Researchers at the Bloomberg-Kimmel Institute for Cancer Immunotherapy in the Johns Hopkins Kimmel Cancer Center discovered inhibiting a previously known protein could reduce tumor burdens and enhance the effectiveness of immunotherapy treatments.

To investigate the role of the Yes-associated protein, or YAP, in T-cells in the cancer setting, scientists used mice genetically engineered to lack YAP in several T-cell populations, including regulatory T-cells, known as Tregs. This was the first time the relationship between YAP and Tregs has been explored.

The study was published in Cancer Discovery on June 15, 2018. Regulatory T cells (Treg) are critical for maintaining self-tolerance and immune homeostasis, but their suppressive function can impede effective antitumor immune responses. FOXP3 is a transcription factor expressed in Tregs that is required for their function.

Tregs are important for health because they prevent autoimmune diseases but can be a major obstacle in the mounting of immune responses to tumors and immunotherapy. YAP can be found in a subset of those regulatory T-cells.

Immunotherapy effectiveness on cancer tumors

Scientists tested the antitumor effects of YAP inhibitors alone and in combination with immunotherapies. Their encouraging results showed YAP plays a role in the suppression of antitumor immunity by Tregs and demonstrated by turning off YAP's abilities, tumor killing with less restrained immune cells is possible.

Fan Pan, M.D., senior author of the study and associate professor of cancer immunology, said blocking YAP or the signaling pathways under its control boosted the effects of both a tumor vaccine and a checkpoint inhibitor (anti-PD1 antibody) to produce even stronger antitumor activity. He said the approach of therapeutically targeting YAP was effective over a broad scope of cancer types in mice.

Since Tregs are notorious for dampening the effectiveness of tumor-directed immunity in cancer patients, this study's finding may pave the way for a new and promising strategy to unleash the patient immune response from the stifling grip of suppressor cell control.

However, the pathways and microenvironmental cues governing FOXP3 expression and Treg function are not completely understood. Herein, we report that YAP, a coactivator of the Hippo pathway, is highly expressed in Tregs and bolsters FOXP3 expression and Treg function in vitro and in vivo. 

Chemical YAP antagonism and knockout or blockade of the YAP-regulated activin receptor similarly improved antitumor immunity. Thus, we identify YAP as an unexpected amplifier of a Treg-reinforcing pathway with significant potential as an anticancer immunotherapeutic target.