Pathophysiology of Alopecia Areata

The exact pathophysiology of alopecia areata remains unknown. The most widely accepted hypothesis is that alopecia areata is a T-cell–mediated autoimmune condition that is most likely to occur in genetically predisposed individuals.[1]

Autoimmunity

Much evidence supports the hypothesis that alopecia areata is an autoimmune condition. The process appears to be T-cell mediated, but antibodies directed to hair follicle structures also have been found with increased frequency in alopecia areata patients compared with control subjects. Using immunofluorescence, antibodies to anagen-phase hair follicles were found in as many as 90% of patients with alopecia areata compared with less than 37% of control subjects. The autoantibody response is heterogeneous and targets multiple structures of the anagen-phase hair follicle. The outer root sheath is the structure targeted most frequently, followed by the inner root sheath, the matrix, and the hair shaft. Whether these antibodies play a direct role in the pathogenesis or whether they are an epiphenomenon is not known.

Regulatory T-cells CD8+ lymphocytes likely play a prominent role in alopecia areata.

Regulatory T-cells CD8+ lymphocytes likely play a prominent role in alopecia areata.

Histologically, lesional biopsy findings of alopecia areata show a perifollicular lymphocytic infiltrate around anagen-phase hair follicles. The infiltrate consists mostly of T-helper cells and, to a lesser extent, T-suppressor cells. CD4+ and CD8+ lymphocytes likely play a prominent role because the depletion of these T-cell subtypes results in complete or partial regrowth of hair in the Dundee experimental bald rat (DEBR) model of alopecia areata. The animals subsequently lose hair again once the T-cell population is replete. The fact that not all animals experience complete regrowth suggests that other mechanisms likely are involved. Total numbers of circulating T lymphocytes have been reported at both decreased and normal levels.

Studies in humans also reinforce the hypothesis of autoimmunity. Studies have shown that hair regrows when affected scalp is transplanted onto SCID (severe combined immunodeficiency) mice that are devoid of immune cells. Autologous T lymphocytes isolated from an affected scalp were cultured with hair follicle homogenates and autologous antigen-presenting cells. Following initial regrowth, injection of the T lymphocytes into the grafts resulted in loss of regrown hairs. Injections of autologous T lymphocytes that were not cultured with follicle homogenates did not trigger hair loss.

A similar experiment on nude (congenitally athymic) mice failed to trigger hair loss in regrown patches of alopecia areata after serum from affected patients was injected intravenously into the mice. However, the same study showed that mice injected with alopecia areata serum showed an increased deposition of immunoglobulin and complement in hair follicles of both grafted and nongrafted skin compared with mice injected with control serum, which showed no deposition.

In addition, research has shown that alopecia areata can be induced using transfer of grafts from alopecia areata–affected mice onto normal mice. Transfer of grafts from normal mice to alopecia areata–affected mice similarly resulted in hair loss in the grafts.

Clinical evidence favoring autoimmunity suggests that alopecia areata is associated with other autoimmune conditions, the most significant of which are thyroid diseases and vitiligo (see History). For instance, in a retrospective cross-sectional review of 2115 patients with alopecia areata who presented to academic medical centers in Boston over an 11-year period, comorbid autoimmune diagnoses included thyroid disease (14.6%), diabetes mellitus (11.1%), inflammatory bowel disease (6.3%), systemic lupus erythematosus (4.3%), rheumatoid arthritis(3.9%), and psoriasis and psoriatic arthritis (2.0%). Other comorbid conditions found included atopy (allergic rhinitis, asthma, and/or eczema; 38.2%), contact dermatitis and other eczema (35.9%), mental health problems (depression or anxiety; 25.5%), hyperlipidemia (24.5%), hypertension (21.9%), and GERD (17.3%).[2,3].

In conclusion, the beneficial effect of T-cell subtype depletion on hair growth, the detection of autoantibodies, the ability to transfer alopecia areata from affected animals to nonaffected animals, and the induction of remission by grafting affected areas onto immunosuppressed animals are evidence in favor of an autoimmune phenomenon. Certain factors within the hair follicles, and possibly in the surrounding milieu, trigger an autoimmune reaction. Some evidence suggests a melanocytic target within the hair follicle. Adding or subtracting immunologic factors profoundly modifies the outcome of hair growth.

Genetics

Many factors favor a genetic predisposition for alopecia areata. The frequency of positive family history for alopecia areata in affected patients has been estimated to be 10-20% compared with 1.7% in control subjects.[1] The incidence is higher in patients with more severe disease (16-18%) compared with patients with localized alopecia areata (7-13%). Reports of alopecia areata occurring in twins also are of interest. No correlation has been found between the degree of involvement of alopecia areata and the type of alopecia areata seen in relatives.

Many factors favor a genetic predisposition for alopecia areata.

Many factors favor a genetic predisposition for alopecia areata.

Several genes have been studied and a large amount of research has focused on human leukocyte antigen. Two studies demonstrated that human leukocyte antigen DQ3 (DQB1*03) was found in more than 80% of patients with alopecia areata, which suggests that it can be a marker for general susceptibility to alopecia areata. The studies also found that human leukocyte antigen DQ7 (DQB1*0301) and human leukocyte antigen DR4 (DRB1*0401) were present significantly more in patients with alopecia totalis and alopecia universalis.[4,5,6]

Another gene of interest is the interleukin 1 receptor antagonist gene, which may correlate with disease severity. Finally, the high association of Down syndrome with alopecia areata suggests involvement of a gene located on chromosome 21.

In summary, genetic factors likely play an important role in determining susceptibility and disease severity. Alopecia areata is likely to be the result of polygenic defects rather than a single gene defect. The role of environmental factors in initiating or triggering the condition is yet to be determined.

Cytokines

Interleukin 1 and tumor necrosis factor were shown to be potent inhibitors of hair growth in vitro. Subsequent microscopic examination of these cultured hair follicles showed morphologic changes similar to those seen in alopecia areata.

Innervation and vasculature

Another area of interest concerns the modification of perifollicular nerves. The fact that patients with alopecia areata occasionally report itching or pain on affected areas raises the possibility of alterations in the peripheral nervous system. Circulating levels of the neuropeptide calcitonin gene-related peptide (CGRP) were decreased in 3 patients with alopecia areata compared with control subjects. CGRP has multiple effects on the immune system, including chemotaxis and inhibition of Langerhans cell antigen presentation and inhibition of mitogen-stimulated T-lymphocyte proliferation.

CGRP also increases vasodilatation and endothelial proliferation. Similar findings were reported in another study, in which decreased cutaneous levels of substance P and of CGRP but not of vasoactive intestinal polypeptide were found in scalp biopsy specimens. The study also noted a lower basal blood flow and greater vasodilatation following intradermal CGRP injection in patients with alopecia areata compared with control subjects. More studies are needed to shed light on the significance of these findings.

Viral etiology

Other hypotheses have been proposed to explain the pathophysiology of alopecia areata, but more evidence is needed to support them. Alopecia areata was believed to possibly have an infectious origin, but no microbial agent has been isolated consistently in patients. Many efforts have been made to isolate cytomegalovirus, but most studies have been negative.[7]


Sources:

  1. van der Steen P, Traupe H, Happle R, Boezeman J, Sträter R, Hamm H. The genetic risk for alopecia areata in first degree relatives of severely affected patients. An estimate. Acta Derm Venereol. 1992 Sep. 72(5):373-5. [Abstract].
  2. Pullen LC. Alopecia Areata Associated With Autoimmune Comorbidity. Available at http://www.medscape.com/viewarticle/804646. Accessed: December 20, 2015.
  3. Huang KP, Mullangi S, Guo Y, Qureshi AA. Autoimmune, Atopic, and Mental Health Comorbid Conditions Associated With Alopecia Areata in the United States. JAMA Dermatol. 2013 May 22. 1-5. [Full Article].
  4. Colombe BW, Lou CD, Price VH. The genetic basis of alopecia areata: HLA associations with patchy alopecia areata versus alopecia totalis and alopecia universalis. J Investig Dermatol Symp Proc. 1999 Dec. 4(3):216-9. [Abstract].
  5. Colombe BW, Price VH, Khoury EL, Garovoy MR, Lou CD. HLA class II antigen associations help to define two types of alopecia areata. J Am Acad Dermatol. 1995 Nov. 33(5 Pt 1):757-64. [Abstract].
  6. Price VH, Colombe BW. Heritable factors distinguish two types of alopecia areata. Dermatol Clin. 1996 Oct. 14(4):679-89. [Abstract].
  7. Jackow C, Puffer N, Hordinsky M, Nelson J, Tarrand J, Duvic M. Alopecia areata and cytomegalovirus infection in twins: genes versus environment?. J Am Acad Dermatol. 1998 Mar. 38(3):418-25. [Abstract].
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