Programmed cell death 1 ligand 1 (PD-L1, B7H1) is a cell surface protein that suppresses the cytotoxic CD8+ T-cell mediated immune response. PD-L1 expression and its clinical relevance in sarcomas are not well understood. Therefore, we sought to measure RNA expression levels for PD-L1 in 38 clinically annotated osteosarcoma tumor samples, and aimed to determine if PD-L1 expression correlates with clinical features and tumor-infiltrating T-lymphocytes (TILs). Quantitative real-time RT-PCR for PD-L1 was optimized in 18 cell lines, of which 5 were osteosarcoma-derived. qRT-PCR results were validated via flow cytometry and immunohistochemistry (IHC) in select cell lines. Total RNA was isolated from 38 human osteosarcoma samples for qRT-PCR analysis. Clinical data was sorted and significance was determined by Student t-test. TILs were examined in patient samples by TMA HE staining. We confirmed constitutive PD-L1 mRNA expression in cell lines by qRT-PCR, flow cytometry, and IHC. Across human osteosarcoma samples, PD-L1 mRNA gene expression ranged over four-log (>5000-fold difference). Relative expression levels were evaluated against clinical factors such as age/gender, metastasis, recurrence, chemotherapy, percent necrosis, and survival and no significant associations were identified. TILs did correspond to high PD-L1 expression (R2=0.37, P=0.01). In summary, we developed an RNA-based assay to determine PD-L1 expression levels and we show for the first time that high levels of PD-L1 are expressed in a subset of osteosarcoma, and PD-L1 expression is positively correlated with TILs. There are multiple agents targeting PD-1/PD-L1 in clinical development, and this may be a novel immunotherapeutic strategy for osteosarcoma clinical trials.
- Received December 18, 2013.
- Revision received March 7, 2014.
- Accepted April 2, 2014.
- Copyright © 2014, American Association for Cancer Research.