SummaryFactor H (fH), a key alternative complement pathway regulator, is a cofactor for factor I-mediated cleavage of C3b. fH consists of 20 short consensus repeat (SCR) domains. Sialic acid binding domains have previously been localized to fH SCRs 6-10 and 13. To examine fH binding on a sialylated microbial surface, we grew Neisseria gonorrhoeae in the presence of 5 Ј -cytidinemonophospho-N -acetylneuraminic acid, which sialylates lipooligosaccharide and converts to serum resistance gonococci previously sensitive to nonimmune serum killing. fH domains necessary for binding sialylated gonococci were determined by incubating organisms with recombinant human fH (rH) and nine mutant rH molecules (deletions spanning the entire fH molecule). rH and all mutant rH molecules that contained SCRs 16-20 bound to the sialylated strain; no mutant molecule bound to serum-sensitive nonsialylated organisms. Sialic acid was demonstrated to be the fH target by flow cytometry that showed a fourfold increase in fH binding that was reversed by neuraminidase-mediated cleavage of sialic acid off gonococci. Functional specificity of fH was confirmed by decreased total C3 binding and almost complete conversion to iC3b on sialylated gonococci. Sialic acid can therefore bind fH uniquely through SCRs 16-20. This blocks complement pathway activation for N. gonorrhoeae at the level of C3.

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SUMMARY The complement system comprises several fluid-phase and membrane-associated proteins. Under physiological conditions, activation of the fluid-phase components of complement is maintained under tight control and complement activation occurs primarily on surfaces recognized as “nonself” in an attempt to minimize damage to bystander host cells. Membrane complement components act to limit complement activation on host cells or to facilitate uptake of antigens or microbes “tagged” with complement fragments. While this review focuses on the role of complement in infectious diseases, work over the past couple of decades has defined several important functions of complement distinct from that of combating infections. Activation of complement in the fluid phase can occur through the classical, lectin, or alternative pathway. Deficiencies of components of the classical pathway lead to the development of autoimmune disorders and predispose individuals to recurrent respiratory infections and infections caused by encapsulated organisms, including Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae. While no individual with complete mannan-binding lectin (MBL) deficiency has been identified, low MBL levels have been linked to predisposition to, or severity of, several diseases. It appears that MBL may play an important role in children, who have a relatively immature adaptive immune response. C3 is the point at which all complement pathways converge, and complete deficiency of C3 invariably leads to severe infections, including those caused by meningococci and pneumococci. Deficiencies of the alternative and terminal complement pathways result in an almost exclusive predisposition to invasive meningococcal disease. The spleen plays an important role in antigen processing and the production of antibodies. Splenic macrophages are critical in clearing opsonized encapsulated bacteria (such as pneumococci, meningococci, and Escherichia coli) and intraerythrocytic parasites such as those causing malaria and babesiosis, which explains the fulminant nature of these infections in persons with anatomic or functional asplenia. Paramount to the management of patients with complement deficiencies and asplenia is educating patients about their predisposition to infection and the importance of preventive immunizations and seeking prompt medical attention.

Certain pathogens recruit host complement inhibitors such as factor H (fH) to evade the immune system. Microbial complement inhibitor-binding molecules can be promising vaccine targets by eliciting antibodies that neutralize this microbial defense mechanism. One such antigen, meningococcal fH-binding protein (fHbp), was used in clinical trials before the protein was discovered to bind fH. The potential effect of fH binding on vaccine immunogenicity had not been assessed in experimental animals because fHbp binds human fH specifically. In this study, we developed a human fH transgenic mouse model. Transgenic mice immunized with fHbp vaccine had 4- to 8-fold lower serum bactericidal antibody responses than control mice whose native fH did not bind the vaccine. In contrast, antibody responses were unimpaired in transgenic mice immunized with a control meningococcal group C polysaccharide-protein conjugate vaccine. In transgenic mice, immunization with an fH non-binding mutant of fHbp elicited antibodies with higher bactericidal activity than fHbp vaccination itself. Antibodies elicited by the mutant fHbp more effectively blocked fH binding to wild-type fHbp than antibodies elicited by fHbp that bound fH. Thus, a mutant fHbp vaccine that does not bind fH, but which retains immunogenicity, is predicted to be superior in humans than an fHbp vaccine that binds human fH. In the case of mutant fHbp vaccination, the resultant antibody responses may be directed more at epitopes in or near the fH-binding site, which result in greater complement-mediated serum bactericidal activity; these epitopes may be obscured when human fH is bound to the wild-type fHbp vaccine.

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Controversy surrounds the association between bacterial vaginosis (BV) and pelvic inflammatory disease (PID). Women (N = 1,140) were ascertained at five US centers, enrolled (1999-2001), and followed up for a median of 3 years. Serial vaginal swabs were obtained for Gram's stain and cultures. PID was defined as 1) histologic endometritis or 2) pelvic pain and tenderness plus oral temperature >38.8 degrees C, leukorrhea or mucopus, erythrocyte sedimentation rate >15 mm/hour, white blood cell count >10,000, or gonococcal/chlamydial lower genital infection. Exploratory factor analysis identified two discrete clusters of genital microorganisms. The first correlated with BV by Gram's stain and consisted of the absence of hydrogen peroxide-producing lactobacillus, Gardnerella vaginalis, Mycoplasma hominis, anaerobic gram-negative rods, and, to a lesser degree, Ureaplasma urealyticum. The second, unrelated to BV by Gram's stain, consisted of Enterococcus species and Escherichia coli. Being in the highest tertile in terms of growth of BV-associated microorganisms increased PID risk (adjusted rate ratio = 2.03, 95% confidence interval: 1.16, 3.53). Carriage of non-BV-associated microorganisms did not increase PID risk. Women with heavy growth of BV-associated microorganisms and a new sexual partner appeared to be at particularly high risk (adjusted rate ratio = 8.77, 95% confidence interval: 1.11, 69.2). When identified by microbial culture, a combination of BV-related microorganisms significantly elevated the risk of acquiring PID.

Gonorrhea, an obligate human infection, is on the rise worldwide and gonococcal strains resistant to many antibiotics are emerging. Appropriate antimicrobial treatment and prevention, including effective vaccines, are urgently needed. To guide investigation, an experimental model of genital tract infection has been developed in female mice to study mechanisms by which Neisseria gonorrhoeae evades host-derived antimicrobial factors and to identify protective and immunosuppressive pathways. Refinements of the animal model have also improved its use as a surrogate host of human infection and accelerated the testing of novel therapeutic and prophylactic compounds against gonococcal infection. Reviewed herein are the (a) history of antibiotic usage and resistance against gonorrhea and the consequences of resistance mechanisms that may increase gonococcal fitness and therefore the potential for spread, (b) use of gonococcal infection in the animal model system to study mechanisms of pathogenesis and host defenses, and

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