Differentially expressed immune-checkpoint genes in TNBC
Among the 79 immune-checkpoint genes, we identified a total of 43 genes from TCGA dataset, and extracted the expression profile of these genes. Compared with normal tissues, 20 immune-checkpoint genes were differentially expressed in TNBC tumor tissues, among which 19 genes were upregulated and 1 gene was downregulated. The results were visualized using the heatmap (Figure 1A) and the volcano plot (Figure 1B).
Functional enrichment analysis of DE-ICGs
To further investigate functions and signaling pathways these DE-ICGs were involved in, GO and KEGG pathway analysis were conducted. The GO terms “T cell activation”, “positive regulation of lymphocyte activation” and “regulation of leukocyte cell-cell adhesion” displayed significant enrichment in the category of biological process. For the category of cellular component, MHC protein complex was found to be enriched. For the molecular function, peptide antigen binding and antigen binding were the most abundant categories (Figure 2A). Meanwhile, KEGG pathway analysis revealed that these DE-ICGs were mainly associated with cell adhesion molecules, antigen processing and presentation, PD-L1 expression and PD-1 checkpoint pathway in cancer (Figure 2B).
Identification of prognosis-related ICGs
Firstly, univariate Cox regression analysis was used to identify candidate ICGs associated with overall survival (OS) of TNBC. 10 ICGs were identified as potential OS predictors of TNBC patients, including IDO1, CD274, PDCD1LG2, PDCD1, CTLA4, ICOS, KIR3DL2, HLA-B, HLA-F, LGA3 (Figure 3A). All these genes were protective factors. Comparison of mRNA expression of these genes in normal and TNBC tissues was shown in Figure 3B, and correlation between ten genes was depicted in Figure 3C. Next, LASSO regression analysis showed that 3 of 10 ICGs were the OS-related ICGs including PDCD1LG2, KIR3DL2 and PDCD1 (Figure 3D-E). Finally, the 3 OS-related ICGs were included in the multivariant Cox regression model (Table 2). The coefficients of the 3 OS-related ICGs were shown in Figure 3F.
Development and validation of an ICGs-based gene signature for individual survival prediction
Based on coefficients and expression level of the 3 genes, the gene signature was generated: Risk score=-0.20*ExpPDCD1LG2-3.20*ExpKIR3DL2-0.17*ExpPDCD1. The median risk score (-0.91) was used as cut-off point, and 113 patients were classified into two prognostic subgroups. As shown in Supplementary Figure 2, clinical parameters distribution and expression patterns of three genes were visualized in the form of heatmap between the two groups. The high-risk group tended to have advanced pathological stages and lower expression level of these hub genes. Next, survival analysis showed that patients in the high-risk group exhibited a worse prognosis (p=4.719E-04, Figure 4A). The distribution of risk score and survival status further corroborated the above conclusion (Figure 4B). We then evaluated the predictive accuracy of this gene signature by operating ROC curve analysis. The AUC for predicting 1-, 2- and 3-year OS was 0.925, 0.822 and 0.835, respectively (Figure 4C). To validate the robustness of this model, we also applied this gene signature to the METABRIC cohort. In the validation cohort, the survival of low-risk patients was superior to high-risk patients (Figure 4D, E). Moreover, ROC curves also indicated a fairly good predictive value of 3-ICGs signature in the validation cohort (Figure 4F).
Establish a nomogram for clinical application
Univariate analysis showed that radiotherapy (p=0.032), AJCC-stage (p<0.001) and risk score (p=0.003) were statistically significant in predicting the prognosis of TNBC patients (Figure 5A). Then, we investigated if the risk score was an independent predictor for clinical outcomes. As indicated by the multivariate analysis (Figure 5B), AJCC-stage (p<0.001) and risk score (p=0.018) were independent predictive factors for TNBC prognosis. By integrating above risk factors, we established a nomogram, which could facilitate clinicians to make risk quantification for TNBC patients (Figure 5C). By adding the points signed for each variable, we can obtain the total points and predict corresponding OS of each patient. Additionally, calibration curves revealed nomogram-predicted survival matched well with the actual observations, which laid a solid foundation for clinical application (Figure 5D-F).
Patterns of immune cell infiltration and immunotherapy response of TNBC patients
To evaluate the differences of immune status in two prognostic subgroups, enrichment scores of immune cell subsets and immune function pathways were quantified. As expected, adaptive immunity cells and innate immunity cells were evidently enriched in low-risk patients, demonstrating a high percentage of CD8+ T cells, B cells, DCs, macrophages and NK cells (Figure 6A). Higher enrichment scores were observed for cytokine-cytokine receptor interaction, check-point, cytolytic activity and T cell co-stimulation signaling pathways in low-risk group. Furthermore, the scores of type Ⅰ and Ⅱ IFN response were significantly higher than that in the high-risk group (Figure 6B). Altogether, these results indicated that effective immune response was more activated in the low-risk group, which may contribute to differences in the therapeutic efficacy. Meanwhile, we compared the immunogenicity in patients between two groups using IPS as a quantitative index for analysis. Our results showed that low-risk group obtained higher IPS of PD-1, CTLA4, PD-L1 and PD-L2 (Figure 6C). Similarly, the gene expression of CTLA4, PD-1, PD-L1 and PD-L2 was elevated in low-risk group (Figure 6D). We could speculate that low-risk group patients might get a favorable response to immunotherapy due to their higher immunogenicity.