Background Over recent decades, there has been a significant global increase in the prevalence of asthma, an inflammatory disease of the respiratory system. While ultraviolet radiation (UV) has been used successfully in the treatment of inflammatory conditions such as psoriasis, studies of UV-induced regulation of allergic respiratory responses have been rare, and have not analysed in vivo measurements of airway hyperresponsiveness (AHR) or the antigen specificity of the UV-induced effects.Objective To investigate the regulatory properties of erythemal ultraviolet B (UVB) irradiation of the skin and the induction of allergen-induced airway immunity in a murine asthma model, and to examine the mechanisms involved.Methods BALB/c mice were exposed to a single erythemal dose of UV 3 days before intraperitonial sensitization (day 0) and boost (day 14) with the antigen, ovalbumin (OVA). Airway-associated, asthma-like responses to aerosolized OVA at day 21 were analysed including (a) AHR measured in vivo, (b) OVA-specific proliferative responses and cytokine production by cells from the lung-draining lymph nodes (LDLN), and (c) inflammatory cells and cytokines in the bronchoalveolar lavage fluid. To determine UVB-induced mechanisms of regulation, LDLN cells from UVB irradiated, OVA-sensitized mice were adoptively transferred into naive BALB/c mice that were subsequently sensitized and challenged with OVA, or a non-specific antigen.Results UVB irradiation of skin significantly suppressed AHR to methacholine and OVA-specific responses in the LDLN and in the lung compartment. Reduced OVA-specific responses by LDLN cells from both UVB irradiated mice and mice that received 5 x 10(6) LDLN cells from UVB irradiated, but not from non-irradiated, OVA-sensitized mice suggested that UVB-induced regulatory cells are responsible for many of the asthma-reducing effects of dorsal UVB exposure.Conclusion UVB irradiation of skin suppresses AHR and cellular responses of the airways to respiratory allergens. Further, this study implicates UVB or its downstream mediators as a potential approach to reducing the severity of asthma.