RESULTS

(1) Sensitization is associated with the presence of a potent antibody that blocks primary mixed lymphocyte response. Primed cells are less susceptible to its antimitogenic action. (2) The blocking antibody activity is present only in sensitized patients who have IgG lymphocytotoxic activity against the same HLA class I antigens. (3) The blocking activity is unequal in the following order: IgG 3 > IgG 1 > IgG 2. (4) Although IgG 1 and 2 fractions contain lymphocytotoxic activity against HLA class I antigens, the IgG 3 fraction does not.

CONCLUSIONS

The differential effect of IgG antibodies on naive and memory T cells may explain why humeral responses to alloantigens can be maintained in the presence of blocking antibodies.

Selection of Opa+ Neisseria gonorrhoeae by limited availability of normal human serum

Experimental infections of human male volunteers with Neisseria gonorrhoeae have provided valuable insights into the early stages of gonorrheal disease. Bacterial variants expressing outer membrane opacity (Opa) proteins appear to be selected from the inoculum during a period in which total recoverable numbers of bacteria decrease rapidly. This apparent survival advantage occurs simultaneously with the onset of an inflammatory response, characterized by local production of interleukin 6 (IL-6) and IL-8 and the appearance of leukocytes in urine. Since the inflammatory response may also result in the presence of serum factors on the mucosal surface, we investigated the possibility that killing in normal human serum (NHS) leads to the selection of Opa+ variants. We therefore studied killing of separate populations and mixtures of Opa- and Opa+ N. gonorrhoeae MS11mk in NHS. Expression of an Opa protein conferred a survival advantage upon the organism; i.e., the Opa+ variants were more serum resistant than their isogenic Opa- counterparts, resulting in a selection for Opa+ phenotypes when a mixture of Opa+ and Opa- gonococci (GC) was exposed to submaximal doses of NHS. This selection was observed in three different lipooligosaccharide (LOS) backgrounds, indicating that it was not due to a difference in LOS expression between Opa- and Opa+ phenotypes. Incubation in NHS of sialylated GC resulted in a similar selection for Opa+ variants. The presence of normal human urine during the serum killing assay had no effect on the selection phenomenon but drastically depleted NHS of bactericidal activity, which was found to be at least partly due to complement inhibition. The results suggest that serum killing may contribute to the transition from Opa- to Opa+ phenotypes during the early stages of infection of the male urethra.

Immunoglobulin G3 blocking antibodies to the fungal pathogen Cryptococcus neoformans

Vaccination and infection can elicit protective and nonprotective antibodies to the fungus Cryptococcus neoformans in mice. The effect of nonprotective antibodies on host defense is unknown. In this study we used mixtures of protective and nonprotective monoclonal antibodies (mAbs) to determine if nonprotective mAbs blocked the activity of the protective mAbs. Antibody isotype and epitope specificity are important in determining the ability to prolong survival in mice given a lethal C. neoformans infection. Three different nonprotective immunoglobulin (Ig) G23 mAbs to cryptococcal capsular polysaccharide were used to study the interaction between the IgG3 isotype and protective IgG1 and IgG2a mAbs in murine cryptococcal infection. One IgG3 mAb reduced the protective efficacy of an IgG1 with identical epitope specificity. A second IgG3 mAb with different epitope specificity also reduced the protection provided by the IgG1 mAb. The protective efficacy of an IgG2a mAb was also dramatically decreased by still another IgG3 mAb. To our knowledge this is the first report of blocking antibodies to a fungal pathogen. The results have important implications for the development of vaccines and passive antibody therapy against C. neoformans.

Mapping of a surface-exposed, conformational epitope of the P6 protein of Haemophilus influenzae

P6 is an outer membrane protein of Haemophilus influenzae that is antigenically conserved and considered a candidate component of future H. influenzae vaccines. P6 contains a surface-exposed epitope recognized by monoclonal antibody (MAb) 3B9. This epitope has been shown to be distinct from that recognized by the P6-specific MAbs 7F3 and 4G4 in a competitive inhibition enzyme-linked immunosorbent assay (ELISA). MAb 3B9 did not bind to synthetic P6-specific sequential and overlapping hexameric peptides. Five peptides made to correspond to P6 sequences with high probabilities of surface exposure did not inhibit binding of MAb 3B9 to P6. An antiserum to one of the peptides, designated SP66, inhibited binding of MAb 3B9 to P6. A rabbit antiserum to P6 bound to sequential hexameric peptides, Gly-87AsnThrAspGluArgGlyThr-94, which were in the SP66 region of P6. This antiserum inhibited the binding of P6 to MAb 3B9 in a competitive inhibition ELISA. P6 mutations with His and Ala substitutions at residues Thr-88 and Asn-89 still bound MAb 3B9. MAb 3B9 reacted with Escherichia coli OmpA and Salmonella typhimurium OmpA. Sequence comparisons of P6 with these proteins indicated that the residue in the SP66 region responsible for binding is either Gly-87, Asp-90, or Gly-93. Mercaptoethanol reduction abolished MAb 3B9 binding to E. coli OmpA and S. typhimurium OmpA. In these proteins, immediately downstream of the second cysteine, there is an ArgArg dipeptide which is identical to and aligns with Arg-147Arg-148 in P6. This dipeptide has a high probability of surface exposure in P6. Mutagenesis of the Arg-147Arg-148 to an AlaAla dipeptide in P6 abolished binding of MAb 3B9, demonstrating that it was either a portion of the epitope or important in the protein folding necessary for expression of this epitope. This study demonstrates that MAb 3B9 recognizes a conserved conformational determinant on the surface of H. influenzae that is composed of two discontinuous regions of P6.

Recognition and control of neisserial infection by antibody and complement

Immunity to neisserial infection involves complex interactions between antibody, complement and bacterial cell-surface molecules. Neisseria species express polysaccharide and glycolipid membrane components, which downregulate complement activation. The pathogenic potential of Neisseria depends on evasion of the complement cascade.

Heparin protects Opa+ Neisseria gonorrhoeae from the bactericidal action of normal human serum

The pathobiological significance of lipooligosaccharide (LOS) and outer membrane opacity protein (Opa) changes in gonorrheal disease are poorly understood. We assessed variants of strain MS11mk with different LOS and Opa phenotypes for their liability to killing by normal human sera. LOS differences correlated with strikingly disparate susceptibilities to serum killing; LOSa variants were serum resistant, LOSb variants were serum sensitive, and sialylation of LOSb variants enhanced their survival (as reported previously). Opa phenotype had little influence on the killing of serum-sensitive LOSb cells that were incubated directly in normal human sera, but preincubation of Opa+ LOSb variants in heparin increased their serum resistance whereas Opa- LOSb variants showed no change. Some Opa proteins conferred slightly higher resistance than others, but heparin preincubation increased serum resistance for variants expressing each of seven Opa proteins. These in vitro phenomena may relate to conditions within the male urethra where sulfate-containing proteoglycans are abundant and where antibody and complement may transude from blood plasma. The results suggest that the selective advantage for Opa+ Neisseria gonorrhoeae bacteria observed in vivo may reflect their ability to utilize host cell components to resist killing by host defenses.

Contradictory roles for antibody and complement in the interaction of Brucella abortus with its host

The ability of serum complement to kill bacteria has been linked to host resistance to Gram-negative bacteria. A mechanism for killing extracellular organisms during early invasion, following release from infected phagocytic cells, or during bacteremia would contribute to a host's ability to resist disease. In fact, the ability of serum complement to kill bacteria has been linked to disease resistance. Brucella abortus are Gram-negative intracellular pathogens. Resistance to these bacteria involves the coordinated activities of the cellular and humoral immune systems. The existence of serum-resistant forms of B. abortus has been established, and it has been shown that these bacteria can resist the killing action of complement even in the presence of specific antibody. Antibody is usually necessary for complement-mediated killing of smooth (virulent) forms of Gram-negative bacteria. An anomolous situation exists with some isolates of smooth B. abortus. Sera containing high titers of specific antibody do not support killing unless they are diluted. In the bovine, this phenomenon is associated with IgG1 and IgG2 antibodies. This finding may account for the lack of positive correlation between antibody levels and resistance to disease, which has led, perhaps wrongly, to the idea that antibody and complement are not important in resistance to brucellosis. Available evidence suggests that antibody may have contradictory roles in the interactions between a host and bacteria. Avirulent (rough) forms of the organism would be rapidly killed by complement shortly after invasion, but serum-resistant smooth forms of the organism would survive and invade resident phagocytic cells. During the process of invasion and phagocytosis, the bacteria would initiate an immune response. With time, some B. abortus organisms would be released from infected phagocytic cells. In the early stages of this process, the bacteria would encounter IgM antibody and low concentrations of IgG antibody. These would cause complement-mediated killing, and infection would be restricted to resident phagocytic cells. However, the immune response to B. abortus antigens would be intensified, and IgG antibody levels would increase. High concentrations of antibody do no support complement-mediated killing of extracellular B. abortus, but the bacteria would be opsonized by antibody and complement component fragments. This would lead to increased phagocytosis of extracellular B. abortus as they appear, and concomitant extension of disease. Because of high levels of antibody would block complement-mediated killing of B. abortus, resistance to disease at this point would be dependent on cell-mediated immunity.

Immunobiology of purified recombinant outer membrane porin protein I of Neisseria gonorrhoeae

Gonococcal porins (Por) from strains FA19 (Por-1, serogroup A), MS11 (Por-2, serogroup B) and FA6434 (Por-5, a hybrid porin containing epitopes from both serogroups), were expressed in Escherichia coli and purified under non-denaturing conditions. Porins were inserted into liposomes, and they were bound by monoclonal antibodies which bind native Por and intact gonococci, but not denatured Por. All three recombinant porins (rPor) were highly immunogenic in rabbits without additional adjuvant. The rPor antisera were specific for Por by Western blotting and whole-cell radioimmunoprecipitation and were broadly cross-reactive within serogroups. Post-immune, but not pre-immune, sera bound to intact gonococci, induced deposition of complement components C3 and C9 onto gonococcal membranes and increased association with and activation of human neutrophils. Gonococci were not killed in bactericidal assays, and there was no phagocytic killing with gonococci opsonized with recombinant antisera. Lack of killing in bactericidal assays was not caused by the presence of blocking antibodies to the outer-membrane protein Rmp.

Antibody to Rmp (outer membrane protein 3) increases susceptibility to gonococcal infection

The severe adverse effects of gonococcal infection on human fertility suggests that Neisseria gonorrhoeae would exert powerful selection for the development of a protective immune response in humans. N. gonorrhoeae is an obligate human pathogen and must persist in humans to survive. Since it is an ecologically successful organism, it must have evolved strategies to evade any human immune response it elicits. In a longitudinal study among 243 women working as prostitutes and experiencing frequent gonococcal infection, younger women, women with HIV infection, and women with antibody to the gonococcal outer membrane protein 3 (Rmp) were at increased risk of infection (adjusted odds ratio 3.4, CI95% 1.1-10.4, P < 0.05). Rmp is highly conserved in N. gonorrhoeae and the blocking of mucosal defences may be one of its functions. As similar proteins occur in many gram negative mucosal pathogens, the enhancing effect of such proteins may be a general strategy whereby bacteria evade human immune responses.

Inhibition of bactericidal activity of anticapsular antibody by nonspecific antibodies reactive with surface-exposed antigenic determinants on Actinobacillus pleuropneumoniae

In an attempt to understand the mechanism of serum resistance in Actinobacillus pleuropneumoniae, in the present study we examined various interactions among the bacterial surface constituents, serum antibodies, and complement. Analysis of swine sera revealed the presence of anticapsular antibodies in convalescent-phase sera but not in preimmune sera. Both types of sera contained antibodies which reacted with each of 14 polypeptides present in saline extracts of the bacteria. Absorption of the preimmune sera with intact bacteria depleted antibodies to two of the polypeptides (27 and 32 kDa) and high-molecular-weight (greater than 97.4,000) components which did not stain with Coomassie blue. Data derived from complement consumption and C3-binding experiments indicated that the organism was capable of initiating complement activation and binding C3 during incubation in preimmune and immune sera. Experiments designed to evaluate the bactericidal effectiveness of anticapsular antibody revealed that the purified antibody was bactericidal only when preimmune sera absorbed with intact bacteria were used as a source of complement. The bactericidal effects of anticapsular antibody and absorbed preimmune sera were inhibited in a dose-dependent manner by heat-inactivated preimmune sera and immunoglobulin G derived from the sera. The inhibitory activity of the preimmune sera was neutralized by preincubating the sera with column fractions of the saline extract which contained either the 27- or the 32-kDa polypeptide. These results indicate that serum resistance in A. pleuropneumoniae 4074 could be related to inhibition of the bactericidal action of anticapsular antibody by nonspecific antibodies which recognize surface-exposed epitopes on the polypeptides.

Sub-minimal inhibitory concentrations of cefmetazole enhance serum bactericidal activity in vitro by amplifying poly-C9 deposition

Serum-resistant organisms grown in sub-minimal inhibitory concentrations (subMICs) of antibiotics in vitro may be rendered sensitive to complement-mediated, serum bactericidal activity. We measured 125I-C3 and 125I-C9 deposition on genetically serum resistant Salmonella montevideo SH5770 (SH5770) that was rendered serum sensitive by growth in sub-MICs of cefmetazole (CMZ), a parenteral, second generation, cephamycin-group antibiotic. Three times as much C3 and over six times as much C9 bound to SH5770 grown in one-fourth the MIC of CMZ compared to broth-grown bacteria. SDS-PAGE analysis and autoradiography showed that neither the ratio of C3b:iC3b (approximately 1:2.5) nor the nature of the C3-bacterial bond was changed by growing the organisms in CMZ. Large amounts of complement membrane attack complexes containing poly-C9 were seen only on CMZ-grown SH5770 by SDS-PAGE and autoradiography. Poly-C9 was also detected only on CMZ-grown bacteria by indirect immunofluorescence and ELISA using a murine monoclonal antibody directed against a neoantigen of poly-C9. Bacterial hydrophobicity increased after growth in CMZ, and transmission electron micrographs of CMZ-grown SH5770 showed cell wall disruption and blebbing. These results indicate that growth in subMICs of CMZ increases bacterial hydrophobic domains available for interacting with the membrane attack complex, C5b-9, allowing formation and stable insertion of bactericidal complexes containing poly-C9.

Human natural anti-Gal IgG regulates alternative complement pathway activation on bacterial surfaces

One percent of circulating IgG in humans recognizes galactose alpha 1,3 galactose residues (anti-Gal) and is synthesized in response to stimulation by enteric bacteria. In this study, we found that the prevalence of binding of anti-Gal to blood isolates is significantly higher than its binding to normal stool isolates. When anti-Gal bound onto the lipopolysaccharide of a representative blood isolate, Serratia marcescens #21, it blocked its alternative complement pathway (ACP) lysis and made the organism serum resistant. In contrast, when anti-Gal bound to the capsular polysaccharide of a serum sensitive Serratia, #7, it increased ACP killing of this strain. The mechanism of blockade of ACP lysis by anti-Gal did not involve a decrease in the number of C3 molecules deposited onto Serratia #21 or an inhibition of the binding of C3b to its LPS, nor did it change the iC3b and C3d degradation products of bound C3b or prevent membrane attack complex formation on this organism. Our findings suggest that the effect of anti-Gal on immune lysis is dependent on the bacterial outer membrane structure to which it binds. We postulate that anti-Gal may play a role in the survival of selected Enterobacteriacae in Gram-negative sepsis by blocking ACP-mediated lysis of such bacteria by the nonimmune host, and that this effect depends on where anti-Gal finds its epitope on the bacterial outer membrane.

The interaction of naturally elaborated blebs from serum-susceptible and serum-resistant strains of Neisseria gonorrhoeae with normal human serum

We studied the interaction of normal human serum immunoglobulins with outer-membrane bleb antigens of Neisseria gonorrhoeae. Gonococcal 68,000 Dalton and Lip (H.8 antigen) outer-membrane proteins were recognized by normal human serum immunoglobulins in blebs from serum-resistant strains, but not in blebs from serum-susceptible strains. The addition of blebs from a serum-resistant strain to bactericidal assays resulted in significantly greater inhibition of serum killing than the addition of blebs from a serum-susceptible strain. Our results indicate that blebs from two serum-resistant gonococcal strains have an enhanced ability to bind and remove cell-targeted bactericidal factors, and that outer-membrane blebbing may contribute to serum resistance.