ASSESSMENT OF THE IMMUNOGENIC POTENTIAL OF RHODOCOCCUS-EQUI VIRULENCE-ASSOCIATED PROTEIN (VAPA) IN MICE

The development of immunity to Rhodococcus equi, particularly to a vir ulence-associated protein (VapA) based antigen,preparation, was examin ed in CD1 and BALB/c mice after intraperitoneal vaccination. Immunizat ion with VapA based antigen without adjuvant markedly enhanced organ c learance in CD1 mice but not in BALB/c mice. Delayed type hypersensiti vity response and antibody titres in VapA based antigen immunized BALB /c mice were less than in CD1 mice. By contrast also to CD1 mice, sera from immunized BALB/c mice did not react as strongly with VapA in wes tern blots. Use of adjuvants (aluminium hydroxide, iscoms) interfered markedly with the immunogenic properties of the VapA based antigen, in the case of aluminium hydroxide by apparently driving a Th2 type of r esponse. Unexpectedly, iscom adjuvants also impaired immunity and, des pite the highest DTH response, produced a low IgG2a response, suggesti ng that iscomization of the antigen produced a low interferon gamma an d high interleukin 2 response. Passive immunization: of BALB/c mice wi th serum from mice immunized with live virulent strain 103 + resulted in only temporary and slight enhancement of organ clearance, supportin g the central importance of cellular immunity to R. equi, Immunization with live virulence plasmid- and VapA-positive R. equi strain 103 res ulted in marked liver clearance, in marked DTH response and high antib ody titres. By contrast, immunization with live virulence plasmid- and VapA-negative strain 103 resulted in slight but variable enhancement of clearance, but insignificant DTH and antibody. The virulence plasmi d and by implication VapA, was thus shown to be critical in determinin g a highly effective protection to live organisms. (C) 1997 Elsevier S cience B.V.