CHARACTERIZATION OF BOVINE TRANSFERRIN RECEPTOR ON NORMAL ACTIVATED AND THEILERIA PARVA-TRANSFORMED LYMPHOCYTES BY A NEW MONOCLONAL-ANTIBODY

A murine IgM monoclonal antibody (mAb), IL-A77, has been generated tha t recognises the bovine transferrin receptor (TfR) and will be a usefu l tool to measure the activation state of bovine lymphocytes and macro phages. The antigen is detected on immature erythroid cells and prolif erating lymphocytes, It is undetectable on resting lymphocytes, but ap pears within 24 h after stimulation with concanavalin A (ConA) or poke weed mitogen (PWM). Immune precipitations of lysates of both labeled a ctivated lymphocytes and bone marrow erythroid cells showed that, simi lar to human TfR, the bovine receptor is a disulfide-bonded dimer of t wo identical chains of M(r) 97 000, A similar 97 000 M(r) protein was eluted from a column containing immobilised bovine transferrin (Tf) us ing conditions known to elute the human TfR, and this protein was reco gnised by mAb IL-A77, proving that it detected bovine TfR. Although th e mAb inhibited binding of transferrin to its receptor, it did not blo ck proliferation of Theileria parva-transformed or ConA-stimulated lym phocytes, When cells were metabolically labeled with S-35-methionine, a second 90 000-M(r) TfR band was detected in Theileria parva-transfor med cells, but not in stimulated lymphocytes. This form of the TfR was not expressed on the cell surface. It may be an unusual precursor of the receptor, a parasite-modified receptor or it may be of parasite or igin and necessary for transfer of iron into the intracellular parasit e.