Immunotherapy Triggers Release of Mitochondrial DNA and Trips Alarm

High-resolution electron microscopy image of a dendritic cell. [NIH]

A place for everything and everything in its place is a mantra that eukaryotic cells ascribe to with some frequency. For instance, DNA is compartmentalized to organelles like the nucleus and mitochondria. The cytosolic space is not typically an area where DNA is present unless the cell is under attack by some form of pathogen, such as a virus. Sensors within the cytosol—namely cyclic GMP-AMP synthase (cGAS)—recognize cytosolic DNA during viral infections and sound the immune system alarm to begin the degradation process. Now, a group of researchers at UT Southwestern Medical Center report a new finding that describes how cancer immunotherapy causes stressed tumor cells to release their mitochondrial DNA into nearby immune cells, triggering a host alert system.    

Findings from the new study—published recently in Immunity in an article entitled “Dendritic Cells but Not Macrophages Sense Tumor Mitochondrial DNA for Cross-Priming through Signal Regulatory Protein α Signaling”—indicate that cGAS plays a critical role in bridging the body's two immune systems: the innate immune system, which senses initial threats, and the adaptive immune system, which in this context specifically amplifies the antitumor response after getting the alert from the innate immune system. The investigators are hopeful that their data could lead to strategies that would improve the effectiveness of immune checkpoint blockade therapies.

Despite dramatic responses in some people, many other patients show no response to immunotherapy, sparking the search for new therapies that target an entirely different type of checkpoint, such as the experimental drug used in this study. The drug targets CD47, a cell-surface protein highly expressed in some tumor cells. CD47 sends a “Do not eat me” signal to block immune cells from killing the cancer cells.

“CD47 is found in every cell of the body, and it has long been known that many kinds of cancer cells produce even higher amounts of CD47 than do healthy cells,” explained senior study investigator Yang-Xin Fu, Ph.D., professor of pathology and immunology at UT Southwestern. “That 'do not eat me' signal helps cancer cells evade detection from the immune system's killer T cells.” In fact, the higher the levels of CD47 found in tumors, the poorer the prognosis for many cancers.

In tests on mouse cells, the researchers found that when cancer cells are stressed by an experimental anti-CD47 immunotherapy, the cancer cells leak DNA into nearby dendritic cells (DCs), which present antigens to killer T cells and are considered a bridge between the innate and adaptive immune systems. The DNA sensor cGAS sounds the alarm inside those DCs, setting off a signaling cascade that alerts the immune system and unleashes the cancer-killing T cells.

“…we have shown that CD47-signal regulatory protein α (SIRPα) axis dictates the fate of ingested DNA in DCs  for immune evasion. Although macrophages were more potent in uptaking tumor DNA, increase of DNA sensing by blocking the interaction of SIRPα with CD47 preferentially occurred in…DCs but not in macrophages,” the authors wrote. “Mechanistically, CD47 blockade enabled the activation of NADPH oxidase NOX2 in DCs, which in turn inhibited phagosomal acidification and reduced the degradation of tumor mitochondrial DNA (mtDNA) in DCs. mtDNA was recognized by cyclic-GMP-AMP synthase (cGAS) in the DC cytosol, contributing to type I interferon (IFN) production and antitumor adaptive immunity.”

The function of cGAS in the immune response to bacterial and viral infection is well established. A study published early this year showed that cGAS is essential for cancer immunotherapy by immune checkpoint blockade. However, the identity of the DNA in the cancer cells that triggers the antitumor immune response was unclear.

“Tumor-originated mtDNA was recognized by the DNA sensor cGAS in DCs during anti-CD47 treatment,” Dr. Fu noted. “Furthermore, we found that the cGAS-STING-IRF3 signaling pathway plays a critical role in driving robust innate and adaptive immune response upon anti-CD47 therapy.”

“Our findings have demonstrated how tumor cells inhibit innate sensing in DCs and suggested that the CD47-SIRPα axis is critical for DC-driven antitumor immunity,” the authors concluded.

Understanding this novel mechanism of anti-CD47 therapy may make it possible to design new combination strategies to improve current immune checkpoint blockade therapies by modulating the innate sensing of mtDNA. Also, this mechanism of releasing mtDNA under stress may apply to many other pathological and pharmacologic conditions.