Stromal tumor infiltrating lymphocytes in hormone receptor positive/HER2 negative metastatic breast cancer.

The immune landscape of hormone receptor positive, HER2-negative metastatic breast cancer (HR+/HER2- mBC), the most common subtype of BC, remains understudied. This is mainly due to reduced sample acquisition opportunities from metastases as compared to primary tumors. In this study, we explored stromal tumor-infiltrating lymphocytes (sTIL) in metastatic samples collected through our post-mortem tissue donation program UPTIDER (NCT04531696). sTIL were scored as a continuous parameter according to international guidelines on 427 metastases and 38 primary untreated tumors acquired from 20 patients with HR+/HER2- mBC. ER-status was evaluated on 362 metastases with a cut-off for positivity set at 1% according to ASCO/CAP guidelines. Our analyses show that 54% and 15% of metastases had sTIL levels >1% and >5% respectively. sTIL levels tended to be lower in metastases as compared to their respective primary tumor (Estimate: -2.83, 95%CI: -5.77-0.11, p:0.07). sTIL levels were lower in metastases from invasive lobular carcinoma than in metastases from invasive breast carcinoma of no special type (Estimate: -1.67, 95%CI: -2.35--0.98, p:<0.001). A loss of ER expression was observed in 14% of all metastases, yet a negative ER-status was not significantly associated with increased sTIL levels. Finally, sTIL levels were significantly higher in lung and axillary lymph node metastases compared to all metastases. While these analyses were conducted on multiple metastases obtained at the end of life after several lines of treatment, the data provides novel and valuable insights into the state of immune infiltration in patients with metastatic HR+/HER2- BC.