Presence of PPE proteins in Mycobacterium avium subsp. paratuberculosis isolates and their immunogenicity in cattle.

Johne's disease or paratuberculosis in cattle is caused by Mycobacterium avium subsp. paratuberculosis (MAP). Although the stages of infection have been well described, very few virulence factors of MAP have been studied in detail. We aimed to study the localization and immunogenicity of members of the polymorphic PPE protein family which is unique to Mycobacteria and has been linked to virulence in Mycobacterium tuberculosis (Mtb). The presence of PPE proteins in the cell wall was investigated by enzymatic digest of surface exposed proteins of live MAP bacteria and analysis by LC-MS/MS. Polyclonal antisera were generated against a recombinant fragment of one PPE protein and a synthetic peptide of the other to confirm their surface exposure. Sera from naturally infected cows were investigated for the presence of specific antibodies against the recombinant PPE protein. Two PPE proteins, Map3420c and Map1506, were detected by mass spectrometry and confirmed to be surface exposed on live MAP cells by immunohistochemistry. The sera from naturally infected animals contained specific antibodies against recombinantly expressed Map3420c as demonstrated by western blotting. These findings show the in vitro expression of two PPE proteins. Additionally the surface exposure and immunogenicity of PPE proteins of MAP was demonstrated.