Mechanistic connection between inflammation and fibrosis

MyD88 signaling pathway is involved in renal fibrosis by favoring a TH2 immune response and activating alternative M2 macrophages

Inflammation contributes to the pathogenesis of chronic kidney disease (CKD). Molecules released by the inflamed injured tissue can activate toll-like receptors (TLRs), thereby modulating macrophage and CD4(+) T-cell activity. We propose that in renal fibrogenesis, M2 macrophages are recruited and activated in a T helper subset 2 cell (T(H)2)-prone inflammatory milieu in a MyD88-dependent manner. Mice submitted to unilateral ureteral ligation (UUO) demonstrated an increase in macrophage infiltration with collagen deposition after 7 d. Conversely, TLR2, TLR4 and MyD88 knockout (KO) mice had an improved renal function together with diminished T(H)2 cytokine production and decreased fibrosis formation. Moreover, TLR2, TLR4 and MyD88 KO animals exhibited less M2 macrophage infiltration, namely interleukin (IL)-10(+) and CD206(+) CD11b(high) cells, at 7 d after surgery. We evaluated the role of a T(H)2 cytokine in this context, and observed that the absence of IL-4 was associated with better renal function, decreased IL-13 and TGF-β levels, reduced arginase activity and a decrease in fibrosis formation when compared with IL-12 KO and wild-type (WT) animals. Indeed, the better renal outcomes and the decreased fibrosis formation were restricted to the deficiency of IL-4 in the hematopoietic compartment. Finally, macrophage depletion, rather than the absence of T cells, led to reduced lesions of the glomerular filtration barrier and decreased collagen deposition. These results provide evidence that future therapeutic strategies against renal fibrosis should be accompanied by the modulation of the M1:M2 and T(H)1:T(H)2 balance, as T(H)2 and M2 cells are predictive of fibrosis toward mechanisms that are sensed by innate immune response and triggered in a MyD88-dependent pathway.

Endothelial-mesenchymal transition and its contribution to the emergence of stem cell phenotype

Vascular endothelial cells can demonstrate considerable plasticity to generate other cell types during embryonic development and disease progression. This process occurs through a cell differentiation mechanism known as endothelial-mesenchymal transition (EndMT). The generation of mesenchymal cells from endothelium is a crucial step in endothelial cell differentiation to several lineages including fibroblasts, myofibroblasts, mural cells, osteoblasts, chondrocytes, and adipocytes. Such differentiation patterns have been observed in systems of cardiac development, fibrosis, diabetic nephropathy, heterotopic ossification and cancer. Here we describe the EndMT program and discuss the current evidence of EndMT-mediated acquisition of stem cell characteristics and multipotent differentiation capabilities.

Loss of vitamin D receptor in chronic kidney disease: a potential mechanism linking inflammation to epithelial-to-mesenchymal transition

Both peritubular inflammation and tubular epithelial-to-mesenchymal transition (EMT) are critical events during the pathogenesis of renal fibrosis. However, the relationship between these two processes is unclear. Here, we investigated the potential role of the vitamin D receptor (VDR) in coupling peritubular inflammation and EMT. In a mouse model of unilateral ureteral obstruction (UUO), loss of VDR was observed as early as 1 day after surgery. In cultured proximal tubular epithelial HK-2 cells, proinflammatory TNF-α inhibited the expression of VDR in a dose- and time-dependant manner. Treatment with TNF-α sensitized HK-2 cells to EMT stimulated by transforming growth factor (TGF)-β1. Ectopic expression of VDR counteracted the synergistic effect of TNF-α and TGF-β1 on EMT. Furthermore, knockdown of VDR using a small interfering RNA strategy mimicked the effect of TNF-α on facilitating EMT. Either TNF-α treatment or a loss of VDR induced β-catenin activation and its nuclear translocation. The VDR ligand calcitriol reversed the VDR loss and inhibited EMT in the mouse UUO model, and late administration of active vitamin D was effective in restoring VDR expression as well, and reduced collagen accumulation and deposition compared with the vehicle control. β-Catenin expression induced by UUO was also significantly inhibited after the late administration of vitamin D. These results indicate that the early loss of VDR in chronic kidney diseases was likely mediated by proinflammatory TNF-α, which renders tubular cells susceptible to EMT. Our data suggest that loss of VDR couples peritubular inflammation and EMT, two key events in renal fibrogenesis.

Activation-induced deaminase contributes to the antibody-independent role of B cells in the development of autoimmunity

B cells contribute to autoimmunity both as secretors of pathogenic antibodies and through the activation of autoreactive T cells. B cells and antibodies acquire higher affinity to self-antigen through a process known as immunoglobulin hypermutation or SHM. The contribution of SHM to pathogenic antibody development in lupus has been established in various autoimmune mouse models and by examining antibodies from patients. However, its role in the antibody-independent contribution of B cells to autoimmunity has not been examined. Herein, we generate lupus-prone MRL/lpr mice with a limited IgM-only B cell repertoire, no secreted antibodies and no SHM. This enabled us to isolate the role of somatic hypermutation in B cell-mediated autoimmunity. We found that SHM-deficiency correlated with a reduction in autoreactive B cells, a decrease in T cell activation and a decrease in kidney lymphocytic infiltration. These data establish AID as an important contributor to the antibody-independent role of B cells in autoimmunity.

IgG4-Related Fibrotic Diseases from an Immunological Perspective: Regulators out of Control?

Patients with autoimmune pancreatitis have a striking polyclonal elevation of total IgG4 in serum. This observation has been confirmed and extended to other fibrotic conditions (that are therefore called IgG4-related disease) but as yet remains unexplained. The affected tissue contains many IgG4-producing plasma cells embedded in a fibrotic matrix originating from activated mesenchymal (stellate) cells. We propose that the process results from an unusual interaction between two regulatory systems: the regulatory arm of the immune system (including Bregs) and the tissue repair regulatory components orchestrated by the activated stellate cell. This interaction results in ongoing mutual activation, generating TGFbeta, IL10, and vitamin D. This environment suppresses most immune reactions but stimulates the development of IgG4-producing plasma cells.

Inflamed tumor-associated adipose tissue is a depot for macrophages that stimulate tumor growth and angiogenesis

Tumor-associated stroma is typified by a persistent, non-resolving inflammatory response that enhances tumor angiogenesis, growth and metastasis. Inflammation in tumors is instigated by heterotypic interactions between malignant tumor cells, vascular endothelium, fibroblasts, immune and inflammatory cells. We found that tumor-associated adipocytes also contribute to inflammation. We have analyzed peritumoral adipose tissue in a syngeneic mouse melanoma model. Compared to control adipose tissue, adipose tissue juxtaposed to implanted tumors exhibited reduced adipocyte size, extensive fibrosis, increased angiogenesis and a dense macrophage infiltrate. A mouse cytokine protein array revealed up-regulation of inflammatory mediators including IL-6, CXCL1, MCP-1, MIP-2 and TIMP-1 in peritumoral versus counterpart adipose tissues. CD11b(+) macrophages contributed strongly to the inflammatory activity. These macrophages were isolated from peritumoral adipose tissue and found to over-express ARG1, NOS2, CD301, CD163, MCP-1 and VEGF, which are indicative of both M1 and M2 polarization. Tumors implanted at a site distant from subcutaneous, anterior adipose tissue were strongly growth-delayed, had fewer blood vessels and were less populated by CD11b(+) macrophages. In contrast to normal adipose tissue, micro-dissected peritumoral adipose tissue explants launched numerous vascular sprouts when cultured in an ex vivo model. Thus, inflamed tumor-associated adipose tissue fuels the growth of malignant cells by acting as a proximate source for vascular endothelium and activated pro-inflammatory cells, in particular macrophages.

Ischemic postconditioning inhibits the renal fibrosis induced by ischemia-reperfusion injury in rats

OBJECTIVE

To investigate whether ischemic postconditioning effects on the development of tubulointerstitial fibrosis follow acute renal ischemia-reperfusion.

METHODS

Rat models of warm renal I/R were established by clamping left pedicles for 45 minutes after right nephrectomy, both with and without treatment with ischemic postconditioning, and then reperfused for up to 12 weeks. Hematoxylin-eosin (H&E) and Masson's trichrome staining were used to assess renal fibrosis. The expression spot and protein levels of α-smooth muscle actin (α-SMA), transforming growth factor-β1 (TGF-β1), and phospho-Smad2 were also analyzed.

RESULTS

Our data showed that patchy inflammation and tubulointerstitial fibrosis were found 12 weeks later in rats subjected to I/R alone or with postconditioning. Tubulointerstitial fibrosis worsened further in rats subjected to 45-minute ischemia-reperfusion, accompanied by the increased expressions of α-SMA, TGF-β1, and phospho-Smad2 at the end of 12 weeks. In contrast, the above histologic changes and molecular expressions were significantly attenuated in rats of ischemic postconditioning group.

CONCLUSION

The results indicated that 45-minute I/R injury may cause tubulointerstitial fibrosis in the long term, and ischemic postconditioning has beneficial effects on renal fibrosis. Its mechanisms may involve inhibition of the TGF-β1/phospho-Smad2 pathway to exert protective effects.

Tissue microenvironments define and get reinforced by macrophage phenotypes in homeostasis or during inflammation, repair and fibrosis

Current macrophage phenotype classifications are based on distinct in vitro culture conditions that do not adequately mirror complex tissue environments. In vivo monocyte progenitors populate all tissues for immune surveillance which supports the maintenance of homeostasis as well as regaining homeostasis after injury. Here we propose to classify macrophage phenotypes according to prototypical tissue environments, e.g. as they occur during homeostasis as well as during the different phases of (dermal) wound healing. In tissue necrosis and/or infection, damage- and/or pathogen-associated molecular patterns induce proinflammatory macrophages by Toll-like receptors or inflammasomes. Such classically activated macrophages contribute to further tissue inflammation and damage. Apoptotic cells and an-tiinflammatory cytokines dominate in postinflammatory tissues which induce macrophages to produce more anti-inflammatory mediators. Similarly, tumor-associated macrophages also confer immunosuppression in tumor stroma. Insufficient parenchymal healing despite abundant growth factors pushes macrophages to gain a profibrotic phenotype and promote fibrocyte recruitment which both enforce tissue scarring. Ischemic scars are largely devoid of cytokines and growth factors so that fibrolytic macrophages that predominantly secrete proteases digest the excess extracellular matrix. Together, macrophages stabilize their surrounding tissue microenvironments by adapting different phenotypes as feed-forward mechanisms to maintain tissue homeostasis or regain it following injury. Furthermore, macrophage heterogeneity in healthy or injured tissues mirrors spatial and temporal differences in microenvironments during the various stages of tissue injury and repair.

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.

PAI-1 in tissue fibrosis

Fibrosis is defined as a fibroproliferative or abnormal fibroblast activation-related disease. Deregulation of wound healing leads to hyperactivation of fibroblasts and excessive accumulation of extracellular matrix (ECM) proteins in the wound area, the pathological manifestation of fibrosis. The accumulation of excessive levels of collagen in the ECM depends on two factors: an increased rate of collagen synthesis and or decreased rate of collagen degradation by cellular proteolytic activities. The urokinase/tissue type plasminogen activator (uPA/tPA) and plasmin play significant roles in the cellular proteolytic degradation of ECM proteins and the maintenance of tissue homeostasis. The activities of uPA/tPA/plasmin and plasmin-dependent MMPs rely mostly on the activity of a potent inhibitor of uPA/tPA, plasminogen activator inhibitor-1 (PAI-1). Under normal physiologic conditions, PAI-1 controls the activities of uPA/tPA/plasmin/MMP proteolytic activities and thus maintains the tissue homeostasis. During wound healing, elevated levels of PAI-1 inhibit uPA/tPA/plasmin and plasmin-dependent MMP activities, and, thus, help expedite wound healing. In contrast to this scenario, under pathologic conditions, excessive PAI-1 contributes to excessive accumulation of collagen and other ECM protein in the wound area, and thus preserves scarring. While the level of PAI-1 is significantly elevated in fibrotic tissues, lack of PAI-1 protects different organs from fibrosis in response to injury-related profibrotic signals. Thus, PAI-1 is implicated in the pathology of fibrosis in different organs including the heart, lung, kidney, liver, and skin. Paradoxically, PAI-1 deficiency promotes spontaneous cardiac-selective fibrosis. In this review, we discuss the significance of PAI-1 in the pathogenesis of fibrosis in multiple organs.

Tubular cell dedifferentiation and peritubular inflammation are coupled by the transcription regulator Id1 in renal fibrogenesis

During renal fibrogenesis, tubular epithelial-mesenchymal transition is closely associated with peritubular inflammation; however, it is not clear whether these two processes are connected. We previously identified the inhibitor of differentiation-1 (Id1), a dominant negative antagonist of basic helix-loop-helix transcription factors, as a major trigger of tubular cell dedifferentiation after injury. Id1 was induced selectively in degenerated proximal tubule and collecting duct epithelia after injury and was present in both the cytoplasm and nucleus, suggesting shuttling between these two compartments. Interestingly, the upregulation of Id1 was associated with peritubular inflammation in mouse and human nephropathies. In vitro, Id1 potentiated NF-κB signaling and augmented RANTES expression in kidney epithelial cells, which led to an enhanced recruitment of inflammatory cells. Id1 also induced Snail1 expression and triggered tubular epithelial dedifferentiation. In vivo, genetic ablation of Id1 in mice reduced peritubular inflammation and decreased tubular expression of RANTES following ureteral obstruction. Mice lacking Id1 were also protected against myofibroblast activation and matrix expression, leading to a reduced total collagen deposition in obstructive nephropathy. Thus, these results indicate that Id1 shuttles between nucleus and cytoplasm and promotes peritubular inflammation and tubular epithelial dedifferentiation, suggesting that these two events are intrinsically coupled during renal fibrogenesis.

Pivotal role of Toll-like receptors 2 and 4, its adaptor molecule MyD88, and inflammasome complex in experimental tubule-interstitial nephritis

Tubule-interstitial nephritis (TIN) results in decreased renal function and interstitial inflammation, which ultimately leads to fibrosis. Excessive adenine intake can cause TIN because xanthine dehydrogenase (XDH) can convert this purine into an insoluble compound, which precipitates in the tubuli. Innate immune sensors, such as Toll-like receptors (TLR) and inflammasome complex, play a crucial role in the initiation of inflammation. The aim of this study was to evaluate the roles of TLR-2 and -4, Myd88 and inflammasome complex in an experimental model of TIN. Here, we show that wild-type (WT) mice fed adenine-enriched food exhibited significant renal dysfunction and enhanced cellular infiltration accompanied by collagen deposition. They also presented higher gene and protein expression of pro-inflammatory cytokines. In contrast, TLR-2, -4, MyD88, ASC and Caspase-1 KO mice showed renoprotection associated with expression of inflammatory molecules at levels comparable to controls. Furthermore, treatment of WT animals with allopurinol, an XDH inhibitor, led to reduced levels of uric acid, oxidative stress, collagen deposition and a downregulation of the NF-kB signaling pathway. We concluded that MyD88 signaling and inflammasome participate in the development of TIN. Furthermore, inhibition of XDH seems to be a promising way to therapeutically target the developing inflammatory process.

A novel U-STAT3-dependent mechanism mediates the deleterious effects of chronic nicotine exposure on renal injury

Previous data from our group have demonstrated (Arany I, Grifoni S, Clark JS, Csongradi, Maric C, Juncos LA. Am J Physiol Renal Physiol 301: F125-F133, 2011) that chronic nicotine (NIC) exposure exacerbates acute renal ischemic injury (AKI) in mice that could increase the risk for development and progression of chronic kidney disease (CKD). It has been shown that proximal tubules of the kidney can acquire characteristics that may compromise structural recovery and favor development of inflammation and fibrosis following injury. Chronic NIC exposure can amplify this epithelial process although the mechanism is not identified. Recently, the unphosphorylated form of signal transducer and activator of transcription-3 (U-STAT3) has emerged as a noncanonical mediator of inflammation and fibrosis that may be responsible for the effects of chronic NIC. We found that levels of transforming growth factor β-1 (TGF-β1), α-smooth muscle actin (α-SMA), fibronectin, monocyte chemotactic protein-1 (MCP-1), and expression of U-STAT3 were increased in the ischemic kidneys of NIC-exposed mice. Chronic NIC exposure also increased TGF-β1-dependent F-actin reorganization, vimentin, fibronectin, and α-SMA expression as well as promoter activity of α-SMA and MCP-1 without significant loss of epithelial characteristics (E-cadherin) in cultured renal proximal tubule cells. Importantly, transduction of cells with a U-STAT3 mimetic (Y705F-STAT3) augmented stress fiber formation and also amplified NIC+TGF-β1-induced expression of α-SMA, vimentin, fibronectin, as well as promoter activity of α-SMA and MCP-1. Our results reveal a novel, chronic NIC-exposure-related and U-STAT3-dependent mechanism as mediator of a sustained transcription of genes that are linked to remodeling and inflammation in the kidney during injury. This process may facilitate progression of AKI to CKD. The obtained data may lead to devising therapeutic methods to specifically enhance the protective and/or inhibit adverse effects of STAT3 in the kidney.

Detection of intrarenal microstructural changes with supersonic shear wave elastography in rats

OBJECTIVES

To evaluate, in a rat model of glomerulosclerosis, whether ultrasonic shear wave elastography detects kidney cortex stiffness changes and predicts histopathological development of fibrosis.

MATERIALS AND METHODS

Three groups were studied transversally: a control group (n = 8), a group after 4 weeks of L-NAME administration (H4, n = 8), and a group after 6 weeks (H6, n = 15). A fourth group was studied longitudinally (n = 8) before, after 4 weeks and after 7 weeks of L-NAME administration. Shear modulus of renal cortex was quantified using supersonic shear imaging technique. Urine was analysed for dosage of protein/creatinine ratio. Kidneys were removed for histological quantification of fibrosis.

RESULTS

Diseased rats showed an increased urinary protein/creatinine ratio. Cortical stiffness expressed as median (interquartile range) was 4.0 kPa (3.3-4.5) in control kidneys. It increased in all but one pathological groups: H4: 7.7 kPa (5.5-8.6) (p < 0.01); H6: 4.8 kPa (3.9-5.9) (not significant); in the longitudinal cohort, from 4.5 kPa (3.1-5.9) to 7.7 kPa (5.9-8.3) at week 4 (p < 0.05) and to 6.9 kPa (6.1-7.8) at week 7 (p < 0.05). Stiffness values were correlated with the proteinuria/creatininuria ratio (r = 0.639, p < 0.001).

CONCLUSIONS

Increased cortical stiffness is correlated with the degree of renal dysfunction. More experience in other models is necessary to understand its relationship with microstructural changes.