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.