ABSTRACT
There is considerable evidence that suppression of the immune system by UVB (280-320 nm UV) irradiation is initiated by UVB-dependent isomerization of a specific skin photoreceptor, urocanic acid (UCA), from the trans to the cis form. Previous studies have confirmed that cis-UCA administration to mice 3-5 days prior to hapten sensitization at a distant site, suppresses the contact hypersensitivity (CHS) response upon challenge. This study demonstrates in mice that cis-UCA, like UVB, suppresses CHS to trinitrochlorobenzene by a mechanism partly dependent on prostanoid production. In vitro experimentation showed that human keratinocytes, isolated from neonatal foreskin, increased prostaglandin E2 (PGE2) production in response to histamine but not UCA alone. However, cis-UCA synergized with histamine for increased PGE2 production by keratinocytes. Cis-urocanic acid also increased the sensitivity of keratinocytes for PGE2 production in response to histamine. Prostaglandin E2 from keratinocytes exposed to cis-UCA and histamine may contribute directly, or indirectly, to the regulation of CHS responses by UVB irradiation.
Subject(s)
Dinoprostone/biosynthesis , Histamine/pharmacology , Immune System/radiation effects , Indomethacin/pharmacology , Keratinocytes/drug effects , Urocanic Acid/pharmacology , Animals , Drug Synergism , Female , Histamine Antagonists/pharmacology , Humans , Immune System/drug effects , Keratinocytes/metabolism , Mice , Mice, Inbred BALB C , Ultraviolet RaysABSTRACT
Photoisomerization of trans-urocanic acid (UCA) in the stratum corneum has been implicated in the immunosuppression detected after irradiation with UVB (UV wavelength of 280-320 nm). In this study, cis-urocanic acid suppressed human monocyte production of TNF-alpha by a PGE2-dependent mechanism. This contrasted with the mechanism involving histamine type 2 receptors by which the UCA structural analogue, histamine, suppressed monocyte TNF-alpha production. Histamine type 1 receptor antagonists were without effect on both the cis-UCA- and histamine-induced suppression of monocyte TNF-alpha levels. As indomethacin can reverse UVB-immunosuppression in murine models, we may have identified one of the cellular mechanisms responsible for reduced delayed-type hypersensitivity responses. Decreased TNF-alpha levels, by restricting further cytokine recruitment, may also limit the development of the inflammatory components of hypersensitivity responses.
