Chemoimmunotherapy reduces the progression of multiple myeloma in a mouse model

Multiple myeloma (MM) is a B-cell malignancy characterized by clonal proliferation of malignant plasma cells in the bone marrow. Recently, we showed a correlation between increased ratios of functional regulatory T cells (Treg) and disease progression in a unique mouse model that mimics the human disease. Cyclophosphamide (CYC) is a cytotoxic alkylating agent widely used in chemotherapeutic regimens. Low-dose CYC was previously reported to selectively reduce Treg levels and to contribute to immunostimulation. Our objectives were (a) to determine whether treatment using a low-dose CYC could reduce MM progression and (b) to further characterize the modes of action underlying these effects. We found that both low- and high-dose CYC given to sick mice with hind limb paralysis resulted in the disappearance of the paralysis, the replacement of plasma tumor cells in the bone marrow by normal cell populations, and a significant prolongation of survival. However, only low-dose CYC treatment decreased the incidence of MM. Low-dose CYC rendered Tregs susceptible to apoptosis because of the downregulation of Bcl-xL and CTLA-4 in these cells, and a decreased production of interleukin 2 by effector CD4 cells. Moreover, using this treatment, we noted the recovery of IFN-γ-producing natural killer T cells and maturation of dendritic cells. Treatment of tumor-bearing mice with repeated administrations of low-dose CYC at longer time intervals (coinciding with the blocked renewal of Tregs) resulted in reduced tumor load, and the prevention or delay of disease recurrence, thereby breaking immune tolerance against MM tumor cells.