High-throughput microarray technology has emerged recently as a fast and effective bioinformatics technique. It has offered a platform for disease diagnosis, treatment, and innovative medication discovery in addition to providing a framework for screening vital genes linked to the onset and progression of a variety of diseases. This is the first study that has applied the LASSO technique for identifying the hub genes related to the progression of the carotid atherosclerotic plaques. 177 DEGs were identified in this report. According to the GO enrichment analysis, DEGs were predominantly enriched in processes related to leukocyte activation and migration, antigen processing and presentation, immunological response, and cytokine generation, all of which are linked to the development of the carotid atherosclerotic plaques [8, 16]. Higher levels of Inflammatory cytokines (IL-1, TNF-α, and IL-6), adhesion molecules, and chemokines have been linked to the recruitment and infiltration of immune cells into the subendothelium, which results in plaque formation, rupture, and thrombus formation [8, 17, 18]. According to the analysis of KEGG signaling pathways, DEGs were mainly enriched in pathways linked to inflammatory diseases and the immune system (i.e., pathways related to NF-κB signaling, TLR signaling, and NET formation). Given that NF-κB plays a crucial regulatory function in immunity, apoptosis, stress responses, and cell differentiation, the NF-κB signaling pathway has long been thought of as a classic proinflammatory signaling system [19, 20]. Earlier research has demonstrated that NF-κB is activated in atherosclerotic lesions [21]. NF-κB is a rapid response transcription factor that plays a role in immunological and inflammatory responses by stimulating the production of many immune cells like growth factors, chemokines, cytokines, cell adhesion molecules, and immunoreceptors [22]. These gene products trigger an immune-inflammatory response that damages vascular walls and affects vascular cell function, resulting in the progression of arteriosclerotic plaques [23]. Recent research has shown that inhibiting the NF-B signaling system can lessen the inflammatory burden and could be a promising anti-atherosclerotic, anti-inflammatory, anti-angiogenic, and anti-apoptotic therapeutic target [24, 25].
TLRs, described as established pattern recognition receptors in the immune system, have the ability to recognize pathogen-associated molecular patterns expressed by a variety of infectious agents and provide a strong link between local innate and adaptive immunity [26]. When these receptors get activated, they trigger the intracellular signaling cascade that is MyD88 or TRIF-mediated, which eventually stimulates the production of the pro- and anti-inflammatory cytokines [27]. TLRs transmit activating signals to secrete pro-inflammatory cytokines in large quantities, thus inducing the transitional activation of inflammation, which can play a vital role in the advancement of early-stage plaques to their advanced stages [28]. Large, extracellular, web-like structures known as NETs are primarily released through a process of neutrophil cell death known as NETosis to trap and kill microbes in particular physiological conditions; NETs are composed of extracellular strands of decondensed DNA in complex with granule proteins and histones [29, 30]. Pathologically speaking, NETs can, however, increase the inflammatory response by inducing the activation of inflammatory factors and cells, which promotes the progression of plaques from their early stage to advanced stages [31, 32]. Additionally, the interaction between the NF-κB signaling pathway and NET aggravated atherosclerosis [33]. The results of the DEG enrichment analysis and the findings from these investigations were in good agreement, indicating that DEGs were primarily responsible for the advancement of the early-stage carotid atherosclerotic plaques to their advanced stage.
WGCNA and LASSO, which are widely used as common methods of bioinformatic analysis, were used for avoiding the drawbacks of the conventional DEG-based screening techniques [34] and improving the accuracy of the screening of target feature-linked genes [35]. The genes whose expression was highly correlated with advanced-stage carotid atherosclerotic plaques identified using WGCNA were matched with previously identified DEGs to identify genes with both differential expression and correlation. Five hub genes, i.e., C3AR1, FERMT3, GIMAP4, SLAMF8, and TMEM176A, were eventually screened via LASSO. The expression levels of the above 5 hub genes showed a significant difference between the early-stage and the advanced-stage carotid atherosclerotic plaques and were validated in an external dataset. Specifically, these five genes showed significantly high expression and good diagnostic efficacy in advanced-stage carotid atherosclerotic plaques.
C3AR1 encodes for an orphan G protein-coupled receptor for the C3a. The gene product can induce inflammation by binding to complement C3a [36]. C3AR1 has a crucial role in TLR activation in innate DCs and influences effector T cell responses [37]. The expression of C3AR1 was higher in carotid plaques than in control arteries [38]. This suggests that C3AR1 overexpression is involved not only in disease onset but also in disease progression. FERMT3 encodes for important cytoplasmic proteins that are needed for platelet aggregation, leukocyte transmigration, integrin activation, and thrombosis [39, 40]. FERMT3 overexpression suppressed NF-κB activation and induced apoptosis [41]. The activation of the integrin-mediated platelet aggregation and adhesion triggered arterial thrombosis and several cardiovascular events [42]. Platelets are seen to play a crucial role in the promotion of inflammation and foam cell formation, leukocyte adhesion, and transmigration in the vessel wall, which, in turn, promoted the carotid atherosclerotic plaque progression [43, 44]. FERMT3 was upregulated in arterial plaques compared with that in healthy controls, and confocal immunofluorescence analysis showed the colocalization of FERMT3 with CD68-positive cells [45]. This is in good agreement with the correlation analysis data for the expression of the 5 hub genes within the immune cells. These data indicate that FERMT3, as an important modulator of integrin-mediated mechanisms, participated in the advancement of the carotid atherosclerotic plaques. Proteins of the GIMAP family were deferentially regulated during the human Th cell differentiation and were associated with immune-mediated disorders in an earlier animal study [46]. GIMAP4 was upregulated by IL-12 and other Th1 differentiation-inducing cytokines in cells differentiating toward a Th1 lineage and downregulated by IL-4 in cells differentiating toward a Th2 [47]. GIMAP4 could accelerate T-cell apoptosis induced by caspase-3 activation and phosphatidylserine exposure, which contributed to the Th-cell subtype-triggered immunological balance [48]. As demonstrated by our results, GIMAP4 expression showed a negative correlation with CD4+T cell abundance. The SLAM family (SLAMF) comprises a group of nine structurally related hematopoietic cell-specific receptors that are differentially expressed and play different roles in many immune cells [49]. SLAMF8, a nonclassical SLAMF member, does not include signaling motifs in its short cytoplasmic tails, in contrast with normal SLAMF receptors. In the past, researchers have shown that a combined SLAMF8 deficiency suppressed the inflammatory responses via the mechanism of downregulating the expression of TLR4 on the macrophages [50, 51]. A deletion in SLAMF8 significantly inhibited TLR4 upregulation and NF-κB activation [51]. TMEM176A is involved in tumor development [52, 53]. TMEM176A inhibits the maturation and activation of DCs to modulate DC function, thereby influencing the development of innate and adaptive immunity [54, 55]. Based on these results, C3AR1 and FERMT3 are significantly involved in the development of the carotid atherosclerotic plaques. Although GIMAP4, SLAM8, and TMEM176A are closely associated with inflammatory responses, their exact roles in the advancement of the carotid atherosclerotic plaques are not clear and need to be investigated further.
Furthermore, the differences noted in the immune cell infiltration between the advanced- and early-stage carotid atherosclerotic plaques were determined using the ssGSEA algorithm. It was noted that the advanced-stage carotid atherosclerotic plaques showed a significantly higher infiltration level of the immune cells like CD8+ T, CD4+ T cells, Tregs, monocytes, macrophages, T helper (Th) cells, and DCs compared to the early-stage, atherosclerotic plaques. Th17 cells are derived from CD4+ T cells, and they got their name because they can secrete high levels of IL-17 [56]. Th17 cells promote plaque fibrosis, whereas Th1 cells promote the formation of atherosclerotic plaques [57]. Tregs are often regarded to exert a protective function in atherosclerotic plaque formation [58]. However, Tregs show the loss of FOXP3 expression and immunosuppressive function during atherosclerosis progression, owing to which a fraction of these cells is transformed into follicular Th cells, which are pro-atherogenic [59]. These findings illustrated the essential role played by the T-cell immune homeostasis disruptions in the development of carotid atherosclerotic plaques.