Ultraviolet radiation (UV), in particular the UVB
range, suppresses the immune system in several ways. UVB inhibits antigen
presentation, induces the release of immunosuppressive cytokines and causes
apoptosis of leukocytes. UVB, however, does not cause general immunosuppression
but rather inhibits immune reactions in an antigen-specific fashion. Application
of contact allergens onto UV-exposed skin does not cause sensitization but
induces antigen-specific tolerance since such an individual cannot be sensitized
against the very same allergen later, although sensitization against other
allergens is not impaired. This specific immunosuppression is mediated by
antigen-specific suppressor/ regulatory T cells. UVB-induced DNA damage
is a major molecular trigger of UV-mediated immunosuppression. Reduction
of DNA damage mitigates UV-induced immunosuppression. Likewise interleukin-12
which exhibits the capacity to reduce DNA damage can prevent UV-induced
immunosuppression and even break tolerance. Presentation of the antigen
by UV-damaged Langerhans cells in the lymph nodes appears to be an essential
requirement for the development of regulatory T cells. Studies addressing
the molecular mechanisms underlying UV-induced immunosuppression will contribute
to a better understanding how UV acts as a pathogen but on the other hand
can be also used as a therapeutic tool. |