It is hypothesized that polymorphic light eruption is characterized by a partial failure of ultraviolet radiation-induced immunosuppression, resulting in a delayed-type hypersensitivity response to photo-induced antigens. We aimed to study the susceptibility of PLE patients to UVR-induced immunosuppression, by measuring the strength of sensitization to 2,4-dinitrochlorobenzene after UVR exposure, and to diphenylcyclopropenone without UVR exposure, in subjects with PLE and controls. Thirteen PLE patients and 11 controls were exposed to 1 minimum erythema dose (MED) of UVR delivered from Waldmann UV-6 bulbs to the upper inner arm. Twenty-four hours later at the same site they were exposed to a sensitizing dose of 2,4-dinitrochlorobenzene. One week later they were exposed to a sensitizing dose of diphenylcyclopropenone at a nonirradiated site. Three weeks later all subjects were challenged with four doses of 2,4-dinitrochlorobenzene and four doses of diphenylcyclopropenone. The resulting increase in skin thickness was measured with Harpenden callipers and summed over the four doses, to give a single value representing the reactivity of the subject to 2,4-dinitrochlorobenzene (Sigma DN) and diphenylcyclopropenone (Sigma DP). Among all subjects, there was a very strong correlation between Sigma DN and Sigma DP (Pearson correlation 0.56, p=0.004). The strength of the reaction to 2,4-dinitrochlorobenzene relative to the reaction to diphenylcyclopropenone was significantly greater among PLE patients than controls (p=0.04 independent samples t test of Sigma DP-Sigma DN). We conclude that induction of sensitization by 2,4-dinitrochlorobenzene is suppressed less by UVR in patients with PLE than in healthy controls.

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This study has (i) validated the concept that a single score can reflect disease severity in PLE by showing that the principal characteristics of the condition, including, for example, the extent of anatomical distribution and the ease of provocation of the eruption, correlate with each other; (ii) formulated the PLESI, which is a simple, valid and reproducible way of assessing disease severity; we suggest it could be used worldwide to determine the severity of PLE among patients enrolled in future PLE research; (iii) shown that the ease with which the eruption is provoked by solar-simulated radiation correlates with the severity of the condition; and (iv) shown that the duration of persistence of the eruption after sun exposure does not correlate with the severity of the condition.