Results

Generation of Mice without CD8 Surface Expression CD8 in the mouse is encoded by two genes, Lyt-2 and Lyt-3 (Zamoyska et al., 1985; Nakauchi et al., 1987), and is expressed on the T cell surface predominantly as a disulfide-linked heterodimer of Lyt-2 and Lyt3 (Ledbetter et al., 1981; Walker et al., 1984a, 1984b). Lyt-2 can also be expressed on the cell surface as a homodimer in transfected cells (Zamoyska et al., 1985; Gorman et al., 1968), but the surface expression of Lyt-3, at least in T cell lines, appears to require the presence of the Lyt-2 molecule (Blanc et al., 1988; Gorman et al., 1988; Schmidt-Ullrich and Eichmann, 1990). Therefore, the Lyt-2 gene was disrupted to generate a mutant mouse strain without cell surface expression of CD8. The Lyt-2 gene contains five exons spanning a region of 4.4 kb (Liaw et al., 1988). To disrupt the Lyt-2 gene by homologous recombination, a 2.2 kb Hindlll-BamHI DNA fragment containing exon 1, 2, and 3 of the Lyt-2 gene was used as a construct and the coding sequence was interrupted by inserting the neomycin resistance gene (Thomas and Capecchi, 1987) into the EcoRl site of exon 1 (Figure 1). This DNA construct was introduced into D3 embryonic stem cells (Doetschman et al., 1985) by electroporation. G418-resistant colonies were screened for the homologous recombination event between the construct, and the endogenous Lyt-2 gene by