The present invention provides novel isolated and purified polynucleotides and polypeptides related to a novel ligand for glucocorticoid-induced TNF receptor (GITR). The invention also provides antibodies to the GITR ligand (GITRL). The present invention also is directed to novel methods for diagnosing, prognosing, monitoring the progress of, and treating disorders arising from disregulation of the immune system (e.g., autoimmune disorders, inflammatory diseases, and transplant rejection, and cancers and infectious diseases) using GITRL and/or modulators of GITRL. The present invention is further directed to novel therapeutics and therapeutic targets and to methods of screening and assessing test compounds for the intervention (treatment) and prevention of said disorders arising from disregulation of the immune system, as related to GITRL and GITR.