Novel type of proliferating lymphoplasmacytoid cell with a characteristic spotted immunofluorescence pattern.

We examined bone marrow from myeloma patients for the presence of cells with the characteristics of the clonogenic cell in the myeloma stem cell assay. We identified a novel type of cell that contained cytoplasmic immunoglobulin of the relevant idiotype located in a cytoplasmic spot. This "spotted" Ig could be located in the rough endoplasmic reticulum. Spotted cells are highly proliferative, as evidenced by the nuclear staining with the antibody Ki67, and were found in the bone marrow from most of the myeloma patients studied. This type of cell was also present in patients with immunocytomas, in some cases of benign monoclonal gammopathy, and in patients in the state of polyclonal hypergammaglobulinemia. IgG subclass distribution of so-called spotted cells and plasma cells, found in a patient with pseudo biclonal gammopathy, indicates that spotted cells are intermediate between B cells and plasma cells. Spotted cells express the B cell-associated antigens HB4 and HB6 but do not express other B cluster of differentiation antigens or plasmacytoid antigens tested.