Direct progenitors of basophils express Ik and are increased in Ik^−/−mice. Bone marrow and spleen leukocytes were isolated from Ik^+/+ (^+/+) and Ik^−/− (^−/−) mice. (A) Total leukocytes in bone marrow (bilateral femoral/tibial) and spleen of ^+/+ and ^−/− animals. Mean ± standard error of the mean (SEM) (n = 10 for ^+/+, n = 11 for ^−/−). (B) Representative plots of Lin⁻ CD34⁺ FceRIa⁺ c-kit⁻ BaPs and Lin⁻ Sca-1⁻ c-kit⁺ CD27⁻ ST2L⁺ β7 integrin⁺ mast cell progenitors in bone marrow and the spleen and Lin⁻ c-kit⁺ CD16/32⁺ β7 integrin^hi BMCP in the spleen of Ik^+/+ and Ik^−/− mice. Numbers represent percentage of parent gate indicated below plots. (C) Quantification of frequencies of indicated cell types as a percentage of total SYTOX⁻ (live) cells in flow sample. Bone marrow and spleen BaP (n = 5 for ^+/+ and ^−/−); bone marrow and spleen MCp (n = 5 for ^+/+ and n = 6 for ^−/−); BMCP (n = 10 for ^+/+ and 11 for ^−/−). (D) Estimated mean absolute cell numbers per organ and 95% confidence interval of mean for indicated cell types, derived from means and SEM of frequency and total cell data in panels A,C. Independence was assumed and SEM were combined by the formula: ⁠. (E) Expression of Ikzf1 (all Ik isoforms) in indicated cell populations assessed by quantitative RT-PCR. CD4⁺ T cells were isolated from naive mice. In vitro bone marrow (BM)-derived basophil cells are isolated from 9-day cultures and mast cells, from 6-week cultures with IL-3 alone. GMPs, BaPs were sorted from bone marrow while purified BMCPs were isolated from spleen. Ex vivo–elicited basophils were derived from the bone marrow and livers of mice after treatment with IL-3 complexes (n = 1 for ex vivo IL-3–elicited complex basophils; n = 3 for other samples). *P < .05; **P < .01.