Herpesvirus Saimiri gene 13 (HVS13) exhibits 57% identity with the predicted sequence of a T cell-derlved molecule termed CTLAI. Recombinant HVS13 and CTLA8 stimulate transcriptional factor NF-KS actlvlty and interleukin-6(IL-6) secretion in flbroblasts, and costimulate T cell proliferation. An HVSlZFc fusion protein was used to isolate a cDNA encodlng a novel receptor that also binds CTLAI. This receptor is unrelated to previously Identified cytokine receptor families. A recombinant soluble receptor inhlblted T cell proliferation and IL-2 production Induced by PHA, concanavalin A (conA), and anti-TM MAb. These! results define CTLA8 and HVS13 as novel cytokines that bind to a novel cytokine receptor. We propose to call these molecules IL-17, vIL-17, and IL-17R, respectively.