Ligand density determines the efficiency of negative selection in the thymus

To study the influence of antigen density on the efficiency of negative sel ection in the thymus, MHC class I (H-2K(b), K-b) transgenic mice were gener ated, which expressed a K-b transgene under the control of its natural prom oter at 33% (Kb-lo) or 150% (Kb-hi) the surface density of K-b in C57BL/6 ( B6, H-2(b)) mice. These mice were crossed to anti-Kb T-cell receptor (Des-T CR) transgenic mice. In Des-TCRxK(b-hi) double transgenic mice, Des-TCR bea ring T cells were completely eliminated during thymocyte maturation. In con trast, in Des-TCRxK(b-lo) double transgenic mice, two populations of Des-TC R T cells were evident, which either expressed the Des-TCR at intermediate density in the absence of CD8 (Des-TCR(int)CD8(-)) or expressed both the De s-TCR and CD8 at low density (Des-TCR(lo)CD8(lo)). In the thymus of both ty pes of double transgenic mice, no Des-TCR(+)CD4(+)CD8(+) thymocytes were de tected, suggesting that deletion of Des-TCR cells occurred before the CD4()CD8(+) stage. Because only very few Des-TCR+ thymocytes were found in Des- TCRxK(b-hi) transgenic mice, deletion of these T cells apparently occurred upon expression of the Des-TCR. By contrast, Des-TCRxK(b-lo) transgenic mic e showed distinct populations of Des-TCR(int)CD4(-)8(-) and Des-TCR(int)CD8 (lo) thymocytes, suggesting that expression of the CD8 coreceptor was requi red to allow negative selection to proceed. Functional analyses showed that sublethally irradiated Des-TCRxK(b-lo) double transgenic mice were protect ed from lethal graft-versus-host disease by injected Des-TCR lymph node cel ls.