Screening candidate DE IRGs
In total, 288 DE IRGs were screened out with the cutoff criteria of |log2FC| > 1 and FDR < 0.01, including 221 upregulated and 67 downregulated genes. For further validation and selection of DE IRGs with significantly characteristic values to classify primary melanoma (PM) and metastatic melanoma (MM), the LASSO algorithm was used to identify a set of 190 DEGs (Fig. 1a, b) and the SVM-RFE algorithm was used to select a set of 55 DEGs (Fig. 1c, d). Finally, after combing the LASSO and SVM-RFE algorithms, the 207 most representative DEGs were identified, with 38 genes selected simultaneously by the two algorithms (Fig. 1e).
Construction of a prognostic immune gene signature
In this study, each DE IRG was first submitted for univariate Cox proportional hazards regression with the criteria of a P value < 0.05 (Fig. S1a, Additional file 1). Then, multivariate Cox regression analysis was performed to develop the prognostic model and the Concordance index was 0.7, which indicated the high predictive accuracy of the signature for survival (P < 0.05; Table 1; Fig. S1b, Additional file 1). The prognostic risk score was calculated as follows: (0.182*S100A12) + (0.390*CCRL2) + (-0.587*CD86) + (-0.290*IL21R) + (0.199*CCR4) + (0.375*FCGR3A) + (-0.350*KLRD1) + (-0.147*IL18RAP) + (0.460*IL2RB) + (-0.211*CCL8).
Melanoma patients were assigned to the high- and low-risk groups based on the risk score model. Distribution of risk score, patients’ survival status, and gene expression profiles associated with the risk score are shown in Fig. 2a. Kaplan-Meier analysis demonstrated that melanoma patients with a high-risk score showed dramatically worse prognosis than those with a low-risk score (Fig. 2b), and the area under the curve values of the time-dependent ROC curve were 0.731, 0.774, and 0.76 for 3-, 5-, and 10-year survival, indicating the high specificity and sensitivity of the prognostic signature (Fig. 2c).
Table 1
Prognostic value of the 10 prognostic IRGs investigated by multivariate Cox regression analysis.
IRGs | coef | exp(coef) | se(coef) | z | P value |
S100A12 | 0.182 | 1.200 | 0.042 | 4.360 | 1.30E-05 |
CCRL2 | 0.390 | 1.477 | 0.112 | 3.483 | 0.000496 |
CD86 | -0.587 | 0.556 | 0.160 | -3.674 | 0.0002383 |
IL21R | -0.290 | 0.748 | 0.087 | -3.334 | 0.0008559 |
CCR4 | 0.199 | 1.220 | 0.070 | 2.856 | 0.0042891 |
FCGR3A | 0.375 | 1.455 | 0.110 | 3.413 | 0.0006419 |
KLRD1 | -0.350 | 0.705 | 0.088 | -3.983 | 6.81E-05 |
IL18RAP | -0.147 | 0.863 | 0.070 | -2.097 | 0.0359954 |
IL2RB | 0.460 | 1.583 | 0.091 | 5.060 | 4.20E-07 |
CCL8 | -0.211 | 0.810 | 0.060 | -3.493 | 0.0004771 |
Stratification analyses of the 10-gene prognostic signature
Considering the potential impacts of clinical characteristics on the risk score of the prognostic model, a Chi-square test was performed. The risk score exhibited a higher level in event and the American Joint Committee on Cancer-Tumor (AJCC-T) subgroups; there were no significant differences in the risk score, AJCC-Nodes (AJCC-N), AJCC-Metastasis (AJCC-M), Stage, Clark, Primary Tumor, and the BRAF and NRAS subgroups (Fig. S2a, Additional file 1). To assess whether the prognostic classifier was an independent indicator in distinct subgroups, we checked the effect of BRAF and NRAS mutation or wild-type on the prognostic ability for melanoma in TCGA cohort. Kaplan-Meier analysis revealed that patients in the high-risk group were associated with a higher mortality risk in the BRAFwt and NRASwt or BRAFmut and NRASmut subgroups (Fig. S2b-S2e, Additional file 1). Additionally, patients in stages I and II or stages III and IV were divided into 2 groups with the median value. We found that patients with high risk were classified into the group with a worse prognosis, compared to patients with a low risk (Fig. S2f, S2g, Additional file 1). These results demonstrated the robust and predictive power of this prognostic model, in which patients high-risk scores had a shorter overall survival than those with low risk scores in each stratum.
Construction of a nomogram model
To develop a clinically relevant quantitative approach for predicting the survival probability of a patient with melanoma, we constructed predictive nomograms. Based on univariate analysis (Fig. 3a), we generated two nomograms to predict the death odds of patients using logistic regression and survival rate with Cox regression analysis (Fig. 3b, c). The calibration plots for the 5-and 10-year survival showed an optimal agreement between the nomogram-predicted and observed OS (Fig. 3d).
Functional traits of the prognostic signature
To explore the potential cause of the prognostic signature, we divided the melanoma patients from TCGA database into high- and low-risk groups, based on the median risk score. After edgeR filtering (|log2FC| > 1 and FDR < 0.05), we screened out 1251 DEGs, including 552 upregulated and 699 downregulated genes (Fig. S3a, Additional file 1). Of these DEGs, 26 genes were immune-related and are highlighted in Fig. S3a. GO enrichment analysis revealed that upregulated genes were significantly enriched in multiple pathways including T cell activation, regulation of T cell activation, and regulation of lymphocytes (P < 0.05; Fig. S3b, Additional file 1). Moreover, downregulated genes were significantly enriched in epidermis development, skin development, and keratinization (P < 0.05; Fig. S3c, Additional file 1). GSVA showed that patients with low risk scores exhibited increased expression of proteins associated with the interferon gamma response, allograft rejection, and interferon alpha response (Fig. S3d, Additional file 1). These findings indicate differences in the immune-related genes and signaling pathways between high- and low-risk groups, which might partly explain the reason for the significant difference in prognosis between the subgroups.
The risk score was associated with immune cell infiltration
Immune cell infiltration status was assessed by applying the ssGSEA approach to the melanoma transcriptomes. Twenty-four immune-related terms were incorporated to deconvolve the abundance of diverse immune cell types in melanoma. The whole cohort was clustered into two clusters in terms of immune infiltration by applying the IRGs signature (Fig. 4a) and the relative immune score in ssGSEA was shown in Fig. S4a. Subsequently, a TME cells network was constructed with three main clusters depicting a comprehensive landscape of tumor-immune cell interactions and cell lineage, and their effects on the OS of melanoma patients (Fig. 4b; Fig. S4b, Additional file 1). ssGSEA was used to assess the relative proportions of the 24 immune cells in each CC sample. The abundance of aDC, B cells, CD8 T cells, cytotoxic cells, DC, macrophages, NK CD56dim cells, pDC, T cells, T helper cells, Tcm, TFH, Th1, and Treg cells was low in the 10-IRG signature high-risk group and associated with better OS. The abundance of eosinophils and mast cells was high in the 10-IRG signature high-risk group and associated with poor OS.
The immune infiltration in melanoma tissues between the high- and low-risk groups was then investigated using the CIBERSORT algorithm. The proportion of 22 immune cells in each subgroup is shown in a bar plot (Fig. 5a). The results revealed that plasma cells, CD8 T cells, CD4 memory activated T cells, follicular helper T cells, and M1 macrophages were negatively correlated with the risk score (Fig. 5b, c) and that M0 macrophages, M2 macrophages, activated DCs, and neutrophils were positively correlated with the risk score (Fig. 5b, c). Figure 5d indicated the poor correlation coefficient between 22 immune cells. The population with negative relation included M0 macrophages and CD8 T cells (r = -0.61), and CD4 memory resting T cells and CD8 T cells (r = -0.57). Further, neutrophils and activated mast cells had a positive relation (r = 0.71). After the ESTIMATE algorithm was processed, a higher Estimate score was found in the low-risk group. Similarly, the fraction of immune and stromal cells was associated with the low-risk group (Fig. 5e).
Different tumor mutation burden (TMB) patterns between two risk groups
We defined and calculated the TMB variable between low- and high-risk groups. We also assessed the correlation between risk score and mutated genes and the mutant rate in these 47 mutants, which were distributed in more than 10% of melanoma samples, and were significantly associated with risk scores at P value < 0.05 (Fig. 6a). The mutational landscape indicated that mutation events occurred more frequently in the low-risk group than in the high group (Fig. 6b, c). Moreover, we further analyzed the survival significance of TMB in melanoma and found that lower TMB levels were associated with worse overall survival outcomes (Fig. 6c).
To substantiate the stability of the prognostic signature, three external validation cohorts were analyzed. For the external validation cohort 1 and 2, GSE19234 and GSE22155 contained 44 and 57 melanoma patients, respectively. Consistent with previous results, the low risk group had higher levels of immune cell infiltration (Fig. S5a, S6a, Additional file 1). The relative immune score is shown in Fig. S5b and S6b. As expected, patients in the high-risk group had a significantly increased mortality risk compared with those in the low-risk group (Fig. S5c, S6c, Additional file 1). For the external validation cohort 3, GSE35640 contained 65 patients with MAGE-A3 antigen-specific cancer immunotherapy. Similar analysis showed that the low-risk group had higher levels of immune cell infiltration (Fig. S7, Additional file 1). Furthermore, the prognostic model demonstrated the potential to predict the effect of MAGE-A3 immunotherapy in melanoma patients (Fig. S7, Additional file 1).