Alopecia areata (AA) is regarded as a nonscarring, in-

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1 The Pathogenesis of Alopecia Areata in Rodent Models REVIEW ARTICLE Kevin J. McElwee, Pia Freyschmidt-Paul, John P. Sundberg n and Rolf Ho mann Department of Dermatology, Philipp University, Marburg, Germany. n The Jackson Laboratory, Bar Harbor, Maine, USA Rodent models of human disease provide an important tool in the investigation of genetic and environmental activation factors, disease pathogenesis, and the development of new and improved treatments. Up to 20% of aged C3H/HeJ mice and 70% of Dundee Experimental Bald Rats (DEBR) develop alopecia areata (AA), a nonscarring, in ammatory hair loss disease with a suspected autoimmune pathogenesis. These rodent models are currently employed in determining the genetic basis of AA, understanding the mechanisms of disease initiation and progression, and de ning potential endogenous and environmental in uences. Induction of AA by skin graft transfer between a ected and una ected mice has been employed to examine skin and immune system changes during AA pathogenesis. Manipulation of in ammatory cells in vivo indicates AA is primarily a cell mediated disease with auto-antibody production as a secondary event. Whether the AA activating factors are exogenous or endogenous antigens, or involve normal or aberrant epitope expression remains to be elucidated. However, current research suggests a self contained disease cycle involving four key events: (1) Failure of the putative anagen stage hair follicle immune privilege and exposure of hair follicle located AA inciting epitopes to the immune system; (2) Antigen presentation, costimulation, and activation of responsive lymphocytes by antigen presenting cells; (3) Activated in ammatory cell migration to, and in ltration of, hair follicles; (4) The subsequent disruptive actions of the in ammatory cell in ltrate on the hair follicles. Each of these events is vulnerable to therapeutic intervention, and rodent models will be fundamentally involved in developing new treatments for AA. Key words: hair follicle/hair disease/in ammation/autoimmunity. JID Symposium Proceedings 8:6 ^11, 2003 Alopecia areata (AA) is regarded as a nonscarring, in- ammatory disease of the hair follicle.while there is no obvious clinically visible in ammation in most AA patients, histopathological examination reveals anagen stage hair follicles to be a ected by a periand intrafollicular in ammatory cell in ltrate of primarily CD4 þ and CD8 þ cells (Perret et al, 1984; Ranki et al,1984).in association with active disease, there is increased expression of class I and II MHC antigens along with increased numbers of antigen presenting cells (macrophages, Langerhans cells) in AA lesions (Wiesner-Menzel and Happle, 1984). Circulating high titer IgG isotype autoantibodies capable of binding hair follicle speci- c antigens have been identi ed in AA a ected individuals and deposits of immunoglobulin and complement around hair follicles, particularly at the margin of active lesions, have been observed (Bystryn et al, 1979; Tobin et al, 1994; Tobin et al, 1997a). E ective therapies for AA involve modulation or suppression of immune cells in the skin (Freyschmidt-Paul et al, 2001). These and other circumstantial and indirect evidence suggest that AA development may involve autoimmune disease mechanisms and targeting of as yet unidenti ed antigens within anagen stage hair follicles (McElwee et al, 1999a). Accepted for publication February 1, 2003 Reprint requests to: Kevin J. McElwee, Philipp University, Department of Dermatology, Deutschhausstrasse 9, Marburg, Germany; kevin@keratin.com Abbreviations: AA, alopecia areata; APC, antigen presenting cell; DEBR, Dundee experimental bald rat; HLA, human leukocyte antigen; MHC, major histocompatibility complex; MoAbs, monoclonal antibodies; SCID, severe combined immunode ciency. Human AA is a complex disease where clinical presentation varies considerably between individuals. In part, this re ects the mixed genetic background of the patients and the dirty environment in which people live involving random exposure to pathogens, toxic compounds, various diets, and more. Inbred rodents living in a controlled environment provide useful models to explore speci c aspects of disease and to perform clean, controlled experiments that cannot be conducted with humans. Information gained from such models helps de ne how disease develops and progresses, and most importantly provides clues as to how they can be treated better. Due to the wide heterogeneous clinical presentation and also probable variations in the pathogenic mechanisms of development in subsets of AA-a ected humans, information derived from inbred rodent models may not be applicable to all instances of human AA. However, an accurate rodent model should provide information relevant to the majority of cases (Fig 1). AA IN RODENT MODELS In addition to isolated case reports of AA in outbred mammals, several inbred mouse strains and substrains have been identi ed with AA-like hair loss of variable expression frequency (McElwee et al, 1998a; McElwee et al, 1999b). However, C3H/HeJ mouse AA is the most extensively characterized and most commonly utilized model. AA-like hair loss was rst observed in a C3H/ HeJ mouse colony with a frequency of 0.25% for females and 0.035% for males, at ve months of age (Sundberg et al, 1994a; Sundberg et al, 1994b). By 18 months, the frequency of AA increased to 20%. Typically rst onset of AA develops from four X/03/$ Copyright r 2003 by The Society for Investigative Dermatology, Inc. 6

2 VOL. 8, NO. 1 JUNE 2003 ALOPECIA AREATA IN RODENT MODELS 7 present in signi cantly elevated concentrations in a ected DEBR and C3H/HeJ mice. As with humans, hair follicle speci c autoantibodies are also occasionally present in rats and mice without hair loss (McElwee et al, 1996a; McElwee et al, 1996b; Tobin et al, 1997a; Tobin et al, 1997b). DISEASE MODELS FOR ALOPECIA AREATA Figure 1. Relevance of rodent models to human alopecia areata. Human alopecia areata involves considerable variation in phenotype and probably also involves genotype heterogeneity within a restricted range. An inbred rodent model may not represent all human phenotype or genotype presentations, but will be relevant to a subset of alopecia areata-affected individuals. Where a rodent model and human phenotype and genotype coincides, each rodent model represents a subset of the human population. Each rodent model may represent unique or overlapping subsets. months of age in females compared to six or more months in males. The lesion is often rst expressed as a large, uniform thinning of hair or as multi focal sites often with symmetrical distribution on the ventrum later progressing to the dorsum. The Dundee experimental bald rat (DEBR) is derived from a spontaneous alopecia trait rst observed in BD-IX inbred rats (Michie et al, 1991). To improve the poor fecundity of these alopecic animals, the BD-IX rats were crossed with Wistar rats and the DEBR descendants of this cross were derived from full sib-matings. Two separate sublines, one brown and one black hooded, have been developed subsequent to the Wistar cross, each isolated and now inbred to generation F 38 þ 0. Each subline has di erent genetic haplotypes, but common disease expression. DEBR rats develop a full coat of hair within two weeks of birth and hair growth continues until the rats are approximately 5 months old. First onset of AA occurs from 5 to 8 months of age with female rats twice as likely as males to express the lesion and more likely to develop extensive AA. Up to 42% of individuals in a DEBR rat colony (70% female) can express a lesion that may persist as a slight thinning of the pelage or extensive patches of hair loss. Near total loss of hair is attained in 15% of a ected rats, almost exclusively female. Typically, alopecia on the head spreads backwards around the eyes to merge with alopecia on the ank and reaches a chronic stable phase at 10^14 months of age. Approximately 25% of lesional DEBR rats have nail deformities with the frequency of expression equally distributed between males and females. The nails typically present as exceptionally long and twisted (onchogryphosis) with minor irregularities on the nail surface. There is no apparent correlation between hair loss extent and nail deformities. Histologically, both AA-a ected mouse and rat skin initially exhibit increased anagen stage hair follicle activity as the lesions aggressively develop and expand. Later, in the chronic, stable disease phase, hair follicles are observed predominantly in a telogen state. Focal peri- and intrabulbar mononuclear cell in ltrates are associated with only anagen stage hair follicles; telogen hair follicles are una ected. The in ltrates consist primarily of CD4 þ, CD8 þ cells and macrophages (Michie et al, 1990; Sundberg et al, 1994a; Sundberg et al, 1994b; Zhang and Oliver, 1994). The perifollicular in ltrate may be located from the region of the bulbar follicular epithelium up to the level of the sebaceous glands. Pigmentary incontinence and follicular dyskeratosis are particularly prominent in rodents with lesions of prolonged duration. Research has revealed hair follicle speci c autoantibodies to be Previously, it has been hypothesized that AA may be modeled on one of three basic mechanisms. (1) AA is the result of infection localized to the hair follicle activating an immune response against the infectious agent with secondary disruption of hair follicle growth (Skinner et al, 1995a; Skinner et al, 1995b); (2) A fundamental defect in the immune system leads to failure of tolerance towards hair follicle speci c antigens and subsequent development of a classic autoimmune disease scenario (Mitchell and Krull, 1984); (3) A fundamental defect in hair follicle function exists in patients with in ammation being a secondary event (Goldsmith, 1991; Nutbrown et al,1996). In each of the above models the AA inciting antigen(s) targeted by the immune system are apparently located within hair follicles and must be exposed to immuno-surveillance for onset of AA to occur. Whether the antigenic target within hair follicles is endogenous or exogenous has yet to be de ned. Exogenous antigen activation of AA such as hair follicle localization of cytomegalovirus has been suggested (Skinner et al, 1995a; Skinner et al, 1995b). However, mouse CMV and other candidate infectious agent contamination of rodent model skin could not be con rmed (McElwee et al, 1998b). There is no circumstantial evidence of direct infectious agent involvement, as AA is not transferred between cohabiting rodent breeding pairs and littermates even after more than a year of continuous contact (personal observations). AA could not be described as an autoimmune disease if hair follicle localized exogenous antigens were identi ed as the direct activator of AA (McElwee et al, 1999a). Equally, an endogenous hair follicle expressed target antigen does not preclude the involvement of environmental antigens in AA onset. Theoretically, exogenous antigens may activate AA by mimicry of endogenous epitopes eliciting an autoimmune reaction. In addition, general viral and vaccine load may be a susceptibility or severity modi er of AA. Transient in ammation of AA a ected hair follicles is readily apparent in rodents, restricted to anagen stage hair follicles. The inciting factor for hair follicle in ammation may be similarly transiently expressed. In ammation could potentially be a purely secondary phenomenon in response to a defect of anagen stage hair follicles or a transient infectious agent that directly promotes AA. However, whether the inciting hair follicle antigen is truly endogenous or exogenous, and AA is truly autoimmune in nature, there is research evidence from rodent models indicating in- ammation is the primary promoter and maintainer of hair loss during development and chronic, persistent phases of AA: (1) Immunosuppressive agents have been used successfully in AA a ected rodents to promote hair regrowth (Sainsbury et al, 1991; Oliver and Lowe, 1995; McElwee et al, 1997; Freyschmidt- Paul, in press). However, such drugs are nonspeci c in their action and may have a direct hair growth promoting action as well as immunosuppressive properties. This is the case for Cyclosporin A and Tacrolimus (Yamamoto et al, 1994;Jiang et al, 1995) and so the bene cial e ects of these agents on AA can only be taken as circumstantial evidence of an in ammatory, controlling mechanism for AA. (2) Of greater signi cance, CD4 þ and CD8 þ cell depletion using monoclonal antibodies in both AA a ected C3H/HeJ mice and DEBR permitted hair regrowth whereas isotype matched irrelevant MoAbs had no e ect on AA (McElwee et al, 1996c; McElwee et al, 1999c; Carroll et al, 2002). This suggests CD4 þ and CD8 þ cells are signi cantly involved in maintaining the AA lesions.

3 8 MCELWEE ET AL JID SYMPOSIUM PROCEEDINGS (3) In addition, inhibiting in ammatory cell migration using monoclonal antibodies (MoAbs) against CD44v10, commonly expressed on activated lymphocytes, blocks development of AA (Freyschmidt-Paul et al, 2000). (4) Inhibition of AA onset has also been demonstrated using MoAbs against costimulatory antigen B7 commonly expressed on antigen presenting cells (APCs) (Carroll et al, 2002). (5) Grafting AA a ected skin to histocompatible C3Smn. CB17-Prkdc scid /J (C3H SCID) mice induced neutrophil in- ammation of anagen stage hair follicles in graft and host skin, but in the absence of CD4 þ and CD8 þ cells hair regrowth was observed from grafted skin and no hair loss developed in host skin. (McElwee et al, 1998c). (6) AA can be transferred from AA a ected mice to una ected mice by injection of skin draining lymph node or spleen derived cells (Carroll et al, 2002). (7) Further con rmation of CD4 þ and CD8 þ cell importance in an in ammatory mediated mechanism of AA may come from a human ^ Prkdc scid /Prkdc scid mouse xenograft model. Human AA a ected skin grafted to Prkdc scid /Prkdc scid mice permits hair regrowth. AA can be re-induced in the skin graft after injection of in vitro activated lymphocytes (Gilhar and Krueger, 1987; Gilhar et al, 1998; Gilhar et al, 1999). However, whether the SCID model approach induces hair loss mimicking AA or whether the hair loss is a scarring alopecia remains to be elucidated. Rodent AA perpetuation is in ammatory cell mediated and targets anagen stage hair follicles, but the studies described above reveal little about the initial events in AA pathogenesis. HYPOTHESES FOR ALOPECIA AREATA PATHOGENESIS Several hypotheses for AA development have been suggested from focal infection and toxic substance exposure to re ex nerve irritation, emotional disturbance and endocrine hormone action (McElwee et al, 1998a). Recently, studies revealed gonadal hormones may play a role in susceptibility to AA onset in C3H/HeJ mice. While estrogen supplementation apparently accelerated the rate of AA progression, dihydrotestosterone supplementation entirely inhibited onset of AA (McElwe et al, 2001). However, while such epigenetic factors may play a signi cant role in disease susceptibility and severity, they are likely secondary disease modi ers to the primary disease mechanism. In light of current evidence, recent AA pathogenesis hypotheses have focused on in- ammatory cell mediation with autoimmune disease as an underlying principle. The most detailed hypothesis to date (Paus et al, 1993) suggested onset of AA may involve a two stage process whereby inciting antigens within the hair follicle are abnormally exposed by subclinical immune system activity and subsequent presentation of these antigens activates a second, much larger immune system response resulting in the overt, clinical hair loss disease. In part this hypothesis relies upon the putative transient immune privilege attributed to anagen stage hair follicles and its breakdown in AA (Christoph et al, 2000). Small skin grafts from rat AA lesions transferred to prelesional DEBR rats enabled regrowth of normal hair from initially dystrophic hair follicles. Reciprocal studies in both rat and mouse models resulted in normal haired skin grafts swiftly developing a dystrophic state when transferred to mature, lesional animals. AA a ected rat skin grafts to nude mice and AA a ected mouse skin grafts to C3H SCID mice enabled the dystrophic hair follicles in the grafts to regrow hair (Oliver, unpublished observations; McElwee et al, 1998c). This demonstrated the importance of an immune component in the perpetuation of hair loss and the importance of systemic immune in uence over the localized a ected hair follicles. The studies suggest that the AA-like lesions in rodents are a true, nonscarring form of alopecia that can potentially be reversed. Intriguingly, grafting skin from AA a ected mice to unaffected C3H/HeJ mice transfers or induces onset of AA typically 8^10 weeks after grafting (McElwee et al, 1998c). Prior to overt hair loss there is a gradual buildup of in ammation in skin, comparatively unfocused at rst, but later forming a peri- and intrafollicular distribution. This time delay between surgery and hair loss may be consistent with the Paus hypothesis (Paus et al,1993). Analysis of this time delay from skin grafting, as the disease activation event, to onset of hair loss should provide signi cant clues as to the mechanisms involved in AA onset. ALOPECIA AREATA PATHOGENESIS IN RODENT MODELS The anagen stage hair follicle has been suggested as a transient immune privileged site (Barker and Billingham, 1977; Westgate et al, 1991; Christoph et al, 2000). Normal anagen hair follicles express very low or no MHC class I or class II antigens, but during human and rodent AA expression signi cantly increases (Messenger and Bleechen, 1985; Br cker et al, 1987; Khoury et al, 1988). Increased MHC expression may result in presentation of hair follicle located antigens not normally exposed to the immune system. However, it has been indicated that MHC expression in hair follicles is a secondary phenomenon, developing subsequent to in ammatory cell in ltration. The hair follicle also incorporates a physical and biochemical barrier to immune surveillance, the basement, or glassy, membrane. For in ammatory cells to penetrate to intrafollicular positions in AA, these barriers must rst be breached. In the mouse model of AA, the putative immune privilege of hair follicles provides little resistance to activated in ammatory cells. Normal haired C3H/HeJ, histocompatible C3H SCID or C3H/OuJ skin grafted to AA a ected skin is rapidly in ltrated by in ammatory cells with almost immediate hair follicle dystrophy and inhibition of visible hair ber growth (McElwee et al, 1998c). This suggests that the exciting antigen targets for activated in ammatory cells are normally expressed in anagen hair follicles of C3H mouse strains. However, it is possible that C3H/HeJ mice and histocompatible substrains inherently express antigens that are not normally expressed in unrelated mouse strains. Equally, it is possible that C3H mouse strains have a defect in the hair follicles immune privilege resulting in a greater general susceptibility towards AA development. While in ammatory cells are apparently the controlling mediators of AA lesion development, the possibility of hair follicle defects cannot be ruled out. The commentary of Goldsmith (Goldsmith, 1991) may yet be applicable to AA with some modi cation. These and other evidence demonstrate rodent AA is an in ammatory mediated, organ speci c disease and are consistent with an autoimmune pathogenesis. Although the inciting agents within anagen hair follicles have yet to be de ned, a basic principle of disease development emerges from rodent model research (Fig 2). Both genetic and environmental factors can contribute to the general degree of AA susceptibility in any one individual. The combined e ects may potentially act on the hair follicle and/or on aspects of the immune system. As environmental factors change with time, gene activity may be modi ed through gene^ environment interaction and additively a ect an individual s susceptibility to AA onset. Once AA has developed, genetic and environment interaction may still in uence the rate of lesion progression, end stage extent of hair loss, lesion duration, and resistance to treatment. Onset of AA requires exposure of unknown inciting epitopes within hair follicles subsequently taken up by antigen presenting cells (APCs) and presented to lymphocytes in draining lymph nodes and possibly in other immune system organs such as the spleen (Fig 2). Activated in ammatory cells migrate from organs of the immune system and in ltrate the skin and anagen stage hair follicles. Their disruptive e ects, mediated in part through Fas ^ Fas ligand binding, result in hair follicle dystrophy and continued antigen exposure in association with MHC expression. This may expose numerous antigenic epitopes for uptake by

4 VOL. 8, NO. 1 JUNE 2003 ALOPECIA AREATA IN RODENT MODELS 9 Figure 2. Alopecia areata pathogenesis and perpetuation. Figure 3. Practical and theoretical therapeutic interventions in alopecia areata pathogenesis. APCs and further stimulation of autoreactive lymphocytes. With repeated cycles of hair follicle disruption, antigen exposure, and lymphocyte activation, epitope spreading may occur and a wider range of antigens and hair follicle structures may be targeted (Chan et al, 1998). This disease cycle is self-perpetuating and the initial disease activating agents need not persist for AA lesions to continue. Hypothetically, the activating antigenic epitopes may only be transiently expressed. PRACTICAL AND HYPOTHETICAL TREATMENT APPROACHES If AA pathogenesis is founded upon an in ammation mediated concept, then there are several possible approaches to new treatment development (Fig 3): (1) Drug treatments for AA are predominantly immunosuppressive or immunomodulatory in their e ect. Immunosuppressive agents restrict in ltration of the skin and hair follicles by activated in ammatory cells (Gupta et al, 1990; Oliver and Lowe, 1995; McElwee et al, 1997), while contact sensitizing agents alter the skin environment such that the action of in- ammatory cells on hair follicle growth is down-regulated (Ho mann et al, 1994; Ho mann and Happle, 1999; Freyschmidt- Paul et al, 1999). Remission of mouse and rat AA has been induced using several immunomodulatory compounds. Intralesional injection of triamcinolone acetonide, PUVA (methoxysporalen plus UV A light), and treatment with the topical sensitizers dinitrochlorobenzene (DNCB), diphenylcyclopropenone (DPCP), and squaric acid dibutyl ester (SADBE) have each induced hair growth in one or both models (Sundberg et al, 1994a; Sundberg et al, 1994b; Oliver and Lowe, 1995; Sundberg et al, 1995; McElwee et al, 1997; Freyschmidt-Paul et al, 1999; Shapiro et al, 1999; Gardner et al, 2000). These and other evidence demonstrate a good correlation between rodent models and human AA and indicate hair loss may similarly be due to in ammatory autoimmune mediated, hair follicle speci c mechanisms. (2) Minoxidil, and also in part cyclosporine and tacrolimus, have direct hair follicle growth promoting e ects (Yamamoto et al, 1994; Jiang et al, 1995). As such, an additional secondary AA therapeutic approach is to treat the symptom of hair loss using agents with direct action on the a ected hair follicles (Fiedler-Weiss, 1987; Price, 1987). However, the relatively nonspeci c action, potential for side-e ects, and limited response in many individuals to current treatments means new therapeutic approaches are required. In the short to mid term, new and improved drug treatments will become available, primarily derived from pharmaceutical research and developments in other autoimmune and in ammatory diseases. With a greater understanding of immunosuppressive agents, contact sensitizers and hair growth promoting drugs and their modes of action, more re ned and potent drugs may become available that are suitable for use in AA. (3) Theoretically, autoimmune or in ammatory disease mechanisms can be modulated in several ways. By extension of our understanding of in ammatory mechanisms and drug action upon them, speci c pathways may be targeted. Observations on contact sensitization treatment have led to speculation that future treatments could involve modifying the skin and hair follicle biochemical environment. Injection of interleukins or application of their respective cdna sequences may be used to block or modulate in ammatory cell in ltration of the skin and hair follicles. Alternatively, similar approaches may directly promote hair growth in spite of hair follicle in ammation. (4) Potentially, the most challenging therapeutic approach is to de ne lymphocyte clones reactive for hair follicle antigen epitopes and either block their production (clonal deletion) or promote tolerance (clonal anergy) (Theo lopoulos, 1995). Oral tolerance has been one suggested approach in autoimmune disease treatment; however, without knowledge of the antigenic targets involved in AA, such treatment approaches are not yet viable. (5) The blockade or modulation of antigen presentation and costimulation by antigen presenting cells to the responsive lymphocyte clones involved in AA may be a further focus of new treatment approaches.

5 10 MCELWEE ET AL JID SYMPOSIUM PROCEEDINGS (6) Once the lymphocytes are activated, it may still be possible to block cell migration from sites of activation to the skin and hair follicles. Preliminary investigations in which CD44v10 was targeted with MoAbs suggest cell migration inhibition may have potential AA treatment bene t (Freyschmit-Paul et al, 2000). Other activated cell surface markers and variants expressed during migration may be additional targets. These approaches may be employed as both treatment and preventative measures in AA susceptible individuals. (7) Once hair follicle in ammation is underway, several treatment approaches are still possible. A potentially complicated therapeutic intervention may involve blocking or modulating antigens expressed within anagen stage hair follicles, that are the apparent target for in ammatory cells, or masking their expression. In addition to modifying the in ammation inciting antigen expression, inhibition of MHC expression within hair follicles may also block AA lesion perpetuation. (8) Finally, the mechanisms by which the in ammatory cells adversely a ect hair follicle activity may be targeted, including disruption of Fas ^ Fas ligand interaction, prevention of granzyme and perforin action, oxygen radical neutralization, and alteration of the cytokine receptor and cytokine milieu. CONCLUSION Human AA is heterogeneous in its clinical presentation and also probably in the pathogenic mechanisms of development. Multiple factors may contribute to disease susceptibility, activation, progression, and perpetuation. In uence on disease may come from the environment and from the genetics of the a ected individual, and changes in the gene^environment interaction over time may modify the nature of the disease. Di erent factors may be important for individual cases; while genetics may be the primary determinant in one individual, the environment may be the major disease factor in another. It is possible that what we currently collectively describe as AA may be a common clinical symptom of several di erent mechanisms of disease pathogenesis. Consequently, rodent models may not be the equivalent of each and every human case, but may represent a subset of the human population. It is possible that di erent rodent models represent di erent human subgroups making comparative research of multiple models desirable. Where the mechanisms of human and rodent disease are similar, so information gained from rodent models will help de ne how AA is initiated and progresses and will provide clues as to how AA can be treated better. De ning what comes rst in the onset of spontaneous AA, hair follicle aberration or in ammatory cell in ltration may not be practical. However, research to understand the downstream cycle of disease pathogenesis after the activation event and disease perpetuation is highly practical when using rodent models.while the initiation of rodent model AA may involve a succession of events, the perpetuation of AA is likely to be a cycle of events and not a cascade. This is particularly relevant when the morphological target, the hair follicle, follows a cycle of proliferation and regression. Within the cycle there are four key events: (1) hair follicle located antigen exposure (be it exogenous, endogenous, normal or aberrant expression) to the immune system; (2) antigen presentation, costimulation, and activation of responsive lymphocytes by APCs; (3) activated in ammatory cell migration to and in ltration of hair follicles; and (4) the action of the in ammatory cell in ltrate on the hair follicles. Each of these events is vulnerable to therapeutic intervention. In addition, peripheral to the disease cycle mechanism, the reactive lymphocyte clones may be tolerized, deleted, or otherwise modulated. Therapeutic intervention in the short term will involve new and improved drug treatments to modulate hair follicle in ammation and to promote hair growth. In the mid term, aspects of the disease cycle may be targeted with cytokines, antigens, antibodies, and other factors. In the long term, treatment may target genes directly involved in the disease cycle mechanism or the contribution of general susceptibility and severity modifying genes. Rodent models will remain vital to understand AA and to develop such treatments. REFERENCES Barker CF, Billingham RE: Immunologically privileged sites. Adv Immunol 25:1^54, 1977 Br cker E-B, Echternacht-Happle K, Hamm H, Happle R: Abnormal expression of class I and class II major histocompatibility antigens in alopecia areata: Modulation by topical immunotherapy. JInvestDermatol88:564^568, 1987 Bystryn JC, Orentreich N, Stengel F: Direct immuno uorescence studies in alopecia areata and male pattern alopecia. J Invest Dermatol 73:317^320, 1979 Carroll JM, McElwee KJ, King LE, JrByrne M, Sundberg JP: Gene array pro ling and immunomodulation studies de ne a cell mediated immune response underlying the pathogenesis of alopecia areata in a mouse model and humans. J Invest Dermatol 119:392^402, 2002 Chan LS, Vanderlugt CJ, Hashimoto T, et al: Epitope spreading: Lessons from autoimmune skin diseases. JInvestDermatol110:103^109, 1998 Christoph T, Muller-Rover S, Audring H, et al: The human hair follicle immune system. cellular composition and immune privilege. Br J Dermatol 142:862^873, 2000 Fiedler-Weiss VC: Topical minoxidil solution (1% and 5%) in the treatment of alopecia areata. J Am Acad Dermatol 16:745^748, 1987 Freyschmidt-Paul P, Ho mann R, Levin E, Sundberg JP, Happle R, McElwee KJ: Current and potential agents for the treatment of alopecia areata. Curr Pharm Des 7:213^230, 2001 Freyschmidt-Paul P, Seiter S, Z ller M, et al: Treatment with an anti-cd44v10-speci c antibody inhibits the onset of alopecia areata in C3H/HeJ mice. JInvest Dermatol 115:653^657, 2000 Freyschmidt-Paul P, Sundberg JP, Happle R, McElwee KJ, Metz S, Boggess D, Ho - mann R: Successful treatment of alopecia, areata-like hair loss with the contact sensitizer squaric acid dibutylester (SADBE) in C3H/HeJ mice. JInvestDermatol 113:61^68, 1999 Gardner S, Freyschmidt-Paul P, Ho mann R, Sundberg JP, Happle R, Lindsey NJ, Tobin DJ: Normalisation of hair follicle morphology in C3H/HeJ alopecia areata mice after treatment with squaric acid dibutylester. Eur J Dermatol 10:443^450, 2000 Gilhar A, Krueger GG: Hair growth in scalp grafts from patients with alopecia areata and alopecia universalis grafted onto nude mice. Arch Dermatol 123:44^50, 1987 Gilhar A, Shalaginov R, Assy B, Sera movich S, Kalish RS: Alopecia areata is a T-lymphocyte mediated autoimmune disease: Lesional human T-lymphocytes transfer alopecia areata to human skin grafts on SCID mice. J Invest Dermatol Symp Proc 4:207^210, 1999 Gilhar A, Ullmann Y, Berkutzki T, Assy B, Kalish RS: Autoimmune hair loss (alopecia areata) transferred by T lymphocytes to human scalp explants on SCID mice. JClinInvest101:62^67, 1998 Goldsmith LA: Summary of alopecia areata research workshop and future research directions. JInvestDermatol96:98S^100S, 1991 Gupta AK, Ellis CN, Cooper KD, et al: Oral cyclosporine for the treatment of alopecia areata. A clinical and immunohistological analysis. JAmAcadDermatol 22:242^250, 1990 Ho mann R, Happle R: Alopecia areata Teil 2: Therapie. Hautarzt 50:310^315, 1999 Ho mann R, Wenzel E, Huth A, van der Steen P, Schaufele M, Henninger HP, Happle R: Cytokine mrna levels in Alopecia areata before and after treatment with the contact allergen diphenylcyclopropenone. JInvestDermatol 103:530^533, 1994 Jiang H,Yamamoto S, Kato R: Induction of anagen in telogen mouse skin by topical application of FK506, a potent immunosuppressant. J Invest Dermatol 104: 523^525, 1995 Khoury EL, Price VH, Greenspan JS: HLA-DR Expression by hair follicle keratinocytes in alopecia areata: Evidence that it is secondary to the lymphoid in ltration. JInvestDermatol90:193^200, 1988 McElwee KJ, Boggess D, Burgett B, Bates R, Bedigan HG, Sundberg JP, King LE: Murine cytomegalovirus is not associated with alopecia areata in C3H/HeJ mice. J Invest Dermatol 110:986^987, 1998b McElwee KJ, Boggess D, King LE Jr, Sundberg JP: Experimental induction of alopecia areata-like hair loss in C3H/HeJ mice using full-thickness skin grafts. J Invest Dermatol 111:797^803, 1998c McElwee KJ, Boggess D, Miller J, King LE Jr, Sundberg JP: Spontaneous alopecia areata-like hair loss in one congenic and seven inbred laboratory mouse strains. J Invest Dermatol Symp Proc 4:202^206, 1999b McElwee KJ, Boggess D, Olivry T, et al: Comparison of alopecia areata in human and nonhuman mammalian species. Pathobiology 66:90^107, 1998a McElwee KJ, Drummond S, Oliver RF: Hair follicle speci c autoantibodies associated with alopecia areata in sera from the DEBR rat model and humans. In: Van Neste D, Randall V (eds). Hair Research for the Next Millennium. Amsterdam: Elsevier, 1996b: p 253^257 McElwee KJ, Pickett P, Oliver RF: The DEBR rat, alopecia areata and autoantibodies to the hair follicle. Br J Dermatol 134:55^62, 1996a

6 VOL. 8, NO. 1 JUNE 2003 ALOPECIA AREATA IN RODENT MODELS 11 McElwee KJ, Rushton DH, Trachy R, Oliver RF: Topical FK506: A potent immunotherapy for alopecia areata? Studies using the Dundee experimental bald rat model. Br J Dermatol 137:491^497, 1997 McElwee KJ, Silva K, Beamer WG, King LE Jr, Sundberg JP: Melanocyte and gonad activity as potential severity modifying factors in C3H/HeJ mouse alopecia areata. Exp Dermatol 10:420^429, 2001 McElwee KJ, Spiers EM, Oliver RF: In vivo depletion of CD8 þ Tcells restores hair growth in the DEBR model for alopecia areata. BrJ Dermatol 135:211^217, 1996c McElwee KJ, Spiers EM, Oliver RF: Partial restoration of hair growth in the DEBR model for Alopecia areata after in vivo depletion of CD4 þ T cells. Br J Dermatol 140:432^437, 1999c McElwee KJ, Tobin DJ, Bystryn JC, King LE Jr, Sundberg JP: Alopecia areata: An autoimmune disease. Exp Dermatol 8:371^379, 1999a Messenger AG, Bleehen SS: Expression of HLA-DR by anagen hair follicles in alopecia areata. JInvestDermatol85:569^572, 1985 Michie HJ, Jahoda CA, Oliver RF, Johnson BE: The DEBR rat: An animal model of human alopecia areata. Br J Dermatol 125:94^100, 1991 Michie HJ, Jahoda CA, Oliver RF, Poulton TA: Immunobiological studies on the alopecic (DEBR) rat. Br J Dermatol 123:557^567, 1990 Mitchell AJ, Krull EA: Alopecia areata. Pathogenesis and treatment. JAmAcad Dermatol 11:763^775, 1984 Nutbrown M, MacDonald-Hull SP, Baker TG, Cunli e WJ, Randall VA: Ultrastructural abnormalities in the dermal papillae of both lesional and clinically normal follicles from alopecia areata scalps. Br J Dermatol 135:204^210, 1996 Oliver RF, Lowe JG: Oral cyclosporin A restores hair growth in the DEBR rat model for alopecia areata. Clin Exp Dermatol 20:127^131, 1995 Paus R, Slominski A, Czarnetzki BM: Is alopecia areata an autoimmune-response against melanogenesis-related proteins, exposed by abnormal MHC class I expression in the anagen hair bulb? Yale J Biol Med 66:541^554, 1993 Perret C, Wiesner-Menzel L, Happle R: Immunohistochemical analysis of T-cell subsets in the peribulbar and intrabulbar in ltrates of alopecia areata. Acta DermVenereol 64:26^30, 1984 Price VH: Topical minoxidil (3%) in extensive alopecia areata, including long-term e cacy. JAmAcadDermatol16:737^744, 1987 Ranki A, Kianto U, Kanerva L, Tolvanen E, Johansson E: Immunohistochemical and electron microscopic characterization of the cellular in ltrate in alopecia (areata, totalis, and universalis). J Invest Dermatol 83:7^11, 1984 Sainsbury TS, Duncan JI, Whiting PH, Hewick DS, Johnson BE, Thomson AW, Oliver RF: Di erential e ects of FK 506 and cyclosporine on hair regrowth in the DEBR model of alopecia areata. Transplant Proc 23:3332^3334, 1991 Shapiro J, Sundberg JP, Bissonnette R, et al: Alopecia areata-like hair loss in C3H/ HeJ mice and DEBR rats can be reversed using topical diphencyprone. JInvest Dermatol Symp Proc 4:239, 1999 Skinner RB Jr, Light WH, Bale GF, Rosenberg EW, Leonardi C: Alopecia areata and presence of cytomegalovirus DNA [letter]. JAMA 273:1419^1420, 1995a Skinner RB Jr, Light WH, Leonardi C, Bale GF, Rosenberg EW: A molecular approach to alopecia areata. J Invest Dermatol (Suppl) 104:3s^4s, 1995b Sundberg JP, Cordy WR, King LE Jr: Alopecia areata in aging C3H/HeJ mice. J Invest Dermatol 102:847^856, 1994a Sundberg JP, Oliver RF, McElwee KJ, King LE Jr: Alopecia areata in humans and other mammalian species. J Invest Dermatol (Suppl) 104:32S^33S, 1995 Sundberg JP, Vallee CM, King LE Jr: Alopecia areata in aging C3H/HeJ mice. In: Sundberg JP (ed). Handbook of Mouse Mutations with Skin and Hair Abnormalities. Boca Ranton: CRC Press, 1994b: p 499^505 Theo lopoulos AN: The basis of autoimmunity: Part I Mechanisms of aberrant self-recognition. ImmunolToday 16:90^98, 1995 Tobin DJ, Hann SK, Song MS, Bystryn JC: Hair follicle structures targeted by antibodies in patients with alopecia areata. Arch Dermatol 133:57^61, 1997a Tobin DJ, Orentreich N, Fenton DA, Bystryn JC: Antibodies to hair follicles in alopecia areata. J Invest Dermatol 102:721^724, 1994 Tobin DJ, Sundberg JP, King LE, Bogess D, Bystryn J-C: Autoantibodies to hair follicles in C3H/HeJ mice with alopecia areata-like hair loss. J Invest Dermatol 109:329^333, 1997b Westgate GE, Craggs RI, Gibson WT: Immune privilege in hair growth. JInvest Dermatol 97:417^420, 1991 Wiesner-Menzel L, Happle R: Intrabulbar and peribulbar accumulation of dendritic OKT 6-positive cells in alopecia areata. Arch Dermatol Res 276:333^334, 1984 Yamamoto S, Jiang H, Kato R: Stimulation of hair growth by topical application of FK506, a potent immunosuppressive agent. JInvestDermatol102:160^164, 1994 Zhang JG, Oliver RF: Immunohistological study of the development of the cellular in ltrate in the pelage follicles of the DEBR model for alopecia areata. Br J Dermatol 130:405^414, 1994