A method of modulating at least one biological activity of a cell expressing a polypeptide designated as A, B, or C, said method comprising contacting said cell with a polypeptide designated as D, E, or F or an anti-A, anti-B, or anti-C polypeptide antibody, whereby said D, E, or F polypeptide or anti-A, anti-B, or anti-C polypeptide antibody binds to said A, B, or C polypeptide, thereby modulating at least one biological activity of said cell, wherein A is a PRO1031 polypeptide (herein also designated IL-17B), B is a PRO10272 polypeptide (herein also designated IL-17E), C is a PRO20110 polypeptide (herein also designated IL-17F), D is a PRO5801 polypeptide (herein also designated IL-17RH1), E is a PRO1 polypeptide (herein known as IL-17R), and F is a PRO20040 polypeptide (herein also designated IL-17RH2).