Mammalian red blood cells are also involved in immunity: Toll-like receptor 9 binds CpG to promote inflammation and activate innate immunity

Red blood cells (RBC) occupies most of the circulating cells in mammals, and are mainly responsible for transporting oxygen to distant cell tissues and are an important part of aerobic respiration

The key mediator molecule TLR9 is expressed on the surface of red blood cells

The evolutionarily conserved nucleic acid sensor Toll-like receptor (TLR) can recognize nucleic acids from pathogens and itself, and by promoting the secretion of inflammatory cytokines, the maturation and proliferation of immune cells, play a central role in the inflammatory response

Previous studies by the Metthew Lam team have shown that red blood cells express intracellular TLR9 under steady-state conditions and clear cell-free (including CpG) mitochondrial DNA (cf-mtDNA)

During sepsis, TLR9 on the surface of red blood cells increases, and red blood cells bind to pathogen DNA.

Immunostimulatory unmethylated CpG motif is a feature of microbial DNA

Mitochondrial DNA containing CpG is a ligand for TLR9 and is elevated in the circulation during sepsis, which is a fatal syndrome defined by the host's dysregulated response to infection

Animal experiments on sepsis mouse models, bacterial pneumonia mouse models, and mouse models of systemic parasites (Legionella pneumophila and Toxoplasma gondii) have shown that mtDNA on red blood cells is increased compared with plasma, indicating that it is in pneumonia, parasites During insect infection and various microbial infections, mtDNA containing CpG is isolated on red blood cells

CPG DNA binding leads to changes in the structure and function of red blood cells

Since changes in red blood cell morphology are a common feature of sepsis and major diseases, the researchers analyzed the effect of excessive free cell CpG on the morphology and function of red blood cells, and used another GpC DNA that binds to TLR9 without causing TLR9 activation or conformational changes.

The binding of red blood cells to CpG results in undetectable CD47 antiphagocytic epitopes.

CpG-bound red blood cells trigger red blood cell phagocytosis to accelerate and initiate innate immune response CpG-induced inflammation depends on RBC-TLR9

The conformational change of CD47 causes red pulp F4/80-positive splenic macrophages (RPM, red pulp F4/80-positive splenic macrophages) to accelerate the phagocytosis of red blood cells

In order to distinguish the role of CpG red blood cells in initiating systemic immunity, the researchers used a simplified reduction model: perfused mice with CpG-carrying red blood cells and analyzed and compared splenic tissue changes

In order to understand the role of red blood cell TLR9 in the innate immune response in vivo, the researchers also customized model mice: red blood cell TLR9 KO mice (Ery tlr9-/- mice (Ery tlr9-/-), red blood cells do not express TLR9

discuss

This study determined the new role of red blood cells in the immune response to infection

Humans produce more than 2 million red blood cells per second, and face the risk of exposure to large amounts of DNA during mitochondrial autophagy and nuclear excretion

The increase in red blood cell-bound mtDNA in COVID-19 patients is related to the severity of the patient's disease and anemia.
These results emphasize the need for further research on red blood cell immune function, which is essential for a comprehensive understanding of the innate immune response to pathogens and sterile damage
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If the CpG transmission of red blood cells drives the production of IL-6 in various inflammatory diseases, then targeting RBC-TLR9 may be an effective way to treat cytokine storms without the immunosuppression associated with monoclonal anti-cytokine antibody therapy
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Alternatively, red blood cell-mediated CpG delivery can be utilized in the development of vaccines and immunotherapy
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This study has several limitations
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Further in vivo studies are needed to fully understand how red blood cell-TLR9 regulates the innate inflammatory response after red blood cell phagocytosis
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Although the masking and morphological changes of CD47 after CpG binding are observed, other mechanisms may also help accelerate the phagocytosis of red blood cells during sepsis
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Analysis of red blood cell phagocytes in the spleen, liver, and bone marrow at the single-cell level is necessary to determine the exact mechanism of CpG-carrying red blood cells for innate immune regulation
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In addition, although cytoplasmic signaling in red blood cells after CpG binding was not observed, the functional and morphological changes through TLR9 may be the result of indirect signaling events on the red blood cell membrane
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Further in-depth structural analysis of RBC-TLR9 is very important for the development of specific inhibitors or therapies for RBC-TLR9
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The author believes that in the resting state, red blood cells use TLR9 expressed on the cell surface as a specific DNA sensor to help promote the removal of trace amounts of CpG to prevent non-specific inflammation
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However, under conditions of excessive circulating CpG, such as sepsis and COVID-19, the combination of RBC-TLR9 and CpG can lead to accelerated red blood cell clearance and inflammation
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This innate immune mechanism may facilitate the elimination of damaged red blood cells, and may lead to the development of systemic inflammation and anemia in a pathological state with elevated free DNA
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Red blood cells use TLR9 to recognize CpG DNA, which provides real evidence for red blood cells to act as immune sentinels
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Research in the past two decades has gradually confirmed the role of enucleated cell platelets in innate immunity and adaptive immunity, which is consistent with the results of studies confirming that red blood cells have immune functions
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Therefore, it is necessary to further study the cooperation of platelets, red blood cells and blood coagulation in the innate immune response in order to truly understand the innate immune mechanism in the blood
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