Dendritic cells in T- and B-cell proliferation in the skin.

In lymphoproliferative diseases of the skin, DC have a key role in T- and B-cell homing. Furthermore, DC alterations may have a pathogenic role in the natural history of specific disorders, either in the neoplastic lymphoid cell progression or in antitumoral lymphocyte reaction. Finally, the morphoantigenic and topographic features of DC may have diagnostic and histogenetic relevance in specific conditions. In CTCL, dermal CD1a+ DC ("indeterminate cells") seem to play a significant role in the neoplastic progression of MF, whereas the possible pathogenetic role of specific alterations of epidermal LC is yet to be proven. Recently, a possible implication of DD (resident, perivascular factor XIIIa+/CD1a- DC) in the pathogenesis of MF has been also suggested. The presence and possible significance of DC in CTCL non-MF are presently poorly studied. At present, DC number, distribution, and phenotype seem possibly useful in the differential diagnosis between CTCL and pseudo-CTCL, but this hypothesis has to be adequately confirmed. CBCL has been recently proposed as a unique type of clinically low-grade lymphoma, namely, skin-associated lymphoid tissue (SALT)-related B-cell lymphoma. Both SALT- and mucosa-associated lymphoid tissue (MALT)-related B-cell lymphoma share with a peculiar nodal lymphoma of follicle mantle origin (parafollicular-monocytoid lymphoma) the nonaggressive clinical behavior and the uniform phenotype (CD5-, CD10-) and genotype (lack of bcl-2 gene rearrangement) of neoplastic B cells, despite the wide variability of cytomorphologic appearances. The putative origin of CBCL is further supported by the typical CD14-, nerve growth factor receptor (NGFr)+ immunophenotype of DRC. Moreover, the immunophenotype and architectural fashion of DRC are interesting clues to the differentiation between neoplastic and true reactive folliclelike nodules and may be of help in the differential diagnosis between CBCL and B-cell pseudolymphoma as well as in the correct interpretation of lesions showing monoclonal proliferations of B cells accompanied by polyclonal follicular reactions.