Selective elimination of a chemoresistant side population of B-CLL cells by cytotoxic T lymphocytes in subjects receiving an autologous hCD40L/IL-2 tumor vaccine

Abstract

Side-population (SP) analysis identifies precursor cells in normal and malignant tissues. Cells with this phenotype have increased resistance to many cytotoxic agents, and in malignant disease may represent a primary drug resistant population. To discover whether drug resistant malignant SP cells are nonetheless sensitive to immune-mediated killing, we first established the presence of a malignant CD5+CD19+ SP subset in the blood of 18/21 subjects with B-CLL. We examined the fate of these cells in 6 of these individuals who received autologous hCD40L/IL-2 gene-modified tumor cells as part of a tumor vaccine study. Vaccinated patients showed an increase in B-CLL-reactive T cells followed by a corresponding decline in circulating CD5+CD19+ SP cells. T cell lines and clones generated from vaccinated patients specifically recognized B-CLL SP tumor cells. Elimination of SP cells is likely triggered by their increased expression of target antigens such as RHAMM following stimulation of the malignant cells by hCD40L, since CD8+ RHAMM-specific T cells could be detected in the peripheral blood of immunized patients and were associated with the decline in B-CLL SP cells. Hence malignant B cells with a primary drug resistant phenotype can be targeted by T cell mediated effector activity following immunization of human subjects.

B-chronic lymphocytic leukemia chemoresistance involves innate and acquired leukemic side population cells

“…Confirming a recent publication, 22 here we reported the existence of few natural SP cells in virtually all B-CLL patients, which harbors phenotypic and cytogenetic hallmarks of the disease. Although they are chemoresistant, B-CLL SP cells are present in patients before any treatments.…”

“…20,21 In this study, we tracked chemoresistant subsets of B-CLL cells by the SP technique to clarify the dynamics of chemoresistance in this disease. Our study confirms the existence of Fludarabine-resistant leukemic SP cells in B-CLL, as established recently by Foster et al 22 In addition to uncovering both innate and acquired cell compartments with SP phenotype, this report addresses the question of polyresistance displayed by the B-CLL SP cells and demonstrates the prevalence of this population in post-therapeutic B-CLL samples. Finally, this study attempted to characterize SP cell dynamics underlying successive B-CLL relapses with progressive chemoresistance.…”

“…33 In complete accordance with our findings, ABCG2 ectopic expression has been shown to mediate Fludarabine resistance, 36 and B-CLL SP cells have been shown to resist Fludarabine treatment. 22 Our study is the first report which, to our knowledge, extends SP resistance to Bendamustin, an alkylating agent, and Rituximab, a monoclonal antibody. Although we assume that the molecular basis of chemoresistance to Fludarabine and Bendamustin is relying on activity of ABC transporters, which efflux drugs and Hoechst 33342 dye, 36 resistance to Rituximab rather involves additional drug-resistance mechanisms, such as the inhibition of the apoptotic process.…”

Cancer Stem Cells: A Review of Potential Clinical Applications

“…We have also shown that treatment of tumor cells with demethylating agents led to upregulation of PRAME expression (C. C. Chang, MD, PhD, unpublished data, November 2011). As mentioned above, studies by Shafer et al 34 and Foster et al 32,33 demonstrated selective susceptibility of SP cells in chronic lymphocytic leukemia and Hodgkin lymphoma to cytotoxic T cells.…”

“…Foster et al 32,33 generated cytotoxic lymphocytes directed against a stem cell-like SP of cells in chronic lymphocytic leukemia. The same group similarly demonstrated the potential cytotoxicity of cytotoxic lymphocytes against SP cells in Hodgkin lymphoma.…”