Nonlymphoid intraepidermal mononuclear cell collections (pseudo-Pautrier abscesses) - A morphologic and immunophenotypical characterization

We evaluated the incidence, morphology, and immunophenotype of intraepiderm al collections of mononuclear cells (ICMC) in a large number of inflammator y dermatosis and cutaneous lymphomas. ICMC appeared as small to large aggre gates of cells, showing a morphology variable from monocytes to obvious den dritic cells, admired with rare lymphocytes, ICMC were recognized in the ep idermis or within hair follicle epithelium, and were either loosely or comp actly arranged. ICMC were identified in 124 of 1,248 skin biopsies (9.9%) o f inflammatory or lymphoid infiltrates, and were particularly frequent in s pongiotic (43.4%) and in lichenoid dermatitis (10%), whereas they were rare ly found in nonspecific superficial dermatitis (3.8%) and in psoriasis (4.7 %). ICMC were also frequent in cutaneous T-cell lymphoma (13.3%), where the y mimicked Pautrier abscesses. The ICMC forming cells showed a unique pheno type: the majority of them expressed CD1a and S-100, and lacked CD14, simil ar to mature Langerhans cells, but they were also strongly labeled by anti- CD11b, anti-CD36, and anti-CD68. Moreover, a subpopulation of them expresse d CD83, an antigen that is usually absent on Langerhans cells. The occurren ce of ICMC is a rather frequent, although hitherto poorly studied, phenomen on, occurring in several dermatosis, but particularly frequent in spongiosi s associated skin reactions. The cells within ICMC are represented by dendr itic cells and dendritic cell precursors, whose phenotype indicates their d erivation from circulating monocytes and differentiation into mature Langer hans cells.