The present invention is based, in part, on the identification of novel human anti-PD-1, PD-L1, and PD-L2 antibodies. Accordingly, the invention relates to compositions and methods for diagnosing, prognosing, and treating conditions that would benefit from modulating PD-1, PD-L1, and / or PD-L2 activity (e.g., persistent infectious diseases, autoimmune diseases, asthma, transplant rejection, inflammatory disorders and tumors) using the novel human anti-PD-1, PD-L1, and PD-L2 antibodies described herein.