Accumulation of IL-2R–expressing immature myeloid cells in the bone marrow of IL-2^−/− mice. Bone marrow cells were isolated from 5-week old wild-type (+/+) and IL-2^−/− (−/−) littermates and stained with Mac-1 and IL-2Rβ or isotype-matched control antibodies before analysis using flow cytometry as described in the legend to Fig 3. (A) IL-2Rβ–expressing myeloid cells in the bone marrow of 6-week old C57BL.6 mice. The boxed region represents the frequency of IL-2Rβ⁺Mac-1^lo cells. Isotype-matched control antibodies (Ctrl, right panel) were used to determine the level of nonspecific staining (boxed region). The results shown are representative of five independent experiments. (B) C57BL.6 (+/+) and IL-2^−/− (−/−) bone marrow cells expressing low levels of Mac-1 (Mac-1^lo) were reanalyzed for expression of IL-2Rβ; the % values represent the frequency of Mac-1^locells that are IL-2Rβ⁺. Note the accumulation of IL-2Rβ–expressing Mac-1^lo cells in IL-2^−/− mice. The results shown are representative of six groups of mice.