Contact sensitivity (CS) is a form of delayed‐type hypersensitivity to haptens applied epicutaneously and is thought to be mediated, like classical delayed‐type hypersensitivity responses, by CD4+ T helper‐1 cells. The aim of this study was to identify the effector T cells involved in CS. We studied CS to the strongly sensitizing hapten dinitrofluorobenzene (DNFB) in mice rendered deficient by homologous recombination in either major histocompatibility complex (MHC) class I, MHC class II, or both, and which exhibited deficiencies in, respectively, CD8+, CD4+, or both, T cells. MHC class I single‐deficient and MHC class I/class II double‐deficient mice, both of which have a drastic reduction in the number of CD8+ T cells, were unable to mount a CS response to DNFB. In contrast, both MHC class II‐deficient mice and normal mice treated with an anti‐CD4 monoclonal antibody (mAb) developed exaggerated and persistent responses relative to heterozygous control littermates. Furthermore, anti‐CD8 mAb depletion of class II‐deficient mice totally abolished their ability to mount an inflammatory response to DNFB. Removal of residual CD4+ T cells in class II‐deficient mice by anti‐CD4 mAb treatment did not diminish the intensity of CS. These data clearly demonstrate that class I‐restricted CD8+ T cells are sufficient for the induction of CS to DNFB, and further support the idea that MHC class II‐restricted CD4+ T cells down‐regulate this inflammatory response.
Contact sensitivity to para-phenylenediamine (PPD) is a frequent delayed-type hypersensitivity resulting in contact dermatitis. The aim of the present study, conducted in 16 patients allergic to PPD (as assessed by a positive patch test), was to get better insight into the mechanism of T-cell activation in PPD contact sensitivity. PPD was unable to induce significant proliferation of T cells from a first set of 9 patients. In 7 further patients, lymphocyte proliferation was assessed using PPD and 2 PPD metabolites, namely Bandrowski’s base (BB) and benzoquinone (BQ). BB specifically stimulated T-cell proliferation in a dose-dependent fashion in all 7 patients whereas BQ, like PPD, was ineffective. The peripheral blood mononuclear cells (PBMC) of 8 PPD nonallergic individuals did not respond to either PPD, BB or BQ. We concluded from this study that: (1) the immunogenic hapten in PPD hypersensitivity is not PPD itself, and (2) BB might be the oxidative derivative of PPD endowed with T-cell-activating properties. Further support to this statement was provided by the observation that a T cell line derived from PBMC of a PPD-allergic patient in the presence of PPD responded to BB but not to PPD. Our in vitro results suggest that PPD is a prohapten which when applied on the skin is metabolized and converted into products (such as BB) which are the immunogenic haptens able to activate specific T cells.
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