Localized radiation therapy (RT) can act as a powerful adjuvant to immunotherapeutic strategies by triggering anti-tumor immune responses to poorly immunogenic tumors. Radiation of tumor cells induces a dose-dependent release of ATP, a molecule that when released in the tumor microenvironment (TME) triggers recruitment and activation of dendritic cells (DCs), including CD103+ DCs recently identified as the key DC subset responsible for cross-presentation of tumor-derived antigens to CD8+ T cells. However, rapid hydrolysis of extracellular ATP by ecto-enzymes CD39 and CD73 results in a local accumulation of immunosuppressive adenosine that inhibits DC activation and CD8+ T cell effector functions and promotes regulatory T cells (Tregs). By blocking CD73 in conjunction with local tumor RT, we tested the hypothesis that adenosine generation limits the ability of RT to trigger anti-tumor immunity.
Wild type (WT) or BATF3-/- mice (CD103+ DC-deficient) were inoculated s.c. with TSA tumor cells (day 0) and assigned to treatment with: (1) control Ab; (2) anti-CD73 Ab (100 μg) (3) RT (20 Gy); (4) RT + anti-CD73 Ab. Antibodies were administered i.p. on day 11, 14, 17 and 20. RT was given locally as single 20 Gy dose on day 12. On day 18, tumors were analyzed by flow cytometry for DC and T cell infiltration. Mice were monitored for tumor progression. Bone marrow-derived DCs (BMDCs) isolated from WT mice (>90% CD103+) were labeled with CFSE and intravenously injected in BATF3-/- recipient mice. Tumors were harvested after 48h and analyzed by flow cytometry for infiltration of CFSE+ DCs.
In irradiated but not mock-treated mice, anti-CD73 Ab resulted in increased infiltration of CD103+DCs (8.9±2.6% of DCs in RT+anti-CD73 v. 3.5±2.8% of DCs in RT, p<0.05) expressing elevated levels of activation marker CD86 compared to mice treated with RT alone. This change was associated with improved CD8+T cell/Treg ratio (5±2.8 in RT+anti-CD73 v. 0.8±0.2 in RT). Importantly, CD73 blockade had no effect by itself but improved significantly radiation-induced tumor control (Tumor size at day 57 post inoculation: 385±525 mm3 in RT+anti-CD73 v. 1036±727 mm3 in RT). Consistent with the hypothesis that CD103+ DCs are essential for anti-tumor responses, the therapeutic effect of RT+CD73 blockade was abrogated in BATF3-/- mice. Moreover, CD73 blockade facilitated recruitment of adoptively transferred CD103+ BMDCs in irradiated tumors in BATF3-/- mice.
Our data support the hypothesis that adenosine generated following RT plays a key role in hindering development of anti-tumor immune responses and identify, as a mechanism of this effect, an abrogated infiltration and activation of CD103+ DCs. Blockade of adenosine generation by anti-CD73 treatment constitutes a promising strategy to enhance radiation-induced anti-tumor immunity.
Citation Format: Erik Wennerberg, Silvia Formenti, Sandra Demaria. Adenosine generation limits the ability of radiation therapy to induce antitumor immunity by abrogating recruitment and activation of CD103+ DCs [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr B05.