Monoclonal antibody Ki-M4 specifically recognizes human dendritic reticulum cells (follicular dendritic cells) and their possible precursor in blood

Ki-M4, a new IgG3 monoclonal antibody, selectively recognizes dendritic reticulum cells (DRC; follicular dendritic cells) in all human lymphatic organs, as tested by the immunoperoxidase method on the light and electron microscopic level. This antibody was raised against separated lysosomes of the 12–0-tetradecanoyl phorbol-13-acetate (TPA) stimulated permanent cell line, U-937, derived from a human histiocytic lymphoma. No cross-reactivity was encountered in epithelial and mesenchymal cells, including macrophages and other cells detectable in bronchial and peritoneal lavages. In the nonadherent fraction of the mononuclear blood cells collected from the interphase of a Ficoll- Urografin gradient (density = 1.077 g/ml), 0.1 per million of the cells hitherto not classified as monocytes or lymphocytes showed a strong reaction. All other separated blood cell types were devoid of any reactivity. The observation that DRC share a highly restricted, and thus specific, antigen with a small but distinct subpopulation of mononuclear leukocytes implies their blood derivation.