Tumor-associated macrophages promote intratumoral conversion of conventional CD4⁺ T cells into regulatory T cells via PD-1 signalling

While regulatory T cells (T_regs) and macrophages have been recognized as key orchestrators of cancer-associated immunosuppression, their cellular crosstalk within tumors has been poorly characterized. Here, using spontaneous models for breast cancer, we demonstrate that tumor-associated macrophages (TAMs) contribute to the intratumoral accumulation of T_regs by promoting the conversion of conventional CD4⁺ T cells (T_convs) into T_regs. Mechanistically, two processes were identified that independently contribute to this process. While TAM-derived TGF-β directly promotes the conversion of CD4⁺ T_convs into T_regs in vitro, we additionally show that TAMs enhance PD-1 expression on CD4⁺ T cells. This indirectly contributes to the intratumoral accumulation of T_regs, as loss of PD-1 on CD4⁺ T_convs abrogates intratumoral conversion of adoptively transferred CD4⁺ T_convs into T_regs. Combined, this study provides insights into the complex immune cell crosstalk between CD4⁺ T cells and TAMs in the tumor microenvironment of breast cancer, and further highlights that therapeutic exploitation of macrophages may be an attractive immune intervention to limit the accumulation of T_regs in breast tumors.