Epidermal T lymphocytes and dendritic cells in chronic plaque psoriasis: the effects of PUVA treatment.

The numbers and HLA-DR expression of T cell subsets and dendritic cells in chronic psoriatic plaques were compared to previously reported findings in spontaneously resolving guttate lesions, and the effects of PUVA treatment on these cell populations studied. The chronic lesions showed a similar T helper/T suppressor (TH/TS) ratio (0.66 +/- 0.10) to resolving guttate lesions. However, in contrast to the resolving lesions which do not contain activated epidermal TH cells, a substantial proportion of the TH cells in the persistent plaques were DR+. Moreover, these persistent lesions contained markedly increased numbers of DR+ dendritic cells, approximately 20% of which were T6 negative. PUVA-induced resolution of chronic lesions was associated with depletion of epidermal TH and TS cells, and a subsequent reduction in DR+ dendritic cells. In each patient the rate of disappearance of both cell types correlated with the rate of resolution. Furthermore, the epidermal T cell depletion preceded the onset of clinical improvement. In contrast, significant reduction of the dendritic cells was generally not observed until the lesions were largely resolved. Dendritic cells decreased faster in uninvolved than in lesional skin and to a subnormal level. Dermal T cells also decreased during PUVA therapy but this did not show any obvious correlation with resolution of the lesions. Blood T cell levels were not significantly affected by the treatment. These findings support the concept that the initiation and maintenance of the psoriatic process requires activation of TH cells in the epidermis via interaction with antigen presenting cells. Furthermore PUVA treatment may clear psoriasis by interfering with such a mechanism through its effects on T lymphocytes.