The rabbit CD11c molecule has been characterized by use of 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 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/CD18 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.

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hucocytes interact with other cells and with the extracellular matrix by the expression of a variety of adhesion receptors (for review see [l]). Of particular importance are the & integrin molecules which form a set of three heterodimers, LFA-1 (CDlla-CD18), Mac-1 (CD11b-CD18) and p159-95 (CDllc-CD18). LFA-1 and Mac-1 have been shown to bind to ICAM-1, which is upregulated by activation of a wide variety of cells, particularly those of the vascular endothelium and the immune system. We have recently demonstrated that a monoclonal antibody (Mab), L13/64, recognises the rabbit equivalent of CD18 and is able to inhibit neutrophil adhesion to gelatin-coated plastic and phorbol ester stimulated T-cell aggregation [2].Balb/c mice were immunised with rabbit peritoneal neutrophils and hybridomas prepared by fusion of spleen cells with SP210 cells. Supernatants from these hybridomas were assayed for their ability to inhibit the phorbol ester-stimulated aggregation of the rabbit T cell line, RL-5. Two such supernatants, from two different fusions, gave essentially complete inhibition. The hybridomas were cloned by limiting dilution and both shown to produce IgG, molecules. One of these, NR185, was chosen for further study.Immunoprecipitation from a detergent lysate of RL-5 cells, metabolically labelled with 35S-methionine, was carried out using both NR185 and L13/64, together with a cross-clearance analysis. The results of this experiment are shown in Fig. 1. In addition to b b kDa -200 -116 -94 -67 -43 a b c d Figure 1. SDS-PAGE (7% gel) of immunoprecipitates from a detergent lysate of 35S-labelled RL-5 cells. (a) Precipitate with NR185. (b) Precipitate with L13/64 following pre-clearance with NR185. (c) Precipitate with L13/64. (d) Precipitate with NR185 following pre-clearance with L13/64. Arrows indicate the positions of CDlla (158 kDa), CDllc (145 kDa), CD18 (95 kDa) and CD18 (90 kDa). c). Re-precipitation, with NR185, of lysate cleared with L13/64 showed complete removal of the 158 and 95 kDa bands (lane d). Conversely, re-precipitation with L13/64 after clearance with NR185 showed bands of 145, 95 and 90 kDa remaining (lane b).Figure 2. Flow cytometry of rabbit peripheral blood leucocytes with NR185. (a) B cells. (b) T cells. (c) Monocytes. (d) Neutrophils. Solid and dotted lines are NR185 and negative control fluorescence, respectively.We have shown previously that the 95 and 90 kDa bands represent CD18 and an intracellular, partially glycosylated form of CD18 (CDl8), respectively and that the 158 and 145 kDa bands are probably CDlla and CDllc [2]. These experiments demonstrate that NR185 recognises rabbit CDlla and also thatthe RL-5 ceil line expresses CDllc. Previous data show that RL-5 does not express CDllb. The expression of CDlla on rabbit peripheral blood leucocytes was investigated by flow cytometry using methods described previously [3]. Rabbit mononuclear cells can be divided, by their scatter properties, into populations containing predominantly B cells, T cells and monocytes. These cells show low, mediu...