title: Urocanic acid suppresses induction of immunity in human skin.
abstract: OBJECTIVE
  Trans-urocanic acid is isomerized to cis-urocanic acid (C-UCA) by ultraviolet radiation. C-UCA suppresses immunity in vitro and in vivo in animals; its effect on human skin is unknown. We sought to determine whether its topical application to normal skin suppresses induction of immunity to dinitrochlorobenzene (DNCB).
  
  
  METHODS
  Forty subjects applied C-UCA (0%, 0.02%, 0.2%, or 2%) for 17 days. A 40-mcg dose of DNCB was then applied to induce immunity. Subjects were challenged for immunity at 6-week follow-up by occluding doses of DNCB (0, 3.125, 6.25, or 12.5 mcg) on untreated normal skin. Induced immunity was measured by area of erythema and induration 2 and 4 days postchallenge.
  
  
  RESULTS
  No significant differences were found in incidence of sensitization by C-UCA concentration (P=.59). DNCB sensitization developed in all 10 subjects induced through 0% C-UCA (placebo); only 23 of 30 patients were sensitized through skin treated with C-UCA. Mean areas of erythema and induration induced through C-UCA-treated skin were less than those in controls (P < 0.05). The number of Langerhans cells in C-UCA-treated skin was unaffected. Laboratory tests of immune function and lymphocyte numbers were unchanged.
  
  
  CONCLUSIONS
  Topically applied C-UCA blunts normal induction responses to a cutaneous sensitizer.
authors.affiliation: Department of Dermatology, Mayo Clinic, Scottsdale, AZ 85259, USA. dahl.markv@mayo.edu
authors.forename: Mark V
authors.initials: MV
authors.lastname: Dahl
authors.forename: Gerald N
authors.initials: GN
authors.lastname: McEwen
authors.forename: H Irving
authors.initials: HI
authors.lastname: Katz
mesh_terms: D000328:Adult; D004137:Dinitrochlorobenzene / Q000008:administration & dosage / Q000276:immunology; D004890:Erythema / Q000276:immunology; D005260:Female; D006801:Humans; D007166:Immunosuppressive Agents / Q000008:administration & dosage / Q000009:adverse effects* / Q000276:immunology; D007801:Langerhans Cells / Q000276:immunology; D008297:Male; D008875:Middle Aged; D012867:Skin / Q000276:immunology*; D013237:Stereoisomerism; D013997:Time Factors; D014466:Ultraviolet Rays / Q000009:adverse effects*; D014560:Urocanic Acid / Q000008:administration & dosage / Q000009:adverse effects* / Q000276:immunology
chemical_list: D004137:Dinitrochlorobenzene; D007166:Immunosuppressive Agents; D014560:Urocanic Acid