OCYTE CELL SURFACE ANTIGENS T h e A - 1 A 5 and A - 3 A 4 Determinants

Human < rodent interspecies hybrids express cell surface molecules derived from both parent species and thus have been useful in genetic analysis of the human cell surface (1, 2). The expressed human antigens serve as genetic markers that are powerful tools in both the analysis and manipulation of the human partial genome present in hybrid cells (2-5), and have recently allowed the development of a new technique for genetic mapping of cell surface antigens that combines somatic cell genetics and fluorescence-activated cell sorting (6, 7). However, the mapping of differentiation-specific surface markers has been hampered by difficulty in the construction of interspecies hybrids that retain the differentiated phenotype. The monoclonal antibody A-1A5 recognizes a cell surface determinant expressed on all tested lymphoid cell lines. It is present at high levels on activated T cells but is only weakly expressed on resting lymphocytes. Nonactivated lymphocytes express a single A-1AS-specified component of 130,000 Mr as determined by immunoprecipitation. Several days after activation with mitogen or alloantigen, however, two additional subunit components of 165,000 and 210,000 Mr are coprecipitated by A-1A5 along with the 130,000 Mr protein (8). The appearance of these higher Mr subunits upon T cell activation occurs after the expression of early activation antigens, such as 4F2, the transferrin receptor, and the interleukin 2 (IL-2) 1 receptor (9), and thus represent late activation antigens. The component of highest molecular weight (210,000 Mr) appears to be recognized by the monoclonal antibody TS2/7, and thus is immunochemically distinct from the 130,000 Mr protein recognized by A-1A5 (10). Further evidence is required to demonstrate that the component of intermediate molecular weight (165,000 Mr) does not represent a modified form of the 130,000 Mr protein. We have recently described (11) the development of a new class of interspecies