The plasmacytoid T cell or plasmacytoid monocyte--a sessile lymphoid cell with unique immunophenotype and unknown function, still awaiting lineage affiliation.

Clusters of cells with a plasmacytoid appearance have been identified in T zones of human lymphoid tissue. These cells were shown to express the CD4 antigen in the absence of B-cell antigens. Thus, they were named "plasmacytoid T cells" (PTCs). Recent studies, however, have suggested a myelomonocytic origin of this cell type, which led to the designation "plasmacytoid monocytes." In order to obtain a comprehensive immunophenotype of this cell, we used frozen serial sections of lymph nodes, an indirect immunoperoxidase technique, and a broad panel of well-defined monoclonal antibodies (mAbs). On PTCs, the antigenic density was high for CD4 and CD26 antigens and low for CD2; they furthermore reacted with the mAb OKM5 recognizing an undefined antigen associated with the myelomonocytic lineage and expressed the monocyte-associated antigen CD68. PTCs expressed HLA-A, B, C, HLA-DR, and the HLA-D associated invariant chain (Ii) while HLA-DP antigens were expressed only in low amounts and HLA-DQ antigens were lacking entirely. PTCs further expressed the adhesion molecule CD11c. The absence of detectable transferrin receptor and the proliferation antigen defined by Ki-67 indicated that PTCs were nonproliferating. Other well-defined antigens confined to the T-, B-, and myelomonocyte lineages could not be detected in the target cell population. For us, antigenic equipment is still suggestive of a sessile, terminally differentiated T cell likely to exert a secretory function as yet unknown.