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.