Spleen cells of phenotypically TL+ mice do not contain intracellular TL antigens

Abstract

The TL (thymus leukemia) antigens are cell surface elements whose expression is normally limited to thymocytes of certain mice, those with the TL § phenotype (Boyse and Old 1969, Boyse et al. 1965). TL antigens may also occur on leukemia cells. Although it is largely the external representation of TL antigens that has been studied in the past, these antigens are not limited to the cell surface (Smith et al. 1974). TL antigens are also detectable in microsomal and mitochondrial fractions obtained from TL + normal thymocytes and TL + leukemia cells. In quantitative terms the mitochondrial and plasma membrane fractions of thymocytes contained similar amounts of TL antigens, and the microsomal fraction somewhat less. Whereas normal adult spleen and bone marrow cells do not bear detectable TL antigens, a few percent of the cells from these organs can be induced, in vitro, to express them (and also some other markers characteristic of thymocytes) by extracts of thymus (Komuro and Boyse 1973) or by thymopoietin, a purified thymic polypeptide (Basch and Goldstein 1974). These observations raise the question as to whether TL antigens are normally expressed internally in bone marrow and spleen cells and whether the induced differentiation might therefore involve only an additional expression at the cell surface. Accordingly, in the present experiments mitochondria and microsomes from spleen cells of phenotypically TL + mice were examined for the presence of TL antigens. A/J mice, 10 weeks old and of both sexes, were obtained from the Jackson Laboratories (Bar Harbor, Maine). The methods used to prepare subcellular fractions and assay the activity of marker enzymes have been described (Smith et al. 1974). In addition the mitochondrial enzyme succinic dehydrogenase was also measured (Green et al. 1955). TL antigens in various cellular compartments were quantified by inhibiting the cytotoxic release of s lCr (Smith et al. 1974, Wigzell 1965). A-strain thymo-