Microbial nucleic acids pay a Toll in kidney disease

IgA Nephropathy: Significance of IgA1-Containing Immune Complexes in Clinical Settings

IgA nephropathy (IgAN) is considered to be an autoimmune disease characterized by the formation of IgA1-containing immune complexes in the circulation and glomerular immunodeposits. Extensive research has identified multiple genetic, immunological, and environmental factors contributing to disease development and progression. The pathogenesis of IgAN is considered a multifactorial process involving the formation of immune complexes wherein aberrantly O-glycosylated IgA1 is recognized as an autoantigen. Consequently, the clinical presentation of IgAN is highly variable, with a wide spectrum of manifestations ranging from isolated microscopic hematuria or episodic macroscopic hematuria to nephrotic-range proteinuria. Whereas some patients may exhibit a slowly progressive form of IgAN, others may present with a rapidly progressive glomerulonephritis leading to kidney failure. Development of the treatment for IgAN requires an understanding of the characteristics of the pathogenic IgA1-containing immune complexes that enter the glomerular mesangium and induce kidney injury. However, not all details of the mechanisms involved in the production of galactose-deficient IgA1 and immune-complex formation are fully understood. Here, we review what we have learned about the characteristics of nephritogenic IgA1 in the half-century since the first description of IgAN in 1968.

The nucleotide-sensing Toll-Like Receptor 9/Toll-Like Receptor 7 system is a potential therapeutic target for IgA nephropathy

The progression determinants of IgA nephropathy (IgAN) are still not fully elucidated. We have previously demonstrated that the mucosal activation of toll-like receptor (TLR) 9, which senses microbial unmethylated CpG DNA, influences progression by producing aberrantly glycosylated IgA. However, numerous recent reports of patients with IgAN presenting with gross hematuria after the mRNA vaccination for coronavirus disease 2019 suggest that the RNA-sensing system also exacerbates IgAN. Here, we investigated whether TLR7, which recognizes microbial RNA, is also involved in IgAN progression using a murine model and tonsil tissue from 53 patients with IgAN compared to samples from 40 patients with chronic tonsillitis and 12 patients with sleep apnea syndrome as controls. We nasally administered imiquimod, the ligand of TLR7, to IgAN-prone ddY mice and found that TLR7 stimulation elevated the serum levels of aberrantly glycosylated IgA and induced glomerular IgA depositions and proteinuria. Co-administered hydroxychloroquine, which inhibits TLRs, canceled the kidney injuries. In vitro, stimulating splenocytes from ddY mice with imiquimod increased interleukin-6 and aberrantly glycosylated IgA levels. The expression of TLR7 in the tonsils was elevated in patients with IgAN and positively correlated with that of a proliferation-inducing ligand (APRIL) involved in the production of aberrantly glycosylated IgA. Mechanistically, TLR7 stimulation enhanced the synthesis of aberrantly glycosylated IgA through the modulation of enzymes involved in the glycosylation of IgA. Thus, our findings suggest that nucleotide-sensing TLR9 and TLR7 play a crucial role in the pathogenesis of IgAN. Hence, nucleotide-sensing TLRs could be reasonably strong candidates for disease-specific therapeutic targets in IgAN.

The Role of the Oxidative State and Innate Immunity Mediated by TLR7 and TLR9 in Lupus Nephritis

Lupus nephritis (LN) is a severe complication of systemic lupus erythematosus (SLE) and is considered one of the leading causes of mortality. Multiple immunological pathways are involved in the pathogenesis of SLE, which makes it imperative to deepen our knowledge about this disease's immune-pathological complexity and explore new therapeutic targets. Since an altered redox state contributes to immune system dysregulation, this document briefly addresses the roles of oxidative stress (OS), oxidative DNA damage, antioxidant enzymes, mitochondrial function, and mitophagy in SLE and LN. Although adaptive immunity's participation in the development of autoimmunity is undeniable, increasing data emphasize the importance of innate immunity elements, particularly the Toll-like receptors (TLRs) that recognize nucleic acid ligands, in inflammatory and autoimmune diseases. Here, we discuss the intriguing roles of TLR7 and TLR9 in developing SLE and LN. Also included are the essential characteristics of conventional treatments and some other novel and little-explored alternatives that offer options to improve renal function in LN.

Mucosal Immune System Dysregulation in the Pathogenesis of IgA Nephropathy

The mucosal immune system, via a dynamic immune network, serves as the first line of defense against exogenous antigens. Mucosal immune system dysregulation is closely associated with the pathogenesis of immunoglobulin A nephropathy (IgAN), as illustrated by IgAN having the clinical feature of gross hematuria, often concurrent with mucosal infections. Notably, previous studies have demonstrated the efficacy of tonsillectomy and found that a targeted-release formulation of budesonide reduced proteinuria in patients with IgAN. However, it remains unclear how exogenous antigens interact with the mucosal immune system to induce or exacerbate IgAN. Thus, in this review, we focus on the dysregulation of mucosal immune response in the pathogenesis of IgAN.

Title IgA Nephropathy and Oral Bacterial Species Related to Dental Caries and Periodontitis

A relationship between IgA nephropathy (IgAN) and bacterial infection has been suspected. As IgAN is a chronic disease, bacteria that could cause chronic infection in oral areas might be pathogenetic bacteria candidates. Oral bacterial species related to dental caries and periodontitis should be candidates because these bacteria are well known to be pathogenic in chronic dental disease. Recently, several reports have indicated that collagen-binding protein (cnm)-(+) Streptococcs mutans is relate to the incidence of IgAN and the progression of IgAN. Among periodontal bacteria, Treponema denticola, Porphyromonas gingivalis and Campylobacte rectus were found to be related to the incidence of IgAN. These bacteria can cause IgAN-like histological findings in animal models. While the connection between oral bacterial infection, such as infection with S. mutans and periodontal bacteria, and the incidence of IgAN remains unclear, these bacterial infections might cause aberrantly glycosylated IgA1 in nasopharynx-associated lymphoid tissue, which has been reported to cause IgA deposition in mesangial areas in glomeruli, probably through the alteration of microRNAs related to the expression of glycosylation enzymes. The roles of other factors related to the incidence and progression of IgA, such as genes and cigarette smoking, can also be explained from the perspective of the relationship between these factors and oral bacteria. This review summarizes the relationship between IgAN and oral bacteria, such as cnm-(+) S. mutans and periodontal bacteria.

Nasal-associated lymphoid tissue is the major induction site for nephritogenic IgA in murine IgA nephropathy

Dysregulation of mucosal immunity may play a role in the pathogenesis of IgA nephropathy (IgAN). However, it is unclear whether the nasal-associated lymphoid tissue (NALT) or gut-associated lymphatic tissue is the major induction site of nephritogenic IgA synthesis. To examine whether exogenous mucosal antigens exacerbate the pathogenesis of IgAN, we assessed the disease phenotypes of IgAN-onset ddY mice housed germ-free. These mice were transferred to a specific pathogen-free environment and divided into three groups: challenged with the Toll-like receptor 9 (TLR9) ligand CpG-oligodeoxynucleotide, fecal transplantation, and the untreated control group. The levels of aberrantly glycosylated IgA and IgG-IgA immune complexes were measured in the serum and supernatant of cultured cells purified from the NALT, mesenteric lymph nodes, and Peyer's patch. Although the germ-free IgAN-onset ddY mice did not develop IgAN, they showed aggravation of kidney injury with mesangial IgA deposition after transfer to the specific pathogen-free state. The NALT cells produced more aberrantly glycosylated IgA than those from the mesenteric lymph node and Peyer's patch, resulting in induction of IgG-IgA immune complexes formation. Additionally, TLR9 enhanced the production of nephritogenic IgA and IgG-IgA immune complexes by nasal-associated lymphoid but not gut-associated lymphatic cells. Furthermore, the germ-free IgAN-onset ddY mice nasally immunized with CpG-oligonucleotide showed aggravation of kidney injury with mesangial IgA deposition, whereas those that received fecal transplants did not develop IgAN. Thus, NALT is the major induction site of the production of aberrantly glycosylated IgA in murine IgAN.

Toll-like receptors in lupus nephritis

The pathogenesis of systemic autoimmune diseases such as systemic lupus erythematosus (SLE) is based on the loss of self-tolerance against ubiquitous autoantigens involving all mechanisms of adaptive immunity. However, data accumulating over the last decade imply an important role also for numerous elements of innate immunity, namely the Toll-like receptors in the pathogenesis of SLE. Here we discuss their role in the most common organ complication of SLE, i.e. lupus nephritis. We summarize experimental and clinical data on the expression and functional contribution of the Toll-like receptors in immune complex glomerulonephritis, and intrarenal inflammation. Based on these discoveries Toll-like receptors are evolving as therapeutic targets for the treatment of SLE and lupus nephritis.

Neutrophils, Dendritic Cells, Toll-Like Receptors, and Interferon-α in Lupus Nephritis

Finding better treatments for lupus nephritis requires an understanding of the pathogenesis of the causative systemic disease, how this leads to kidney disease, and how lupus nephritis progresses to end-stage kidney disease. Here, we provide a brief conceptual overview on the related pathomechanisms. As a main focus we discuss in detail the roles of neutrophils, dendritic cells, Toll-like receptors, and interferon-α in the pathogenesis of lupus nephritis by separately reviewing their roles in extrarenal systemic autoimmunity and in intrarenal inflammation and immunopathology.

Toll-like receptor activation in the pathogenesis of lupus nephritis

The pathogenesis of systemic lupus erythematosus (SLE) and lupus nephritis is complex but no longer enigmatic. Much progress has been made to on the polygenetic origin of lupus in identifying gene variants that permit the loss of tolerance against nuclear autoantigens. Along the same line in about 50% of lupus patients additional genetic weaknesses promote immune complex glomerulonephritis and filtration barrier dysfunction. Here we briefly summarize the pathogenesis of SLE with a focus on loss of tolerance and the role of toll-like receptors in the "pseudo"-antiviral immunity concept of systemic lupus. In addition, we discuss the local role of Toll-like receptors in intrarenal inflammation and kidney remodeling.

Activation of toll-like receptor-7 exacerbates lupus nephritis by modulating regulatory T cells

BACKGROUND

Toll-like receptor-7 (TLR7), which recognizes viral single-stranded RNA, can trigger immune complex glomerulonephritis in experimental lupus erythematosus. However, whether it modulates dendritic cells (DCs) phenotype and regulatory T cells (Treg) function is incompletely understood.

METHOD

Splenocytes and bone marrow DCs were obtained from 5- and 20-week-old female MRL(lpr/lpr) mice and C57BL/6 mice. In addition, to understand the response of Treg and DCs to TLR7 ligation in vivo, 16-week-old female MRL(lpr/lpr) and C57BL/6 mice were distributed into two groups with or without intraperitoneal injections of TLR7 ligand every other day.

RESULTS

After activation with the TLR7 ligand imiquimod in vivo and vitro, DCs from imiquimod-treated MRL/lpr mice showed an altered costimulatory profile, with decreased induction of CD80, CD86, and MHCII expression, comparing to age-matched C57BL/6 control mice. There was no significant difference in the numbers of CD4+CD25+Foxp3+ cells after TLR7 ligation by imiquimod in MRL(lpr/lpr) and control mice. Immunostaining of kidney sections of nephritic MRL/lpr mice revealed that CD11c was expressed in the infiltrated tubulointerstitial cells, and confocal microscopic analysis of renal CD11c+MHCII+, CD11c+CD80+, and CD11c+)CD86+ cells showed an immature phenotype with low levels of CD80, CD86, and MHCII in imiquimod-treated MRL/lpr mice. There was no difference in the number of Foxp3 positive cells in kidneys between the imiquimod and vehicle-treated groups.

CONCLUSIONS

Our results suggest that activation of TLR7 exacerbated lupus nephritis by modulating the abnormally costimulatory phenotype of dendritic cells and functions of Treg in MRL/lpr mice.

Danger control programs cause tissue injury and remodeling

Are there common pathways underlying the broad spectrum of tissue pathologies that develop upon injuries and from subsequent tissue remodeling? Here, we explain the pathophysiological impact of a set of evolutionary conserved danger control programs for tissue pathology. These programs date back to the survival benefits of the first multicellular organisms upon traumatic injuries by launching a series of danger control responses, i.e., 1. Haemostasis, or clotting to control bleeding; 2. Host defense, to control pathogen entry and spreading; 3. Re-epithelialisation, to recover barrier functions; and 4. Mesenchymal, to repair to regain tissue stability. Taking kidney pathology as an example, we discuss how clotting, inflammation, epithelial healing, and fibrosis/sclerosis determine the spectrum of kidney pathology, especially when they are insufficiently activated or present in an overshooting and deregulated manner. Understanding the evolutionary benefits of these response programs may refine the search for novel therapeutic targets to limit organ dysfunction in acute injuries and in progressive chronic tissue remodeling.

Estrogen receptor-alpha mediates Toll-like receptor-2 agonist-induced monocyte chemoattractant protein-1 production in mesangial cells

TLR2 agonists are well known for inducing NF-kB activation and inflammation, while estrogen receptor-alpha (ER-α) is a regulator of estrogen-mediated anti-inflammatory responses. In the present work, we determined the role of ER-α and phosphorylated ER-α in TLR2 agonist-induced MCP1 production in mesangial cells. We found that TLR2 agonists induced nuclear localization of phospho-ER-α (serine 118), and estrogen and TLR2 agonists both induced phosphorylation of ER-α at the serine 118 and 104/106 positions. Incubation of MRL/lpr mesangial cells with estrogen was found to attenuate TLR2 agonist-mediated MCP1 production. To determine the mode of action of ER-α/pER-α (serine-118), we used the ER-α inhibitor MPP and transfected mesangial cells with ER-α siRNA. ER-α inhibition was found to decrease MCP1 production in mesangial cells. Thus, ER-α/pER-α is an intermediate regulator for both TLR2-mediated MCP1 production during inflammation and estrogen-mediated anti-inflammatory signals in mesangial cells.

Four danger response programs determine glomerular and tubulointerstitial kidney pathology: clotting, inflammation, epithelial and mesenchymal healing

Renal biopsies commonly display tissue remodeling with a combination of many different findings. In contrast to trauma, kidney remodeling largely results from intrinsic responses, but why? Distinct danger response programs were positively selected throughout evolution to survive traumatic injuries and to regenerate tissue defects. These are: (1) clotting to avoid major bleeding, (2) immunity to control infection, (3) epithelial repair and (4) mesenchymal repair. Collateral damages are acceptable for the sake of host survival but causes for kidney injury commonly affect the kidneys in a diffuse manner. This way, coagulation, inflammation, deregulated epithelial healing or fibrosis contribute to kidney remodeling. Here, I focus on how these ancient danger response programs determine renal pathology mainly because they develop in a deregulated manner, either as insufficient or overshooting processes that modulate each other. From a therapeutic point of view, immunopathology can be prevented by suppressing sterile renal inflammation, a useless atavism with devastating consequences. In addition, it appears as an important goal for the future to promote podocyte and tubular epithelial cell repair, potentially by stimulating the differentiation of their newly discovered intrarenal progenitor cells. By contrast, it is still unclear whether selectively targeting renal fibrogenesis can preserve or bring back lost renal parenchyma, which would be required to maintain or improve kidney function. Thus, renal pathology results from ancient danger responses that evolved because of their evolutional benefits upon trauma. Understanding these causalities may help to shape the search for novel treatments for kidney disease patients.

Role of Uropathogenic Escherichia coli Virulence Factors in Development of Urinary Tract Infection and Kidney Damage

Uropathogenic Escherichia coli (UPEC) is a causative agent in the vast majority of urinary tract infections (UTIs), including cystitis and pyelonephritis, and infectious complications, which may result in acute renal failure in healthy individuals as well as in renal transplant patients. UPEC expresses a multitude of virulence factors to break the inertia of the mucosal barrier. In response to the breach by UPEC into the normally sterile urinary tract, host inflammatory responses are triggered leading to cytokine production, neutrophil influx, and the exfoliation of infected bladder epithelial cells. Several signaling pathways activated during UPEC infection, including the pathways known to activate the innate immune response, interact with calcium-dependent signaling pathways. Some UPEC isolates, however, might possess strategies to delay or suppress the activation of components of the innate host response in the urinary tract. Studies published in the recent past provide new information regarding how virulence factors of uropathogenic E. coli are involved in activation of the innate host response. Despite numerous host defense mechanisms, UPEC can persist within the urinary tract and may serve as a reservoir for recurrent infections and serious complications. Presentation of the molecular details of these events is essential for development of successful strategies for prevention of human UTIs and urological complications associated with UTIs.

Effect of activation of viral receptors on the gelatinases MMP-2 and MMP-9 in human mesothelial cells

BACKGROUND

Extracellular matrix (ECM) not only provides molecular and spatial information that influence cell proliferation, differentiation and apoptosis but also has the potential to bind and present or release cytokines and cytotactic factors. Synthesis and degradation of extracellular matrix components are balanced by matrix metalloproteinases (MMP) and their inhibitors. In the pericardium as well as in the pleural and peritoneal cavities a multitude of clinically relevant disease states ranging from inflammation to fibrosis and tumor invasion result from altered regulation of MMP activity and are known to be associated with viral disease.

METHODS

Therefore, the functional linkage between viral receptors of the innate immune system, the toll-like receptors (TLR), and control of MMP activity was exemplarily analyzed by stimulating human mesothelial cells with poly (I:C) RNA.

RESULTS

We hereby show that human mesothelial cells (MC) express TLR3. After stimulation of MC with the cytokines TNF-alpha, IL-1beta and IFN-gamma alone or in combination to simulate a proinflammatory milieu as would occur during immune-mediated inflammatory disease, an upregulation of TLR3 is seen. Furthermore, a selectively TLR3 mediated, time- and dose-dependent upregulation of MMP-9 and TIMP-1 is found, whereas MMP-2 expression is not significantly affected by TLR3 stimulation.

CONCLUSIONS

With these results we provide evidence for a mechanism by which infectious agents can mediate processes of the final common path of inflammation as fibrosis via regulation of MMP and TIMP.

Toll-like receptors (TLRs) and Nod-like receptors (NLRs) in inflammatory disorders

Toll-like receptors (TLRs) and Nod-like receptors (NLRs) are two major forms of innate immune sensors, which provide immediate responses against pathogenic invasion or tissue injury. Activation of these sensors induces the recruitment of innate immune cells such as macrophages and neutrophils, initiates tissue repair processes, and results in adaptive immune activation. Abnormalities in any of these innate sensor-mediated processes may cause excessive inflammation due to either hyper responsive innate immune signaling or sustained compensatory adaptive immune activation. Recent gene association studies appear to reveal strong associations of NLR gene mutations and development of several idiopathic inflammatory disorders. In contrast, TLR polymorphisms are less often associated with inflammatory disorders. Nevertheless, TLRs are up-regulated in the affected tissue of most inflammatory disorders, suggesting TLR signaling is involved in the pathogenesis of chronic and/or idiopathic inflammatory disorders. NLR signaling results in the formation of a molecular scaffold complex (termed an inflammasome) and orchestrates with TLRs to induce IL-1beta and IL-18, both of which are important mediators in the majority of inflammatory disorders. Therefore, understanding the roles of TLRs and NLRs in the pathogenesis of chronic and idiopathic inflammatory disorders may provide novel targets for the prevention and/or treatment of many common and uncommon diseases involving inflammation.

Toll-like receptor 9 affects severity of IgA nephropathy

Environmental pathogens are suspected to aggravate renal injury in IgA nephropathy (IgAN), but neither underlying mechanisms nor specific exogenous antigens have been identified. In this study, a genome-wide scan of ddY mice, which spontaneously develop IgAN, was performed, and myeloid differentiation factor 88 (MyD88) was identified as a candidate gene for progression of renal injury (chi(2) = 21.103, P = 0.00017). For evaluation of the potential influence of environmental pathogens on progression of renal injury, ddY mice were housed in either conventional or specific pathogen-free conditions. Expression of genes encoding toll-like receptors (TLR) and the signaling molecule MyD88 were quantified by real-time reverse transcription-PCR in splenocytes. Although the housing conditions did not affect the prevalence of IgAN, the severity of renal injuries was higher in the conventionally housed group. Mice that had IgAN and were housed in conventional conditions had higher levels of TLR9 and MyD88 transcripts than mice that had IgAN and were housed in specific pathogen-free conditions. Furthermore, nasal challenge with CpG-oligodeoxynucleotides, which are ligands for TLR9, aggravated renal injury, led to strong Th1 polarization, and increased serum and mesangial IgA. For investigation of whether these results may be generalizable to humans, single-nucleotide polymorphisms in the TLR9 and MyD88 genes were analyzed in two cohorts of patients with IgAN; an association was observed between TLR9 polymorphisms and disease progression. In summary, these findings suggest that activation of the TLR9/MyD88 pathway by common antigens may affect the severity of IgAN.

Nucleic acid recognition receptors in autoimmunity

Recent studies in mouse models of systemic autoimmune diseases have drawn attention to the involvement of Toll-like receptors (TLRs) in the generation of autoreactive immune responses. The endosomally localized TLRs7 and 9 are activated by autoimmune complexes containing self DNA and RNA in B lymphocytes and dendritic cells. These endogenous TLR ligands act as autoadjuvants providing a stimulatory signal together with the autoantigen and thus contribute to break peripheral tolerance against self antigens in systemic lupus erythematosus (SLE), for example. In vivo studies in SLE mouse models demonstrate an essential role for TLR7 in the generation of RNA-containing antinuclear antibodies and deposition of pathogenic immune complexes in the kidney. TLR9, however, appears to have immunostimulatory as well as regulatory functions in SLE mouse models. Type I Interferon, which is produced by plasmacytoid dendritic cells in response to autoimmune complexes containing RNA and DNA recognized by TLR7 and 9 acts as a potent amplifier of the autoimmune response. TLR-independent recognition of self nucleic acids by cytosolic RNA and DNA sensors may also play a role in the generation of autoimmune responses. Defects in protective mechanisms, which normally prevent immunostimulation by self nucleic acids in healthy individuals, promote the development of autoimmune diseases. For example, defects in nucleases that clear nucleic acids derived from apoptotic material, changes in the level and localization of TLR expression, defects in negative regulators of TLR signaling, or changes in the posttranscriptional modification of mammalian DNA and RNA may contribute to autoreactive responses. A better understanding of the exact function of different nucleic acid recognition receptors in the development of systemic autoimmunity will allow targeting of these innate immune receptors for the therapy of patients with systemic autoimmune diseases.

Innate pathogen recognition in the kidney: Toll-like receptors, NOD-like receptors, and RIG-like helicases

How do infections trigger or aggravate renal pathology? The discovery of the toll-like receptors, and more recently, the retinoic-acid-inducible protein-like helicases and the nucleotide-binding oligomerization domain-like receptors may offer new concepts to answer this question. Common pathogen-associated molecules are recognized by these receptors in several cellular compartments of immune cells and non-immune cells inside the kidney. This article summarizes ligand-receptor interactions and their known or potential significance in kidney diseases.

Virus-induced cellular immune mechanisms of injury to the kidney

Cellular immune systems play an important role in determining renal outcomes in virus-induced kidney diseases. Highlighted briefly are five different locations along the development of adaptive immune responses to viral infection that may promote injury to the renal parenchyma and the loss of renal function. This may occur because adaptive immune cells directly target infected renal parenchymal cells or because the kidney becomes a bystander organ of adaptive immune cell-mediated injury. Examples from recent studies are provided to illustrate how this may lead to clinically relevant renal disease.

TLR-dependent and TLR-independent pathways of type I interferon induction in systemic autoimmunity

We formulate a two-phase paradigm of autoimmunity associated with systemic lupus erythematosus, the archetypal autoimmune disease. The initial Toll-like receptor (TLR)-independent phase is mediated by dendritic cell uptake of apoptotic cell debris and associated nucleic acids, whereas the subsequent TLR-dependent phase serves an amplification function and is mediated by uptake of TLR ligands derived from self-antigens (principally nucleic acids) complexed with autoantibodies. Both phases depend on elaboration of type I interferons (IFNs), and therapeutic interruption of induction or activity of these cytokines in predisposed individuals might have a substantial mitigating effect in lupus and other autoimmune diseases.

Chemokines and Chemokine Receptors as Therapeutic Targets in Lupus Nephritis

Recruitment of leukocytes is a characteristic feature of tissue injury in systemic lupus erythematosus, including lupus nephritis. Locally secreted chemokines and their receptors are important mediators of leukocyte recruitment to the specific sites of immune complex injury, and contribute to renal inflammatory disease in the initiation and progression phase. Therefore, chemokines and chemokine receptors represent potential therapeutic targets in lupus nephritis. In this review we summarize available experimental and human data supporting their functional role in lupus nephritis. Moreover, interventional studies with chemokine and chemokine receptor antagonists that show the therapeutic potential of chemokine antagonists in experimental models of lupus nephritis and potentially in human renal disease are discussed.