Investigating Human T Cell Response to CD1a and Contact Dermatitis Allergens in Botanical Extracts and Commercial Skin Care Products

The big idea:

During industrialisation, humans have been exposed to increasing numbers of foreign chemicals.
Failure of the immune system to tolerate drugs, cosmetics, and other skin products causes allergic contact dermatitis: a T cell-mediated disease with rising prevalence.
Thus, this study explains why some creams and cosmetics may cause eczema breakouts.

What the study asked:

The goal of the study was to determine whether known contact allergens can bind to CD1a and stimulate a CD1a-dependent T cell response.

Explanation of key terms:

T cells:
- A type of white blood cell which is part of the immune system. An allergic reaction begins when the immune system’s T cells recognise a chemical as foreign.
CD1a:
- A molecule that is abundant on Langerhans cells (immune cells in the skin’s outer layer), which the study suspected to be responsible for making contact allergens detectable to T cells.
Antigenic:
- A substance (such as protein, virus, or bacteria) that is capable of triggering an immune response, specifically by prompting the body to produce antibodies or activating T-cells.

What the study did:

The study tested whether CD1a, an abundant MHC1-like protein in human skin, mediates contact allergen recognition.
They did so with human cells in tissue culture, by using CD1a-autoreactive human alpha-beta T cell clones to screen clinically important allergens present in skin patch testing kits.

What the study found:

The study identified responses to balsam of Peru, a tree oil widely used in cosmetics and toothpaste.
- Additional purification identified benzyl benzoate and benzyl cinnamate (chemicals used in skin care products) as the antigenic compounds found in balsam of Peru.
- Screening of structurally related compounds revealed additional stimulants of CD1a-restricted T cells, including farnesol and coenzyme Q2 (more chemicals used in skin care products, specifically for fragrance and anti-oxidant, anti-aging properties respectively).
Certain general chemical features controlled response:
- Small size, extreme hydrophobicity, and chemical constraint from rings and unsaturations.
- Normal lipids vs Farnesol:
  - Normal lipids:
    - Sticks out of CD1a enabling T cells to recognise the lipid, and the T cell responds to the foreign lipid itself.
  - Farnesol:
    - Hides inside CD1a and causes T cells to recognise CD1a itself instead, which can trigger an immune response, and thus eczema.

What this means for eczema:

This study identifies molecular connections between CD1a and hypersensitivity to consumer products, defining a mechanism that could plausibly explain the many known T cell responses to oily substances, especially in skin care.
Furthermore, this discovery raises the possibility that allergic contact dermatitis could be stopped by applying competing lipids to the skin to displace those triggering the immune reaction.
- This is given that from previous studies done, several lipids that can bind to CD1a but will not activate T cells have been identified.

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