A novel polypeptide ligand, p30, or LIGHT, for herpes virus entry mediator, HVEM, is provided. LIGHT is useful for modulating immune responses and in inhibiting infection and/or subsequent proliferation by herpesvirus. HVEM fusion proteins are also provided. Methods for treating subjects with lymphoid cell disorders, tumors, autoimmune diseases, inflammatory disorders or those having or suspected of having a herpesvirus infection, utilizing p30 and the fusion proteins of the invention, are also provided.