In this study, 97 DEGs, of which 50 were upregulated and 47 were downregulated, were identified in progressive CKD samples compared with stable CKD samples in the array dataset GSE45980. Enrichment analysis showed that upregulated DEGs were significantly enriched in pathways related to the immune response, inflammatory response, and NF-κB signaling pathway, which was also recognized as the key process for the progression of CKD to end-stage renal disease. The gene CCR7, which was upregulated in progressive CKD samples, was enriched in the above GO-terms and KEGG pathways, and presented the highest interaction degree in the PPI network. Using an in vitro model of renal interstitial fibrosis model, we identified that the expression level of CCR7 was concomitantly increased with the aggravation of oxidative stress.
Chemokines play a key role in numerous immune processes, including immune cell development, immune response initiation, and the pathophysiological recruitment of immune cells in infections and diseases. Functionally, chemokines can be divided into two categories: inflammatory chemokines, which are mainly expressed during activation of the immune response, and steady-state chemokines, which are constitutively expressed in discrete locations without obvious activation stimuli . Nonetheless, most chemokines have both functions. CCR7 is an essential chemokine necessary to maintain homeostasis. Under homeostatic conditions, low levels of CCR7 in the lymph nodes contributes to peripheral tolerance. However, when encountering inflammatory mediators, pathogens or tissue damage, dendritic cells (DCs) express high amounts of CCR7, which in turn induces an immune response [20, 22].
CCR7 has only two constitutive ligands, CCL19 (also known as Eb11 ligand chemokine or ELC) and CCL21 (also known as secondary lymphochemokine or SLC). CCL21 has a distinctive 32 amino acids-long C-terminal tail. Twelve of the C-terminal amino acids are basic amino acid residues that can bind to glycosaminoglycans and other molecules . Whereas CCL21 mediates the migration effects in CCR7, CCL19 plays a complementary role and mediates non-migration signals, such as promoting cell survival . Both these ligands, CCL21 and CCL19, were also identified as upregulated DEGs.
The CCR7-CCL19/CCL21 axis can promote the retention of CD4+ T lymphocytes at the reconstruction site of collateral arteries, thereby promoting arteriogenesis . Liu et al. found that Baicalin inhibited the levels of CCR7 and NF-κB simultaneously when studying the anti-inflammatory mechanism of Baicalin in asthma . Qian et al. found that Newcastle disease virus-like particles (NDV VLPs) activate DCs through TLR4/NF-κB pathway, and promote DC migration through the CCR7-CCL19/CCL21 axis . These results show that CCR7-CCL19/CCL21 axis may be related to the NF-κB pathway, which plays an important role in the cellular inflammatory response and immune response . In addition, McNamee et al. found that CCL19, CCL21 and CCR7 were involved in the induction and maintenance of chronic inflammation in a Crohn's-like ileitis mice model . These results are consistent with the results of the GO term and KEGG pathways enrichment analysis.
Several studies have found that CCL21 and CCR7 are expressed in interstitial fibroblasts and tubular epithelial cells in transplanted kidneys , and the CCL21-CCR7 axis can promote renal fibrosis . Early studies have shown that lymph angiogenesis is closely related to the progression of tubulointerstitial fibrosis . Pei et al. identified that that lymphatic vessels were significantly increased in CKD biopsy samples relative to normal samples . In addition, CCL21 was highly expressed in lymphatic vessels, with CCR7+ lymphocytes and DCs clustered near lymphatic vessels. The same results were observed in unilateral ureteral obstruction animal models, in which blocking CCR7 inhibits lymph angiogenesis and reduces injury-induced inflammation and fibrosis. These results suggest that the recruitment of lymphocytes and DC in the kidney depends on the CCL21-CCR7 axis. Altogether, these results indicate that continuous damage to the kidney leads to lymphatic vessels angiogenesis and the recruitment CCR7+ cells into the kidney, which in turn and accelerates intrarenal inflammation. This suggests that there is a relationship between the expression level of CCR7 and the severity of CKD. Studies have found that CCL19 can inhibit cell viability and promote cellular inflammation and fibrosis in DN cells . Mesenchymal stem cells can also inhibit renal inflammation by reducing the expression levels of CCL19 . At present, there are few reports about CCL19, since the CCL19-CCR7 axis is commonly overlooked in comparison with the CCL21-CCR7 axis. However, recent reports show that the CCL19-CCR7 axis plays an important role in renal inflammation. Limited sample data and related experimental results show that, compared with CCR7, CCL19 and CCL21 are still unstable markers for predicting the progress of CKD.
In conclusion, CCR7 may play an important role in the progression of CKD through the immune system. The expression level of CCR7 may be related to the severity of CKD. We found that the expression level of CCR7 was independent of gender and age, indicating that it may be a prognostic marker for CKD progression in individuals of different age and gender. However, more experiments and larger multicenter prospective studies are required to validate the utility of this marker, as well as the threshold for predicting CKD progression and adverse events. As the only ligands of CCR7, the roles of CCL19 and CCL21 in the progression of CKD still need further study. Recent results indicate that the CCR7-CCL19/CCL21 axis may be a therapeutic target for chronic kidney disease and end-stage renal disease.