Identification and enrichment analysis of differentially expressed IRGs
The RNA-Seq data from TCGA consisted of 263 cases, including 241 male GC cases and 22 male non-tumor cases. Compared to male non-tumor tissues, a total of 276 differentially expressed IRGs including 87 downregulated and 189 upregulated were screened out, with the cut-off criterion of |log2 fold change (FC)| > 1 and false discovery rate (FDR) < 0.05 (Fig. 1a).
Additionally, to further investigate potential functions of the differentially expressed IRGs in male GC patients, GO and KEGG analysis were perform in R software. As shown in Fig 1b, the results of GO enrichment analysis indicated that these IRGs can be significantly enriched in several important immune responses, including cell chemotaxis, leukocyte chemotaxis, myeloid leukocyte migration, leukocyte migration, granulocyte chemotaxis, neutrophil chemotaxis, granulocyte migration and neutrophil migration. KEGG analysis highlighted that the differentially expressed IRGs were mainly enriched in immune responses and tumor-related signaling pathways (Fig. 1c).
Identification of OS-related IRGs
A total of 20 differentially expressed IRGs were identified to be significantly associated with OS of male GC patients based on the result of univariate Cox regression analysis (Fig. 2a). Then, by performing multivariate Cox regression analysis, a prognostic signature consisting of seven IRGs (LCN12, CCL21, RNASE2, CGB5, NRG4, AGTR1 and NPR3) was selected to construct a prediction model (Table. 1). All the seven IRGs were associated with high risk with hazard ratios (HR) > 1. Among these IRGs, three genes (LCN12, RNASE2, and CGB5) were upregulated and four genes (CCL21, NRG4, AGTR1, and NPR3) were downregulated in male GC tissues compared to the normal tissues based on the TCGA dataset (Fig. 2b).
Establishment of the seven-IRGs risk signature
Subsequently, we constructed a prognostic model on the basis of the seven-IRGs. The risk score of each male GC patient was calculated as follows: risk score = expression level of LCN12 × 0.13933 + expression level of CCL21 × 0.00181 + expression level of RNASE2 × 0.01554 + expression level of CGB5 × 0.01036 + expression level of NRG4 × 0.34521 + expression level of AGTR1 × 0.14512 + expression level of NPR3 × 0.23201. The male GC patients were classified into high- and low-risk groups according to the median risk score. The distribution of risk scores and the survival status of male GC patients was displayed in Fig. 3a. In addition, the heatmap revealed the differentially expressed levels of the seven-IRGs in the high- and low-risk groups (Fig. 3b).
Next, the prognostic value of risk score was evaluated. Univariate Cox regression analysis showed that risk score (P < 0.001) was significantly correlated with OS of male GC patients (Fig. 4a). Notably, as shown in Fig. 4b, risk score could be an independent prognostic indicator. The Kaplan–Meier curve demonstrated that the male GC patients with high risk score had a shorter survival time than those with low risk score (logrank P < 0.001, Fig. 4c). The area under the curve (AUC) for risk score at 1-, 3- and 5-year in predicting OS was 0.73, 0.633 and 0.745, respectively (Fig. 4d). Moreover, compared to other clinical parameters, risk score had the highest performance in the survival prediction of male GC patients (Fig. 4e). Taken together, the above results indicated that the risk score performed well at predicting OS in the TCGA dataset.
Construction of theprognostic nomogram
To better predict the prognosis of male GC patients, we established a nomogram to predict the OS probability at 1-, 3- and 5-years (Fig. 5a). The variables of age, grade, stage and risk score were enrolled in the prediction model. The C-index of the nomogram was 0.695 (95% CI: 0.632–0.759). As shown in Fig. 5b, the calibration plots demonstrated the favorable agreement between predicted probabilities from the nomogram and observed outcomes. Collectively, these results implied that the nomogram had good reliability in predicting survival for male GC patients.
External validation of the seven-IRGs prognostic signature
To further examine the performance of the seven IRGs-based model, the gene expression data and survival outcomes from GSE15460 were used for external validation. We calculated the risk score with the same formula for each male GC patient, and then divided them into high‐ and low‐risk groups according to the median risk score. Compared with patients in low‐risk group, the male GC patients with high risk scores significantly suffered more survival risks (Fig. 6a). Similar to the above mentioned, the male GC patients in high-risk group were associated with poor survival outcomes (Fig. 6b). As presented in Fig. 6c, the AUCs of 1-, 3- and 5-year in predicting OS was 0.595, 0.621 and 0.657, respectively. Furthermore, the calibration plots indicated that quite good agreement between prediction and observation for the 3- and 5-year OS probabilities of the patients (Fig. 6d).
Associations of the risk signature with tumor-infiltrating immune cells
In order to investigate distinct patterns of immune infiltration, we used CIBERSORT algorithm to estimate the composition of 22 infiltrating immune cells types in each male GC sample. As presented by radar plot in Fig. 7a, the abundance of the 22 infiltrative immune cells were significantly different between high- and low-risk groups. Specifically, the infiltration levels of resting memory CD4 T cells (P = 0.034), activated NK cells (P = 0.003), regulatory T cells (Tregs) (P = 0.002), monocytes (P = 0.004) and resting mast cells (P = 0.008) were significantly higher in high-risk group compared with those in low-risk group, whereas the infiltration levels of activated memory CD4 T cells (P < 0.001), resting NK cells (P < 0.001), follicular helper T cells (P = 0.009) and M1 macrophages (P = 0.006) were opposite (Fig. 7b).