Streptococcus suis bacterin and subunit vaccine immunogenicities and protective efficacies against serotypes 2 and 9

Abstract

Streptococcus suis causes numerous diseases in pigs, most importantly, meningitis, arthritis, septicemia, and bronchopneumonia. One of the major problems in modern swine production is the lack of a vaccine protecting against more than one S. suis serotype. The objective of this study was to determine the protective efficacy of a serotype 2 murein-associated protein (MAP) fraction subunit vaccine in comparison to that of a bacterin against experimental challenge with serotype 2 (containing muramidase-released protein [MRP], extracellular factor, and suilysin [SLY]) and serotype 9 (containing MRP variant MRP* and SLY) strains. MAP was shown to include different surface-associated proteins, such as the MRP and surface antigen one (SAO) expressed by both pathotypes used for challenge. The results of this study demonstrated that the serotype 2 bacterin induced protective immunity against homologous challenge. In contrast, the protective efficacy of the MAP subunit vaccine was low, though MAP immunization resulted in high serum immunoglobulin G2 titers against MRP and SAO. Importantly, immunization with bacterin but not with MAP induced opsonizing antibody titers against the serotype 2 strain, and these antibody titers were found to correlate with protection. However, after absorption with a nonencapsulated isogenic mutant, the sera from bacterin-immunized piglets failed to facilitate neutrophil killing, indicating that antibodies directed against capsule may not have been essential for opsonophagocytosis. Furthermore, induction of opsonizing antibodies against serotype 9 was not detectable in the group receiving bacterin or in the group receiving the MAP vaccine. In agreement, protection against the heterologous serotype 9 strain was low in both groups. Thus, identification of an antigen protecting against these two important S. suis pathotypes remains an important goal of future studies.

FIG. 1.

MAP fraction of S. suis serotype 2 strain 10 (A) and serotype 9 strain A3286/94 (B) separated by 8% SDS-PAGE (silver stained, left panel) and analyzed via Western blots probed with anti-rMRP (3), anti-rSAO, or anti-rFBPS antisera as indicated. The sample for strain 10 shown in lanes A was a 1:100 dilution of the MAP fraction used for preparation of the subunit vaccine in the presented study.

FIG. 2.

Kaplan-Meier survival diagrams of piglets challenged with S. suis after application of a bacterin, a MAP subunit vaccine, or a placebo on the 7th and 18th days postweaning. Challenge was performed either intranasally with 10⁹ CFU of S. suis strain 10 (A) or intravenously with 10⁸ CFU of MRP* SLY⁺ serotype 9 S. suis strain A3286/94 (B), both on the 32nd day postweaning. The P values of the comparison between the bacterin- and MAP-immunized groups with the placebo group are indicated.

FIG. 3.

Serum IgG responses against surface-associated proteins MRP (A and B) and SAO (C) in pigs immunized with a bacterin or a MAP subunit vaccine. The serum samples were drawn before immunization and 14 days after the second immunization, prior to challenge either with serotype 2 (ST2) strain 10 (A and C) or with serotype 9 (ST9) strain A3286/94 (B). (See also Fig. S1 and S2 in the supplemental material.)

FIG. 4.

Opsonophagocytic killing by porcine neutrophils in the presence of serum of piglets immunized with a bacterin or a MAP subunit vaccine. (A) The survival factors of strain 10 were determined in neutrophil killing assays with sera taken either prior to immunization (pre) or two weeks after the second immunization (post). (B) Correlation of the induction of opsonizing antibodies as determined by the ratio of postimmunization survival factors to preimmunization survival factors as shown in panel A and development of disease after homologous strain 10 challenge. Triangles and circles represent bacterin- and MAP-immunized piglets, respectively. Data for one MAP-immunized piglet (open circle) were not included in the calculation of the regression coefficient. (C) Comparison of the induction of opsonizing antibodies against serotype 9 (ST9) strain A3286/94 and serotype 2 (ST2) strain 10 as determined by the ratio of respective survival factors in the presence of postimmunization and preimmunization sera. The sera were from the piglets challenged with S. suis A3286/94.

FIG. 5.

Antibody responses against serotype 2 (ST2) capsular polysaccharides in piglets immunized with a bacterin, a MAP subunit vaccine, or, for comparison, BSA-conjugated ST2 capsular polysaccharides. (A) Serum IgG responses against ST2 capsular polysaccharides. (B) Opsonophagocytic killing by porcine neutrophils in the presence of absorbed sera from immunized piglets. Sera had been absorbed with S. suis strain 10 (ST2) or the isogenic capsule mutant 10cpsΔEF (Δcps2) as indicated below the x axis and were then tested in neutrophil killing experiments with S. suis strain 10.