PET imaging of the natural killer cell activation receptor NKp30

Abstract

Redirecting the immune system in cancer treatment has led to remarkable responses in a subset of patients. Natural killer (NK) cells are innate lymphoid cells being explored as they engage tumor cells in different mechanisms compared to T cells, which could be exploited for treatment of nonresponders to current immunotherapies. NK cell therapies are monitored through measuring peripheral NK cell concentrations or changes in tumor volume over time. The former does not detect NK cells at the tumor site(s), and the latter is inaccurate for immunotherapies because of pseudoprogression. Therefore, new imaging methods are required as companion diagnostics for optimizing immunotherapies. Methods: Here we develop and complete pre-clinical in vivo validation of two antibody-based PET probes specific for NKp30, an activation natural cytotoxicity receptor expressed by human NK cells. Quantitative, multicolor flow cytometry during a variety of NK cell activation conditions was completed on primary human NK cells and the NK92MI cell line. Human renal cell carcinoma (RCC) tumors were stained for the NK cell receptors CD56, NKp30, and NKp46 to determine expression on tumor-infiltrating NK cells. An NKp30 antibody was radiolabeled with ⁶⁴Cu or 89Zr and evaluated in subcutaneous xenografts and adoptive cell transfer mouse models. Results: Quantitative flow cytometry showed consistent expression of the NKp30 receptor during different activation conditions. NKp30 and NKp46 costained in RCC samples, demonstrating the expression of these receptors on tumor-infiltrating NK cells in human tumors, while tumor cells in one RCC sample expressed the peripheral NK marker CD56. Both PET tracers showed high stability and specificity in vitro and in vivo. Notably, 89Zr- NKp30Ab had higher on-target contrast compared to ⁶⁴Cu-NKpAb at their respective terminal time points. ⁶⁴Cu-NKp30Ab delineated NK cell trafficking to the liver and spleen in an adoptive cell transfer model. Conclusion: The consistent expression of NKp30 on NK cells makes it an attractive target for quantitative imaging. Immunofluorescence staining on human RCC samples demonstrated the advantages of NKp30 targeting versus the traditional CD56 for detection of tumor infiltrating NK cells. This work advances PET imaging of NK cells and supports the translation of imaging agents for immunotherapy monitoring.