Programmed death ligand 1-positive immune cells in primary tumor or metastatic axillary lymph nodes can predict prognosis of triple-negative breast cancer even when present at < 1% in the tumor region

Background The efficacy of pre-operative systemic treatment (PST) combined with immune checkpoint inhibition (ICI) for triple-negative breast cancer (TNBC) has been recognized recently as being independent of the degree of programmed death ligand-1 (PD-L1) positivity of infiltrating immune cells, especially for patients with axillary lymph node metastasis (ALNM). Methods TNBC patients with ALNM were treated surgically between 2002 and 2016 in our facility (n = 109), of whom 38 received PST before resection. The presence of tumor-infiltrating lymphocytes (TILs) expressing CD3, CD8, CD68, PD-L1 (detected by antibody SP142) and FOXP3 at primary and metastatic LN sites was quantified. Results The size of invasive tumor and the number of metastatic axillary LN were confirmed as prognostic markers. The numbers of both CD8+ and FOXP3+ TILs at primary sites were also recognized as prognostic markers, especially for overall survival (OS) (CD8, p = 0.026; FOXP3, p < 0.001). The presence of CD8+, FOXP3+ and PD-L1+ cells was better maintained in LN after PST and may contribute to improved antitumor immunity. Provided they were present as clusters of ≥ 70 positive cells, even < 1% of immune cells expressing PD-L1 at primary sites predicted a more favorable prognosis for both disease-free survival (DFS) (p = 0.004) and OS (p = 0.020). This was the case not only for 30 matched surgical patients, but also in all 71 surgical only patients (DFS: p < 0.001 and OS: p = 0.002). Conclusions PD-L1+ , CD8+ or FOXP3+ immune cells in the tumor microenvironment (TME) at both primary and metastatic sites are significant on prognosis, which could be a clue to expect the potential for better responses to the combination of chemotherapy and ICI, especially for patients with ALNM.