The academic vaccinologist assumes the challenge of marshalling a candidate antigen from discovery through proof-of-principle and preclinical testing in a suitable animal model in order to evaluate its potential as a vaccine. Positive findings in preclinical immunogenicity, and in some cases, safety, tests could lead to evaluation of the candidate vaccine in phase 1 and phase 2 clinical trials to determine if it is safe and immunogenic in healthy humans.17
Armed with calls from advocacy groups for a viable vaccine and a clear mandate from the National Academy of Sciences, Institute of Medicine ranking an effective GBS vaccine a Level I (most favorable) priority,18
we sought to further the progress of a GBS vaccine beyond clinical trials in healthy non-pregnant adults to healthy pregnant women, the population targeted to be immunized in order to protect newborns against GBS disease. The path toward this goal led from the generation of a conjugate vaccine prepared under cGMP with transfer of technology from the Channing Laboratory, a research laboratory, to the Salk Institute, a vaccine production facility. Although tolerance ranges for each vaccine parameter were not established, there was remarkable similarity between the prototype lot 91-1 and lot 3-1-96, especially with respect to the size of type III CPS and the degree of sialic acid oxidation, parameters that impact cross-linking of the CPS with the tetanus toxoid carrier protein.19
Experience with lyophilized CPS antigens and conjugate vaccines at the Channing Laboratory suggested that a lyophilized preparation might offer longer stability and potency than if the vaccine was vialed as an aqueous preparation, a hypothesis that was later proven correct by extended potency testing of GBS vaccines in mice10
and chemical analyses of lyophilized Haemophilus influenzae
type b conjugate vaccines.20
Although most licensed conjugate vaccines are now vialed as aqueous preparations, GBS III-TT lot 3-1-96 was vialed as a lyophilized preparation with sucrose excipient with no thimerosal or other preservatives.
III-TT lot 3-1-96 was the first GBS conjugate vaccine to be examined as a reproductive and developmental teratogen. The vaccine itself, as well as antibody to it, were evaluated for potential harmful effects on developmental events occurring from preconception and implantation to early- and late-fetal development, including organogenesis and newborn health and survival rates. Tested at 100 times the maximum anticipated dose in humans on a weight basis, there were no deleterious effects attributed to the III-TT vaccine in comparison with the placebo group given saline and alum adjuvant.
Differences in pregnancy and abortion rates among the four groups were within the variation observed at the testing facility. For example, in 78 studies performed between January 2000 and January 2002 involving 1,187 rabbits, the pregnancy rate was 90.9%, 0.8% died, 1.9% spontaneously aborted, and 0.3% delivered prematurely (Christian MS and Hoberman AM, unpublished data). The sole statistical difference among the four groups in our study was the overall incidence in the variation of fetal skull ossification. We consider this variation to be of minor importance because there was no differences in the subcategory analyses and because the observations are within historical values of the testing facility.
Serum type III CPS-specific IgG confirmed that the III-TT lot 3-1-96 vaccine was active in rabbits, with the highest levels detected in sera from the two groups that were given the vaccine prior to insemination (IIA and IIB). At gestation day 29, transfer of type III CPS-specific IgG from does to kits in the three vaccine groups was apparent, with persistence of specific antibody in sera of 21-day-old kits. Transfer of functional specific maternal IgG following vaccination with a GBS conjugate vaccine has been demonstrated in several animal models including mice, rabbits and baboons.21-23
GMP lot 3-1-96 was compositionally similar to the prototype III-TT vaccine lot 91-1, which allowed for direct comparison of safety and immunogenicity. Lot 3-1-96 was evaluated in two phase 1/2 clinical trials in healthy non-pregnant adults.15,16
These trials showed that III-TT lot 3-1-96 was safe and immunogenic and that the vaccine-induced, CPS-specific antibody was functionally active in vitro, findings sufficient to forward this lot of vaccine for phase 1 evaluation as a maternal vaccine in healthy pregnant women. A preliminary report from a double-blinded, placebo-controlled trial involving pregnant women (18 to 45 years of age) between 30 and 32 weeks of gestation described the III-TT lot 3-1-96 vaccine as well-tolerated and immunogenic and that it elicited high levels of maternal antibody that were functionally active in vitro.24
Importantly, there was a strong, positive correlation of type III CPS-specific IgG in maternal and delivery-cord blood and infant sera: >0.5 μg/ml of specific IgG promoted >1 log10
reduction of GBS type III colony forming units by human polymorphonuclear leukocytes, suggesting that functional immune protection in the infant was passively acquired from the mother.
III-TT lot 3-1-96 was one of three lots prepared at the Salk Institute under cGMP; the other two lots, prepared with CPS purified from the same GBS culture, were denoted lots 1-1-95 and 2-1-96. All three lots were potent as determined with the mouse maternal vaccination-neonatal pup challenge model of GBS disease10
and could be used in clinical trials. Experiences gained from producing cGMP lots of GBS conjugate vaccine, the fact that lot 3-1-96 was not teratogenic, and the preliminary report of the safety and immunogenicity of III-T T lot 3-1-96,24
advanced our understanding and expanded our knowledge of GBS vaccines, as the task of providing a safe and efficacious GBS vaccine is now transferred from academia to industry.