Serving as the body's defenders, immune cells swiftly mobilize in response to foreign threats, attracting their cellular partners to address infections and other dangers. Their concerted work with stromal cells, neurons, and microbes drives several crucial processes, including wound healing, pain modulation, and response to cancer immunotherapy.
Download this ebook from Drug Discovery News to learn about the latest research on the crosstalk between immune cells, neurons, stromal cells, and microbes during pain, tissue repair, and cancer.
The Dual Role of Microbiota in Cancer Immunotherapy
BY LUISA TORRES, PhD
Gut bacteria influence the already complex relationship between immune cells and cancer. Understanding this three-way communication sheds light on why immunotherapies fail in some patients but succeed in others.
Immunotherapies transformed cancer treatment by enhancing the immune system's ability to detect and kill cancer cells. However, immunotherapies have not been universally effective for all patients or for all cancer types. Emerging research suggests that the gut microbiome's influence on the immune response may play a significant role in this variation (1). While some bacteria contribute to immunotherapy success in some cancers, others inhibit the immune response that immunotherapies stimulate.
A gentle nudge
Gut bacteria’s influence isn't confined to the gut. For instance, in mice with melanoma, gut bacteria can amplify the effectiveness of immunotherapies. “I don’t think it’s a far stretch to say that a bug in your gut could modulate the development of a cancer in your gut. But if you say that a bug in your gut can help you fight melanoma on your skin, it just doesn’t make sense,” said Andrew Koh, a microbiologist at the University of Texas Southwestern Medical Center.
Koh’s research team conducted a study to explore how healthy gut bacteria escape the gut and influence immunotherapy results in distant tumors such as those in the skin (2). The team focused on melanoma-afflicted mice receiving immune checkpoint inhibitor therapy, a form of immunotherapy that prods the immune system into attacking cancer cells.
They found that the presence of skin tumors didn't prompt gut bacteria to migrate to the tumor site. However, immunotherapy treatment triggered inflammation in the mouse digestive system, leading bacteria to travel from the gut to nearby lymph nodes, similar to what happens during a bacterial infection. This bacterial movement increased the number and diameter of the lymph nodes' blood and lymphatic vessels, which then transported the bacteria selectively to skin tumors. This exciting discovery illustrates how gut bacteria can end up in tumors located elsewhere in the body.
Bacteria are potent activators of the innate immune system, which then leads T cells to attack malignant cells (3). Once bacteria arrived at the tumor, they activated dendritic cells, which primed T cells to kill skin cancer cells. Gut bacteria seemed to rouse the immune system following immunotherapy, making the treatment more effective. “Just like you need an adjuvant in a vaccine to get an optimal response, you may need an adjuvant to help prime T cells for any type of immunotherapy,” said Koh.
Enterococcus faecalis and Lactobacillus johnsonii were the primary bacterial species making the journey from the gut to the lymph nodes and the tumor. Both are natural gut residents and associate with gut health (4,5). Other bacteria such as Akkermansia muciniphila and Bacteroides thetaiotaomicron, which enhance the response to cancer immunotherapies (6,7), also migrated to the tumor.
When bacteria are not friends
While bacteria's presence in skin tumors might be beneficial by activating the T- cell response against tumors, this isn't a universal rule. Patients with high Fusobacterium nucleatum levels within their tumors experience a robust immune reaction that paradoxically hampers immune cell infiltration into the tumor. “We’ve known for thousands of years that the immune system protects us from bacteria, so it's not surprising that immune cells inside the tumor react to them,” said Jorge Galeano Niño, a cancer immunologist at the Fred Hutchinson Cancer Center. “However, that response is not always beneficial in cancer because it impairs the T cell response that clears cancer cells.”
F. nucleatum, which is common in the oral cavity of healthy individuals, appears to thrive within tumors of the colon, where it arrives through the circulatory system during oral infections (8). Galeano Niño assessed the effect of F. nucleatum on colorectal and oral cancer progression in a 2022 study published in Nature (9).
He found that the macrophages and neutrophils that responded to bacteria produced anti-inflammatory molecules such as arginase and Interleukin-10, which prevented T cells from entering the tumor. “We believe this is the reason cancers are refractory to immunotherapies,” he said.
Bacterial cells also directly contribute to chemotherapy resistance and possibly metastasis. Galeano Niño found that tumor bacteria reduced the ability of cancer cells to proliferate. While this might initially sound beneficial, it would make the tumor less sensitive to chemotherapies since they target cells that are actively dividing. This might also result in cancer relapse if chemotherapy-resistant areas of the tumor get reactivated (10). Moreover, conditions that allow cancer cells to stop dividing could also promote metastasis by encouraging cell migration (11).
Antibiotic assistance
If bacteria drive colorectal and oral cancer progression, a reasonable strategy might be to use antibiotics to make the tumors sensitive to immunotherapies. The antibiotic metronidazole can reduce Fusobacterium levels and tumor growth in mice with colon cancer (12). However, applying a similar approach in humans won’t be as simple given that bacteria prefer hard to reach areas of the tumor with poor circulation, which would make antibiotic delivery challenging.
In the case of melanoma, Koh’s group found that giving antibiotics to the melanoma-bearing mice receiving immunotherapy decreased the number of bacteria that moved from the gut. This meant fewer bacteria entered the lymph nodes and the tumor itself, which caused a weaker activation of the immune system against cancer cells.
Whether reducing or eliminating bacteria will be harmful or beneficial for the outcome of immunotherapy treatment might depend on the bacteria type that the tumor harbors. According to Galeano’s data, Fusobacterium, Bacteroides, and Triponema bacteria trigger a potent inflammatory response that neutralizes T cells, while other bacteria might not interfere with T cell responses. The outcome might also depend on the tumor. “Bacteria are present in 33 types of solid tumors,” said Galeano Niño, “And it seems that the microbiome is different in every cancer type.”
In the end, each tumor might require a unique approach tailored to its specific microbiome profile. Understanding and leveraging these microbial differences could pave the way for breakthroughs in patient-specific treatments and offer new hope in the ongoing fight against cancer.
References
Lu, Y., Yuan, X., Wang, M. et al. Gut microbiota influence immunotherapy responses: mechanisms and therapeutic strategies. J Hematol Oncol 15, 47 (2022).
Choi, Y., Lichterman, J.N., & Coughlin, L.A. et al. Immune checkpoint blockade induces gut microbiota translocation that augments extraintestinal antitumor immunity. Science immunology 8(81), (2023).
Hobohm, U., Stanford, J. L. & Grange, J. M. Pathogen-associated molecular pattern in cancer immunotherapy. Crit Rev Immunol 28, 95-107 (2008).
Krawczyk, B., Wityk, P., Gałęcka, M. & Michalik, M. The many faces of Enterococcus spp.-commensal, probiotic and opportunistic pathogen. Microorganisms 9, 1900 (2021).
Dempsey, E. & Corr, S. C. Lactobacillus spp. for gastrointestinal health: current and future perspectives. Front Immunol 13, 840245 (2022).
Routy, B., Le Chatelier, E., Derosa, L., Duong, C. P. M. & Alou, M. T. et al. Gut microbiome influences efficacy of PD-1-based immunotherapy against epithelial tumors. Science 359, 91-97 (2018).
Huang, J., Zheng, X., Kang, W., Hao, H. & Mao, Y. et al. Metagenomic and metabolomic analyses reveal synergistic effects of fecal microbiota transplantation and anti-PD-1 therapy on treating colorectal cancer. Front Immunol 13, 874922 (2022).
Abed, J., Maalouf, N., Manson, A. L., Earl, A. M. & Parhi, L. et al. Colon cancer-associated Fusobacterium nucleatum may originate from the oral cavity and reach colon tumors via the circulatory system. Front Cell Infect Microbiol 10, 400 (2020).
Galeano Niño, J.L., Wu, H., & LaCourse, K.D. et al. Effect of the intratumoral microbiota on spatial and cellular heterogeneity in cancer. Nature 611, 810–817 (2022).
Mitola, G., Falvo, P. & Bertolini, F. New insight to overcome tumor resistance: an overview from cellular to clinical therapies. Life 11, 1131 (2021).
Fares, J., Fares, M. Y., Khachfe, H. H., Salhab, H. A. & Fares, Y. Molecular principles of metastasis: a hallmark of cancer revisited. Signal Transduct Target Ther 5, 28 (2020).
Bullman, S., Pedamallu, C. S., Sicinska, E., Clancy, T. E. & Zhang, X. et al. Analysis of Fusobacterium persistence and antibiotic response in colorectal cancer. Science 358, 1443-1448 (2017).
The Cellular Players That Enable Wound Healing
BY LUISA TORRES, PhD
A coordinated effort between the stroma, the immune system, and the skin microbiome restores damaged skin.
From Cut to Cure
Healing skin injuries such as cuts, burns, or puncture wounds requires diverse cell types and signaling molecules, including mesenchymal stem cells (MSCs), immune cells, and commensal skin bacteria. Interactions between these key players result in effective tissue repair and help restore the skin’s integrity and functionality.
Cleaning Up
Injured cells release signaling molecules such as alarmins that activate MSCs to produce chemokines, which attract macrophages and neutrophils to the injury site. Macrophages and neutrophils clear debris and dead cells, causing swelling, heat, redness, and pain (1,2).
Closing the Wound
Macrophages secrete growth factors, and commensal skin bacteria activate CD8+ T cells that prompt keratinocytes, specialized epithelial cells, to proliferate. Keratinocytes migrate from the wound edges to rapidly cover the wound surface (3,4).
Letting Nutrients In
MSCs, macrophages, and neutrophils produce vascular endothelial growth factor (VEGF), which helps form new blood vessels. These vessels deliver nutrients, oxygen, and anti-inflammatory immune cells that facilitate tissue regrowth (1,5).
Reducing Wound Size
Macrophages release cytokines that help MSCs differentiate into myofibroblasts, which form a collagen-rich layer that provides a scaffold for tissue regeneration (5). They also help contract the wound and apply tension to the wound edges, reducing wound size.
Making a Scar
Making a Scar Myofibroblasts produce new collagen fibers that form strong and flexible scar tissue (2). Macrophages prevent excessive scarring by engulfing surplus collagen.
Fully Healed
As the scar matures, most blood vessels, myofibroblasts, and inflammatory cells disappear through apoptosis. The final scar primarily consists of collagen and other structural components (5).
References
1. Harrell, C.R., Djonov, V., & Volarevic, V. The cross-talk between mesenchymal stem cells and immune cells in tissue repair and regeneration. Int J Mol Sci 22(5), 2472 (2021).
2. Guillamat-Prats, R. The role of MSC in wound healing, scarring, and regeneration. Cells 10(7), 1729 (2021).
3. Pastar, I. et al. Epithelialization in wound healing: a comprehensive review. Advances in wound care 3(7), 445-464 (2014).
4. Johnson, T. R. et al. The cutaneous microbiome and wounds: new molecular targets to promote wound healing. International Journal of Molecular Sciences 19(9), 2699 (2018).
5. Gonzalez, A.C., Costa, T.F., Andrade, Z.A., & Medrado, A.R. Wound healing - a literature review. Anais brasileiros de dermatologia 91(5), 614–620 (2016).
Deciphering Three-way Cellular Communication
INTERVIEWED BY YUNING WANG, PhD
Isaac Chiu delves into the intricate interplay between neurons, immune cells, and microbes to understand how pain and infection occur.
Despite being classified as distinct entities, the nervous and immune systems work interdependently. In the brain, skin, and gastrointestinal tract, the interplay between the nervous and immune systems, known as the neuro-immune axis, orchestrates a variety of physiological processes.
“It fascinated me how cells in these two systems talk to each other,” said Isaac Chiu, an immunobiologist at Harvard Medical School who spent graduate school investigating the crosstalk between T cells and microglia in neurodegeneration.
Further research led him to an even more complex situation. During his postdoctoral research on bacterial skin infections, Chiu discovered that bacteria could directly interact with sensory neurons, causing pain and suppressing immune responses. This unexpected finding introduced a third player, microbes, into the mix.
“The finding that nerves can directly sense microbes opens up so many questions,” Chiu said, “How do neurons sense microbes? How do neurons participate in an infection? Does pain affect host defense against pathogens? How do neurons, microbes, and immune cells interact altogether?”
To answer these questions, Chiu launched his lab combining immunology, neurobiology, and microbiology in 2014. Recently, Chiu and his team uncovered novel mechanisms of pain and inflammation in various microbial infections such as meningitis, anthrax, and colitis. By establishing the link between neurons, immune cells, and microbes, Chiu’s research may inspire new therapeutic strategies to treat these infections.
What are the biological mechanisms underlying pain?
As an unpleasant sensation, pain is the body’s fundamental ability to sense danger. Pain is mediated by neurons called nociceptors, which have long branches going from the spinal cord all the way to tissues such as the skin, the gut, and the joints. Once nociceptors activate, they send signals to the brain, and the brain processes that information. Different pain-mediating neurons sense cold, heat, and mechanical force in different ways. Pain is intimately associated with the immune system. Inflammation involves pain, redness, heat, and swelling. When tissues are full of inflammatory mediators and immune cells, pain occurs.
How do nociceptors interact with the immune system?
There is bidirectional crosstalk between immune cells and nociceptors. Immune cells produce cytokines, prostaglandins, and other lipid mediators that can act directly on the nerves to make them fire more easily. Immune cells also sense neuronal mediators from nociceptors. In their nerve terminals, nociceptors store neuropeptides such as calcitonin gene-related peptide (CGRP), and many different types of immune cells express receptors for these neuropeptides.
Our research team has focused a lot on CGRP and its interaction with activity-modifying protein 1 (RAMP1) and the calcitonin receptor-like receptor, which are highly expressed in macrophages, neutrophils, monocytes, T cells, and B cells. We recently found that their interactions could suppress cytokine production or polarize macrophages to a more wound-repair and less proinflammatory phenotype. The interaction also regulates neutrophil recruitment into tissues and acts on dendritic cells to affect downstream T cell responses. These neuropeptides are powerful regulators of immunity.
What roles do microbes play in neuro-immune interactions during pain?
Pain fibers can be hijacked by bacteria in a clever way. We recently studied meningitis, where bacteria invade the brain through the cerebrospinal fluid. We found that nociceptor neurons were exploited by two bacterial pathogens, Streptococcus pneumoniae and Streptococcus agalactiae, which are the leading causes of meningitis in humans. These bacteria induce pain directly by activating nociceptor neurons with a pore-forming toxin, pneumolysin. Pneumolysin induces the neurons to fire and produce CGRP. CGRP then acts on the macrophages next to the nerves, suppressing the production of protective cytokines against the bacteria. By shutting down the immune response, the bacteria survive better in the meninges and more easily invade the brain.
When we removed the nerves or CGPR in the meninges using genetic or pharmacological approaches, we enhanced the immune system’s ability to fight bacterial pathogens that cause meningitis. The macrophages produced higher levels of cytokines and chemokines, which helped recruit neutrophils and monocytes to kill the bacteria.
How do you study the crosstalk between microbes, neurons, and immune cells?
We use a combination of in vivo and in vitro approaches, but we prioritize in vivo experimentation because it captures all neuroimmunological factors.
We use animal models to look at what happens to one system when we manipulate another. For example, we genetically target specific microbial factors to see which microbe toxin causes pain. We also target specific immune cell types to see whether they contribute to pain or host defense against the microbe. Additionally, by using chemogenetic or optogenetic methods, we can temporarily activate or silence neurons and see what happens to the immune system or microbes.
To study specific molecular interactions, we perform in vitro assays in a dish. For example, we grow nerves in a dish to see how they respond to the microbe or culture immune cells and see if they respond to a neurotransmitter or neuropeptide in a certain way.
How does neuro-immune crosstalk work in the body?
We study these interactions in the gut and skin, which are two major sites where nerves, immune cells, and microbes closely interact. The situation is different in each context.
In the gut, mucus forms a barrier and protects the gut from harmful substances. Inadequate mucus production can lead to inflammatory bowel diseases. Last year, we discovered that nociceptors in the gut regulate goblet cells, which produce mucus. The gut microbiome helps this process by stimulating nociceptors to produce CGRP, which triggers mucus production. Meanwhile, mucus keeps the microbiome healthy as well. We found in mice that this mechanism protects against colitis.
Microbes can silence neurons as well. We studied anthrax, a skin infection in humans and livestock that does not cause any pain. Anthrax bacteria produce a toxin that acts on the nociceptors and blocks pain. It is fascinating that microbes can exploit different pathways not just to survive in the body, like in meningitis, but also to help them spread.
Can targeting pain pathways help treat infections?
There is a lot of promise there. By blocking CGRP, we can enhance immune responses against microbes. In fact, there are many drugs now on the market that block CGRP or its receptor RAMP1 to treat headaches and migraine. We can repurpose these drugs to treat infections. For example, in bacterial meningitis, we could use CGRP blockers to enhance antibiotic treatment. In addition, CGRP also regulates immune responses during invasive skin infections, such as necrotizing fasciitis. Necrotizing fasciitis is a horrible disease caused by aggressive Streptococcus pyogenes infection. We are interested in exploring the potential of using CGRP blockers to treat this infection.
What is the most exciting aspect of this research?
I'm excited about how microbes affect neurons. The interaction between microbes and neurons plays a role in pain and in the gut-brain axis, where gut microbes affect the brain. We think it affects many different neurological diseases such as Alzheimer's disease, Parkinson's disease, and depression. Interestingly, some evolutionary biologists working with Caenorhabditis elegans found that neurons might be the most ancient defense against pathogens. These microbe-neuro-immune circuits may have evolved to protect us or to activate when we're sick. It is essential to gain insights into the underlying causes of neurodegeneration and infection.
This interview has been condensed and edited for clarity.