Mycoplasma gallisepticum virulence factors and vaccine design

Previous work has shown that the virulent strain R of M. gallisepticum (Rlow) lost its pathogenicity after 164 successive passages in vitro. Molecular characterization revealed that at least three proteins were absent in the avirulent strain Rhigh compared to Rlow. These proteins are the major cytadhesin GapA, cytadherence related molecule CrmA and a component of an ABC transporter system, HatA. Complementation of Rhigh with wild-type gapA restored expression in the transformant (GT5) but did not restore the cytadherence phenotype and remained avirulent for chickens. These results suggested that CrmA might play an essential role in the M. gallisepticum cytadherence process. CrmA is encoded by the second gene in the gapA operon and shares significant sequence homology to the ORF6 gene of M. pneumonia, which has been shown to play an accessory role in the cytadherence process. Complementation of Rhigh with wildtype crmA resulted in the transformant (SDCA), which lacked the cytadherent and virulent phenotype comparable to Rhigh and GT5. In contrast, complementation of Rhigh with the entire wild-type gapA operon resulted in the transformant (GCA1) which restored cytadherence to the level of wild type Rlow. In vivo pathogenesis trials revealed that GCA1 had regained virulence, causing airsacculitis but not tracheitis in chickens. These results demonstrate that both GapA and CrmA are required for M. gallisepticum cytadherence and pathogenesis. The HatA− phenotype exhibited a diminished growth rate in vitro as well as diminished virulence in vivo. ^ Birds vaccinated with GT5 were protected upon challenge with the virulent low passage R strain, as evidenced by the absence of tracheal lesions of 2 and 4 weeks post-challenge, in contrast to sham-immunized-then-challenged control birds.