Targeted α-Therapy Using<sup>225</sup>Ac Radiolabeled Single-Domain Antibodies Induces Antigen-Specific Immune Responses and Instills Immunomodulation Both Systemically and at the Tumor Microenvironment
Thomas Ertveldt, Ahmet Krasniqi, Hannelore Ceuppens, Janik Puttemans, Yana Dekempeneer, Kevin De Jonghe, Wout de Mey, Quentin Lecocq, Yannick De Vlaeminck, Robin Maximilian Awad, Cleo Goyvaerts, Kim De Veirman, Aliyah Morgenstern, Frank Bruchertseifer, Marleen Keyaerts, Nick Devoogdt, Matthias D’Huyvetter, Karine Breckpot
Abstract
Targeted radionuclide therapy (TRT) using targeting moieties labeled with α-particle–emitting radionuclides (α-TRT) is an intensely investigated treatment approach as the short range of α-particles allows effective treatment of local lesions and micrometastases. However, profound assessment of the immunomodulatory effect of α-TRT is lacking in literature. <b>Methods:</b> Using flow cytometry of tumors, splenocyte restimulation, and multiplex analysis of blood serum, we studied immunologic responses ensuing from TRT with an antihuman CD20 single-domain antibody radiolabeled with <sup>225</sup>Ac in a human CD20 and ovalbumin expressing B16-melanoma model. <b>Results:</b> Tumor growth was delayed with α-TRT and increased blood levels of various cytokines such as interferon-γ, C-C motif chemokine ligand 5, granulocyte-macrophage colony-stimulating factor, and monocyte chemoattractant protein-1. Peripheral antitumoral T-cell responses were detected on α-TRT. At the tumor site, α-TRT modulated the cold tumor microenvironment (TME) to a more hospitable and hot habitat for antitumoral immune cells, characterized by a decrease in protumoral alternatively activated macrophages and an increase in antitumoral macrophages and dendritic cells. We also showed that α-TRT increased the percentage of programmed death-ligand 1 (PD-L1)–positive (PD-L1<sup>pos</sup>) immune cells in the TME. To circumvent this immunosuppressive countermeasure we applied immune checkpoint blockade of the programmed cell death protein 1–PD-L1 axis. Combination of α-TRT with PD-L1 blockade potentiated the therapeutic effect, however, the combination aggravated adverse events. A long-term toxicity study revealed severe kidney damage ensuing from α-TRT. <b>Conclusion:</b> These data suggest that α-TRT alters the TME and induces systemic antitumoral immune responses, which explains why immune checkpoint blockade enhances the therapeutic effect of α-TRT. However, further optimization is warranted to avoid adverse events.