Interdigitating reticulum cells in the dermal infiltrate of mycosis fungoides. An ultrastructural and immunohistochemical study.

In order to provide insight into the role of accessory cells in lymphoproliferative neoplasms, 7 cases of mycosis fungoides at various clinical stages--patches, plaques and nodules--were studied ultrastructurally and immunohistochemically. The aim was to establish whether interdigitating reticulum cells are a constant finding in the dermal infiltrate. Their possible relationships with mycosic cells were also investigated. This study revealed that interdigitating reticulum cells were present in all the skin lesions examined, were present in considerable number in the patches and plaques and became sparse in the nodules. Furthermore, in the lesions at various clinical stages these cells showed varying ultrastructural features, probably related to different developmental stages. The close contacts between interdigitating reticulum cells and mycosic cells, the expression of antigenic markers of activation by mycosic cells and the morphological and immunohistochemical signs of progressive de-differentiation of mycosic cells in the more advanced stages suggest that interdigitating reticulum cells are involved in stimulating proliferation and--possibly--neoplastic progression of mycosic cells. A role for the T-cell microenvironment created in the dermis by lymphoid infiltrate in inducing the differentiation of interdigitating reticulum cells from their precursors is proposed.