Plasmacytoid T cells: a cell population normally present in the reactive lymph node. An immunohistochemical and electronmicroscopic study.

Plasmacytoid T cells (PTCs) are medium-sized cells characterized by abundant rough endoplasmic reticulum. They occur in the thymic-dependent area in human lymph nodes. PTCs are hardly identified in routinely stained sections. We studied their occurrence in 100 reactive lymph nodes with the use of monoclonal antibodies MB2, MT1, LN1, LN2, reactive on paraffin-embedded tissue, and with electron microscopy in nine selected cases. PTCs strongly reactive with MT1 and LN2 were found in 87 of 100 lymph nodes. They were observed in clusters, loose aggregates, and as singular cells. An association between PTCs, postcapillary venules, small T lymphocytes, and interdigitating reticulum cells (IDRCs) was found. Our results indicate that PTCs are normally present in the human lymph node. Their immunophenotype suggests a relationship with a monocyte/macrophage lineage, but does not rule out a T cell origin. If the various distribution patterns represent the morphologic substrate of functional stages of PTCs it can be assumed that PTCs play a role in T cell-mediated immune response.