The present invention provides antibodies that bind to the T-cell co-inhibitor programmed death- 1 (PD-1 ) protein, and methods of use. In various embodiments of the invention, the antibodies are fully human antibodies that bind to PD-1. In certain embodiments, the present invention provides multi-specific antigen-binding molecules comprising a first binding specificity that binds to PD-1 and a second binding specificity that binds to an autoimmune tissue antigen, another T- cell co-inhibitor, an Fc receptor, or a T-cell receptor. In some embodiments, the antibodies of the invention are useful for inhibiting or neutralizing PD-1 activity, thus providing a means of treating a disease or disorder such as cancer or a chronic viral infection. In other embodiments, the antibodies are useful for enhancing or stimulating PD-1 activity, thus providing a means of treating, for example, an autoimmune disease or disorder.