Differentiation of interdigitating reticulum cells and Langerhans cells in the human skin with T-lymphoid infiltrate. An immunocytochemical and ultrastructural study.

The differentiation of two types of T-lymphocyte accessory cells, i.e., interdigitating reticulum cells and Langerhans cells, was studied immunocytochemically and ultrastructurally on cutaneous lesions from patients with mycosis fungoides, a neoplasm of mature T-lymphocytes. In such a condition the lymphoid infiltrate creates, adjacent to the epidermis, a microenvironment in the dermis similar to that of T-cell areas of lymphoid organs. Immunocytochemistry revealed that CD11c+ CD1a- putative monocytic cells co-exist with CD11c+ CD1a+ putative mature accessory cells. By electron microscopy, large numbers of interdigitating reticulum cells in the dermal infiltrate and Langerhans cells in the epidermis were found. Furthermore, monocytes were frequently observed, at times with cells showing intermediate features between monocytes and interdigitating reticulum cells on the one hand and Langerhans cells on the other. In the absence of proliferative phenomena of the above cells, it is conceivable that both interdigitating reticulum cells and Langerhans cells originate from locally migrated monocytes. A possible role of the local tissue micro-environment--namely the T-lymphoid microenvironment for interdigitating reticulum cells and the epidermal microenvironment for Langerhans cells--in inducing the differentiation of monocytes into the two kinds of accessory cells is proposed.