INFLUENCE OF ANTIGEN DENSITY ON DEGREE OF CLONAL DELETION IN T-CELL RECEPTOR TRANSGENIC MICE

The extent of peripheral clonal deletion of the T cell antigen receptor (TCR) from a CD8-dependent cytotoxic T lymphocyte of H-2k origin with alloreactivity for H-2K(b) Was measured in a TCR transgenic (Tg) model by use of a clonotype-specific mAb (anti-Ti mAb). Deletion varied depending on whether the TCR Tg was expressed in mice heterozygous (H-2kxb) or homozygous (H-2bxb) for the H-2K(b) antigen. CD8 surface staining and functional analyses of peripheral T cells stimulated with polyclonal activators in bulk culture or by limiting dilution confirmed that in the H-2bxb mice few Ti+ cells were generated as measured by anti-Ti mAb-dependent target cell killing; while such cells could readily be detected in the H-2kxb mice. The latter maintained non-responsiveness to H-2K(b), as measured by cytolysis of H-2K(b)-expressing tumor target cells, due to their down-regulation of surface CD8 expression. The question of whether the difference between H-2bxb and H-2kxb mice was due to the influence of positive selection imposed by the k haplotype in the H-2kxb hybrid, or to the lower antigen density in heterozygous than homozygous mice was addressed by analysis of H-2bxd Tg mice, H-2d being a non-selecting haplotype. Results obtained were similar for H-2bxd and H-2kxb Tg mice, suggesting that the density of the H-2K(b) antigen may be one of the parameters controlling the extent of clonal deletion in the thymus.