The Sensitization Phase of Allergic Contact Dermatitis Consists of at Least 2 Steps and Its Completion Plays a Crucial Role in the Response to All Antigens

We examined whether 0.1% 2,4-Dinitorofluorobenzene (DNFB), which is a concentration insufficient to induce contact dermatitis with a single application, can induce dermatitis with repeated applications. 0.1% DNFB was painted on the shaved backs of C57BL/6 mice 2 consecutive days a week and 6 applications (3 weeks) induced allergic contact dermatitis. A hapten-specific lymphocyte proliferative response was observed in regional lymph node cells obtained 2 applictions (1 week) of 0.1% DNFB. Four applications (2 weeks) of 0.1% DNFB did not elicit dermatitis, despite the appearance of hapten-specific lymphocytes; therefore, immunological memory is not sufficient to complete the sensitization phase. Interferon (IFN)-γ was produced at 1 week and in significantly higher amounts at 2 weeks. The redox status of regional lymph node cells was examined, because reductive cells have been reported to produce IFN-γ. The number of reductive cells was also increased at 1 week and significantly higher at 2 weeks. Most of the increased reductive cells were considered to be antigen non-specific because even repeated immunization can not induce such a high number of antigen specific cells. If the higher number of antigen non-specific reductive cells play a crucial role in completing the sensitization phase, even irrelevant antigens could induce dermatitis after 4 applications (2 weeks) of DNFB. Although 2.5% or 3.5% oxazolone could not induce dermatitis with a single application, it could induce dermatitis, if applied following 4 applications (2 weeks) of 0.1% DNFB. These results demonstrate that the sensitization phase of allergic contact dermatitis consists of at least 2 steps: the first step, which is antigen specific, is the appearance of antigen specific lymphocytes in regional lymph nodes, and the second step, which is antigen non-specific, is increases in IFN-γ production and the number of reductive cells. Entry of an antigen to the skin plays a crucial role in the response to all antigens.