Streptococcus agalactiae, also known as Group B Streptococcus (GBS), is a commensal bacteria that is commonly found in the vagina of approximately 30% of healthy women. While GBS does not normally pose a health risk to humans, GBS infection is a serious risk for neonatal infants. This otherwise innocuous bacterium is the leading cause of neonatal septicemia, pneumonia, and meningitis. Neonatal GBS infection carries with it a 10% mortality rate, and meningitis caused by GBS leaves 25 to 35% of survivors with permanent neurological damage. Current treatment of GBS is focused on the identification of “at-risk” mothers and the prevention of mother-infant transmission through intrapartum antibiotic prophylaxis (IAP). In IAP, a mother who has been identified as a GBS carrier is treated with antibiotics during her pregnancy, particularly during the third trimester. While this preventative treatment has been effective in reducing the incidence of early-onset disease (EOD), it is not without its problems. IAP has been unable to prevent a rise in infant mortality in the last decade due to late-onset disease (LOD), which occurs after the first week of life. Unfortunately, LOD is the most damaging form of GBS infection, as it tends to cause meningitis. At the same time, the rise of antibiotic-resistant strains of GBS has been linked to the widespread use of IAP as a preventative measure (11, 12).
Due to the problems with current GBS treatments, current research is focused on the development of an effective vaccine. While the fragility of the fetal and neonatal immune system makes immunizing the fetus itself impractical, neonates may develop immunity to diseases against which their mother has been vaccinated. This occurs through a phenomenon known as passive immunity, in which antibodies produced by the mother cross the placenta and enter the fetal bloodstream. Previous attempts to design vaccines for GBS have been met with difficulty due to the wide variety of infectious GBS strains. Ideally, a vaccine would target a protein that is both required for infection – also known as a virulence factor – and that is shared by all infectious strains of GBS. A second difficulty with promoting GBS immunity in infants is that the reasons for neonatal susceptibility to GBS have not previously been well-understood. While it is clear that GBS is able to prevent an effective immune response, the methods by which it does so had not been characterized.
In this paper, published in PLoS: Pathogens this November, Madureira, et al. were able to address both of these problems, identifying both the cause of neonatal susceptibility to GBS and a virulence factor that is conserved among all infectious strains of GBS. Additionally, the researchers were able to demonstrate that this structurally conserved virulence factor is a viable target for vaccination, reliably producing passive immunity in the offspring of vaccinated mice. Together, these findings represent a significant step forward in developing an effective vaccine against GBS.
Madureira, et al began their investigation of GBS infection by examining a protein involved in GBS metabolism: glyceraldehyde-3-phosphate dehydrogenase (GAPDH). In a previous study, the researchers had identified extracellular GAPDH as a GBS virulence factor. In order for GAPDH to be a viable target for vaccine research, however, its structure needed to be conserved among all eight infectious strains of GBS. To confirm that GAPDH is present in all GBS strains, the researchers cultured each of the eight strains and tested the supernatant fluid from each culture with antibodies for GAPDH. In every case, the cultures tested positive for the presence of GAPDH. Additionally, analysis of each GBS genome confirmed that the GAPDH structure was almost identical in each GBS strain. Finally, the researchers demonstrated that the GAPDH produced by GBS was distinct enough from that of humans, mice and rabbits that antibodies produced for GBS-GAPDH would not cross-react with the GAPDH of other species.. Together, these findings suggested that GAPDH was a worthwhile target for continued vaccine research.
Having identified a vaccine target, the researchers moved on to testing whether or not maternal immunization against GAPDH could effectively counteract GBS infection in neonates. In order to test this, the authors treated pregnant mice with either heat-denatured rGAPDH or a placebo, and infected their offspring with a virulent strain of GBS. The results showed a significant reduction in mortality in the rGAPDH immunized offspring: 95% of those born from immunized mothers survived GBS infection, while only 18.5% of the control mice survived. To further confirm these data, the researchers repeated this experiment with a “hyper virulent” strain of GBS. Again, the results were conclusive: none of the control offspring survived infection with the hyper virulent GBS, while 78.6% of immunized offspring survived. While these results clearly indicated that vaccination against rGAPDH greatly reduced GBS mortality, the mechanism by which the vaccination – and the infection itself – were operating was still unclear.
Previous research had identified GBS-GAPDH as a trigger for the secretion of interleukin 10 (IL-10), an immunosuppressive cytokine in adult mice, but this research had not been applied to a neonatal model (29). In order to confirm that GAPDH was acting as an IL-10 agonist in infants, the authors injected neonatal mice with rGAPDH and measured the amount of IL-10 in the bloodstream. The researchers observed a significant increase in IL-10 secretion in the injected mice, compared to a control group. Additionally, Madureira et al confirmed that the production of IL-10 was essential to fatal GBS infection. When mice unable to produce IL-10 were infected with GBS, they were significantly more resistant to the infection than were their IL-10 producing counterparts.
The authors hypothesized that IL-10 secretion was increasing GBS virulence by inhibiting the function of neutrophils, an essential part of the immune response to bacteria. Neutrophils are a type of immune cell that moves through the circulatory and lymphatic systems, searching for and destroying extracellular pathogens. Ideally, these neutrophils would be able to seek out and destroy the GBS bacteria before a significant infection was established. In GBS-infected individuals, however, neutrophil activity appeared to be reduced. To test this hypothesis, the authors infected mice with no neutrophils with GBS. In the absence of neutrophils, the researchers were unable to prevent GBS mortality; neither treatment with anti-rGAPDH antibodies nor IL-10 blocking drugs were able to stop the infection from progressing. These data strongly suggest that rGAPDH increases GBS virulence by reducing the ability of neutrophils to clear the infection.
Taken together, the results of this research are a significant advancement in both the understanding and prevention of neonatal GBS infection. With the identification of GAPDH as a viable target for vaccines, and the elucidation of the methods by which GBS subverts the neonatal immune response, the prospect of a maternal GBS vaccine are brighter than ever. With luck, this research will be applied to produce a vaccine that can replace the current, somewhat flawed standard treatments, and effectively end GBS infection as a source of infant mortality.
Madureira P, Andrade EB, Gama B, Oliveira L, Moreira S, et al. (2011) Inhibition of IL-10 Production by Maternal Antibodies against Group B Streptococcus GAPDH Confers Immunity to Offspring by Favoring Neutrophil Recruitment. PLoS Pathog 7(11): e1002363. doi:10.1371/journal.ppat.1002363
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Baltimore RS (2007) Consequences of prophylaxis for group B streptococcal infections of the neonate. Semin Perinatol 31: 33–38.
Castor ML, Whitney CG, Como-Sabetti K, Facklam RR, Ferrieri P, et al. (2008) Antibiotic resistance patterns in invasive group B streptococcal isolates. Infect Dis Obstet Gynecol 727505 2008: 727505.