Antigenic analysis of Rhodococcus equi preparations using different horse sera

An R. equi vaccine, prepared under conditions which induce the expression of many antigens, and which has given encouraging results in field trials, was analyzed by SDS-PAGE and immunoblots and compared with other R. equi preparations: a preparation made in with the same technique from a nonvirulent isolate (virulence associated protein negative, VapA-negative); a whole cell preparation of a VapA-positive R. equi, prepared as a standard bacterin; and a semipurified VapA preparation (APTX). The antigens in these preparations were analyzed using hyperimmune sera (from adult horses vaccinated with the R. equi vaccine), passively and actively immunized foals' sera, asymptomatic but serologically positive foals' sera sera from R. equi pneumonic foals, an equine APTX antiserum, and a VapA monoclonal antibody (Mab). The vaccine under study had many proteins in high concentrations. Hyperimmune sera reacted strongly with vaccine antigens in the high molecular weight regions. In the low molecular weight range, it reacted in the 14 and less kDa zone. Sera from passively immunized foals reacted similarly but not so strongly. Actively immunized foals gave very weak reactions. With the APTX extract, the Mab reacted with bands at 15-17, 44 and 66 kDa; it reacted weakly with the whole cell and not with the VapA-negative preparations. The APTX antiserum and the Mab reacted strongly with the vaccine at the 14 and less kDa zone, and also with bands at 21, 44 and 66 kDa and very tenuously at 18 kDa, but not in the expected 15-17 kDa zone, suggesting that the native form of VapA is altered but without loss of antigenicity in the vaccine preparation. Our results suggest that other higher molecular weight antigens, in addition to VapA, may be important in inducing antibodies that protect young foals from R. equi pneumonia. These antigens are in high concentrations and in an immunogenic form in the vaccine.