INDUCIBLE NITRIC-OXIDE SYNTHASE AND PROINFLAMMATORY CYTOKINE EXPRESSION BY HUMAN KERATINOCYTES DURING ACUTE URTICARIA

Background: IgE/allergen-dependent activation of skin mast cells is in volved in acute urticaria and leads to their IL-4 release, Previously we have demonstrated in vitro the induction of the low-affinity recept or for IgE (CD23/Fc epsilon RII) in human keratinocytes (HK) upon stim ulation with IL-4. In addition, we have observed that ligation of CD23 on keratinocytes induced type II nitric oxide synthase (iNOS), leadin g to the release of nitric oxide (NO) and proinflammatory cytokines (T NF-alpha, IL-6). According to these in vitro data, we explored whether keratinocytes could also express iNOS, TNF-alpha, IL-6, and CD23 in a cute urticaria, an in vivo model in which activation of mast cells by IgE/allergen immune complexes is involved. Materials and Methods: iNOS , TNF-alpha, IL-6, and CD23 expression by keratinocytes was studied in acute urticaria (n = 11) in biopsies from lesional and autologous nor mal skin by immunohistochemistry, in situ hybridization, or RT-PCR. Ni trites and TNF-alpha synthesis were assayed in supernatants of culture d lesional keratinocytes. Results: INOS mRNA expression was demonstrat ed with RT-PCR in 10 biopsies out of 11 sections of acute urticaria le sional skin. Immunohistochemistry showed that this iNOS positivity ori ginated from keratinocytes located close to the dermoepidermal junctio n; TNF-alpha and IL-6 mRNA transcription was observed in all but one i NOS(+) biopsy. Immunostaining and in situ hybridization with CD23-spec ific probes were strong in all but one iNOS(+) skin biopsy. Noninflame d autologous, skin was negative for iNOS (except for a weak positivity in one case), cytokines, and CD23. Conclusion: The colocalization of iNOS, proinflammatory cytokines, and CD23 within keratinocytes in acut e urticaria demonstrates that these cells play an important role in th e initiation and maintenance of the inflammatory reaction during this disease in humans through activation of the iNOS pathway by CD23 ligat ion with IgE/allergen immune complexes.