Standardising assays for the assessment of serocorrelates of protection of antibodies against key GBS proteins to facilitate vaccine licensure

Summary

Group B Streptococcus (GBS) is a leading cause of neonatal infections and causes stillbirths and preterm births.  GBS can also cause infections in the elderly and immunocompromised host. Vaccine development is currently undergoing clinical trials and there are two vaccine candidates, one developed to make antibodies against the GBS coat (the capsule) and one against key proteins on the GBS surface. The capsule changes with each of the 10 types of GBS, but key proteins are the same for all types of GBS. Although GBS is a leading cause of disease, it is still relatively rare and testing a vaccine in such a rare disease would be too complex and too costly. It is now widely agreed that giving a vaccine against to pregnant women is a priority and that the vaccine could be licenced based on measuring the amount of antibodies needed to protect against infection using standardised laboratory tests followed by a large clinical trial after the vaccine is licenced to look at vaccine effectiveness. Such an approach has been used for vaccines like the meningitis B vaccine given to children under the age of 1 year. However, to develop this laboratory test we need to develop standard reagents that every laboratory wishing to test antibodies can use. We are already developing reagents to study antibodies against the capsule and this proposal aims to now develop reagents against key proteins so that we can finally get GBS vaccines to those who need them most.

Project Outcomes

We have successfully developed an assay to simultaneously measure levels of antibodies against the N-terminal domains of three key proteins on the surface of GBS; AlphaC, Alp2 and Alp3 proteins (Alp2 and Alp3 have the same N-terminal domain (Alp2/3-N) meaning that the current version of the assay measures two specificities covering three proteins). These proteins are members of the Alpha-like protein (Alp) family, also containing Rib, Alp1 and Alp4. The Alps are expressed by GBS variant strains as allelic variants and essentially all invasive strains encode at least one family member. Alp4 is however extremely rare. The Alps are thought to be involved in helping GBS to cross into the bloodstream and cause disease in babies and mothers.

The multiplex assay developed is adapted to easily incorporate measurement of antibodies also against Rib-N and Alp-1, yielding a multiplex assay that can measure antibodies against all clinically relevant Alps/GBS strains. When we developed the assay, we were however aware of that some cross-reactivity between the different Alps exists and while we now have ruled out that this cross-reactivity interferes with the assay’s ability to determine the concentration of  AlphaC-N and Alp2/3-N binding  antibodies, it remains to be demonstrated that cross-reactivity does not impact on accuracy when measuring concentration of antibodies against the remaining Alp-Ns.  However, one of the reagents we used (avidin beads) did give us some incorrect antibody readings. So, we developed an improved method of measuring the antibodies, to make sure that the results of our assay show only antibodies against the  proteins of interest. In the future, the reagents we have developed will be available via NIBSC together with a standard protocol for the assessment of antibodies against key proteins on GBS. Combining all proteins into an antibody detection test may be achievable and could be used to measure the amount of antibodies that a pregnant woman needs to pass to her baby to protect against GBS disease (serocorrelates of protection) in the first months of life.