This study compares the ability of two ultraviolet (UV) B-absorbing sunscreens, 2-ethylhexyl p-methoxycinnamate (2-EHMC) and 2-ethylhexyl p-aminobenzoate (Padimate O), and two physical sunscreens, microfine titanium dioxide (MTD) and zinc oxide, to protect the skin immune system from chronic (4 weeks) solar-simulated UV irradiation. Mice were exposed to suberythemal doses of UV before assessing local and systemic immunosuppression and tolerance to a contact sensitizer. Using a UV protocol that induced local but not systemic immunosuppression or tolerance in BALB/c mice, it was shown that Padimate O made the immunosuppression worse, whereas 2-EHMC and MTD protected the immune system. When the cumulative dose was increased by 12.7%, causing systemic immunosuppression and tolerance, none of the sunscreens protected from immunosuppression, but 2-EHMC provided partial, and MTD gave complete protection from tolerance. To examine this apparent lack of protection from systemic immunosuppression, C3H/HeJ mice were used. These mice had a minimal erythema dose similar to that of the BALB/c mice but were systemically immunosuppressed, with only 44% of the UV dose required to immunosuppress BALB/c mice. 2-EHMC provided some protection, whereas MTD provided complete protection from systemic immunosuppression in C3H/HeJ mice. Hence, sunscreens can protect from local and systemic immunosuppression, although this protection is limited and is not related to the sun protection factor of the sunscreens or the minimal erythema dose of the mouse strain. Instead, protection seems to be related to the sunscreens' having a broad absorption spectrum.

It has previously been demonstrated that chronic low-dose solar-simulated UV radiation could induce both local and systemic immunosuppression as well as tolerance to a topically applied hapten. In this study, we have used a chronic low-dose UV-irradiation protocol to investigate the effects of UVA on the skin immune system of C3H/ HeJ mice. Irradiation with UVA + B significantly suppressed the local and systemic primary contact hypersensitivity (CHS) response to the hapten 2,4,6-trinitrochlorobenzene. Furthermore UVA + B reduced Langerhans cell (LC) and dendritic epidermal T cell (DETC) densities in chronically UV-irradiated mice. Ultraviolet A irradiation induced local, but not systemic, immunosuppression and reduced LC (32%) but not DETC from the epidermis compared to the shaved control animals. Treatment of mice with both UVA + B and UVA radiation also induced an impaired secondary CHS response, and this tolerance was transferable with spleen cells. These results suggest that depletion of LC, but not DETC, may be involved in UVA-induced local immunosuppression in our model, and that tolerance was induced in the presence of normal numbers of DETC. Hence exposure of C3H/HeJ mice 5 days per week for 4 weeks with UVA can induce local immunosuppression and tolerance.

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