Figure 6.

Bond formation rates and apparent off-rate of L-selectin or P-selectin microbeads interacting with PSGL-1. L-selectin- (A and C) or P-selectin- (B and D) coated microbeads were flowed over 1 ng/ml (circles), 10 ng/ml (squares), or 100 ng/ml (diamonds) PSGL-1 over a range of wall shear stresses. Using statistically determined definitions, bond formation rates were measured on a per time (A and B) or per distance (C and D) scale. Note that the y axes are different for C and D. The bond formation rates increased for L-selectin and decreased for P-selectin over the range of wall shear stresses tested. The inset in D shows bond formation rates for P-selectin on 1 ng/ml PSGL-1 using the same scale as C. At 1 and 10 ng/ml, P-selectin had the higher bond formation rate up to 0.7 dyn/cm² wall shear stress. Above 0.7 dyn/cm², L-selectin had the higher rate. Apparent off-rates were measured (E and F) and, at each PSGL-1 site density, the apparent off-rate for L-selectin was 2–10-fold higher than P-selectin. Catch bond behavior was recorded for L-selectin at all PSGL-1 site densities, but not for P-selectin.