CHARACTERIZATION OF CD4(-BG MICE()CD8(+) THYMOCYTES OBSERVED IN SCID)

Previously, we reported that a strain of severe combined immunodeficie nt (SCID) mice, namely SCID-bg, spontaneously develop CD4(+)CD8(+) dou ble positive (DP) thymocytes despite their scid mutation. In the prese nt study, we intend to further characterize the DP thymocyte populatio n in SCID-bg mice with Southern hybridization and flow cytometry. Sout hern hybridization analyses of sorted DP thymocytes in SCID-bg mice sh owed rearrangement of T cell receptor (TCR) beta and TCR gamma gene se gments, although the expression of TCR beta and gamma delta TCR molecu les was absent. The phenotype of the DP thymocytes in SCID-bg mice was CD44(-) heat-stable antigen (HSA)(+)CD25(-), and they did not express CD69. Interestingly, the expression of thymus leukaemia (TL) antigen was observed in the DP thymocytes of SCID-bg mice but not in their CD4 (-)CD8(-) double negative (DN) fraction. Fas expression was higher and bcl-2 expression was downregulated in the DP thymocytes as compared t o the DN thymocytes in SCID-bg mice, suggesting that they an not immor talized cells having escaped from apoptosis. Taken together, these res ults show that the phenotype of the DP thymocytes in SCID-bg mice is s imilar to that of the earliest phase of the DP thymocytes in normal mi ce, although the expression of the molecules in the pre-TCR complex is absent.