Myeloid-derived suppressor cells exhibit two bioenergetic steady-states in vitro

Growing tumours have acquired several mechanisms to resist to immune recognition. Among these strategies, myeloid-derived suppressor cells (MDSCs) contribute to tumour escape by suppressing T-cell specific anti-tumoural functions. The development of therapies that could specifically inhibit MDSC maturation, recruitment, accumulation and immunosuppressive functions is thus of great interest. This requires the identification of valuable biomarkers of MDSC behaviour in vitro. As for immune cells, whose energetic state is known as a biomarker of their functionality, we have characterized in vitro the metabolic and energetic behaviour of MSC-1 cells, an immortalized cell line derived from mouse MDSCs and used as model cell line. Combined results from in vitro(31)P-NMR with living cells and HPLC-MS analyses from cell extracts allowed to identify two distinct bioenergetic steady-states that coincided with exponential and stationary growth phases. While the adenylate energy charge remained constant throughout the culture duration, both the percentage of total pyrimidines, the UTP-to-ATP and PME (phosphomonoesters)-to-NTP ratios were higher at the exponential growth phase compared to the plateau phase, suggesting metabolically active cells and the production of growth-related molecules. Conversely, the NTP ratio increased at the entry of the stationary phase revealing the deterioration of the global bioenergetic status and the arrest of anabolic processes.