THRESHOLD TOLERANCE IN H-2K(B)-SPECIFIC TCR TRANSGENIC MICE EXPRESSING MUTANT H-2K(B) - CONVERSION OF HELPER-INDEPENDENT TO HELPER-DEPENDENT CTL

To evaluate the role of the structure of the class I molecule and associated peptide(s) in intrathymic selection and tolerance, mice expressing as a transgene (tg) a TCR specific for the H-2K(b) alloantigen were crossed with mice expressing the mutant class I molecule H-2K(bm1) or H-2K(bm8). In H-2(k/k) TCR tg mice (in a situation of exclusive positive selection), peripheral tg TCR expressing (Ti+) CD8+ T cells showed high, suboptimal, and an absence of reactivity for H-2K(b), H-2K(bm1), and H-2K(bm8), respectively. in the peripheral lymphoid organs of TCR tg H-2(k/k), H-2(k/bm8), H-2(k/bm1, and H-2(k/b) MiCe respectively, the tg TCR was expressed on T cells with decreasing intensity of surface CD8. Thymic subpopulations of TCR tg mice presented a pattern of negative selection with decreasing intensity from H-2(k/b) to H-2(k/bm1) and H-2(k/bm8). This suggests that a weak interaction between the TCR and H-2K(bm8) exists which partially results in negative, but not in positive, intrathymic selection. Results further indicate that expression of H-2K(bm8) does not induce tolerance to H-2K(b). In H-2(k/bm1) mice, the peripheral Ti+ CD8lo cells express two distinct types of 'threshold' tolerance in vitro: (i) they generate cytotoxic T lymphocytes (CTL), in the presence of exogenous IL-2, which fall to respond to H-2K(bm1) but remain reactive to H-2K(b); and (ii) they do not make significant titers of IL-2 and do not significantly proliferate in response to H-2K(b), unlike the Ti+ CD8+ T cells from H-2(k/k) TCR tg mice which respond efficiently. These results show that tolerance is induced up to a level of non-reactivity within a given MHC environment: for the same TCR, CTL reactivity to H-2K(bm1) is totally lost, whereas CTL reactivity to H-2K(b) is only slightly reduced. Additionally, proliferation and IL-2 production by Ti+ CD8+ cells in response to H-2K(b) were strongly affected in H-2(k/bm1) mice. Thus, in H-2(k/k) mice the Ti+ CD8+ cells behave as helper-independent, whereas in H-2(k/bm1) mice CD8+ Cells expressing the same TCR behave as helper-dependent CTL.