Sequential binding of CD11a/CD18 and CD11b/CD18 defines neutrophil capture and stable adhesion to intercellular adhesion molecule–1

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Abstract
The relative contributions of CD11a/CD18 and CD11b/CD18 to the dynamics and strength of neutrophil adhesion to intercellular adhesion molecule (ICAM)-1–transfected cells were examined over the time course of chemotactic stimulation. Suspensions of neutrophils and transfectants were sheared in a cone-plate viscometer, and formation of heterotypic aggregates was measured by 2-color flow cytometry. The 2-body collision theory was used to compute adhesion efficiency, defined as the proportion of collisions between neutrophils and target cells that resulted in capture. ICAM-1 surface density and shear rate both regulated adhesion efficiency. Target cells expressing approximately 1000 ICAM-1 sites/μm2 (Ilow) were captured with an efficiency of 0.15 at 100 s−1, which decreased to zero at 300 s−1. At 8-fold higher ICAM-1 expression (Ihigh) corresponding to levels measured on interleukin-1–stimulated endothelium, efficiency was 0.3 at 100 s−1 and remained above background to 900 s−1. Shear alone was sufficient for CD11a/CD18-mediated adhesion to ICAM-1, and stimulation with formyl-methionyl-leucyl-phenylalanine boosted capture efficiency through CD11a/CD18 by 4-fold. In comparison, CD11b/CD18 supported one third of this efficiency, but was necessary for aggregate stability over several minutes of shear and at shear stresses exceeding 5 dyne/cm2. Hydrodynamics influenced capture efficiency predominantly through the collisional contact duration, predicted to be approximately 9 milliseconds for successful capture of Ilow and 4 milliseconds for Ihigh. The implication is that an increase in ICAM-1 from resting levels to those on inflamed endothelium effectively increases the permissible shear in which capture through β2-integrins may occur. Neutrophil adhesion to ICAM-1 appears to be a cooperative and sequential process of CD11a-dependent capture followed by CD11b-mediated stabilization.

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