PD-L1 expression is associated with higher residual cancer burden in triple-negative breast cancers with residual disease after neoadjuvant chemotherapy

The consequences of neoadjuvant chemotherapy (NAC) for PD-L1 activity in triple-negative breast cancers (TNBC) are not well understood. This is an important issue as immune checkpoint inhibitors (ICI) are undergoing rapid development and could be beneficial in patients who do not achieve a pathological complete response. We used immunohistochemistry to assess PD-L1 expression (E1L3N clone, cutoff for positivity: ≥ 1%) on both tumor (PD-L1-TC) and immune cells (PD-L1-IC) from a cohort of surgical specimens of T1-T3NxM0 TNBCs treated with NAC. PD-L1-TC was detected in 17 cases (19.1%) and PD-L1-IC in 14 cases (15.7%). None of the baseline characteristics of the tumor or the patient were associated with PD-L1 positivity, except for pre-NAC stromal TIL levels, which were higher in post-NAC PD-L1-TC-positive than in negative tumors. PD-L1-TC were significantly associated with a higher residual cancer burden ( p =0.035) and aggressive post-NAC tumor characteristics, whereas PD-L1-IC were not. PD-L1 expression was not associated with DFS ( p =0.38) or OS ( p =0.48), but high Ki67 levels after NAC were strongly associated with a poor prognosis (DFS p =0.0014 and OS p =0.001). A small subset of TNBC patients displaying PD-L1 expression in the context of an extensive post-NAC tumor burden could benefit from ICI treatment after standard NAC.