On the role of MHC class II molecules in the survival and lymphopenia-induced proliferation of peripheral CD4+ T cells.

CD4(+) T cells expand after transfer into lymphopenic H-2(b) A(beta)-/- mice (I-A(beta)-, I-E(alpha)-deficient mice) but not after transfer into lymphopenic MHC II(Delta/Delta) mice (I-A(alpha)-, I-A(beta)-, I-E(alpha)-, and I-E(beta)-deficient mice), implying that in H-2(b) A(beta)-/- mice, A(alpha) chain and E(beta) chain associate to form a hybrid A(alpha)E(beta) MHC class II molecule. In light of this unexpected result, we reexamined the MHC class II requirement in the survival and lymphopenia-induced proliferation of CD4(+) T cells. Here we show that expansion, but not short-term survival, of CD4(+) T cells depends on interactions with MHC class II molecules in lymphopenic mice. Nevertheless, interactions with classical MHC class II molecules are required for CD4(+) T cells to survive in CD8(+) T-cell-containing mice.