JPC SYSTEMIC PATHOLOGY
INTEGUMENTARY SYSTEM
September 2022
I-M27

Signalment (JPC #2185555): A horse

 

HISTORY: None

 

HISTOPATHOLOGIC DESCRIPTION: Haired skin: There are multifocal sub-basilar clefts, up to 1.5 mm long, that separate the epidermis from the dermis.  Clefts contain small amounts of eosinophilic fluid (serum), eosinophilic fibrillar material (fibrin), and low numbers of erythrocytes. The basement membrane zone of the less affected areas at the dermal-epidermal junction frequently is expanded by multiple small, clear, variably sized vacuoles (subepidermal vacuoles). There is multifocal to coalescing mild epidermal hyperplasia (acanthosis) and mild intercellular edema, forming prominent intercellular bridging (spongiosis). Multifocally, there is mild parakeratotic hyperkeratosis with multiple small intracorneal pustules containing necrotic neutrophils. The superficial and perivascular dermis is infiltrated by moderate numbers of neutrophils, lymphocytes, and plasma cells admixed with low numbers of reactive fibroblasts, increased fibrous connective tissue (fibrosis), mild hemorrhage, and increased clear space (edema). Multifocally, apocrine glands and hair follicles are mildly ectatic.

 

MORPHOLOGIC DIAGNOSIS:  Haired skin: Subepidermal clefts, multiple, with subepidermal vacuoles, moderate lymphoplasmacytic superficial interstitial and perivascular dermatitis, and intracorneal pustules, breed unspecified, equine.

 

ETIOLOGIC DIAGNOSIS:  Immune-mediated dermatopathy

 

CONDITION:  Bullous pemphigoid

 

GENERAL DISCUSSION: 

• Autoimmune bullous diseases (ABDs) occur as a result of the binding of autoantibodies; characterized by blisters on the skin and mucous membranes; two groups of ABDs:
• Pemphigus group: Autoantibodies to desmosomal components leads to inhibition of cell adhesion and/or abnormal signal transduction (i.e. pemphigus vulgaris, pemphigus foliaceus)
• Desmosomes: adhesion between keratinocytes
• Subepidermal ABD group: Autoantibodies to hemidesmosomal components in the epidermal basement membrane zone; generally need inflammatory event in addition to autoantibodies to develop skin lesions (i.e. bullous pemphigoid, epidermolysis bullosa acquisita)
• Hemidesmosomes: adhesion between the epidermis and underlying dermis
• Bullous pemphigoid (BP) is a rare, chronic, autoimmune, subepithelial vesiculobullous disease of dogs, cats, Yucatan minipigs, and horses
• Many previous cases of BP were diagnosed prior to the ability to identify the target protein, and therefore may not have truly been BP
• Collies previously thought to be predisposed, but it is now thought that the disease process in collies is more consistent with vesicular cutaneous lupus erythematosus
• A bulla is a collection of fluid (subcorneal, suprabasilar, subepidermal, etc.) larger than 1cm; a vesicle is smaller than 1cm

 

PATHOGENESIS: 

• Autoantibodies (IgG) are directed against bullous pemphigoid antigen 2 (BPAG 2 or BP180) which is a transmembrane form of type XVII collagen (COL17) in basal keratinocyte hemidesmosomes (of skin or mucosal epithelium) > epidermal-dermal separation of basal epithelial cells from upper lamina lucida of the basement membrane
• In addition to anti-BPAG2 antibodies, complement and neutrophil activation are required for development of clinical disease:
• Binding of complement-fixing antibody to the antigen of the hemidesmosome > complement fixation and activation > activation of mast cells and release of cytokines (IL-1, -5, -6, and -8) > attract neutrophils and eosinophils > proteolytic enzymes released from infiltrating leukocytes disrupt dermo-epidermal cohesion > subepidermal/sub-basilar vesicle formation
• The cause of auto-antibody production is unknown; in some cases, drugs (sulfonamides, penicillins, and furosemide) and UV light may be involved
  

TYPICAL CLINICAL FINDINGS:

• Horses: Lesions are often generalized with severe sloughing of the oral mucosa, squamous lining of the esophagus and stomach, and skin; often accompanied by systemic illness
• Pain, pruritus, secondary pyoderma
• Clin Path: Mild leukocytosis and neutrophilia, mild nonregenerative anemia, mild hypoalbuminemia, and mild hyperglobulinemia (peripheral eosinophilia is rare); circulating antibasement membrane autoantibodies have been detected

 

TYPICAL GROSS FINDINGS: 

• Vesiculobullous, ulcerative, crusting lesions in skin/oral mucosa
• Bullae are less transient than those in intradermal autoimmune diseases
• Smaller vesicles develop near the main vesicle
• Collarettes form after vesicle ruptures and ulcer develops

 

TYPICAL LIGHT MICROSCOPIC FINDINGS: 

• Subepidermal clefts/vesicles/bullae (dermal-epidermal junction); may rupture leading to ulceration; may extend to follicular infundibula; basal cells line the roof of the bulla and the basement membrane zone lies on the floor of the bulla
• Vesicles usually contain fibrin, neutrophils, mononuclear cells, and +/- eosinophils; eosinophils are not always present (as they are in humans)
• Subepidermal vacuoles (“bubbles”) are early pre-vesicle findings
• Acantholytic keratinocytes are not present
• Mild dermal inflammation and marked edema, often with perivascular inflammation; eosinophils may line up in rows beneath basement membrane
  • +/- mild to moderate lichenoid interface dermatitis within mucocutaneous regions

 

ULTRASTRUCTURAL FINDINGS:

• Smudging, thickening, and interruption of basement membrane zone with separation within the lamina lucida
• Fragmentation and disappearance of anchoring fibers and hemidesmosomes
• Basal cell degeneration

 

ADDITIONAL DIAGNOSTIC TESTS: 

• Immunofluorescence and immunohistochemistry: Linear IgG, IgA, or IgM autoantibodies or complement (C3) that targets bullous pemphigoid antigens at the basement membrane

 

DIFFERENTIAL DIAGNOSIS:

Histologic differentials for BP:

• Pemphigus vulgaris (dogs, cats, horses) (I-M25): Suprabasilar acantholysis (separation of epidermis occurs between stratum spinosum and stratum basale); no apoptosis; basal keratinocytes arranged as “row of tombstones” along basement membrane with loss of intercellular contact; usually affects mucous membranes, particularly in middle-aged dogs
• Epidermolysis bullosa acquisita (dogs): Autoantibodies against type VII collagen (COL7) within anchoring fibrils of basement membrane zone; subepidermal clefts and vesicle formation in areas of concurrent marked neurophilic superficial dermatitis; recognized in dogs, young Great Danes are predisposed (<1y); affects similar areas as BP, but more consistently affects footpads.
• Vesicular cutaneous lupus erythematosus: Middle-aged to older dogs (especially rough-coated collies, border collies, and Shetland sheepdogs), cats (Siamese), and horses; autoantibodies target nuclear antigens similar to that seen in CLE; vesicles, bullae, erosions, and ulcers, predominantly in inguinal and axillary regions, +/- mucocutaneous junctions; interface lymphocytic dermatitis with keratinocyte apoptosis, hydropic degeneration of basal cells, and vesicles/bullae at dermoepidermal junction
• Mucous membrane pemphigoid (AKA cicatricial pemphigoid): Subepidermal clefting with varying degress of inflammation; localized to mucosa and mucocutaneous junctions and most often occurs in German shepherd dogs; lesions typically arise on the nasal planum, medial canthus, oral cavity, ear canals, and genitalia; multiple target antigens associated with the basement membrane have been identified including BPAG 2, type XVII collagen, laminin-5, laminin-6, type VII collagen, and β-4 integrin
• Toxic epidermal necrolysis (I-M29): Apoptotic epidermal cells at all levels of the epidermis and follicular epithelium with few lymphocytes surrounding apoptotic keratinocytes (lymphocytic satellitosis); subepidermal cleft and vesicle formation at basement membrane zone; lymphohistiocytic cytotoxic (interface) dermatitis; hydropic degeneration of basal cells; suspected misdirected cell-mediated (type IV) immune response targeting keratinocytes
• Thermal burns (I-M04): Coagulative necrosis that extends into deep dermis, depending on the burn severity; vesicles and bullae containing serum or debris form in second degree burns

 

Other causes of non-infectious vesicular and bullous formation:

• Congenital epidermolysis bullosa: Mechanobullous genodermatoses  characterized by skin and mucous membrane blistering and ulceration; lesions occur in response to minor mechanical trauma (often over prominences, footpads, palate, etc); develop shortly after birth; histologically characterized by subepidermal clefts with minimal inflammation (typically require additional immunofluorescence, immunohistochemistry, or ultrastructural examination to differentiate from BP); divided into three groups:
• Epidermolysis bullosa simplex: Cytolysis of the basal keratinocytes > intraepidermal clefting; mutations in keratins 5 & 14
• Junctional epidermolysis bullosa: Clefting occurs within the lamina lucida; deficiency of one of hemidesmosome-associated proteins (laminin-5, collagen XVII, integrin α6β4, or LAD-1)
• Dystrophic epidermolysis bullosa: Split in superficial dermis below lamina densa; mutation in anchoring fibril protein type VII collagen
• Mutations in other structural proteins of the basement membrane zone & cytoskeleton of basal keratinocytes; mutation in various genes:
• Plakophilin 1 (PKP1), desmoplakin (DSP), keratins 5 & 14, plectin (PLEC1), αbβ5 integrin (ITGA6), laminin, collagen types XVII & VII (COL17A1, COL7A)
• Linear IgA bullous dermatosis: The dog is a natural model for Linear IgA Disease (LAD) of humans
• IgG and/or IgA autoantibodies against extracellular form of type XVII collagen (LAD-1)
• BP antigen is also a type XVII collagen, but recognizes the transmembrane form (vs. the extracellular form in LAD)
• Four criteria to differentiate from other subepidermal blistering diseases: (1) Crusts and ulcers on the extremities, face, and oral cavity; (2) Non-inflammatory subepidermal clefting; (3) IgA and/or IgG basement membrane antibodies; and (4) IgA and IgG circulating autoantibodies that target LAD-1
• Ulcerative dermatosis of collies and Shetland sheepdogs: Necrosis is primarily confined to the basal layer

 

COMPARATIVE PATHOLOGY: 

• Pigs and dogs have more inflammatory cells in lesions than horses and cats
• Yucatan minipigs: Lesions most often occur on the rump and back (mucosal lesions not described)
• Dogs: Vesicle formation (or ulceration if they rupture) on skin of the face, back, concave surface of ears, axilla, groin, less frequently on mucous membranes (especially of the oral cavity in approximately half of dogs with this condition), mucocutaneous junctions (lips), and rarely the footpads; usually mildly affected
• Cats: Few lesions limited to the oral commissures, oral mucosa (hard and soft palate), and the hairless regions of the concave aspect of the pinnae

 

REFERENCES:

1. Gross TL, Ihrke PJ, Walder EJ, Affolter VK. Skin Diseases of the Dog and Cat. 2nd ed. Ames, IA: Blackwell; 2005: 27-30.
2. Mauldin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016: 533-534, 564-566, 602-604, 607, 609-610.
3. Scott DW, Miller WH. Equine Dermatology. 2nd ed. Maryland Heights, MO: Elsevier Saunders; 2011: 83.
4. Welle MM and Linder KE. The Integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7^th St. Louis, MO: Elsevier: 2022: 1190, 1195-1196.

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