The programmed cell death-1 receptor (PD-1) is an immune checkpoint inhibitor which is expressed on the surface of immune effector cells. It is activated mainly by PD-L1 which can be expressed by all human cells. The PD-1/PD-L1 pathway plays a subtle role in maintaining peripheral T-lymphocyte tolerance and regulating inflammation. In cancer, the expression of PD-L1 seems to be one of the major immune escape mechanisms. Many studies have shown efficacy of blocking PD-1 or PD-L1 with specific antibodies like pembrolizumab or atezulizumab. In breast cancer, potential response was demonstrated in metastatic triple-negative breast cancers.

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In metastasized breast cancer patients, adoptive transfer of BM TCs can induce the presence of tumor antigen-reactive type-1 TCs in the peripheral blood. Patients with immunologic response after ADI show a significantly longer overall survival. Patients with bone metastases significantly less frequently respond to the treatment and, therefore, might not be optimal candidates for ADI. Although the present study does not yet prove the therapeutic effect of ADI, these findings shed light on the relation between immune response and cancer prognosis and suggest that transfer of reactivated BM TCs might bear therapeutic potential.