A monoclonal antibody, 3/22, to rabbit CD11c which induces homotypic T cell aggregation: evidence that ICAM‐1 is a ligand for CD11c/CD18

The rabbit CD11c molecule has been characterized by use of a new monoclonal antibody, mAb 3/22. Expression of the p150,95 integrin (CD11c/CD18) has been shown by flow cytometry and immunohistochemistry to be restricted to monocytes, macrophages, dendritic cells and a small population of lymphocytes in peripheral blood. No expression on neutrophils could be demonstrated. Incubation of the newly derived CD8+ T cell line, BJ/873, with mAb 3/22 causes homotypic aggregation, which has been shown to be a cell surface event that is not dependent on intracellular signaling or on receptor cross‐linking. Inhibition studies show that the ligands responsible for this aggregation are CD11c/CD 18 and ICAM‐1, both of which are expressed on BJ/873. One other rabbit T cell line, K34, that also expresses p150,95 and ICAM‐1, shows a similar aggregation response when stimulated with 3/22. Cell lines that express p150,95 but not ICAM‐1 do not aggregate. These observations suggest that ICAM‐1 is a ligand for activated p150,95.