9.1 Pemphigus Group: Pemphigus Vulgaris, Foliaceus & Paraneoplastic Pemphigus

9.1 Pemphigus Group: Pemphigus Vulgaris, Foliaceus & Paraneoplastic Pemphigus

Molecular Pathophysiology & Desmosomal Biology

The pemphigus diseases represent a group of potentially life-threatening autoimmune intraepidermal blistering dermatoses characterized by loss of cell-cell adhesion between keratinocytes, a process designated acantholysis. Acantholysis is mediated by pathogenic polyclonal IgG autoantibodies (predominantly the IgG4 subclass during active clinical disease, alongside IgG1) directed against calcium-dependent transmembrane cell adhesion glycoproteins within desmosomes.

The Desmosomal Cadherin Complex

Desmosomes are specialized intercellular anchoring junctions that confer mechanical tensile strength to stratified squamous epithelia by tethering intermediate keratin filaments of adjacent cells. The structural core comprises:

1. Transmembrane Cadherins: Desmogleins (Dsg1, Dsg2, Dsg3, Dsg4) and Desmocollins (Dsc1, Dsc2, Dsc3). In human skin and mucous membranes, Dsg1 and Dsg3 constitute the primary antigenic targets in autoimmune pemphigus.
2. Armadillo Plaque Proteins: Plakoglobin (gamma-catenin) and Plakophilins (PKP1–3), which bind directly to the cytoplasmic tails of desmogleins and desmocollins.
3. Plakin Family Cytoskeletal Linkers: Desmoplakin I and II, Envoplakin, Periplakin, and Plectin, which anchor the armadillo plaque complex to the intracellular keratin intermediate filament cytoskeleton (keratins 5/14 in basal keratinocytes; keratins 1/10 in suprabasal keratinocytes).

Mechanisms of Autoantibody-Mediated Acantholysis

Autoantibodies binding to the extracellular domain 1 (EC1) of desmoglein molecules trigger keratinocyte detachment through dual pathways:

• Steric Hindrance: Direct physical disruption of trans-adhesive homophilic (Dsg-Dsg) and heterophilic (Dsg-Dsc) trans-interactions bridging the intercellular space.
• Intracellular Signal Transduction: Autoantibody ligation induces intracellular signaling cascades, notably p38 mitogen-activated protein kinase (p38 MAPK) activation, protein kinase C (PKC) phosphorylation, and intracellular calcium influx. This cascade prompts endocytic internalization and lysosomal degradation of non-assembled desmoglein molecules, destabilizing the desmosomal architecture and causing retraction of keratin tonofilaments from the cell membrane (collapse of the intermediate filament network).

The Desmoglein Compensation Theory

The clinical phenotype and the precise histological level of intraepidermal cleavage in pemphigus are dictated by the anatomical and epithelial distribution of Dsg1 and Dsg3, conceptualized as the Desmoglein Compensation Theory (Amagai and colleagues).

Differential Tissue Expression Profiles

• Cutaneous Epidermis:
  • Desmoglein 1 (Dsg1): Expressed throughout all living epidermal layers, with maximum expression in the superficial spinous and granular layers, tapering toward the basal layer.
  • Desmoglein 3 (Dsg3): Expressed intensely in the basal, suprabasal, and lower spinous layers, but diminishes rapidly toward the upper spinous and granular layers.
• Stratified Squamous Mucosa (Oral, Pharyngeal, Esophageal):
  • Desmoglein 3 (Dsg3): Expressed abundantly throughout the entire thickness of the mucosal epithelium (basal to superficial layers).
  • Desmoglein 1 (Dsg1): Expressed at negligible or very low levels throughout mucous membranes.

Clinical and Histological Correlations of Compensation

1. Mucosal-Dominant Pemphigus Vulgaris (Anti-Dsg3 antibodies alone):

   • In mucous membranes, Dsg3 is the dominant cadherin, while Dsg1 is absent. Inactivation of Dsg3 by anti-Dsg3 antibodies leaves no functional cadherin to maintain mucosal adhesion, causing extensive, painful mucosal erosions.
   • In the skin, although basal Dsg3 is neutralized, abundant co-expressed Dsg1 in the epidermis provides functional compensation, preventing cutaneous blister formation. Cutaneous involvement is absent.

2. Mucocutaneous Pemphigus Vulgaris (Anti-Dsg3 and Anti-Dsg1 antibodies):

   • Autoantibodies neutralize both Dsg3 and Dsg1. In mucous membranes, loss of Dsg3 causes extensive erosions. In the skin, neutralization of both basal Dsg3 and suprabasal Dsg1 removes all compensatory adhesion, producing suprabasal cleavage and extensive cutaneous flaccid bullae alongside mucosal ulceration.

3. Pemphigus Foliaceus (Anti-Dsg1 antibodies alone):

   • In the skin, anti-Dsg1 neutralizes Dsg1 in the superficial epidermis. Because Dsg3 is absent in the upper spinous and granular layers, there is no cadherin to compensate, resulting in subcorneal intraepidermal acantholysis.
   • In the deeper cutaneous epidermis and throughout mucous membranes, abundant Dsg3 provides complete compensation. Thus, mucous membranes are strictly spared, and cutaneous blisters are entirely superficial.

Pemphigus Subtypes and Clinical Presentations

Pemphigus Vulgaris (PV): Clinical Morphology

• Mucosal Lesions: In over 50–70% of cases, pemphigus vulgaris begins in the oral cavity months before cutaneous blisters appear. Painful, persistent erosions affect the buccal mucosa, soft and hard palate, ventral tongue, and gingiva (desquamative gingivitis). Lesions extend to the pharynx, larynx (causing hoarseness), esophagus (causing severe odynophagia), and anogenital mucosa. Intact oral bullae are almost never observed because the non-keratinized mucosal roof is extraordinarily fragile and ruptures under minor masticatory friction.
• Cutaneous Lesions: Clear, flaccid, thin-walled bullae arise on non-inflamed or mildly erythematous skin, most commonly on the scalp, face, neck, upper chest, back, and intertriginous flexures. The blister fluid rapidly becomes seropurulent. Bullae rupture spontaneously to leave raw, weeping, tender erosions that expand centrifugally with ragged epidermal collarettes. Erosions show minimal tendency to heal spontaneously and are prone to secondary bacterial infection (Staphylococcus aureus, Pseudomonas aeruginosa) and sepsis.

Pemphigus Foliaceus (PF) and Fogo Selvagem

• Pemphigus Foliaceus: Presents insidiously with small, flaccid vesicles that rupture so rapidly that primary blisters are rarely captured. The eruption manifests as scaly, crusted, erythematous plaques with a greasy, cornflake-like scale distributed across seborrheic areas (scalp, face, sternum, interscapular back). Severe cases progress to exfoliative erythroderma. Patients complain of burning and stinging rather than pruritus. Mucous membranes are strictly unaffected.
• Fogo Selvagem ("Wildfire"): An endemic variant of pemphigus foliaceus concentrated in rural river basins of Brazil (states of São Paulo, Minas Gerais, Mato Grosso do Sul) and Colombia. Epidemiological studies long linked it to bites of black flies (Simulium nigrimanum). Later work showed that anti-Dsg1 autoantibodies cross-react with the salivary protein LJM11 of the sand fly Lutzomyia longipalpis. Disease develops in genetically susceptible individuals carrying HLA-DRB1*04:04, *14:02, or *14:06 alleles.

Pemphigus Vegetans

Pemphigus vegetans accounts for 1–2% of pemphigus cases. Two clinical variants are recognized:

• Neumann Variant: Begins as typical pemphigus vulgaris bullae that rupture and subsequently develop exuberant, vegetating, papillomatous, hyperkeratotic granulation tissue studded with pustules in intertriginous regions (groin, axillae, perianal folds, umbilicus).
• Hallopeau Variant: A more indolent form presenting initially with grouped pustules that rapidly evolve into vegetating plaques without a preceding bullous phase. Both types exhibit prominent pseudoepitheliomatous hyperplasia and intraepidermal eosinophilic microabscesses on biopsy.

Paraneoplastic Pemphigus (PNP / PAMS)

Paraneoplastic Pemphigus, also designated Paraneoplastic Autoimmune Multiorgan Syndrome (PAMS), is an aggressive, life-threatening autoimmune condition triggered by an underlying neoplasm—predominantly of hematologic or lymphoid lineage in over 80% of cases.

Underlying Neoplastic Associations

1. Castleman Disease (Angiofollicular Lymph Node Hyperplasia): The most frequent underlying cause in pediatric and young adult PNP patients; predominantly the localized hyaline vascular subtype.
2. Chronic Lymphocytic Leukemia (CLL): The most common neoplasm associated with PNP in older adults.
3. Non-Hodgkin Lymphoma (NHL): Diffuse large B-cell lymphoma, follicular lymphoma, and mantle cell lymphoma.
4. Thymoma & Thymic Carcinomas.
5. Waldenström Macroglobulinemia.
6. Retroperitoneal / Mesenteric Sarcomas: Particularly follicular dendritic cell sarcomas.

Target Antigens in Paraneoplastic Pemphigus

Unlike classic pemphigus, the autoantibody repertoire in PNP targets both desmosomal cadherins and the intracellular plakin cytoskeletal linker family:

• Envoplakin (210 kDa) and Periplakin (190 kDa) — considered the most specific serological biomarkers for PNP.
• Desmoplakin I (250 kDa) and Desmoplakin II (215 kDa).
• Plectin (500 kDa) and BP230 (230 kDa).
• Alpha-2-Macroglobulin-like-1 (A2ML1): A 170-kDa broad-spectrum protease inhibitor expressed in stratified epithelia.
• Desmoglein 3 and Desmoglein 1.

Clinical Presentation and Lethal Respiratory Complications

• Intractable Stomatitis: The earliest and most constant feature (100% of cases). Severe, painful, ulcerating, hemorrhagic stomatitis involving the entire oral cavity, extending onto the vermilion border of the lips with hemorrhagic crusted cheilitis, tongue, pharynx, and conjunctiva. Mucosal erosions are excruciating, non-healing, and refractory to conventional immunosuppressive therapies.
• Polymorphous Cutaneous Eruption: Skin lesions demonstrate striking morphological heterogeneity: flaccid or tense bullae, erythema multiforme-like targetoid lesions, lichenoid flat-topped violaceous papules, and widespread graft-versus-host disease (GVHD)-like desquamation. Severe palmoplantar erythema and targetoid lesions are characteristic.
• Bronchiolitis Obliterans (Constrictive Bronchiolitis): The primary cause of mortality in PNP (mortality rates exceed 75–90%). Autoantibodies target plakin proteins expressed in respiratory columnar epithelium, causing detachment of respiratory epithelial cells, intense submucosal inflammation, and progressive fibrous obliteration of terminal bronchioles. Patients develop progressive, irreversible dyspnea, wheezing, and hypoxemia leading to respiratory failure. Bronchiolitis obliterans is characteristically resistant to systemic immunosuppression and rituximab, persisting even after surgical resection of the underlying tumor.

Diagnostic Maneuvers & Laboratory Workup

Bedside Clinical Maneuvers

• Nikolsky Sign I (Direct Nikolsky Sign): Elicited by applying firm, sliding tangential lateral pressure with an index finger or thumb onto normal-appearing perilesional skin. A positive sign is defined by epidermal shearing, dislodgement, or blister formation under the shearing force. It reflects active intraepidermal acantholysis with diminished desmosomal cohesion. Highly specific for pemphigus and toxic epidermal necrolysis (TEN).
• Nikolsky Sign II (Indirect Nikolsky / Asboe-Hansen Sign): Elicited by applying gentle, direct downward vertical pressure with a gloved finger to the roof of an intact blister. A positive sign is defined by lateral centrifugal extension of the blister margin into surrounding, unblistered skin. It indicates intraepidermal or subepidermal fluid dissection.
• Tzanck Smear: A rapid bedside test. The roof of a fresh intact vesicle is unroofed, excess fluid blotted, and the base of the erosion gently scraped with a sterile scalpel blade. Material is smeared onto a glass slide and stained with Giemsa, Wright, or toluidine blue. Examination under light microscopy reveals acantholytic keratinocytes (Tzanck cells): rounded, solitary or clustered keratinocytes featuring enlarged hyperchromatic nuclei, prominent nucleoli, and a condensed peripheral rim of basophilic cytoplasm ("perinuclear halo").

Histopathology

• Biopsy Technique: Biopsy for routine light microscopy must be taken from an early, small, intact primary vesicle or blister edge containing both the blister roof and base. Biopsying an older, ulcerated erosion leads to loss of the epidermal roof and non-diagnostic re-epithelialization.
• Pemphigus Vulgaris: Demonstrates suprabasal intraepidermal acantholysis. Cleavage occurs immediately above the basal cell layer. The single layer of basal keratinocytes remains tethered to the underlying basement membrane zone by intact hemidesmosomes, producing the characteristic "row of tombstones" (tombstoning) appearance. Acantholytic cells float freely within the blister cavity. Prominent eosinophilic spongiosis (spongiosis with intraepidermal infiltration of eosinophils) is an early, highly sensitive finding.
• Pemphigus Foliaceus: Demonstrates subcorneal acantholysis localized to the granular layer or immediately beneath the stratum corneum. Acantholytic cells are found within the superficial blister cavity or attached to the stratum corneum roof. Resembles staphylococcal scalded skin syndrome (SSSS) and bullous impetigo.
• Paraneoplastic Pemphigus: Demonstrates a distinct hybrid histopathology: suprabasal acantholysis combined with a lichenoid / vacuolar interface dermatitis, extensive individual necrotic/apoptotic keratinocytes (satellitosis), and a dense band-like dermal lymphocytic infiltrate.

Direct Immunofluorescence (DIF)

• Specimen Handling: The gold standard diagnostic modality. The biopsy MUST be taken from perilesional, normal-appearing skin or mucosa (within about 1–2 cm of an active lesion). Biopsying an active blister cavity yields false-negative results due to secondary inflammatory enzymatic degradation of antibodies.
• Immunofluorescence Pattern: In PV and PF, DIF reveals continuous, intercellular deposition of IgG (specifically IgG4 in active disease; IgG1) and frequently C3 throughout the epidermis, creating a classic "chicken-wire", "fishnet-like", or honeycomb lattice pattern outlining individual keratinocyte cell membranes.
• In PV, intercellular staining is present throughout the full thickness of the epidermis (or concentrated in the lower suprabasal layers in mucosal-dominant PV). In PF, intercellular staining is concentrated in the upper spinous and granular layers.

Indirect Immunofluorescence (IIF) and Rat Bladder Testing

• Standard IIF: Serum is incubated on epithelial tissue substrates. Monkey esophagus is the preferred substrate for detecting anti-Dsg3 antibodies in pemphigus vulgaris, whereas guinea pig esophagus is optimal for detecting anti-Dsg1 in pemphigus foliaceus.
• Rat Bladder IIF (PNP Hallmark): Rat bladder transitional epithelium expresses rich concentrations of plakin family proteins (envoplakin, periplakin, desmoplakins) but lacks desmoglein 1 and desmoglein 3. Therefore, serum IIF demonstrating intercellular staining on rat bladder transitional epithelium has a specificity approaching 100% for paraneoplastic pemphigus, distinguishing PNP from classic pemphigus vulgaris.

Enzyme-Linked Immunosorbent Assay (ELISA)

Recombinant human Dsg1 and Dsg3 ELISA assays provide sensitive and quantitative measurement of circulating IgG autoantibodies:

• Quantitative index values correlate tightly with clinical disease severity, blister count, and treatment response.
• Monitoring Clinical Course: Normalization of anti-Dsg ELISA titers correlates with complete clinical remission. Rising titers herald an impending clinical relapse, often preceding visual blister recurrence by several weeks to months, providing an objective biomarker for rituximab maintenance dosing.

European Management Guidelines (EDF / EADV Consensus)

Historically, pemphigus carried a mortality rate approaching 70–90% within 2 years, predominantly from electrolyte depletion, cutaneous protein loss, and bacterial sepsis. The introduction of systemic corticosteroids reduced mortality to 10–15%, but high-dose, long-term steroid therapy introduced devastating iatrogenic morbidity (opportunistic infections, sepsis, osteoporosis, avascular bone necrosis, diabetes, hypertension, and peptic ulceration).

The Ritux 3 European Landmark Trial & First-Line Biologic Therapy

The treatment paradigm of pemphigus was fundamentally transformed by the Ritux 3 French multicenter randomized controlled trial published by Joly and colleagues in The Lancet (2017). This trial compared first-line rituximab combined with short-course prednisone versus prednisone monotherapy in newly diagnosed patients with moderate-to-severe pemphigus vulgaris and pemphigus foliaceus:

• Primary Endpoint: Complete remission off all therapy at month 24 was achieved by 89% of patients in the rituximab + prednisone group compared to only 34% in the prednisone-alone group ($p < 0.0001$).
• Steroid Sparing: The cumulative prednisone dose in the rituximab cohort was less than one-third of that in the steroid monotherapy arm, with a 50% reduction in severe (Grade 3–4) adverse events.
• Consequently, the European Dermatology Forum (EDF) and European Academy of Dermatology and Venereology (EADV) consensus guidelines now recommend Rituximab combined with systemic corticosteroids as the preferred first-line therapy for moderate-to-severe pemphigus.

First-Line Treatment Protocol

1. Rituximab (Anti-CD20 Chimeric Monoclonal Antibody):

   • Dosing Regimen: Two intravenous infusions of 1000 mg administered on Day 0 and Day 14 (rheumatoid arthritis / pemphigus protocol). Alternatively, 375 mg/m² body surface area weekly for 4 consecutive weeks (lymphoma protocol).
   • Maintenance / Relapse Redosing: A consolidation dose of 500 mg IV at Month 12 and Month 18 (or 1000 mg upon clinical relapse guided by anti-Dsg titers).
   • Pre-Medication: Intravenous methylprednisolone (100 mg), oral paracetamol (1000 mg), and an antihistamine (clemastine or diphenhydramine) 30–60 minutes prior to infusion to prevent acute cytokine-release syndrome.
   • Mandatory Pre-Treatment Screening:
     • Full serological screening for Hepatitis B (HBsAg, anti-HBc total, anti-HBs) and Hepatitis C to prevent fulminant hepatic reactivation.
     • HIV screening.
     • QuantiFERON-TB Gold or T-SPOT for latent tuberculosis.
     • Complete blood count and quantitative serum immunoglobulin levels (IgG, IgM, IgA) to baseline B-cell status.
   • Infection Prophylaxis: Concomitant Trimethoprim-Sulfamethoxazole (Co-trimoxazole) (800/160 mg three times weekly or 400/80 mg daily) for Pneumocystis jirovecii pneumonia (PJP) prophylaxis while receiving high-dose systemic steroids and rituximab until CD4 counts recover.

2. Systemic Corticosteroids:

   • Initial Dosing: Oral prednisolone 1.0 mg/kg/day (or up to 1.5 mg/kg/day in fulminant severe cases; 0.5 mg/kg/day in mild disease).
   • Tapering Protocol: Once disease control is achieved (no new lesions appear and existing lesions begin to re-epithelialize, typically within 2–4 weeks), the prednisolone dose is tapered rapidly over 3 to 6 months in patients receiving concurrent rituximab, aiming for complete cessation by month 6.

Second-Line Steroid-Sparing Immunosuppressants

When rituximab is strictly contraindicated (e.g., active uncontrolled infection, severe hypogammaglobulinemia) or unavailable, standard oral immunosuppressants are combined with systemic corticosteroids:

• Azathioprine: 1.0–3.0 mg/kg/day. Mandatory pre-treatment measurement of Thiopurine S-methyltransferase (TPMT) enzymatic activity or genotyping. Patients with low or absent TPMT activity are at catastrophic risk of fatal, irreversible bone marrow aplasia and severe myelosuppression.
• Mycophenolate Mofetil (MMF): 2.0–3.0 g/day (or enteric-coated mycophenolate sodium 1440–2160 mg/day). Reversibly inhibits inosine monophosphate dehydrogenase (IMPDH), selectively blocking de novo purine synthesis in proliferating T and B lymphocytes. Preferred in patients with hepatic impairment or TPMT deficiency.

Refractory and Rescue Modalities

In severe, fulminant, or therapy-refractory pemphigus:

• Intravenous Immunoglobulin (IVIg): Administered at a total dose of 2.0 g/kg body weight divided over 2 to 5 consecutive days monthly. IVIg acts by saturating the neonatal Fc receptor (FcRn), markedly accelerating the catabolic clearance of pathogenic anti-Dsg IgG autoantibodies, while providing passive antimicrobial protection.
• Immunoadsorption (IA): An extracorporeal apheresis technique utilizing tryptophan or protein A columns that selectively extracts circulating immunoglobulins from plasma while returning albumin and coagulation factors. Provides rapid, life-saving clearance of pathogenic autoantibodies within 48–72 hours in critical ICU-level cases.