Definitive activation of endogenous antitumor immunity by repetitive cycles of cyclophosphamide with interspersed Toll-like receptor agonists.

// Soraya Zorro Manrique 1 , Ana L. Dominguez 1 , Noweeda Mirza 1 , Christopher D. Spencer 1 , Judy M. Bradley 1 , James H. Finke 2 , James J. Lee 3,4 , Larry R. Pease 1 , Sandra J. Gendler 1,3,5 and Peter A. Cohen 1,5 1 Department of Immunology, Mayo Clinic in Arizona, Scottsdale, AZ, USA 2 Department of Immunology, Lerner Research Institute, Cleveland, OH, USA 3 Department of Biochemistry and Molecular Biology, Mayo Clinic in Arizona, Scottsdale, AZ, USA 4 Division of Pulmonary Medicine, Mayo Clinic in Arizona, Scottsdale, AZ, USA 5 Division of Hematology/Oncology, Mayo Clinic in Arizona, Scottsdale, AZ, USA Correspondence to: Peter A. Cohen, email: // Keywords : cancer immunotherapy, chemotherapy, TLR agonists, MDSCs, Tregs, Immunology and Microbiology Section, Immune response, Immunity Received : February 17, 2016 Accepted : April 29, 2016 Published : June 21, 2016 Abstract Many cancers both evoke and subvert endogenous anti-tumor immunity. However, immunosuppression can be therapeutically reversed in subsets of cancer patients by treatments such as checkpoint inhibitors or Toll-like receptor agonists (TLRa). Moreover, chemotherapy can leukodeplete immunosuppressive host elements, including myeloid-derived suppressor cells (MDSCs) and regulatory T-cells (Tregs). We hypothesized that chemotherapy-induced leukodepletion could be immunopotentiated by co-administering TLRa to emulate a life-threatening infection. Combining CpG (ODN 1826) or CpG+poly(I:C) with cyclophosphamide (CY) resulted in uniquely well-tolerated therapeutic synergy, permanently eradicating advanced mouse tumors including 4T1 (breast), Panc02 (pancreas) and CT26 (colorectal). Definitive treatment required endogenous CD8+ and CD4+ IFNγ-producing T-cells. Tumor-specific IFNγ-producing T-cells persisted during CY-induced leukopenia, whereas Tregs were progressively eliminated, especially intratumorally. Spleen-associated MDSCs were cyclically depleted by CY+TLRa treatment, with residual monocytic MDSCs requiring only continued exposure to CpG or CpG+IFNγ to effectively attack malignant cells while sparing non-transformed cells. Such tumor destruction occurred despite upregulated tumor expression of Programmed Death Ligand-1, but could be blocked by clodronate-loaded liposomes to deplete phagocytic cells or by nitric oxide synthase inhibitors. CY+TLRa also induced tumoricidal myeloid cells in naive mice, indicating that CY+TLRa’s immunomodulatory impacts occurred in the complete absence of tumor-bearing, and that tumor-induced MDSCs were not an essential source of tumoricidal myeloid precursors. Repetitive CY+TLRa can therefore modulate endogenous immunity to eradicate advanced tumors without vaccinations or adoptive T-cell therapy. Human blood monocytes could be rendered similarly tumoricidal during in vitro activation with TLRa+IFNγ, underscoring the potential therapeutic relevance of these mouse tumor studies to cancer patients.