Impaired contact hypersensitivity in untreated psoriasis and the effects of photochemotherapy and dithranol/UV-B.

Cell mediated immune reactivity was studied in eighty-five psoriasis patients and twenty-five healthy controls by an improved (quantitative) method for measuring contact hypersensitivity to 2,4 dinitrochlorobenzene (DNCB). Patients were sensitized with 50 micrograms DNCB, the lowest dose found to sensitize all healthy subjects. Responses to epicutaneous challenge with a series of concentrations of DNCB were measured as volumes calculated from diameter and thickness and the various groups were compared by the differences in the log dose-response curves. Patients with untreated psoriasis were less responsive than healthy controls; responses were less still in patients treated with dithranol/UV-B/tar and they were least of all in patients treated with photochemotherapy with 8-methoxypsoralen and UV-A (PUVA), particularly in those who pigmented least. Sensitization and challenge at different stages of treatment showed that both induction and elicitation of sensitization were impaired by PUVA. The possible relationship of these changes to DNCB metabolism, Langerhans cell damage and a decrease in circulating T cells is discussed. Although the clinical significance of the findings is unknown, treatment with dithranol/UV-B/tar has proved safe over many years of use.