The present invention provides Immunosuppressive 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 signalling induced by PD-1, PD-L1, or PD-L2 and therapies using them, immunopotentiative substrates included as the active ingredient. Further, the invention provides pharmaceutical compositions comprising the Immunosuppressive peptide compounds or modified peptide moieties for preventive and/or therapeutic agents for cancer, cancer metastasis, immunodeficiency, an infectious disease or the like and an application of PD-1 or PD-L1 as a testing or diagnostic agent or a research agent for such a disease.