Rethinking Alopecia Areata: Different Biological Types and Treatment Implications

Table of Contents

• Introduction: Challenging Traditional Views
• Is Alopecia Areata One Disease or Many?
• Autoimmune vs. Non-Autoimmune Alopecia Areata
• The Critical Need for Better Biomarkers
• Why Prognosis Differs So Much Between Patients
• Looking Beyond CD8+ T Cells: Other Immune Players
• The Central Role of Hair Follicle Immune Privilege
• What This Means for Future Treatments
• Source Information

Introduction: Challenging Traditional Views

This perspective article, based on discussions at the 2018 National Alopecia Areata Foundation Summit Meeting, reexamines fundamental questions about what causes alopecia areata. The author argues that the traditional view of AA as a single disease entity no longer fits with what researchers have learned about its varying presentations and outcomes.

Researchers now recognize that the characteristic AA hair loss pattern requires four specific conditions to develop. First, there must be an inflammatory cell infiltrate around hair follicle bulbs that secretes interferon-gamma (IFNg). Second, the hair follicle's natural immune privilege must collapse. Third, major hair follicle dystrophy must occur leading to hair shedding. Fourth, premature regression of hair follicles typically happens.

What's particularly interesting is that research shows this AA hair loss pattern can sometimes develop without a specific autoimmune response against hair follicle antigens. This suggests that AA might not always be a distinct disease entity but rather a stereotypic response pattern that even healthy hair follicles can demonstrate if these four conditions coincide.

Is Alopecia Areata One Disease or Many?

The highly variable presentation and prognosis of different AA types has long suggested it might not be a single disease. While the field continues to distinguish AA based on extent and distribution of hair loss (focal, multifocal, totalis, universalis, diffuse), these clinical classifications may not reflect underlying biological differences.

Research indicates that with increasing chronicity, the AA phenotype changes dynamically. Acute, rapidly progressing AA shows anagen effluvium (hair shedding during growth phase), while chronic AA demonstrates telogen effluvium (shedding of resting hairs) with premature exogen (hair shedding).

We know that CD8+ T cells alone can trigger AA-like hair loss lesions, while autoantibodies against hair follicle antigens fail to do so. However, certain subpopulations of natural killer (NK) cells can also induce AA lesions without requiring a specific genetic predisposition or autoantigens. This supports the idea that non-specific hair follicle damage might suffice to trigger AA in some cases.

Autoimmune vs. Non-Autoimmune Alopecia Areata

The article proposes distinguishing between autoimmune alopecia areata (AAA) and non-autoimmune forms. Only in AAA patients is there evidence for a specific CD8+ T cell-dependent autoimmune response where researchers can demonstrate autoreactive T cells against hair follicle-associated autoantigens.

Patients with a positive personal or family history of other autoimmune disorders, frequently relapsing AA, or rapidly progressive AA may be top candidates for showing the AAA variant. Only in this subpopulation does it make sense to search for pathogenic autoantigens and autoreactive T cells.

For non-AAA forms, the AA response pattern may result from non-specific hair follicle damage that leads to immune privilege collapse and sufficient hair follicle dystrophy. This distinction has major implications for treatment approaches, as discussed later in this article.

The Critical Need for Better Biomarkers

Identifying reliable biomarkers to distinguish the AAA subgroup from other AA types is clinically crucial. Only in autoimmune AA does developing curative therapy aimed at reestablishing tolerance against hair follicle autoantigens or eliminating autoreactive CD8+ T cells make sense for achieving lasting treatment success.

For non-autoimmune AA forms, symptomatic therapy that reestablishes and maintains hair follicle immune privilege may be sufficient. The field needs molecular biomarkers that can reliably distinguish AAA patients from non-autoimmune forms with better prognosis.

Currently, clinical indicators provide the most reliable prognostic information. These include:

• Extent, distribution and progression of hair loss lesions
• Presence of ophiasis (band-like pattern around hairline)
• Nail dystrophy or pitting
• Presence of atopy (allergic tendencies)
• Age at first AA onset
• Family history of AA or associated autoimmune disease

Any proposed molecular biomarkers must demonstrate they are more informative, reliable, and cost-effective than these clinical indicators.

Why Prognosis Differs So Much Between Patients

AA research must explain why not only the extent and distribution of lesions but also disease prognosis differs substantially between individuals. The chance of spontaneous hair regrowth ranges from relatively high in non-atopic patients whose first episode occurred after puberty without nail dystrophy or family history, to poor in pre-pubertal atopic patients with family history of AA or other autoimmune diseases.

We still lack satisfactory explanations for why certain lesion patterns (like ophiasis) or nail involvement are reliable prognostic markers, or why comorbidities like Down Syndrome and lupus erythematosus associate with negative prognosis. The molecular basis of "atopy" remains poorly understood, and may involve neurophysiological abnormalities like defective beta-2 adrenergic signaling found in many atopic dermatitis patients.

Mast cells and eosinophils (immune cells associated with allergies) may be important in understanding why atopic AA patients have worse prognosis. Research shows:

• The number of perifollicular eosinophils and mast cells is significantly increased in many lesional AA skin samples
• These classical atopy-associated immune cells may play a more important role in AA than previously appreciated
• Beta-2 adrenergic signaling defects in mast cells and/or eosinophils might contribute to poorer prognosis in atopic AA patients

Looking Beyond CD8+ T Cells: Other Immune Players

While much research has focused on CD8+ T cells, other immune cells deserve attention. The number and percentage of degranulated perifollicular mast cells are significantly higher in lesional compared to non-lesional AA skin. These cells proliferate abnormally, show increased physical contacts with CD8+ T cells, and switch from immunoinhibitory to proinflammatory, autoimmunity-promoting phenotypes.

The role of the hair follicle microbiome and its potential dysbiosis (imbalance) in maintaining or damaging hair follicle immune privilege represents an essentially uncharted frontier in AA research. The potential therapeutic effect of fecal microbiota transplants and discussed role of intestinal dysbiosis in AA further support exploring microbiological factors.

Other immune cells beyond CD8+ T cells contribute to AA:

• CD4+ T cells drive disease severity and extent
• Regulatory T cells may play important modulatory roles
• Resident memory T cells might be involved in AA recurrence in the same locations
• NK cells have major functional roles in AA pathobiology
• Gamma and/or delta T cells appear to play important, non-antigen-specific roles in triggering immune privilege collapse

The Central Role of Hair Follicle Immune Privilege

All evidence suggests the AA hair loss pattern doesn't develop without first collapsing the hair follicle's relative physiological immune privilege. Therapeutically, the primary challenge remains preventing immune privilege collapse (to stop progression) and restoring it lastingly (to enable regrowth without immediate reattack).

JAK inhibitors likely achieve this effectively—though not in all patients, typically only temporarily, and with potential adverse effects. The top priority should be developing universally effective, well-tolerated, cost-efficient immune privilege guardians and understanding how hair follicles produce such agents.

Promising candidate therapeutics include topical FK506 (tacrolimus) in vehicles that deliver effective drug concentration to hair bulbs, and systemic "superpotent" alpha-melanocyte-stimulating hormone analogs. The most appealing strategy might be developing topical agents that upregulate production of endogenous immune privilege guardians like alpha-MSH, TGFb1/2, IL-10, and VIP (vasoactive intestinal peptide).

What This Means for Future Treatments

This new understanding suggests future AA management should be tailored to the specific pathogenesis pathway in each patient. This would require diagnostic biomarkers that identify the distinct pathways underlying the AA response pattern in individual patients.

Since AA is profoundly territorial (location-specific), this will likely require skin biopsies, with blood/serum biomarkers serving as auxiliary parameters. The ultimate goal is gaining mastery over protecting and restoring hair follicle immune privilege, which would represent a major advancement in AA treatment.

Key treatment implications include:

1. Autoimmune AA may require therapies that reestablish tolerance against specific autoantigens
2. Non-autoimmune forms may respond to treatments that maintain immune privilege without targeting specific immune pathways
3. Short Jak inhibitor courses may be justified even in non-AAA patients with extensive, therapy-resistant hair loss
4. Long-term systemic therapy may not be justified in non-AAA subgroup due to relapse after discontinuation
5. Treatment should be personalized based on the specific pathogenesis pathway in each patient

Source Information

Original Article Title: The Evolving Pathogenesis of Alopecia Areata: Major Open Questions

Author: Ralf Paus

Publication: Journal of Investigative Dermatology Symposium Proceedings (2020) 20, S6-S10

DOI: 10.1016/j.jisp.2020.04.002

This patient-friendly article is based on peer-reviewed research from a leading dermatology journal. The original perspective article was published as part of a supplement sponsored by the National Alopecia Areata Foundation following their 2018 Summit Meeting.