Crescentic Glomerulonephritis: Pathogenesis and Therapeutic Potential of Human Amniotic Stem Cells.

Chronic kidney disease (CKD) leads to significant morbidity and mortality worldwide. Glomerulonephritis (GN) is the second leading cause of CKD resulting in end stage renal failure. The most severe and rapidly progressive type of GN is characterized by glomerular crescent formation. The current therapies for crescentic GN, which consist of broad immunosuppressive drugs, are partially effective, non-specific, toxic and cause many serious side effects including infections, cancer, and cardiovascular problems. Therefore, new and safer therapies are needed. Human amniotic epithelial cells (hAECs) are a type of stem cell which are isolated from the placenta after birth. They represent an attractive and novel therapeutic option for the treatment of various inflammatory conditions owing to their unique and selective immunosuppressive ability, as well as their excellent safety profile and clinical applicability. In this review, we will discuss the immunopathogenesis of crescentic GN, issues with currently available treatments and how hAECs offer potential to become a new and harmless treatment option for this condition.

Emerging Cellular Therapies for Anti-myeloperoxidase Vasculitis and Other Autoimmune Diseases.

Anti-myeloperoxidase vasculitis (MPO-AAV) is a life-threatening autoimmune disease which causes severe inflammation of small blood vessels, mainly in the kidney. As for many other autoimmune diseases, current treatments, which consist of general immunosuppressants, are partially effective, toxic and broadly immunosuppressive, causing significant and serious adverse effects in many patients. Therefore, there is an urgent need for more targeted and less harmful therapies. Tolerogenic dendritic cells, regulatory T cells and stem cells have emerged as attractive, new and safer options for the treatment for various autoimmune diseases due to their unique and selective immunosuppressive capacity. In this review, we will discuss how these cellular therapies offer potential to become novel and safer treatments for MPO-AAV.

Tolerogenic Dendritic Cells Attenuate Experimental Autoimmune Antimyeloperoxidase Glomerulonephritis.

Background Because of their capacity to induce antigen-specific immunosuppression, tolerogenic dendritic cells are a promising tool for treatment of autoimmune conditions, such as GN caused by autoimmunity against myeloperoxidase (MPO). METHODS: We sought to generate tolerogenic dendritic cells to suppress anti-MPO GN by culturing bone marrow cells with an NFκB inhibitor (BAY 11-7082) and exposing them to a pulse of MPO. After administering these MPO/BAY dendritic cells or saline to mice with established anti-MPO or anti-methylated BSA (mBSA) immunity, we assessed immune responses and GN. We also examined mechanisms of action of MPO/BAY dendritic cells. RESULTS: MPO/BAY dendritic cells decreased anti-MPO immunity and GN without inhibiting immune responses against mBSA; they also induced IL-10-producing regulatory T cells in MPO-immunized mice without affecting IL-10+ CD4+Foxp3- type 1 regulatory T cells or regulatory B cells. MPO/BAY dendritic cells did not inhibit anti-MPO immunity when CD4+Foxp3+ cells were depleted in vivo, showing that regulatory T cells are required for their effects. Coculture experiments with dendritic cells and CD4+Foxp3- or CD4+Foxp3+ cells showed that MPO/BAY dendritic cells generate Foxp3+ regulatory T cells from CD4+Foxp3- cells through several pathways, and induce IL-10+ regulatory T cells via inducible costimulator (ICOS), which was confirmed in vivo. Transfer of MPO/BAY dendritic cell-induced regulatory T cells in vivo, with or without anti-IL-10 receptor antibody, demonstrated that they suppress anti-MPO immunity and GN via IL-10. CONCLUSIONS: MPO/BAY dendritic cells attenuate established anti-MPO autoimmunity and GN in an antigen-specific manner through ICOS-dependent induction of IL-10-expressing regulatory T cells. This suggests that autoantigen-loaded tolerogenic dendritic cells may represent a novel antigen-specific therapeutic option for anti-MPO GN.

OX40 ligand is inhibitory during the effector phase of crescentic glomerulonephritis.

BACKGROUND: The functional relevance of OX40 ligand (OX40L) in the effector phase of crescentic glomerulonephritis (GN) is unknown. These studies defined the role of endogenous OX40L during the effector stage of murine crescentic GN. METHODS: GN was induced by immunization with sheep globulin/adjuvant on Day 0 and injection of sheep anti-mouse glomerular basement membrane immunoglobulin (Ig) on Day 10. Rat IgG or neutralizing anti-OX40L antibody was administered on Days 10-18 and immune responses and renal injury assessed on Day 20. RESULTS: Compared with naïve animals, OX40L was upregulated in the lymph nodes (LNs) and on leucocytes and resident non-immune cells in the kidneys of mice with GN. Inhibition of OX40L in GN augmented renal injury, as indicated by increased crescent formation, proteinuria and glomerular leucocyte accumulation. In line with increased injury, anti-OX40L treatment increased proliferation and decreased apoptosis of CD4 T cells in the LNs, without affecting LN CD4 cytokine production and CD8 T-cell responses. Blockade of OX40L decreased LN regulatory T-cell (Treg) proliferation, transforming growth factor ß production and foxp3 expression. OX40L inhibition did not affect B cell expansion or circulating antibody levels. In the kidney, neutralization of OX40L augmented interferon γ (IFNγ) expression by CD4 and CD8 T cells and shifted macrophage polarization towards the pro-inflammatory M1 phenotype. CONCLUSIONS: OX40L is protective during the effector phase of murine crescentic GN by reducing the expansion of CD4 T cells and enhancing Treg responses in the LNs, and by locally inhibiting T-cell IFNγ production and pro-inflammatory macrophage phenotype in the kidney.

Formyl peptide receptor activation inhibits the expansion of effector T cells and synovial fibroblasts and attenuates joint injury in models of rheumatoid arthritis.

The effects of formyl peptide receptors (FPRs) on effector T cells and inflammation-causing tissue-resident cells are not well known. Here, we explored the effect of FPR activation on efferent T cell responses in models of rheumatoid arthritis (RA) and on the expansion of fibroblast-like synoviocytes (FLS). Compound 43 (Cpd43; FPR1/2 agonist) was administered to mice with collagen-induced arthritis (CIA) or antigen-induced arthritis (AIA) after disease onset. Joint inflammation/damage and immunity were assessed. FLS were cultured with Cpd43 to test its effects on cell apoptosis and proliferation. To explore the effects of endogenous FPR2 ligands on FLS proliferation, FLS FPR2 was blocked or Annexin A1 (AnxA1) expression silenced. Cpd43 reduced arthritis severity in both models. In CIA, Cpd43 decreased CD4 T cell proliferation and survival and increased the production of the protective cytokine, IFNγ, in lymph nodes. In AIA, Cpd43 increased CD4 apoptosis and production of the anti-inflammatory IL-4, while augmenting the proportion of splenic regulatory T cells and their expression of IL-2Rα. In both models, Cpd43 increased CD4 IL-17A production, without affecting humoral immunity. FPR2 inhibitors reversed Cpd43-mediated effects on AIA and T cell immunity. Cpd43 decreased TNF-induced FLS proliferation and augmented FLS apoptosis in association with intracellular FPR2 accumulation, while endogenous AnxA1 and FPR2 reduced FLS proliferation via the ERK and NFκB pathways. Overall, FPR activation inhibits the expansion of arthritogenic effector CD4 T cells and FLS, and reduces joint injury in experimental arthritis. This suggests the therapeutic potential of FPR ligation for the treatment of RA.

C5a receptor 1 promotes autoimmunity, neutrophil dysfunction and injury in experimental anti-myeloperoxidase glomerulonephritis.

The prospects for complement-targeted therapy in ANCA-associated vasculitis have been enhanced by a recent clinical trial in which C5a receptor 1 (C5aR1) inhibition safely replaced glucocorticoids in induction treatment. C5aR1 primes neutrophils for activation by anti-neutrophil cytoplasmic antibody (ANCA) and is therefore required in models of glomerulonephritis induced by anti-myeloperoxidase antibody. Although humoral and cellular autoimmunity play essential roles in ANCA-associated vasculitis, a role for C5aR1 in these responses has not been described. Here, we use murine models to dissect the role of C5aR1 in the generation of anti-myeloperoxidase autoimmunity and the effector responses resulting in renal injury. The genetic absence or pharmacological inhibition of C5aR1 results in reduced autoimmunity to myeloperoxidase with an attenuated Th1 response, increased Foxp3+ regulatory T cells and reduction in generation of myeloperoxidase-ANCA. These changes are mediated by C5aR1 on dendritic cells, which promotes activation, and thus myeloperoxidase autoimmunity and glomerulonephritis. We also use renal intravital microscopy to determine the effect of C5aR1 inhibition on ANCA induced neutrophil dysfunction. We found that myeloperoxidase-ANCA induce neutrophil retention and reactive oxygen species burst within glomerular capillaries. These pathological behaviors are abrogated by C5aR1 inhibition. Thus, C5aR1 inhibition ameliorates both autoimmunity and intra-renal neutrophil activation in ANCA-associated vasculitis.

Pathogenic Role for γδ T Cells in Autoimmune Anti-Myeloperoxidase Glomerulonephritis.

Myeloperoxidase (MPO) anti-neutrophil cytoplasmic Ab (ANCA)-associated vasculitis results from autoimmunity to MPO. IL-17A plays a critical role in generating this form of autoimmune injury but its cell of origin is uncertain. We addressed the hypothesis that IL-17A-producing γδ T cells are a nonredundant requisite in the development of MPO autoimmunity and glomerulonephritis (GN). We studied MPO-ANCA GN in wild type, αß, or γδ T cell-deficient (C57BL/6, ßTCR-/- , and δTCR-/- respectively) mice. Both T cell populations played important roles in the generation of autoimmunity to MPO and GN. Humoral autoimmunity was dependent on intact αß T cells but was unaffected by γδ T cell deletion. Following MPO immunization, activated γδ T cells migrate to draining lymph nodes. Studies in δTCR-/- and transfer of γδ T cells to δTCR-/- mice show that γδ T cells facilitate the generation of anti-MPO autoimmunity and GN. δTCR-/- mice that received IL-17A-/- γδ T cells demonstrate that the development of anti-MPO autoimmunity and GN are dependent on γδ T cell IL-17A production. Finally, transfer of anti-MPO CD4+ T cell clones to naive δTCR-/- and wild type mice with planted glomerular MPO shows that γδ T cells are also necessary for recruitment of anti-MPO αß CD4+ effector T cells. This study demonstrates that IL-17A produced by γδ T cells plays a critical role in the pathogenesis of MPO-ANCA GN by promoting the development of MPO-specific αß T cells.

Neutrophil-Mediated Regulation of Innate and Adaptive Immunity: The Role of Myeloperoxidase.

Neutrophils are no longer seen as leukocytes with a sole function of being the essential first responders in the removal of pathogens at sites of infection. Being armed with numerous pro- and anti-inflammatory mediators, these phagocytes can also contribute to the development of various autoimmune diseases and can positively or negatively regulate the generation of adaptive immune responses. In this review, we will discuss how myeloperoxidase, the most abundant neutrophil granule protein, plays a key role in the various functions of neutrophils in innate and adaptive immunity.

Endogenous Toll-Like Receptor 9 Regulates AKI by Promoting Regulatory T Cell Recruitment.

Toll-like receptor 9 (TLR9) enhances proinflammatory responses, but whether it can act in a regulatory capacity remains to be established. In experimental murine AKI induced by cisplatin, Tlr9(-/-) mice developed enhanced renal injury and exhibited fewer intrarenal regulatory T cells (Tregs) compared with genetically intact mice. A series of reconstitution and depletion studies defined a role for TLR9 in maintaining Treg-mediated homeostasis in cisplatin-induced AKI. When Rag1(-/-) mice were reconstituted with nonregulatory CD25(-) splenocytes from wild-type (WT) or Tlr9(-/-) mice, AKI was similarly enhanced. However, when Rag1(-/-) mice were reconstituted with CD4(+)CD25(+) regulatory cells, WT CD4(+)CD25(+) cells were more renoprotective and localized to the kidney more efficiently than Tlr9(-/-) CD4(+)CD25(+) cells. In Treg-depleted Foxp3(DTR) mice, reconstitution with naive WT CD4(+)CD25(+) cells resulted in less severe AKI than did reconstitution with Tlr9(-/-) Tregs. Tlr9(-/-) mice were not deficient in CD4(+)CD25(+) cells, and WT and TLR9-deficient Tregs had similar suppressive function ex vivo. However, expression of adhesion molecules important in Treg trafficking was reduced on peripheral CD4(+)CD25(+) cells from Tlr9(-/-) mice. In conclusion, we identified a pathway by which TLR9 promotes renal Treg accumulation in AKI.

Mast Cell Stabilization Ameliorates Autoimmune Anti-Myeloperoxidase Glomerulonephritis.

Observations in experimental murine myeloperoxidase (MPO)-ANCA-associated vasculitis (AAV) show mast cells degranulate, thus enhancing injury as well as producing immunomodulatory IL-10. Here we report that, compared with biopsy specimens from control patients, renal biopsy specimens from 44 patients with acute AAV had more mast cells in the interstitium, which correlated with the severity of tubulointerstitial injury. Furthermore, most of the mast cells were degranulated and spindle-shaped in patients with acute AAV, indicating an activated phenotype. We hypothesized that the mast cell stabilizer disodium cromoglycate would attenuate mast cell degranulation without affecting IL-10 production. We induced anti-MPO GN by immunizing mice with MPO and a low dose of anti-glomerular basement membrane antibody. When administered before or after induction of MPO autoimmunity in these mice, disodium cromoglycate attenuated mast cell degranulation, development of autoimmunity, and development of GN, without diminishing IL-10 production. In contrast, administration of disodium cromoglycate to mast cell-deficient mice had no effect on the development of MPO autoimmunity or GN. MPO-specific CD4(+) effector T cell proliferation was enhanced by co-culture with mast cells, but in the presence of disodium cromoglycate, proliferation was inhibited and IL-10 production was enhanced. These results indicate that disodium cromoglycate blocks injurious mast cell degranulation specifically without affecting the immunomodulatory role of these cells. Thus as a therapeutic, disodium cromoglycate may substantially enhance the regulatory role of mast cells in MPO-AAV.

Glucocorticoid-induced leucine zipper (GILZ) inhibits B cell activation in systemic lupus erythematosus.

OBJECTIVES: Systemic lupus erythematosus (SLE) is a serious multisystem autoimmune disease, mediated by disrupted B cell quiescence and typically treated with glucocorticoids. We studied whether B cells in SLE are regulated by the glucocorticoid-induced leucine zipper (GILZ) protein, an endogenous mediator of anti-inflammatory effects of glucocorticoids. METHODS: We conducted a study of GILZ expression in blood mononuclear cells of patients with SLE, performed in vitro analyses of GILZ function in mouse and human B cells, assessed the contributions of GILZ to autoimmunity in mice, and used the nitrophenol coupled to keyhole limpet haemocyanin model of immunisation in mice. RESULTS: Reduced B cell GILZ was observed in patients with SLE and lupus-prone mice, and impaired induction of GILZ in patients with SLE receiving glucocorticoids was associated with increased disease activity. GILZ was downregulated in naïve B cells upon stimulation in vitro and in germinal centre B cells, which contained less enrichment of H3K4me3 at the GILZ promoter compared with naïve and memory B cells. Mice lacking GILZ spontaneously developed lupus-like autoimmunity, and GILZ deficiency resulted in excessive B cell responses to T-dependent stimulation. Accordingly, loss of GILZ in naïve B cells allowed upregulation of multiple genes that promote the germinal centre B cell phenotype, including lupus susceptibility genes and genes involved in cell survival and proliferation. Finally, treatment of human B cells with a cell-permeable GILZ fusion protein potently suppressed their responsiveness to T-dependent stimuli. CONCLUSIONS: Our findings demonstrated that GILZ is a non-redundant regulator of B cell activity, with important potential clinical implications in SLE.

Suppression of Autoimmunity and Renal Disease in Pristane-Induced Lupus by Myeloperoxidase.

OBJECTIVE: Myeloperoxidase (MPO) locally contributes to organ damage in various chronic inflammatory conditions by generating reactive intermediates. The contribution of MPO in the development of experimental lupus is unknown. The aim of this study was to define the role of MPO in murine lupus nephritis (LN). METHODS: LN was induced in C57BL/6 wild-type (WT) and MPO knockout (MPO(-/-) ) mice by intraperitoneal injection of pristane. Autoimmunity and glomerulonephritis were assessed 20 and 40 weeks after pristane administration. Cell apoptosis, leukocyte accumulation, and cytokine levels in the peritoneal cavity of WT and MPO(-/-) mice were assessed 3 or 6 days after pristane injection. RESULTS: MPO(-/-) mice developed more severe nephritis than did WT mice 20 and 40 weeks after pristane injection, despite having reduced glomerular deposition of antibody and complement and diminished levels of markers of oxidative stress (oxidized DNA and glutathione sulfonamide). Enhancement of renal disease in MPO-deficient mice correlated with increased accumulation of CD4+ T cells and macrophages in glomeruli, which, in turn, was associated with augmented generation of CD4+ T cell responses and increased activation and migration of dendritic cells in secondary lymphoid organs. In addition, the enhanced renal injury in MPO(-/-) mice was associated with increased glomerular accumulation of neutrophils and deposition of neutrophil extracellular traps. MPO deficiency also increased early cell apoptosis, leukocyte accumulation, and proinflammatory cytokine expression in the peritoneum. CONCLUSION: MPO attenuates pristane-induced LN by inhibiting early inflammatory responses in the peritoneum and limiting the generation of CD4+ T cell autoimmunity in secondary lymphoid organs.

T cell mediated autoimmune glomerular disease in mice.

Many forms of glomerulonephritis are mediated by autoimmunity. While autoantibodies are often pathogenic, cell-mediated immunity plays an important role in a number of forms of rapidly progressive glomerulonephritis. This unit describes the induction of cell-mediated autoimmune glomerular disease in mice. One disease model, experimental anti-glomerular basement membrane (GBM) disease, features autoreactivity to a well-defined component of type IV collagen found in the GBM, α3(IV)NC1. The other models the cell-mediated effector response in forms of renal vasculitis, where autoantibodies to myeloperoxidase result in systemic neutrophil activation, resulting in their localization to the glomerulus and the subsequent deposition of myeloperoxidase within glomerular capillaries. There, myeloperoxidase acts as a "planted" autoantigen and is recognized by effector autoreactive myeloperoxidase-specific T cells. These models are useful both in defining mechanisms germane to the development of autoimmunity to α3(IV)NC1 and myeloperoxidase, and in dissecting the role of cell-mediated responses in effecting glomerular injury.

Glomerulonephritis Induced by Heterologous Anti-GBM Globulin as a Planted Foreign Antigen.

The glomerulonephritides are diseases characterized by immune-mediated glomerular inflammation. Most severe and rapidly progressive forms of glomerulonephritis feature the participation of injurious leukocytes that localize to glomeruli. This unit describes classical models of rapidly progressive glomerulonephritis in mice, induced by injecting heterologous globulin (raised in sheep) that binds to the glomerular basement membrane. These models have been particularly useful in defining the participation of effector leukocytes in severe glomerular disease. In these models, injury typically occurs in two phases. In the initial, heterologous phase, injury is mediated by the globulin bound within the glomerulus acting as an antibody. The later, autologous phase of injury is mediated by the host's adaptive immunity to the heterologous globulin now functioning as a planted foreign antigen within glomeruli. As autologous phase injury is driven by immunity to sheep globulin, assessment of antigen-specific systemic immunity to sheep globulin is critical when using this model.

FcγRIIB regulates T-cell autoreactivity, ANCA production, and neutrophil activation to suppress anti-myeloperoxidase glomerulonephritis.

Anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis involves innate and adaptive immune cells in the induction of autoimmunity and in autoimmune effector responses. Most Fcγ receptors (FcγRs) activate immune cells, but FcγRIIB, found in humans and mice on B cells and innate cells, is an inhibitory receptor. Here we tested whether endogenous FcγRIIB negatively regulates autoreactivity and effector responses in experimental anti-myeloperoxidase (MPO) glomerulonephritis, using wild-type and FcγRIIB(-/-) mice. After MPO immunization, FcγRIIB(-/-) mice developed higher MPO-ANCA titers and increased anti-MPO T-cell responses. Transfer of FcγRIIB-deficient dendritic cells loaded with a nephritogenic MPO peptide (MPO409-428) into wild-type mice induced stronger autoimmunity than dendritic cells derived from wild-type mice. Transferring anti-MPO antibodies into lipopolysaccharide-primed mice resulted in increased glomerular neutrophil accumulation and injury in FcγRIIB(-/-) mice, showing a role for FcγRIIB in suppressing neutrophil activation. Inducing active autoimmunity to MPO followed by triggering T cell-mediated glomerular injury by transfer of sub-nephritogenic doses of lipopolysaccharide and anti-MPO antibodies resulted in more disease in FcγRIIB(-/-) mice. Thus, endogenous FcγRIIB negatively regulates anti-MPO autoimmunity and glomerulonephritis by dendritic cells, B cells, and neutrophils to limit MPO-ANCA production, T-cell responses, and neutrophil activation.

Endogenous myeloperoxidase is a mediator of joint inflammation and damage in experimental arthritis.

OBJECTIVE: Myeloperoxidase (MPO) is implicated as a local mediator of tissue damage when released extracellularly in many chronic inflammatory diseases. The purpose of this study was to explore the role of endogenous MPO in experimental rheumatoid arthritis (RA). METHODS: K/BxN serum-transfer arthritis was induced in C57BL/6 wild-type (WT) and MPO knockout (MPO(-/-) ) mice, and disease development was assessed. MPO activity was measured in joint tissues from mice with or without K/BxN arthritis. Collagen-induced arthritis (CIA) was induced in WT and MPO(-/-) mice, and disease development and immune responses were examined. MPO expression was assessed in synovial biopsy samples from patients with active RA, and the effect of MPO on synovial fibroblasts was tested in vitro. RESULTS: MPO was up-regulated in the joints of mice with K/BxN arthritis, and MPO deficiency attenuated the severity of the disease without affecting circulating cytokine levels. In CIA, MPO(-/-) mice had enhanced CD4+ T cell responses and reduced frequency of regulatory T cells in the lymph nodes and spleen, as well as augmented interleukin-17A and diminished interferon-γ secretion by collagen-stimulated splenocytes, without an effect on circulating anticollagen antibody levels. Despite enhanced adaptive immunity in secondary lymphoid organs, CIA development was attenuated in MPO(-/-) mice. Intracellular and extracellular MPO was detected in the synovium of patients with active RA, and human MPO enhanced the proliferation and decreased the apoptosis of synovial fibroblasts in vitro. CONCLUSION: MPO contributes to the development of arthritis despite suppressing adaptive immunity in secondary lymphoid organs. This suggests distinct effects of local MPO on arthritogenic effector responses.

Innate IL-17A-producing leukocytes promote acute kidney injury via inflammasome and Toll-like receptor activation.

In acute kidney injury, which is a significant cause of morbidity and mortality, cytokines and leukocytes promote inflammation and injury. We examined the pathogenic role of IL-17A in cisplatin-induced acute kidney injury. Intrarenal IL-17A mRNA transcription and protein expression were increased in wild-type mice after cisplatin-induced renal injury. An important role for IL-17A in the nephrotoxicity of cisplatin was demonstrated by observing protection from cisplatin-induced functional and histological renal injury in Il17a(-/-) and Rorγt(-/-) mice, as well as in mice treated pre-emptively with anti-IL-17A antibodies. Both renal injury and renal IL-1ß and IL-17A production were attenuated in Asc(-/-) and Tlr2(-/-) mice, suggesting that cisplatin induces endogenous TLR2 ligand production and activates the ASC-dependent inflammasome complex, resulting in IL-1ß and injurious IL-17A production. Neutrophils and natural killer cells are the likely targets of these pathways, because combined depletion of these cells was strongly protective; anti-IL-17A antibodies had no additional effect in this setting. Although IL-17A can also be produced by CD4(+) and γδ T cells, IL-17A from those cells does not contribute to renal injury. Cisplatin-induced injury was unchanged in γδ T-cell-deficient mice, whereas Il17a(-/-) CD4(+) T cells induced similar injury as did wild-type CD4(+) T cells on transfer to cisplatin-injected Rag1(-/-) mice. These studies demonstrate an important role for TLR2, the ASC inflammasome, and IL-17A in innate leukocytes in cisplatin-induced renal injury.

Neutrophil myeloperoxidase regulates T-cell-driven tissue inflammation in mice by inhibiting dendritic cell function.

Myeloperoxidase (MPO) is important in intracellular microbial killing by neutrophils but extracellularly causes tissue damage. Its role in adaptive immunity and T-cell-mediated diseases is poorly understood. Here, T-cell responses in lymph nodes (LNs) were enhanced by MPO deletion or in vivo inhibition, causing enhanced skin delayed-type hypersensitivity and antigen (Ag)-induced arthritis. Responses of adoptively transferred OT-II T cells were greater in MPO-deficient than wild-type (WT) recipients. MPO, deposited by neutrophils in LNs after Ag injection, interacted with dendritic cells (DCs) in vivo. Culture of murine or human DCs with purified MPO or neutrophil supernatant showed that enzymatically dependent MPO-mediated inhibition of DC activation occurs via MPO-generated reactive intermediates and involves DC Mac-1. Transfer of DCs cultured with WT, but not MPO-deficient, neutrophil supernatant attenuated Ag-specific immunity in vivo. MPO deficiency or in vivo inhibition increased DC activation in LNs after immunization. Studies with DQ-ovalbumin showed that MPO inhibits Ag uptake/processing by DCs. In vivo DC transfer and in vitro studies showed that MPO inhibits DC migration to LNs by reducing their expression of CCR7. Therefore, MPO, via its catalytic activity, inhibits the generation of adaptive immunity by suppressing DC activation, Ag uptake/processing, and migration to LNs to limit pathological tissue inflammation.

Mast cells contribute to peripheral tolerance and attenuate autoimmune vasculitis.

Mast cells contribute to the modulation of the immune response, but their role in autoimmune renal disease is not well understood. Here, we induced autoimmunity resulting in focal necrotizing GN by immunizing wild-type or mast cell-deficient (Kit(W-sh/W-sh)) mice with myeloperoxidase. Mast cell-deficient mice exhibited more antimyeloperoxidase CD4+ T cells, enhanced dermal delayed-type hypersensitivity responses to myeloperoxidase, and more severe focal necrotizing GN. Furthermore, the lymph nodes draining the sites of immunization had fewer Tregs and reduced production of IL-10 in mice lacking mast cells. Reconstituting these mice with mast cells significantly increased the numbers of Tregs in the lymph nodes and attenuated both autoimmunity and severity of disease. After immunization with myeloperoxidase, mast cells migrated from the skin to the lymph nodes to contact Tregs. In an ex vivo assay, mast cells enhanced Treg suppression through IL-10. Reconstitution of mast cell-deficient mice with IL-10-deficient mast cells led to enhanced autoimmunity to myeloperoxidase and greater disease severity compared with reconstitution with IL-10-intact mast cells. Taken together, these studies establish a role for mast cells in mediating peripheral tolerance to myeloperoxidase, protecting them from the development of focal necrotizing GN in ANCA-associated vasculitis.