Galectin-1 mediated chronic tumoral-STING activation promotes metastasis 1 through MDSC recruitment

To identify the differences in the MDSC population in the tumor verus pre-metastatic niche site, we performed RNA sequencing of isolated CD11b+ Ly6G+ MDSCs and analyzed the differentially expressed genes in PMN-MDSCs isolated from MOC2 tumors and corresponding lung tissues from the same mice To compare the MDSC RNA-level expression with neutrophil expression, a public mouse neutrophil microarray dataset was used (GSE60336). Our MDSC RNA-seq and the public mouse neutrophil data sets were merged using COMBAT55. DEGs for three groups (MDSC lung, MDSC tumor and neutrophil) were calculated using SAM56 multiple class comparison with false discovery rate smaller than 0.05. A total of 5 samples were generated. RNA was isolated from a minimum of 2x10^6 sorted PMN-MDSCs from tumors or lungs using a RNeasy micro Kit (Qiagen). Library preparation and RNA sequencing were performed by Novogene via Illumina platforms based on mechanism of SBS (Sequencing by synthesis).Expression levels of RNA sequencing data were quantified using a quasi-mapping two phase inference algorithm implemented in Salmon 635 (version 0.9.1). Transcript sequences for mouse M24 from Gencode (https://www.gencodegenes.org) were used for reference transcriptome indexing. Transcript-per-million (TPM) value was used as normalized expression level unit. Quantified gene expressions were then log transformed for downstream analysis. Differentially expressed gene (DEG) analysis between MDSC tumor group and MDSC lung group was performed on each gene expression and the p-values were corrected using false discovery rate. DEGs were defined by false discovery rate smaller than 0.05 and fold change higher than 1.5.