Cutting edge: Identification of GL50, a novel B7-like protein that functionally binds to ICOS receptor

By the genetic selection of mouse cDNAs encoding secreted proteins, a B7-li ke cDNA clone termed mouse GL50 (mGL50) was isolated encoding a 322-aa poly peptide identical with B7h. Isolation of the human ortholog of this cDNA (h GL50) revealed a coding sequence of 309 aa residues with 42% sequence ident ity with mGL50, Northern analysis indicated GL50 to be present in many tiss ues including lymphoid, embryonic yolk sac, and fetal liver samples. Of the CD28, CTLA4 and ICOS fusion constructs tested, flow cytometric analysis de monstrated only mouse ICOS-IgG binding to mGL50 cell transfectants. Subsequ ent phenotyping demonstrated high levels of ICOS ligand staining on splenic CD19(+) B cells and low levels on CD3(+) T cells. These results indicate t hat GL50 is a specific ligand for the ICOS receptor and suggest that the GL 50-ICOS interaction functions in lymphocyte costimulation.