Circulating myeloid-derived suppressor cells and survival in prostate cancer patients: systematic review and meta-analysis

Background Immunotherapy has not achieved improvement of survival in prostate cancer patients. Myeloid-derived suppressor cells (MDSCs) in tumor microenvironment can hamper its efficacy. Some preclinical studies explored the role of MDSCs in prostate cancer development. We aimed to verify the availability of studies exploring the prognostic effect of circulating MDSCs in prostate cancer patients. Methods We systematically selected studies for a meta-analysis, which compares survival between prostate cancer patients with high vs low circulating MDSC levels. We extracted or calculated hazard ratios (HRs) and relative 95% confidence intervals (CIs) in terms of overall survival (OS) from selected studies. We calculated the pooled HR and relative 95% CIs and estimated publication bias. Results Among 133 studies retrieved from search on Pubmed, 5 eligible studies (236 prostate cancer patients) met inclusion criteria. High circulating MDSC levels are associated with a worse OS (HR = 2.19; 95%CI = 1.51-3.17). Heterogeneity was not significant (I-2 = 0%; p = 0.64). Publication bias was also not significant (Egger's test, p = 0.09). Conclusions High levels of circulating MDSCs induce a worse OS in prostate cancer patients than in those with low levels. This finding supports the importance of MDSC detection and targeting also in prostate cancer patients.