Abstract 2369: HPRT overexpression may contribute to the immunosuppressive tumor microenvironment
Authors: Townsend Michelle H. , Ewell Zachary D. , Freitas Claudia M. Tellez , Larsen Dallas J. , Bitter Eliza L. , Bennion Kelsey B. , Piccolo Stephen R. , Weber K Scott , Robison Richard A. , O'Neill Kim L. 
Published: August 2019

Abstract
The purpose of this study was to examine the possible effects that Hypoxanthine Guanine Phosphoribosyltransferase (HPRT) may have on the immune response in the tumor microenvironment. HPRT is a salvage pathway enzyme responsible for recycling guanosine throughout the cell cycle, and we have shown that it is significantly upregulated in numerous solid malignancies. As a necessary component of the S phase in the cell cycle, guanosine has also been regarded as a potential neuroprotectant in the central nervous system (CNS). We thus hypothesized that there may be a correlation between the observed upregulation of HPRT and the relative downregulation of the immune system mediated by the differential regulation of guanosine. Using data from The Cancer Genome Atlas (TCGA), we analyzed over 200 cytokine genes and found a negative correlation between the level of HPRT expression in cancer and the general expression level of both pro-inflammatory and anti-inflammatory cytokine expression. We further analyzed data showing the relative infiltration level of individual immune cell subsets in solid tumors compared with elevated HPRT levels. We found the most significant negative correlations between HPRT expression and immune cell infiltration for B-cells (p &lt; 0.001), CD8+ T-cells (p &lt; 0.01), CD4+ T-cells (p &lt; 0.001), macrophages (p &lt; 0.001), neutrophils (p &lt; 0.001), and dendritic cells (p &lt; 0.001) in lung squamous cell carcinoma. Other significant negative correlations were found in kidney renal clear cell carcinoma, prostate adenocarcinoma, and breast invasive carcinoma. To further analyze this reduction in infiltration, we examined the relationship between HPRT expression and both costimulatory and coinhibitory molecules and found a significant negative correlation. Because of these observed associations between HPRT levels and cytokine expression, we hypothesized that the increased guanosine production was the mechanism by which HPRT was affecting the immune system in the tumor microenvironment. To test this, we examined the level of Ca2+ influx using Fluo-4 assays of B-cell (Raji), T-cell (Jurkat), and monocyte (THP-1) cell lines when treated with guanosine as a measure of overall cell activation. We then compared this activation to untreated cells and cells treated with adenosine, a known anti-inflammatory agent. When treated with guanosine, a significant reduction in activation was found in THP-1s (p &lt; 0.01) and Rajis (p &lt; 0.01), but not in Jurkats. However, under hypoxic and hypoglycemic conditions similar to those commonly found within the tumor microenvironment, Jurkat activation was significantly reduced (p &lt; 0.05) when treated with guanosine. From this data, we conclude that guanosine levels significantly reduce immune cell activation in the tumor microenvironment and that the upregulation of HPRT in malignant tissue may be one of the contributing factors of the immunosuppressive nature of the microenvironment.
Citation Format: Michelle H. Townsend, Zachary D. Ewell, Claudia M. Tellez Freitas, Dallas J. Larsen, Eliza L. Bitter, Kelsey B. Bennion, Stephen R. Piccolo, K Scott Weber, Richard A. Robison, Kim L. O'Neill. HPRT overexpression may contribute to the immunosuppressive tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2369.