Background: Recent data indicate a promising response to immune checkpoint inhibition in patients with metastatic TNBC. Ample research showed that PD-L1, a PD-1 ligand, is expressed in multiple tumor types, including TNBC, and may be a predictor of response to PD-1/PD-L1 blockade. Quantification of the stromal composition, particularly PD-1 and PD-L1 expression, continues to be controversial in its relationship to immune checkpoint inhibition in several cancer types, and it remains unclear whether PD-L1 expression is necessary to predict response. Here, we aimed to determine the distribution of PD-1 and PD-L1 in a large set of centrally ascertained specimens of TNBC.

Methods: The study cohort consisted of 993 tumor samples (both primary and metastatic TNBC) analyzed for either PD-1 or PD-L1 expression in one laboratory (Caris Life Sciences; Phoenix, AZ). Estrogen receptor and progesterone receptor status was assessed by immunohistochemistry (IHC). HER2/Neu expression or amplification was assessed by either IHC or in-situ hybridization. PD-1 and PD-L1 expression were confirmed using IHC with validated antibodies. For PD-L1, clone SP142 (Roche Diagnostics) was utilized and a sample was considered positive if there was > 5% membranous staining of tumor cells. For PD-1, clone EH21.1 (BD Biosciences) was used. Tumor infiltrating lymphocytes (TILs) expressing PD-1 were counted and a sample was considered positive if there was at least one PD-1 positive TIL per 40x microscopic field.

Results: The median age in this cohort was 56 years (range: 22 – 88). A total of 363 TNBC specimens were tested for PD-1 via IHC. One hundred fifty eight (158; 43.5%) were negative for PD-1 expression. Two hundred five (205; 56.5%) were positive for PD-1. Of those that were PD-1 positive, 116 (56.6%), were in samples from a primary site (breast) and 89 (43.4%) in samples from a metastatic site. A total of 630 TNBC specimens were tested for PD-L1 via IHC. Five hundred seventy four (574; 91.1%) were negative for PD-L1. Fifty-six (56; 8.9%) were positive for PD-L1. Of those that were PD-L1 positive, were equally distributed between primary site and metastatic sites (28/324, and 28/306, respectively).

Conclusion: In this retrospective analysis, we describe, to the best of our knowledge, the distribution of PD-1 and PD-L1 expression in one of the largest datasets reported in TNBC. Unlike prior reports showing a high PDL-1 expression in excess of 50% in TNBC, this analysis show a low distribution of PD-L1 positivity. Our cohort represents a biased sample as those were unselected patients with recurrent breast cancer. Additionally, other factors can be implicated, including a change in the antibody used. These findings call for future standardization of the PD-L1 assay, particularly if further exploration showed PD-L1 to be a predictive or prognostic biomarker in mTNBC, particularly in relationship to therapy with immune checkpoint blockade.

Citation Format: Baker TM, Gatalica Z, Goldstein LJ, Obeid E. Programmed death 1 (PD-1) and PD-1 ligand (PD-L1) distribution in triple negative breast cancer (TNBC) [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P2-05-09.