Figure 4.

The pattern of MM14.4 T cell selection and lineage commitment reflects their dual MHC class I and MHC class II recognition. (A) CD4⁺ and CD8⁺ cells were positively selected in MM14.4.690 transgenics bred to Rag1-deficient background. The MHC and Ii genotypes are shown, as well as total cellularity [× 10⁻⁶] of the thymus and peripheral lymph nodes. Selection into CD4⁺ lineage was dependent on A^bEp expression, while selection into CD8⁺ lineage was not. Expression of variable A^b/peptide repertoire in C2⁺Ii⁺ mice (A^b-positive, Ii-positive), A^bEp⁺Ii⁺ mice, and C2⁺Ii° (A^b-positive, Ii-negative) mice led to negative selection of MM14.4⁺ T cells. (B) Reactivity of the lymph node cells from Rag1° MM14.4.690 transgenic mice with stimulators of variable MHC background confirms the pattern of positive and negative selection of the MM14.4 TCR. Mice used as donors of responding cells (FACS analysis is shown in A) were A^bEp mice (A^b-negative, A^bEp-positive, Ii-negative), C2° mice (mice with no MHC class II), or C2⁺Ii⁺ mice (A^b-positive, Ii-positive). Proliferation of 3 × 10⁵ responders mixed with 5 × 10⁵ of B6 (A^b-positive, left) or B10.BR (H-2K^k- positive, right) irradiated splenocytes. (C) Selection of CD8⁺ MM14.4⁺ T cells required expression of MHC class I molecules. MM14.4 mice lacking MHC class I because of genetic deletion of the β2-microglobulin had very few CD8 single-positive cells in the thymus and in the periphery independently of A^bEp expression (top and middle), as compared with MHC class I–sufficient, MHC class II–negative mice (bottom). Numbers in brackets show absolute numbers of CD8⁺,Vα2⁺ cells (×10⁻⁶) in the thymi and lymph nodes. Coreceptor FACS dot-plots are shown for cells gated on Vα2-positive cells.