Pathogenic effects of antimyeloperoxidase antibodies in patients with microscopic polyangiitis

Neutrophils in ANCA-associated vasculitis: Mechanisms and implications for management

Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a group of systemic autoimmune diseases, which is typified by inflammatory necrosis predominantly affecting the small vessels and often accompanied by positive ANCA. Clinically, AAV primarily includes microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA), and eosinophilic granulomatosis with polyangiitis (EGPA). It has been found that in AAV pathogenesis, both innate and adaptive immunity are related to neutrophil function mutually. Many proteins, such as myeloperoxidase (MPO) and proteinase 3 (PR3), in neutrophil cytoplasm lead to the production of proteins such as MPO-ANCA and PR3-ANCA by activating adaptive immunity. In addition, through the process of neutrophil extracellular trap (NET) formation, activation of an alternative complement pathway and the respiratory burst can stimulate the neutrophils close to vascular endothelial cells and will participate the vessel inflammation. This review aims to reveal the potential mechanisms regulating the association between the neutrophils and various types of AAVs and to emphasize the results of recent findings on these interactions. Moreover, multiple underlying signaling pathways involved in the regulation of neutrophils during AAV processes have also been discussed. The ultimate goal of this review is to identify novel biomarkers and therapeutic targets for AAV management in the future.

The presentation, etiologies, pathophysiology, and treatment of pulmonary renal syndrome: A review of the literature

Pulmonary renal syndrome (PRS) is a constellation of different disorders that cause both rapidly progressive glomerulonephritis and diffuse alveolar hemorrhage. While antineutrophil cytoplasmic antibody associated vasculitis and anti-glomerular basement membrane disease are the predominant causes of PRS, numerous other mechanisms have been shown to cause this syndrome, including thrombotic microangiopathies, drug exposures, and infections, among others. This syndrome has high morbidity and mortality, and early diagnosis and treatment is imperative to improve outcomes. Treatment generally involves glucocorticoids and immunosuppressive agents, but treatment targeted to the underlying disorder can improve outcomes and mitigate side effects. Familiarity with the wide range of possible causes of PRS can aid the clinician in workup, diagnosis and early initiation of treatment. This review provides a summary of the clinical presentation, etiologies, pathophysiology, and treatment of PRS.

[Pathogenesis of ANCA-associated vasculitides in 2021: An update]

Anticytoplasmic neutrophil antibodies (ANCA)-associated vasculitis (AAV) are rare systemic immune-mediated diseases characterized by small vessel necrotizing vasculitis and/or respiratory tract inflammation. Over the last 2 decades, anti-MPO vasculitis mouse model has enlightened the role of ANCA, neutrophils, complement activation, T helper cells (Th1, Th17) and microbial agents. In humans, CD4T cells have been extensively studied, while the dramatic efficacy of rituximab demonstrated the key role of B cells. Many areas of uncertainty remain, such as the driving force of GPA extra-vascular granulomatous inflammation and the relapse risk of anti-PR3 AAV pathogenesis. Animal models eventually led to identify complement activation as a promising therapeutic target. New investigation tools, which permit in depth immune profiling of human blood and tissues, may open a new era for the studying of AAV pathogenesis.

The Onset of Systemic Oxidative Stress Associated with the Accumulation of Lipid Peroxidation Product Acrolein in the Skin of Patients with Small-Vessel Vasculitis

Small-vessel vasculitis (SVV) is the inflammation of the vessel wall that can result in hemorrhage and/or ischemia. Among the histological findings in SVV are increased infiltrating neutrophils, which, due to their oxidative burst and myeloperoxidase activity, release excessive reactive oxygen species, triggering a chain reaction of lipid peroxidation and yielding reactive aldehydes such as acrolein. The implication of oxidative stress in the pathogenesis of SVV was studied, focusing on acrolein immunohistochemistry in the affected skin vessels and systemic stress response. Samples from SVV patients and healthy subjects were collected and analyzed for total serum peroxides, total antioxidant capacity, inflammatory and immunological parameters, as well as for the presence of acrolein–protein adducts in the skin tissue specimens. The obtained data showed that systemic redox homeostasis and iron metabolism are altered in SVV patients. Possible biomarkers in the evaluation of oxidative status, disease activity and prevalence were indicated. Furthermore, a strong correlation between the accumulation of acrolein–protein adducts in the skin and the progression of the disease was revealed. Thus, the results of this study demonstrate that SVV is not only associated with systemic oxidative stress but also with tissue-specific oxidative stress that promotes acrolein formation and protein modification correlating with the severity of cutaneous vasculitis.

Antimyeloperoxidase and proteinase 3 antibodies for nephritis flare prediction in ANCA-associated-vasculitis

BACKGROUND

The value of myeloperoxidase and proteinase 3 antibodies titers in the assessment of renal disease activity and flare prediction in patients with ANCA-associated-vasculitis (AAV) is not well-known.

METHODS

Retrospective study including 113 AVV patients with a renal biopsy-proven pauci-immune necrotizing glomerulonephritis from seven Spanish hospitals. The main inclusion criteria were assessment of MPO antibodies (MPOab) using multiplex flow immunoassay and PR3 antibodies (PR3ab) measurements using immunoassay chemiluminescence with an identical range of values for all participating centers.

RESULTS

Serum MPOab, 3 ± 1.2 months before relapse, was higher in patients who relapsed (19.2 ± 12.2 vs 3.2 ± 5.1 AI, p < 0.001). The discrimination value of MPOab 3 months before renal relapse had an AUC of 0.82 (95%CI 0.73-0.92; p < 0.001). ΔMPOab (change in antibodies titration 6 months before relapse) was higher in patients who relapsed [8.3 ± 12 vs 0.9 ± 3.1 AI, p = 0.001) (AI; antibody index unit). The discrimination value of ΔMPO had an AUC of 0.76 (95%CI 0.63-0.88; p < 0.001). The positive predictive value of renal relapse in PR3 patients is 100% and the negative predictive value of renal relapse in patients with PR3 positive titers is 57.1%. Serum PR3ab was higher in patients who relapsed 2.8 ± 1.4 months before relapse (58.6 ± 24.6 vs 2.0 ± 0.6 AI, p < 0.001).

CONCLUSIONS

MPO antibody level monitorization using multiplex flow immunoassay and PR3 measurements using immunoassay chemiluminescence are useful and sensitive tools for the prediction of renal relapse in the follow-up of AAV patients with renal disease, and relevant surrogate markers of renal disease activity.

Proposal for a more practical classification of antineutrophil cytoplasmic antibody-associated vasculitis

The nomenclature for antineutrophil cytoplasmic antibody (ANCA)-associated kidney disease has evolved from honorific eponyms to a descriptive-based classification scheme (Chapel Hill Consensus Conference 2012). Microscopic polyangiitis, granulomatosis with polyangiitis and eosinophilic granulomatosis with polyangiitis do not correlate with presentation, response rates and relapse rates as when comparing myeloperoxidase versus leukocyte proteinase 3. Here we discuss the limitations of the currently used classification and propose an alternative, simple classification according to (i) ANCA type and (ii) organ involvement, which provides important clinical information of prognosis and outcomes.

Microscopic polyangiitis: an incidental finding in a patient with stroke

Microscopic polyangiitis (MPA) is a primary systemic vasculitis characterized by inflammation of small-sized vessels associated with the presence of anti-neutrophilic cytoplasmic antibodies. We report a case of a 39-year-old female diagnosed with microscopic polyangiitis as an incidental finding who presented with signs and symptoms of a stroke at a young age. Usually, it presents with fever, malaise, skin rash, weight loss, mononeuritis multiplex, and arthralgia/myalgia. Very rarely, it can involve meninges to cause meningeal vasculitis which can present as a febrile seizure. The most frequent neurological manifestation is peripheral neuropathy. Cerebral infarction or hemorrhage as an isolated finding is very rarely observed in the patient with MPA as was seen in our patient.

Refractory optic perineuritis related to granulomatosis with polyangiitis treated with intensive immunosuppressive therapy combined with plasma exchange

Optic perineuritis (OPN), which is an inflammatory disorder affecting the optic nerve sheath, is one of the rare complications in granulomatosis with polyangiitis (GPA). Although several groups have reported that immunosuppressive therapies are generally effective against GPA-associated OPN, so far, there is little information as to other options for refractory cases. Here we demonstrate a case of GPA-associated OPN, which is refractory to potent immunosuppressive therapy including high-dose glucocorticoid, intravenous cyclophosphamide and rituximab therapy, and effective application of therapeutic plasma exchange. We also report here that CSF IL-6 levels may serve as a new biomarker for GPA-associated OPN.

Impact of MPO-ANCA-mediated oxidative imbalance on renal vasculitis

Glomerulonephritis is a severe complication of microscopic polyangiitis (MPA), a small-vessel vasculitis associated with anti-myeloperoxidase antibodies (MPO-ANCA). We previously showed the pathogenic effects of MPO-ANCA that activate MPO to trigger an oxidative burst mainly through HOCl production, contributing to endothelial injury and lung fibrosis. The aim of this study was to investigate the relationship between MPO-induced oxidative stress, anti-oxidant defenses and renal histological lesions in MPA patients. We therefore analyzed histological data from a prospective cohort of ANCA-associated glomerulonephritis. Serum-mediated HOCl production, advanced oxidation protein products (AOPP), and thiol concentration in sera were determined. From 38 patients included, histological classification noted 50% focal glomerulonephritis, 15.8% crescentic-glomerulonephritis, and 34.2% mixed-glomerulonephritis. MPA patients’ sera displayed higher HOCl production by MPO ( P < 0.001), higher AOPP ( P < 0.001) and thiol ( P < 0.01) levels, compared with healthy subjects. The presence of cellular crescents was associated with higher serum-mediated HOCl production ( P = 0.049) and lower thiol levels ( P = 0.022) at disease onset. Higher thiol concentrations were associated with focal glomerulonephritis ( P = 0.042), less interstitial fibrosis ( P = 0.039) and hyalinosis ( P = 0.066). In remission, HOCl production was decreased ( P < 0.01), and thiol concentration remained high ( P = 0.39). Our findings suggest that HOCl production by activated MPO could contribute to the very early stage of glomerulonephritis, whereas thiol may exert a protective effect against the development of renal vasculitis and glomerulosclerosis. This study highlights the importance of oxidative defenses to counteract the process of MPO-ANCA associated glomerulonephritis.

Pathogenesis of ANCA-Associated Pulmonary Vasculitis

Antineutrophil cytoplasmic antibodies (ANCAs) are autoantibodies specific for antigens located in the cytoplasmic granules of neutrophils and lysosomes of monocytes. ANCAs are associated with a spectrum of necrotizing vasculitis that includes granulomatosis with polyangiitis, microscopic polyangiitis, and eosinophilic granulomatosis with polyangiitis. Pulmonary vasculitis and related extravascular inflammation and fibrosis are frequent components of ANCA vasculitis. In this review, we detail the factors that have been associated with the origin of the ANCA autoimmune response and summarize the most relevant clinical observations, in vitro evidence, and animal studies strongly indicating the pathogenic potential of ANCA. In addition, we describe the putative sequence of pathogenic mechanisms driven by ANCA-induced activation of neutrophils that result in small vessel necrotizing vasculitis and extravascular granulomatous necrotizing inflammation.

ANCA-associated vasculitis - clinical utility of using ANCA specificity to classify patients

The anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitides (AAV) are a heterogeneous group of rare syndromes characterized by necrotizing inflammation of small and medium-sized blood vessels and the presence of ANCAs. Several clinicopathological classification systems exist that aim to define homogeneous groups among patients with AAV, the main syndromes being microscopic polyangiitis (MPA), granulomatosis with polyangiitis (GPA) and eosinophilic GPA (EGPA). Two main types of ANCA can be detected in patients with AAV. These ANCAs are defined according to their autoantigen target, namely leukocyte proteinase 3 (PR3) and myeloperoxidase (MPO). Patients with GPA are predominantly PR3-ANCA-positive, whereas those with MPA are predominantly MPO-ANCA-positive, although ANCA specificity overlaps only partially with these clinical syndromes. Accumulating evidence suggests that ANCA specificity could be better than clinical diagnosis for defining homogeneous groups of patients, as PR3-ANCA and MPO-ANCA are associated with different genetic backgrounds and epidemiology. ANCA specificity affects the phenotype of clinical disease, as well as the patient's initial response to remission-inducing therapy, relapse risk and long-term prognosis. Thus, the classification of AAV by ANCA specificity rather than by clinical diagnosis could convey clinically useful information at the time of diagnosis.

Proteinase 3-ANCA Vasculitis versus Myeloperoxidase-ANCA Vasculitis

In patients with GN or vasculitis, ANCAs are directed against proteinase 3 (PR3) or myeloperoxidase (MPO). The differences between PR3-ANCA-associated vasculitis (AAV) and MPO-AAV described in the past have been supplemented during the last decade. In this review, we discuss the differences between these two small-vessel vasculitides, focusing especially on possible etiologic and pathophysiologic differences. PR3-AAV is more common in northern parts of the world, whereas MPO-AAV is more common in southern regions of Europe, Asia, and the Pacific, with the exception of New Zealand and Australia. A genetic contribution has been extensively studied, and there is a high prevalence of the HLA-DPB1*04:01 allele in patients with PR3-AAV as opposed to patients with MPO-AAV and/or healthy controls. Histologically, MPO-AAV and PR3-AAV are similar but show qualitative differences when analyzed carefully. Clinically, both serotypes are difficult to distinguish, but quantitative differences are present. More organs are affected in PR3-AAV, whereas renal limited vasculitis occurs more often in patients with MPO-AAV. For future clinical trials, we advocate classifying patients by ANCA serotype as opposed to the traditional disease type classification.

[Endometriosis: increasing concentrations of serum interleukin-1β and interleukin-1sRII is associated with the deep form of this pathology]

OBJECTIVES

To assess interleukin-1β (IL-1β) and its inhibitory soluble interleukin-1 receptor type II (IL-1sRII) levels into the serum of patients with various forms of endometriosis and normal women, and investigate the correlation with disease activity.

PATIENTS AND METHODS

In this prospective laboratory study (2005-2010), 510 women with histologically proven endometriosis and 93 endometriosis-free controls have been enrolled. Laparoscopic complete exploration of the abdominopelvic cavity and blood samples have been performed in each patient. For each serum, IL-1β and IL-1sRII have been evaluated using Elisa.

RESULTS

IL-1β and IL-1sRII have been respectively detectable in 64% and 54.6% of serum samples from all 603 women studied. IL-1β was higher in women with deep infiltrating endometriosis (DIE) (mean 10.0pg/mL [0.005-416.2]) than in endometriosis-free women (mean 0.5pg/mL [0.01-1.7], P<0.01) or in women with superficial endometriosis (SUP) (mean 0.6pg/mL [0.1-2.9], P<0.01). Also, IL-1sRII was higher in DIE (mean 236.7pg/mL [0.9-6975]) than in the witness group (mean 85.0pg/mL [1-235.2], P<0.05) or in SUP (mean 85.1pg/mL [0.6-302], P<0.01).

CONCLUSION

This study highlights both a marked significant increase in serum IL-1β and IL-1sRII levels in DIE compared to SUP and normal women and suggests that a defect in the control of IL-1 can impact the pathophysiology of endometriosis.

Utility of immunologic testing in suspected rheumatologic disease

The use of diagnostic testing in the clinical practice of medicine has been a shifting landscape from the time that the first blood test was utilized. This is no different in the field of immunology and in particular rheumatology. As the field of immunology is relatively young, the clinical tests are not well established and therefore guidelines for use are still under debate. In this review, we seek to look at some of the key autoantibodies, as well as other tests that are available to diagnose suspected rheumatologic disease, and examine how to best use these tests in the clinic. In particular, we will focus on the anti-nuclear antibodies, anti-neutrophil cytoplasmic antibodies, complement, cryoglobulins, rheumatoid factor, and anti-citrullinated protein antibodies.

Ficolin-1 is up-regulated in leukocytes and glomeruli from microscopic polyangiitis patients

Microscopic polyangiitis (MPA) is a systemic autoimmune disease that often has a fatal outcome. Although delineating the molecular pathogenesis is essential for its remedy, an understanding of its molecular mechanism has remained elusive. To search for new markers of active lesions that might help better understand the molecular basis of MPA and aid in its diagnosis, we here performed DNA microarray analysis with peripheral blood mononuclear cells (PBMCs). Compared to normal control, several genes were up- or down-regulated in MPA patients, including up-regulation of the mRNA level of ficolin-1 (FCN1 or M-ficolin), an innate pattern recognition complement molecule. The amount of ficolin-1, as detected by immunohistochemistry, was higher in the glomeruli of another group of MPA patients than in the glomeruli of control patients who harbored almost normal glomeruli. Many of the ficolin-1 dots were also positive for CD68, suggesting that the ficolin-1-positive cells were monocytes, such as macrophages or dendritic cells. This is not due to the difference in the number of neutrophil or monocytes in the blood samples of MPA and control patients. Taken together, we conclude that increased ficolin-1 expression could serve as a new marker for the characterization of MPA, especially when it is associated with local active lesions.

Antineutrophil cytoplasmic antibody-associated vasculitides: is it time to split up the group?

Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitides are a heterogeneous group of diseases corresponding to necrotising inflammation of small vessels with a wide range of clinical presentations. At least two of the diseases are believed to exhibit a common ground of pathophysiological mechanisms. These are granulomatosis with polyangiitis (GPA, formerly known as Wegener's granulomatosis) and microscopic polyangiitis (MPA). ANCA directed against proteinase 3 (PR3) are preferentially associated with GPA, and anti-myeloperoxidase (MPO) ANCA are associated mainly with MPA and eosinophilic GPA (formerly known as Churg-Strauss syndrome). Anti-MPO and anti-PR3 antibodies can activate neutrophils in vitro. In vivo data are available for humans and mice on the pathogenicity of anti-MPO but it is more controversial for PR3-ANCA. A recent genome-wide association study of patients with ANCA-associated vasculitides confirmed the genetic contribution to the pathogenesis of these conditions, with significant association of PR3-ANCA and human leukocyte antigen-DP and the genes encoding α1-antitrypsin and PR3. MPO-ANCA were significantly associated with human leukocyte antigen-DQ. Thus, recent results from epidemiological studies, genome-wide association study and therapeutic trials have suggested that these entities are, in fact, distinct. We have summarised these results and discuss the idea that these two entities should be studied separately as the nature of the two auto-antigens suggests at a molecular level despite shared ANCA involvement.

The advantage of specific intravenous immunoglobulin (sIVIG) on regular IVIG: experience of the last decade

During the last decade it has been shown that some components of intravenous immunoglobulin (IVIG) are responsible for their broadly therapeutic application. Currently, such specific subfractions are defined as specific IVIG (sIVIG) and are affinity-purified from commercial IVIGs that target specific antigens/antibodies related to a specific autoimmune disease. A remarkable example of the therapeutic potential of sIVIG is the proven enhanced anti-inflammatory potency of sialylated and recombinant sialylated IVIG obtained from total IVIG. In other experimental models, it has also been demonstrated that sIVIG work in many other contrivances, such as revealing anti-idiotypic networks blocking pathogenic antibodies ameliorating disease activity. sIVIG has also been shown to exert its action by modulating specific receptors expressed on immune cells in both inflammatory and autoimmune diseases. Indeed, sIVIG has emerged as a novel approach to treat different immune-mediated conditions in a more accurate antigen-specific manner. Herein we review experimental evidence supporting sIVIG-efficacy in treating autoimmune diseases and inflammation.

An autopsy-proven case of myeloperoxidase-antineutrophil cytoplasmic antibody-positive polyarteritis nodosa with acute renal failure and alveolar hemorrhage

An 80-year-old woman positive for myeloperoxidase-antineutrophil cytoplasmic antibody (MPO-ANCA) was admitted with a 3-month history of fever, general malaise, and weight loss, after unsuccessful treatment with antibiotics. Upon admission, her fever persisted, and there was concomitant deterioration of renal function without active urine sediments. Furthermore, she developed hemoptysis, and chest computed tomography (CT) scan revealed bilateral diffuse alveolar hemorrhage. Although a renal biopsy was not performed because of her dementia, we initially suspected microscopic polyangiitis (MPA) on the basis of her clinical course. Because of her poor general condition, she was administered a low dose of prednisolone. Although her fever subsided, she suffered from intractable alveolar hemorrhage and eventually died from respiratory failure. During the autopsy, fibrinoid necrosis was restricted to medium-sized arteries, including the arcuate arteries of the kidneys and the bronchial arteries, without necrotizing crescentic glomerulonephritis and alveolar capillaritis. Therefore, polyarteritis nodosa (PAN) was diagnosed. It is important to distinguish between MPA and PAN because they can lead to life-threatening complications, and their treatment strategies and prognosis are different. When a patient presents with MPO-ANCA, alveolar hemorrhage, and acute renal failure with little evidence of glomerulonephritis, a differential diagnosis of PAN should be made; however, it is difficult to do so without pathological findings. Therefore, pathological examination should be carried out whenever possible.

ANCA-associated vasculitides--advances in pathogenesis and treatment

Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitides (AAV) include Wegener granulomatosis, microscopic polyangiitis, Churg-Strauss syndrome and renal-limited vasculitis. This Review highlights the progress that has been made in our understanding of AAV pathogenesis and discusses new developments in the treatment of these diseases. Evidence from clinical studies, and both in vitro and in vivo experiments, supports a pathogenic role for ANCAs in the development of AAV; evidence is stronger for myeloperoxidase-ANCAs than for proteinase-3-ANCAs. Neutrophils, complement and effector T cells are also involved in AAV pathogenesis. With respect to treatment of AAV, glucocorticoids, cyclophosphamide and other conventional therapies are commonly used to induce remission in generalized disease. Pulse intravenous cyclophosphamide is equivalent in efficacy to oral cyclophosphamide but seems to be associated with less adverse effects. Nevertheless, alternatives to cyclophosphamide therapy have been investigated, such as the use of methotrexate as a less-toxic alternative to cyclophosphamide to induce remission in non-organ-threatening or non-life-threatening AAV. Furthermore, rituximab is equally as effective as cyclophosphamide for induction of remission in AAV and might become the standard of therapy in the near future. Controlled trials in which specific immune effector cells and molecules are being therapeutically targeted have been initiated or are currently being planned.

[Pathogenic role of antimyeloperoxidase antibodies]

Antimyeloperoxidase antibodies are a variety of antineutrophil cytoplasm antibodies (Anca), which can be detected in systemic small-sized vessel vasculitides such as microscopic polyangiitis, Wegener's granulomatosis and Churg-Strauss syndrome. Antimyeloperoxidase antibodies have been also associated with the development of lung fibrosis. Their pathogenic role has been well established, both in vitro and in vivo. These autoantibodies can activate neutrophils and trigger their oxidative burst leading to the release of free oxygen species and cytotoxic proteins. The oxidative burst is deleterious for the endothelium. Another mechanism by which antimyeloperoxidase may act is the activation of myeloperoxydase leading to an increased production of hypochlorous acid, which is highly toxic for the endothelial cells. These mechanisms contribute to the development of vasculitis.

Natural autoantibodies to myeloperoxidase, proteinase 3, and the glomerular basement membrane are present in normal individuals

Anti-neutrophil cytoplasmic antibodies (ANCAs) have a pathogenic role in ANCA-associated vasculitis. The origin of ANCAs and anti-glomerular basement membrane (GBM) antibodies, however, is unknown. In this study, we determined whether natural autoantibodies against myeloperoxidase (MPO), proteinase 3 (PR3), and GBM were present in each of 10 healthy Chinese and Swedish individuals, negative for all three antigens by routine ELISA. Antibodies were purified from isolated IgG by antigen-specific affinity columns. Natural anti-GBM autoantibodies gave a linear staining pattern along the GBM of human renal sections. On ethanol-fixed granulocytes, both natural anti-MPO and anti-PR3 autoantibodies gave cytoplasmic staining. The titers of natural anti-MPO/PR3 autoantibodies were significantly lower than those from patients with vasculitis. In competition ELISA, the binding of natural anti-MPO autoantibodies could be inhibited by MPO, but not by PR3 or noncollagenous domains from type IV collagen. The same specificity results were found for natural anti-PR3 and anti-GBM autoantibodies. Overall, individuals of the Chinese origin had more natural autoantibodies than did those of the Swedish origin, but no other differences were found. Hence, our study shows that healthy individuals have masked circulating, noncross-reactive, antigen-specific natural autoantibodies against MPO, PR3, and GBM in their serum and IgG fractions. Further studies are needed to determine their role if any in the etiology of ANCA-associated vasculitis and anti-GBM disease.

Pathogenesis of ANCA-associated vasculitis

Antineutrophil cytoplasm antibody (ANCA)-associated vasculitis (AAV) comprises a group of systemic inflammatory vasculitides associated with circulating autoantibodies directed against the neutrophil granule components proteinase 3 and myeloperoxidase. ANCA interact with their target antigens on cytokine primed neutrophils, causing neutrophil activation via several signaling pathways that culminates in endothelial interaction, degranulation, cytokine production, and endothelial and tissue damage. The presence of autoantibodies implies the assistance of autoreactive T-helper cells and B cells, and a failure of regulatory mechanisms. This article reviews the current evidence for the pathogenic mechanisms culminating in autoantibody production, the effects of ANCA-neutrophil and neutrophil-endothelial interactions, and the mechanisms of tissue damage.

Microscopic polyangiitis

In 1923, Friedrich Wohlwill described two patients with a "microscopic form of periarteritis nodosa," which was distinct from the classical form. This disease, now known as microscopic polyangiitis (MPA), is a primary systemic vasculitis characterized by inflammation of the small-caliber blood vessels and the presence of circulating antineutrophil cytoplasmic antibodies. Typically, microscopic polyangiitis presents with glomerulonephritis and pulmonary capillaritis, although involvement of the skin, nerves, and gastrointestinal tract is not uncommon. Treatment of MPA generally requires use of a cytotoxic agent (such as cyclophosphamide) in addition to high-dose glucocorticoids. Recent research has focused on identifying alternate treatment strategies that minimize or eliminate exposure to cytotoxic agents. This article reviews the history, pathogenesis, clinical manifestations, and treatment of MPA.

Prompt plasmapheresis successfully rescue pulmonary-renal syndrome caused by ANCA-negative microscopic polyangiitis

Pulmonary-renal syndrome (PRS) associated with antineutrophil cytoplasmic antibodies (ANCA)-negative microscopic polyangiitis (MPA) is relatively rare, and the effects of plasmapheresis on these patients remain unclear. Here, we report the case of a 66-year-old man who presented with fever, acute renal failure, thrombocytopenia, and sudden onset of diffuse pulmonary hemorrhage. Prompt plasmapheresis and concurrent pulse therapy with methylprednisolone effectively rescued his pulmonary-renal syndrome. The patient was then diagnosed with MPA on the basis of typical histological findings and the absence of surrogate markers of Wegener's granulomatosis and Churg-Strauss syndrome. This case demonstrates the therapeutic effects of plasmapheresis on ANCA-negative MPA and highlights the necessity of prompt plasmapheresis for not only resolving pulmonary hemorrhage but also increasing the likelihood of renal function restoration in patients with PRS.

Successful pregnancy and delivery of a healthy newborn despite transplacental transfer of antimyeloperoxidase antibodies from a mother with microscopic polyangiitis

Myeloperoxidase (MPO)-specific antineutrophil cytoplasmic antibodies have been proposed as pathogenic for microscopic polyangiitis. Supporting this hypothesis, a case report of transplacental anti-MPO antibody transfer presumably causing a vasculitis-like syndrome in the newborn is cited frequently. Here, we report a case of transplacental transfer of high levels of anti-MPO antibodies not resulting in clinical compromise in the newborn. The mother developed microscopic polyangiitis 5 years before the pregnancy. After induction therapy, remission was maintained with low-dose prednisone and azathioprine for 4.5 years despite high levels of anti-MPO antibodies (>100 U/mL). The patient elected to become pregnant, immunosuppression was maintained during pregnancy, and a normal-term neonate was delivered. The newborn's venous blood anti-MPO antibody levels decreased gradually from greater than 100 U/mL at birth to undetectable by day 120. No clinical manifestation of vasculitis developed in the newborn. This case supports that anti-MPO antibodies alone are not pathogenic without additional cofactors.

Update on pathogenic mechanisms of systemic necrotizing vasculitis

Systemic necrotizing vasculitis is rare but can have serious sequelae. Despite recent advances in cellular and molecular immunology and genetics, the causes of vasculitic syndromes remain largely undefined. Although mechanisms of blood vessel damage in systemic necrotizing vasculitis are complex, recent studies have provided significant insights.

Natural and disease associated anti-myeloperoxidase (MPO) autoantibodies

Myeloperoxidase (MPO) is a cationic protein present in primary azurophilic granules of neutrophils and monocytes. MPO produces a highly deleterious reactive oxygen species, the hypochlorous acid (HOCl), using hydrogen peroxide (H(2)O(2)) and chloride ions as substrate. Anti-MPO antibodies (Abs) are present in 70% of the cases in patients with microscopic polyangiitis (MPA), a small-sized vessel vasculitis. Anti-MPO Abs from patients with MPA can trigger the release of MPO by neutrophils and monocytes. Anti-MPO Abs can activate MPO to generate an oxidative stress deleterious for the endothelium. Thus, we recently demonstrated that MPA sera with anti-MPO Abs activated MPO in vitro, and generated hypochlorous acid, whereas sera from MPA patients with no anti-MPO Abs or healthy individuals did not. Both hypochlorous acid production and endothelial lysis were abrogated by N-acetylcysteine (NAC), an antioxidant molecule. Thus, anti-MPO Abs could play a pathogenic role in vivo by triggering an oxidative burst leading to severe endothelial damages.