Distribution of Immunocompetent Cells in Equine Lymphoid Tissues

Abstract

Distribution of immunocompetent cells in lymphoid tissues of healthy nulliparous horses were revealed by the double-staining indirect immunofluorescence antibody technique with monoclonal antibodies. In the thymus, immature T cells (CD4⁺ 8⁺ T cells) aggregated in the cortex, while a few mature T cells (CD4⁺ 8^- or CD4^- 8⁺ T cells) were scattered in the medulla. MHC class II⁺ cells aggregated in the cortico medullary region were considered to be macrophages and dendritic cells, while the MHC class II⁺ cells in the cortex were thought to be macrophages, dendritic cells or cortical epithelial cells. The existence of T cell-dependent areas in the spleen, lymph nodes and ileum was confirmed by the presence of CD5⁺ T cells. The number of helper T cells (CD4⁺ 5⁺ T cells) was slightly greater than that of killer T cells (CD5⁺ 8⁺ T cells) in these areas. Within the follicles of the spleen, lymph nodes and ileum, aggregated MHC class II⁺ cells were considered to be B cells and dendritic cells. Almost all the IgM⁺ MHC class II⁺ B cells aggregated within the center of the follicles, whereas IgM^- MHC class II⁺ B cells were found in the marginal zone of the follicles. These findings suggest that activated B cells are contained in the center of the follicle and inactivated B cells in the marginal zone of the follicle and that the follicles are adjoined to T cell-dependent areas.