Ontogeny, morphology and tissue distribution of a unique subset of CD4-CD8- sheep T lymphocytes

The morphology, distribution and ontogeny of the 197+ T lymphocyte subpopulation is described. Cells were identified by immunofluorescent and immunoenzymic staining of cell suspensions and tissue sections. In the circulation, 197+ cells are small and regular, indistinguishable in size from peripheral CD4+ and CD8+ cells, with little cytoplasm and a large amount of condensed chromatin. In the post-natal animal they are unevenly distributed, being most common in the circulatory pathways, particularly in the blood. In the thymus they chiefly localize in the regions around Hassal's corpuscles and medullary blood vessels. They are totally absent from B cell areas in all tissues. They are the last of the T cells to appear in ontogeny, and are first detected in the thymus. Their frequency in blood is age-dependent with peak levels occurring perinatally. There is a post-natal decline in the frequency of 197+ cells in ileocaecal but not in prescapular lymph nodes. We conclude that these T cells differ from the more commonly described T cells, not only in their surface expression of the CD4, CD8 and T19 antigens, but also in the time of their first appearance, their age-related prevalence and their distribution between and within tissues.