Specific Imaging of CD8+ T-Cell Dynamics with a Nanobody Radiotracer against Human CD8β

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 via llama immunizations and subsequent biopanning. The lead anti-human CD8β nanobody was characterized in vitro on binding, specificity, stability and toxicity. The lead nanobody was labelled with ^99mTc and ⁶⁸Ga for non-invasive imaging of human T-cell lymphomas and CD8⁺ T cells in human CD8 transgenic mice and non-human primates via SPECT or PET/CT. Repeated imaging of CD8⁺ T cells in MC38 tumor-bearing mice was performed to visualize 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 via 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 Overall, this study describes the development of a more specific human CD8⁺ T-cell-targeting immunotracer, allowing follow up of immunotherapy responses via non-invasive imaging of human CD8⁺ T-cell dynamics.