DEG screening and pathway enrichment analysis
A total of 88 up-regulated and 43 down-regulated genes were identified between AS and healthy samples in GSE73754 (Sup.Figure 1A), and 94 up-regulated and 107 down-regulated genes were identified in GSE25101 (Sup.Figure 1B). GO enrichment analysis of DEGs suggested the involvement of NK cell-mediated cytotoxicity (Figure 1A) in GSE73754 and positive regulation of cytokine production (Figure 1C) in GSE25101. GSEA analysis further confirmed the enrichment of NK cell-mediated immunity in both GSE73754 and GSE25101 datasets, with statistical significance (Figure 1B, 1D). These results suggested that innate immunity, especially NK cells, played a crucial role in the pathogenesis of AS.
Construction of co-expression network
We selected the dataset GSE73754 to further explore the role of the NK cell-mediated immune pathway in AS. Firstly, we calculated the enrichment scores of this pathway for each sample, and then identified 27 co-expressed gene modules through WGCNA analysis. After removing four poorly conserved gene modules, we obtained 23 co-expressed gene modules. We identified the gene module correlated with the enrichment scores by correlation analysis (Figrue 2A-2C), and found that the MElightyellow module was the most relevant to the NK cell-mediated immune pathway according to the heatmap (Figure 2D) (r=0.52, p=3e-6).
Diagnostic predictive value of selected genes
Using a Venn diagram, we identified 6 key genes (IL2RB, CD247, PLEKHF1, EOMES, S1PR5, FGFBP2) in the MElightyellow module that showed differential expression in both datasets (Figure 3A). We evaluated the diagnostic value of these genes using ROC curves and AUC values, and found AUC values ranging from 0.65 to 0.87, indicating moderate to high diagnostic value (Figures 3B, 3C). We then constructed a diagnostic prediction model using logistic regression and visualized the results using a nomogram (Figures 3D, 3F). Through calibration testing, the p-values of 0.935 (GSE73754) and 0.812 (GSE25101) indicated good model fit. The AUC values of 0.878 in GSE7375, 0.805 in GSE25101 demonstrated excellent discriminatory ability (Figures 3E, 3G). Overall, the predictive capability of the model was considered to be excellent.
We further analyzed the expression of these 6 genes in both datasets and their correlation with enrichment scores. Among them, IL2RB, CD247, S1PR5 and FGFBP2 showed differential expression between the AS and healthy groups in both datasets (Figures 4A-4B). Additionally, the expression of IL2RB, PLEKHF1, EOMES, S1PR5, and FGFBP2 showed significant correlation with the enrichment score of the NK-mediated immune pathway (Figures 4C-4D). These results indicated that IL2RB, CD247, PLEKHF1, EOMES, S1PR5, FGFBP2 might involve in dysfunction of NK cells leading to the pathogenesis of AS.
ScRNA-seq profiling, clustering and gene expression
We explored the role of the 6 genes mentioned above in NK cells using scRNA-seq data from GSE194315. After preprocessing scRNA-seq data based on the stringent quality control metrics as noted above, 15926 AS cells and 66433 control cells were included in the analysis (sup Figure 2). All cells were divided into eight clusters using PCA, tSNE, and UMAP classification methods (sup Figure 3). It was found that except for EOMES, the average expression level of the 6 genes in NK cells was higher than in other cells (Figure 5A). Further analysis of gene expression levels in NK cells revealed statistically significant differences in CD247, PLEKHF1, EOMES, S1PR5, and FGFBP2 between AS and control groups (Figure 5B).
Experimental validation of hub gene expression
We quantified the expression levels of these 6 genes in the peripheral blood of AS patients and healthy controls. And the results showed that the most highly expressed gene was FGFBP2, while the least expressed gene was PLEKHF1 (Figure 6). Notably, the expression of 5 of the 6 genes was significantly reduced in AS patients compared to healthy donors, which were CD247, EOMES, FGFBP2, IL2RB and S1PR5.
Gene upregulation after TNF‐α blocker therapy
In order to clarify the effect of TNF-α blocker on NK cells, we analyzed Jing Liu’s scRNA data from NK cells of AS patients treated with TNF-α blocker22. After a standard Seurat treatment pipeline, 7426 pre-treatment and 7648 post-treatment NK cells were included in further analysis (sup. Figure 4). Expression of PLEKHF1, S1PR5, and FGFBP2 was upregulated in post-treatment AS patients compared to pre-treatment AS patients, indicating that TNF-α blocker therapy reversed NK cell dysregulation in AS (Figure 7).