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Lab Med
Lab Med
labmed
Laboratory Medicine
0007-5027
1943-7730
Oxford University Press US

38493322
10.1093/labmed/lmae015
lmae015
Overview
AcademicSubjects/MED00690
Noninvasive biomarkers for lupus nephritis
Liu Ting MD, PhD Department of Laboratory Medicine, West China Second University Hospital, Sichuan University, Chengdu, China
Key Laboratory of Birth Defects and Related Diseases of Women and Children (Ministry of Education), West China Second University Hospital, Sichuan University, Chengdu, China
State Key Laboratory of Biotherapy and Cancer Center/National Collaborative Innovation Center for Biotherapy, Sichuan University, Chengdu, China

Yang Yun-long MD Department of Laboratory Medicine, West China Second University Hospital, Sichuan University, Chengdu, China
Key Laboratory of Birth Defects and Related Diseases of Women and Children (Ministry of Education), West China Second University Hospital, Sichuan University, Chengdu, China

Zhou Yan MD Department of Laboratory Medicine, West China Second University Hospital, Sichuan University, Chengdu, China
Key Laboratory of Birth Defects and Related Diseases of Women and Children (Ministry of Education), West China Second University Hospital, Sichuan University, Chengdu, China

Jiang Yong-mei MD, PhD Department of Laboratory Medicine, West China Second University Hospital, Sichuan University, Chengdu, China
Key Laboratory of Birth Defects and Related Diseases of Women and Children (Ministry of Education), West China Second University Hospital, Sichuan University, Chengdu, China

Corresponding author: Yong-mei Jiang; jiangym_scu@163.com
9 2024
17 3 2024
17 3 2024
55 5 535542
© The Author(s) 2024. Published by Oxford University Press on behalf of American Society for Clinical Pathology.
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (https://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com

Abstract

Lupus nephritis (LN) is one of the most severe clinical manifestations of systemic lupus erythematosus (SLE). Notably, the clinical manifestations of LN are not always consistent with the histopathological findings. Therefore, the diagnosis and activity monitoring of this disease are challenging and largely depend on invasive renal biopsy. Renal biopsy has side effects and is associated with the risk of bleeding and infection. There is a growing interest in the development of novel noninvasive biomarkers for LN. In this review, we summarize most of the LN biomarkers discovered so far by correlating current knowledge with future perspectives. These biomarkers fundamentally reflect the biological processes of kidney damage and repair during disease. Furthermore, this review highlights the role of urinary cell phenotype detection in the diagnosis, monitoring, and treatment of LN and summarizes the limitations and countermeasures of this test.

lupus nephritis
laboratory medicine
noninvasive biomarker
urinary biomarker
Sichuan Science and Technology Program 2023YFS0186 2023YFS0222 Clinical Research Foundation of West China Second University Hospital KL076
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pmcIntroduction

Lupus nephritis (LN) is one of the most severe clinical manifestations in patients with systemic lupus erythematosus (SLE), which has a significantly high morbidity and mortality.1-3 Generally, about 30%-60% of adults and up to 70% of children with SLE will develop related nephritis as the disease progresses.4-7 Additionally, 10%-30% of LN patients eventually develop end-stage renal disease (ESRD), requiring dialysis and transplantation.8-10 As a chronic autoimmune disease, the pathogenesis of LN involves multiple factors, including the deposition of autoantibodies and complements in the glomerulus and the activation of the innate and adaptive immunity, which ultimately lead to glomerular, interstitial, tubular, and vascular damage. It is worth mentioning that the clinical manifestations of LN are not always consistent with the histopathological findings.11,12 At present, the clinical diagnosis of LN mainly depends on renal histopathological changes. Renal biopsy is the gold standard for diagnosis and classification of the histopathological changes of the kidneys, but it is an invasive procedure that is unsuitable for continuous monitoring of patients.13,14 Additionally, there are puncture-related side effects, such as bleeding and infection, associated with renal biopsy. Therefore, there is an urgent need to identify novel noninvasive biomarkers that can effectively reflect the activity and severity of LN.

Noninvasive biomarkers are laboratory indicators of biological, biochemical, or molecular substances obtained by noninvasive methods. They can be qualitatively and quantitatively detected by laboratory techniques, and their changes are closely related to the occurrence and development of diseases. Compared with renal biopsy, noninvasive biomarkers in blood and urine can significantly increase the frequency of kidney damage assessment, aid in monitoring disease activity, and guide clinical treatment in patients with LN.15-17 A large number of studies have been conducted to develop novel noninvasive biomarkers for the biological processes in LN, especially those in urine, which fundamentally reflect the biological processes of kidney damage and repair during disease, such as inflammatory cell recruitment, immune cell activation and differentiation, and renal vascular damage and repair, etc.18-20 In addition, it is well known that the choice of treatment strategy for LN patients mainly depends on nephritis severity, and late diagnosis of LN is associated with a high frequency of ESRD.21-23 Therefore, early diagnosis, continuous monitoring, and early intervention of LN are important to significantly improve prognosis. To address these issues, there has been a growing interest in the development of novel noninvasive LN biomarkers over the past decade. The in-depth multidimensional study of this disease has led to the gradual incorporation of many noninvasive biomarkers, particularly urinary biomarkers, into the diagnostic strategy for LN. In this review, we mainly discuss the possible roles of potential noninvasive biomarkers in the early diagnosis, activity monitoring, and treatment guidance of LN by correlating the current knowledge with future perspectives (TABLE 1).

TABLE 1. Summary of biomarkers revieweda

Biomarker	Sample type	Key points	Reference	
Gene				
HLA-DR15 haplotype	Blood	Associated with diffuse proliferative glomerulonephritis	24	
IFN-I polymorphisms	Blood	Associated with immune-mediated glomerular damage	25	
miRNA	Blood	mir-125a-5p, mir-146a-5p, and mir-221-3p suggested therapeutic effect	26	
Autoantibody				
Anti-NCS	Serum	Associated with renal disease and deserves further investigation	27	
Anti-C1q	Serum	Associated with renal disease and deserves further investigation	28	
Anti-α-actinin	Serum	Associated with renal disease and deserves further investigation	29	
AECA	Serum	Associated with endothelial damage	30	
Cytokines				
BLyS	Serum	Associated with the frequency of LN	31,32	
IFNAR1	Serum	Associated with the frequency of LN	31	
IL-16	Urine	Associated with the disease activity of LN	33,34	
IFN-I	Urine	Associated with the extent of kidney damage	20	
IL-35	Serum/Urine	Associated with renal involvement	35,36	
Chemokines				
CCL2 (MCP-1)	Serum/Urine	Associated with active LN patients	37,38	
CCL3 (MIP-1α)	Serum/Urine	Associated with renal disease activity	39,40	
CCL5 (RANTES)	Serum/Urine	Associated with active LN patients	41,42	
CCL7 (MCP-3)	Urine	Associated with renal disease activity	43	
CCL8 (MCP-2)	Urine	Associated with renal disease activity	43	
CCL19 (MIP-3β)	Serum	Associated with active LN patients	41	
CXCL8 (IL-8)	Serum	Associated with active LN patients	44	
CXCL9 (MIG)	Serum/Urine	Associated with active LN patients	41	
CXCL10 (IP-10)	Serum/Urine	Associated with renal disease activity	45,46	
CXCL11 (I-TAC)	Serum	Associated with renal disease activity	39	
CXCL13 (BCA-1)	Serum	Associated with renal disease activity	47	
CXCL16 (SR-PSOX)	Serum/Urine	Associated with renal disease activity	48	
CX3CL1	Urine	Associated with renal disease activity	43	
Other proteins				
TWEAK	Urine	Associated with renal involvement	49,50	
OPG	Urine	Associated with renal involvement	49,51	
UFCR	Urine	Significantly higher than that of SLE patients without LN	52	
IGFBP-2	Serum	Associated with renal histology	53,54	
TNFR1/TNFR2	Serum	Associated with renal histology	55,56	
VCAM-1	Serum	Associated with damage and repair of renal vascular endothelium	57,58	
NRP-1	Urine	Early prognostic biomarker in LN	59	
AXL	Serum	Associated with renal histology	53,60	
TGF-β	Urine	Associated with the disease activity of LN	33	
Immune cells				
CD8+T cells	Urine	Associated with renal disease activity	61	
CD4/CD8 ratio	Urine	Distinguishes LN from other inflammatory kidney diseases	62	
CD14+ monocytes	Urine	Associated with class IV nephritis	63	
TFH cells	Urine	Elevated in biopsies and deserves further investigation	64	
AECA, anti-endothelial cell antibodies; anti-NCS, antinucleosome; anti-C1q, anti-complement 1q; AXL, a receptor tyrosine kinase; BCA-1, B cell attracting chemokine 1; BLyS, B-lymphocyte stimulator protein; CX3CL1, C-X3-C motif chemokine ligand 1; HLA, human leukocyte antigen; IFN-I, type I interferon; IFNAR1, interferon alpha/beta receptor 1; IGFBP2, insulin like growth factor binding protein 2; IP-10, IFN gamma inducible protein10; I-TAC, IFN inducible T cell α chemoattractant; MCP, monocyte chemoattractant protein; MIG, monokine induced by gamma interferon; MIP, macrophage inflammatory protein; NRP-1, neuropilin-1; OPG, osteoprotegerin; RANTES, regulated upon activation, normal T cell expressed and secreted; SR-PSOX, scavenger receptor for phosphatidylserine and oxidized low-density lipoprotein and bacteria; TFH cells, T follicular helper cells; TGF-β, transforming growth factor-β; TNFR, tumor necrosis factor receptor; TWEAK, TNF-like weak inducer of apoptosis; UFCR, urinary ferritin/creatinine ratio; VCAM-1, vascular cell adhesion molecule-1.

aIt must be taken into account that these data are only available in 1-2 studies, and these findings should be updated as more validation experiments are conducted.

Conventional Laboratory Biomarkers for LN

The conventional laboratory markers used for LN diagnosis and monitoring mainly include proteinuria, protein/creatinine ratio, glomerular filtration rate, plasma complement levels (C3 and C4), anti-double-stranded (ds)DNA antibody, anti-cardiolipin antibody, rheumatoid factor, serum immunoglobulin, blood cell counts, as well as erythrocyte sedimentation rate and so on. Among these markers, the diagnostic specificity and sensitivity of the protein/creatinine ratio, plasma complement, anti-dsDNA antibody and serum immunoglobulins (IgG and IgA) for LN are relatively higher than those of the others. However these markers are still insufficient to diagnose renal damage or to monitor disease activity, and patients still need further renal biopsy.25,65,66 For example, it is difficult to distinguish between irreversible glomerular capillary damage and ongoing LN activity in most patients with persistent proteinuria.25 Moreover, Pinheiro et al67 reported that LN may recur in the absence of an elevated level of anti-dsDNA. Likewise, Soliman et al18 found that the diagnostic efficacy of anti-dsDNA varied across different assay platforms, making it difficult to achieve uniformity in clinical judgment. Based on these findings, we believe that LN can develop before the levels of any of these commonly used laboratory markers are altered and can continue to develop without obvious clinical signs. Correlations between these conventional laboratory biomarkers and LN are incomplete, and their utility in reflecting disease activity and predicting prognosis remains controversial.68 Therefore, there is an urgent need to develop novel noninvasive biomarkers that can replace kidney biopsy and conventional biomarkers to better reflect the extent of kidney damage in patients with LN.

Blood Biomarkers for LN

The biggest problem with renal biopsy is that it can cause a variety of complications, including hematuria, kidney damage and loss, and even death.69 Also, due to differences in infrastructure and personnel qualifications in different regions, not all the medical institutions can perform this test.70,71 Compared with renal biopsy, side effects caused by venous blood collection are almost negligible. Of note, the detection of blood biomarkers requires relatively low infrastructure and personnel qualifications, making it easier to perform, especially in remote areas. Great effort has been made to identify novel noninvasive biomarkers in blood. To date, several studies have screened candidate biomarkers, including genes, cytokines, chemokines, autoantibodies, adhesion molecules, and factors involved in kidney damage and repair.72,73

Genes as Biomarkers for LN

Genetic factors are important in the pathogenesis of LN. Specifically, differences in levels of protein biomarkers, such as cytokines, chemokines, and autoantibodies in the serum and urine of LN patients can reflect genetic and epigenetic differences. In recent decades, numerous studies have evaluated the potential roles of genes in predicting LN susceptibility. On one hand, detection of changes in gene sequence, such as single nucleotide polymorphisms, is expected to be a novel biomarker indicating susceptibility to LN. For example, Marchini et al24 found that the interactions of HLA-DQA and HLA-DR alleles in an Italian population confer a susceptibility to LN in SLE patients; the HLA-DR15 haplotype is especially associated with diffuse proliferative glomerulonephritis. Besides, Lodi et al74 reported that type I interferon (IFN-I) apparently promoted immune-mediated glomerular damage in LN. Therefore, gene polymorphisms that enhanced IFN-I signaling were identified as risk factors for SLE and LN.25 On the other hand, epigenetic modifications by DNA methylation, histone modification, and micro (mi)RNA can also influence gene expression and alter cell function without changing the genome sequence, thereby affecting LN susceptibility.75 Su et al26 used next-generation sequencing technology and real-time quantitative polymerase chain reaction technology to screen the different expression levels of miRNAs in the peripheral blood mononuclear cells (PBMCs) of SLE patients with and without LN. Ultimately, the study revealed that mir-125a-5p, miR-146a-5p, and mir-221-3p were significantly decreased in PBMCs of SLE patients with LN and then significantly increased after treatment, suggesting their role as biomarkers in early diagnosis and treatment guidance of LN.26

Serum Protein as Biomarkers for LN

In addition to cells, blood also contains a large number of bioactive substances, such as proteins. In recent decades, several studies have been conducted to screen serum biomarkers in LN patients based on proteomics.18,39,76 Various soluble serum proteins, including cytokines, chemokines, adhesion molecules, and autoantibodies, were found to be correlated with LN disease severity and activity. For cytokines, Chun et al77 found that SLE patients had higher serum levels of cytokines such as interleukin (IL)-6, IL-10, IL-12, and IFN-γ compared with normal healthy controls, but no significant differences in levels were observed between SLE patients with and without LN. These findings suggest that due to the common molecular pathways among various inflammatory diseases, the efficacy of serum cytokines in the diagnosis and differential diagnosis of LN is limited; more studies focusing on the correlations between cytokines in urine (or specific cells) and disease are being conducted. Notably, López et al31 analyzed in depth the expression levels of pathogenic IFNα, B-lymphocyte stimulator protein (BLyS), and IL-17 axis in various blood cell types such as T and B lymphocytes, monocytes, dendritic cells, and neutrophils. They found that tumor necrosis factor (TNF) superfamily cytokine BLyS and related ligand IFN alpha/beta receptor 1 (IFNAR1) levels on monocytes or in the serum were associated with the frequency of nephritis.31,32 For chemokines, numerous studies have shown that serum chemokines, including CCL3 (MIP-1α), CXCL10 (IP-10), CXCL11 (I-TAC), and CXCL13 (BCA-1), are strongly associated with disease activity of LN.39,45,47 Also, Fu et al41 reported that the transcriptional levels of interferon-inducible chemokines, including CCL2 (MCP-1), CCL5 (RANTES), CCL19 (MIP-3β), CXCL8 (IL-8), and CXCL9 (MIG), are also strongly associated with active LN patients.37,41,44 Compared with cytokines, the levels of chemokines are more significantly changed in the blood of LN patients, and their potential as specific biomarkers is greater. For adhesion molecules, Yao et al57 found that serum vascular cell adhesion molecule-1 (VCAM-1) was significantly increased in proliferative LN, suggesting that it can be used as a potential biomarker to assess the onset and remission of proliferative LN. It is reported that VCAM-1 is involved in the damage and repair of renal vascular endothelium during the disease course of LN patients.58 For autoantibodies, a large number of studies have revealed the strong correlation between antinucleosome, anti-complement 1q, and anti-α-actinin and renal disease in SLE patients.27-29 However, their role in assessing the severity and activity of LN remains to be further validated. Kondo et al30 developed a solubilized cell surface protein capture enzyme-linked immunosorbent assay to detect the serum antibodies in LN patients and further found a significant correlation between anti-endothelial cell antibodies (AECA) and glomerular hypercellularity, indicating that AECA, particularly IgA-AECA, are associated with endothelial damage. For other proteins, Wu et al53 found that serum AXL (a receptor tyrosine kinase), insulin-like growth factor binding protein 2 (IGFBP2), and TNF receptor (TNFR) were significantly elevated in patients with active LN and showed significant positive correlations with the renal pathology activity index.53 AXL and IGFBP2 are involved in regulating the innate immune response of monocytes and T cells, respectively, and seem to be biomarkers of particular interest in LN studies,53,54,60 whereas TNFR1 and TNFR2 are found predominately in glomerular cells and tubular cells, respectively.55 Once TNF-α binds to its receptors, a complex signaling cascade involving apoptosis and inflammation occurs.56 Ultimately, the expression levels of specific serum proteins are closely related to the kidney damage, and its detection can be used to monitor the disease activity and guide treatment. However, the usefulness of serum protein detection in assessing LN susceptibility in SLE patients is limited.

Urinary Biomarkers for LN

Like blood, urine also contains a variety of components, such as epithelial cells, proteins, glucose, inorganic salts, and trace elements, which can be used as biomarkers for laboratory medicine. Arazi et al20 used single cell transcriptomics to analyze kidney and urine samples from LN patients and healthy individuals. Their finding revealed similarities in the molecular signatures of urine and kidney tissues, highlighting the superiority of urine samples in understanding kidney damage. Additionally, numerous studies have demonstrated that the phenotype and composition of urine cells were similar to those cells observed in renal biopsies, suggesting that the immune cells examined in urine originated from the kidney.18 Of note, urine collection is clinically very easy with few side effects; hence, it can be used to significantly improve the frequency of continuous monitoring in LN patients.33,78,79 Therefore, urine testing has the potential to serve as a surrogate for renal biopsy in assessing the extent of kidney damage and inflammation level.

Urinary Immune Cells as Biomarkers for LN

Numerous studies have reported that urinary cell phenotype correlates with renal interstitial infiltration, which can reflect the disease activity and kidney damage.80 Normally, there are no blood cells in the urine of healthy people, but some T cells, B cells, and macrophages may be observed in the urine of LN patients and other patients with proliferative renal diseases.43,49,63 Therefore, the classification and counting of cells in urine may be a potential noninvasive biomarker for identifying and diagnosing of LN. Urine sediment analysis, including erythrocyte and leukocyte counts, as well as morphological analysis, can indicate the presence of renal disease and inflammation. However, this test has a very limited role in the differential diagnosis of renal diseases. Some researchers have introduced flow cytometry to further analyze the urine cell phenotype and preliminarily obtained a urinary cellular profile of SLE patients with LN. Abdelati et al63 found that the numbers of CD3+CD4+, CD3+CD8+, and CD14+ cells in the urine of SLE patients with LN were significantly higher than those patients without LN. In particular, CD14+ monocyte counts were much higher in the urine of patients with class IV LN than those with class III LN and could serve as a potentially sensitive and specific biomarker for detecting proliferative LN.63 In another study, a low CD4/CD8 ratio was found to be a specific manifestation of LN, suggesting that the shift of urinary T cells into CD8+ T cells is a marker that distinguishes LN from other inflammatory kidney diseases, such as diabetic nephropathy.62 Therefore, both the absolute number and the proportion of urinary CD8+ T cells may be considered as promising biomarkers for assessing renal activity in patients with LN.61 This finding was supported in the study by Kopetschke et al.62 They found that the numbers of urinary T cells, B cells, and macrophages were correlated with the disease activity of LN; however, due to the low number of B cells and macrophages in urine, T cells remained the best cellular biomarker for LN. Of course, with the development of laboratory medicine technologies, it is conceivable that there will be more research focus on cells that are less abundant in urine, such as B cells and T follicular helper cells (TFH). Several studies have already confirmed that B cells and TFH cells are elevated in lupus biopsies and their number alterations in urine may be consistent with those in kidney tissue, which deserve further investigation.64,81

Urinary Proteins as Biomarkers for LN

It is well known that the activation of inflammatory pathways within the kidneys are early responses to kidney damage. During the inflammatory process, cytokines are secreted by immune cells such as T cells and macrophages and participate in the chemoattraction, migration, and activation of inflammatory cells. Fava et al33 analyzed the urinary proteomic profiles in patients with LN and found that urinary IL-16, CD163, and TGF-β have the ability to mirror nephritis activity. Of note, IL-16 is a CD4 ligand with proinflammatory and chemotactic properties. Single cell RNA sequencing also showed that a large number of immune cells capable of producing IL-16 infiltrated at the inflammatory site of the kidney in LN patients, which supports the idea of IL-16 as a potential noninvasive marker for LN diagnosis and treatment monitoring.33 Furthermore, Häyry et al34 detected high levels of IL-16 expression in the plasma, urine, and kidney tissues in SLE patients, and only urinary IL-16 could differentiate patients with proliferative LN from those SLE patients without LN. Therefore, it is necessary to study the expression levels of cytokines and other inflammatory substances in the urine of LN patients. Similarly, Arazi et al20 analyzed kidney samples and urinary cells from patients with LN using single cell RNA sequencing. The researchers found that a variety of immune cells, including myeloid cells, T cells, B cells, and natural killer cells in the kidney had a clear interferon response, and the gene expression of urine cells was highly consistent with that of kidney cells. Therefore, the IFN-I test in urinary cells can distinguish LN from other forms of nephritis and truly reflect the extent of kidney damage. Also, IL-35 has immunosuppressive and anti-inflammatory effects and is significantly elevated in the blood and urine of LN patients, which may aid in assessing renal involvement in SLE patients.35 Cai et al36 demonstrated that LAIR1 could be used as a novel potential target of the IL-35-regulated JAK/STAT signaling pathway, and its expression level in urine was significantly correlated with clinical inflammatory parameters of LN. For chemokines, Klocke et al43 found that 9 urinary chemokines were significantly elevated in LN patients and correlated with disease activity and urinary cell counts. These chemokines include CCL2, CCL3, CCL5, CCL7, CCL8, CXCL9, CXCL10, CXCL16, and CX3CL1.42,46,48 This finding was corroborated in the study by Kulkarni and Anders.40 Additionally, Kulkarni and Anders40 and Singh et al38 reported that CCL2 and CCL3 were highly expressed in both glomeruli and urine of LN patients, suggesting that the expression of chemokines in urine was consistent with that in kidney tissues. However, there was no significant correlation between urinary chemokine concentration and serum chemokine levels. These results indicate that elevated chemokines in urine mainly originate from the kidney rather than the blood. For other proteins, El-Shehaby et al49 found that urinary levels of TNF-like weak inducer of apoptosis and osteoprotegerin were positively correlated with renal involvement as assessed by the renal disease activity index. These proteins were involved in the harmful physiological responses of cell survival, proliferation, differentiation, angiogenesis, migration and apoptosis, and then contributed to kidney damage in LN.50,51 Kidney damage leads to the impairment of glomerular filtration function, which ultimately results in increased levels of protein substances in the urine. Qi et al52 evaluated the clinical significance of urinary ferritin/creatinine ratio (UFCR) in LN, and found that the UFCR level in LN patients was significantly higher than that in patients without LN. Because ferritin cannot be filtered by normal glomeruli, damage of glomerular filtration should be the cause of elevated UFCR.52 Moreover, Torres-Salido et al59 found that the transmembrane receptor neuropilin-1 (NRP-1) is highly expressed in urine and renal tissue of LN patients, which could be used as an early prognostic biomarker of LN. In the kidney, NRP-1 could promote renal recovery through endothelial proliferation and migration, mesangial migration, and local T cell cytotoxicity.

Conclusion

A large number of potential noninvasive biomarkers targeting candidate genes, immune responses, inflammation, and kidney damage have been reported (FIGURE 1). Compared with renal biopsy, noninvasive biomarkers have many advantages. First, renal biopsy has high requirements for medical environment, facilities, equipment, and personnel qualification, and in addition to possible puncture-related complications, up to 35% of renal biopsies may fail to obtain adequate samples,82 whereas the collection procedure for noninvasive biomarkers, especially from urine, is straightforward and efficient, with almost no unqualified specimen. Second, renal biopsy requires a clear clinical indication of kidney damage; therefore, renal biopsy is a lagging indicator. In contrast, the alterations signaled by noninvasive biomarkers can occur before kidney damage and therefore can be applied to assess and predict the risk of LN in patients with SLE. Third, the contraindications for renal biopsy include coagulation disorders, heart disease, mental illness, solitary kidney, and kidney atrophy, thus making it unsuitable for all patients. However, there are few contraindications for noninvasive biomarker tests, and urine tests in particular are suitable for almost everyone. Finally, histological classification artificially divides disease types based on a point in time, whereas the detection of noninvasive biomarkers can dynamically and continuously reflect the specific changes in the kidneys of LN patients. Based on these advantages, the development of noninvasive biomarkers has been very rapid in recent decades.

FIGURE 1. Summary of noninvasive biomarkers for lupus nephritis. AECA, anti-endothelial cell antibodies; anti-NCS, antinucleosome; anti-C1q, anti-complement 1q; AXL, a receptor tyrosine kinase; HLA, human leukocyte antigen; IFN-I, type I interferon; IFNAR1, interferon alpha/beta receptor 1; IGFBP2, insulin-like growth factor binding protein 2; IL, interleukin; mBLyS, membrane B-lymphocyte stimulator protein; miRNA, microRNA; NRP-1, neuropilin-1; OPG, osteoprotegerin; TFH, T follicular helper; TGF-β, transforming growth factor-β; TNFR, tumor necrosis factor receptor; TWEAK, TNF-like weak inducer of apoptosis; UFCR, urinary ferritin/creatinine ratio; VCAM-1, vascular cell adhesion molecule-1; VEGF, vascular endothelial growth factor.

We also are aware that the noninvasive biomarkers in these studies have some disadvantages. More effort should be made to apply noninvasive biomarkers to clinical practice or incorporate them into clinical treatment guidelines for LN. For urinary cell detection, we find that the number of immune cells in LN patients varied slightly across different studies.16,61-63 These differences could be explained by the differences in sample volume obtained, sample preparation techniques, gating strategy and patient ethnicity. Therefore, a standardized detection process must be established to apply urinary cell detection in clinical practice and ensure consistent results from different laboratories. In addition, assuming that urinary immune cells originated from the kidney, their role and clinical significance in renal disease remain unclear and must be evaluated. For serum protein detection, most studies have validated the efficacy of noninvasive biomarkers in SLE populations. If these biomarkers are to be applied in clinical practice, they need to be further validated in other types of nephritis to evaluate their potential in differential diagnosis. For urinary protein detection, it is necessary to consider the influence of urinary tract infection on the specificity of the detection biomarkers and further evaluate its efficacy. Because most inflammatory diseases of the urinary system share a common molecular pathway, it is conceivable that potential biomarkers in urine may not be specific to a particular disease.83 Above all, more clinical trials are needed to further validate the diagnostic efficacy of novel noninvasive biomarkers and therefore promote the translation of basic research into clinical practice.

In conclusion, noninvasive biomarkers may provide a new turning point for disease diagnosis, monitoring, and treatment guidance in patients with LN, avoiding the hazards of repeat renal biopsy and untimely diagnosis. A better understanding of the distinct biological role and molecular mechanisms of noninvasive biomarkers in LN is also crucial for the development of possible therapeutic targets and strategies. Therefore, in addition to aiding in the diagnosis, monitoring, and treatment guidance of LN, noninvasive biomarkers may act as possible therapeutic targets for LN and must be investigated in detail in the future.

Abbreviations:

LN lupus nephritis

SLE systemic lupus erythematosus

ESRD end-stage renal disease

dsDNA double-stranded DNA;

IFN-I type I interferon;

miRNA microRNA

PBMCs peripheral blood mononuclear cells;

IL , interleukin

BLyS B-lymphocyte stimulator protein;

TNF tumor necrosis factor

IFNAR1 IFN alpha/beta receptor 1

VCAM-1 vascular cell adhesion molecule-1

AECA anti-endothelial cell antibodies

AXL a receptor tyrosine kinase;

IGFBP2 insulin like growth factor binding protein 2

TNFR TNF receptor

TFH T follicular helper cells

UFCR urinary ferritin/creatinine ratio

NRP-1 neuropilin-1

Funding

This work was supported by Sichuan Science and Technology Program (Grant No. 2023YFS0186 and 2023YFS0222) and the Clinical Research Foundation of West China Second University Hospital (Grant No. KL076).

Conflict of Interest Disclosure

The authors have nothing to disclose.
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