title: Treg depletion inhibits efficacy of cancer immunotherapy: implications for clinical trials.
abstract: BACKGROUND
  Regulatory T lymphocytes (Treg) infiltrate human glioblastoma (GBM); are involved in tumor progression and correlate with tumor grade. Transient elimination of Tregs using CD25 depleting antibodies (PC61) has been found to mediate GBM regression in preclinical models of brain tumors. Clinical trials that combine Treg depletion with tumor vaccination are underway to determine whether transient Treg depletion can enhance anti-tumor immune responses and improve long term survival in cancer patients.
  
  
  RESULTS
  Using a syngeneic intracrabial glioblastoma (GBM) mouse model we show that systemic depletion of Tregs 15 days after tumor implantation using PC61 resulted in a decrease in Tregs present in tumors, draining lymph nodes and spleen and improved long-term survival (50% of mice survived >150 days). No improvement in survival was observed when Tregs were depleted 24 days after tumor implantation, suggesting that tumor burden is an important factor for determining efficacy of Treg depletion in clinical trials. In a T cell dependent model of brain tumor regression elicited by intratumoral delivery of adenoviral vectors (Ad) expressing Fms-like Tyrosine Kinase 3 ligand (Flt3L) and Herpes Simplex Type 1-Thymidine Kinase (TK) with ganciclovir (GCV), we demonstrate that administration of PC61 24 days after tumor implantation (7 days after treatment) inhibited T cell dependent tumor regression and long term survival. Further, depletion with PC61 completely inhibited clonal expansion of tumor antigen-specific T lymphocytes in response to the treatment.
  
  
  CONCLUSIONS
  Our data demonstrate for the first time, that although Treg depletion inhibits the progression/eliminates GBM tumors, its efficacy is dependent on tumor burden. We conclude that this approach will be useful in a setting of minimal residual disease. Further, we also demonstrate that Treg depletion, using PC61 in combination with immunotherapy, inhibits clonal expansion of tumor antigen-specific T cells, suggesting that new, more specific targets to block Tregs will be necessary when used in combination with therapies that activate anti-tumor immunity.
authors.affiliation: Department of Biomedical Sciences, Gene Therapeutics Research Institute, Cedars Sinai Medical Center, Los Angeles, California, United States of America.
authors.forename: James F
authors.initials: JF
authors.lastname: Curtin
authors.forename: Marianela
authors.initials: M
authors.lastname: Candolfi
authors.forename: Tamer M
authors.initials: TM
authors.lastname: Fakhouri
authors.forename: Chunyan
authors.initials: C
authors.lastname: Liu
authors.forename: Anderson
authors.initials: A
authors.lastname: Alden
authors.forename: Matthew
authors.initials: M
authors.lastname: Edwards
authors.forename: Pedro R
authors.initials: PR
authors.lastname: Lowenstein
authors.forename: Maria G
authors.initials: MG
authors.lastname: Castro
mesh_terms: D000256:Adenoviridae; D000818:Animals; D001932:Brain Neoplasms / Q000276:immunology / Q000473:pathology / Q000628:therapy*; D002986:Clinical Trials as Topic; D003713:Dendritic Cells / Q000276:immunology; D005260:Female; D005909:Glioblastoma / Q000276:immunology / Q000473:pathology / Q000628:therapy*; D006801:Humans; D007109:Immunity; D016219:Immunotherapy, Adoptive; D008198:Lymph Nodes / Q000276:immunology / Q000473:pathology; D008213:Lymphocyte Activation; D008212:Lymphocyte Depletion; D016246:Lymphocytes, Tumor-Infiltrating / Q000276:immunology; D008264:Macrophages / Q000276:immunology; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D012074:Remission Induction; D050378:T-Lymphocytes, Regulatory / Q000276:immunology*; D013937:Thymidine Kinase / Q000378:metabolism; D016896:Treatment Outcome
chemical_list: D008565:Membrane Proteins; C084881:flt3 ligand protein; D013937:Thymidine Kinase