Fig. 5.

Effect of different genetic backgrounds on the Chk1 T cell phenotype. (A) Partial rescue of T cell numbers in Lck-Chk1^fl/fl mice induced by loss of Chk2 or expression of bcl2. Lck-Chk1^fl/fl mice were backcrossed to mice of different genetic backgrounds, including Chk2-, p53-, Brca1-, p21-null, and bcl2-tg mice. The total number of T cells in the thymi of the indicated double-mutant offspring and controls was counted by flow cytometry. ∗, P < 0.05. (B) Flow cytometric analysis of DN thymocytes of different genetic backgrounds. After depletion of CD4⁺, CD8⁺, and B220⁺ cells with Dynabeads, staining was performed with anti-CD25, anti-CD44, and anti-Lin Abs. Lin⁻ cells of the indicated double-mutant mice and controls were analyzed for CD25 and CD44 expression. (C) Apoptosis in DN thymocyte subpopulations in Lck-Chk1^fl/fl;Brca^fl/fl double-mutant mice. Thymocytes from the indicated mutant mice and controls (at least three per group) were analyzed for DN stage and stained with Annexin V. Results shown are flow cytometric determinations of the percentages of Annexin V⁺ cells in the DN2, DN3E, and DN3L subpopulations.