Affinity, lateral mobility, and clustering contribute independently to β₂-integrin-mediated adhesion

Abstract

Affinity changes and avidity modulation both contribute to activation of beta(2)-integrin-mediated adhesion, an essential, early step in inflammation. Avidity modulation, defined as an increase in adhesiveness independent of integrin conformational changes, might be due to integrin clustering, motion, or both. Increased integrin diffusion upon leukocyte activation has been demonstrated, but whether it is proadhesive in itself, or just constitutes a mechanism for integrin clustering, remains unclear. To understand the proadhesive effects of integrin affinity changes, clustering, and motion, an experimental system was devised to separate them. Clustering and integrin motion together were induced by cytochalasin D (CD) without inducing high-affinity; integrin motion could then be frozen by fixation; and high affinity was induced independently by Mn(2+). Adhesion was equivalent for fixed and unfixed cells except following pretreatment with CD or Mn(2+), which increased adhesion for both. However, fixed cells were less adhesive than unfixed cells after CD, even though integrin clustering was similar. A simple explanation is that CD induces both clustering and integrin motion, fixation then stops motion on fixed cells, but integrins continue to diffuse on unfixed cells, increasing the kinetics of integrin/ICAM-1 interactions to enhance adhesion. Affinity changes are then independent of, and additive to, avidity effects.

SDF-1α (CXCL12) regulation of lateral mobility contributes to activation of LFA-1 adhesion

“…This is thought to be because increased lateral mobility facilitates encounters between the integrin and its ligands, consistent with a diffusion-limited adhesion theory (1). In addition, release of LFA-1 from cytoskeletal constraints on its movements, even by disruption of integrin-cytoskeleton connections by cytochalasin D (22) or by truncation of the ␤ 2 -cytoplasmic domain (20), also leads to LFA-1 clustering. Theory predicts that clustering alone can lead to increased avidity, due to cooperative effects on resistance to bond breakage (21), and this has been confirmed by atomic force microscopy (4).…”

“…Adhesion was to a mixed substrate of P-selectin (to mediate capture and rolling adhesion) and ICAM-1 (a ligand for LFA-1 that mediates firm adhesion). Firm adhesion of both native and transfected LFA-1 was increased by low-dose cytochalasin D, which activates adhesion by integrin rearrangement without conformational changes (22), and by SDF-1␣, a physiological activator of LFA-1.…”

“…Theory predicts that clustering alone can lead to increased avidity, due to cooperative effects on resistance to bond breakage (21), and this has been confirmed by atomic force microscopy (4). Recently, it was shown that increased binding kinetics due to enhanced lateral mobility, clustering, and affinity adjustment can each increase cell adhesion independently (22). Since LFA-1 activation includes components of both affinity and avidity modulation, any comprehensive model requires an understanding of both.…”

Chemerin Contributes to Inflammation by Promoting Macrophage Adhesion to VCAM-1 and Fibronectin through Clustering of VLA-4 and VLA-5

“…For example, CXCL12 promotes the rapid clustering of VLA-4 on lymphocytes to achieve adhesion to VCAM-1 while having no effect on VLA-4 affinity (35). Integrin clustering alone is sufficient to support cell adhesion under the flow conditions that model the environment found within blood vessels (54).…”

Intracellular Modelling of Cell-Matrix Adhesion during Cancer Cell Invasion

“…They observed that integrin clusterings in the periphery of the cell occur within minutes, starts about after 3 minutes. There are also many recent studies on integrin clustering, such as by [19,48,39,52,55,2,25,45].…”