A lipid core peptide construct containing a conserved region determinant of the group A streptococcal M protein elicits heterologous opsonic antibodies

Abstract

The study reported here investigated the immunogenicity and protective potential of a lipid core peptide (LCP) construct containing a conserved region determinant of M protein, defined as peptide J8. Parenteral immunization of mice with LCP-J8 led to the induction of high-titer serum immunoglobulin G J8-specific antibodies when the construct was coadministered with complete Freund's adjuvant (CFA) or administered alone. LCP-J8 in CFA had significantly enhanced immunogenicity compared with the monomeric peptide J8 given in CFA. Moreover, LCP-J8/CFA and LCP-J8 antisera opsonized four different group A streptococcal (GAS) strains, and the antisera did not cross-react with human heart tissue proteins. These data indicate the potential of an LCP-based M protein conserved region GAS vaccine in the induction of broadly protective immune responses in the absence of a conventional adjuvant.

FIG. 1.

Chemical structure of the LCP-J8 construct. Four identical peptide J8 sequences were synthesized directly onto the two amino groups of each lysine (Lys) of a branched polylysine core. The lipophilic anchor of the LCP-J8 construct contains three 2-amino-dodecanoic lipoamino acids (shown as branched alkyl side chains) attached to the polylysine core, with one glycine amino acid spacer between the resin and the first lipoamino acid and a second between the second and third lipoamino acids.

FIG. 2.

Serum IgG antibody response in B10.BR mice immunized parenterally with the LCP-J8 construct in the presence and absence of CFA for exp 1 (A) and exp 2 (B). Antibody titers to the J8 peptide for individual mice are shown, with the average titers (geometric means) represented as bars. Antibody titers to J8 are shown for individual mice from the control group that had been immunized with J8 in CFA. The two experiments were performed separately and differ in that mice received four booster injections of immunogen in exp 1 and five booster injections in exp 2. Using analysis of variance (with Tukey's post hoc method for multiple comparisons), antibody titers were significantly greater in mice immunized with LCP-J8/CFA than in those immunized with J8 in CFA alone (exp 1; ∗, P < 0.001). Significant differences in antibody titers were also observed as indicated (∗∗, P < 0.001; ∗∗∗, P < 0.001; ∗∗∗∗, P < 0.001).

FIG. 3.

Opsonization of GAS by LCP-J8/CFA, LCP-J8/PBS, and control (J8/CFA and pepM1/CFA) B10.BR mouse antisera for exp 1 (panels A and C to E) and exp 2 (B). The average percentage of opsonization (measured as the percentage of reduction in bacterial population [in CFU]) for each individual group is shown with the standard error of the mean represented as a bar. Opsonization was determined by an indirect bactericidal assay which compares the growth of bacterial population (in CFU) following incubation in the presence of immune sera to that of control normal mouse sera obtained from mice immunized with adjuvant in PBS alone. Using analysis of variance (with Tukey's post hoc method for multiple comparisons), opsonization against M1 and BSB24 GAS was significantly greater in mice immunized with LCP-J8/CFA and LCP-J8/PBS than in those immunized with J8 in CFA alone. ∗, P = 0.051; ∗∗, P = 0.008; ∗∗∗, P = 0.031; ∗∗∗∗, P = 0.013.

FIG. 3.

Opsonization of GAS by LCP-J8/CFA, LCP-J8/PBS, and control (J8/CFA and pepM1/CFA) B10.BR mouse antisera for exp 1 (panels A and C to E) and exp 2 (B). The average percentage of opsonization (measured as the percentage of reduction in bacterial population [in CFU]) for each individual group is shown with the standard error of the mean represented as a bar. Opsonization was determined by an indirect bactericidal assay which compares the growth of bacterial population (in CFU) following incubation in the presence of immune sera to that of control normal mouse sera obtained from mice immunized with adjuvant in PBS alone. Using analysis of variance (with Tukey's post hoc method for multiple comparisons), opsonization against M1 and BSB24 GAS was significantly greater in mice immunized with LCP-J8/CFA and LCP-J8/PBS than in those immunized with J8 in CFA alone. ∗, P = 0.051; ∗∗, P = 0.008; ∗∗∗, P = 0.031; ∗∗∗∗, P = 0.013.