Integrin activation by divalent ions affects neutrophil homotypic adhesion.

Activation of beta 2 integrins on leukocytes, via receptor clustering or affinity changes, is a key step in the inflammatory response. To evaluate the importance of integrin affinity changes for the formation of adhesive contacts, divalent cations were used to modulate beta 2-integrin affinity without causing a general activation of neutrophils. Two neutrophils were brought into repeated contact with micropipettes and the probability of forming an adhesive contact was measured for neutrophils exposed to different cations. In the presence of magnesium and manganese, which induce a high affinity integrin conformation, the adhesion frequency was 12%-14%. In the presence of calcium (which stabilizes the low-affinity state) the frequency of adhesion was reduced (2.4 +/- 2.1%) and in the absence of divalent cations there were no adhesive contacts. The adhesive probability was not affected by addition of soluble VCAM-1, but was significantly reduced to 7 +/- 3% in the presence of magnesium plus a soluble form of ICAM-1. Addition of a blocking anti-beta 2-integrin Fab fragment also significantly reduced the probability of forming an adhesive contact to 4 +/- 2%. Thus increased beta 2-integrin affinity promotes homotypic neutrophil adhesion during cell collisions, but the adhesion strength is weak compared to forces observed in cell arrest and migration.