GUT MICROBIOME AND RESPONSE TO IMMUNOTHERAPY

Immunologic fitness orchestrated by the gut microbiome is strongly suspected of mediating response to cancer treatments, according to an area of research that is heating up quickly. Experimental studies published by independent research groups over the past 5 years have made this hypothesis increasingly plausible. Not least intriguing, the specific mix of commensal intestinal microflora may explain why only a limited proportion of patients achieve long-term responses to checkpoint inhibitors.

"I want to convince you that the microbiome can actually dictate the cancer immune set-point," said Laurence Zitvogel, MD, PhD, group leader, Tumor Immunology and Immunotherapy, Institut Gustave Roussy, Villejuif, France. As one of the scientists who is active in this area of research, she was given the task of summarizing progress in a special symposium on immunotherapy biomarkers at the 2017 annual meeting of the European Society for Medical Oncology.

Several studies in experimental models have demonstrated an interaction between the intestinal microbiome and the antitumor activity of cytotoxic agents with immunostimulatory effects, according to Dr Zitvogel. In one early example, gut dysbiosis induced by vancomycin was shown to greatly inhibit the antitumor effect of cyclophosphamide.^[1] This change in antitumor effect has been linked subsequently to several changes in type 1 helper (Th1) CD4+ cell activity, according to Dr Zitvogel, but the reduced trafficking of these cells into the tumor bed may be particularly relevant to immunotherapies.

"One of the dominating mechanisms appears to involve how the gut microbiome affects expression of CCR9," Dr Zitvogel explained. CCR9, a chemokine receptor, is most closely associated with lymphocyte homing to the gut, but the ligand for CCR9, CCL25, can also be expressed in tumors. There is evidence to suggest that CCR9 priming by gut microflora plays a role in sending lymphocytes to cancers and in initiating signaling that "transforms a cold tumor into a hot tumor," said Dr Zitvogel, referring to antitumor immunologic activity. In one set of experiments tracing the effect of CCR9 expression, the ratio of antitumor CD4+ T cells to suppressor T-regulatory cells was "tremendously increased" in tumors of mice with a healthy gut microbiome relative to those in which the gut microbiome had been compromised.

Understanding the impact of the gut microbiome on immune function may be the key to developing consistently effective immunotherapy.

According to Dr Zitvogel, understanding the impact of the gut microbiome on immune function may be the key to developing consistently effective immunotherapy. In about 20%-30% of patients, checkpoint inhibitors have produced what appears to be cure in an array of cancer types. A defect in immunologic function mediated by the gut microbiome may be the explanation for the limited or absent response in the remaining 70%-80%.

The basis for this hypothesis begins with the overwhelming evidence of symbiosis between gut microbiota and the immune system. Although the impact on the gut microbiome for regulating disease risk is the focus in almost every corner of clinical medicine, ranging from obesity to mood disorders, there appears to be a special relationship between the gut microbiome and immune surveillance and activation. The ability of the immune system to tolerate the vast number of commensal microflora in the gut without losing its ability to recognize and eliminate pathogens must require a complex interaction, according to Dr Zitvogel.

"Immunologists will tell you that most of the anticancer immunity takes place in the lymph nodes, but actually we now know that [the] intestinal barrier contains the vast majority of the immune system spread over a 200-m² surface and is constantly interacting with bacteria, parasites, and other bugs. On this surface, a mutual symbiosis has developed [to permit the immune system] to recognize antigens," Dr Zitvogel said. She believes the evidence also supports a role for this symbiosis in maturation of the immune system as well as functions beyond immunologic activity, including basic metabolism.

As has been repeatedly noted in the increasing interest in the role of the gut microbiome, the human intestine contains tens of trillions of microbes. Most are bacteria, but there are a variety of other organisms representing thousands of different species. The intestinal metagenome of these organisms has been estimated to outnumber the size of the human genome by a factor of 150, according to Dr Zitvogel. In the series of studies suggesting a relationship among the microbiome, immune function, and the efficacy of cancer treatments, the challenge has been to unravel the signaling pathways. These are not expected to be simple.

Greater numbers of organisms favorable to a healthy microbiome have been associated with greater anticancer effect from immunotherapy.

"The algorithm is complex because there is a delicate triangle among the gut microbiome, the intestinal immune system, and systemic immune function," Dr Zitvogel said. When the natural balance in this triangle is disturbed, one of the potential consequences appears to be a reduction in effective immunosurveillance. In other words, the usual immune set-point that characterizes a well-functioning system to activate normal immune-driven anticancer activity is lost, according to Dr Zitvogel.

The adverse impact of vancomycin on the efficacy of cyclophosphamide has been reproduced with other immunostimulatory drugs, which now includes checkpoint inhibitors. Citing several studies, Dr Zitvogel noted that overall survival and progression-free survival has been found to be lower in patients exposed to antibiotics just before anti–PD-1 therapy relative to those who were not. In one study that tested this association in a multivariate regression analysis, recent exposure to antibiotics, which presumably alters the microbiome, remained a strong risk factor for a poor response.

Of many directions of research, one has been to isolate commensal bacterial species that predict an improved response to checkpoint inhibitors. In a series of studies conducted in animal models with the anti–CTLA-4 monoclonal antibody ipilimumab at Dr Zitvogel's center, the introduction of Bacteroides fragilis and Bacteroides thetaiotaomicron were associated with restoration of anti–CTLA-4 anticancer responses in mice who had been poorly responsive after previously induced dysbiosis.^[2] It is notable that other centers performing related experiments have associated some of the same bacteria with an improved response to checkpoint inhibition.

"With thousands of different species in the gut, it is interesting that different teams are identifying the same bugs," Dr Zitvogel said. She also noted that several groups, including her own, have found a dose effect for this relationship so that greater numbers of organisms favorable to a healthy microbiome have been associated with greater anticancer effect from immunotherapy.

In an even more impressive demonstration, response to checkpoint inhibitors in a mouse model can be altered with gut bacteria from patients who did or did not respond to a checkpoint inhibitor, according to Dr Zitvogel. Describing this experiment, Dr Zitvogel explained that feces from checkpoint inhibitor responders or nonresponders were introduced into the gut of germ-free mice. After a delay to allow the gut microbiome to adjust, cancer treatment was initiated with a checkpoint inhibitor. According to Dr Zitvogel, the mice that received feces from patients who progressed on a checkpoint inhibitor achieve a smaller response than those that received feces from responders.

Blocking checkpoint proteins has been compared with removing the brake on a car. Like a car on a flat surface, removing the brake on immune response may not be enough. Rather, some push or acceleration is needed. This may be the role of a healthy balance of gut microflora, according to Dr Zitvogel. She suggested that a microbiome in balance with the immune system increases T-cell activity and sends T cells into tumors to attack cancer cells. She believes that the gut microbiome controls the immune set-point, which triggers the immune response.

"To have immune fitness, you need a healthy gut. This is the take-home message," Dr Zitvogel said. Although she conceded that there are many unanswered questions about how to define dysbiosis and how best to restore the gut microbiome to health once dysbiosis is found, she thinks this area of investigation is fundamental to the future of immuno-oncology.

Dr Zitvogel has disclosed no relevant financial relationships.