Vaccination With a Latch Peptide Provides Serotype-Independent Protection Against Group B Streptococcus Infection in Mice

Abstract

Streptococcus agalactiae (group B streptococcus [GBS]) is a leading cause of invasive diseases in neonates and severe infections in elderly individuals. GBS serine-rich repeat glycoprotein 1 (Srr1) acts as a critical virulence factor by facilitating GBS invasion into the central nervous system through interaction with the fibrinogen Aα chain. This study revealed that srr1 is highly conserved, with 86.7% of GBS clinical isolates expressing the protein. Vaccination of mice with different Srr1 truncated peptides revealed that only Srr1 truncates containing the latch domain protected against GBS meningitis. Furthermore, the latch peptide alone was immunogenic and elicited protective antibodies, which efficiently enhanced antibody-mediated opsonophagocytic killing of GBS by HL60 cells and provided heterogeneous protection against 4 different GBS serogroups. Taken together, these findings indicated that the latch domain of Srr1 may constitute an effective peptide vaccine candidate for GBS.

Keywords: Streptococcus agalactiae; latch; serine-rich repeat; vaccine.

Figure 1.

Protective effects of vaccination with Srr1 truncates against group B streptococcus (GBS). Mice (n = 10) were vaccinated with Srr1 truncates N2N3, N2, N3, and N2N3∆latch, with 2% Alhydrogel adjuvant. A, Serum levels of immunoglobulin G (IgG) were measured 7 days after the second vaccination. B, Seven days after the second vaccination, mice were intravenously challenged with GBS CNCTC10/84 (10⁷ colony-forming units [CFU]), and mouse survival was recorded. **P < .01 and ***P < .001, compared with mice vaccinated with adjuvant. C, Brains of mice (n = 5) were dissected at the time of death or upon euthanasia at the end of experiment and homogenized in phosphate-buffered saline. GBS counts were measured in the serially diluted brain tissue homogenates. ***P < .001, compared with mice vaccinated with adjuvant. D, Human brain microvascular endothelial cell line monolayers were infected with GBS at a multiplicity of infection of 0.1 for 30 minutes, followed by washing and incubation for 30 minutes with penicillin (100 μg/mL). Cells were then lysed with 0.1% Triton X-100 and plated onto blood agar plates to count GBS. GBS organisms were preincubated for 30 minutes with immune serum from mice vaccinated with adjuvant, N2N3, N2, N3, or N2N3∆latch before cell infection. Data are expressed as means ± SD. ***P < .001, compared with cells incubated with adjuvant-vaccinated serum.

Figure 2.

Protective effects of latch vaccination against group B streptococcus (GBS) infections in mice. Mice (n = 5) were vaccinated with bovine serum albumin–conjugated latch peptide (BSA-latch) twice at 14-day intervals, followed by challenge intravenously with 10⁷ colony-forming units of GBS CNCTC10/84. A, Survival of mice was recorded for 3 days. B–D, Brains of mice were dissected at the time of death or after euthanasia at the end of the experiment. Brain tissue specimens were homogenized in phosphate-buffered saline, and GBS counts were measured in the serially diluted brain tissue homogenates (B). Formalin-fixed, paraffin-embedded brain tissue sections were stained with hematoxylin and eosin (C) and Gram stain (D). *P < .05 and ***P < .001, compared with adjuvant vaccinated mice.

Figure 3.

Protective effects of latch-specific antibodies against group B streptococcus (GBS). Mice (n = 5) were vaccinated with indicated concentrations of bovine serum albumin–conjugated latch peptide (BSA-latch) twice intraperitoneally at 14-day intervals. A and B, Serum levels of latch-specific immunoglobulin G (IgG; A) and immunoglobulin M (IgM; B) were analyzed 7 days after the second vaccination. Data are expressed as means ± SD. **P < .01 and ***P < .001, compared with adjuvant (Adj). C, GBS CNCTC 10/84 (10⁹ colony-forming units) was incubated with 10 μL of immune serum from the mice vaccinated with Adj, BSA, or BSA-latch and added to the well coated with human fibrinogen. After incubation for 30 minutes, wells were washed and stained with 0.5% crystal violet. Data are expressed as means ± SD. **P < .01, compared with Adj. D, Serum (100 μL) from naive or BSA-latch–vaccinated mice was intraperitoneally inoculated into naive CD-1 mice (n = 5). At 12 hours after inoculation, mice were challenged intravenously with GBS CNCTC 10/84, and mouse survival was monitored for 72 hours. *P < .05, compared with unvaccinated mice.

Figure 4.

Protective effect of latch-specific CD4⁺ T cells against group B streptococcus (GBS). Mice (n = 5) were vaccinated with 10 µg of bovine serum albumin–conjugated latch peptide (BSA-latch) twice intraperitoneally at 14-day intervals, and the spleen was dissected from the mice 7 days after vaccination. A, Splenocytes were incubated with 1 or 10 µg of latch peptide for 48 hours, and proliferation of splenocytes was measured. Data are expressed as means ± SD. B, Splenocytes were incubated with 10 µg of latch peptide, and percentages of interferon γ (IFN-γ)–secreting CD4⁺ T cells were measured by flow cytometry. Data are expressed as means ± SD. C, Splenocytes were incubated with 10 µg of latch peptide, and expression of interleukin 2 (IL-2), interleukin 4 (IL-4), interleukin 5 (IL-5), and IFN-γ were measured in culture supernatants by enzyme-linked immunosorbent assay. Data are expressed as means ± SD. D, Splenic CD4⁺ T cells from naive mice (n = 5) or BSA-latch–vaccinated mice were pooled and transferred intraperitoneally to CD-1 mice (n = 5). At 12 hours after inoculation, mice were challenged intravenously with GBS CNCTC10/84, and mouse survival was monitored for 72 hours. *P < .05, **P < .01, and ***P < .001, compared with unvaccinated mice. Adj, adjuvant; ConA, concanavalin A.

Figure 5.

Cross-serotype protection against Srr1-expressing group B streptococcus (GBS) serotypes by latch vaccination. Mice (n = 5) were vaccinated intraperitoneally with 10 µg of bovine serum albumin–conjugated latch (BSA-latch) twice at 14-day intervals, and mice were challenged intravenously with GBS strains 2603V/R (ST-V; A), CNCTC1/82 (ST-IV; B), H36B (ST-1b; C), or NEM316 (ST-III; D) 7 days after the second vaccination. Survival of mice was monitored for 72 hours. *P < .05, compared with mice vaccinated with adjuvant.