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The monoclonal antibodies 25.91 and 20.27 define two lymphocyte cell surface antigens of sheep. 25.91 is reactive with 60-80% of lymphocytes and 98% of thymocytes, and only stains surface immunoglobulin-negative peripheral lymphocytes. 25.91 immuno-precipitates a 67,000 MW protein from lymphocyte lysates under both reducing and non-reducing conditions, whereas immunoprecipitation of thymocyte lysates reveals a 67,000, 62,000 MW complex. The tissue distribution and molecular weight analysis reported here for the antigen recognized by 25.91 indicate that this antigen is the sheep homologue of the human T1 and mouse Ly 1 antigens. The monoclonal antibody 20.27 is reactive with 80% of thymocytes and the majority of cell surface immunoglobulin-positive peripheral blood lymphocytes (B cells), but is unreactive with peripheral blood T cells. 20-27 also stains Langerhans cells in skin tissue sections and large dendritic-like cells in the paracortex sections and large dendritic-like cells in the paracortex of lymph node tissue sections. Immunoperoxidase staining of thymus tissue sections with 20.27 shows intense staining of cortical thymocytes and an absence of staining within the medulla. Molecular weight analysis of the 20.27 antigen reveals two major bands of 46,000 and 12,000 MW under both reducing and non-reducing conditions. The 20.27 antigen has properties resembling MHC class I-like antigens such as T6 in the human and TL in the mouse.

Three monoclonal antibodies, SBU.I 41-17, 41-19 and 41-28, have been produced which recognize sheep Class I major histocompatibility complex antigens (OLA). All three antibodies are able to precipitate a heavy chain of 44,000 MW and a smaller beta 2-microglobulin of 12,000 MW from 125I-surface labelled lymphocytes. The antibodies have been used to localize OLA Class I antigens in lymphoid and non-lymphoid tissues using indirect immunoperoxidase histological staining and cytofluorograph analyses. Evidence suggests that the three antibodies are directed against monomorphic determinants but that they recognize different epitopes.

Major differences in the maternal immune response to the fetus were observed in the placentomes and in the interplacentomal regions of the pregnant sheep uterus. Firstly, fewer lymphocytes were detected in the placentomes compared to the interplacentomal regions and to non-pregnant uterine tissue (Lee, Gogolin-Ewens & Brandon, 1988). Secondly, a large population of CD45R+ granulated lymphocytes was uniformly distributed in the interplacentomal uterine epithelium throughout pregnancy but never in the syncytial layer of the placentomes. Thirdly, monoclonal antibodies specific for the CD5 antigen consistently stained the endothelium of blood vessels within the placentomes but never blood vessels in the interplacentomal areas. Finally, OLA class I antigens were present on the interplacentomal uterine epithelial cells and on the maternal stromal cells, but no staining of the trophoblast or syncytium was observed. These observations suggest that different mechanisms to prevent immune rejection of the fetus may operate in the placentomes where trophoblast invasion of the maternal tissue occurs compared to the interplacentomal regions.