Interactions between immunoglobulin evasion systems in Mycoplasma gallisepticum and their impact on antibody-dependent complement-mediated killing

Mycoplasma gallisepticum (MG) causes chronic respiratory disease, which is thought to be contributed to by its capacity to evade the immune response of infected birds. It possesses multiple copies of the genes encoding the mycoplasma immunoglobulin binding-protease (MIB-MIP) system, which binds and cleaves avian immunoglobulins (Igs), but also encodes homologs of the Ig-binding Protein-M of M. genitalium and M. pneumoniae and the Ig-cleaving CysP of M. synoviae . However, the interactions between these homologs, their host-specificity, and their interplay with the MIB-MIP system have not been defined. Here, the putative Protein-M and CysP in MG were expressed as recombinant glutathione-S-transferase (GST)-fusion proteins, functionally characterized, and their interactions with different MIB-MIP system components, as well as their impact on antibody-dependent complement-mediated killing of MG, examined. GST-Protein-M bound avian and mammalian immunoglobulins, but did not interact with the MIP proteases. GST-CysP selectively cleaved free chicken IgY and IgY complexed with GST-MIB or GST-Protein-M. GST-Protein-M, GST-CysP, and GST-MIB-MIP proteins all bound ATP, but only GST-CysP had ATPase activity. Sera from MG-infected chickens contained antibodies against a subset of MIP proteins, but not antibodies against Protein-M or CysP. The capacity of rabbit anti-MG antibodies to enhance complement-mediated killing of MG was inhibited by GST-MIB-MIP and GST-Protein-M. GST-CysP could not cleave rabbit IgG and had no detectable effect in the antibody-dependent complement-mediated killing assays. Collectively, these findings establish that Protein-M and CysP function as additional Ig-targeting systems in M. gallisepticum alongside the MIB-MIP system through distinct but complementary mechanisms, enhancing immune evasion and persistence of this important pathogen.