Dissociation between increased surface expression of gp165/95 and homotypic neutrophil aggregation

Whether homotypic neutrophil aggregation depends on the quantitative increase of gp165/95 molecules (Mac 1, CR3) recruited to the cell surface during activation was studied using mAb of the CD11b group that recognize distinct epitopes encoded by the alpha-subunit of this glycoprotein. After the addition of antibody MN41, neutrophils did not aggregate in response to a chemoattractant, FMLP. Blockade of preexisting surface gp165/95 by mAb MN41, followed by removal of the excess antibody from the mixture, was used to show that the molecules of gp165/95 newly expressed in response to stimulation by a chemoattractant were incapable of effectively mediating the induced cell-cell interactions of aggregation. Flow cytometry studies confirmed that binding of unlabeled antibody MN41 did not block further increases in surface expression of gp165/95 after stimulation with FMLP. These data suggest that molecules of gp165/95 exhibit two functionally distinct forms, one, present on the surface of freshly isolated neutrophils, that becomes competent to mediate the aggregation response upon activation by a stimulus and a second form that can be translocated to the cell surface by the stimulus but is greatly diminished if not lacking in the ability to participate in that aggregation event.