SURVIVAL OF MATURE CD4 T-LYMPHOCYTES IS DEPENDENT ON MAJOR HISTOCOMPATIBILITY COMPLEX CLASS II-EXPRESSING DENDRITIC CELLS

Thymic T cell development is controlled by T cell receptor (TCR)-major histocompatibility complex (MHC) interactions, whereas a further depe ndence of peripheral mature T cells on TCR-MHC contact has not been de scribed so far. To study this question, CD4 T cell survival was survey ed in mice lacking MHC class II expression and in mice expressing MHC class II exclusively on dendritic cells. Since neither of these mice p ositively select CD4 T cells in the thymus, they were grafted with MHC class II-positive embryonic thymic tissue, which had been depleted of bone marrow derived cells. Although the thymus grafts in both hosts w ere repopulated with host origin thymocytes of identical phenotype and numbers, an accumulation of CD4(+) T cells in peripheral lymphoid org ans could only be observed in mice expressing MHC class II on dendriti c cells, but not in mice that were completely MHC class II deficient. As assessed by histology, the accumulating peripheral CD4 T cells were found to be in close contact with MHC class II+ dendritic cells, sugg esting that CD4 T cells need peripheral MHC class II expression for su rvival and that class II+ dendritic cells might play an important role for the longevity of CD4 T cells.