The NCI Molecular Imaging Program (MIP) develops new diagnostics and image guided therapies. One of the most exciting projects from the MIP is Photoimmunotherapy (PIT) in which an antibody conjugated to a special photoabsorber (IR700) is injected intravenously. The antibody conjugate accumulates in the tumor and subsequent light exposure in the near infrared spectrum results in rapid tumor killing. The initial human testing of PIT is in Phase 3 trials and will likely be approved in the US within 2 years. We have been working to augment the response of PIT by adding additional antibody conjugates that target the immune suppressive environment often found around tumors. Specifically, a cell type known as a T-regulatory cell is often present. This cell expresses CD25 on its surface. In animal studies the combination of an anti-tumor antibody conjugate and anti CD25 antibody conjugate exposed to near infrared light resulted not only in killing of the exposed tumors but also killed tumors that were not irradiated with light AND prevented reimplantation of the tumor a month after therapy. This implies a strong immune response. Thus, developing an antiCD25 antibody to target Tregs is a high priority for the program.