Quantitation of IgG antibody to the type-specific polysaccharide of group B streptococcus type 1b in pregnant women and infected infants

Opsonophagocytic Antibodies to Serotype Ia, Ib, and III Group B Streptococcus among Korean Infants and in Intravenous Immunoglobulin Products

Group B streptococcus (GBS) infection is a leading cause of sepsis and meningitis among infants, and is associated with high rates of morbidity and mortality in many countries. Protection against GBS typically involves antibody-mediated opsonization by phagocytes and complement components. The present study evaluated serotype-specific functional antibodies to GBS among Korean infants and in intravenous immunoglobulin (IVIG) products. An opsonophagocytic killing assay (OPA) was used to calculate the opsonization indices (OIs) of functional antibodies to serotypes Ia, Ib, and III in 19 IVIG products from 5 international manufacturers and among 98 Korean infants (age: 0-11 months). The GBS Ia, Ib, and III serotypes were selected because they are included in a trivalent GBS vaccine formulation that is being developed. The OI values for the IVIG products were 635-5,706 (serotype Ia), 488-1,421 (serotype Ib), and 962-3,315 (serotype III), and none of the IVIG lots exhibited undetectable OI values (< 4). The geometric mean OI values were similar for all 3 serotypes when we compared the Korean manufacturers. The seropositive rate among infants was significantly lower for serotype Ia (18.4%), compared to serotype Ib and serotype III (both, 38.8%). Infant age of ≥ 3 months was positively correlated with the seropositive rates for each serotype. Therefore, only a limited proportion of infants exhibited protective immunity against serotype Ia, Ib, and III GBS infections. IVIG products that exhibit high antibody titers may be a useful therapeutic or preventive measure for infants. Further studies are needed to evaluate additional serotypes and age groups.

Group B Streptococcus vaccine development: present status and future considerations, with emphasis on perspectives for low and middle income countries

Globally, group B Streptococcus (GBS) remains the leading cause of sepsis and meningitis in young infants, with its greatest burden in the first 90 days of life. Intrapartum antibiotic prophylaxis (IAP) for women at risk of transmitting GBS to their newborns has been effective in reducing, but not eliminating, the young infant GBS disease burden in many high income countries. However, identification of women at risk and administration of IAP is very difficult in many low and middle income country (LMIC) settings, and is not possible for home deliveries. Immunization of pregnant women with a GBS vaccine represents an alternate pathway to protecting newborns from GBS disease, through the transplacental antibody transfer to the fetus in utero. This approach to prevent GBS disease in young infants is currently under development, and is approaching late stage clinical evaluation. This manuscript includes a review of the natural history of the disease, global disease burden estimates, diagnosis and existing control options in different settings, the biological rationale for a vaccine including previous supportive studies, analysis of current candidates in development, possible correlates of protection and current status of immunogenicity assays. Future potential vaccine development pathways to licensure and use in LMICs, trial design and implementation options are discussed, with the objective to provide a basis for reflection, rather than recommendations.

Maternal antibody at delivery protects neonates from early onset group B streptococcal disease

BACKGROUND

Further reduction in the group B streptococcal (GBS) disease burden in neonates in the United States awaits an additional prevention strategy, such as maternal immunization.

METHODS

We performed a prospective, multicenter, case-control study of 33 mothers delivering neonates with early onset GBS infection (cases), and 99 age- and ethnicity-matched mothers colonized with the same capsular polysaccharide (CPS) types delivering healthy neonates (controls). Relative risk and absolute risk were calculated for early onset disease associated with concentrations of type Ia, III, or V CPS-specific antibody in maternal serum.

RESULTS

For GBS types Ia and III, maternal CPS-specific antibody concentrations of ≥ 0.5 µg/mL were associated with a relative risk of approximately 0.1 (95% confidence intervals [CIs], .01-.74 and 0-.72, respectively; P = .02 for each), corresponding to a 90% risk reduction (by logistic regression). For type V, the relative risk was 0.3 (95% CI, .01-3.1), corresponding to a 70% risk reduction. By Bayesian modeling, the risk of early onset disease would decrease by 70% if maternal CPS-specific antibody concentrations for these 3 GBS types were ≥ 1 µg/mL.

CONCLUSIONS

Maternal CPS-specific antibody serum concentrations of ≥ 1 μg/mL at the time of delivery appear to protect most neonates from early onset GBS type Ia and III disease.

Group B streptococcal conjugate vaccines elicit functional antibodies independent of strain O-acetylation

Recently, it was discovered that sialic acid residues on group B streptococcal (GBS) capsular polysaccharides (CPS) are O-acetylated. Since GBS vaccine development has focused on de-O-acetylated CPS, it became germane to investigate the influence of de-O-acetylated GBS vaccine formulations on functional activity of sera against strains that bear the O-acetyl modification. Post-immunization sera from healthy adult recipients of de-O-acetylated GBS CPS-tetanus toxoid conjugate vaccines were evaluated in opsonophagocytosis assays using 20 GBS clinical isolates representing type Ia, Ib, II, III, or V CPS that varied in amount of O-acetylation from 2% to 40%. Ninety percent or greater opsonophagocytosis and killing of all strains were achieved, using CPS type-specific post-immunization sera. These data indicate that de-O-acetylated CPS-conjugate vaccines contain immunogenic epitopes that offer protection against GBS, independent of O-acetyl CPS modifications. Thus, presence of O-acetyl groups on the GBS CPS is not essential for functional antibodies to be elicited by GBS glycoconjugate vaccines.

Alpha C protein-specific immunity in humans with group B streptococcal colonization and invasive disease

Alpha C protein, found in 76% of non-type III strains of group B Streptococcus (GBS), elicits antibodies protective against alpha C-expressing strains in experimental animals, making it an appealing carrier for a GBS conjugate vaccine. We determined whether natural exposure to alpha C elicits antibodies in women. Geometric mean concentrations of alpha C-specific IgM and IgG were similar by ELISA in sera from 58 alpha C GBS strain colonized and 174 age-matched non-colonized women (IgG 245 and 313 ng/ml; IgM 257 and 229 ng/ml, respectively), but acute sera from 13 women with invasive alpha C-expressing GBS infection had significantly higher concentrations (IgM 383 and IgG 476 ng/ml [p=0.036 and 0.038, respectively]). Convalescent sera from 5 of these women 16-49 days later had high alpha C-specific IgM and IgG concentrations (1355 and 4173 ng/ml, respectively). In vitro killing of alpha C-expressing GBS correlated with total alpha C-specific antibody concentration. Invasive disease but not colonization elicits alpha C-specific IgM and IgG in adults.

Clinical microbiology of bacterial and fungal sepsis in very-low-birth-weight infants

Twenty percent of very-low-birth-weight (<1500 g) preterm infants experience a serious systemic infection, and despite advances in neonatal intensive care and antimicrobials, mortality is as much as threefold higher for these infants who develop sepsis than their counterparts without sepsis during their hospitalization. Outcomes may be improved by preventative strategies, earlier and accurate diagnosis, and adjunct therapies to combat infection and protect the vulnerable preterm infant during an infection. Earlier diagnosis on the basis of factors such as abnormal heart rate characteristics may offer the ability to initiate treatment prior to the onset of clinical symptoms. Molecular and adjunctive diagnostics may also aid in diagnosing invasive infection when clinical symptoms indicate infection but no organisms are isolated in culture. Due to the high morbidity and mortality, preventative and adjunctive therapies are needed. Prophylaxis has been effective in preventing early-onset group B streptococcal sepsis and late-onset Candida sepsis. Future research in prophylaxis using active and passive immunization strategies offers prevention without the risk of resistance to antimicrobials. Identification of the differences in neonatal intensive care units with low and high infection rates and implementation of infection control measures remain paramount in each neonatal intensive care unit caring for preterm infants.

Colonization in the rectum and uterine cervix with group B streptococci may induce specific antibody responses in cervical secretions of pregnant women

We have studied the relationships between genital or rectal carriage of group B streptococci (GBS) with the levels of systemic and mucosal antibodies to GBS in 200 women at about week 17 of pregnancy. Secretions from the uterine cervix were collected with absorbent cylindrical wicks for quantification of antibody levels with whole cell enzyme-linked immunosorbent assay. GBS were cultured from the cervix (with or without concomitant rectal colonization) of 13.5%, from the rectum (with or without concomitant cervical colonization) of 12%, and from both culture sites of 8.5% of the women. Serotypes Ia, II, and III were predominant. Compared with culture-negative women, the group of women colonized rectally had markedly elevated levels of both immunoglobulin A (IgA) and IgG antibodies to GBS in cervical secretions and also had a moderate but significant elevation of IgA antibodies in sera. Women colonized only in the cervix had increases of specific IgA and IgG antibodies in cervical secretions, but their serum antibody levels were not elevated. In cervical secretions, the increase in antibody levels in the groups of colonized women was most pronounced for the IgG isotype, indicating a mucosal immune response involving IgG as well as IgA. A close correlation was found among the levels of antibodies to each of the three GBS serotypes tested. Evidence for such cross-reacting antibodies to different serotypes of GBS, as well as to group A streptococci, was also obtained from absorption experiments. Altogether, our results show that undiluted secretions for antibody determination can be easily collected from the uterine cervix with absorbent wicks and demonstrate that colonization of GBS in the rectum and the uterine cervix may induce a systemic as well as a pronounced local immune response in the female genital tract. The findings may have implications for the development of a mucosal vaccine against GBS disease.

A simple, quantitative, reproducible avidin-biotin ELISA for the evaluation of group B streptococcus type-specific antibodies in humans

Type-specific antibodies to the capsular polysaccharides (CP) of group B Streptococcus (GBS) are protective. Historically, the radioactive antigen-binding assay (RABA) has been used to determine GBS antibody levels. This method measures total immunoglobulin and employs the use of radioactive materials. We have developed an avidin-biotin ELISA that is less hazardous and is able to measure GBS Ia, Ib, II or III CP specific IgG. To avoid inconsistent binding to the plate, the CPs from GBS Ia, Ib, II and III were derivatized using adipic acid dihydrazide (ADH) and subsequently biotinylated without altering their antigenic epitopes and bound to avidin coated plates. Plasma from three different human subjects immunized with a tetravalent CP vaccine were used to prepare IgG references for Ia, II and III, respectively, thus rendering the assay quantitative for those types. The assay is able to detect nanograms per milliliter of GBS Ia, Ib, II or III specific antibody. This method is reproducible, sensitive and correlates with RABA by 76%.

Immunologic basis for increased susceptibility of the neonate to infection

Neonatal infection with the GBS occurs in a small fraction of exposed infants who lack specific antibody. Diminished influx of PMNs to sites of infection as a result of abnormalities in chemotaxis, bone marrow exhaustion, and to a lesser degree relative complement deficiency and decreased microbicidal activity of PMNs may be additional predisposing factors. Infection with HSV occurs more often in infants born to mothers with primary rather than secondary infection; the lack of passively acquired antibody in such infants is a possible but unproved susceptibility factor. The failure of neonates to control HSV may also be related to decreased production of or response to interferon or to decreased activity of nonimmune and immune cellular cytotoxic mechanisms. Similarly, infection with Toxoplasma and intracellular bacterial pathogens, such as Listeria, may be more severe because of the decreased generation of lymphokines and interleukins, which attract macrophages to the site of infection and enable them to kill these organisms. Much of this analysis based on in vitro and animal studies summarizes current information in a rapidly changing field rather than stating established fact. The precise age at which most of the immune functions discussed reach maturity is unknown. However, the risk of severe infection with these pathogens appears to wane by 2 to 3 months of age. Although this may partly reflect decreased exposure, we might hypothesize that immune functions that are mature by this age are those most critical for protection. Future studies focusing on changes in immune function during the first months of life may provide useful insights into the immunobiology of these diseases and direct attention to the most fruitful areas for immunologic intervention.

Immunoprophylaxis and immunotherapy of neonatal group B streptococcal infections

With emphasis on work from our laboratory, this paper briefly reviews previous studies which have established the basis for immunity to group B streptococcal infections. Quantitative data are presented on the concentration of antibody to the type-specific polysaccharides of group B streptococci in normal adults and infected infants, the protective level in experimental animals, and the influence of prematurity on transplacental passage of antibody. The role of the polymorphonuclear leukocyte in immunity to group B streptococcal infections is critical as supported by in vitro and in vivo experiments as well as clinical observations. Strategies for prevention include antimicrobial chemoprophylaxis and active immunization. Alternative approaches to adjunctive therapy such as administration of specific immune globulin, polymorphonuclear leukocytes, and exchange transfusions are discussed.