A prime-boost strategy using the novel vaccine candidate, LemA, protects hamsters against leptospirosis

Abstract

Toward developing an effective vaccine capable of conferring heterologous protection, the putative lipoprotein LemA, which presents an M3 epitope similar to that of Listeria, was evaluated as a vaccine candidate in the hamster model of leptospirosis. LemA is conserved (>70% pairwise identity) among the pathogenic Leptospira spp., indicating its potential in stimulating a cross-protective immune response. Using different vaccination strategies, including prime-boost, DNA vaccine, and a subunit preparation, recombinant LemA conferred different levels of protection in hamsters. Significant protection against mortality was observed for the prime-boost and the DNA vaccine strategies, which showed 87.5% (P < 0.01) and 62.5% (P < 0.05) efficacy, respectively. Although the subunit vaccine preparation protected 50.0% of immunized hamsters, the level of protection was not significant. None of the hamsters in the control groups survived challenge with a virulent strain of Leptospira interrogans serogroup Icterohaemorrhagiae. Characterization of the immune response found that the strongest antibody response was stimulated by the subunit vaccine preparation, followed by the prime-boost strategy. The DNA vaccine failed to elicit an antibody response in immunized hamsters.

Fig 1

rLemA vaccine preparation. (A) SDS-PAGE of the rLemA protein expressed in E. coli and purified by IMAC using Ni²⁺ Sepharose HisTrap columns. Lanes 1 to 6, purified rLemA eluted with 250 mM imidazole (17 kDa). (B) Expression of LemA in Vero cells visualized in a Western blot assay using anti-LemA mouse serum. Lane 1, pellet of cells transfected with pTARGET and incubated with anti-LemA serum; lane 2, pellet of cells transfected with pTARGET-lemA and incubated with anti-LemA serum. Lanes M, molecular mass protein ladder (kDa).

Fig 2

Systemic antibody levels in hamsters inoculated with rLemA vaccines and controls. Recombinant rLemA expressed by E. coli was used as the antigen in the ELISA. Mean values were calculated from serum samples assayed in triplicate. Results are expressed as the mean absorbance ± standard deviation, and significant differences, at a P value of <0.05 in comparison to the control group, are shown by an asterisk. OD₄₉₂, optical density at 492 nm.

Fig 3

Survival of hamsters challenged with virulent L. interrogans after immunization with the various rLemA vaccine preparations. Postchallenge survival is shown for a representative vaccine evaluation (experiment 1; data are given in Table 1).