The present invention provides a newly identified B7 receptor, zB7R1 that functions as lymphocyte inhibitory receptor, which is a PD-1-like molecule and is expressed on T cells. The present invention also provides the discovery of zB7R1's ability to bind to CD155. Methods and compositions for modulating zB7R1-mediated negative signaling and interfering with the interaction of its counter-receptor for therapeutic, diagnostic and research purposes are also provided.