Abstract 3978: B7-H1/PD-L1 soluble expression in multiple cancer subtypes: High sensitivity measurement by immunoassay

Aberrant expression of the B7-H1/PD-L1 immunomodulatory molecule is evident in many forms of cancer. The evidence for increased protein levels of B7-H1/PD-L1 comes largely from immunohistochemistry analysis of surgical and biopsy specimens. While several studies attempt to correlate B7-H1/PD-L1 expression with poor prognosis, there are studies that conclude no correlation based on technical difficulties associated with detecting B7-H1/PD-L1 protein expression on the cell surface. We were able to quantifiably measure soluble B7-H1/PD-L1 in conditioned medium from tumor cell cultures using two new immunoassays, a Quantikine ® ELISA and a Simple Plex™ Assay. We show that breast, prostate, glioma, and non-Hodgkin’s lymphoma cells express various levels of B7-H1/PD-L1 which correlate to their PTEN or proliferation status. This is in agreement with previous findings showing that increases in B7-H1/PD-L1 expression levels are frequently coincident with loss of the PTEN tumor suppressor or increased activation of the phosphoinositide 3-kinase (PI 3-K) pathway. The androgen-responsive LNCaP prostate cancer cell line does not exhibit measurable B7-H1/PD-L1 in any instance, while the androgen-independent prostate cancer cell line PC-3 expresses measurable B7-H1/PD-L1 that is further inducible with PMA treatment. In all cell lines included in our study, incubation with PMA increases the level of B7-H1/PD-L1 released into the culture media, while pretreatment with the PI 3-K inhibitor, LY294002, abrogates this increase. Importantly, the Simple Plex platform is able to quantify very low levels of B7-H1/PD-L1 that are below detection by ELISA. Our data suggest that combining the Quantikine ELISA and Simple Plex platforms is a powerful strategy for reliably and quantitatively assessing soluble B7-H1/PD-L1 levels. Citation Format: Justin Haworth, Rainer Grant, Samuel Thayer, Melissa Floren, Nathan Steere, Isabel O9Brien, Paul Younge, Michael Anderson, Greta Wegner, Kathryn Brumbaugh. B7-H1/PD-L1 soluble expression in multiple cancer subtypes: High sensitivity measurement by immunoassay [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3978. doi:10.1158/1538-7445.AM2017-3978