Good afternoon, I am very happy to have the opportunity to talk with you today about "two or three things about intestinal fungi". I am Wang Tingting from Nanjing University School of Medicine.
There is such a thing as a creature: it is independent of animals and plants, it becomes a realm on its own; it seems to be invisible, but it is everywhere; it can soar freely in the sky, and it can go down to five kilometers underground. This class of creatures is our protagonist today, the fungus. Its English name is Fungus, which is derived from Latin and originally means "mushroom".
In fact, fungi are ubiquitous in our lives. As we all know, our edible fungi, including mushrooms; our medicinal fungi, with Cordyceps sinensis; and the stinky tofu that everyone likes to eat, are also fermented by fungi.
In fact, there are still many unknown topics about the origin of fungi. What we now know is that fungi are a class of eukaryotes with a complete cell wall and nucleus structure, and there are many organelles in fungi, including the endoplasmic reticulum, Golgi, mitochondria, and in the DNA of fungi, there are also a large number of non-coding DNA.
There are some of these fungi—probably hundreds—that are closely related to disease. Diseases caused by these fungi are also known as fungal infectious diseases. From the point of view of the site of fungal infection, we divide it into shallow fungal infection and deep fungal infection.
Superficial fungal infections refer to fungal infections on the surface of our skin, including tinea capitis, tinea corporis, tinea femoritis, tinea pedunculitis and so on. What we often know as athlete's foot is a kind of tinea pedis.
The so-called deep fungal infection refers to the invasion of fungi into human organs, such as oral mucosal infection, Cryptococcal encephalopathy, pulmonary aspergillosis, fungal enteritis, and histoplasmosis in HIV patients.
Let's take a look at the distribution of fungi in our human body. Although the abundance and number of fungi are relatively small compared to bacteria, it is widely distributed in our bodies. In addition to being distributed on the surface of the skin, it is widely distributed in our brain, lungs, liver, intestines, and so on.
In addition to being associated with some of the above infectious diseases, there are also many studies reporting that fungal and metabolic diseases such as asthma and obesity are also closely related.
Why do normal people rarely develop fungal infections? A fungal infection is an opportunistic infection because the body has a very strong immune system that can control fungal infections.
This is a timeline of fungal research, and you can see that the red part above is about the study of fungi and disease, while the blue part below is about the study of fungi and the immune system.
We can see that the first pattern recognition receptor we found with respect to fungal immunity was the Dectin-1 receptor identified in 2001. By 2006, we had discovered a linker protein downstream of it called the Card9 molecule. By 2013, Dectin-2 and Dectin-3 molecules were first discovered, and they could form a dimer. Since then, more and more immune cells have been found to be closely associated with fungal infections.
Let's take a look at how our bodies recognize and respond to fungi.
Fungi it is magical, there are two forms - mycelium and sporophytes. In the case of adequate nutrition, the fungus will proliferate and form a bead-like structure, which is the mycelium; and once the nutrition is insufficient, under harsh conditions, it will form a round sporophyte.
On the surface of these mycelium and spores, most of them are polysaccharide structures, including chitosan, mannan and dextran as we know it, which are polysaccharides on the surface of fungi, and our body recognizes fungi by identifying these polysaccharides.
This is the main receptor that immune cells recognize for fungi, called the C-type lectin receptor. Type C lectin, a protein that can bind to polysaccharides, has a calcium-dependent sugar recognition zone on its surface.
Type C lectin receptors are mainly distributed on the surface of macrophages and dendritic cells, and the C-type lectin receptors we currently know include the Dectin-1, Dectin-2, Dectin-3 family, and some C-type lectin receptors specifically secreted on the surface of DC (dendritic cells).
Let's take the surface of the lungs as an example to see how immune cells combine natural immune responses and adaptive immune responses to inhibit fungal proliferation.
We can see that epithelial cells, and some of the mucus they secrete, can effectively stop the invasion of pathogenic microorganisms. The fungal surface can complete its invasion by releasing proteases, destroying these epithelial cells.
After the fungus passes through the epithelial cells, it can release a class of inflammatory factors called alarms, including interleukin-25, interleukin-33, and so on.
On the one hand, these cytokines can induce an immune response, causing our neutrophils to degranate, causing type conversion of antibodies to secrete IgE. On the other hand, DC cells play an important role in this process, and DC cells can recognize mycelium and spores, thereby inducing Th1 and Th2 immune responses. In addition, neutrophils also play a very important role in this process.
It is precisely because of the cooperation of many immune cells that our body is free from fungal infections.
Let's take a look at intestinal fungi.
In fact, there are also a large number of fungi in our intestines. The 2013 Science paper provides strong evidence that by utilizing a fungus-specific antibody, a large number of fungi are found on the surface of our intestines, especially at the distal end of the colon.
The 2018 Science article further identified and isolated an intestinal immune defender, which is a mononuclear phagocyte expressing CX3CR1.
As we can see from this diagram, the blue stain is the nucleus, and the green dye is this type of intestinal immune guard - that is, CX3CR1-positive monocyte macrophages, and the red dye is the well-known Candida albicans. We can see that such cells can firmly grasp Candida albicans and annihilate them.
Once the CX3CR1 molecule is genetically mutated, it will affect the effective production of antibodies to fungi in our body, thereby inducing the occurrence of diseases.
Let's take a look at several types of diseases closely related to fungi, the first is inflammatory bowel disease IBD, which mainly includes Crohn's disease and ulcerative colitis.
There have been growing reports confirming that intestinal fungi are strongly associated with inflammatory bowel disease. Mice with gene knockouts of fungal pattern recognition receptors Dectin-1 and Dectin-3 were susceptible to inflammatory bowel disease. Further in the case of these diseases, its genome was studied and it was found that one of the fungi called Candida tropicalis had a significantly increased abundance. Dectin-1 and Dectin-3 were also confirmed to be susceptible genes for inflammatory bowel disease through GWAS experiments.
Our research group also found that the main mechanism by which intestinal fungi cause inflammatory bowel disease is due to the abnormality of the repair of intestinal epithelial cells that induce it.
The relationship between intestinal microecology and colorectal tumors is also of great concern to everyone at present, and a large number of studies have confirmed that the intestinal microbiota is closely related to the occurrence and development of tumors.
In colorectal cancer patients, there is also a general decrease in microflora diversity, a decrease in beneficial bacteria, an increase in pathogenic bacteria, and abnormalities in intestinal mucosal immunity. However, most studies focus on bacteria, and there are many unknowns about the relationship between fungi and intestinal tumors.
Our research group has been studying intestinal fungus and tumorigenesis since 2013.
We first constructed a tumor model of colon cancer on the card9 gene knockout mice of the intestinal fungus, and found that the tumors of card9 knockout mice increased significantly.
After further analysis of the microbiota, we found that the role of bacteria in this is minimal, mainly caused by fungi, which is what we have just called Candida tropicalis. We have also confirmed the carcinogenic effects of Candida tropicalis through a large number of experiments, including fecal transplantation experiments, co-housed (co-caged) experiments in mice, and candida tropical gastric irrigation experiments.
On the other hand, we used antibiotic therapy and found that antibiotics have a very limited therapeutic effect on such mice, while antifungal therapy can effectively inhibit the occurrence of tumors.
The above positive and negative experiments have strongly confirmed that Candida tropicalis is a pathogenic bacteria that can promote the occurrence and development of colorectal cancer.
What is the pathogenic mechanism of this tropical Candida?
We screened for immune cells and found that Candida tropicalis can effectively induce differentiation and proliferation of one of these immune cells, called MDSCs. MDSCs can further inhibit the immune function of T cells, thereby promoting the occurrence and development of tumors.
This part of the work, which first linked the development of intestinal fungi and colorectal cancer, was published in the journal Immunity in 2018.
Coincidentally, in another article published at the same time as ours, also in the mice that Card9 knocked out, an increase in fungi was found, an increase in tumors was found, and its regulatory effect on the inflammasome was also found.
In addition to Candida tropicalis, do other fungi we know as well have carcinogenic effects?
Our research group continued to study Candida albicans, and we were surprised to find that Candida albicans can also promote the occurrence and development of tumors, which can effectively promote the secretion of interleukin-22 cytokines in tumors.
We further identified the source of interleukin-22 and found that it originated from a class of immune cells called ILC3. Interestingly, the surface of ILC3 does not express the pattern recognition receptor we just talked about, and it does not recognize Candida albicans.
So we further explored and found that Candida albicans further promotes the proliferation of intestinal epithelium and the occurrence of colorectal tumors by regulating the secretion of interleukin-7 by macrophages, which in turn can regulate the secretion of interleukin-22 by ILC3 cells.
Therefore, we currently have some preliminary answers about the occurrence of fungal and colorectal tumors.
First, fungi can induce the development of colorectal tumors by regulating host immunity, including inducing the production of inflammasomes, inducing differentiation of MDSC cells, inducing mechanisms such as ILC3 cells secreting interleukin-22.
In addition, due to the large size of the fungus, it can competitively bind with bacteria in the intestine, inhibiting the proliferation of probiotics, thereby promoting the occurrence and development of tumors.
Does the fungus have any clinical application value?
In 2019, the article published by Professor Yu Jun's team at the University of Chinese in Hong Kong in Gut also gave us a good answer. They conducted a validated cohort study and found that in tumors, fungi were abnormally increased, and found a combination of 14 fungi that could effectively indicate tumorigenesis and recurrence.
Therefore, we can also say that intestinal fungi can be the next generation of novel tumor markers.
In addition to being associated with the intestine, fungi are also closely related to many lung diseases.
We know that the lungs are in direct contact with the outside world through the respiratory tract, so in the healthy upper and lower respiratory tracts, there are many microorganisms, and fungi are also in them. In the upper respiratory tract, aspergillus is predominantly increased, while in the lower respiratory tract, Malassezia is predominantly.
Does fungi play a role in lung diseases? Related studies have also been reported.
In allergic bronchopulmonary aspergillosis, Aspergillus fumigatus can induce activation of RelB, a nonclassical pathway of the NF-κB pathway, through Card9, thereby secreting interleukin-5 and inducing a Th2-type immune response. That is, fungal and respiratory asthma diseases are closely related.
Finally, let's take a look at the detection methods of fungi.
At present, in the hospital, it still maintains a very traditional test, mainly including the smear of some specimens, microscopic observation, to find this hyphae and sporophytes.
In addition to this, there are new techniques, including the use of ELISA methods to detect antibody-related molecules of fungi.
The current genetic testing method and sequencing method provide us with a powerful means to conduct fungal research. Through 18S, ITS2 and ITS1 detections, we can systematically study a certain kind of bacteria and a certain type of bacteria in the fungus, so as to conduct more in-depth research.
At present, in the research on fungi, we only know the fur, for example, whether the fungus is related to immunotherapy? What is the interaction between fungi and bacteria? All this is expected of us more research.
Let's pay attention to intestinal fungi together and protect intestinal health, thank you!