The present invention provides immunosuppression compounds capable of inhibiting the programmed cell death 1 (PD1) signalling pathway. The present invention further provides peptide based compositions for treatment of cancer or treatment of infections via immunopotentiation caused by inhibition of immunosuppressive signaling induced by PD-1, PD-L1, or PD-L2 and therapies using them, immunopotentiative substrates included as the active ingredient. Further, the invention provides an application of the compositions containing the peptide moieties for preventive and/or therapeutic agents for cancer, cancer metastasis, immunodeficiency, an infectious disease or the like and an application of peptide moieties as a testing or diagnostic agent or a research agent for such a disease.