I. CLASSIFICATION AND ETIOLOGY OF LICHEN SCLEROSUS
LS occurs in genital skin of children, adolescents and adults of both sexes. LS begins with uncharacteristic symptoms and advances slowly to atrophic disease with white plaques and areas of lichenification. In vulvar LS, synechia of the labia minora and stenosis of the introitus vaginae represent the irreversible late stages of LS with high morbidity. End stage LS has been known for over 100 years under various names (ichthyosis, leukoplakia, craurosis, lichen sclerosus et atrophicus, white spot disease, dystrophy), but the preferred terminology for all disease stages is Lichen sclerosus [15] . The etiology of LS remains unclear. Familial cases of LS in twin sisters, siblings, mothers and daughters, and HLA class II DQ antigen restrictions support the notion of a genetic predisposition [3, 8] . Hormonal factors and the spirochete borrelia have been suggested but not substantiated as triggers for the onset of LS. The high incidence of concomitant autoimmune diseases (thyroiditis, alopecia areata, vitiligo, pernicious anemia) with tissue and organ-specific auto-antibodies in patients with LS and their first degree relatives links LS to autoimmune diseases [9] . Although no disease-specific auto-antibodies or antigens have been identified in LS, an "immune dysregulation" in the etiology of LS is presently the most accepted theory. Fetal microchimerism (transfer of fetal progenitor cells into maternal circulation and engraftment into maternal tissues during pregnancy) has been implicated in the development of autoimmune diseases, especially Hashimoto's thyroiditis and systemic sclerosis [1, 5] . Despite the strong association of LS with autoimmune thyroid disease and some similarities of LS with systemic sclerosis, fetal microchimerism is common in vulvar skin and appears to be etiologically irrelevant for the development of LS [14] .

II. HISTOLOGICAL FEATURES OF VULVAR LS
End stage LS with an atrophic flat epidermis, hyperkeratotic hair follicles and acrosyringia, dystrophic hair, extensive dermal sclerosis with scarce, highly sclerotic and ectatic vessels, and a lymphocytic infiltrate of varying degrees poses clinically and diagnostically no problem. In contrast, early LS presents clinically with uncharacteristic symptoms. Histopathologically, early LS shows a variety of typically subtle features, which may involve all or individual compartments of the skin / mucosa (Table 1):

Table 1 - Histological Features of Early Lichen Sclerosus

Epidermis	Normal or
	Mild acanthosis with slightly irregular plump rete ridges
	Intraepithelial lymphocytes, mostly in basal or suprabasal location
	In the presence of intraepidermal lymphocytes: Spongiosis, Vacuolar basal keratinocyte and melanocyte degeneration can be found
Basement membrane	Normal or focally thickened (PAS-reaction!)
	May be obscured by band-like lichenoid lymphocytic infiltrate
Dermis	Homogenized collagen of papillary dermis
	Melanophages
	Interstitial lymphocytic infiltrate
Vessels	Normal numbers or slightly reduced
	Ectatic capillaries in dermal papillae, often located immediately beneath the basement membrane
	Hyalinized vessel walls
	Perivascular lymphohistiocytic infiltrates
	Vasculitis: lymphocytic - lymphohistiocytic - rarely leukocytoclastic vasculitis
Inflammatory Infiltrate	Intraepidermal lymphocyte exocytosis with epidermal spongiosis and keratinocyte vacuolization
	Dermal infiltrates: lichenoid with intraepidermal extension - interstitial - nodular perivascular
Hair/Folliculo-sebaceous Unit	Acanthosis of epithelium of follicular ostiae and sebaceous glands isthmus
	Luminal hyperkeratosis and hypergranulosis
	Perifollicular basement membrane thickening
	Dystrophic hair shafts
Sweat glands	Acanthosis of acrosyringial epithelium with
	Luminal hyperkeratosis and hypergranulosis of ostiae
	Basement membrane thickening

The changes of early LS are focal and serial sectioning of small biopsies and PAS-reactions are necessary for their identification. Early LS typically involves the adnexal structures, and in biopsies of early LS the changes of adnexal structures are more prominent and observed before the interfollicular epidermis shows the characteristic features. The modified mucosa and epidermis are either of normal thickness with regular keratinization or show a mild acanthosis and irregular rete ridges. The ostiae of hair follicles, sebaceous glands and sweat glands show epithelial acanthosis with prominent luminal hyperkeratosis and hypergranulosis. The basement membrane displays a focal subtle thickening, which is best demonstrated in a PAS-reaction. The papillary dermis / submucosa appear homogenized with prominent ectatic hyalinized capillaries, some of which are located immediately beneath the basement membrane. The lymphocytic infiltrate is either focal or diffuse. Lichenoid infiltrates are often associated with intraepidermal exocytosis of lymphocytes which may result in intraepidermal edema (spongiosis), squamatization or vacuolization of basal keratinocytes. The dermis in early LS may contain numerous melanophages, reflecting the destruction of melanocytes and subsequent phagocytosis of melanosomes. Vascular changes in of all stages of LS include a lymphohistiocytic ("granulomatous") and a lymphocytic vasculitis. Very rarely, a leukocytoclastic vasculitis can be observed [12].

The histopathological differential diagnosis of an early LS includes a Candida infection, which must be excluded with a PAS-reaction. Lichen planus is in the differential diagnosis of early LS with a band-like lymphoid infiltrate, but the rete ridges are saw-tooth-like elongated in lichen planus rather than flattened or plump as typical for LS. Lichen planus features a wedge-shaped epidermal hypergranulosis and shows no adnexal involvement. Epidermal vesiculation and occasional eosinophilic granulocytes are indicative for hypersensitivity reactions and contact allergies. Vulvar psoriasis demonstrates the strikingly regular psoriasiform epidermal hyperplasia with intraepidermal pustules containing neutrophilic granulocytes. The para- and hyperkeratosis is less prominent in vulvar mucosa and modified mucosa than in keratinized skin.

III. IMMUNE DYSREGULATION IN LICHEN SCLEROSUS
LS is a progressive lymphocyte-mediated dermatosis. In early LS, the lymphocytic infiltrate is typically scant. The dense lichenoid and interstitial lymphoid infiltrates in fully developed LS result in keratinocyte destruction with epithelial atrophy, massive basement membrane thickening, dermal sclerosis with rarefied sclerotic vessels. In late stage of LS, the lymphocytic infiltrate can vary. Lymphocytic and lymphohistiocytic vasculitis is a feature of all disease stages of LS [12] . Between 20% and 50% of vulvar and penile LS harbor T-cells with a monoclonally rearranged T-cell receptor γ-chain gene (mTCRγ). mTCRγ can be found in all disease stages of LS, in young and old patients, and biopsies with dense or scant lymphocytic infiltrates [7, 11, 13] . The amount of clonal DNA with mTCRγ is typically below 20% of total analyzed DNA. In our hands, the amount of clonal DNA ranges between 1.4% and 23.1% DNA and can be observed in biopsies and excision of LS of children and adults (age range 4 - 82 years) ^{personal observation}. The lymphocytic infiltrates in biospies with mTCRγ consist of B-cells, CD4 > CD8 +T-cells, dendritic cells expressing CD21, CD35, CD106, and granzyme-B and TIA-1-positive cytotoxic T-cells [13] . The monoclonal T-cells in LS represents a reactive infiltrate with an immunoprofile highly indicative of an antigen-mediated immune response. Currently, the antigen stimulating the TCR modulation is unknown and we do not know at which time point in the evolution of LS the T-cell clones emerge. An exceptional serological demonstration of clonal DNA initially demonstrated in a biopsy of LS of the foreskin has been observed in a 31-year-old man who presented 13 years later with the same clone in the blood and in other skin lesions [10, 11] . The appropriate clinical management of patients with LS containing monoclonal T-cells in the tissue infiltrates of LS is challenging, mainly due to the lack of experience and long-term follow up of these patients.

IV. THE RELATIONSHIP OF LS TO VULVAR CANCER
It is believed that longstanding vulvar LS carries a slightly increased risk for development of a vulvar SCC [9]. The association of LS with SCC is only known in genital skin. The exact incidence of SCC arising in LS is unknown, but estimated to be around 4-5% of women suffering from LS. The risk to develop a vulvar SCC for a woman with LS is about 15-times increased compared to the "normal" population. LS is observed in up to 60% of vulvectomy specimens for SCC, and represents often an incidental finding [2]. Vulvar SCC may be considered a complication of advanced untreated LS, since LS-associated SCC develops typically in older women suffering from late stage advanced LS with atrophic, lichenified, cigarette-paper thin skin, resorption and synechiae of the labia minora and secondary changes after decades of disease progression . However, depending on the age at onset of LS and the rapidity of disease progression, patients with SCC arising in vulvar LS may be as young as 25 years. Evidence is accumulating that SCC in LS may develop very rapidly, usually within less than 6 months, which makes yearly appointments for vulvar cancer screening in these patients questionable. Presently, it is not clear if LS should be classified as precancerous lesion, since there are no documented clinical long term experiences about the natural course of untreated LS. Presently, only a correct early diagnosis will ensure early treatment of LS and prevent or at least delay the progression to atrophic mutilating LS and possibly to SCC.

Several theories have been raised to explain the carcinogenesis in LS:

1. SCC has been postulated to arise from localized squamous hyperplasia within a lesion of LS [2, 17] . Immunohistochemical demonstration of p53 in SCC and adjacent hypertrophic / hyperplastic LS has lead to the assumption that hyperplastic lesions are precursors of SCC [16] . However, immunohistochemical demonstration of p53 is a common finding in LS. In our own survey of 200 patients with LS, which included pediatric and adult patients at various disease stages, p53-staining of nuclei of basal keratinocyte was observed commonly. While early LS and young patients revealed mostly individual positive nuclei, about 60% of fully developed LS showed nuclear basal keratinocyte staining. Hypertrophic LS revealed staining in 70% basal and suprabasal keratinocyte nuclei. p53-staining was associated with sclerosis of dermis and blood vessels, and lymphoid infiltrates and explained by inflammation and ischemic stress due to poor oxygenation secondary to rarified sclerotic blood vessels and vasculitis. p53-expression is a poor screening tool for identifying precancerous lesions, since p53-expression does not correlate with dysplasia (unpublished personal data, abstract /oral presentation section D, Monday afternoon) or p53 gene mutations, which are late events in vulvar carcinogenesis [18] . True neoplastic hypertrophies and hyperplasias in LS have to be distinguished from reactive pseudo-epitheliomatous hyperplasias in the vicinity of ulcerations, edges of epithelial regeneration and ruptured hair follicles, particularly when fibrin and inflammation is present. Pseudoepitheliomatous hyperplasia is a self-limited condition without progression to SCC [6] and typically observed in advanced LS complicated by secondary changes.
   
   
2. Differentiated vulvar intraepithelial neoplasia (d-VIN) has been postulated to be a precursor lesion for non-HPV-related vulvar cancer with a rapid progression to invasive SCC. d-VIN has been described as acanthotic epidermis with elongated and anastomosing rete ridges and lack of HPV-associated cell changes. The enlarged atypical basal keratinocytes are p53-positive, show occasional mitoses and premature eosinophilic cytoplasmic differentiation. d-VIN can be observed in the immediate vicinity of SCC in about 25% of vulvectomy specimens for SCC [4, 19] . In a survey of over 200 patients, only 2% of biopsies and excisions of lone LS revealed d-VIN. However, we were able to identify d-VIN in 8/33 (24%) vulvectomy specimens for SCC arising in the background of LS (unpublished personal data, abstract /oral presentation section D, Monday afternoon). d-VIN may also be considered a complication of longstanding LS which occurs in endstage disease independent of the patients age. Our youngest patients with d-VIN adjacent to the SCC in the background of LS were 25 and 23 years resp. In the vast majority of cases, d-VIN is identified retrospectively adjacent to vulvar SCC. In my opinion, d-VIN has only a limited value as a prospective screening marker for assessing the cancer risk in an individual patient. Again, there are no long-term clinical follow-up data on the natural course of LS and d-VIN, and the importance and significance of d-VIN as a precancerous lesion is still undefined.
   
   
3. The observed local immune dysregulation in LS may lead to a reduction of TCR diversity in the lymphoid infiltrate. The demonstration of mTCRg gene in the lymphoid infiltrate in LS [7, 11, 13] is interpreted as antigen-driven selection of T-cells after prolonged exposure of the host immune system to a local antigen. The restricted TCR-usage may result in subsequent immune dysregulation and inadequate tumor response of the tumor infiltrating lymphocytes, creating a permissive environment for the development of SCC in LS patients. All investigators of LS agree on the non-neoplastic nature of the infiltrate despite the lack of a thorough understanding of the etiology of LS. Antigen-driven selection of T-cells and restricted TCR-usage may reflect prolonged exposure of the host immune system to a local (? infectious, ? putative LS-associated) antigen. The lack of long-term follow-up of LS patients with documented mTCRg , however, defers a final judgment on the clinical significance of our observations [10].

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