Surface expression of activated α2 integrins is down-regulated in leukocytes during hematopoiesis in vivo. (A) Expression of surface α2 integrins and GFP in white blood cells from lethally irradiated mice reconstituted with hematopoietic stem cells lacking both α2 and GPVI that were exposed to empty vector (vector) or vectors expressing the indicated α2 integrins. (B) Quantitation of leukocyte surface α2 integrin levels. Mean fluorescence intensity (MFI) was measured in the indicated cells stained with anti-α2 antibody. N = 3-5; mean and SD are shown. *P < .05, **P < .01 compared with vector control. (C) Expression of surface α2 in GFP⁺ CD41⁻ bone marrow cells harvested from lethally irradiated mice reconstituted with α2-deficient hematopoietic cells exposed to empty vector (vector) or vector expressing wild-type α2 integrins (α2WT), α2E318A integrins, and α2GFFKRΔ integrins is shown. Top panels show expression immediately after harvest from the bone marrow, middle panels show expression after 72 hours of ex vivo culture in the absence of collagen, and bottom panels show expression after 72 hours of ex vivo culture on collagen-coated plates. (D) Quantitation of α2 surface expression in the GFP⁺ CD41⁻ bone marrow cells described in panel A. N = 5; mean and SD are shown.