Characterization of Clonal Pre-B Cell Progenitors of Plasma Cells In Multiple Myeloma

Abstract 1921

The lack of specific molecules to define the multiple myeloma (MM) malignant progenitor cells responsible for the development of the disease has hampered the evaluation of minimal residual disease (MRD) in MM. Syndecan (CD138) expression is limited to terminally differentiated plasma cells (PC) and studies of myeloma cell biology using CD138+ selected cells are limited in scope since earlier PC progenitors involved in the disease process are likely CD138 negative. We have identified a bone marrow (BM) CD138- subset that co-express CD19 with identical kappa or lambda light chain restriction as the abnormal plasma cells, as previously shown by others. Further characterization of CD138-/CD19+ cells (23% ± 18% of total BM cells) has shown that this subset co-expresses Notch-1 (90 ± 5%), c-Kit (20 ± 5%), CD20 (5 ± 2%), CD27 (20 ± 13%), CD34 (21 ± 15%) and lack CD56. CD138-/CD19+ cells represent two distinctive populations being CD34+/CD20- or CD34-/CD20+. A small percentage (1.1 ± 0.4%) of CD138-/CD19+ cells showed aldehyde dehydrogenase (ALDH) activity. To further study MM BM progenitor we developed a multicolor flow cytometry assay to flow sort MM BM cell subsets allowing > 95% purity using a FACS Aria. Isolated BM populations were grown in methylcellulose supplemented with 5% PHA-leukocyte conditioned medium to detect progenitor cells committed to differentiate into mature PC. CD138+ cells did not form colonies, whereas CD138-/CD38+/CD19+/light chain+ regardless of CD34 expression grew colonies with a low efficiency of 1 in 25,000. However, only CD138-/CD38+/CD19+, but not CD34+ (HSC), differentiated into a mature syndecan (CD138+) expressing PC. Isolated CD138-/CD38+/CD19+ cells were relatively bortezomib and melphalan resistant when compared to CD138+ plasma cells. We hypothesize that the CD138-/CD38+/CD19+/CD34+ population contains earlier progenitor B cells that differentiate into the malignant PC. Surrogate assays for stem cell activity and xenotransplant models should determine cancer stem cell activity of these cells. Ongoing experiments will test whether Notch-1, CD34 or c-Kit expression is required for malignant PC progenitor function. Research studies of these CD138- MM putative progenitor cells will allow developing novel treatments to eradicate the MM minimal residual disease reservoir.