Three distinct subpopulations of sheep T lymphocytes

Monoclonal antibodies reactive with distinct T lymphocyte subpopulations have been described in man, mouse and rat and structural analyses of these antigens have demonstrated a high degree of evolutionary conservation. This report describes the reactivity of three monoclonal antibodies (mAb), 19-19, alpha SBU-T4 and alpha SBU-T8, which define T cell subpopulations in the sheep. The mAb alpha SBU-T4 and alpha SBU-T8 define the sheep CD4 and CD8 molecules, respectively. These two antigens show similar tissue distributions, molecular weights and fluorescence-activated cell sorter profiles to human, mouse and rat CD4 and CD8 molecules. The mAb 19-19 is reactive with a subpopulation of T lymphocytes which displays a tissue distribution unlike that reported for a T cell subset in any other species. 19-19 stains 7% of efferent lymph lymphocytes, 15% of peripheral blood lymphocytes but only 1-3% of lymph node lymphocytes. Two-color immunofluorescence demonstrates that the 19-19+ T cell subset is SBU-T4- and SBU-T8-, and thus defines a third T cell subpopulation in sheep. Immunohistology on frozen lymph node tissue sections localizes 19-19 mAb-reactive cells to the subcapsular region of the lymph node and lymph node trabeculae. Only 1% of thymocytes are 19-19+ and these cells are located mainly in the medulla and often arranged as foci around blood vessels. The 19-19 mAb immunoprecipitates from sheep lymphocytes an antigen with an apparent molecular mass of 215 kDa under both reducing and nonreducing conditions. It is concluded that alpha SBU-T4 and alpha SBU-T8 recognize the sheep homologues of the human T4 and T8 antigens, respectively, whereas 19-19 recognizes an antigen (termed SBU-T19) which has not been reported in any other species.