The antigen has been assigned to the CDw93 cluster, the epitope has not been further characterized. Antigen distribution: Isolated cells: Positive with all granulocytes, with staining increasing transiently upon maturation. Antigen expression is increased by TNFa, GM-CSF, (fMLP) treatment. No effect on superoxide production by binding to neutrophils has been noted. PMA stimulation causes down regulation of the antigen. Positive with monocytes following fMLP, TNF, GM-CSF treatment. Antigen expression decreases on cultured monocytes. Positive on AML cases with monocytic component. Positive on cell lines HL-60, Namalva, K562, THP-1, U937, negative on HMC-1 (human mast cell line). Tissue sections: Monoclonal antibody X-2 stains epitheloid cells and Rosai-Dorfman cells and most endothelial cells in the synovial membrane of RA patients. Specificity: Human: granulocytes, monocytes, endothelial cells (main cellular reactivity). Other: not tested.