Antibodies and antigen binding fragments thereof are provided that immunospecifically bind to PD-1, preferably human or mouse PD-1, and induce or promote an immune response that activates immune cell proliferation or activity. Contrary to the existing paradigm that PD-1 exclusively promotes a suppressive immune response, the disclosed antibodies and antigen binding fragments thereof, immunospecifically bind to PD-1 and cause an activating signal to be delivered to the immune cell that activates the immune cell rather than suppressing the immune cell. In one embodiment, the disclosed antibodies and antigen binding fragments thereof specifically bind to PD-1 expressed on immune cells. The binding of the disclosed antibodies and antigen binding fragments thereof to PD-1 on immune cells causes an activating signal to be transmitted into the immune cell, for example a signal that enhances or promotes cytokine production and/or activation of immune cell proliferation. Immune cells that express PD-1, include but are not limited to B and T cells as well as myeloid-derived cells. In one embodiment, the immune cell is a T cell, preferably a CD8+ T cell.
