Expression of the 50-kDa integrin-associated protein on myeloid cells and erythrocytes.

Integrin-associated protein (IAP) is a 50-kDa intrinsic membrane protein that is involved in signal transduction during neutrophil activation by a variety of Arg-Gly-Asp-containing ligands. However, IAP does not itself directly bind these ligands, which are instead recognized by the leukocyte response integrin (LRI). In fact, IAP is more widely expressed than the LRI and is even on erythrocytes, which express no known integrins. This suggests that IAP may have additional functions besides signal transduction in association with the LRI. In this work we have quantitated IAP expression on several myeloid cells and cell lines, as well as erythrocytes, using a mAb which recognizes this protein. These data show that there are about 10,000 IgG-binding sites on each erythrocyte and 20 times that number on the myeloid cell lines U937 and HL60. Normal PMN and monocytes express about 25,000 IgG-binding sites. There are twice as many Fab'-binding sites on each cell, suggesting that each IgG binds via both Ag-combining sites. Binding data using several mAb to IAP suggest that IAP undergoes a temperature-dependent conformational change. Unlike some integrin receptors, IAP is not stored in a regulated secretory compartment in polymorphonuclear leukocytes (PMN). In addition, there is no evidence for internalization or shedding of IAP upon PMN activation. These data show that IAP is expressed at significant levels on myeloid cells and erythrocytes and that its expression is unaffected by the state of PMN activation.