Snail promotes ovarian cancer progression by recruiting myeloid-derived suppressor cells via CXCR2 ligand upregulation

Snail is a major transcriptional factor that induces epithelial-mesenchymal transition (EMT). In this study, we explore the effect of Snail on tumor immunity. Snail knockdown in mouse

ovarian cancer cells suppresses tumor growth in immunocompetent mice, associated with an increase of CD8+ tumor-infiltrating lymphocytes and a decrease of myeloid-derived suppressor cells

(MDSCs). Snail knockdown reduces the expression of CXCR2 ligands (CXCL1 and CXCL2), chemokines that attract MDSCs to the tumor via CXCR2. Snail upregulates CXCR ligands through NF-kB

pathway, and most likely, through direct binding to the promoters. A CXCR2 antagonist suppresses MDSC infiltration and delays tumor growth in Snail-expressing mouse tumors. Ovarian cancer

patients show elevated serum CXCL1/2, which correlates with Snail expression, MDSC infiltration, and short overall survival. Thus, Snail induces cancer progression via upregulation of CXCR2

ligands and recruitment of MDSCs. Blocking CXCR2 represents an immunological therapeutic approach to inhibit progression of Snail-high tumors undergoing EMT.

Emerging evidence suggests that the acquisition of invasiveness in cancer is accompanied by the loss of epithelial features and the gain of a mesenchymal phenotype, a process known as

epithelial-to-mesenchymal transition (EMT)1,2. In previous reports, gene expression clustering in ovarian cancer showed that the mesenchymal subtype, comprising enriched EMT-related gene

signatures, had poor survival compared to other subtypes1,3,4. We discovered that there is decreased number of intraepithelial CD8+ tumor-infiltrating lymphocytes (CD8+TILs) in the

mesenchymal subtype4. Thus, immune evasion might be taking place in the tumor undergoing EMT, although the mechanism of suppression of anti-tumor immunity in the state of EMT remains

unclear.

Immune evasion is one of the major hallmarks of cancer5, often accomplished via the recruitment of immunosuppressive cells such as myeloid-derived suppressor cells (MDSCs) or through the

expansion of an immune checkpoint signal, namely the programmed death 1 (PD-1)/PD-1 ligand 1 (PD-L1) axis6,7,8,9,10,11. MDSCs represent a heterogeneous immature immunosuppressive myeloid

cell population that expands during cancer progression and has the remarkable ability to suppress T cell functions in the tumor microenvironment7. Since MDSCs were officially described in

2007, an increasing number of studies have reported the biological and clinical significance of MDSCs6,7,12,13. We previously reported that MDSC infiltration was inversely correlated with

CD8+TIL numbers and shorter overall survival in advanced ovarian cancer14. These reports indicated that MDSCs, as key players in cancer immune evasion, might be used both as prognostic

factors and as therapeutic targets in cancer treatment.

Here, we focus on Snail, a key transcriptional repressor of E-cadherin during EMT15,16, and explore the influence of Snail on MDSC infiltration into ovarian tumors. Elucidating the

mechanisms of Snail-induced immune evasion leads to the potential development of novel treatment strategies for tumor undergoing EMT.

We first analyzed the dataset of high-grade serous ovarian cancer (HGSOC) from The Cancer Genome Atlas (TCGA) (n = 266). Application of a generic gene signature of EMT3 to score the EMT

status of the TCGA samples revealed that Snail expression was positively correlated with EMT scores (Fig. 1a). TCGA samples were divided into four molecular subtypes (differentiated,

immunoreactive, mesenchymal, and proliferative) based on unsupervised hierarchical clustering using gene expression analysis1. Among the four subtypes, the mesenchymal subtype, which is

associated with the poorest prognosis and has an EMT-related expression signature, had the highest Snail expression (Fig. 1b).

Snail expression is related to epithelial-to-mesenchymal transition and poor prognosis in ovarian cancer. a Correlation between generic epithelial-to-mesenchymal transition (EMT) scores for

266 high-grade serous ovarian cancer (HGSOC) samples from TCGA dataset1 and Snail expression. The EMT signature was acquired from a previous report by Tan et al.3. Generic EMT scores were

calculated following the method of single-sample Gene Set Enrichment Analysis (ssGSEA). The correlation coefficient (R) and P-value were based on Pearson’s product-moment correlation

analysis; R = 0.41 P