PD-L1 and CD8 are associated with deficient mismatch repair status in triple-negative and HER2-positive breast cancers

Summary Triple-negative and HER2-positive breast cancers (BCs) are more aggressive than hormone receptor–positive/HER2-negative BCs and show higher levels of tumor-infiltrating lymphocytes (TILs) and PD-L1 expression. Recently, US Food and Drug Administration approved anti–PD-L1 immunotherapy for solid tumors with deficient mismatch repair (MMR). In this study, we aimed to examine the prevalence of deficient MMR and its association with checkpoint immune markers in BCs. Immunohistochemistries (IHCs) with anti–MMR proteins (MLH1, PMS2, MSH2 and MSH6) and multiplex IHCs with anti–PD-L1, anti-CD8 or anti-CD163 were performed on tissue microarrays (TMAs) with 101 triple-negative BCs (TNBC) and 197 HER2-positive BCs. Additional IHCs for MMR proteins were also performed on whole-tissue sections from selected cases. Thirteen cases (4.4%) showed complete loss of MMR protein on TMAs, including 7 TNBCs (6.9%) and 6 HER2-positive BCs. On whole-tissue sections, only one of 13 cases showed complete loss of MMR proteins, while the other 12 cases showed partial loss. PD-L1 expression was identified in 37% of cases and was significantly higher in TNBCs than in HER2-positive BCs (71% versus 19%). Furthermore, BCs with complete/partial loss of MMR demonstrated significantly more PD-L1 and CD8 expressions than BCs with preserved MMR proteins. Although complete loss of MMR proteins exists in an extremely low frequency, partial loss is not uncommon in BCs. The association of partial loss of MMR proteins with increased PD-L1 and CD8 expression suggests a potential use of MMR testing as a screening method for anti–PD-L1 immunotherapy in BCs.