CD8 INHIBITS SIGNAL-TRANSDUCTION THROUGH THE T-CELL RECEPTOR IN CD4-CD8- THYMOCYTES FROM T-CELL RECEPTOR TRANSGENIC MICE RECONSTITUTED WITHA TRANSGENIC CD8-ALPHA MOLECULE

T cell repertoire selection processes involve intracellular signaling events generated through the TCR. The CD4 and CD8 coreceptor molecules can act as positive regulators of TCR signal transduction during thes e developmental processes. In this report, we have used TCR transgenic mice to determine whether TCR signaling can be modulated by the CD8 c oreceptor molecule. These mice express on the majority of their T cell s a TCR specific for the male (H-Y) Ag presented by the H-2D(b) MHC cl ass I molecule. We show that CD4-CD8-, but not CD4-CD8+, thymocytes ex pressing the H-Y TCR responded with high intracellular calcium fluxes to TCR/CD3 stimulation without extensive receptor cross-linking. To ex amine the effects of CD8 expression on intracellular signaling respons es in the CD4-CD8- cells, the H-Y TCR transgenic mice were mated with transgenic mice that constitutively expressed the CD8alpha molecule on all T cells. The expression of the CD8alphaalpha homodimer in the CD4 -CD8- thymocytes led to impaired intracellular calcium responses and l ess efficient protein tyrosine phosphorylation of substrates after TCR engagement. In male H-2b H-Y transgenic mice, the majority of thymocy tes have been deleted with the surviving cells expressing a high densi ty of the transgenic TCR and exhibiting either a CD4-CD8- or CD4-CD8lo phenotype. It has been postulated that these cells escaped deletion b y down-regulating the CD8 molecule. In the H-Y TCR/CD8alpha double tra nsgenic male mice, the CD4-CD8lo cells were completely eliminated as a result of CD8alpha expression. However, the CD4-CD8- T cells were not deleted despite normal levels of the CD8alpha transgene expression. T hese results suggest that the CD4-CD8- thymocytes may not be susceptib le to the same deletional mechanisms as other thymocytes expressing TC R-alphabeta.