Nonlymphoid intraepidermal mononuclear cell collections (pseudo-Pautrier abscesses): a morphologic and immunophenotypical characterization.

We evaluated the incidence, morphology, and immunophenotype of intraepidermal collections of mononuclear cells (ICMC) in a large number of inflammatory dermatosis and cutaneous lymphomas. ICMC appeared as small to large aggregates of cells, showing a morphology variable from monocytes to obvious dendritic cells, admixed with rare lymphocytes. ICMC were recognized in the epidermis or within hair follicle epithelium, and were either loosely or compactly arranged. ICMC were identified in 124 of 1,248 skin biopsies (9.9%) of inflammatory or lymphoid infiltrates, and were particularly frequent in spongiotic (43.4%) and in lichenoid dermatitis (10%), whereas they were rarely found in nonspecific superficial dermatitis (3.8%) and in psoriasis (4.7%). ICMC were also frequent in cutaneous T-cell lymphoma (13.3%), where they mimicked Pautrier abscesses. The ICMC forming cells showed a unique phenotype: the majority of them expressed CD1a and S-100, and lacked CD14, similar to mature Langerhans cells, but they were also strongly labeled by anti-CD11b, anti-CD36, and anti-CD68. Moreover, a subpopulation of them expressed CD83, an antigen that is usually absent on Langerhans cells. The occurrence of ICMC is a rather frequent, although hitherto poorly studied, phenomenon, occurring in several dermatosis, but particularly frequent in spongiosis-associated skin reactions. The cells within ICMC are represented by dendritic cells and dendritic cell precursors, whose phenotype indicates their derivation from circulating monocytes and differentiation into mature Langerhans cells.