Abstract: The present disclosure is directed to a protein binding agent, or an antibody or an antigen-binding fragment thereof, which binds to a programmed death-1 (PD-1) protein. The present disclosure further provides a polynucleotide sequence encoding the protein binding agent, the antibody or the antigen-binding fragment thereof, a vector comprising the polynucleotide sequence, or a host cell comprising the vector. Also, the present disclosure provides a pharmaceutical composition or a kit, which comprises the protein binding agent, the antibody or the antigen-binding fragment thereof.