The present invention relates to an ICOS binding protein, or antigen binding portion thereof that is an agonist human ICOS and does not induce complement, ADCC, or CDC when placed in contact with a T cell in vivo and methods of treating cancer, infectious disease and/or sepsis with said ICOS binding protein or antigen binding portion thereof. Further the ICOS binding proteins or antigen binding portions thereof of the present invention are capable of activating a T cell when placed in contact with said T cell; stimulating T cell proliferation when placed in contact with said T cell and/or inducing cytokine production when placed in contact with said T cell. The present invention relates to ICOS binding proteins or antigen binding portions thereof comprising one or more of: SEQ ID NO:1; SEQ ID NO:2; SEQ ID NO:3; SEQ ID NO:4; SEQ ID NO:5; and/or SEQ ID NO:6.