Deletion of α4 gene in hematopoietic and nonhematopoietic cells in Tie2Cre⁺α4^f/f mice. (A) Surface expression of α4 integrin on circulating WBCs in Tie2Cre⁺α4^f/f (open histogram, thick line) and control (filled histogram) mice. FACS analysis with PS/2, anti-α4 antibody showed that in Tie2Cre⁺α4^f/f mice, 3.1% ± 0.2% (n = 20) of circulating WBCs and 2.6% ± 0.6% (n = 4) of BM cells (not shown) are α4⁺. Isotype-matched immunoglobulin (open histogram, dashed line) served as a negative control. (B) Deletion of α4 integrin in endothelial cells (E), fibroblasts (F), and hematopoietic BM cells (H) at the genomic level (left panel) and at the mRNA level (right panel). Note only partial deletion of α4 gene in fibroblasts. (C) Deletion of α4 integrin results in increase in WBCs and in circulating hematopoietic progenitors. Circulating CFU-C levels in Tie2Cre⁺a4^f/f mice (2446 ± 256, n = 17) are significantly increased compared with control levels (338 ± 113, n = 9, P < .01). Circulating WBC and CFU-C levels in Tie2Cre⁺VCAM-1^f/f mice are shown for comparison. The asterisk indicates significant difference over controls, P < .01; ‡, significant difference over Tie2Cre⁺VCAM-1^f/f mice, P < .05. Error bars indicate standard error of the mean (SEM).