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There is a need to produce a vaccine against 'Rhodococcus equi ' pneumonia in foals. Immunity against infection is largely based on a Th1 cell-mediated response. C57BL/6 and BALB/c mice were immunized with a DNA vaccine constructed from 'vapA' incorporated into pcDNA3.1. The plasmid construct expressed VapA in a COS-7 cell line. Intramuscular immunization of mice resulted in enhanced clearance of 'R. equi' from the liver of intravenously challenged mice compared to non-immunized controls. This effect was more marked when pORF-'IL-12' was included with the vaccine. Antibody developed to VapA with an IgG2a response being more marked in mice immunized with pcDNA-'vapA' than in non-immunized or in mice immunized with the mixed 'vapA' and 'IL-12 ' plasmid constructs. The plasmid constructs were electroporated into an attenuated 'Salmonella' Typhimurium strain MGN 707 ([Delta]' cya'[Delta]'crp'-'pabA') and both strains of mice were immunized twice orally with the vaccine vector. Following intravenous challenge with 'R. equi', the VapA ELISA showed a slight immune response only in mice immunized with a 50:50 mixture of 'Salmonella ' containing either 'vapA' or 'IL-12' gene constructs. Mice developed a marked IgG antibody response to the ' Salmonella'. Foals administered the 'Salmonella' vector (without the plasmid) orally showed no ill effect and failed to shed the organism. In conclusion, this study has shown for the first time that DNA immunization with 'vapA' enhances the immune responses of mice against ' R. equi' infection, and that this can be enhanced by injection of the 'IL-12' gene. Incorporation of these DNA constructs into a 'Salmonella' vaccine vector induced only a marginal immune response. |
