Defective triggering of VLA-4 avidity to VCAM-1 by chemokine in LAD PBLs. (A) SDF-1–augmented VLA-4–mediated capture and arrest of control and patient (LAD) PBLs to purified VCAM-1 under shear flow. (i) Frequency of PBL tethers to purified sVCAM-1 (2 μg/mL) with functional (+) or heat-inactivated (-) SDF-1 (2 μg/mL). The different tether categories were determined in 2 fields, and results are an average and range of each tether category. Mean duration of transient tethers is shown on top of bars. (ii) FACS staining of α4 integrins using the α4 subunit–specific mAb HP1/2. Background staining is indicated by the thin black lines. This mAb fully blocked all PBL adhesion to the different VCAM-1/SDF-1 substrates characterized in panels A-B (not shown). (B) SDF-1–triggered VLA-4–mediated adhesion of PBLs to VCAM-1 at rapid stationary contacts. Resistance to detachment by incremented shear forces developed by control or LAD patient PBLs settled for 1 minute at stasis on sVCAM-1 (2 μg/mL) coimmobilized with functional (+) or heat-inactivated (-) SDF-1 (2 μg/mL). Results are given as mean ± range of determinations in 2 fields of view. A representative experiment of 3 is shown.