Immunohistologic studies in pityriasis rosea. Evidence for cellular immune reaction in the lesional epidermis.

Biopsy specimens of the skin lesions of pityriasis rosea obtained from 15 patients were studied. Characteristic histologic changes were composed of focal intercellular edema, epidermotropism of mononuclear cells often associated with the formation of focal intraepidermal collections of mononuclear cells, and of a perivascular lymphohistiocytic cell infiltration in the superficial dermis. Immunologic analysis using monoclonal antibodies showed that large numbers of lymphoid cells in the perivascular infiltrate reacted with anti-pan-T-cell, anti-helper-inducer subset, and anti-HLA-DR monoclonal antibodies, while the epidermotropic mononuclear cells consisted of helper-inducer cells or suppressor-cytotoxic cells without any predominance pattern. In addition to epidermal Langerhans' cells, some of the dermal infiltrating cells were reactive with monoclonal antibody OKT6. Moreover, there was localized expression of HLA-DR antigen on the keratinocytes. We think that cellular immune reactions are taking place in the lesional epidermis of pityriasis rosea.