With the clinical impact of CTLA-4 and PD-1/PD-L1 immune checkpoint therapies, widespread interest in cancer immunotherapy has been ignited. However, the rate and extent of clinical responses to approved therapies are limited and often non-existent in many solid tumors. This is partially because immune checkpoint therapies are most effective against T-cell inflamed tumors, and non-T-cell inflamed or T-cell excluded tumors remain a significant barrier. New strategies are needed to overcome immune resistance mechanisms that arise during tumor development which result in T-cell exclusion. Approaches may need to be combined with conventional therapies such as chemotherapy, radiotherapy, and molecularly targeted therapy and many clinical trials are ongoing. This review discusses the challenge of T-cell exclusion and innate oncologic pathways that contribute to this problem, including β-catenin, STAT3, NF-κB, PTEN, and AXL tyrosine kinase. The GAS6/AXL pathway is of interest immunologically as it's targeting can lead to greater antitumor immune responses after radiation. In addition, several targeted therapies that are selective and non-selective for AXL are in preclinical and clinical development in AML and renal cell cancer. There remains much to learn but the future is bright for anti-AXL therapies, though effective combinations and their impact may not be realized for years to come.
- Received January 21, 2017.
- Revision received February 10, 2017.
- Accepted March 8, 2017.
- Copyright ©2017, American Association for Cancer Research.