Integrin α₄β₁ regulates CD34⁺ and CD34^– cell homing to tumor vasculature. (A) Cryosections of N202 breast carcinomas from mice injected with CD34⁺ cells (red, arrowheads) and with either saline or anti-human α₄β₁ antibodies. Blood vessels were identified by CD31 staining (green, arrows) and CD34⁺ cells by CMTMR labeling (red). (B) Average number of CD34⁺ cells ± SEM per ×200 field. *P < 0.0007. (C) Cryosections of LLC lung carcinomas from mice injected with CD34⁺ cells (red, arrowheads) and with saline, anti-human α₄β₁ antibodies, or isotype-matched control anti-integrin antibody (cIgG, anti-α_vβ₃). Blood vessels were identified by CD31 staining (green, arrows). (D) Average number of CD34⁺ cells ± SEM per ×200 field. **P < 0.0006. (E) Video microscopy of tumors in animals injected with CMTMR-labeled CD34^– human cells in the presence of saline, anti-α₄β₁, or isotype-matched control anti-integrin antibody (anti-α_vβ₃). (F) Average number of CD34^– cells ± SEM per ×200 field. ^†P < 0.011. Scale bars: 50 μm.