Implication of (99m)Tc-sum IL-2 SPECT/CT in immunotherapy by imaging of tumor-infiltrating T cells

BACKGROUND: Although immune checkpoint blockade (ICB) and adoptive T cell transfer (ACT) therapy have achieved impressive clinical outcomes, majority of patients do not respond to immunotherapy. Tumor-infiltrating T cells, a critical factor to immunotherapy, is dynamically changing. Therefore, a reliable real-time in vivo imaging system for tumor-infiltrating T cells, but not immunohistochemical analyses, will be more valuable to predict response and guide immunotherapy. In this study, we developed a new SPECT/CT imaging probe (99m)Tc-sum IL-2 targeting the IL-2Rβ/IL-2Rγ (CD122/CD132) receptor on tumor-infiltrating T cells, and evaluated its application in predicting the immune response to anti-PD-L1 (αPD-L1) therapy as well as tracking infused T cells in ACT therapy. METHODS: The binding affinity of the super mutated IL-2 (sum IL-2) in various T cell subtypes was measured. Sum IL-2 was subsequently labeled with (99m)Tc through Sortase-A mediated site-specific transpeptidation. SPECT/CT imaging and biodistribution studies of (99m)Tc-sum IL-2 were performed in a MC38 mouse model. Wild type IL-2 (IL-2) was used as control in the above studies. Finally, we evaluated (99m)Tc-sum IL-2 SPECT/CT for the detection of tumor-infiltrating T cells in the context of αPD-L1 immunotherapy and ACT therapy. RESULTS: Sum IL-2 preferentially bound to CD8(+) T cells, especially activated CD8(+) T cells, while IL-2 showed biased binding to Treg cells. As a result, (99m)Tc-sum IL-2 could detect tumor-infiltrating T cells. In the MC38 tumor model, SPECT/CT imaging showed the increased tumor uptake of (99m)Tc-sum IL-2 after αPD-L1 treatment, suggesting that the treatment significantly increased tumor-infiltrating T cells, resulting in a correspondingly significant curative effect. In addition, (99m)Tc-sum IL-2 SPECT/CT could also track the infiltration of antigen-specific cytotoxic CD8(+) T cells during ACT therapy. CONCLUSION: (99m)Tc-sum IL-2 has great clinical potential for non-invasive and specific SPECT/CT imaging of tumor-infiltrating T cells as well as for timely prediction and evaluation of the therapeutic efficacy of ICB and ACT therapy.