Different strains of mice have varying susceptibilities to ultraviolet radiation (UV) of wavelength 280–320 nm (UVB) for 50% suppression of systemic contact hypersensitivity (CHS) responses. Prevalence of histamine-staining dermal mast cells in different strains of mice (C57BL/ 6J, DBA/2, BALB/c) correlated directly with their susceptibility to UVB-induced systemic immunosuppression. BALB/c mice carrying Uvs1, a major locus for susceptibility to UV-induced immunosuppression, contained greater numbers of dermal mast cells than BALB/c mice of the same parental origin. Strains of mice that were differentiated on their susceptibility to UVB-induced downregulation of systemic CHS responses were similar in their susceptibility to histamine-induced immunomodulation. Histamine, but not UVB irradiation, decreased systemic CHS responses in mast cell–depleted mice (W  f/W  f). Reconstitution of the dorsal skin of W  f/W  f mice with bone marrow–derived mast cell precursors from nonmutant mice rendered the mice susceptible to UVB irradiation for systemic suppression of CHS responses. UVB irradiation did not suppress delayed type hypersensitivity responses to allogeneic spleen cells in W  f/W  f mice. In contrast, UV irradiation suppressed CHS responses in W  f/W  f mice when hapten was applied to the irradiated site. This study demonstrates that dermal mast cells are necessary for the induction of systemic suppression of CHS responses by UVB radiation, and suggests that mast cell– derived histamine is one component of this UVB-induced systemic immunosuppression.

SUMMARYStudies in experimental models have implicated histamine and prostanoids in ultra-violet B ( UVB)-and cis-urocanic acid ( UCA)-induced systemic immunosuppression. This study examined the hypothesis that UVB irradiation and cis-UCA suppressed contact hypersensitivity responses to hapten by induction of histamine, which in turn evoked a prostanoid-dependent component of immunosuppression. BALB/c mice were administered with a cis-UCA monoclonal antibody, a combination of histamine types 1 and 2 receptor antagonists, or indomethacin. Mice were sensitized to 2,4,6-trinitrochlorobenzene (TNCB) on their ventral surface 5 days after UVB irradiation, or cis-UCA or histamine administration. Ears were challenged with TNCB 5 days later. Cis-UCA antibody inhibited the suppressive eÂects of UVB by approximately 60% and confirmed that suppression of contact hypersensitivity responses by UVB was due, at least in part, to mechanisms involving cis-UCA. Histamine suppressed contact hypersensitivity responses and the eÂects of cis-UCA and histamine were not cumulative, suggesting that cis-UCA and histamine signal largely through the same pathway. The immunosuppressive eÂects of histamine were not aÂected by the cis-UCA antibody, consistent with the model that histamine acts downstream of cis-UCA. Administration of histamine receptor antagonists and indomethacin each approximately halved the UVB-and cis-UCA-induced systemic suppression of contact hypersensitivity responses. The eÂects of the reagents that inhibited the action of histamine and prevented prostanoid production were not cumulative, and suggested involvement in the same pathway. These results support the involvement of cis-UCA, histamine and prostanoids, in a sequence, in UVB-induced systemic suppression of contact hypersensitivity responses. INTRODUCTIONwhich, in turn, initiates immunosuppressive signals. There is some evidence that DNA may be a UVB photoreceptor,6,7 Ultra-violet B ( UVB) irradiation (wavelength 280-320 nm) is but trans-urocanic acid (deaminated histidine), a molecular immunosuppressive and allows the growth of highly antigenic species located superficially in the stratum corneum of the skin UV-induced tumours.1,2 The immunosuppression can be both and which isomerizes to its cis form on UVB irradiation, has local and systemic, and results in reduced expression of contact also been implicated in the mechanisms whereby UV hypersensitivity (CHS) and delayed-type hypersensitivity irradiation generates systemic immunosuppression.8-12 Skin ( DTH) responses to a variety of antigens in mice and humans.painting or parenteral inoculation with cis-UCA can reduce Examination of the suppression of CHS responses to haptens systemic CHS responses and is associated with an alteration in experimental animals has allowed some dissection of the in antigen-presenting cell ability in vivo. However, in vitro mechanisms of the UVB-induced eÂects.2-4 studies have shown that the defect is not due to the direct As less than 10% of UVB irradiation reaches the dermis,5 eÂec...

SUMMARYDifferences in dermal mast cell prevalence for adult mice of different strains have been reported previously. In this study, the dermal mast cell prevalence for BALB/c and C57BL/6 mice at 6 weeks of age was similar but as BALB/c mice matured from 6 to 10 weeks of age, their dermal mast cell prevalence halved. In contrast, there was no signi®cant difference in the dermal mast cell prevalence of 6-and 10-week-old C57BL/6 mice. These differences determined the degree of susceptibility of BALB/c and C57BL/6 mice of different ages to UVB (UV radiation of wavelength 280±320 nm)-induced systemic immunosuppression. Expression of the receptor for stem cell factor, Kit protein, was examined on mast cells under conditions in which the dermal mast cell prevalence varied. A signi®cant correlation was observed between Kit expression by mast cells from adult BALB/c, DBA/2 and C57BL/6 mice and dermal mast cell prevalence. In BALB/c mice, mast cell Kit expression decreased as the mice matured from 6 to 10 weeks of age and correlated with the reduction in dermal mast cell numbers. Kit levels on dermal mast cells from C57BL/6 mice were consistently higher than on mast cells from BALB/c mice although signi®cant reductions in Kit were also measured with ageing from 6 to 10 weeks. We hypothesize that regardless of the extent of Kit expression, the dermal mast cell populations were maximally expanded in C57BL/6 mice. We suggest that BALB/c mice of 6 and 10 weeks of age are useful hosts in which to quantitatively evaluate mast cell involvement in a range of functional assays involving skin.

The mechanisms by which UV radiation is immunosuppressive are controversial, but there is growing evidence that processes of UVB-induced suppression of the immune response towards a sensitizing antigen are different if this antigen is applied to irradiated compared with non-irradiated sites. Consistent with this is our recent observation (Hart et al., J. Exp. Med. 1998. 187: 2045-2053) that the prevalence of dermal mast cells determines the extent of susceptibility of different mouse strains to UVB-induced systemic, but not local, immunosuppression. Using C57BL/6 and BALB/c mice exposed to low and high doses of UVB, respectively, in the presence of a polyclonal anti-TNF antibody, we found that TNF is directly involved as a mediator in the suppression by UVB of local immune responses. To determine whether TNF indirectly regulates UVB-induced systemic immunomodulation by altering the prevalence of dermal mast cells, dermal mast cell numbers in gene-targeted mice deficient in TNF or TNF receptors (p55/p75-/- mice) were quantified by video image analysis. A reduced dermal mast cell prevalence in these mice correlated with decreased susceptibility for systemic immunosuppression caused by UVB. We hypothesize that TNF is one molecule that controls dermal mast cell prevalence by as yet unknown mechanisms. However, it is the mediators released from mast cells upon UVB-induced degranulation, which do not include TNF, that directly signal suppressive events relevant to systemic immunosuppression.