Intratumoral Treg ablation is sufficient to mediate tumor control systemically without autoimmunity

Regulatory T cells (T _regs ) infiltrate most tumors, yet whether they suppress immune responses directly within tumor tissues is untested. We used intratumoral (IT) delivery of diphtheria toxin (DT) in Foxp3 ^DTR mice to deplete IT T _regs while leaving peripheral T _regs intact. IT delivery of DT reduced T _reg frequencies in the tumor, which promoted potent tumor control without autoimmunity. Interestingly, this control was principally mediated by CD4 ⁺ T cells, whereas CD8 ⁺ T cells only contributed when CD4 ⁺ T cells were absent. While conventional dendritic cells (cDCs) were required to clear tumors, Batf3 ⁺ cDC1s were dispensable. Distant secondary tumors, mimicking metastases, were also controlled by IT T _reg ablation. Mechanistically, IT T _regs suppressed antitumor T cell responses by blocking the acquisition of tumor antigen by cDC2s. Importantly, similar mechanisms of control were observed using a clinically translatable IT T _reg -depleting anti-CCR8 antibody and reveal a distinct therapeutic strategy that leverages cDC2s and CD4 ⁺ T cells.