CD4 and breast cancer: PD‐L1 is programmed cell death ligand 1.[74] Studies have demonstrated that B cells also express PD‐L1 and modulate tumor immunity.[79] A subset of tumor‐activated PD‐L1+ B cells promotes the conversion of CD4+ T cell to Tregs through the TGF‐β pathway, which in turn promotes breast cancer progression and metastasis.[29] A subsequent study by Zhang et al.