CD123-positive plasmacytoid dendritic cells in primary cutaneous marginal zone B-cell lymphoma: diagnostic and pathogenetic implications.

The histogenesis of primary cutaneous marginal zone B-cell lymphoma (PCMZL) has not yet been clarified. Plasma-cytoid dendritic cells (PDC) play a crucial role in the initiation of immune responses and activation of T-cells and B-cells. We analyzed the presence of PDC by immunohistochemistry in various forms of primary cutaneous B-cell lymphomas and in B-cell pseudolymphomas. Clusters of CD123+PDC were observed in all PCMZL (23 of 23 cases; 100%) and in two-thirds of cutaneous B-cell pseudolymphomas (14 of 22; 64%), but only in a minority of primary cutaneous follicle center lymphoma (4 of 31; 13%) and not in primary cutaneous diffuse large B-cell lymphoma, leg type. CD123+ PDC clusters were found in close proximity to monoclonal plasma cells and T-cells and were already present at high numbers in early lesions and initial recurrences of PCMZL. In conclusion, the detection of larger clusters of PDC in PCMZL provides a useful adjunctive diagnostic marker and suggests a pathogenetically relevant role of these cells in PCMZL. Furthermore, the occurrence of PDC could explain the high number of T-cells typically found in PCMZL. We propose considering PCMZL as a unique and potentially antigen-driven neoplasm with PDC, T-cells, and B-cells as the constitutive tumor components.