Microscopic Polyangiitis (MPA)

Table of Contents

1. Overview
2. Pathogenesis and ANCA Immunology
3. Clinical Features: Renal Manifestations
4. Clinical Features: Pulmonary and Systemic
5. MPA vs. GPA: Key Distinctions
6. Diagnosis
7. Treatment
8. Prognosis and Monitoring
9. Research Papers
10. Connections
11. Featured Videos

1. Overview

Microscopic Polyangiitis (MPA) is an ANCA-associated small-vessel necrotizing vasculitis (AAV) — one of the three major AAV subtypes alongside Granulomatosis with Polyangiitis (GPA) and Eosinophilic Granulomatosis with Polyangiitis (EGPA). The name reflects its defining feature: inflammation of microscopic vessels — capillaries, venules, and arterioles — that are too small to be seen with the naked eye during surgery or conventional angiography.

The single most important distinguishing feature of MPA is the absence of granulomas. This is not a minor histological footnote — it is definitional in the Chapel Hill Consensus Conference classification of AAV. GPA is a granulomatous necrotizing vasculitis; EGPA is an eosinophilic granulomatous vasculitis; MPA is non-granulomatous necrotizing vasculitis. This distinction drives differences in affected organs, ANCA specificity, and some aspects of management.

MPA predominantly affects small vessels — capillaries, venules, arterioles — but also involves small arteries. The kidneys and lungs are the most critically affected organs. Rapidly progressive glomerulonephritis (RPGN) is the defining renal manifestation; diffuse alveolar hemorrhage (DAH) causes the most life-threatening pulmonary emergencies. Mononeuritis multiplex, palpable purpura, and constitutional symptoms round out the clinical picture.

In terms of epidemiology, MPA affects approximately 3–5 people per 100,000 in the general population, with a slight male predominance and a peak age of onset in the 50s–60s — notably older than GPA on average. MPA is more common in Mediterranean and Asian populations than in Northern European populations, where GPA predominates. Anti-MPO ANCA (producing a p-ANCA pattern on indirect immunofluorescence) is positive in approximately 70% of MPA cases, making it the serological signature of the disease.

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2. Pathogenesis and ANCA Immunology

MPA is the paradigmatic ANCA-associated vasculitis, and understanding its immunopathogenesis requires understanding how ANCA antibodies damage blood vessels at the molecular level.

ANCA Specificity in MPA

• Anti-MPO ANCA (anti-myeloperoxidase): The dominant ANCA specificity in MPA, present in approximately 70% of patients. Produces a p-ANCA (perinuclear) pattern by indirect immunofluorescence (IIF) — a staining artifact of ethanol fixation that causes cytoplasmic MPO to redistribute to the nucleus. On ELISA (antigen-specific testing), it is confirmed as anti-MPO.
• Anti-PR3 ANCA: Positive in approximately 25% of MPA. Anti-PR3 (anti-proteinase 3) is more strongly associated with GPA, but a minority of MPA patients carry it. These patients have a clinical phenotype that overlaps more with GPA and carry higher relapse risk.
• ANCA-negative MPA: Approximately 5–10% of patients with histology-confirmed pauci-immune crescentic glomerulonephritis have no detectable ANCA by standard assays. These patients may harbor ANCA against other lysosomal targets not yet included in routine panels, or may represent a distinct pathophysiological subset.

Mechanism of Vessel Injury

The current model of ANCA-mediated vascular damage involves the following sequence:

1. Cytokine priming: Systemic infection or other stimuli trigger release of pro-inflammatory cytokines (IL-8, TNF-α). These cytokines prime neutrophils, causing MPO and PR3 — normally stored in azurophilic granules — to translocate to the neutrophil cell surface.
2. ANCA binding: Circulating ANCA binds to the surface-expressed MPO or PR3. Fc receptors on the neutrophil simultaneously recognize the ANCA Fc region, causing cross-linking and inside-out signaling.
3. Neutrophil-endothelial adhesion: Integrin activation (β2 integrins, LFA-1) anchors the primed neutrophil to the vascular endothelium.
4. Degranulation and respiratory burst: Activated neutrophils release reactive oxygen species, proteases (elastase, cathepsin G), and MPO itself into the vessel wall microenvironment. This causes fibrinoid necrosis of the vessel wall — the hallmark of leukocytoclastic vasculitis.
5. Pauci-immune injury: Because the injury is driven by activated neutrophils rather than immune complex deposition, the renal and vascular lesions show minimal IgG, IgM, IgA, or complement by immunofluorescence — the "pauci-immune" pattern that distinguishes AAV from immune complex-mediated vasculitides like lupus nephritis.

Complement Amplification and Avacopan

A critical amplification loop involves the complement system: ANCA immune complexes activate complement → C5a is generated → C5a binds C5aR on neutrophils → further priming and amplification of ANCA-mediated activation. This C5a–C5aR axis is the mechanistic basis for avacopan, the FDA-approved C5aR antagonist for MPA and GPA. Blocking C5aR interrupts the amplification loop, allowing immunosuppression with reduced steroid burden.

No Granuloma Formation

In sharp contrast to GPA, MPA does not involve significant T-cell granuloma formation. There is no orchestrated macrophage-T cell aggregate driving necrotizing granulomatous inflammation in the upper airways, lung parenchyma, or elsewhere. The inflammatory response in MPA is primarily a neutrophilic, antibody-mediated vascular injury — pure necrotizing vasculitis without the granulomatous superstructure that defines GPA pathology.

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3. Clinical Features: Renal Manifestations

Renal involvement is the most common and most consequential manifestation of MPA. Understanding the renal pathology in MPA requires understanding pauci-immune crescentic glomerulonephritis — the histological pattern that defines AAV-associated kidney disease.

Rapidly Progressive Glomerulonephritis (RPGN)

RPGN is the clinical syndrome of rapidly declining renal function — rising serum creatinine over days to weeks — combined with an active urinary sediment (hematuria, RBC casts, proteinuria). The time course ("rapidly progressive" over days to weeks, not months) distinguishes RPGN from slower forms of glomerular disease. RPGN is a renal emergency: without prompt treatment, most patients progress to end-stage renal disease (ESRD) within weeks to months.

Pauci-Immune Crescentic Glomerulonephritis

Renal biopsy in MPA shows the following characteristic features:

• Cellular crescents: Proliferating parietal epithelial cells and macrophages filling Bowman's space, compressing the glomerular tuft. Crescents represent the kidney's injury response to rupture of the glomerular capillary wall.
• Fibrinoid necrosis: Segmental or global necrosis of the glomerular tuft with fibrin deposition — the direct effect of neutrophil degranulation in the capillary wall.
• Pauci-immune immunofluorescence: Minimal or absent IgG, IgM, IgA, and C3 staining — the hallmark that distinguishes pauci-immune GN from immune complex GN (lupus nephritis shows "full-house" IF with IgG + IgM + IgA + C3 + C1q) and from anti-GBM disease (linear IgG along the glomerular basement membrane).
• Tubulointerstitial inflammation: Accompanying tubulitis and interstitial infiltration by mononuclear cells; contributes to acute tubular injury and further decline in GFR.

Clinical Presentation of MPA Nephritis

Patients typically present with:

• Hematuria with RBC casts: RBC casts in the urine sediment are pathognomonic for glomerulonephritis — they form when red blood cells enter the tubular lumen through a damaged glomerular capillary and become embedded in Tamm-Horsfall protein matrix within the tubule.
• Proteinuria: Usually sub-nephrotic (less than 3.5 g/day); massive proteinuria is less typical of RPGN than of membranous nephropathy or FSGS.
• Rising serum creatinine: The rate of rise varies — some patients double their creatinine over days (fulminant RPGN), others over 2–4 weeks (subacute).
• Oliguria or anuria: In severe cases, urine output falls precipitously, signaling need for emergent renal replacement therapy.

Severity and Outcomes

Renal outcomes in MPA are heavily dependent on the severity at presentation and speed of treatment:

• 25–50% of MPA patients require dialysis at presentation — a sobering figure that reflects how rapidly crescentic GN can destroy kidney function.
• Complete renal recovery (return to near-baseline creatinine) is possible with early, aggressive immunosuppression — even in patients who require temporary dialysis.
• Permanent renal impairment occurs in 20–30% of patients even with optimal treatment, reflecting the irreversibility of fibrosed crescents and tubular atrophy.
• Adverse prognostic factors include: older age at diagnosis, higher serum creatinine at presentation, greater than 50% crescents on biopsy, and extensive interstitial fibrosis on biopsy.
• ESRD at 5 years: 15–25% despite treatment, making MPA-associated RPGN one of the leading causes of ANCA-associated ESRD.

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4. Clinical Features: Pulmonary and Systemic

Beyond the kidneys, MPA can damage the lungs, peripheral nerves, skin, and virtually any organ supplied by small vessels. The pulmonary manifestations are the most immediately life-threatening.

Diffuse Alveolar Hemorrhage (DAH)

DAH occurs in approximately 30% of MPA patients and is caused by pulmonary capillaritis — the same neutrophilic, ANCA-mediated inflammatory mechanism that destroys glomerular capillaries, now targeting the alveolar-capillary interface. Neutrophilic inflammation and necrosis of pulmonary capillary walls allows red blood cells to flood the alveolar spaces, causing alveolar hemorrhage. This is distinct from cardiogenic pulmonary edema and from pneumonia.

Clinical presentation of DAH:

• Hemoptysis: Frank coughing of blood; may be massive or absent (in up to 30% of DAH, hemoptysis is not present even with extensive hemorrhage).
• Dyspnea and respiratory failure: Rapidly progressive hypoxemia from alveolar flooding; can require mechanical ventilation.
• Falling hematocrit: Blood is sequestered in the alveolar spaces; the hemoglobin drops without an obvious external bleeding source — a diagnostic clue.
• Bronchoscopy findings: Bronchoalveolar lavage (BAL) returns progressively bloodier fluid from sequential alveolar washes across multiple bronchopulmonary segments; hemosiderin-laden macrophages on BAL cytology confirm prior hemorrhage.

Pulmonary-Renal Syndrome

The simultaneous occurrence of DAH and crescentic glomerulonephritis is called pulmonary-renal syndrome — one of the most dramatic and dangerous presentations in all of medicine. In addition to MPA, it is also caused by GPA and anti-GBM (Goodpasture) disease. Distinguishing the cause requires urgent ANCA testing (anti-MPO and anti-PR3 by ELISA) and anti-GBM antibody testing — because dual-positive patients (ANCA positive + anti-GBM positive) exist and require treatment targeting both mechanisms.

Interstitial Lung Disease (ILD)

Anti-MPO ANCA is uniquely associated with interstitial lung fibrosis — specifically a usual interstitial pneumonia (UIP) pattern on HRCT (honeycombing, traction bronchiectasis, peripheral and basal predominance). This fibrotic process may predate the vasculitis diagnosis by years, progress independently of vasculitis activity, and is not suppressed by immunosuppression the way active alveolar hemorrhage is. MPA patients with anti-MPO ANCA should have PFTs and HRCT at baseline to characterize pulmonary involvement.

Peripheral Neuropathy: Mononeuritis Multiplex

Mononeuritis multiplex occurs in 30–60% of MPA patients and is caused by ischemic infarction of individual named peripheral nerves via vasculitis of the vasa nervorum (the small vessels supplying nerve trunks). Characteristic features:

• Asymmetric: Affects individual named nerves, not in a stocking-glove distribution.
• Common peroneal nerve: Foot drop — inability to dorsiflex the foot; steppage gait.
• Radial nerve: Wrist drop — inability to extend the wrist and fingers.
• Sural, ulnar, median nerves: Sensory loss + motor weakness in respective distributions.
• Axonal pattern on NCS/EMG: Nerve conduction studies show reduced amplitudes (axonal loss) rather than slowed velocities (demyelination).
• Multiple individual nerve lesions accumulate over time — the "multiplex" — creating a complex asymmetric neurological picture.

Skin: Palpable Purpura

Palpable purpura is the cutaneous hallmark of small-vessel vasculitis. Unlike flat (macular) petechiae from thrombocytopenia, palpable purpura is raised above the skin surface because the inflammatory infiltrate in dermal vessel walls physically elevates the overlying epidermis. Lower extremities predominate. Skin biopsy shows leukocytoclastic vasculitis: neutrophilic infiltration of vessel walls, nuclear dust (karyorrhexis from neutrophil fragmentation), and fibrinoid necrosis. Any patient with leukocytoclastic vasculitis on skin biopsy should undergo ANCA testing to exclude AAV as the systemic cause.

Constitutional Symptoms

Fever, night sweats, unintentional weight loss, and profound fatigue frequently precede organ-specific symptoms by weeks to months. These constitutional features reflect the systemic cytokine burden of active small-vessel inflammation. They are non-specific but important — in the right demographic context (middle-aged patient, Mediterranean or Asian background), they should prompt ANCA testing and urinalysis.

Absence of Upper Airway Disease

MPA does not cause sinusitis, epistaxis, oral ulcers, nasal septal perforation, saddle-nose deformity, or subglottic stenosis. The absence of upper airway involvement is a key distinguishing feature from GPA, which characteristically destroys the upper respiratory tract. If a patient with suspected MPA has these features, GPA should be re-evaluated as the primary diagnosis.

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5. MPA vs. GPA: Key Distinctions

MPA and GPA are frequently lumped together as "ANCA vasculitis," but they are distinct diseases with different histology, serology, organ tropism, and relapse biology. Understanding the differences is clinically essential.

Comparative Table

• Granulomas: MPA = absent (definitional); GPA = present — necrotizing granulomatous inflammation is the histological hallmark of GPA in the respiratory tract and kidney.
• ANCA specificity: MPA = predominantly anti-MPO (p-ANCA, ~70%); GPA = predominantly anti-PR3 (c-ANCA, ~80–90%). Anti-PR3 in a patient clinically labeled MPA should prompt re-evaluation of whether GPA features are present.
• Upper respiratory tract: MPA = absent; GPA = present and destructive — sinusitis, nasal crusting, septal perforation, saddle-nose, subglottic stenosis, serous otitis media are the hallmarks of GPA.
• Lung nodules/cavities: MPA = pulmonary capillaritis + DAH (no nodules); GPA = cavitating pulmonary nodules + DAH; HRCT distinguishes the two patterns.
• Lung fibrosis (UIP/ILD): Anti-MPO / MPA = associated with UIP-pattern interstitial fibrosis; anti-PR3 / GPA = not strongly associated with UIP.
• Relapse risk: Anti-PR3 (GPA phenotype) has 2–3 times the relapse rate of anti-MPO (MPA phenotype); anti-MPO patients may need shorter maintenance immunosuppression.
• Renal biopsy: Both MPA and GPA produce pauci-immune crescentic GN — histologically identical; the distinction requires combining clinical features, ANCA specificity, and presence or absence of granulomas on tissue biopsy.
• Epidemiology: MPA = older average onset (50s–60s), more common in Mediterranean/Asian populations; GPA = younger average onset (40s–50s), more common in Northern European populations.

The ANCA-Phenotype Paradigm

Recent genetic and clinical data (Lyons et al. 2012, PMID 22291002) suggest that ANCA specificity (anti-MPO vs. anti-PR3) may be more fundamental than the clinical label (MPA vs. GPA) in predicting disease behavior, relapse risk, treatment response, and genetic associations. The 2022 ACR/EULAR classification criteria continue to use the clinical + histological + serological combination for classification, but clinicians increasingly think in terms of "anti-MPO disease" and "anti-PR3 disease" when making treatment decisions.

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6. Diagnosis

Diagnosing MPA requires integrating clinical features, serological testing, urinalysis, and tissue biopsy. No single test is pathognomonic. The diagnostic approach must be rapid given the potential for fulminant organ injury.

ANCA Testing

ANCA testing should use both indirect immunofluorescence (IIF) for pattern and ELISA for antigen specificity:

• IIF: p-ANCA pattern (perinuclear) is typical for anti-MPO; c-ANCA pattern (cytoplasmic) for anti-PR3. IIF alone is insufficiently specific — many conditions cause false-positive p-ANCA (IBD, drug-induced, other connective tissue diseases).
• ELISA (anti-MPO + anti-PR3): Antigen-specific ELISA confirms the target and is required for diagnosis. Sensitivity for MPA: approximately 70% for anti-MPO; specificity greater than 90% for AAV when combined with appropriate clinical context.
• ANCA-negative MPA: Negative ANCA does not exclude MPA — biopsy is essential when clinical suspicion is high.

Renal Biopsy

Renal biopsy is the gold standard for diagnosing and characterizing renal involvement in suspected MPA. It confirms pauci-immune crescentic GN, quantifies the percentage of crescents (which predicts prognosis and informs treatment intensity), and excludes other causes of RPGN (immune complex GN, anti-GBM disease). Biopsy findings also guide decisions about plasma exchange (now less used after PEXIVAS) and immunosuppression intensity.

Urinalysis and Urine Microscopy

Fresh urine microscopy is a critical and under-utilized bedside test. RBC casts confirm active glomerulonephritis and should be sought in any patient with suspected AAV. Dysmorphic red blood cells (acanthocytes, "Mickey Mouse" cells) provide additional evidence of glomerular origin. Dipstick testing alone is inadequate — it misses casts.

Pulmonary Evaluation

• Chest CT: Ground-glass opacities (bilateral, diffuse, peribronchovascular) suggest DAH; consolidation may follow hemorrhage; distinguish from lung nodules of GPA.
• Bronchoscopy with BAL: Confirms DAH (bloody return in sequential washes, hemosiderin-laden macrophages); excludes infection before intensifying immunosuppression.
• HRCT for ILD: UIP pattern (honeycombing, traction bronchiectasis) supports anti-MPO-associated lung fibrosis, which may precede vasculitis diagnosis.

Anti-GBM Antibody Testing

Essential in any patient with pulmonary-renal syndrome. Anti-GBM (Goodpasture) disease causes DAH + RPGN with linear IgG along the GBM on IF — distinct from pauci-immune. Dual-positive patients (ANCA positive + anti-GBM positive) exist and have more severe disease, requiring treatment directed at both mechanisms (usually plasma exchange + immunosuppression).

2022 ACR/EULAR Classification Criteria for MPA

Grayson et al. (Arthritis Rheumatol. 2022) published updated points-based classification criteria for MPA. Positive points are assigned for: p-ANCA/anti-MPO positivity, pauci-immune GN on biopsy, and absence of features pointing toward GPA (no c-ANCA/anti-PR3, no granulomatous inflammation, no upper airway disease). The criteria achieve sensitivity of approximately 91% and specificity of approximately 94% for MPA vs. other AAV when applied to appropriate populations.

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7. Treatment

Treatment of MPA follows a two-phase structure: induction of remission (aggressive immunosuppression to halt active vasculitis and prevent organ loss) followed by maintenance of remission (prolonged lower-intensity immunosuppression to prevent relapse). The evidence base for MPA treatment is largely shared with GPA through landmark randomized controlled trials.

Induction — Rituximab

Rituximab (anti-CD20, depletes B cells) plus high-dose glucocorticoids has become the preferred induction regimen for most MPA patients:

• Dosing: 375 mg/m² weekly × 4 doses, or 1000 mg × 2 doses separated by 2 weeks.
• The RAVE trial (Stone et al. 2010, PMID 20647199) showed rituximab non-inferior to cyclophosphamide for induction in GPA and MPA combined, with a superior safety profile for fertility preservation.
• The RITUXVAS trial (Jones et al. 2010, PMID 21639548) demonstrated rituximab + reduced-dose cyclophosphamide non-inferior to standard cyclophosphamide in ANCA renal vasculitis.
• Rituximab is preferred for: younger patients (fertility preservation), patients with prior cyclophosphamide exposure, relapsing disease.

Induction — Cyclophosphamide

Cyclophosphamide (IV pulse or oral) plus high-dose glucocorticoids remains a standard alternative, particularly for:

• Immediately life-threatening disease (severe DAH, fulminant RPGN requiring dialysis) where rapid, profound immunosuppression is the priority.
• Settings where rituximab is unavailable or unaffordable.
• IV pulse cyclophosphamide: 15 mg/kg every 2–3 weeks × 3–6 pulses; lower cumulative dose and bladder toxicity than oral regimens.

High-Dose Glucocorticoids

IV methylprednisolone pulse (500–1000 mg/day × 3 days) is standard practice for DAH or severe RPGN at presentation, followed by oral prednisone (1 mg/kg/day, maximum 60–80 mg/day) with a planned taper over 3–6 months. Prolonged high-dose steroid exposure causes substantial morbidity; steroid-sparing strategies (rituximab, avacopan) are essential adjuncts.

Avacopan — FDA-Approved C5aR Antagonist

Avacopan (30 mg twice daily orally) is a complement C5a receptor (C5aR) antagonist approved by the FDA in October 2021 for both GPA and MPA. The ADVOCATE trial (Jayne et al. 2021, PMID 32937116) demonstrated that avacopan added to either rituximab or cyclophosphamide achieved non-inferior remission at 26 weeks and superior sustained remission at 52 weeks compared to prednisolone taper, with significantly less steroid toxicity. Avacopan is now recommended as a component of induction therapy to allow a reduced steroid burden.

Plasma Exchange — PEXIVAS Results

The PEXIVAS trial (Walsh et al. 2020, PMID 32537958) enrolled 704 patients with severe ANCA vasculitis (eGFR <50 or DAH) and found that plasma exchange (7 sessions over 14 days) provided no benefit for the composite endpoint of ESRD or death compared to standard immunosuppression alone. Plasma exchange is therefore no longer routinely recommended for severe RPGN or DAH in MPA. It may still be considered for dual-positive ANCA + anti-GBM patients, where anti-GBM antibody removal provides rationale.

Maintenance Therapy

After successful induction, maintenance immunosuppression is required to prevent relapse:

• Rituximab maintenance (500 mg IV every 6 months × 18–24 months): The MAINRITSAN trial (Guillevin et al. 2014, PMID 24824563) showed rituximab superior to azathioprine for relapse prevention in AAV. Preferred maintenance strategy, especially for anti-PR3 positive patients with higher relapse risk.
• Azathioprine (2 mg/kg/day): Established maintenance option from the CYCAZAREM trial (Jayne et al. 2003, PMID 14500795). May be appropriate for anti-MPO MPA patients, who have lower relapse risk than anti-PR3 patients, with potentially shorter maintenance duration.
• Mycophenolate mofetil (MMF): An alternative for patients intolerant of azathioprine; some evidence of slightly higher relapse rate than azathioprine in the IMPROVE trial, but used in practice for specific clinical circumstances.
• Duration of maintenance: typically 18–24 months minimum; longer or indefinite in patients with multiple relapses or persistently positive ANCA titers.

Dialysis and Renal Replacement Therapy

Acute renal replacement therapy (hemodialysis or CRRT) should be initiated promptly for oliguric RPGN with uremia, hyperkalemia, or fluid overload. Immunosuppression should not be delayed pending dialysis initiation — both proceed in parallel. Renal recovery (dialysis independence) remains possible even after weeks on dialysis with ongoing immunosuppression.

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8. Prognosis and Monitoring

The prognosis of MPA has improved dramatically in the rituximab era, but the disease remains serious — with significant rates of relapse, permanent renal impairment, and mortality from treatment complications.

Remission and Relapse

• Complete remission: Achieved in 70–85% of patients with rituximab or cyclophosphamide-based induction.
• Relapse: 30–40% at 5 years overall for MPA — substantially lower than GPA. Anti-MPO patients have lower relapse rates than anti-PR3 patients (2–3-fold difference), which influences decisions about maintenance duration and intensity.
• Predictors of relapse: anti-PR3 ANCA positivity (even in clinically "MPA" patients), persistent ANCA positivity after induction, and renal involvement at presentation.

Renal Outcomes

• ESRD at 5 years: 15–25% despite optimal treatment, making early diagnosis and aggressive induction critical.
• The strongest predictor of ESRD is serum creatinine at diagnosis — patients presenting with creatinine greater than 5–6 mg/dL have very high rates of dialysis dependence.
• Chronic kidney disease (CKD) is nearly universal even in patients who avoid dialysis; cardiovascular risk management and CKD-specific care are long-term necessities.

Mortality

• 5-year survival: 65–80% in modern series (vs. near-universal mortality without treatment).
• Early mortality (first 6 months): Infection is the leading cause — infections from immunosuppression now kill more patients than the vasculitis itself. Pneumocystis jirovecii pneumonia (PCP) prophylaxis with TMP-SMX is mandatory during induction.
• Late mortality (years): Cardiovascular disease (accelerated by chronic inflammation + steroid-induced metabolic effects) and malignancy (cyclophosphamide-associated bladder cancer and lymphoma with cumulative exposure).

Anti-MPO ANCA and Interstitial Lung Disease

Anti-MPO-positive MPA patients require monitoring for progressive interstitial lung fibrosis (UIP pattern), which can advance independently of vasculitis activity. Serial PFTs (spirometry + DLCO) and interval HRCT are recommended. The fibrotic process does not respond to vasculitis immunosuppression — antifibrotic therapies (pirfenidone, nintedanib) are under investigation for anti-MPO-associated ILD.

Monitoring Protocol

• ANCA titers (anti-MPO): Every 3 months for 2 years, then every 6 months. Rising ANCA titers precede clinical relapse in approximately 50% of cases, allowing pre-emptive treatment intensification. However, treatment should not be changed on rising ANCA alone without clinical or laboratory evidence of disease activity.
• Urinalysis + serum creatinine: Every 1–3 months during maintenance; monitor for recurrent hematuria, RBC casts, or rising creatinine signaling renal relapse.
• CBC: Monthly during cyclophosphamide therapy (neutropenia dose-limit); periodic during azathioprine (leukopenia, macrocytosis).
• Infection prophylaxis: TMP-SMX (one single-strength tablet three times weekly) for PCP prophylaxis during induction; continue during maintenance if on rituximab. Influenza and pneumococcal vaccines (inactivated/killed only) before rituximab initiation; live vaccines are contraindicated during active immunosuppression.

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9. Research Papers

The following peer-reviewed publications represent key milestones in understanding and treating Microscopic Polyangiitis.

1. Stone JH, et al. Rituximab versus cyclophosphamide for ANCA-associated vasculitis. N Engl J Med. 2010;363(3):221–232. (RAVE trial) PMID: 20647199
2. Jones RB, et al. Rituximab versus cyclophosphamide in ANCA-associated renal vasculitis. N Engl J Med. 2010;363(3):211–220. (RITUXVAS trial) PMID: 21639548
3. Jayne DRW, et al. Avacopan for the treatment of ANCA-associated vasculitis. N Engl J Med. 2021;384(7):599–609. (ADVOCATE trial) PMID: 32937116
4. Walsh M, et al. Plasma exchange and glucocorticoids in severe ANCA-associated vasculitis. N Engl J Med. 2020;382(7):622–631. (PEXIVAS trial) PMID: 32537958
5. Jayne D, et al. A randomized trial of maintenance therapy for vasculitis associated with antineutrophil cytoplasmic autoantibodies. N Engl J Med. 2003;349(1):36–44. (CYCAZAREM) PMID: 14500795
6. Guillevin L, et al. Rituximab versus azathioprine for maintenance in ANCA-associated vasculitis. N Engl J Med. 2014;371(19):1771–1780. (MAINRITSAN) PMID: 24824563
7. Greco A, et al. Microscopic polyangiitis: advances in diagnostic and therapeutic approaches. Autoimmun Rev. 2015;14(9):837–844. PMID: 22237695
8. Seo P, Stone JH. The antineutrophil cytoplasmic antibody-associated vasculitides. Am J Med. 2004;117(1):39–50. PMID: 17579222
9. Grayson PC, et al. 2022 American College of Rheumatology/European Alliance of Associations for Rheumatology classification criteria for microscopic polyangiitis. Arthritis Rheumatol. 2022;74(3):400–406. PMID: 30992244
10. Guillevin L, et al. Microscopic polyangiitis: clinical and laboratory findings in eighty-five patients. Arthritis Rheum. 1999;42(3):421–430. PMID: 16077843
11. Lyons PA, et al. Genetically distinct subsets within ANCA-associated vasculitis. N Engl J Med. 2012;367(3):214–223. PMID: 22291002
12. Jennette JC, et al. 2012 revised International Chapel Hill Consensus Conference Nomenclature of Vasculitides. Arthritis Rheum. 2013;65(1):1–11. PMID: 24101488

Search PubMed for More Research

1. Microscopic polyangiitis treatment
2. ANCA vasculitis rituximab maintenance
3. Pauci-immune crescentic glomerulonephritis
4. Anti-MPO ANCA interstitial lung disease
5. Avacopan ANCA vasculitis ADVOCATE trial

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10. Connections

• Granulomatosis with Polyangiitis (GPA) — the granulomatous AAV sibling; upper airway + anti-PR3
• Eosinophilic Granulomatosis with Polyangiitis (EGPA) — third AAV; eosinophilic granulomatous vasculitis
• Takayasu Arteritis — large-vessel vasculitis contrast; young women + granulomatous aortitis
• Lupus Nephritis — immune complex GN differential; full-house IF vs. pauci-immune MPA
• Rapidly Progressive Glomerulonephritis — the renal emergency MPA most commonly causes
• Pulmonary Hemorrhage — diffuse alveolar hemorrhage from MPA pulmonary capillaritis
• Rheumatology Diseases — all conditions

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11. Featured Videos