Simple SummaryThe immunological profile of pleomorphic invasive lobular cancer is poorly investigated. pILC is characterized by more aggressive behavior and a worse prognosis; however, this rare subtype lacks a specific treatment approach. Here, we investigated the expression of sTILs and analyzed the PD-L1 expression levels from sixty-six patients with pILC. Moreover, we analyzed the association between sTILs and PD-L1 expression with other prognostic or predictive biomarkers and correlated sTILs and PD-L1 expression with survival outcomes. sTILS (≥1%) was present in 64% of the patients, and 36% of the tumors demonstrated a positive PD-L1 using SP142 (≥1%) and 28% had a positive PD-L1 score of ≥1 using 22C3. We found no differences between the molecular subtypes, the clinicopathological features, and the immune parameters, probably due to the small sample size of the HER2+ and TN subgroups. Larger trials on the immune composition of the subtypes of lobular breast cancer are needed.AbstractBackground: The prognostic and predictive role of stromal tumor-infiltrating lymphocytes (sTILs) is undetermined in pleomorphic invasive lobular cancer (pILC). The same applies for the expression of PD-1/PD-L1 in this rare breast cancer subtype. Here, we aimed to investigate the expression of sTILs and analyze the PD-L1 expression levels in pILC. Methods: Archival tissues from sixty-six patients with pILC were collected. The sTIL density was scored as a percentage of tumor area using the following cut-offs: 0%; <5%; 5–9%; and 10–50%. The PD-L1 expression was analyzed using IHC on formalin-fixed, paraffin-embedded tissue sections using SP142 and 22C3 antibodies. Results: A total of 82% of the sixty-six patients were hormone receptor positive and 8% of cases were triple negative (TN), while 10% showed human epidermal growth factor receptor 2 (HER2) amplification. sTILs (≥1%) were present in 64% of the study population. Using the SP142 antibody, 36% of tumors demonstrated a positive PD-L1 score of ≥1%, and using the 22C3 antibody, 28% had a positive PD-L1 score of ≥1. There was no correlation between sTILs or PD-L1 expression and tumor size, tumor grade, nodal status, expression of estrogen receptor (ER), or amplification of HER2. Our data did not show any difference in survival between the three molecular subtypes of pILC with respect to sTILs and PD-L1 expression. Conclusion: This study shows that pILCs show some degree of sTILs and PD-L1 expression; however, this was not associated with a survival improvement. Additional large trials are needed to understand immune infiltration in lobular cancer, especially in the pleomorphic subtype.