The immune system maintains a delicate balance between immunity and tolerance, protecting the host from pathogens while minimizing local tissue damage. The immunoregulatory enzyme Indolamine 2,3 dehydrogenase (IDO) is a critical endogenous cellular factor that contributes to immune suppression. IDO activity constitutes a counter regulatory mechanism induced by pro-inflammatory signals. This is also a crucial mechanism in cancer and cancer patients exhibit IDO elevation, which can be detected both in immune cells and on tumor cells. Thus, IDO is involved in the regulation of the immune response by suppressing T-cell function and enabling local tumor immune escape.

IDO is expressed in many different forms of cancer and hence treatment has the potential to be widely used.

It has already been verified that IDO-specific T-cells can be isolated from cancer patients and that these T-cells possess killing activity against tumor cells. Furthermore, these IDO specific T-cells can enhance other T-cell responses directly or indirectly.

Thus, IDO may serve as a widely applicable target for immunotherapeutic strategies.