Molecular modeling of group B Streptococcus type II and III capsular polysaccharides explains low filter retention of type II and lack of cross-reactivity with type III

Group B Streptococcus (GBS) is a bacterial pathogen associated with significant morbidity and mortality in pregnant women and infants, particularly in resource-limited settings. A hexavalent vaccine candidate in development incorporates the capsular polysaccharides (CPSs) from the most prevalent serotypes: Ia, Ib, II, III, IV, and V. Vaccine production is facilitated by a standardized CPS purification process. In the final purification step, a 30 kDa membrane filter gives high-yield recovery for five of the six CPSs, but <50 % for type II (GBSII), despite similar CPS structure and size. However, a smaller 10 kDa membrane improves recovery to about 90 %, suggesting that CPS conformation affects retention. Here comparative molecular modeling - corroborated by through-space NMR correlations - reveals that GBSII forms compact, globular conformations, while type III (GBSIII) forms an elongated zig-zag. This explains GBSII's poor retention during filtration: GBSII's compact globules pass through the 30 kDa membrane more easily than GBSIII's elongated forms. Additionally, we identify distinct epitopes and compare their interactions with a GBSIII-specific fragment antibody to clarify the lack of cross-reactivity between GBSII and GBSIII. This work provides valuable mechanistic insight into physically observed behavior to inform development of multivalent GBS vaccines to reduce maternal and infant mortality.