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

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 18 F-fluorodeoxyglucose-positron-emmission-tomograpy ( 18 F-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.