Expression of leptospiral immunoglobulin-like protein by Leptospira interrogans and evaluation of its diagnostic potential in a kinetic ELISA.

The search for novel antigens suitable for improved vaccines and diagnostic reagents against leptospirosis led to the identification of LigA and LigB. LigA and LigB expression were not detectable at the translation level but were detectable at the transcription level in leptospires grown in vitro. Lig genes were present in pathogenic serovars of Leptospira, but not in non-pathogenic Leptospira biflexa. The conserved and variable regions of LigA and LigB (Con, VarA and VarB) were cloned, expressed and purified as GST-fusion proteins. Purified recombinant LigA and LigB were evaluated for their diagnostic potential in a kinetic ELISA (KELA) using sera from vaccinated and microscopic agglutination test (MAT)-positive dogs. Sera from vaccinated dogs showed reactivity to whole-cell antigens of leptospires but did not show reactivity in the KELA assay with recombinant antigens, suggesting a lack of antibodies to Lig proteins in the vaccinated animals. The diagnostic potential of recombinant Lig antigens in the KELA assay was evaluated by using 67 serum samples with MAT > or =1600, which showed reactivity of 76, 41 and 35% to rConA, rVarA and rVarB, respectively. These findings suggest that recombinant antigen to the conserved region of LigA and LigB can differentiate between vaccinated and naturally infected animals.