The present disclosure relates to compositions and therapeutic methods for activating an immune response in a patient in need thereof. In a preferred embodiment, the subject methods and compositions are able to antagonize the activity of VISTA, a naturally occurring “checkpoint” protein which contributes to immune tolerance, optionally in combination with an antagonist of a second checkpoint pathway such as PD-1. For example, such methods and compositions may be suitable for preventing and treating colon cancer or another cancer. An exemplary VISTA antagonist, specifically, an anti-VISTA antibody, is demonstrated herein to activate an immune response against cancer cells in vitro and in vivo, thereby conferring protective anti-tumor immunity which decreased tumor burden. Additionally, an additive benefit was observed when a VISTA antagonist was used in combination with a second checkpoint protein antagonist, specifically, an antibody against PD-1 ligand (PD-L1).