A case of primary plasma cell leukemia with hairy-cell morphology and lambda-type Bence-Jones protein. Immunohistochemical and molecular analysis.

A case of primary plasma cell leukemia with hairy-cell morphology and lambda-type Bence-Jones protein is reported. Most of the atypical cells in the peripheral blood of this case were small lymphoid cells or lymphoplasmacytoid lymphocytes with numerous cytoplasmic hairy projections. Plasmablastic cells and 'tadpole'-like cells were also present in the bone marrow. Immunohistochemically, these atypical cells expressed the cytoplasmic lambda light chain and surface CD38 proteins but were negative for B-cell markers such as CD19, CD20 and CD79a. VLA-5 (CD49e), which is supposed to be expressed in mature populations of plasma cells, was negative. A sequence analysis of the variable region gene in the light-chain (V(L)) and heavy-chain (V(H)) loci of immunoglobulin demonstrated significant somatic hypermutation and intraclonal nucleic acid sequence variations. To our knowledge, the intraclonal diversity of these loci has been previously reported in some cases of monoclonal gammopathy of undetermined significance (MGUS), but never in a case of multiple myeloma. The immunohistochemical and molecular characteristics of this case allowed us to delineate the origin of the leukemic cells with hairy cell-morphology as germinal center B-cells, which would be at a more immature stage than the presumable origin of multiple myeloma.