Molecular characterization and expression of a novel human leukocyte cell-surface marker homologous to mouse Ly-9

Ly-9 is a mouse cell-surface glycoprotein that is selectively expressed on thymocytes and on mature T and B lymphocytes, Ly-9 belongs to the CD2 subse t of the immunoglobulin superfamily, an emerging family of cell signaling r eceptors, Recently, a partial human Ly-9 complementary DNA (cDNA) sequence has been described. Full-length cDNA clones were isolated that included the initiation codon, the sequence encoding the full signal peptide, and 14 am ino acids more in the cytoplasmic domain than in the previously reported cl one. The predicted extracellular domain of human Ly-9 contains 4 immunoglob ulinlike domains, similar to those in mouse Ly-9, Northern blot analysis re vealed that the human Ly-9 messenger RNA (2.6 kb) is expressed predominantl y in lymph node, spleen, thymus, and peripheral blood leukocytes, Four mono clonal antibodies (mAbs) were raised against human Ly-9 by immunizing mice with the pre-B-cell line 300.19 stably transfected with human Ly-9 full-len gth cDNA, These mAbs strongly stained the surfaces of cells transfected wit h human Ly-9 cDNA but not of untransfected cells. Human Ly-9 expression was restricted to T and B lymphocytes and thymocytes, with the highest levels of expression on CD4(+)CD8(-) and CD4(-)CD8(+) thymocytes. Monocytes, granu locytes, platelets, and red blood cells were uniformly negative for Ly-9. T hese mAbs immunoprecipitated major polypeptides of 120 kd from the transfec ted cells and 120 kd and 100 kd from B-cell line Daudi, probably because of the cell-surface-expressed isoforms. These data demonstrate that human Ly- 9 is a new marker for the study of normal and malignant leukocytes, (Blood, 2001;97:3513-3520) (C) 2001 by The American Society of Hematology.