Repeated exposure to hair dye induces regulatory T cells in mice

We have recently shown that commercial p-phenylenediamine (PPD)-containing hair dyes are potent immune activators that lead to severe contact hypersensitivity in an animal model. However, only a minority of people exposed to permanent hair dyes develops symptomatic contact hypersensitivity. This suggests that the majority of people exposed to hair dyes does not become sensitized or develop immunological tolerance.

To study the immune response in mice repeatedly exposed to PPD-containing hair dye in a consumer-like manner.

A commercial hair dye containing PPD was tested in C57BL/6 mice. The local immune response was measured by ear swelling and by histological examinations. The immune response in the draining lymph nodes was analysed by flow cytometry.

The hair dye induced local inflammation as seen by swelling and cell infiltration of the treated ears. In addition, exposure to hair dye caused T-cell activation as seen by T-cell proliferation and production of interferon-γ and interleukin (IL)-17 within the draining lymph nodes. The inflammatory response peaked at the fourth exposure to hair dye. From this point on, an upregulation of regulatory T cells and IL-10-producing cells was seen.

This study shows that PPD-containing hair dyes strongly affect the immune system. In addition to being potent skin sensitizers that activate inflammatory T cells, hair dyes also induce anti-inflammatory mechanisms. This might explain why many consumers can use hair dyes repeatedly without developing noticeable allergies, but it also raises the question whether the immune modulatory effects of hair dyes might influence the development of autoimmune diseases and cancers.