MALA-2, mouse homologue of human adhesion molecule ICAM-1 (CD54).

In humans, lymphocyte adhesion to cells is mediated by the protein heterodimer CD11a/CD18 (Leu-CAMa, LFA-1) and its ligand CD54 (ICAM-1). Although the murine CD11a/CD18 is well characterized, the mouse homologue of human ICAM-1 has not been identified. In the present study a rat monoclonal antibody to the murine lymphocyte activation antigen MALA-2 was found to inhibit in a dose-dependent manner the phorbol ester-enhanced aggregation of mouse lymphoblasts, an adhesion-specific assay, and hence to define an adhesion molecule. By immunofluorescence flow cytometry the antigen expression was low on spleen cells but it largely increased after stimulation with mitogens. The antigen was expressed by some, but not all, lymphoid cell lines, and myelomonocytic and mastocytoma cells were also positive. In frozen tissue sections MALA-2 was mainly detected on germinal center B cells, dendritic cells, macrophages and vascular endothelium, including high endothelial venules. Cell surface labeling followed by immunoprecipitation and gel electrophoresis indicated that the antigen is a sialoglycoprotein which has a relative molecular mass of 95 kDa and displays a faster electrophoretic mobility under non-reducing conditions. The function, cellular distribution and molecular properties of MALA-2 are indistinguishable from those of human ICAM-1.