The present invention provides compositions and methods for modulating lymphocyte activation. Nucleic acids encoding proteins and proteins so encoded which are capable of modulating lymphocyte activation are provided. Compositions and methods for the treatment of disorders related to lymphocyte dysfunction or dysregulation are also provided. Prophylactics and methods for the prevention of such disorders are also provided. Also provided are compositions and methods for diagnostic and prognostic determination of such disorders. Further provided are assays for the identification of bioactive agents capable of modulating lymphocyte activation.