Review
The challenges of UV-induced immunomodulation for children’s health

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Abstract

Exposure to solar ultraviolet radiation (UVR) is recognised to have both beneficial and harmful effects on human health. With regard to immune responses, it can lead to suppression of immunity and to the synthesis of vitamin D, a hormone that can alter both innate and adaptive immunity. The consequences in children of such UV-induced changes are considerable: first there are positive outcomes including protection against some photoallergic (for example polymorphic light eruption) and T cell-mediated autoimmune diseases (for example multiple sclerosis) and asthma, and secondly there are negative outcomes including an increased risk of skin cancer (squamous cell carcinoma, basal cell carcinoma and cutaneous malignant melanoma) and less effective control of several infectious diseases. Many uncertainties remain regarding the amount of sun exposure that would provide children with the most effective responses against the variety of immunological challenges that they are likely to experience.

Introduction

Exposure to sunlight has both beneficial and harmful outcomes. The sun provides warmth and a sense of well-being, as well as being essential for controlling biorhythms. It is also required for the synthesis of vitamin D in the skin. Solar ultraviolet radiation (UVR) can suppress immunity to antigens experienced within a short period after the exposure. This was first demonstrated about 35 years ago by Margaret Kripke and colleagues using highly antigenic tumour cells derived from chronically irradiated mice which were transferred to syngeneic naïve mice: if these recipients were UV-irradiated prior to the transfer, the cells were not rejected but grew into tumours (Kripke and Fisher, 1976). Other antigens including contact sensitisers, alloantigens and microorganisms have been tested since then in model systems, and the same down-regulation found. More limited approaches in human subjects indicate that a similar response is induced as in the models. Exposure to solar UVR is thought to induce much of the vitamin D in most individuals. Vitamin D has many effects on human health including the capacity to affect both innate and adaptive immune responses. The consequences of the immunosuppression and the vitamin D production that follow UVR are complex, some considered positive and some negative.

In this article, the effects of UVR on human immune responses are assessed, particularly those relating to children’s health. The first section below contains overviews of the pathways leading to UV-induced immunosuppression and to UV-induced vitamin D together with its immunological properties. This is followed by illustrations of the protective role of UVR against some photoallergic and autoimmune diseases and against asthma. Next illustrations of the detrimental aspects of UVR on immunity are described, namely the increased risk of skin cancer and the less effective control of infections. Lastly some conclusions are reached concerning the best advice to offer to the public regarding the sun exposure that would provide children with the optimal “immunological health”.

Section snippets

Effects of UVR on immune responses

These fall into two categories, first the cascade that leads to suppression of immunity and secondly the production of cutaneous vitamin D which can subsequently modulate immune responses. Brief descriptions of both aspects follow.

Beneficial effects of UV-induced immunosuppression on the health of children

There are several instances where the down-regulation in immunity as a result of UVR can confer positive health outcomes in children. First there is protection against photosensitive disorders: polymorphic light eruption (PLE) is used as an example (see Section 3.1 below). Secondly there is protection against the T cell-mediated autoimmune diseases: multiple sclerosis (MS) is used as an example (see Section 3.2 below). Thirdly asthma is considered (see Section 3.3 below).

Detrimental effects of UV-induced immunosuppression on the health of children

While the sections above describe some potential advantages of UV-induced immunomodulation on the health of children, there can also be significant adverse effects, as illustrated by the induction of skin cancer and by the reduced control of selected infectious diseases and efficacy of vaccination.

Conclusions

This review has assessed the evidence available currently regarding the effects of solar UVR on human health, especially relating to the health of children. UVR is recognised to suppress T cell immunity and to induce vitamin D which itself alters many immune parameters. There are robust results indicating several beneficial outcomes of UVR including protection against photoallergy and some autoimmune diseases, and several adverse outcomes including the increased risk of skin cancer. Other

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