Human B Cell Responses to Dominant and Subdominant Antigens Induced by a Meningococcal Outer Membrane Vesicle Vaccine in a Phase I Trial

Abstract

Neisseria meningitidis outer membrane vesicle (OMV) vaccines are safe and provide strain-specific protection against invasive meningococcal disease (IMD) primarily by inducing serum bactericidal antibodies against the outer membrane proteins (OMP). To design broader coverage vaccines, knowledge of the immunogenicity of all the antigens contained in OMVs is needed. In a Phase I clinical trial, an investigational meningococcal OMV vaccine, MenPF1, made from a meningococcus genetically modified to constitutively express the iron-regulated FetA induced bactericidal responses to both the PorA and the FetA antigen present in the OMP. Using peripheral blood mononuclear cells collected from this trial, we analyzed the kinetics of and relationships between IgG, IgA, and IgM B cell responses against recombinant PorA and FetA, including (i) antibody-secreting cells, (ii) memory B cells, and (iii) functional antibody responses (opsonophagocytic and bactericidal activities). Following MenPF1vaccination, PorA-specific IgG secreting cell responses were detected in up to 77% of participants and FetA-specific responses in up to 36%. Memory B cell responses to the vaccine were low or absent and mainly detected in participants who had evidence of preexisting immunity (P = 0.0069). Similarly, FetA-specific antibody titers and bactericidal activity increased in participants with preexisting immunity and is consistent with the idea that immune responses are elicited to minor antigens during asymptomatic Neisseria carriage, which can be boosted by OMV vaccines. IMPORTANCE Neisseria meningitidis outer membrane vesicles (OMV) are a component of the capsular group B meningococcal vaccine 4CMenB (Bexsero) and have been shown to induce 30% efficacy against gonococcal infection. They are composed of multiple antigens and are considered an interesting delivery platform for vaccines against several bacterial diseases. However, the protective antibody response after two or three doses of OMV-based meningococcal vaccines appears short-lived. We explored the B cell response induced to a dominant and a subdominant antigen in a meningococcal OMV vaccine in a clinical trial and showed that immune responses are elicited to minor antigens. However, memory B cell responses to the OMV were low or absent and mainly detected in participants who had evidence of preexisting immunity against the antigens. Failure to induce a strong B cell response may be linked with the low persistence of protective responses.

Keywords: Neisseria; bacteria; genetic modification; infection; meningitidis; meningitis; meningococcal; outer membrane; outer membrane proteins; outer membrane vesicles; vaccine; vesicles.

FIG 1

Kinetics of IgG-producing antibody-secreting cells (ASC) detected by ex vivo ELISPOT before vaccination and 7 days after each vaccine dose of 25 or 50 μg of MenPF1 vaccine (administered at time point 0, week 8, and week 16). Individual IgG secreting cell counts specific to PorA (A), FetA (B), and OMVs (C) in participants immunized with 25 μg (closed circles) or 50 μg (open circles) of MenPF1 are represented. The horizontal red bar represents the geometric means and 95% CI. (D) Relation between PorA and FetA-specific IgG ASC responses 7 days after the first injection. (E) Relation between PorA and OMV-specific ASC responses 7 days after the first injection.

FIG 2

IgA-producing ASC numbers detected by ex vivo ELISPOT before and 7 days after the second and/or third vaccine dose of the MenPF1 vaccine. Individual IgA responses specific to PorA (A), FetA (B), and OMVs (C) are represented at the time points indicated. The horizontal red bar represents the geometric mean and 95% CI. NT = not tested. (D) Relation between PorA-specific IgG and IgA ASC responses 7 days after the third injection. (E) Relation between OMV-specific IgG and IgA ASC responses 7 days after the third injection.

FIG 3

Kinetics of IgG, A, and M memory B cell numbers detected by cultured ELISPOT before and 28 days after each vaccine injection of 25 or 50 μg of MenPF1 vaccine (administered at time point 0, week 8, and week 16). Individual IgG memory B cell counts specific to PorA (A), FetA (B), and OMVs (C) are represented. The horizontal red bar represents the geometric mean and 95% CI. The blue bars denote statistical significance compared with baseline. IgA responses are represented in (D to F) and IgM responses in (G to I). (J) Relation between PorA and OMV-specific memory B cell responses after the third dose. (K) Relation between PorA and FetA-specific IgG memory B cell responses after the third dose. (L) Influence of OMV-specific IgG memory B cell response at baseline on the induction of IgG ASC responses. The individual OMV IgG ASC responses are shown for participants in each dose group according to the absence (<5) of presence (≥5) of preexisting memory B cell response to OMVs at baseline.

FIG 4

Kinetics of FetA-specific IgG, IgA, and IgM serum antibody titers before (time point 0) and a month after the second (week 12) and third vaccine doses (week 20) of 25 or 50 μg of MenPF1 vaccine (administered at time point 0, week 8, and week 16). Individual IgG (A), IgA (B), and IgM (C) are represented. The horizontal red bar represents the geometric mean of the group and 95% confidence interval.

FIG 5

SBA titers before (time point 0) and a month post the second (week 12) and third injections (week 20) of MenPF1 vaccine (administered at time point 0, week 8, and week 16) in both dose groups. The individual SBA titers are represented against the wild-type 44/76 strain (WT; [A]) and the isogenic modified strains PorA_on FetA_off (B), 44/76 PorA_on FetA_on (C), and PorA_off FetA_off (D), 44/76 PorA_off FetA_on (E). The geometric mean per group and 95% confidence intervals are shown in red. The horizontal dotted line represents the putative cutoff for protection (titer of 1:4). (F) shows the relation between the individual PorA-specific and FetA specific titers at week 20 (both doses).

FIG 6

Immune responses at week 20 (4 weeks post third vaccine dose, y-axis) according to absence or presence of preexisting responses at baseline in each dose group (x-axis). Individual SBA and geometric mean titers against the wild-type strain (WT) (A) and against the PorA_on FetA_off (B), or 44/76 PorA_off FetA_on (C) are represented with participants separated in the x-axis according to the absence of baseline response before vaccinations (<1:4) or presence of preexisting SBA response (≥1:4) in each dose. The horizontal dotted line represents the putative cutoff for protection (titer of 1:4). Serum IgG ELISA titers and geometric mean against FetA (D). The horizontal line represents the cutoff for positivity (3 × 10⁶ ELISA titer), and participants are separated on the x-axis according to the absence of response at baseline ELISA titer) or preexisting response (≥3 × 10⁶ ELISA titer). The IgG memory B cell responses to PorA (E), FetA (F), and OMV (G), the horizontal dotted line represents a response >5 spots per million cells (after background subtraction) with participants separated in the x-axis according to the absence of baseline response before vaccinations (<5) or presence of preexisting response (≥5), in each dose. * and ** represent statistical significance as described in the text.