Beta 2-microglobulin deficient mice lack CD4-8+ cytolytic T cells.

Mice homozygous for a beta 2-microglobulin gene disruption do not express any detectable beta 2-m protein. They express little if any functional major histocompatibility complex (MHC) class I antigen on the cell surface yet are fertile and apparently healthy. They show a normal distribution of gamma delta, CD4+8+ and CD4+8- T cells, but have no mature CD4-8+ T cells and are defective in CD4-8+ T cell-mediated cytotoxicity. Our results strongly support earlier evidence that MHC class I molecules are crucial for positive selection of T cell antigen receptor alpha beta+ CD4-8+ T cells in the thymus and call into question the non-immune functions that have been ascribed to MHC class I molecules.