Cutaneous Cytokines and Allergic Contact Dermatitis

Craig A. Elmets, M.D.
University of Alabama School of Medicine

  1. Allergic contact dermatitis
    1. Cutaneous T lymphocyte mediated immunological reaction to topically applied, low molecular weight haptens.
    2. Skin is an active participant in its development.
    3. A common human experience that is responsible for considerable morbidity.
    4. A problem for the dermatologist who must care for the patient and who must diagnose the disease.
    5. A major impediment to the development of new cosmetics, personal hygiene products and topical medications.
  2. Predictive tests for contact allergens
    1. Have relied almost exclusively on in vivo procedures in humans and animals.
    2. Predictive tests for contact allergies have lagged far behind other medical and toxicological disciplines in which in vitro assays have, to a large extent, supplanted animal testing.
    3. Animals are less than ideal for a number of reasons: marked species differences between man and rodents; expense of animals; ethical issues associated with the use of animals.
  3. Pathophysiology of allergic contact dermatitis
    1. Epicutaneous application of hapten results in avid binding to protein and cell membrane carriers within the epidermis to form a complete antigen.
    2. The complete antigen is taken up by epidermal Langerhans cells.
    3. In the regional lymph nodes, Langerhans cells present haptens to specific clones of T cells that bear T cell receptors for antigen.
    4. T cells proliferate and differentiate and return to the site of hapten application where they are responsible for creating an inflammatory response that is recognized clinically as allergic contact dermatitis.
    5. Keratinocytes contribute to allergic contact dermatitis by synthesizing and secreting a variety of cytokines
      1. Some of which promote the inflammatory response, and others confine the process
      2. Cytokines controlling the overall magnitude of such reactions.
      3. Keratinocytes produce many cytokines including IL-la and IL-1B, IL-6, and TNF-a; the chemokine IL-8; and the colony stimulating factor GM CSF.
  4. Cytokines and allergic contact dermatitis to urushiol (poison ivy)
    1. Efforts in the Elmets' laboratory to develop an assay that might serve as an in vitro alternative to animal testing have focussed on individual differences in cutaneous cytokine expression following contact with allergenic compounds. For this purpose, urushiol, the active moiety in poison ivy/oak, has been employed as a prototypic contact allergen and the cytokines IL-la, IL-lB, IL-6, IL-8, GM-CSF and TNF-a have been examined as cytokines whose synthesis is likely to be altered following urushiol exposure.
    2. In situ detection of cytokine mRNA in normal human skin
      1. By in situ hybridization, IL-la, IL-lB, and TNF-a, messenger RNAs were detected at multiple sites including the epidermis, hair follicles, sebaceous glands, the dermal microvasculature, and arrectores pilorum smooth muscle.
      2. Within the epidermis, transcripts for the cytokines were localized primarily within the granular and basal layers.
      3. IL-la, IL-1B, and TNF-a mRNAs were also present in normal human eccrine sweat glands and in secretory coil epithelium.
    3. Upregulation of cytokine mrnas in human skin treated with urushiol
      1. In subjects allergic to urushiol, significant elevations of IL-ls mRNA were detected as early as six hours after urushiol application, and had returned to background levels by 72 hours.
      2. Elevation of IL-la and TNF-a mRNAs was not present until 24 hours after urushiol application, coincident with clinical and histological evidence of contact dermatitis. Levels of IL-la and TNF-a mRNAs continued to rise in epidermis for at least the next 48 hours.
      3. In subjects in whom no clinical or histological evidence of reactivity to urushiol was present, no elevation in cytokine mRNAs was observed, indicating that the changes in cytokine mRNAs correlated well with the contact allergic reaction.
    4. Cytokine RNA profiles in cultured keratinocytes exposed to poison ivy in vitro
      1. In cultures of normal human keratinocytes exposed to 5 u molar extracts of urushiol, a modest increase in the expression of IL-la and IL-lB mRNAs was observed
      2. A much greater increase in IL-6 and IL-8 mRNAs was present and the maximum increase tended to occur at a later timepoint than that of IL- 1 a and IL-1B mRNAs.
      3. GM-CSF was not upregulated following urushiol exposure.
      4. When cytokine mRNA expression in keratinocyte cultures from donors who were allergic to urushiol was compared to donors who were not allergic to urushiol, cytokines were only upregulated in keratinocyte cultures derived from allergic donors and not from keratinocyte cultures from non-allergic subjects.
      5. This model may therefore be useful in further development of an in vitro correlate of the in vivo contact dermatitis response.