A group B Streptococcus alpha-like protein subunit vaccine induces functionally active antibodies in humans targeting homotypic and heterotypic strains

Abstract

Maternal vaccination is a promising strategy for preventing neonatal disease caused by group B Streptococcus. The safety and immunogenicity of the prototype vaccine GBS-NN, a fusion protein consisting of the N-terminal domains of the alpha-like proteins (Alp) αC and Rib, were recently evaluated favorably in healthy adult women in a phase 1 trial. Here we demonstrate robust immunoglobulin G (IgG) and immunoglobulin A (IgA) responses against αC and Rib, as well as against the heterotypic Alp family members Alp1-Alp3. IgA and heterotypic IgG responses are more variable between subjects and correlate with pre-existing immunity. Vaccine-induced IgG mediates opsonophagocytic killing and prevents bacterial invasion of epithelial cells. Like the vaccine-induced response, naturally acquired IgG against the vaccine domains is dominated by IgG1. Consistent with the high IgG1 cross-placental transfer rate, naturally acquired IgG against both domains reaches higher concentrations in neonatal than maternal blood, as assessed in a separate group of non-vaccinated pregnant women and their babies.

Trial registration: ClinicalTrials.gov NCT02459262.

Keywords: antibodies; group B Streptococcus; maternal immunization; neonatal disease; opsonophagocytosis; vaccines.

Conflict of interest statement

B.J.-L. reports personal fees and grants from MinervaX ApS during the conduct of the study. P.B.F. reports grants and personal fees from MinervaX ApS during the conduct of the study and personal fees from MinervaX ApS outside of the submitted work. In addition, P.B.F. has a pending patent application relating to use of GBS-NN as a vaccine candidate. M.D. reports personal fees from MinervaX ApS during the conduct of the study. G.K. reports personal fees from MinervaX ApS during the conduct of the study and personal fees from various clients outside of the submitted work.

© 2022 The Author(s).

Figures

Graphical abstract

Figure 1

Vaccination with GBS-NN elicits persistent IgG responses against homotypic and heterotypic Alp-Ns (A) Serum concentration of IgG against the indicated Alp-N before vaccination and on the indicated days after the first dose. Gray symbols represent results obtained 4 weeks after primary immunization and prior to the second dose. Results show individual subject concentrations and GMC (n = 45). (B) Pearson correlations between pre- and post-vaccination (day 57) IgG against indicated Alp-N (n = 45). (C) Serum concentration of IgG1 and IgG2 against the indicated Alp-N before vaccination and 4 weeks after the second dose (day 57). Results show GMCs with 95% CI (n = 20). See also Table S1.

Figure 2

Primary immunization with GBS-NN elicits persistent IgA responses against homotypic and heterotypic Alp-Ns (A) Serum concentration of IgA against the indicated Alp-N before vaccination and on the indicated days after the first dose. Gray symbols represent results obtained 4 weeks after primary immunization and prior to the second dose. Results show individual subject concentrations and GMC (n = 45). (B) Pearson correlations between pre- and post-vaccination (day 57) IgA against the indicated Alp-N (n = 45). See also Table S1.

Figure 3

Vaccine-induced Alp-N-specific IgG correlates with an increase in IgG binding to intact bacteria and detects Rib protein on the surface of clinical isolates with different CPS types (A and B) Binding of human serum IgG to strain A909 (A) and BM110 (B) was analyzed by flow cytometry before and after vaccination, and the association between the vaccine-induced increase in GMFI (ΔGMFI) and vaccine-induced increase in corresponding IgG concentration was assessed by Pearson correlation. (C) Flow cytometry analysis of Rib protein on the surface of 31 clinical EOD and LOD isolates possessing the Rib gene and indicated CPS types, using a fixed dilution (1:3,200) of a rabbit GBS antiserum. The model strain BM110 was included as reference. Results show geometric mean fluorescence intensities (GMFIs). The asterisk for strain 1,021 indicates bimodal expression (one negative and one positive GBS population; it is not possible to report GMFI).

Figure 4

GBS-NN elicits an Ab response that mediates OPk of homotypic and heterotypic GBS strains Pre- and post-vaccination sera were assessed for the ability to mediate OPk of the indicated GBS strains. OPkA titer is defined as the reciprocal serum dilution required to mediate 50% bacterial killing relative to killing in the absence of human serum. (A) Total OPk with pre- and post-vaccination sera. Results show titers for individual sera and GMTs. (B) Percentage of subjects reaching the indicated OPkA titer thresholds before and after vaccination. (C and D) Representative OPk killing curves, showing colony-forming units (CFUs) for the A909 (C) and BM110 (D) strains after incubation with pre-vaccination (brown curves) or post-vaccination (blue curves) serum samples in the absence (filled circles) or presence (open circles) of 50 µg/ml soluble GBS-NN (adsorptions [Ads]). The top graphs show results for pre- versus post-vaccination sera in the absence of inhibitor. The center graphs show post-vaccination sera in the absence or presence of inhibitor. The bottom graphs show pre-vaccination sera in the absence or presence of inhibitor. (E–H) Pearson correlations between ΔOPkA titer and concentrations of IgG (left), IgA (center), and IgM (right) specific for the strain homologous Alp-N. See also Table S2.

Figure 5

Post-vaccination sera prevent GBS invasion of cervical epithelial cells (A) Post-vaccination sera from four subjects with intermediate to high levels of IgG and/or IgA against αC-N and Rib-N were compared with the paired pre-vaccination sera for the ability to prevent A909 (left) and BM110 (right) invasion of human cervical epithelial ME180 cells. All sera were diluted 1/100. Pooled results from three independent experiments. Error bars indicate SD. For information on αC-N- and Rib-N-specific Ab concentrations in selected sera, see Table S3. (B) Post-vaccination sera were diluted to a target concentration of 100 ng/mL IgG plus IgA against αC-N (left) or Rib-N (right) and compared with corresponding pre-vaccination sera for the ability to prevent invasion of ME180 cells (n = 28). Each pre-vaccination serum was diluted by the same dilution factor as the paired post-vaccination serum. The geometric mean serum dilution factor for A909 experiments was 116 (range, 14–898). The geometric mean serum dilution factor for BM110 experiments was 38 (range, 11–204). All results are normalized to invasion in the absence of human serum.

Figure 6

Naturally acquired IgG against αC-N and Rib-N accumulates in neonatal blood and persists for at least 2 months (A and B) Serum concentrations of IgG against αC-N (A) and Rib-N (B) in a cohort of paired mothers and neonates. Maternal venous blood and cord blood were collected at partum (n = 152), as well as approximately 1 month (n = 105), and approximately two months (n = 61) after delivery, respectively (STAR Methods). Results shown are IgG GMC ±95% CI. Statistical analysis was performed by paired t test on logarithmically transformed concentration values. For each comparison, only paired values were included in the analysis (maternal partum versus cord blood, n = 152; partum/cord versus 1 month, n = 105; 1 month versus two months, n = 61).