Identification of tissue-infiltrating lymphocytes expressing PEN5, a mucin-like glycoprotein selectively expressed on natural killer cells

PEN5 is a carbohydrate epitope selectively expressed on peripheral blood natural killer cells. We have used a monoclonal antibody reactive with PEN5 to survey the expression of PEN5+ large granule lymphocytes in a variety of human tissues. PEN5+ cells are scattered throughout lymphoid (eg, lymph node, tonsil, thymus, spleen, and intestine) and nonlymphoid (eg, liver, esophagus, lung, and uterus) organs. Due to their relatively abundant cytoplasm, these cells are somewhat larger than resting tissue lymphocytes. The majority of splenic (87 +/- 13%, n = 5) and hepatic (92 +/- 6%, n = 5) PEN5+ lymphocytes coexpress TIA-1, a cytotoxic granule protein found in natural killer cells. In some tissues (eg, tonsil and Peyer's patch), however, relatively few PEN5+ lymphocytes coexpress TIA-1, possibly reflecting different stages of activation or differentiation. Our results indicate that PEN5 may be a useful marker of tissue-infiltrating natural killer cells and reveal these cells to be surprisingly abundant in lymphoid tissues.