Targeted radionuclide therapy using anti-CD20 sdAbs: assessment of immune-activation post treatment with beta-emitting radionuclides

Targeted radionuclide therapy using anti-CD20 sdAbs: assessment of immune-activation post treatment with beta-emitting radionuclides

Ertveldt, T.1; Krasniqi, A.2; Keyaerts, M.2,3; Goyvaerts, C.1; Lecocq, Q.1; De Vlaeminck, Y.1; Awad, R.M.1; De Beck, L.1; Dekempeneer Y.2; Devoogdt, N.2; D’Huyvetter, M.2,Δ; Breckpot, K1,Δ.
1Laboratory of Molecular and Cellular Therapy (LMCT), Vrije Universiteit Brussel (VUB), Belgium
2In Vivo Cellular and Molecular Imaging (ICMI), VUB, Belgium
3Department of Nuclear Medicine, UZ Brussel, Belgium
ΔContributed equally to this work.
Introduction: Targeted radionuclide therapy (TRT) is a systemic treatment with radiolabeled cancer-specific probes purposed to selectively hit diseased cells. As radioactive labels, beta-emitters, such as 90Yttrium and 177Lutetium, are studied to cause irreparable DNA damage. Beta-TRT using camelid single domain antibodies (sdAbs) is studied at the VUB, showing high potential in controlling tumor growth. Recently, immune activation after beta-TRT was reported. Therefore, we studied stimulation of CD8+ T cells and upregulation of inhibitory immune checkpoints after sdAb-mediated beta-TRT.

Materials and methods: C57BL/6 mice were inoculated with B16-melanoma cells expressing human CD20 and ovalbumin. After engraftment, mice received fractionated treatment with non-targeting sdAb or anti-CD20 sdAbs, labeled with the radionuclide 177Lutetium. Different readouts were evaluated: (1) tumor volume, measured by caliper, (2) gene expression profiling of the tumor microenvironment (TME), evaluated using RT-qPCR, (3) immune cell composition of the TME, evaluated using flow cytometry and (4) systemic immune responses, evaluated by stimulation of CD8+ splenocytes with tumor antigens.

Results: A reduced tumor progression was observed upon treatment of CD20+ melanoma-bearing mice. Despite this tumor control, we were not able to document immune activation. Gene expression and flow cytometry analysis did not reveal changes in CD8+ T cells or the inhibitory immune checkpoint PD-1/PD-L1 in the TME. Moreover, CD8+ splenocytes did not show specificity for the antigen ovalbumin, which serves as a surrogate tumor antigen.

Conclusion: Although beta-TRT with 90Yttrium coupled to a tumor-targeting alkylphosphocholine in a model non-Hodgkin Lymphoma was shown to induce immune responses, we were not able to show similar immune activation with 177Lutetium-coupled anti-CD20 sdAbs in a melanoma model. Further research to confirm and study the reason for these contradicting results is required.