Serotype specific anti-capsular polysaccharide IgG associated with colonizing and invasive group B streptococcus in pregnant woman-newborn dyads

Maternal IgG against group B streptococcus (GBS) capsular polysaccharide (CPS), is the principal placental mediator of neonatal protection, yet naturally acquired anti-CPS immunity has not been characterized in North African populations. To address this gap, we conducted a prospective study in a Moroccan mother-newborn cohort to quantify the serotype-specific anti-CPS IgG levels in relation to colonizing versus invasive GBS serotypes. Additionally, we assessed the transplacental transfer of anti-CPS IgG and identified potential correlates of invasive disease. We examined serotype-specific anti-CPS IgG (serotypes Ia-V) in the plasma from maternal and cord blood of 103 GBS-colonized pregnant women, 35 uncolonized controls and their paired newborns (90 paired colonized dyads, including five newborns with invasive GBS disease) using a standardized six-plex immunoassay. Transfer ratios were compared with 1.0 and cord concentrations with published serotype- and onset-specific risk-reduction thresholds. Maternal anti-CPS IgG concentrations varied by serotype, with the highest median for serotype II(1.36g/mL) and the lowest for serotype Ib(0.003g/mL). They were higher in GBS-colonized than uncolonized mothers for serotypes Ia, Ib and II. Maternal and cord blood IgG concentrations were correlated across serotypes, but placental transfer was incomplete, with median ratios of 0.50-0.80. Among 77 newborns, 13.0%(n=10) reached early-onset and 44.2%(n=34) achieved late-onset disease risk-reduction thresholds. The serotype IV was the most identified among neonatal infection disease cases, and this was associated with sub-protective anti-serotype IV antibody. This suggests that while naturally acquired anti-serotype II antibodies were predominant, the low level of anti-serotype IV antibodies could contribute to neonatal infection diseases in mother-newborn pairs.