Intratumoral Regulatory T-cell Ablation Mediates Tumor Control Systemically without Autoimmunity.

Regulatory T cells (Treg) infiltrate most tumors, and increased Treg infiltration is correlated with reduced survival in patients with cancer. Although Tregs suppress antitumor immunity, they are also integral for preventing harmful inflammation in nontumor tissues, hindering the development of cancer therapeutic strategies targeting all Tregs systemically. In this study, we used intratumoral (IT) delivery of diphtheria toxin (DT) in Foxp3DTR mice to deplete Tregs within tumor tissues while leaving peripheral Tregs intact. IT delivery of DT reduced Treg frequencies in the tumor, which promoted potent tumor control without autoimmunity. Interestingly, tumor control was principally mediated by CD4+ T cells, whereas CD8+ T cells only contributed when CD4+ T cells were absent. Although conventional dendritic cells (cDC) were required to clear tumors, either type I DCs or type II DCs (cDC2) alone were sufficient to promote tumor control. Distant secondary tumors, mimicking metastases, were also controlled by IT Treg ablation in the primary tumor. Mechanistically, IT Tregs suppressed antitumor T-cell responses by blocking the acquisition and presentation of tumor antigen by cDC2s. Importantly, similar mechanisms of control were observed using a clinically translatable IT Treg-depleting anti-CCR8 antibody. Collectively, these findings reveal a distinct therapeutic strategy that leverages CD4+ T cells upon ablation of Tregs within tumors. SIGNIFICANCE: Localized ablation of intratumoral Tregs activates an alternative arm of the immune system by acting on type II conventional dendritic cells and CD4+ T cells, inducing prolonged suppression of tumor growth.