Immunotherapy is revolutionizing the approach to cancer treatment. By modulating the patient’s immune system, it represents a good therapeutic option. A form of immunotherapy is the treatment based on the use of drugs called immune checkpoint inhibitors. These drugs target specific molecules expressed on immune cells –the checkpoint proteins– that need to be activated or inactivated to start an immune response. For instance, the PD-1 molecule is expressed on cells of the immune system –the T cells– and, by interacting with PD-L1 protein expressed on normal and cancer cells, prevents immune system response, thus enabling cancer cells to avoid and survive the attack of the immune system. The block of the PD-1/PD-L1 interaction with immune checkpoint inhibitors makes the immune system capable of attacking cancer cells. Similarly, CTLA-4 is expressed on T cells and prevents the immune system response. Antibodies targeting CTLA-4 enable the immune system response against cancer cells.
Immune checkpoint inhibitor based-therapy is widely used in the treatment of melanoma, lung, kidney cancer and is currently being tested for several other tumor types. However, not all the tumors tested react with similar efficiency to this therapy, showing limited response and high variability among patients; moreover, cancer cells frequently develop resistance against these treatments.
Why there are such differences in the response to immune checkpoint inhibitors? It has been proposed that the gut microbiota (living bacteria and other microorganisms in the intestine actively interacting with the host) can affect anti-tumor immune response and likely predict the outcome of immune checkpoint inhibitor therapy. Specifically, some types of bacteria may enforce the immune checkpoint inhibitor-based treatment and enable to overcome drug resistance. What is the mechanism underlying the ability of gut bacteria to potentiate the effect of immune checkpoint inhibitor treatment?
Main findings. The authors show that the presence in the gut of specific bacteria increased efficacy of immune checkpoint inhibitor-based immunotherapy, through the release of Inosine that, by binding the adenosine receptor, starts the cascade leading to the activation of T cells, which ultimately potentiates anti-tumor immunity.
Experimental details. In a mouse model of colorectal cancer (CRC), treatment with immune checkpoint inhibitors reduced the size and the number of tumors, reduced the number of cancer stem cells (subpopulation of cancer cells responsible for tumor progression), and increased immune cell infiltration in the tumor mass. Some bacteria were enriched in tumor bearing animals treated with immune checkpoint inhibitors compared to the control animals. In particular, seven bacterial types were found only in the treated animals (so associated with animal responsiveness to treatment), while four were present only in the control.
Validation experiments showed that transfer in a different mouse CRC model of three bacteria types isolated from treated animals increased the efficacy of immune checkpoint inhibitor therapy, significantly enhancing T cell activation. Among these, Bifidobacterium Pseudolongum exerted the strongest effect and was exploited to further study the mechanism involved.
When serum from mice colonized with Bifidobacterium Pseudolongum was transferred in tumor-bearing mice treated with immune checkpoint inhibitors, it reduced tumor growth and induced an anti-tumor immune response, suggesting that the underlying mechanism involved soluble factors either derived from the bacteria or induced by the bacteria. A metabolomic analysis of the serum highlighted several metabolites whose levels were increased in Bifidobacterium Pseudolongum colonized mice compared to other bacteria not affecting the treatment with immune checkpoint inhibitor. Among all, the metabolite Inosine was significantly more abundant. Inosine was also present in the culture medium of Bifidobacterium Pseudolongum bacterial cultures, suggesting that it was directly produced by the bacteria.
How does Inosine work? In vitro experiments showed that Inosine acts on T cells rather than directly on tumor cells, binding the adenosine receptor and, at the physiological concentrations found in the mice colonized with Bifidobacterium Pseudolongum, initiates the downstream cAMP signalling, with the activation of PKA protein and the phosphorylation of CREB, a factor involved in the differentiation of T cells.
Can Inosine administration by itself enforce immune checkpoint inhibitor therapy and reduce tumor growth in absence of the bacteria? In vivo experiments demonstrate that the effect of Inosine is context-dependent, namely administration of Inosine increased anti-tumor immunity and reduced tumor growth when immune checkpoint inhibitors were co-administered with an additional “stimulus” (CpG, which stimulates innate immune response, the aspecific immune response involving immune cells such as neutrophils, macrophages, NK cells,…), otherwise the opposite effect was exerted. Physiologically, the co-stimulation is likely linked to the involvement of another type of immune system cells: the dendritic cells. Depletion of dendritic cells, indeed, prevented the anti-tumor immunity elicited by the bacteria potentiating the immune checkpoint inhibitor therapy, resulting into larger tumors and reduced anti tumor immunity.
Inosine enhanced the effect of immune checkpoint inhibitor therapy also in another model of CRC (though not in all the CRC models tested), in models of bladder cancer and melanoma.
Conclusions. Even though the bacteria studied were isolated in mice, they are also present in humans (albeit at low abundance in adults), suggesting the translational potential of these findings. Moreover, analyses of dataset of patients’ fecal microbiota show a higher –though not significant– presence of Bifidobacterium Pseudolongum in responders compared to non-responder cancer patients. Therefore, these data suggest that modification of the microbiota or specific bacterial therapies may represent a promising adjuvant therapy in CRC and other cancers.
Reference: Microbiome-derived inosine modulates response to checkpoint inhibitor immunotherapy. Mager, Burkhard, Pett, […],McCoy. Science 2020.