Recognition of soluble Theileria parva antigen by bovine helper T cell clones: characterization and partial purification of the antigen.

Theileria parva-specific bovine BoT4+ Th cell clones were used to characterize Ag associated with T. parva schizont-infected lymphoblastoid cells. All of the clones tested responded to cells infected with the immunizing (Muguga) as well as heterologous stocks of T. parva, indicating that the T cells are specific for an Ag shared by several geographically diverse parasites. The response was apparently MHC-restricted, and induced by Ag expressed on the infected cell surface. In the presence of autologous APC, the clones were also stimulated by a soluble high speed supernatant (HSS), but not by a schizont membrane-enriched, subcellular fraction prepared from homogenates of infected cells. The clones produced IFN-gamma and T cell growth factor in response to HSS. The soluble Ag was absent in cells from which schizonts had been eliminated by treatment with the anti-theilerial drug, parvaquone. Fractionation of HSS by hydroxylapatite chromatography revealed two antigenic peaks that separated from the majority of the protein. Fractionation of HSS by gel filtration with the use of HPLC revealed several peaks of activity ranging in Mr from 270 kDa to less than 5 kDa. Further fractionation of HSS by both hydroxylapatite chromatography and gel filtration yielded three major peaks of activity (Mr 43, 12, 4.2 kDa). We conclude that a T cell-dependent schizont-associated soluble Ag is also expressed on the surface of T. parva-infected cells.