The present invention provides methods for increasing expression of ICOS on an effector T cell comprising contacting said effector T cell with an anti-PD-1 antibody. The present invention also provides methods for decreasing expression of ICOS on a regulatory T cell comprising contacting said regulatory T cell with an anti-PD-1 antibody. The present invention provides methods for increasing sensitivity to an agent directed to ICOS in a human comprising administering to the human an anti-PD1 antibody. The present invention also provides methods of treating cancer in a human in need thereof comprising administering an anti-PD-1 antibody and an anti-ICOS antibody to said human, wherein the anti-PD-1 antibody increases T cell sensitivity to the ICOS antibody.