JPC SYSTEMIC PATHOLOGY
Signalment (JPC #2185555): A horse
HISTOPATHOLOGIC DESCRIPTION: Haired skin: There are multifocal sub-basilar clefts, up to 2.5 mm long, that separate the epidermis from the dermis. Clefts contain small amounts of eosinophilic fluid (serum), 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 acanthosis and spongiosis. Multifocally, there is mild parakeratotic hyperkeratosis with multiple small intracorneal pustules containing degenerate neutrophils. The superficial and perivascular dermis is expanded by moderate numbers of neutrophils, lymphocytes, and plasma cells admixed with low numbers of reactive fibroblasts and mild fibrosis. The superficial dermis is multifocally expanded by increased clear space (edema). There is mild multifocal apocrine gland and follicular ectasia.
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
· Autoimmune bullous diseases (ABDs) occur as a result of the binding of autoantibodies; they are 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, pigs, 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 lupus erythematosus
· A bulla is a collection of fluid (subcorneal, suprabasilar, subepidermal, etc.) larger than 1cm; a vesicle is smaller than 1cm
· The cause of auto-antibody production is unknown; in some cases, drugs (sulfonamides, penicillins, and furosemide) and UV light may be involved
· 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 at the lamina lucida
· 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 > vesicle formation
TYPICAL CLINICAL FINDINGS:
· Horses: Lesions are often generalized with sloughing of the oral mucosa and systemic illness
· Pain, pruritus, secondary pyoderma
· Clin Path: Mild leukocytosis and neutrophilia, mild nonregenerative anemia, mild hypoalbuminemia, and mild hyperglobulinemia (peripheral eosinophilia is rare)
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, eosinophils, and mononuclear cells; however eosinophils are not always present (as they are humans)
· Subepidermal vacuoles (“bubbles”) are early prevesicle 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
· 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
· Salt-split skin (split through lamina lucida) is used as a substrate for immunofluorescence testing; allows identification of autoantibodies that bind the roof (epidermal), floor (dermal), or both
Histologic differentials for BP:
· Pemphigus vulgaris (dogs, cats, horses)(I-M25): Suprabasilar acantholysis, no apoptosis, usually affects mucous membranes
· Epidermolysis bullosa acquisita (dogs): Autoantibodies against type VII collagen (COL7) within anchoring fibrils of basement membrane zone; subepidermal clefts associated with neutrophil-rich vesicles and a mild dermal infiltrate of neutrophils, lymphocytes and plasma cells; can be systemically ill and often have footpad lesions
· Vesicular lupus erythematosus: Dogs (especially Collies, Shetland sheepdogs), cats (Siamese), and horses
· Mucous membrane pemphigoid: Affects mucosa and mucocutaneous junctions and most often occurs in German shepherd dogs; 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
· Lesions typically arise on the nasal planum, medial canthus, oral cavity, ear canals, and genitalia
· Toxic epidermal necrolysis (I-M29): Apoptotic epidermal cells within all layers of epidermis, subepidermal cleft and vesicle formation at basement membrane zone; lymphohistiocytic interface dermatitis; hydropic degeneration of basement membrane
· Thermal burns (I-M04): Coagulative necrosis that extends into deep dermis, depending on the burn severity
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; 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: (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
· Dogs: Vesicle formation (or ulceration if they rupture) on mucous membranes (especially of the oral cavity), mucocutaneous junctions (lips), and skin of the face, back, concave surface of ears, axilla, groin, and rarely the paws
· Cats: Lesions of the oral commissures, oral mucosa (hard and soft palate), and the hairless regions of the concave aspect of the pinnae
· Yucatan pigs: Lesions most often occur on the rump and back (mucosal lesions not described)
· Rats: Hereditary junctional epidermolysis bullosa & dystrophic epidermolysis bullosa reported in Sprague-Dawley rats
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