Diversity in Naturally Acquired Immunity to Group B Streptococcus: A Comparative Study of Women From Bangladesh, Malawi, and the United Kingdom

Abstract

Background: Significant disparities in group B Streptococcus (GBS) colonization and neonatal disease rates have been documented across different geographic regions. For example, Bangladesh reports notably lower rates as compared with the United Kingdom and Malawi. This study investigates whether this epidemiologic variability correlates with the immune response to GBS in these regions.

Methods: Qualitative and quantitative analyses of naturally acquired immunoglobulin G (IgG) antibodies against GBS capsular polysaccharide and the Alp protein family were conducted in serum samples from women of childbearing age in the United Kingdom, Bangladesh, and Malawi. The efficacy of these antibodies in clearing vaginal colonization or protecting newborns from GBS infection was assessed with humanized mouse models.

Results: Bangladeshi women displayed the highest diversity in serotype distribution, with elevated IgG levels in the serum against GBS capsular polysaccharides Ia, Ib, II, III, IV, and V, as well as Alp family proteins. In contrast, Malawian sera demonstrated the weakest antibody response. Bangladeshi sera also showed heightened IgG-mediated complement deposition, opsonophagocytic killing, and neonatal Fc receptor binding while tested against capsular polysaccharide Ib. In a humanized neonatal Fc receptor mouse model, Bangladeshi sera led to faster clearance of GBS virulent serotype Ib vaginal colonization. Additionally, offspring from dams passively immunized with Bangladeshi sera demonstrated notably increased survival rates.

Conclusions: This study demonstrates significant variability in the immune response to GBS across different geographic regions. These findings underscore the importance of understanding GBS-induced immune response in diverse populations, which may significantly affect vaccine efficacy in these regions.

Keywords: Alp protein; antibody diversity; capsular polysaccharide; group B Streptococcus; mouse model.

Figure 1.

Diverse seroprevalence to group B Streptococcus (GBS) capsular polysaccharide observed in the United Kingdom, Bangladesh, and Malawi. A, Presence of immunoglobulin G (IgG) against GBS serotypes Ia, Ib, II, III, IV, and V in serum samples collected from the United Kingdom (n = 100), Bangladesh (n = 100), and Malawi (n = 88). The bar plot represents the percentage of women with IgG concentration equal to or above the lower limit of quantification (LLOQ) for the assay. B, Simultaneous presence of IgG antibodies against multiple serotypes of GBS in a single woman in 3 countries. The analysis included serum samples with a concentration equal to or above the LLOQ. Box and whisker plots indicate median, IQR, and minimum/maximum values. *P < .05. ****P < .0001. C, Reverse cumulative distribution curves show IgG levels in the serum samples. Analysis included all serum samples in the study. Sample concentrations below the LLOQ were assigned half the LLOQ value for the corresponding serotypes. Green, blue, and yellow lines represent the sera from the United Kingdom, Bangladesh, and Malawi, respectively. BD, Bangladesh.

Figure 2.

Subclass distribution of capsular polysaccharide (CPS) Ib–specific immunoglobulin G (IgG) and IgG functionality across 3 countries shows dominance of the Bangladeshi sera. A, Group B Streptococcus CPS Ib–specific IgG subclasses (IgG1, IgG2, IgG3, and IgG4) in serum samples collected from the United Kingdom, Bangladesh, and Malawi (n = 55/country). Data are depicted as mean fluorescence intensity (MFI). B–D, Effector functions of CPS Ib–specific IgG were determined as neonatal Fc receptor (FcRn) binding, antibody-dependent complement deposition (ADCD), and opsonophagocytic killing (OPK) in the serum samples (n = 55/country). The Luminex-based assay was used to measure FcRn binding and ADCD, and the results are expressed as MFI. OPK was determined by in vitro OPK assay, and the results are expressed as OPK titer, determined as 50% killing by HL60 neutrophils vs no-serum control. Samples with titers below the lowest value were assigned half the minimum detectable titer. Statistical analysis to evaluate intercountry variation was performed by a nonparametric Kruskal-Wallis test with Dunn multiple-comparison tests. *P < .05. **P < .01. ***P < .001. ****P < .0001. Box and whisker plots indicate median, IQR, and minimum/maximum values. BD, Bangladesh; ns, not significant.

Figure 3.

Differential seroprevalence and immunoglobulin G (IgG) subclass distribution observed against Alp-N–specific IgG across the countries. A, IgG against group B Streptococcus (GBS) Alp proteins in the serum samples from the United Kingdom, Bangladesh, and Malawi (n = 100/country) where the bar plot represents the percentage of women possessing IgG above or equal to the lower limit of quantification (LLOQ) against GBS surface protein antigens Alp1-N, Alp2/3-N, αC-N, and Rib-N. B, Reverse cumulative distribution curves show IgG levels against GBS Alp proteins in the serum samples of all study participants (n = 100/country). Sample concentrations below the LLOQ were assigned half the LLOQ value of the corresponding serotypes. Maroon, red, and pink lines represent the sera from the United Kingdom, Bangladesh, and Malawi, respectively. C, Subclass distribution of Alp-specific IgG in the serum samples (n = 55/country) where data are presented as mean fluorescence intensity (MFI). Statistical significance was determined by the Kruskal-Wallis test with Dunn multiple-comparison test. *P < .05. **P < .01. ***P < .001. ****P < .0001. Box and whisker plots indicate median, IQR, and minimum/maximum values. Alp, alpha-like protein; BD, Bangladesh; ns, not significant.

Figure 4.

Variable immunoglobulin G (IgG) effector functions observed against Alp-N proteins in different country settings. A, Antibody-dependent complement deposition (ADCD) exerted by IgG against Alp1-N, Alp2/3-N, αC-N, and Rib-N proteins in the serum of women from the United Kingdom, Bangladesh, and Malawi. B, Neonatal Fc receptor (FcRn) binding of IgG against 4 Alp proteins across the 3 countries. Statistical significance was determined by Kruskal-Wallis test with Dunn multiple-comparison test. *P < .05. **P < .01. ***P < .001. ****P < .0001. n = 55/country for both assays. Box and whisker plots indicate median, IQR, and minimum/maximum values. Alp, alpha-like protein; BD, Bangladesh; MFI, lower limit of quantification; ns, not significant.

Figure 5.

Bangladeshi serum conferred superior protection against group B Streptococcus (GBS) vaginal colonization and neonatal sepsis. A, Schematic representation of GBS vaginal colonization in female BALB/c mice treated with serum samples (n = 5/group). B, Bacterial load in the mouse vaginal tissues is plotted, where each dot represents a single mouse. Data are expressed as mean ± SEM. Colony-forming units (CFU) in the serum-treated groups were compared with the group treated with phosphate-buffered saline at each time point by 2-way analysis of variance with Dunnett test for multiple comparisons. *P < .05. C, Schematic representation of neonatal protection from GBS infection following passive immunization of pregnant mice with serum samples (n = 5/group). D, Kaplan-Meier survival curve shows the survival of pups born from dam that received the UK sera (n = 17), Bangladeshi sera (n = 15), and Malawi sera (n = 14) or PBS (n = 14). The survival curves were compared by the log-rank Mantel-Cox test. Data were pooled from 3 independent experiments. E, The violin plot shows the IgG transfer ratios of pups to dam for the United Kingdom, Bangladesh, and Malawi serum measured in humanized neonatal Fc receptor pregnant mice (n = 3 per country) and their pups (n = 7 per country). Statistical analysis was performed by Kruskal-Wallis 1-way analysis of variance with Dunn test for multiple comparisons. BD, Bangladesh; IP, intraperitoneal; ns, not significant.