Phenotypic Expression of T Lymphocytes in Thymus and Peripheral Lymphoid Tissues

The pattern of expression of cell surface antigens and their relationship to the sequence of T-cell differentiation were delineated by the application of a series of monoclonal antibodies against T cells on tissue sections. The morphologic features and location of T-cell differentiation can be categorized into four stages: 1) subcapsular thymocytes, 2) cortical thymocytes, 3) medullary thymocytes, and 4) peripheral T cells. Phenotypically, on the basis of reactivity with monoclonal antibodies, T cells could be separated into two groups: immature and mature T cells. All stages of T cells expressed T200, Lyt 3, Leu 1, OKT3, Leu 9, and TA1, although staining intensities varied between thymic cortex and medulla. Mature T cells (medullary thymocytes and peripheral T cells) stained more intensely than immature T cells for some antigens, such as Leu 1, OKT3, and Leu 9, and, therefore, probably have a greater antigen density. Immature T cells, exemplified by subcapsular and cortical thymocytes, were characterized by the expression of Tdt, Leu M3, OKT6, OKT9, J5, BA-2, OKT10, and usually coexpressed both helper (T4/Leu 3a) and suppressor (T8/Leu 2a) antigens. With further maturation, medullary thymocytes and peripheral T cells lost reactivity for the above-mentioned markers, acquired A1G3 and Leu 8, and segregated into either T4+/Leu 3a+ or T8+/Leu 2a+ cells. A subpopulation of T cells in germinal centers differs from the majority of peripheral T cells by virtue of an absence of expression of Leu 8/A1G3. This histologically localized subset of T cells may be responsible for the mediation of helper function within the germinal center.