Monoclonal circulating B cells in multiple myeloma. A continuously differentiating, possibly invasive, population as defined by expression of CD45 isoforms and adhesion molecules.

The origin of the malignant stem cell in multiple myeloma, despite years of investigation by many laboratories, remains elusive. We have described a population of monoclonal circulating B-lineage lymphocytes that has been detected in all myeloma patients analyzed, both at diagnosis and after chemotherapy, and that has many properties consistent with its definition as either a stem cell compartment or an intermediate between the stem cell and the bone marrow localized plasma cells. On average, 40% to 50% of peripheral blood mononuclear cells are abnormal B cells that express CD10 and PCA-1 in conjunction with B-lineage markers CD19, CD20, and CD24 and variable expression of CD5. The B cells are monoclonal by Southern blot analysis and represent a highly pleiomorphic population. The migratory patterns of these cells are unknown, and their presence in blood may reflect cells in transit from a parent organ such as spleen to bone marrow for terminal differentiation, or they may originate in the bone marrow prior to circulation and seeding of other skeletal or extraskeletal sites. The working hypothesis underlying this work postulates that these abnormal B cells originate outside the marrow, giving rise to plasma cells only after migration to the bone marrow, which provides a microenvironment conducive to terminal plasma cell differentiation. Bone marrow plasma cells do not include an actively proliferating component and are terminally differentiated end stage cells. In contrast, the circulating abnormal B cells include proliferating cells and appear to be heterogeneous in differentiation stage. Analysis of CD45 isoform expression indicates a population continuously differentiating from a late B-cell stage through the early plasma cell stages to an end stage plasma cell. Quantitative and qualitative expression of CD45 has been shown to characterize B-cell development, with a high density of the CD45RA isoform on mature resting B cells, a transition to CD45R0 on activated B cells, and a gradual loss of total CD45, predominantly of the CD45R0 isoform, during plasma cell development until, on end stage plasma cells, all CD45 expression is lost. In myeloma patients, all of these B-cell stages are represented, with the least differentiated B cells occurring in blood, intermediate stages in both blood and marrow, the most differentiated B and/or plasma cells in the bone marrow.(ABSTRACT TRUNCATED AT 400 WORDS)