Characterization of a new human B7-related protein: B7RP-1 is the ligand to the co-stimulatory protein ICOS

Optimal T cell activation requires the interactions of co-stimulatory molec ules, such as those in the CD28 and B7 protein families. Recently, we descr ibed the co-stimulatory properties of the murine ligand to ICOS, which we d esignated as B7RP-1, Here, we report the co-stimulation of human T cells th rough the human B7RP-1 and ICOS interaction. This ligand-receptor pair inte racts with a K-D similar to 33 nM and an off-rate with a t(1/2) > 10 min. I nterestingly, tumor necrosis factor (TNF)alpha differentially regulates the expression of human B7RP-1 on B cells, monocytes and dendritic cells (DC). TNF-alpha enhances B7RP-1 expression on B cells and monocytes, while it in hibits it on DC. The human B7RP-1-Fc protein or cells that express membrane -bound B7RP-1 co-stimulate T cell proliferation in vitro. Specific cytokine s, such as IFN-gamma and IL-10, are induced by B7RP-1 co-stimulation, Altho ugh IL-2 levels are not significantly increased, B7RP-1 co-stimulation is d ependent on IL-2, These experiments define the human ortholog to murine B7R P-1 and characterize its interaction with human ICOS.