The subject invention provides novel soluble PD-1 (sPD-1) proteins, nucleic acids, and fusion constructs thereof, for enhancing humoral and cell-mediated immunity of a subject. Also provided are therapeutic compositions comprising the sPD-1 proteins, nucleic acids, and fusion constructs of the subject invention. In a preferred embodiment, the therapeutic composition is formulated as a vaccine composition. Advantageously, the sPD-1 proteins, nucleic acids, and therapeutic compositions provide protective immunity against pathogenic infection including HIV infection. In addition, the subject invention can be used in the prevention and/or treatment of tumor or cancer.