Renal fibrosis is a complex and prolonged course characterized by infiltration of inflammatory cells and damage to the renal parenchyma and interstitium, gradually leading to the formation of scar tissue [30]. The aberrant tissue repair allows excessive accumulation of ECM, including collagen (types I, III, and IV), fibronectin, and proteoglycans. Meanwhile, activated fibroblasts are transformed into myofibroblasts, and α-SMA is induced to generate constriction tension [31]. It seems difficult to change the irreversible fate of fibrosis simply by inhibiting the ECM sedimentation.
In recent years, NK cells have attracted increasing attention in the field of fibrosis. In 2017, elevated IFN-γ levels of CD56dim NK cells and CD56bright NK cells in specimens from patients with renal fibrosis were found for the first time to be associated with decreased renal function [8]. In response to cytokine stimulation, CD56+ cells carry cytotoxicity and proliferate in the renal tubule interstitial matrix, which in turn receives activation signals and exerts anti-inflammatory (IFN-γ) effects in CKD, which indicates that this is a pathogenic phenotype. In addition, the activated receptor CD335 and the differentiation marker CD117 were selectively co-expressed on CD56bright NK cells, and this co-expression is thought to be a source of pro-inflammatory cytokines as well as an intermediate factor in the development of fibrosis and deterioration of renal function [9, 32–34]. Zhang et al. found that NK cells can directly damage renal tubular epithelial cells in vitro and that perforin is also involved in their cytotoxic acquisition and deeply involved in fibrosis during their evolution [35–36]. Similarly, the accumulation of NK cells in renal tubules leads to irreversible renal fibrosis [37–39]. This is consistent with our data suggesting a correlation between renal fibrosis and increased perforin and IFN-γ.
It has been indicated that cGAMP and STING can activate NK cells [13, 40–41], and are correlated with CD4+ and CD8+ recruitment [42–43]. In our study, we found that STING/p-TBK1/p-IRF3 pathway protein expression was significantly higher in the FA group than in the NC and FA+ Rh groups, as was fibrotic pathology and ECM protein expression, which indicates that the fibrotic environment exerts an activating effect on STING and its downstream molecules and that this may be responsible for the recruitment of NK cells, CD8+ and CD4+ cells and the release of perforin and IFN-γ. A groundbreaking study demonstrated that the STING signaling is involved in the progression of renal fibrosis, but they focused on the upstream cGAS of STING, while the downstream of STING failed to be explored [44]. Together with their findings, we can draw the conclusion that CKD is accompanied by STING signaling activation, but the exact level of activation may be related to the modeling method and CKD staging. Furthermore, STING activation was also observed in the patients with liver fibrosis and animal models, while liver fibrosis and inflammatory responses were alleviated after STING downregulation [45], which is consistent with our experimental data, indicating that there exists a trigger for STING signaling in the fibrotic environment, and the specific molecule remains to be further screened. Given the role of STING in regulating multiple inflammatory and immune responses, an increasing number of studies suggest that STING is a promising new target for the treatment of liver, lung, and cardiac fibrosis [46–48]. The experiment in vitro showed that knockdown of STING could reduce TGF-β-promoted ECM deposition, and conversely, STING can accelerate TGF-β-induced fibrosis. Our study revealed in greater depth the activation of STING and its downstream in renal fibrosis patients and animal models, which may be responsible for the release of perforin and IFN-γ by immune cells. Therefore, new therapeutic strategies for renal fibrosis and chronic kidney injury could revolve around the inhibition of immune signaling pathways, including STING and its upstream and downstream, which may be of greater therapeutic significance in immune-related nephritis, such as lupus nephritis.
Previous studies have shown that Rh can significantly inhibit the compensatory hypertrophy and hardening of the glomerulus and the inflammatory response during the renal fibrosis progress. Meanwhile, it can reduce the excretion of urine protein and effectively improve the permeability of the glomerular filtration membrane and attenuate the loss of nephron [49–50]. Our study also demonstrated the renoprotective effects of Rh. Further investigation of the regulatory mechanisms of how Rh inhibits the release of cellular granules and inflammatory factors from immune cells may better predict its renoprotective outcome, which would allow us to more effectively utilize immunomodulatory therapeutic strategies to prevent kidney injury. Further investigation of the regulatory network and potential functions of immune cells in RIF requires the development of specific therapeutic agents capable of acting on the STING/TBK1/IRF3 signaling pathway for patients with fibrotic kidney disease.