Role of Thymus T Cell Differentiation

Abstract

The first part of the Workshop was addressed to a number of questions concerning the role of the thymus in the generation of the T cell repertoire. In particular, the way in which the thymic environment might influence tolerance and MHC restriction of emerging T cell populations was discussed and evaluated. A first question — to what extent does contact of differentiating thymic T cells with MHC antigens expressed on thymic stromal cells determine restriction? — was taken up by Golding and Singer (NIH, U.S.A.). They indicated that whereas Ia-restricted T helper cells are restricted by the thymic Ia haplotype, class I restricted cytotoxic T cells are not necessarily restricted by the thymic haplotype. They argued that this discrepancy is due to an inherent difference between the ontogeny of T helper and T cytotoxic precursors. In the discussion that followed a number of points emerged — (i) it is still unclear whether class II MHC antigens on thymic epithelial cells or on thymic dendritic (interdigitating) cells might direct restriction of T helper cells (ii) difficulties in resolving the roles of different thymic stromal types in restriction are due to uncertainties concerning the regenerative capacity of dendritic cell populations following thymic transplantation or irradiation. Thus, in instances where thymic epithelial cells have been implicated, the restriction patterns seen might be due to persisting dendritic cells and their precursors.