Pemphigus

(Ref. Pathology of the skin, PH McKee, 3rd Ed. Elsevier-Mosby)

Acantholytic Lesions

The term acantholysis derives from the Greek akantha, a thorn or prickle, and lysis, a loosening. In its simplest definition, the term is used to reflect a primary disorder of the skin (and sometimes the mucous membranes) characterized by separation of the keratinocytes at their desmosomal junctions . A wide range of conditions are characterized by this feature, from inherited disorders such as Darier's disease and Hailey-Hailey disease in which a calcium pump gene mutation results in desmosomal instability through to the autoimmune pemphigus group of diseases whereby autoantibodies directly damage desmosomes with resultant keratinocyte separation and blister for­mation. Desmosomes may also be damaged by secondary phenomena, for example following severe edema, either intercellular (spongiosis) or intracellular (e.g. ballooning degeneration as is seen in various viral infections). Such processes, however, are not included in the acantholytic category and are discussed elsewhere. The histological features of the conditions described in this chapter show considerable overlap. The diagnosis is therefore dependent upon adequate clinical information and the results of immunofluorescence investigations.

 

Pemphigus

Pemphigus (Gr. pemphix, blister) refers to a group of chronic blistering diseases which develop as a consequence of autoantibodies directed against a variety of desmosomal proteins. The condition as a whole rare, with an annual incidence ranging from 0.1 to 0.5/100,000 of the general population. It is commoner in the Jewish population in white annual incidence rises to 1.6-3.2/100.000. Ashkenazi Jews are the most frequently affected. There is no sex predilection.

The clinical features and, therefore, classification of these disorders depends upon the level of separation within the epidermis:

• In pemphigus vulgaris (p. vulgaris) and pemphigus vegetans (p. vegetans) the blisters are suprabasal.

• ln pemphigus foliaceus (p. foliaceus), pemphigus erythematosus (p. erythematosus) and fogo selvagem, the blisters are situated more superficially.

Pemphigus vulgaris is by far the most common variant, accounting for 80% of cases. In addition to affecting humans, pemphigus has been described in a variety of animals including dogs, cars, goats and horses.

 

 

Pemphigus vulgaris

 

Clinical features

Pemphigus vulgaris (p. vulgaris) particularly affects the middle aged (onset typically at 40-60 years of age) although occasionally children are affected. Self-limiting neonatal disease through transplacental transfer of maternal autoantibodies has also rarely been documented.

The disease begins in the mouth in 50-70% of patients with painful erosions or bullae and, after a period of weeks or months, the blisters spread to involve the skin. Oral lesions most commonly affect the buccal, palatine and gingival mucosae.

The typical skin lesion is a fragile, flaccid blister, which develops on normal or erythematous skin, and readily ruptures, leaving a painful crusted, raw, bloody erosion. Lesions are most often seen on the scalp, face, axillae and groins, although in some patients they are generalized. Blisters can be induced by rubbing the adjacent, apparently normal skin with a finger, the Nikolsky sign. Direct pressure applied to the center of the blister is also followed by lateral extension, the Asboe-Hansen sign. Healing is often accompanied by postinflammatory hyperpigmentation but scarring is not a feature.

Before the introduction of corticosteroid therapy, the lesions usually became more extensive and in the past often led eventually to death. Treatment with high doses of corticosteroids and immunosuppressants, such as azathioprine, has significantly reduced the mortality to 5-15% and prolonged remissions without treatment are now being reported. A considerable proportion of the deaths that do occur, however, is due to the side-effects of therapy and include staphylococcal infections and, to a lesser extent, pulmonary embolism.

Rarely, nail involvement in the form of hemorrhagic paronychia, chronic paronychia, trachyonychia, onycholysis, or onychomadesis, is encountered in patients with p. vulgaris.

Occasional modes of presentation include linear lesions, postsurgical, postburn and postirradiation pemphigus.

In addition to oral and cutaneous involvement, lesions have been described at a wide variety of sites including the pharynx, larynx, oesophagus, eye, external genitalia, urethra and anal mucosa. Oesophageal lesions, although originally thought to be rare, have more recently been documented in as many as 63-87% of patients. Erosions and ulcers are typically found and intact blisters are rare. Exceptionally, the whole mucosa may be affected with subsequent sloughing, esophagitis dissecans superficialis. Ocular lesions are usually restricted to the conjunctiva, presenting as conjunctivitis or small vesicles that rapidly rupture. Very rarely, scarring may develop and corneal ulceration with perforation bas been described. Vulval, vaginal and cervical lesions are well recognized. Exceptionally, the vagina may be the sole site of involvement. Penile lesions most commonly affect the glans. They arc not usually followed by any significant sequelae.

The development of pemphigus may bc associated with a variety of disorders including other autoimmune bullous dermatoses, particularly bullous pemphigoid, lupus erythematosus, thymoma and myasthenia gravis. As in the many other diseases with immunological patho­genesis, pemphigus is accompanied by an increased incidence of internal malignancy including thymoma, lymphoma and Kaposi's sarcoma (sec paraneoplastic pemphigus).

 

Pathogenesis and histological features

Pemphigus is an immunologically mediated disease. Examination of perilesional skin by direct immunofluorescent techniques reveals in vivo-bound immunoglobulin (usually IgG) and often complement (C3) in the intercellular region of the epidermis. Abundant antigen in the follicular outer root sheath and germinal matrix may account for the marked scalp involvement typical of pemphigus. The in vivo-bound IgG is mainly of the IgG1 and IgG4 subclasses.

Indirect immunofluorescent techniques show that the serum of patients with pemphigus contains an IgG antibody that reacts with the intercellular region of normal squamous epithelium - the intercellular substance (pemphigus) antibody. This antibody is, however, not entirely specific as it may be found in a variety of other conditions, such as severe burns, penicillin drug reactions and following radiation therapy. Circulating antibodies are predominantly of the IgG1 and IgG4 subclasses; IgG3 is much less often identified.

Circulating IgG is pathogenic. The level of the antibody titer closely parallels the clinical state of the disease. IgG4 titers diminish during remission whereas circulating IgG1 may continue to be present. Relapse is commonly preceded by rising IgG4 antibody titers.

P. vulgaris very occasionally may be evident in a neonate born of a mother with active p. vulgaris. Such autoantibodies cross the placenta inducing disease in the infant. The disease is, however, short lived, and with lesions disappearing as the maternal antibodies are catabolized. Passive transfer of IgG4 into neonatal mice results in the development of blisters. Purified IgG from pemphigus induces acantholysis in human skin explants and keratinocyte cultures.

The pemphigus antibody binds to the full thickness of the epidermis. Compared with p. vulgaris, immunofluorescence studies on the sera of p. foliaceus patients tend to show more staining in the superficial epidermis, correlating with the level of the split. Conversely, the sera from patients with p. vulgaris show more affinity for the lower epidermis. Despite these trends, we generally do not base diagnoses on these (often subtle) differences in immunofluorescence staining distribution.

The p. vulgaris antibody is directed at the extracytoplasmic domain of the 130 kD epithelial desmosomal cadherin, desmoglein 3 (Dsg3), which forms a complex with plakoglobin (85 kD). The P. vulgaris antibody, however, does not recognize the latter. Many patients also have antibodies that bind to the p. foliaceus antigen, desmoglein 1 (Dsg1), a 160 kD polypeptide. Dsg3 is expressed primarily in the oral mucosa and therefore antibodies directed against this antigen result in mucosal pemphigus. In contrast, Dsg1 is a cutaneous antigen and, therefore, antibodies directed against it result in lesions affecting the skin but not the mucosa (cutaneous pemphigus).

Antibodies reactive to a number of other proteins including desmo­collins, pemphaxin and acetylcholine receptor have been demonstrated in the sera of p. vulgaris patients.

The pathogenesis of the acantholysis is uncertain. It is likely, that direct binding of antibody to the desmosomal cadherins is of major importance. There is also some evidence to suggest that the process may also involve, at least secondarily, the action of local proteolytic enzymes. Pemphigus antibody induces expression of plasminogen activator receptor on the surface of keratinocytes. Binding of plasminogen activator to its keratinocyte cell membrane receptor results in plasminogen activation with resultant production of plasmin. This latter has non-specific proteolytic activity, which may be responsible at least in part for the dissolution of the desmosomes. P. vulgaris antibodies stimulate production of keratinocyte phospholipase C, mositol 1,4,5-triphosphate and increase intracellular calcium. Protein kinase C activation results in release of keratinocyte plasminogen activator and increased expression of plasminogen activator receptor. Other factors, however, must be of greater importance since p. vulgaris IgG can induce acantholysis in plasminogen activator knockout mice. Complement appears not to be essential for acantholysis and it is thought that any involvement is secondary, perhaps accelerating or extending the process.

T-cells are also critical to the development of the antibody-mediated acantholysis. CD4+ memory T-cells are predominantly involved and both T-helper 1 (Th1) and Th2 Dsg3‑specific subtypes are represented. Th1 T-cell-derived interferon stimulates production of IgG1, and Th2 cells produce interleukin (IL)4 and IL-13 which are responsible for secretion of B-cell derived IgG4. Both populations are therefore of importance in stimulating production of p. vulgaris antibody. In addition, there is evidence that tumor necrosis factor 1 (TNF-1) and IL-1 are also of importance in the development of acantholysis. Knockout mice for both these cytokines show diminished acantholysis in passive antibody transfer experiments.

There is considerable evidence of a genetic background influenc­ing susceptibility to pemphigus as shown by strong associations with human leukocyte antigen (HLA) DRP1*0402, HLA DRP1"1401 and HLA DQ01*0503. Perhaps surprisingly, however, there are only occasional documented reports of familial occurrence.

Pemphigus blisters rupture easily. It is therefore essential to biopsy an early lesion to establish the correct diagnosis. The characteristic acantholysis develops because of damage to the intercellular bridges. Acantholytic cells are rounded and have intensely eosinophilic cytoplasm, pyknotic nuclei and perinuclear halos. An early lesion of p. vulgaris shows a slit-like suprabasal cleft or vesicle containing occasional acantholytic cells. The established blister contains acantholytic cells in clumps and in isolation. Characteristically, the floor of the cavity is lined by a single layer of intact basal cells, the so-called 'tombstone' pattern. The acantholytic process frequently involves the epithelium of the adnexae which can be a useful diagnostic clue in those lesions which lack the roof of the blister. The dermal papillary outline is usually maintained and, frequently, the papillae protrude into the blister cavity. Sometimes the features of eosinophilic spongiosis are seen on biopsy, particularly in early lesions.' The blister cavity often contains a few inflammatory cells (notably eosinophils) and, in the dermis, there is a moderate perivascular chronic inflammatory cell infiltrate with conspicuous eosinophils although sometimes these are scanty or even absent. Mucous membrane lesions show similar histology.

Ultrastructurally, there is dilatation of the intercellular space with consequent stretching of the desmosomal attachment points. With progression these separate and eventually disappear, residual cell membranes often showing a pseudovillous morphology. Hemidesmosomes are morphologically normal. Immunoelectron microscopy confirms that the immunoreactants are located within the intercellular space.

 

Differential diagnosis

The differential diagnosis of p. vulgaris includes a variety of conditions such as Darier's disease, Halley-Hailey disease and transient acantholytic dermatosis (Grover's disease). In the absence of clinical information or without immunofluorescence studies, it may be impossible to establish a definitive diagnosis. Darier’s and Hailey-Hailey diseases are not associated with immunoreactants.

Dyskeratosis in the form of corps ronds and grains is typical of Darier's disease, but is rarely seen in Hailey-Hailey disease, and is not a feature of pemphigus. In Hailey-Hailey disease, the perivesicular epithelium is likened to a dilapidated brick wall, an effect sometimes seen in p. vulgaris. More frequently, however, the epithelium overlying and adjacent to the blister is essentially intact.

Acantholysis involving the follicular epithelium is often seen in pemphigus, but usually not in Hailey-Hailey disease. The pemphigus-like variant of Grover's disease is histologically indistinguishable from pemphigus, but the clinical history, minute size of the lesions as viewed by the microscope, and negative immunofluorescence findings make distinction relatively easy. Extreme degrees of acantholysis in acantholytic solar keratosis may on rare occasions be confused with the previously mentioned acantholytic disorders. Similarly, it is important not to mis­interpret the trivial finding of incidental focal acantholytic dyskeratosis in a skin specimen removed or biopsied for an unrelated finding.