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 F-fluorodeoxyglucose-positron-emmission-tomograpy (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. 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´) fragment-based tracers were produced and characterized, focusing on functional and T-cell analyses to exclude any impact of T-cell targeting on cell survival and antitumor efficacy. T cells incubated with anti-CD2 and anti-CD7 F(ab´) showed no major modulation of functionality , 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´) had no long-term impact on T-cell functionality , anti-CD2 F(ab´) caused severe T-cell depletion and failure of tumor rejection. 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.