Abstract
BackgroundRegulatory 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.FindingsUsing 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.ConclusionsOur 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.
Highlights
Glioblastoma multiforme (GBM) is a deadly primary brain tumor which is highly invasive with tumor cells infiltrating the surrounding healthy brain tissue [1]
We 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
In combination with intratumoral delivery of AdFlt3L and adenoviral vectors (Ad)-Type 1Thymidine Kinase (TK), we found that PC61 administration 24 days after tumor implantation (7 days after treatment) completely suppressed adaptive immune responses against GL26 tumor antigens
Summary
Glioblastoma multiforme (GBM) is a deadly primary brain tumor which is highly invasive with tumor cells infiltrating the surrounding healthy brain tissue [1]. (ii) Macrophages (MW) were assessed by gating live leukocytes with CD45, plotting CD11b against I-Ab. The red box outlines the population of tumor infiltrating macrophages (CD11b+ CD45+ I-Ab+). CD3e+ live leukocytes were gated and dot plots display Foxp against CD4 staining. (vi) Tumor infiltrating B cells are visualized by gating for live leukocytes, plotting CD45 against CD19. The percentages of each immune cell population infiltrating the tumor with respect to the total number of tumor CD45+ cells is indicated in representative dot plots. The percentages of CD25+ cells population with respect to the total number of CD3e+ cells in the tumor, draining lymph nodes or spleen are indicated in representative dot plots. 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
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