Expression of RT6 on activated rat T cells.

The gene products of the RT6 system have been demonstrated to be expressed on the majority of resting peripheral T cells but not on fully activated cytotoxic T cells and in vitro generated long term T cell lines. This has lead to the conclusion that T cell activation causes a loss of RT6 expression. In the present study this question was examined by analysing the expression of RT6 on T cell populations activated in vitro by the T cell mitogen ConA or by allogeneic stimulator cells and subsequently maintained in culture for several weeks. The results showed an initial increase in RT6 expression on most cells during the first two days of culture and a subsequent loss of RT6 expression in many activated cells. Substantial numbers of both CD4+ and CD8+ T cells, however, remained clearly RT6 positive. These cells were present during the entire observation period. The RT6 positive cells did not represent persisting unstimulated cells since they coexpressed CD25 and exhibited typical blast morphology. It is concluded that activation of T cells leads to loss of RT6 expression only in subsets of the CD4 and CD8 T cell populations. The previously reported finding that long term cultures contain only RT6 negative blast cells may indicate preferential survival of RT6 negative blasts rather than obligatory loss of RT6 expression after activation. The possibility is discussed that activated RT6-CD4+ T cells may be inflammatory Th1 cells and activated RT6+CD4+ T lymphoblasts may represent regulatory Th2 cells.