Defense from the Group A Streptococcus by active and passive vaccination with the streptococcal hemoprotein receptor

Abstract

Background: The worldwide burden of the Group A Streptococcus (GAS) primary infection and sequelae is considerable, although immunization programs with broad coverage of the hyper variable GAS are still missing. We evaluate the streptococcal hemoprotein receptor (Shr), a conserved streptococcal protein, as a vaccine candidate against GAS infection.

Methods: Mice were immunized intraperitoneally with purified Shr or intranasally with Shr-expressing Lactococcus lactis. The resulting humoral response in serum and secretions was determined. We evaluated protection from GAS infection in mice after active or passive vaccination with Shr, and Shr antiserum was tested for bactericidal activity.

Results: A robust Shr-specific immunoglobulin (Ig) G response was observed in mouse serum after intraperitoneal vaccination with Shr. Intranasal immunization elicited both a strong IgG reaction in the serum and a specific IgA reaction in secretions. Shr immunization in both models allowed enhanced protection from systemic GAS challenge. Rabbit Shr antiserum was opsonizing, and mice that were administrated with Shr antiserum prior to the infection demonstrated a significantly higher survival rate than did mice treated with normal rabbit serum.

Conclusions: Shr is a promising vaccine candidate that is capable of eliciting bactericidal antibody response and conferring immunity against systemic GAS infection in both passive and active vaccination models.

Figure 1.

Anti-streptococcal hemoprotein receptor (Shr) response in serum after intraperitoneal vaccination. Mice were administrated rShr (arrow heads), and the resulting serum immunoglobulin (Ig) G titers were determined by enzyme-linked immunosorbent assay performed in quadruplicates. Each datum point represents the average response in individual animals. A, Shr antibody development. Shr-specific Ig G levels were determined in serum samples collected from 4 representative mice on days 0, 7, 21, 35, and 49. B, Endpoint anti-Shr IgG response. The anti-Shr IgG titers found in serum samples collected on day 49 from mice administered rShr (Shr-immuned; n = 19) or with phosphate-buffered saline (mock; n = 10) are shown. The horizontal bar indicts the mean titer response.

Figure 2.

Protection of mice from systemic group A Streptococcus (GAS) challenge after intraperitoneal vaccination with streptococcal hemoprotein receptor (Shr). Immunized mice were infected intraperitoneally with 5 × 10⁸ CFU. Kaplan-Meier survival curves of antigen-immunized mice (shr; n = 18) and mock-vaccinated mice (phosphate-buffered saline [PBS]; n = 14) are shown. The statistical significance (P = .002) was determined by the log-rank test. Two independent experiments producing similar results were conducted; the data shown are from a representative experiment.

Figure 3.

Streptococcal hemoprotein receptor (Shr) intranasal vaccination and protective immune response. Mice were immunized with 2 × 10⁹ CFU of Lacococcus lactis (MG1363) or L. lactis expressing Shr (MG1363/pXL14). Endpoint antibody response (on days 50 and 52, respectively) was determined by enzyme-linked immunosorbent assay preformed in quadruplicate. A, Shr-specific immunoglobulin (Ig) A level in undiluted lung lavage specimens from mice treated with MG1363 (n = 2) or MG1363/pXL14 (n = 13). Each datum point represents the mean response in individual animals. The statistical significance (P = .01) was determined by the Student t test. B, Shr-specific IgG titer in serum for individual mice treated with MG1363 (n = 15) or MG1363/pXL14 (n = 15). The statistical significance (P < .0001) was determined by the Student t test. C, Kaplan-Meier survival curves of vaccinated mice after systemic group A Streptococcus challenge. Mice immunized with MG1363 (n = 8) or MG1363/pXL14 (n = 8) were intraperitoneally received 1 × 10⁸ CFU. The statistical significance (P =.014) was determined by the log-rank test. The data shown are pooled data from 2 independent experiments.

Figure 4.

Protection of mice from systemic group A Streptococcus (GAS) infection with passive immunization with streptococcal hemoprotein receptor (Shr). Kaplan-Meier survival curves are shown for mice challenged with 5 × 10⁷ CFU 4 h after intraperitoneal administration of rabbit Shr antiserum (anti-Shr) or with normal rabbit serum (NRS). A, MGAS5005 challenge. The results are representative of 2 independent experiments (n = 10 for both groups; P = .0308). B, MGAS315 challenge. The data shown are pooled from 3 independent experiments (n = 15 for both groups; P = .034). Statistical significance was determined by the log-rank test.

Figure 5.

Serum mediation of phagocytosis and killing of bacteria with anti–streptococcal hemoprotein receptor (Shr). ZE491 cells were incubated in rabbit whole blood in the presence of Shr antiserum or normal rabbit serum (NRS). Bacterial growth after 3 h of incubation with Shr-antiserum was compared with that of control NRS. The mean opsonization percentage (measured as the percentage reduction in CFU) derived from 4 independent experiments is shown; the standard deviation is represented by the error bar (P < .001).