T-cell functionality testing is highly relevant to developing novel immuno-tracers monitoring T cells in the context of immunotherapies and revealed CD7 as an attractive target

Abstract

Cancer immunotherapy has proven high efficacy in treating diverse cancer entities by immune checkpoint modulation and adoptive T-cell transfer. However, patterns of treatment response differ substantially from conventional therapies, and reliable surrogate markers are missing for early detection of responders versus non-responders. Current imaging techniques using 18F-fluorodeoxyglucose-positron-emmission-tomograpy (18F-FDG-PET) cannot discriminate, at early treatment times, between tumor progression and inflammation. Therefore, direct imaging of T cells at the tumor site represents a highly attractive tool to evaluate effective tumor rejection or evasion. Moreover, such markers may be suitable for theranostic imaging. Methods: We mainly investigated the potential of two novel pan T-cell markers, CD2 and CD7, for T-cell tracking by immuno-PET imaging. Respective antibody- and F(ab´)2 fragment-based tracers were produced and characterized, focusing on functional in vitro and in vivo T-cell analyses to exclude any impact of T-cell targeting on cell survival and antitumor efficacy. Results: T cells incubated with anti-CD2 and anti-CD7 F(ab´)2 showed no major modulation of functionality in vitro, and PET imaging provided a distinct and strong signal at the tumor site using the respective zirconium-89-labeled radiotracers. However, while T-cell tracking by anti-CD7 F(ab´)2 had no long-term impact on T-cell functionality in vivo, anti-CD2 F(ab´)2 caused severe T-cell depletion and failure of tumor rejection. Conclusion: This study stresses the importance of extended functional T-cell assays for T-cell tracer development in cancer immunotherapy imaging and proposes CD7 as a highly suitable target for T-cell immuno-PET imaging.