Chemical compounds found in skin creams and other personal care products can cause an allergic reaction on the skin, a condition known as allergic contact dermatitis. Although this condition is on the rise, particularly in industrialized countries, how these compounds trigger a reaction remains unknown.
Most allergic reactions involving T cells are attributed to proteins or peptide antigens that activate the immune system. It was thought, however, that chemical compounds in personal care products escape detection by T cells because they are smaller and structured differently than immune-triggering antigens and proteins. However, a study by a team of HMS scientists at Brigham and Women’s Hospital and investigators at Columbia University in New York City and Monash University in Melbourne, Australia, has changed that thinking. The team identified a new molecular mechanism that, when set in motion by common components of consumer products, triggers a T cell-mediated immune response. The mechanism is mediated by a protein called CD1a, a molecule in the immune cells that form the outer layer of human skin.
The researchers tested whether CD1a could bind directly to the allergens in personal care products and present these molecules to the immune system, eliciting a reaction. When they exposed T cells to material from skin patch testing kits, they found that T cells responded to certain substances, including balsam of Peru, widely used for fragrance in cosmetics and toothpaste. In addition, benzyl benzoate and benzyl cinnamate, substances in balsam of Peru, were directly responsible for stimulating the T cell response. Investigators also tested similar substances and found a dozen small molecules, including farnesol, that appeared to elicit a response. Crystallographic analyses showed that when farnesol forms a complex with CD1a, it kicks out naturally occurring human lipids, making the protein more visible to T cells and leading to T cell activation.
The authors note that while their work shows that fragrances in personal care products can directly initiate a T cell response, further investigation is needed to understand whether patients with allergic contact dermatitis commonly have T cells that recognize molecules like farnesol.
Nicolai S et al., Science Immunology, January 2020