Antibodies to meningococcal class 1 outer membrane proteins in South African complement-deficient and complement-sufficient subjects

Complete deficiency of the sixth complement component (C6Q0), susceptibility to Neisseria meningitidis infections and analysis of the frequencies of C6Q0 gene defects in South Africans

Complete complement component 6 deficiency (C6Q0) is a co-dominant genetic disease presenting as increased susceptibility to invasive Neisseria meningitidis infections. Affected individuals have two affected alleles which can be homozygous or compound heterozygous for the particular gene defects they carry. This disorder has been diagnosed relatively frequently in Western Cape South Africans. Affected patients are prescribed penicillin prophylaxis. In 2004 we commenced a clinical follow-up study of 46 patients. Of these, 43 had family age-matched C6 sufficient controls. Participants were classified as either (i) well, or (ii) having a serious illness (SI) or died (D). An SI was a long-term illness that did not allow the performance of normal daily activities. Among 43 patients, 21 were well and 22 were SI/D, while among 43 matched controls, 35 were well and eight were SI/D. This difference is highly significant. Among all 46 C6Q0 patients, those who had had recurrent infection had significantly more SI/D than those who had suffered none or one infection. Thus, this work demonstrates the long-term serious outcome of repeated meningococcal disease (MD) episodes. We investigated the frequencies of four C6Q0 pathogenic mutations known to affect Cape patients (828delG, 1138delC, 821delA and 1879delG) in 2250 newborns. A total of 103 defective alleles (2·28%) and three affected C6Q0 individuals were detected. For all defects combined, 5·24 affected subjects (C6Q0) are expected among 10,000 individuals. What is still unknown is the number of C6Q0 individuals who suffer MD or other infectious diseases.

Infections of people with complement deficiencies and patients who have undergone splenectomy

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.

Inherited complement deficiencies and bacterial infections

A wide variety of bacteria are recognized by the complement system through the early components that trigger the three pathways of complement activation, leading to the release of biologically active products involved in opsonization, recruitment of phagocytes and bacterial killing. Deficiencies of complement components and regulators provide a model to understand the in vivo role of complement as a defense system against bacterial infections. An increased susceptibility to these types of infections is frequently seen in individuals with C2, C3, late component, properdin and factor I deficiencies. The identification of these deficiencies is essential for the adoption of preventive measures aimed to reduce the risk of bacterial infections. Vaccination represents the treatment of choice to protect these subjects, although further studies on a large number of C-deficient individuals are needed to prove the protective effect of vaccines.

Immunogenicity and safety of a hexavalent meningococcal outer-membrane-vesicle vaccine in children of 2-3 and 7-8 years of age

To study the reactogenicity and immunogenicity of a hexavalent meningococcal outer-membrane-vesicle vaccine (OMV), two different dosages of this vaccine (7.5 and 15 microg of individual PorA proteins) consisting of vesicles expressing class 1 outer-membrane proteins (OMPs) of subtypes P1.7,16; P1.5,2; P1.19,15 and P1.5(c), 10; P1.12,13; P1.7(h),4 were administered to a group of 7-8 year (n=165) and a group of 2-3 year old children (n=172). Control groups of children with similar ages were vaccinated against hepatitis B. All participants received three injections. Pre- and postimmunisation sera were tested for bactericidal antibodies against six isogenic meningococcal vaccine strains expressing different PorA proteins. Antibody titres against OMP of the two different vesicles (PL16215 and PL10124) were measured by ELISA. The meningococcal hexavalent OMV vaccine was well tolerated. No statistically significant differences were seen between the high and low dose of hexavalent meningococcal OMV vaccine. The percentage of children showing a fourfold increase of bactericidal antibody titres against the specific serosubtype varied in toddlers from 28 to 98% and in older children from 16 to 100%. Both ELISA antibody titres and bactericidal activity showed the highest level in the youngest age-group.

Phagocytic killing and antibody response during the first year after tetravalent meningococcal vaccine in complement-deficient and in normal individuals

Seven individuals with late complement component (LCC) deficiency and seven control subjects were vaccinated with tetravalent meningococcal vaccine. The response to vaccination was evaluated by measuring the antibody titer and the phagocyte killing of the bacteria, before, 5-7 weeks, and 12-14 months after vaccination. Prior to vaccination, no phagocytic killing and a low titer of antibody was found in the LCC-deficient group and a low killing (mean of 40-58%, according to the serogroup) in normal controls. The phagocytic killing increased significantly 5-7 weeks after vaccination. However, while in normal controls the phagocytic killing was close to 100% after 5-7 weeks and decreased only slightly during the first year, the mean killing of the various meningococcal subgroups in LCC-deficient individuals was 70-89% and dropped to only 53-71% one year after vaccination. Six weeks after vaccination the mean antimeningococcal antibody titer increased similarly in the sera of LCC-deficient patients and controls. One year after vaccination the controls maintained the high concentration, while the LCC-deficient patients had tendency toward a decrease. In addition, the interpersonal variability of the antibody concentration, both in LCC-deficient individuals and in normal controls, was much higher than the phagocytic killing, with only a very mild increase in some individuals. Thus, it is possible that in spite of adequate increase of antimeningococcal antibody titer after vaccination of LCC-deficient individuals their immunity against the bacteria may not be optimal. Our data show also that phagocytic killing of meningococci is probably a more consistent assay than antibody titer levels for antimeningococcal immunity, especially in LCC-deficient patients.

Specificity of bactericidal antibody response to serogroup B meningococcal strains in Brazilian children after immunization with an outer membrane vaccine

Pre- and postvaccination serum samples from 77 children aged 2 to 6 years, who received the Cuban BC vaccine (B:4:P1.15), were analyzed for bactericidal antibodies against a local B:4:P1.15 strain (N44/89). Sera from 16 individuals with bactericidal antibodies against the B:4:P1.15 strain were tested against 23 Brazilian isolates. These include B:4 strains of distinct serosubtypes: P1.15, P1.7,1, P1.3, P1.9, P1.nt, and a B:8,19,23:P1.16 strain. A Cuban B:4:P1.15 strain (Cu385/83) was also included in the study. The specificities of bactericidal antibodies were analyzed by using mutant strains lacking a class 1 protein (PorA protein) or a class 5 protein or both. The results indicated that PorA and class 5 proteins are the main targets recognized by the bactericidal antibodies of vaccinees. Nonetheless, a complex pattern of recognition by bactericidal antibodies was found, and vaccinees were grouped according to antibody specificity. Antibodies from some individuals recognized PorA of serosubtype P1.15. However, antibodies from these individuals could not kill all P1.15 strains tested. Antibodies from a second group recognized both PorA and class 5 proteins, and antibodies from a third group recognized an as yet unidentified target antigen. The results demonstrate the importance of determining the fine epitope specificity of bactericidal antibodies to improve the existing vaccines against B meningococci.

Immune responses against major outer membrane antigens of Neisseria meningitidis in vaccinees and controls who contracted meningococcal disease during the Norwegian serogroup B protection trial

Sera from vaccinees and controls who contracted serogroup B meningococcal disease during the blinded and open parts of a two-dose protection trial in Norway were compared for antigen-specific and bactericidal antibodies against vaccine strain 44/76 (B:15:P1.7,16). From 16 of 20 (80%) vaccinees and 26 of 35 (74%) controls, one or more serum samples (n = 104) were collected during the acute phase (1 to 4 days), early convalescent phase (5 to 79 days), and late convalescent phase (8 to 31 months) after onset of disease. Binding of immunoglobulin G (IgG) to the major outer membrane antigens (80- and 70-kDa proteins, class 1, 3, and 5 proteins, and lipopolysaccharide [LPS]) on immunoblots was measured by digital image analysis. Specific IgG levels in vaccinees increased from acute to early convalescent phases, followed by a decline, while controls showed a small increase over time. Vaccinees had significantly higher levels than controls against class 1 and 3 porins and LPS in acute sera, against all antigens during early convalescence, and against class 1 and 3 porins in the later sera. Vaccinees who were infected with strains expressing subtype P1.7,16 proteins demonstrated a level of IgG binding to protein P1.7,16 with early-convalescent-phase sera that was fourfold higher than that of those infected with other strains. Bactericidal titers in serum against the vaccine strain were 192-fold higher for vaccinees than those for controls during early convalescence, but similarly low levels were found during late convalescence. A vaccine-induced anamnestic response of specific and functional antibody activities was thus shown, but the decrease in protection over time after vaccination indicated that two vaccine doses did not induce sufficient levels of long-term protective antibodies.

Vaccination of patients deficient in a late complement component with tetravalent meningococcal capsular polysaccharide vaccine

Eighteen patients with late complement component deficiency (LCCD) were immunized with meningococcal capsular polysaccharide vaccine. The LCCD patients had experienced one-to-five meningococcal infections before vaccination, but their immunological and clinical status was normal at the time of immunization. Serum samples from vaccinated complement-sufficient relatives of the LCCD patients and healthy Russian male adults were used as controls. Total and immunoglobulin-specific concentrations of antibodies to group A, C, W135, and Y capsular polysaccharides were determined by enzyme immunoassay in serum samples taken before and 1-108 weeks after immunization. The individual preimmunization and post-immunization antibody concentrations varied greatly. The median antibody concentrations of the LCCD patients increased significantly after vaccination, and were not significantly different from those of the control groups. The antibody concentrations remained elevated for at least 1 year after vaccination. The post-immunization antibody concentrations correlated with the number of meningococcal infections within 10 years before vaccination. In spite of the vaccination two LCCD patients experienced a meningococcal disease 9 and 12 months, respectively, after vaccination.

Current status of meningococcal group B vaccine candidates: capsular or noncapsular?

Meningococcal meningitis is a severe, life-threatening infection for which no adequate vaccine exists. Current vaccines, based on the group-specific capsular polysaccharides, provide short-term protection in adults against serogroups A and C but are ineffective in infants and do not induce protection against group B strains, the predominant cause of infection in western countries, because the purified serogroup B polysaccharide fails to elicit human bactericidal antibodies. Because of the poor immunogenicity of group B capsular polysaccharide, different noncapsular antigens have been considered for inclusion in a vaccine against this serogroup: outer membrane proteins, lipooligosaccharides, iron-regulated proteins, Lip, pili, CtrA, and the immunoglobulin A proteases. Alternatively, attempts to increase the immunogenicity of the capsular polysaccharide have been made by using noncovalent complexes with outer membrane proteins, chemical modifications, and structural analogs. Here, we review the strategies employed for the development of a vaccine for Neisseria meningitidis serogroup B; the difficulties associated with the different approaches are discussed.

Humoral immune response to class 1 outer membrane protein during the course of meningococcal disease

We have determined the amounts of specific anti-class 1 outer membrane protein antibodies in sera from 25 patients during the course of systemic meningococcal disease, using purified class 1 protein as the sensitizing antigen in an enzyme-linked immunosorbent assay. The class 1 protein was obtained from a variant of strain 44/76 (B:15:P1.7,16) lacking class 3 and class 4 outer membrane proteins. Specific anti-class 1 (serosubtype P1.7,16) outer membrane protein immunoglobulin G (IgG) antibody levels increased significantly in 12 patients (12 of 25; 48%), regardless of the serotype of the infecting strain, indicating that the antibodies reacted in part with epitopes not determined by the monoclonal antibodies used for serotyping. Most patients had low levels of anti-class 1 IgG antibodies during the acute illness. The antibody levels peaked during the second week of disease and returned to near baseline levels in sera collected 6 weeks to 12 months after the onset of the disease. The majority of the specific anti-class 1 IgG antibodies bound to surface-exposed epitopes on whole bacteria and belonged to the IgG1 and IgG3 subclasses. Anti-class 1 IgA and IgM antibodies were not detected in any of the patient sera. Prior to disease, seven patients had been immunized with a meningococcal outer membrane vesicle vaccine developed from strain 44/76 (P1.7,16). None of these patients was infected with meningococcal strains containing class 1 protein homologous or partly homologous to that of the vaccine strain, indicating serosubtype-specific protection. The highest anti-class 1 IgG antibody peak levels were seen in immunized patients infected with strains of heterologous serotype, suggesting an anamnestic response. However, these patients were not protected from meningococcal disease after immunization.

Complement deficiency states and meningococcal disease

Analysis of complement deficiency states has supported the role of complement in host defense and elucidated diseases associated with defective complement function. Although neisserial infection plays a prominent role in these deficiency states, examination of individuals with late complement component deficiency (LCCD) reveals a particular propensity for recurrent meningococcal disease and provides important clues to the role of complement in neisserial infections. In response to meningococcal disease, LCCD individuals produce significantly greater amounts of antilipooligosaccharide (LOS) antibody which can kill group B meningococcus in a complement-sufficient in vitro system. Further studies of antibody cross-reactivity to other meningococci has led to a clearer understanding of its epitopic specificity. Nevertheless, epidemiologic evidence is consistent with the relative absence of protective immunity in LCCD persons following an episode of infection and supported by quantitation of antibody to capsular polysaccharide. However, compared to anti-LOS antibodies, anticapsular antibodies can offer immune protection to LCCD individuals via complement-dependent opsonophagocytosis--the only form of complement-mediated killing available to these persons. Thus vaccination of LCCD persons with capsular antigens is considered an important means of protecting these high-risk individuals against meningococcal disease.