A flow chart of this study
According to the process of this study, we drew a flow diagram (Fig. 1). By TCGA database, we successfully obtained the transcriptome data and corresponding clinical data of glioma. In total, we obtained data from 701 tumors and 5 normal samples. To evaluate the immune status of TME in glioma, we calculated the ImmuneScore, StromalScore, and ESTIMATEScore of each specimen by the ESTIMATE algorithm. To detect the prognostic significance of the three scores, survival analyses were performed, respectively. According to ImmuneScore and StromalScore, two kinds of DEGs were obtained and shared DEGs were identified by intersection analysis. To further screen candidate genes from the shared DEGs, we conducted univariate Cox regression analysis and PPI analysis towards the shared DEGs. The thirty most significant genes were selected from the univariate Cox regression analysis and PPI analysis, respectively. Three candidate genes, including BST2, CCL2, and RSAD2, were identified by intersection analysis. Next, we focus on BST2, and to further verify its expression and clinical prognostic significance by public database and our validation cohort, differential expression analysis, survival analysis, and correlation analyses were carried out. To investigate the mechanisms, GSEA was conducted and results indicated that BST2 might be involved in the regulation of tumor immunity. Relative contents of 22 TICs in the TME of every glioma tissue were calculated using the CIBERSORT algorithm. To detect the key TICs related to BST2, we conducted differential analyses of TICs contents according to BST2 expression, followed by correlation analyses between BST2 expression and TICs contents. Seven key TICs were identified by intersection analysis, and five of them, including M2 macrophages, regulatory T cells, eosinophils, resting mast cells, and activated mast cells, were found to be associated with prognosis by survival analyses. Thus, the TME-related gene BST2 might regulate the composition of TICs in glioma to induce deterioration.
ImmuneScore, StromalScore, and ESTIMATEScore were negatively correlated with OS in patients with glioma respectively
To evaluate the immune status of TME, we calculated the ImmuneScore, StromalScore, and ESTIMATEScore of every specimen by the ESTIMATE algorithm. In the light of the median of each score, we divided glioma samples into two groups respectively, followed by Kaplan-Meier survival analyses. As shown in Fig. 2, the three scores were negatively related to the OS respectively, i.e., compared with the low score groups, the overall survival time of the high score groups was shorter. Thus, the status of TME may influence the prognosis of glioma.
The three scores were related to the clinical characteristics of patients with glioma
Corresponding clinical data was successfully downloaded from the TCGA database. Correlation analyses between the three scores and clinical features were conducted respectively. Results showed that glioma patients over the age of 60 had significantly higher scores than those under the age of 60 (Fig. 3A, D, and G), and male glioma patients had significantly higher scores than female glioma patients (Fig. 3B, E, and H). Of particular importance, these three scores were positively correlated with tumor grades respectively, i.e., scores increased significantly as grade went from G2 to G4 (Fig. 3C, F, and I).
ImmuneScore- and StromalScore-related DEGs were involved in immune regulation
In the light of the medians of ImmuneScore and StromalScore, we divided glioma samples into two groups respectively, followed by gene difference expression analyses. As shown in heatmaps and Venn diagrams, there were 1642 DEGs in terms of ImmuneScore, 1202 of which were up-regulated and 440 down-regulated, and there were 1979 DEGs in terms of StromalScore, 1630 of which were up-regulated and 349 down-regulated. In addition, by intersection analysis of the two kinds of DEGs, we identified 1387 shared DEGs, 1100 of which were up-regulated and 287 down-regulated. The shared DEGs might be key genes regulating the TME (Fig. 4A, B, and C).
To explore the molecular biological functions of the shared DEGs, GO and KEGG analyses were performed. As shown in the bubble diagram and circos diagram of the GO analysis (Fig. 4D and E), shared DEGs were mainly enriched in GO items related to immune regulation, including leukocyte-mediated immunity, the external side of the plasma membrane, and antigen binding. In addition, as shown in the bubble diagram and circos diagram of KEGG analysis (Fig. 4F and G), shared DEGs are significantly enriched in signaling pathways related to cytokine-cytokine receptor interaction.
Further identification of candidate genes from shared DEGs
To further identify candidate genes from the shared DEGs, both univariate Cox regression analysis and PPI analysis towards DEGs were conducted (Fig. 5). As shown in the bar chart derived from PPI analysis, 30 genes with the most nodes were obtained. In addition, as shown in the forest plot derived from univariate Cox regression analyses, the 30 most significant genes were obtained. Finally, as shown in the Venn diagram of intersection analysis, three candidate genes, including BST2, CCL2, and RSAD2, were identified.
Verification of BST2 expression and its clinical prognostic significance via a public database and our validation cohort
To verify BST2 expression and its clinical prognostic significance, we performed differential expression analysis, survival analysis according to BST2 expression, and correlation analyses between the BST2 expression and clinical features. As shown in Fig. 6A, compared with normal specimens, BST2 expression in mRNA level was significantly increased in glioma samples. In our cohort, compared with corresponding adjacent normal tissues, the protein expression level of BST2 was higher in glioma tissues (Fig. 6F and G). In the light of the median of BST2 expression level, glioma samples of public datasets and our validation cohort were divided into two groups respectively, followed by survival analyses. Results suggested that the BST2 expression in mRNA and protein levels were negatively correlated with OS respectively (Fig. 6B and H). In correlation analyses between BST2 expression and clinical characteristics, BST2 expression was correlated with age to some extent, and was positively correlated with tumor grades, but not with gender (Fig. 6C-E).
Primary mechanism investigation via GSEA of BST2
To investigate the signaling pathways related to BST2 expression, GSEAs of high and low expression were performed respectively. High expression of BST2 mainly enriched in signaling pathways related to immune regulation, such as cytokine-cytokine receptor interaction, Toll-like receptor pathway, B cell receptor pathway, antigen processing, and presentation, and NK mediated cytotoxicity (ten significant signaling pathways are shown in Fig. 7A). In addition, low expression of BST2 is mainly enriched in signaling pathways associated with metabolisms, such as butanoate metabolism, vasopressin-regulated water reabsorption, and inositol phosphate metabolism (five significant pathways are shown in Fig. 7B). Thus, high expression of BST2 is closely related to the regulation of tumor immunity.
BST2 regulates the composition of TICs in TME
To evaluate the composition of TICs in the TME of glioma, the relative contents of 22 kinds of TICs in every glioma tissue were calculated using the CIBERSORT algorithm. The composition of TICs in glioma tissues as well as correlations between every two TICs were shown in Fig. 8. To verify BST2 participates in the regulation of TICs composition, difference analysis of TICs contents according to BST2 expression, as well as correlation analyses between BST2 expression and TICs contents were conducted respectively. Results indicated that 7 kinds of TICs showed significant differences (Fig. 9A). Furthermore, 8 kinds of TICs correlated (6 positively and 2 negatively) with the expression level of BST2 (Fig. 9B). Seven shared TICs, including M2 macrophages, regulatory T cells, eosinophils, M1 macrophages, activated mast cells, resting mast cells, and CD8+ T cells were identified by intersection analysis (Fig. 9C). According to the median of the content of each shared TIC, we divided glioma samples into two groups respectively, followed by survival analyses. Results indicated that patients with a higher content of M2 macrophages or resting mast cells or regulatory T cells had a shorter overall survival time, while those with higher content of activating mast cells had a longer OS time (Fig. 10). In conclusion, BST2 might influence the overall survival time in glioma by regulating the composition of TICs in TME.