U5 monoclonal antibody identifies a novel lymphocyte surface antigen preferentially expressed in human circulating natural killer cells with high cytotoxic activity

U5 monoclonal antibody (mAb), developed against Japanese monkey lymphocytes , identified a glycoprotein of 32 000 MW that is expressed in a subset of h uman circulating natural killer (NK) cells. The distribution of U5 molecule s was restricted among CD16<SUP>+</SUP> cells, and U5 antigen was preferent ially expressed in the CD38<SUP>+</SUP> subset. U5<SUP>+</SUP> CD16<SUP>+</ SUP> CD56<SUP>+</SUP> cells were highly active on NK assay against K562 tar get cells. Variations in cytolytic activities and mRNA expression of perfor in, granzyme B and Fas ligand (FasL) were observed in U5<SUP>-</SUP> CD16<S UP>+</SUP> CD56<SUP>+</SUP> cells depending on the donor. We found that in some donors, a phenotypically mature (CD16<SUP>+</SUP> CD56<SUP>+</SUP>) bu t functionally immature subset was present in the peripheral circulation. T he U5- CD16<SUP>+</SUP> CD56<SUP>+</SUP> cells of some donors exhibited neg ligible cytolytic activity with no detectable expression of the above mRNAs , whereas those of the other donors had a significant but lower cytolytic a ctivity with a reduced expression of granzyme B mRNA as compared with those of U5<SUP>+</SUP> CD16<SUP>+</SUP> CD56<SUP>+</SUP> cells. Concanavalin A (Con A) stimulation induced an expression of U5 antigen in U5- CD16<SUP>+</ SUP> CD56<SUP>+</SUP><INF></INF> cells accompanied by an up-regulation of g ranzyme B mRNA expression. These findings suggest that U5 antigen may be a novel molecule involved in the maturation or differentiation of human circu lating NK cells.