Targeted radionuclide therapy (TRT) using probes labeled with Lutetium-177 represents a new and growing type of cancer therapy. We studied immunological changes in response to TRT with Lutetium-177 labeled anti-human CD20 camelid single domain antibodies (sdAbs) in a B16-melanoma model transfected to express human CD20, the target antigen, and ovalbumin, a surrogate tumor antigen. High-dose TRT induced melanoma cell death, calreticulin exposure and ATP-release in vitro. Melanoma-bearing mice received fractionated low and high-dose TRT via tumor targeting anti-human CD20 sdAbs, as opposed to control sdAbs. Tumor growth was delayed with both doses. Low and high-dose TRT increased interleukin-10 serum levels. Low-dose TRT also decreased CCL5 serum levels. At the tumor, high-dose TRT induced a type I interferon gene signature, while low-dose TRT induced a pro-inflammatory gene signature. Low and high-dose TRT increased the percentage of PD-L1pos and PD-L2pos myeloid cells in tumors with a marked increase in alternatively activated macrophages after high-dose TRT. The percentage of tumor-infiltrating T-cells was not changed, yet a modest increase in ovalbumin-specific CD8pos T-cells was observed after low-dose TRT. Contradictory, low and high-dose TRT decreased CD4pos T helper 1 (Th1)-cells in addition to double negative T-cells. In conclusion, these data suggest that low and high-dose TRT induce distinct immunological changes, which might serve as an anchoring point for combination therapy.

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