A. Introduction

The progressive diversification of T -lymphocytes begins within the thymus gland. Since cell-cell interactions may playa major role in this process, the study of the expression of thymic surface markers would aid our understanding of thymic differentiation. On murine thymocytes, selective expression of genes coding for cell surface markers has been studied with alloantisera [I]. More recently, monoclonal antibodies have made possible the study of selective expression of cell surface glycoproteins on human thymocytes. Thus, as in the mouse, early and late events in the thymic differentiation have been recognized [2]. Although the precise function of these thymic differentiation antigens remains to be determined, it seems plausible that they may govern associative recognition among cooperative cells sets. One could therefore surmise that inappropriate expression of some of the thymic differentiation antigens may playa role in leukemogensis. The majority of human thymocytes (75%-80% ) express the markers T4, T6, and T8, whereas only the most mature thymocytes (15%) can be decorated with the monoclonal reagent anti- T3 tagged with fluorescein-isothiocyanate [2]. T6 has never been found on peripheral blood T cells, whereas T3 is present on all T -lymphocytes. T4 and T8 are present on different subsets of T cells and on T3+ T6- thymocytes [2]. As the thymocyte is exported into the peripheral T -cell compartment, it is again involved in a variety of cell-cell interactions. Interestingly, the T -cell markers T3 and T8 appear to serve functions which are specific for interactions of the cell type which expresses them.

Here we discuss the preliminary structural analysis of T3, T6, and T8 and the possible role ofT3 and T8 in the recognition of target cells by cytotoxic T -lymphocytes. Studies involving the glycoprotein structure. biosynthesis, and membrane insertion of T3, T6, and T8 will, in our opinion, facilitate investigations that develop our understanding of the differentiative pathways of human thymus-derived lymphocytes; aid in the molecular description of T -cell functions; and make further classification of T -cell leukemias possible.