Poultry infected with Mycoplasma gallisepticum and Mycoplasma synoviae produce antibodies to their cysteine protease CysP Major poultry pathogens, Mycoplasma gallisepticum and Mycoplasma synoviae possess cysteine protease CysP, which in vitro cleaves chicken immunoglobulin G (IgG) into Fab (antigen-binding fragment) and Fc (crystallisable region fragment). We used recombinant CysP of M. synoviae (rCysP) in enzyme immunoassays to detect antibodies to CysP in sera, synovial fluids, and in washings of respiratory tract and oviducts of chickens and turkeys experimentally or naturally infected with M. gallisepticum or M. synoviae. In poultry infected with M. synoviae, 70.4 % of samples contained antibodies reacting with rCysP. In birds infected with M. gallisepticum we detected CysP antibodies in 63.1 % of samples. Our data demonstrate that CysP is immunogenic for chickens and turkeys and indicate that M. gallisepticum and M. synoviae infecting chickens and turkeys synthesise CysP in vivo. This is the first study demonstrating that proteases of any Mycoplasma species can induce production of specific antibodies in the natural host. Perutnina ob okužbi z bakterijo Mycoplasma gallisepticum ali Mycoplasma synoviae proizvede specifična protitelesa proti njuni cisteinski proteazi CysP Mycoplasma gallisepticum in Mycoplasma synoviae sta pomembna patogena mikroorganizma pri perutnini. Sintetizirata cisteinsko proteazo CysP, ki in vitro razgrajuje kokošje imunoglobuline G na Fab in Fc fragment. Za dokazovanje protiteles proti CysP z encimsko imunskimi testi v serumih, sinovialnih tekočinah in v izpirkih dihalnih traktov in jajcevodov ptic naravno ali eksperimentalno okuženih z M. synoviae ali M. gallisepticum, smo uporabili rekombinanten protein CysP (rCysP). Pri perutnini okuženi z M. synoviae je protitelesa proti CysP vsebovalo 70,4 % vzorcev, pri perutnini okuženi z M. gallisepticum pa smo protitelesa proti CysP dokazali pri 63,1 % vzorcih. Pridobljeni podatki potrjujejo, da je CysP za perutnino imunogen protein in nakazujejo, da M. gallisepticum in M. synoviae ob okužbi perutnine sintetizirata CysP in vivo. Z našo študijo prvič dokazujemo, da lahko mikoplazemska proteaza inducira nastanek specifičnih protiteles v naravnem gostitelju.