Alopecia areata im Tiermodell – Neue Einblicke in Pathogenese und Therapie einer T‐Zell‐vermittelten Autoimmunerkrankung

Alopecia areata is a common disease, but for ethical reasons it seems difficult to perform large‐scale studies to elucidate the pathogenesis and to develop new therapeutic approaches in man. It is therefore helpful to develop appropriate animal models. The Dundee experimental bald rat (DEBR) and the C3H/HeJ mouse are well‐established animal models for alopecia areata and can be used for the study of genetic aspects, pathogenesis and therapy of the disease. In C3H/HeJ mice alopecia areata can be experimentally induced by grafting lesional skin from an affected mouse to a histocompatible recipient which offers the possibility to study the influence of various factors on the development of the disease. Studies on the C3H/HeJ mouse and the DEBR have corroborated the concept that alopecia areata is a T‐cell mediated autoimmune disease and various steps and aspects of the pathogenesis have been elucidated. Based on this knowledge new therapeutic options may be developed such as inhibition of lymphocyte‐homing by an anti‐CD44v10 antibody, or inhibition of costimulation by monoclonal antibodies. Therapeutic studies in the C3H/HeJ mouse and the DEBR suggest that alopecia areata can be treated by topical tacrolimus but treatment in humans may only be successful after development of an improved vehicle that facilitates penetration of tacrolimus down to the hair bulb. Current investigations in mice are designed to elucidate the mechanisms how contact sensitizers act in the treatment of alopecia areata, and this will hopefully lead to the development of more specific approaches based on the beneficial effect of contact sensitizers.     

Maternal IL‐6 can cause T‐cell‐mediated juvenile alopecia by non‐scarring follicular dystrophy in mice

Aiming to decipher immunological mechanisms of the autoimmune disorder alopecia areata (AA), we hypothesized that interleukin‐6 (IL‐6) might be associated with juvenile‐onset AA, for which there is currently no experimental model. Upon intramuscular transgenesis to overexpress IL‐6 in pregnant female C57BL/6 (B6) mice, we found that the offspring displayed an initial normal and complete juvenile hair growth cycle, but developed alopecia around postnatal day 18. This alopecia was patchy and reversible (non‐scarring) and was associated with upregulation of Ulbp1 expression, the only mouse homolog of the human AA‐associated ULBP3 gene. Alopecia was also associated with inflammatory infiltration of hair follicles by lymphocytes, including alpha‐beta T cells, which contributed to surface hair loss. Despite these apparently shared traits with AA, lesions were dominated by follicular dystrophy that was atypical of human AA disease, sharing some traits consistent with B6 alopecia and dermatitis. Additionally, juvenile‐onset alopecia was followed by complete, spontaneous recovery of surface hair, without recurrence of hair loss. Prolonging exposure to IL‐6 prolonged the time to recovery, but once recovered, repeating high‐dose IL‐6 exposure de novo did not re‐induce alopecia. These data suggest that although substantial molecular and cellular pathways may be shared, functionally similar alopecia disorders can occur via distinct pathological mechanisms.     

Major locus on mouse chromosome 17 and minor locus on chromosome 9 are linked with alopecia areata in C3H/HeJ mice

Alopecia areata is an autoimmune disease that targets actively growing (anagen) hair follicles in humans, mice, rats, dogs, horses, and cattle. C3H/HeJ mice spontaneously develop alopecia areata from 5 mo of age and older in females and later in males. Frequency of disease approached 20% in a colony by 18 mo of age. C57BL/6J mice do not develop alopecia areata. A segregating F2 population of female mice (n=1096) was generated from crossing these two strains. Alopecia areata (n=138) and clinically normal (n=214) mice were genotyped at 12 mo of age using 211 microsatellite probes. The peak logarithm of odds ratio score on mouse chromosome 17 (10.9) was around marker D17Mit134 at 16.9 cM from the centromere. The mouse histocompatibility locus, H2, the mouse equivalent of human leukocyte antigen in humans, was a likely candidate. Twelve-month-old C3H.SW-H2b/SnJ mice (C3H/HeJ congenic mice in which the H2k purported susceptibility locus was replaced with the H2b purported resistance locus) did not develop alopecia areata, supporting this locus as being important in alopecia areata. A suggestive linkage was also found on mouse Chromosome 9 (logarithm of odds ratio score 2.0) around D9Mit206, 20 cM from the centromere. The interval on mouse Chromosome 17 contains several orthologous genes potentially associated with human alopecia areata.     

Gene array profiling and immunomodulation studies define a cell-mediated immune response underlying the pathogenesis of alopecia areata in a mouse model and humans

Alopecia areata is a suspected autoimmune hair loss disease. In a rodent model, alopecia areata can be induced in normal haired C3H/HeJ mice by transfer of skin grafts from mice with spontaneous alopecia areata. At weeks 2, 4, 6, and 10 after surgery, grafted mice were euthanized, skin collected and processed for histology, and RNA extracted. Age-matched sham-grafted mice, and mice with and without spontaneous alopecia areata, were similarly processed. For comparison, skin biopsies from alopecia areata and androgenetic alopecia affected humans were also collected. Skin mRNA processed to cDNA was analyzed using Affymetrix mouse 11K and human 6800 gene chip(R) array technology. Microarray results indicated 42 known genes upregulated or downregulated during onset of mouse alopecia areata consistent with an inflammatory cell-mediated disease pathogenesis involving antigen presentation, costimulation, and a T helper 1 lymphocyte response. In contrast, 114 genes, many regulating immunoglobulin response, were altered late in disease development. In alopecia areata affected humans, 95 genes were significantly modulated. As confirmation of microarray analysis results, lymph node and spleen cells from alopecia areata affected mice injected into normal haired littermates transferred the alopecia areata phenotype. Alopecia areata onset could be inhibited in skin-grafted mice by modulation with B7.1- and B7.2-specific monoclonal antibodies. In addition, depletion of CD4+ CD8+ expressing cells in chronic alopecia areata affected mice using monoclonal antibodies permitted hair regrowth. The results consistently demonstrated the importance of an immune cell-mediated disease mechanism in alopecia areata pathogenesis and suggested targeting antigen-presenting cells and reactive lymphocytes may be effective in alopecia areata treatment.     

Treatment of alopecia areata: “What is new on the horizon?”

New drug treatment opportunities based on the results of a genome-wide association study, which implicate T cell and natural killer (NK)-cell activation pathways, are leading to new approaches in future clinical trials of alopecia areata. Special attention is being given to the UL 16-binding protein (ULBP3) gene cluster on chromosome 6q25, as these genes make the NKG2D-activating ligand or signal that can trigger the NKG2D receptor, initiating an autoimmune response. A greater expression of ULBP3 has also been found in hair follicles in scalp biopsy specimens from patients with active disease. It is now postulated that the characteristic T cell "swarm of bees" infiltrate seen in alopecia areata is the result of T cells being attracted to the hair follicle by NKG2D-activating ligands. Future treatment approaches for alopecia areata include use of drugs that: (i) block the NKGD-activating ligand and NKG2D receptor interaction, (ii) halt activated T cells, or (iii) modification of the inflammatory cytokine network. Many drugs currently being used or being evaluated for other autoimmune diseases that work through these mechanisms might prove to be very effective in alopecia areata.     

Normalisation of hair follicle morphology in C3H/HeJ alopecia areata mice after treatment with squaric acid dibutylester

Alopecia areata is a non-scarring, reversible disorder, presumably caused by an autoimmune attack on anagen hair follicles. Treatments are numerous, and most of these are ineffective. However, the elicitation of contact dermatitis on the affected skin is commonly associated with hair regrowth. A major advance in the study of alopecia areata has been the introduction and characterisation of the C3H/HeJ mouse model that exhibits many features of the human disease. In this study we examined the effects of squaric acid dibutylester treatment on hair follicles and the associated leukocyte infiltrate in alopecia areata mice by light and transmission electron microscopic analysis. This was compared with unaffected normal mice and alopecic untreated mice. Experimental mice were treated unilaterally with the contact allergen squaric acid dibutylester and the skin was assessed after hair regrowth. The characteristic pathological picture of alopecia areata was observed in alopecic but not normal mice. Nine of eleven experimental mice regrew hair on the treated side only and this was associated with a reduction in peri/intrafollicular inflammatory cell infiltrates, hair follicle dystrophy, melanin incontinence/clumping, and an increase in the numbers of hair follicles in full anagen. This normalisation of hair follicle status after treatment reflects the successful reversal of disease in these mice. The mechanism of action of topical immunotherapy with a potent contact allergen such as squaric acid dibutylester still needs to be elucidated, but an altered immune milieu is suspected. This study further validates the C3H/HeJ mouse model of alopecia areata in the search for therapeutic interventions in this common hair follicle disorder.     

Resistance to alopecia areata in C3H/HeJ mice is associated with increased expression of regulatory cytokines and a failure to recruit CD4+ and CD8+ cells

Grafting alopecia areata affected C3H/HeJ mouse skin to littermates induces alopecia areata, but high dietary soy oil reduces alopecia areata susceptibility. Alopecia areata affected and resistant mice were characterized to evaluate possible mechanisms involved in alopecia areata resistance. Of 44 mice that received alopecia areata affected skin grafts but failed to develop alopecia areata, only two of 22 receiving further alopecia areata affected skin grafts developed alopecia areata, whereas 39 of 44 controls developed alopecia areata. Alopecia areata affected skin contained increased numbers of CD4+ and CD8+ cells, increases in pro inflammatory T helper 1 and T helper 2 type cytokines, and upregulation of CD28, CD40L, and their ligands. In draining lymph nodes, a relatively high number of antigen-presenting cells was recovered, whereas several CD44v variants were downregulated. In contrast, alopecia areata resistant mouse skin did not display increased numbers of CD4+ and CD8+ cells, whereas counter-regulatory cytokines interleukins 4 and 10 were upregulated. High expression of CD28, CD80, CD86, CD40, CTLA4, CD44v variants, and FasL occurred in alopecia areata resistant mouse spleens. In vitro, lymph node cells of susceptible and resistant mice responded equally to a mitogenic stimulus, but only lymph node cells from alopecia areata affected mice displayed an increased response with T cell receptor stimulation via anti-CD3 cross-linking. These results suggest alopecia areata is a cell-mediated autoimmune disease, but alopecia areata affected skin graft hosts may resist alopecia areata onset through active counter-regulatory mechanisms. Because alopecia areata resistant mice showed unimpaired responsiveness and a transient inflammatory response towards the graft, it is suggested that alopecia areata develops as a consequence of an inappropriate immune response regulation.     

5‐Bromo‐2′‐deoxyuridine induced effluvium via p53‐mediated CD326‐positive keratinocyte apoptosis in C57BL/6 mice

Anagen effluvium develops because of disturbances in the hair follicle cycle, leading to acute and severe hair loss in humans. The objective of this study was to establish a mouse model of anagen effluvium by 5‐bromo‐2′‐deoxyuridine (BrdU) treatment, and evaluate the pathological changes and underlying mechanisms. We treated 9–10‐day‐old pups and 3–7‐week‐old C57BL/6 mice with BrdU. After successfully inducing hair loss in the neonatal pups, microscopic, immunohistochemical and flow cytometry analyses were conducted. BrdU induced early onset alopecia in neonates and caused epidermal thickening and hair shaft breakage. BrdU appeared to incorporate the CD326‐positive keratinocyte layer and induced p53‐related apoptosis. Keratinocyte apoptosis caused immune cell infiltration in the dermal region; M2 macrophages and neutrophils were dominant. The BrdU‐induced hair loss was dose‐dependent, and alopecia was visible at a dose range of 25–200 μg/g bodyweight. The BrdU‐induced anagen effluvium mouse model is novel and easily established by administrating four simple BrdU injections to pups; these mice showed synchronized onset of alopecia symptoms with little individual variation. Moreover, this model showed an alopecia phenotype similar to that of human anagen effluvium with acute, severe and widespread hair loss.     

Fas-deficient C3.MRL-Tnfrsf6(lpr) mice and Fas ligand-deficient C3H/HeJ-Tnfsf6(gld) mice are relatively resistant to the induction of alopecia areata by grafting of alopecia areata-affected skin from C3H/HeJ mice

Alopecia areata is suspected to be a T cell-mediated autoimmune disease of the hair follicle, where Fas is expressed on hair follicles and Fas ligand on perifollicular infiltrates. To elucidate whether the Fas/Fas ligand pathway is of pathogenetic significance in alopecia areata, we investigated whether alopecia areata can be induced in Fas-deficient and Fas ligand-deficient mice and whether alopecia areata develops in Fas-deficient and Fas ligand-deficient skin. Therefore, we induced alopecia areata by grafting alopecia areata-affected C3H/HeJ mouse skin on to C3H/HeJ mice (control), on to Fas ligand-deficient C3H/HeJ-Tnfsf6(gld) mice or Fas-deficient C3.MRL-Tnfrsf6(lpr) mice. All control mice developed alopecia areata, whereas no Fas-deficient mice showed hair loss and two of seven Fas ligand-deficient mice developed only transitory, limited alopecia areata. Moreover, skin from C3H/HeJ mice (control), C3H/HeJ-Tnfsf6(gld) mice, and C3.MRL-Tnfrsf6(lpr) mice was grafted on to C3H/HeJ mice with extensive alopecia areata. Skin grafts from control mice developed hair loss, whereas Fas-deficient and Fas ligand-deficient skin grafts were spared from alopecia areata. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling and immunofluorescence studies revealed an increased number of apoptotic cells and expression of Fas on hair follicles as well as expression of Fas ligand on cells of the perifollicular infiltrate in C3H/HeJ mice with alopecia areata, whereas in Fas-deficient and Fas ligand-deficient mice apoptotic cells were virtually absent in hair follicles. The results suggest that the Fas/Fas ligand pathway plays an important pathogenetic role in alopecia areata.     

Die Excimer‐Laser‐Therapie der Alopecia areata – Halbseitige Evaluation eines repräsentativen Areals

We report for the first time on hair regrowth in alopecia areata of the scalp achieved with the 308‐nm xenon‐chloride excimer laser in a prospective side‐by‐side trial. The alopecia areata had shown progression over a period of three years, and various treatments had not been effective. Out of a number of affected areas, one representative lesion was chosen; one half of it was treated, the other half remained untreated. After 27 sessions (200 – 4000 mJ/cm², cumulative dose 52.6 J/cm²) over 3 months, only the treated area showed hair growth; which suggests that this was most probably not a spontaneous remission.     

Therapy of alopecia areata: on the cusp and in the future

Over the past decade, basic research has established alopecia areata as a T cell-mediated autoimmune disease and has clarified many of its genetic, cellular, and molecular aspects. Perifollicular and intrafollicular mononuclear cell infiltrates directed at anagen hair bulbs are characteristic and striking histologic features in early alopecia areata. The inflammatory infiltrate is composed predominantly of activated CD4+ and CD8+ T cells, together with macrophages and Langerhans cells. The initiation phase of alopecia areata is mediated by type 1 cytokines, including interleukin-2, interferon-gamma, and tumor necrosis factor-alpha. Like other diseases with a strong autoimmune component, alopecia areata has associated with it specific human leukocyte antigens, which determine susceptibility, severity, chronicity, and resistance. New topical immunomodulating drugs and biologic therapies that have been developed, or that are in development, for the treatment of other immune-mediated inflammatory skin diseases will likely be effective in alopecia areata as well. The present discussion addresses the treatment of alopecia areata within the framework of these new modalities.     

Ocular findings in alopecia areata

Alopecia areata is a T cell mediated disease with which many disorders may be associated. There are few studies reporting ocular findings in alopecia areata. The aim of the study is to assess tear function and ocular surface pathologies in alopecia areata. Thirty‐two patients with alopecia areata and 20 age‐ and sex‐matched healthy controls were enrolled in the study. Ocular surface disease index questionnaire, Schirmer, tear break‐up time, and corneal staining stage tests were done. The data was analyzed using SPSS 10.0 software. One‐way variance analysis and Chi‐square tests were used as tests of significance. The patient group had significantly higher ocular surface disease index questionnaire and corneal staining stage test scores and lower tear break‐up time test scores compared with the control group (P < 0.05). Dry eye disease (DED) was diagnosed in 27 (84%) of 32 alopecia areata patients and in only 3 (15%) of 20 controls, and there was a significant difference between the groups (P < 0.01). T cell mediated autoimmunity has a prominent role in the etiopathogenesis of alopecia areata and dry eye disease. We think that inflammatory mechanisms causing alopecia areata may trigger dry eye disease or vice versa. All patients with AA should be referred to an ophthalmologist for the evaluation of DED and other possible eye pathologies.     

Treatment with an anti-CD44v10-specific antibody inhibits the onset of alopecia areata in C3H/HeJ mice

A murine CD44v10-neutralizing antibody has been reported to impair delayed-type hypersensitivity reactions. Because alopecia areata is characterized by a delayed-type hypersensitivity-like T cell mediated immune response, we addressed the question whether an anti-CD44v10-antibody influences the onset of alopecia areata. Therefore, we used the C3H/HeJ mouse model with the induction of alopecia areata in unaffected mice by the grafting of lesional alopecia areata mouse skin. Six grafted mice were injected (intraperitoneally) with anti-CD44v10, six grafted mice with anti-CD44standard, and six with phosphate-buffered saline only. After 11 wk phosphate-buffered saline injected animals on average had developed alopecia areata on 36.8% of their body. The onset of hair loss was slightly delayed and its extent reduced to 17.2% of their body in anti-CD44standard-treated mice. By contrast, five of six anti-CD44v10-treated mice did not show any hair loss and one mouse developed alopecia areata on only 1% of the body. Immunohistochemical examination revealed a marked reduction of perifollicular CD8+ lymphocytes and, to a lesser degree, CD4+ cells as well as a decreased expression of major histocompatibility complex class I on hair follicle epithelium in anti-CD44v10-treated mice as compared with phosphate-buffered saline or anti-CD44 standard-treated mice. Our data show that anti-CD44v10 is able to inhibit the onset of alopecia areata in C3H/HeJ mice. This might be accomplished by an anti-CD44v10-triggered impairment of immune cell homing (e.g., CD8+ T cells), resulting in a decrease of their number in target tissues.     

When your patient's parent asks: “My child's alopecia areata is not getting better. Should he or she get tested for celiac disease?”

The relationship between alopecia areata and celiac disease has been suspected for more than 20 years. For a particular individual with alopecia areata, the risk of coexisting celiac disease and the potential benefit of a gluten‐free diet on hair loss or regrowth is known. Despite this limited information, patients often aske dermatologists whether they should undergo celiac disease testing or begin a gluten‐free diet. We reviewed all available studies of the relationship between alopecia areata and celiac disease to provide guidance to practicing clinicians.     

Recurrence of alopecia areata in a patient receiving etanercept injections

BACKGROUND: Alopecia areata is a common condition of patchy hair loss that has been postulated to have an autoimmune pathogenesis involving inflammatory cytokines, including tumor necrosis factor (TNF) alpha. Etanercept is a novel medication that blocks TNF-alpha-mediated processes. We report a case involving the recurrence of alopecia areata in a patient receiving etanercept. OBSERVATIONS: We describe a 49-year-old man with a history of rheumatoid arthritis and alopecia areata who developed a recurrence of his alopecia areata while being treated with etanercept for more than 2 years. CONCLUSIONS: The anti-TNF-alpha effect of etanercept therapy may not be sufficient to prevent the recurrence of alopecia areata. The possible role of TNF-alpha in the pathogenesis of alopecia areata may be called into question if our observation is repeated.     

Alopecia in IL‐10‐deficient mouse pups is c‐kit‐dependent and can be triggered by iron deficiency

Please cite this paper as: Alopecia in IL‐10‐deficient mouse pups is c‐kit‐dependent and can be triggered by iron deficiency. Experimental Dermatology 2010; 19: 518–526. Abstract: Hair loss (alopecia) can result from a variety of metabolic, endocrine, immunologic, and environmental causes. This investigation was undertaken to determine the mechanisms underlying the sporadic development of alopecia in litters from C57BL/6 interleukin‐10‐deficient (Il10^?/?) mice. All pups in affected litters demonstrated alopecia by postnatal days 17–19, with hair loss from their trunks but not from their head, base of tail, or feet. Histopathology revealed distorted hair follicles containing broken hair shafts and prominent dermal infiltrates containing increased numbers of activated mast cells. Hair re‐growth began soon after weaning, suggesting that the alopecia was triggered by factors transmitted during lactation. Milk from Il10^?/? dams induced macrophage secretion of pro‐inflammatory cytokines in vitro regardless of whether or not their pups developed alopecia. Feeding dams a diet containing 3–6 ppm iron increased the percentage of litters with alopecia to 100% for pups with mast cells, with 0% alopecia in mast cell‐deficient pups. When dams were fed a diet containing 131 ppm iron, significantly lower haemoglobin and hematocrit values were observed in pups from litters with alopecia (71%; 5 of 7 litters) compared to litters without alopecia. Genetic or pharmacologic inhibition of c‐kit that resulted in depletion of mast cells in pups prevented hair loss in at‐risk litters. These studies demonstrate that maternal iron‐restricted diets enhance the incidence of alopecia in IL‐10‐deficient mouse pups and suggest mast cells as potential effector cells. Further studies are indicated to further explore the mechanisms involved and to determine how mast cells may contribute to alopecia in humans.     

Transient CD44 variant isoform expression and reduction in CD4(+)/CD25(+) regulatory T cells in C3H/HeJ mice with alopecia areata

Alopecia areata, an autoimmune disease affecting anagen stage hair follicles, can be induced by grafting spontaneous alopecia areata affected skin to normal-haired C3H/HeJ mice. As the onset of alopecia areata can be significantly retarded by anti-CD44 variant isoform 10 treatment, it was interesting to explore the underlying disease mechanism. Two weeks after transplanting alopecia areata affected skin, expression of CD44 variant isoforms 3, 6, 7, and 10 was strikingly upregulated as compared with sham-grafted mice. By 6 wk after grafting, CD44 variant isoform levels had returned to normal, whereas in draining lymph nodes, CD44 variant isoform expression was slightly decreased. Leukocytes in the skin of mice with chronic alopecia areata expressed a hematopoietic isoform of CD44 and CD44 variant isoform 6 at an elevated level, but CD44 variant isoform 3 expression was reduced. Cytokine expression in leukocytes of chronic alopecia areata affected skin was higher than in normal-haired controls. Cytokine expression also increased postsurgery in sham and alopecia areata grafted mice, but remained elevated only in mice receiving alopecia areata affected skin. Finally, from the skin of mice with chronic alopecia areata and of mice transplanted with alopecia areata affected skin, an increased number of CD4(+) and CD8(+) cells, but a strongly decreased number of CD4(+)/CD25(+) regulatory T cells was recovered. Thus, expression of CD44 variant isoforms is important for the migration of leukocytes during the initial period of alopecia areata. CD44, however, is apparently not involved in the maintenance of the disease state, which is characterized by high cytokine expression levels, an increased number of CD4(+) and CD8+ cells, but a low level of CD4(+)/CD25(+) suppressor cells.     

Alopecia areata as another immune‐mediated disease developed in patients treated with tumour necrosis factor‐α blocker agents

Background Tumour necrosis factor antagonists (anti‐TNF‐α) have demonstrated the efficacy in different chronic immune inflammatory disorders. Within the spectrum of adverse events, autoimmune diseases have been observed, including cases of alopecia areata (AA). Objectives The objective of the study is to characterize AA developed during anti‐TNF‐α therapy. Methods We present five new cases and review all the cases reported in the literature (eleven). Results One third of the cases had a positive (personal or family) history of AA. Most of them presented with rapid extensive AA, usually involving the ophiasis area. Prognosis was usually poor, with slight response to treatments. In the cases where anti‐TNF‐α therapy was maintained, the course did not seem to change. Conclusions Although rare, AA developed during anti‐TNF‐α therapy might be more frequent than suggested by reports of isolated cases. Personal and family history of autoimmune disease might alert clinicians to their possible development or relapse once the anti‐TNF‐α therapy is started.     

A natural canine homologue of alopecia areata in humans

Background Alopecia areata (AA) is suspected to be an autoimmune disease directed preferentially against hair follicles (HF) affecting both humans and various mammalian species. Recently, two rodent models of AA were described, namely the ageing C3H/HeJ mouse and the DEBR rat. Despite several case reports of canine AA in the literature, there has been no systematic assessment of the disease in these companion animals, and it is also not known whether dogs with AA could be useful as an outbred homologue of this disease in humans. Objectives To evaluate the clinical, histopathological and immunopathological features of 25 dogs with AA and compare these data with those found in the human disease. Patients/methods Twenty‐five client‐owned dogs exhibiting macroscopic alopecia with peri‐ or intrabulbar lymphocytic infiltrates were selected for study. Biopsies and sera were obtained and assessed by histopathology, direct immunofluorescence of immunoreactant deposition, immunohistochemistry for lymphocyte markers, indirect immunofluorescence and immunoblotting analysis of circulating serum IgG, selective immunoprecipitation of HF proteins by serum IgG, and passive transfer of purified canine IgG into naïve C57BL/10 mice. Results Clinical signs including alopecia, skin hyperpigmentation and leucotrichia usually developed during adulthood and were first seen on the face, followed by the forehead, ears and legs. Spontaneous remission of alopecia occurred in 60% of dogs and regrowing hair shafts were often non‐pigmented. Histological examination of skin biopsy specimens revealed peri‐ and intrabulbar mononuclear cell infiltrates affecting almost exclusively anagen HF. Direct immunofluorescence analysis detected HF‐specific IgG in 73% of dogs, while indirect immunofluorescence revealed circulating IgG autoantibodies to the HF inner and outer root sheaths, matrix and precortex. Immunoblotting analysis revealed IgG reactivity to proteins in the 45–60 kDa molecular weight range and with a 200–220 kDa doublet. The latter was identified as trichohyalin by selective immunoprecipitation. Purified HF‐reactive IgG, pooled from AA‐affected dogs, was injected intradermally to the anagen skin of naïve mice where it was associated with the local retention of HFs in an extended telogen phase in AA‐treated skin compared with that seen in controls. Conclusions These findings are very similar to those reported for human AA patients; therefore, they support the consideration of dogs with AA as a useful homologue for the study of the pathogenesis of this common autoimmune disease of humans.     

Autoimmunity: alopecia areata

Strong direct and indirect evidence supports an autoimmune etiology for alopecia areata. T lymphocytes that have been shown to be oligoclonal and autoreactive are predominantly present in the peribulbar inflammatory infiltrate. Alopecia areata frequently occurs in association with other autoimmune diseases, such as thyroiditis and vitiligo, and autoantibodies to follicular components have been detected. Finally, the use of immune modulating drugs, including corticosteroids and contact sensitizers such as dyphencyprone, can be beneficial in the management of this disease. Recent studies have demonstrated that alopecia areata scalp skin grafted onto nude mice with severe combined immunodeficiency grow hair and that infiltrating lymphocytes in the graft are lost. It is now also possible to induce alopecia areata in human scalp explants on these mice by injecting T lymphocytes with scalp homogenate. Neuropeptides produced by cutaneous nerves are known to modify immune reactivity and, in all likelihood, affect the alopecia areata process. Future studies may show that modulation of neuropeptide expression is associated with hair regrowth. Likewise, testing the efficacy of the newly developed immunomodulatory agents in patients with alopecia areata may lead to the introduction of novel therapies for this immune-mediated disease of the hair follicle.