Abstract
PURPOSE: While immunotherapy has revolutionized the oncology field, variations in therapy responsiveness limit the broad applicability of these therapies. Diagnostic imaging of immune cell, and specifically CD8(+) T cell, dynamics could allow early patient stratification and result in improved therapy efficacy and safety. In this study, we report the development of a nanobody-based immunotracer for non-invasive SPECT and PET imaging of human CD8(+) T-cell dynamics. METHODS: Nanobodies targeting human CD8β were generated by llama immunizations and subsequent biopanning. The lead anti-human CD8β nanobody was characterized on binding, specificity, stability and toxicity. The lead nanobody was labeled with technetium-99m, gallium-68 and copper-64 for non-invasive imaging of human T-cell lymphomas and CD8(+) T cells in human CD8 transgenic mice and non-human primates by SPECT/CT or PET/CT. Repeated imaging of CD8(+) T cells in MC38 tumor-bearing mice allowed visualization of CD8(+) T-cell dynamics. RESULTS: The nanobody-based immunotracer showed high affinity and specific binding to human CD8 without unwanted immune activation. CD8(+) T cells were non-invasively visualized by SPECT and PET imaging in naïve and tumor-bearing mice and in naïve non-human primates with high sensitivity. The nanobody-based immunotracer showed enhanced specificity for CD8(+) T cells and/or faster in vivo pharmacokinetics compared to previous human CD8-targeting immunotracers, allowing us to follow human CD8(+) T-cell dynamics already at early timepoints. CONCLUSION: This study describes the development of a more specific human CD8(+) T-cell-targeting immunotracer, allowing follow-up of immunotherapy responses by non-invasive imaging of human CD8(+) T-cell dynamics.