Identification of a CLL Cancer Stem Cell

To date cancer research and therapeutic approaches have mainly targeted the cellular mechanisms of aberrant proliferation. These treatments do shrink tumors, but after a remission of variable length, most patients relapse with frequent drug resistance and metastatic dissemination. These features indicate that within a tumor exists a population of cells that is inherently resistant to therapies and are able to reinitiate tumor growth and development. This is the basis of the cancer stem cell (CSC) theory. Targeted elimination of CSCs is actively being explored as a new therapeutic approach in hemato-oncology with a focus on cell surface antigens expressed on CSCs for antibody- or ligand-based therapeutic approaches. Most studies have defined CSCs functionally by their tumor initiating ability, when inoculated into immune-deficient (NSG) mice. These xenograft models have been powerful preclinical tools in a number of malignancies, with the notable exception of chronic lymphocytic leukemia (CLL).

We have recently isolated a primary cell line from the bone marrow BM of a patient with CLL, which we have termed MDA-BM5, and have successfully passaged this line through six serial transplants in NSG mice. The mice die of disease around 1 month post transplant and analysis of organs has demonstrated the presence of CD19+CD5+ cells with Ig light chain restricted to lambda which is consistent with the original patient samples. BM cells from tertiary transplant NSG mice demonstrate a stem cell phenotype; including expression of CD34 and aldehyde dehydrogenase (ALDH). Treatment of MDA-BM5 cells in vitro with inbrutinib is effective in killing 99% of cells, however, continued culture after removal of ibrutinib results in propagation of a "resistant" subpopulation. Based upon these data we propose that the novel cell line (MDA-BM5) contains a CSC subpopulation which replicates human CLL in NSG mice. Further characterization of the CLL-CSC is ongoing to identify potential therapeutic targets.