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 with 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, and the latter is inaccurate for
immunotherapies because of pseudoprogression. Therefore, new imaging methods are required as companion diagnostics for optimizing
immunotherapies. Methods: In this study, we developed and completed preclinical in vivo validation of 2 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, whereas
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 than 64Cu-NKp30Ab 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 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.