Identification and characterization of Tregs recognizing tumor-associated antigens.

Foxp3-expressing CD25+CD4+ regulatory T cells (Tregs) are abundantly infiltrating into tumor tissues, hindering effective anti-tumor immunity. Here, assuming that such tumor Tregs were clonally expanding by recognizing tumor-associated antigens, we attempted to characterize them by their gene expression profiling and T-cell receptor (TCR) clonotyping at the single-cell level. We found that they predominantly expressed the chemokine receptor CCR8 and few tumor-infiltrating conventional T cells expressed CCR8 in mice and humans. CCR8+ tumor Tregs were phenotypically highly differentiated and functionally stable. Treatment of tumor-bearing mice with one injection of anti-CCR8 mAb with high antibody-dependent cellular cytotoxic activity was indeed able to specifically deplete clonally expanded tumor Tregs and thereby eradicate established tumors. The depletion changed the phenotype of tumor-infiltrating CD8+ effector T cells, which were different in activated or exhausted patterns from those seen after immune checkpoint blockade by anti-PD-1 mAb. The treatment also evoked potent secondary immune responses against the same tumor cell line inoculated several months after tumor eradication, indicating that the anti-CCR8-induced memory-type tumor-reactive effector T cells were, once induced, resistant to recovered Treg suppression. In contrast to such evocation of potent tumor immunity, anti-CCR8 treatment scarcely elicited discernable autoimmunity in mice, contrasting with a similar treatment for systemic whole Treg depletion, which induced histologically and serologically evident autoimmunity. Thus, temporary and specific depletion of clonally expanded Tregs in tumor tissues by cell-depleting anti-CCR8 mAb is sufficient to evoke potent tumor immunity with long-lasting memory and without serious autoimmunity.