Learn the antiviral mechanism of bats, maybe one day humans can also be invincible?

#头条创作挑战赛 #

More than 1,400 species of bats have been discovered, making it the second most diverse mammal group on Earth. They live on every continent except Antarctica and carry many different types of viruses, and studies have shown that a bat can carry nearly 100 viruses, more than 60 of which are zoonotic infectious diseases that can be transmitted directly or indirectly to humans. Bats carry so many pathogenic viruses without showing any adverse symptoms, how do bats do it? If we can understand how bats' immune systems protect them from viral infections, we may find clues that help humans better fight viral infections.

The team led by Professor Wang Linfa, director of the Institute for Emerging Infectious Diseases at Duke-NUS School of Medicine, used malacca virus (PRV3M, a double-stranded RNA virus) to infect bats, and found that the virus can replicate in bat lungs without causing symptoms in bats, so they used single-cell transcriptome sequencing (scRNA-seq) to analyze the immune response of bat lungs after infection with Malacca virus, and found that immune cells in bat lungs have a very unique response. It provides us with a promising clue to the antiviral ability of bats, and the results were published in the journal Immunology, a well-known immunology journal.

The main findings of the study are:

1. White blood cells are the second line of defense against microbial invasion and are an important part of the immune system. Among them, neutrophils have strong chemotaxis and phagocytosis function, and play a very important role in resisting bacterial infection and protecting the body. However, little is known about the role of neutrophils in viral infections. This study found that high levels of IDO1 were expressed in neutrophils in bat lung after viral infection, and previous studies have found that abnormally high expression of indolamine 2,3-dioxygenase 1 (IDO1) is strongly associated with tumor immune evasion and is an important target of tumor immunotherapy. Therefore, the scientists believe that the expression of IDO1 in neutrophils in bat lungs may play an important role in limiting inflammation after viral infection.

2. After viral infection, the transcriptome of bat lung monocytes and alveolar macrophages underwent extensive changes, and the changed genes were enriched in antivirus-related functional pathways, indicating that monocytes and alveolar macrophages played an important role in bats fighting viruses.

3. Natural killer cells (NK) are important immune cells of the body, an important part of the antiviral response in innate immunity, and are related to the survival of virus-infected cells. The NK cells of bats show a very different immune response from other mammals, that is, NK cells are widely activated after viral infection and produce antigen-presenting chemokines to activate the body's anti-infection activity.

The virus can replicate in bats without causing symptoms

Infecting 4 cave nectar bats with Malacca virus through intranasal infection, the researchers detected viral excretion in both the mouth and rectum of bats, and the amount of virus excreted in the mouth of 4/4 bats and the rectum of 3/4 bats showed an upward trend between 1 dpi (1 day after infection) and 3 dpi, indicating that the virus can replicate in bats. Both detection routes found that the peak of virus excretion in 4 bats occurred at 3 ~ 5 dpi, but during the entire viral infection process, scientists did not find that bats had adverse symptoms, such as lethargy, activity restriction and runny nose.

Using another batch of bats to model, and then 2dpi and 4dpi respectively to sacrifice some bats, autopsy found that viral RNA is most common in bats' lungs, small intestine and large intestine, and 4dpi lung tissue virus content is higher than 2dpi, 4 dpi lung tissue immunohistochemical (IHC) staining also found viral antigen staining positive small lung cell foci, indicating that the virus can replicate in the lungs of bats, which has aroused the interest of scientists, bats are known to carry a variety of coronaviruses, And a variety of coronaviruses, including SARS-Cov-2, which is still circulating, can invade the respiratory system of the human body, causing pneumonia and other symptoms, how do bats resist the virus infection and pneumonia? Scientists thought that the booming single-cell sequencing technology in recent years might answer this question.

Immune cell homology is high in bats and human lungs

Single-cell level transcriptome sequencing was performed on lung tissues of uninfected and virus-infected bats by single-cell sequencing, followed by annotation of bat lung cells such as CD247, GIMAP7, TCRAVL and TCRBVL-labeled NK cells, CD79B, MS4A1, and FCMR-labeled B cells, and MRC1, CD163, and MARCO-labeled alveolar macrophages by typical Marker genes. By combining previously reported human lung single-cell sequencing data, human lung immune cell annotation of the bat scRNA-seq transcriptome using the SingleR algorithm, and scoring the transcriptional similarity between bats and human lung immune cells using the AddModuleScore function on Seurat, it was found that the myeloid and lymphoid cell types of bats were mapped to human myeloid and lymphocyte cell types, respectively, while in the Seurat score, classical monocytes, macrophages, pDC (plasmacytic-like dendritic cells) and B cells have high homology scores between humans and bats, indicating that the transcription gene tags of these immune subsets between humans and bats are highly conserved.

IDO1-mediated kynurenine-AHR axis is a potential antiviral pathway

Scientists have found high expression of IDO1 in bat neutrophils, which are specifically expressed in bat neutrophils, and previous studies have confirmed that this enzyme plays a role in mediating immune tolerance, and in humans, IDO1 is expressed in subsets of antigen-presenting cells and regulated by the tissue microenvironment and immune activation state, although no previous studies have explored the role of IDO1 in neutrophils. Further analysis of the most significant batch of differentially expressed genes in the bat neutrophil cluster identified two other genes, ALAS1 and SLC16A10, associated with tryptophan degradation, but neither gene expression was observed in neutrophils from human, mouse, or porcine lung tissue.

ALAS1 (aminovalerate synthetase) is a nuclear-encoded mitochondrial enzyme responsible for the first and rate-limiting steps in the mammalian heme biosynthesis pathway. SLC16A10 is a membrane transporter responsible for the transport of aromatic amino acids such as tryptophan. These two genes, together with IDO1, are involved in the tryptophan-kynurenine metabolic pathway (TRP-KYN), and kynurenine and its metabolites can act as potent agonists of aromatic hydrocarbon receptors (AHRs) to regulate gene expression. Therefore, IDO1 can achieve gene regulation through the kynurenine-AHR axis. These findings suggest that IDO1-mediated tryptophan degradation produces kynurenine as a potent immunomodulator, and that the role of the kynurenine-AHR axis in regulating inflammation (e.g., limiting excessive inflammation in bats) warrants further study.

Macrophages and classical monocytes play a key role in the immune defense of bats

The authors next examined changes in lung immune cell composition following viral infection in bats, with the largest increase in the proportion of COL+ myeloid cells of the 14 immune cell types identified, with an increase of about 300% post-infection, and despite relatively limited changes in overall immune cell composition, extensive transcriptional responses were produced in each immune cell type, particularly antiviral gene tags observed from myeloid cell subsets. Enrichment analysis revealed significant enrichment of genes for "cellular response to type I interferon", "response to interferon γ", and "response to virus", with typical interferon-stimulating genes including MX1, MX2, IFIT2, IFIT3, and IRF7 showing gene-specific induction patterns in different cell types, but they were induced primarily in myeloid cells, particularly from macrophages and classical monocytes. Thus, although there are no significant pathological or symptomatic changes after infection in vivo, we show that bat lung immune cell types produce a robust transcriptional response, in which macrophages and classical monocytes play a key role in the bat's innate immune defense against viral infection.

T cells and NK cells are widely activated in response to infection

The authors examined the transcriptional response after infection with the bat NK & T cell cluster. Genes associated with T cell activation, including IER2, nuclear receptor NR4A1, lysine demethylase KDM68, are expressed increased in post-infection NK and T cell clusters. The authors also observed evidence of altered effector cell function at the time of infection, and in bat NK & T cell clusters, particularly in the CD8+ effector NK & T cell population, the expression levels of cytotoxic granulin NKG7 showed very significant changes before and after infection. In humans, NKG7 is highly expressed on NK cells and some CD8+ T cells, and expression is further increased under IL-2 stimulation. At the same time, the authors observed the expansion of NK & T cells expressing XCL1. In humans, XCL1 produced by activated CD8+ T cells and NK cells is a potent antigen-presenting chemokine and therefore plays an important role in the adaptive immune response to viral infection. These findings provide valuable information for us to target the activation of immunity.

Summary and outlook

Scientists used bat virus infection models and single-cell sequencing to explore why bats can not develop dysfunction in the case of virus replication, and we found that the immune system of bats plays an antiviral role efficiently and accurately, which is sufficient to resist the harm of the virus and does not cause excessive immune response to cause inflammatory storms, and the discovery of kynurenine-AHR axis and targets such as NKG7 and XCL1 also provides valuable clues for further research.