Apoptosis assay in spleen tissue. (A-C) Histopathologic sections of spleen in the same experiments as those described in Figure 2 were stained with TUNEL kit as described in “Materials and methods” (A, original magnification × 40/0.75 PH2 objective lens). Splenocytes were isolated from mice ii and iv, and annexin V⁺ cells were detected on flow cytometry (B). Splenocytes were isolated from all mice and PARP proteins were detected by Western blots (C). MW indicates molecular weight. (D) Leukemic mice, which were fed in normal oxygen for 24 days after injection of leukemic cells, were housed into intermittent hypoxia for 1 to 3 days, and the distribution of cell cycle–related nuclear DNA contents of splenocytes were analyzed in flow cytometry (left) and the percentages of G₁-, S-, and G₂/M-phase cells were shown (right). Symbols i to v represent mice with the same treatment as in Figure 2. In addition, BM cells were also analyzed in the same experiments, and similar results were obtained (data not shown).