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Analysis of Programmed Death-Ligand 1 Expression, Stromal Tumor-Infiltrating Lymphocytes, and Mismatch Repair Deficiency in Invasive Micropapillary Carcinoma of the Breast

Introduction: Invasive micropapillary carcinoma (IMPC) of the breast is a rare and aggressive subtype of invasive ductal carcinomas, associated with poor prognosis and without a well-established treatment. Programmed death-ligand 1 (PD-L1) expression, high tumor-infiltrating lymphocytes (TILs), and microsatellite instability have recently been linked to susceptibility to immunotherapies against PD-1/PD-L1 axis. No exhaustive data is available on the status of these predictive markers in IMPCs of the breast. The aim of our study is to analyze PD-L1 expression, stromal TIL (sTIL), and mismatch repair (MMR) gene status in IMPCs of the breast, to extend the therapeutic possibilities of these rare aggressive tumors. Materials and Methods: Thirty-seven cases of IMPCs diagnosed in two European institutions between 2003 and 2017 with detailed clinical and pathologic data were analyzed. sTILs were assessed in hematoxylin and eosin-stained sections. MMR deficiency was tested by either immunohistochemistry (IHC) for MMR proteins (MLH1, MSH2, MSH6, and PMS2) or capillary electrophoresis for microsatellite instability using a standardized panel of five loci (Bat25, Bat26, D2S123, D5S346, and D17S250). For PD-L1, expression in both tumor cells (TCs) and immune cells (ICs) was determined using the antibody clone SP263. Results: The median sTILs was 3% (mean: 6%, range: 0–40). Thirty-one cases (84%) showed ≤10% of sTILs and only one case had 40% of sTILs. Higher median TILs were more frequently observed in lymph node metastases. PD-L1 expression (≥1%) was observed in 4 (11%) and 14 (38%) cases in TCs and ICs, respectively. None of the tumors showed PD-L1 expression in >1% of TCs. Only three cases showed expression in >10% of ICs. All cases were microsatellite stable by either IHC or polymerase chain reaction analyses. Conclusions: IMPCs of the breast are microsatellite-stable and immune desert tumors with low PD-L1 expression, thus arguing against the use of immune-checkpoint inhibitors in these patients. Active immunotherapy strategies attempting to stimulate self-immune system to attack tumor are needed.