Single cell studies on the immunological marker profile of plasmacytoid T-zone cells

Plasmacytoid T cells (PTC) are known to home to thymic (T) zones in human lymph nodes and are characterized by their abundant, concentrically layered, rough endoplasmic reticulum. These cells have been found in reactive and neoplastic conditions. Three cases of PTC lymphomas have so far been reported. All of them were complicated by a myelomonocytic leukemia leading to the assumption of a functional relationship between PTC and the myeloid system. The immunologic phenotype of PTC, as revealed on frozen tumor tissue sections, comprised the expression of CD5 (T1), CD4 (T4), and HLA-DR, but not CD8 (T8) and CD2 (T11) and suggested an affiliation to the T cell system. Extending our previous report on one of these cases we here present the first study on the immunological marker profile of suspended PTC. The employment of unfractionated or PTC-enriched tumor cell suspensions rendered possible the application of a panel of monoclonal antibodies (moAbs) on both fixed and unfixed cells and enabled us to allocate various markers either to the intracytoplasmic or surface domain of this cell type. Our results suggest that PTC from our case rest in the G0/G1 phase of the cell cycle. They express the transferrin receptor, but not the Il-2 receptor (CD25) or the nuclear antigen Ki-67. No T cell antigen was demonstrated on the surface of unfixed suspended PTC. Under these conditions only HLA-DR and a predominantly monocytic antigen (CD36/moAb 5F1) were identified. Fixed cells, however, showed a weak cytoplasmic reactivity for CD5 and two myelomonocytic antigens (CD15/moAb 1G10 and CD14/moAb My4). Our findings do not sustain positive evidence for a T cell nature of PTC. Whether their phenotypical pattern indicates terminal differentiation with concomitant loss of T cell antigens or points to a cytogenetic relationship of PTC to the myeloid system, remains speculative. Until the cytogenesis of PTC is clarified we propose the noncommitted term "plasmacytoid T-zone cells" for this elusive cell type.