Advances in Immunology: 77
Numerous studies have shown that plant polysaccharides can regulate the immune system in multiple ways and at multiple levels. They not only activate immune cells, including T cells, B lymphocytes, macrophages, and natural killer cells, but they also activate complements and promote the production of cytokines, thus showing regulatory effects on the immune system in multiple ways 25 , 31 — Innate immune regulation has an important impact on the host's ability to respond rapidly to pathogens.
As important members of the immune defense system of the host, macrophages can collaborate with other types of cells such as neutrophils to resist external adverse factors 35 — The immunomodulatory effects of plant polysaccharides on macrophages are mainly achieved through the generation of reactive oxygen species ROS , the secretion of cytokines, cell proliferation, and the phagocytic activity of macrophages NO is an important biologically active substance.
In addition to being an important signal transducer in the central nervous system, it is widely involved in the physiological and pathological processes of multiple systems, including immune responses, and inflammatory reactions 26 , 32 , 41 — In the immune system, when macrophages are stimulated and activated, a large amount of nitric oxide NO is released, which can kill microorganisms, parasites, and tumor cells; it can also induce inflammatory reactions and protect the body from external adverse factors A study by Luo et al.
Polysaccharides extracted from Pterospartum tridentatum L. Cytokines play an important role in the regulation of cell-cell interactions, cell growth, and cell differentiation. They play important roles in the regulation of inflammatory responses and immune responses, and they have regulatory effects on both innate immunity and adaptive immunity Thus, plant polysaccharides can simultaneously regulate the expression of pro- and anti-inflammatory cytokines IL acts as a negative feedback factor to prevent the over-activation of macrophages in the hyper-inflammatory response , which suggests that the body has a self-regulatory mechanism to maintain an equilibrium state Kouakou et al.
The synergistic effect of cytokines and plant polysaccharides on macrophages can also activate the immune response of macrophages. Im et al. In addition, the synergistic effect caused changes in the differentiation antigens CD11b, CD18, and CD24, which are expressed by macrophages The study of mechanisms of immunoregulation by plant polysaccharides has reached the molecule and receptor level.
Most research has shown that plant polysaccharides activate macrophages by recognizing and binding to specific receptors on the surfaces of macrophages, which initiates the immune response and exerts an immunomodulatory effect. These macrophage receptors are called pattern recognition molecules. The activation of macrophage receptors can initiate a series of intracellular signaling cascades, leading to the transcriptional activation and production of inflammation-related cytokines 56 — 60 , as shown in the Figure 1 Figure 1. Signal transduction pathway related to the immune-regulation of botanical polysaccharides in macrophage activation.
These receptors can work separately, and some different receptor types can cooperate with each other forming clusters of signaling complexes. Ultimately, these activating factors enter the nucleus cause induction of gene transcription. TLR4 is a very important class of membrane receptors that are expressed on the surfaces of macrophages and mediate the activation of macrophages by converting extracellular signals 61 — The binding of TLR4 to the ligand results in the formation of a complex between the TLR cytoplasmic domain, the adaptor protein myeloid differentiation primary response gene 88 MyD88 , and the interleukin-1 receptor-associated kinase IRAK , which in turn activates tumor necrosis factor receptor-associated factor 6 TRAF6 and finally initiates the mitogen-activated protein kinase MAPK signaling pathway 65 — Because activator protein 1 AP-1 and MAPK are important downstream signaling molecules that mediate the activation of macrophages and the expression of inflammatory genes, T.
Yeo Dae Yoon et al. When the induction effect of PG was detected after pretreatment of macrophages with specific inhibitors of these three kinases, PG did not show an activation effect, and the DNA binding activity of AP-1 was inhibited by the MAPK inhibitors. Wang's research showed that HRWP-A could recover the body condition and activated macrophage in Cyclophosphamide CTX -induced immunosuppressed mice which may depended on mouse peritoneal macrophages.
Studies have shown that many different receptors work together through pairing and cooperation.
CD14 is known to be a high-affinity receptor for LPS-activated macrophages. Both are involved in the immune response to plant polysaccharides. CD14 antibody and CR3 antibody significantly reduced the amounts of NO released in PG-induced macrophages, suggesting that the surfaces of these two types of molecules may have binding sites for plant polysaccharides Plant polysaccharides also affect the expression of receptors on the macrophage surface. For example, Panax ginseng polysaccharide significantly upregulates the expression of CD14 in mouse peritoneal macrophages and downregulates the expression of CR3 at the same time Scavenger receptors have multiple ligands, so these receptors play important roles in pathogen clearance, host defense, and the signal transduction of macrophages PKC is a protein kinase that is commonly expressed in mammals.
Nakamura et al.
The mannose receptor is an important member of the C-type lectin-like receptor family. It can recognize glycosylated molecules that contain a mannose residue, a trehalose group, or an N-acetyl glucose residue Many mannose receptors are expressed on the surfaces of alveolar macrophages, peritoneal macrophages, and mononuclear macrophages in blood, and they play an important role in the early immune response. Dectin-1 is a surface receptor that is expressed in immune cells to defend against fungal pathogens.
It is a type II membrane protein with a molecular weight of 28 kDa.
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Dectin-1 can also form a signaling complex with TLR2 to transmit the stimulatory signals of macrophages. During the signal transduction, the expression of Dectin-1 in the activated macrophages is significantly increased compared with that found in cells in a resting state In addition to interacting with receptors to activate macrophages, plant polysaccharides can reach the interior of macrophages via phagocytosis.
Unlike starch and glycogen, the phagocytosed plant polysaccharide molecules are not easily digested completely; therefore, the incompletely degraded polysaccharides can activate macrophages as a co-stimulatory signal Although multiple signaling pathways are involved in the activation of macrophages, almost all such signals are ultimately attributed to the expression of inflammation-related cytokines and inducible nitric oxide synthase iNOS.
It mainly plays a role in regulating cell proliferation and apoptosis and immune and inflammatory responses, and it regulates the transcription of a variety of pro-inflammatory genes. The developments in mass spectrometry, GC-MS, X-ray fiber diffraction, nuclear magnetic resonance, electron diffraction, and other analytical techniques have made it possible to obtain structural information about plant polysaccharides and their mechanisms.
However, there are still many unresolved issues. First, the relationship between the structure and function of plant polysaccharides is still not fully understood. The activities of polysaccharides originating from the same plant are also very different, suggesting that the structures of polysaccharides are directly related to the biological activities However, due to the complex structure and wide variety of polysaccharides, the isolation, purification, and structure determination of polysaccharides are very difficult. Therefore, the structural characteristics of most of the reported active polysaccharides remain unclear.
Second, a large number of inflammatory mediators released in macrophages after activation by plant polysaccharides may cause excessive inflammation in the body. The appropriate amount of NO released by macrophages can exert beneficial functions and protect the body from adverse factors. If the macrophages receive continuous stimulation and release excessive NO, this may lead to sepsis and local or systemic inflammatory disorders. In particular, excessive inflammation is seriously harmful to human health.
A large amount of experimental and clinical evidence indicates that long-term inflammation can increase the risk of cancer because a variety of inflammatory mediators can directly regulate the expression of oncogenes. In addition, key inflammation-related enzymes such as iNOS and COX-2 are associated with cancer induced by long-term inflammation 86 , Therefore, it is important to continue studying whether the pro- and anti-inflammatory factors induced by plant polysaccharides can maintain equilibrium in the body without causing harm due to excessive activation.
The impact of this factor should be considered in clinical applications.
Volume 145. Advances in Immunology in China - Part B
Third, the problem of endotoxin contamination in active plant polysaccharides needs to be excluded. However, studies have shown that polymyxin B has different levels of impact on iNOS expression and cytokine production in addition to inhibiting LPS activity Therefore, this detection method is also controversial. Finding safer and more effective methods for the detection of endotoxin contamination is another urgent problem to be solved.
Fourth, plant polysaccharides and microorganism-derived polysaccharides can bind to the same receptors on the surfaces of macrophages and induce similar immune responses, suggesting that polysaccharides from different sources may be highly conserved throughout evolutionary history. Therefore, we speculate that the response of macrophages to plant polysaccharides may mimic their natural immune responses to microbial pathogens. Although plant polysaccharides can bind to different receptors on the surfaces of macrophages, they activate similar signaling pathways.
Does this mean that there is coordination or crosstalk between the signals transmitted by different receptors? These problems need to be further investigated.
With the rapid development of biology and medicine, the study of plant polysaccharides is receiving more extensive attention. Although there are still many unsolved puzzles in the field, the clinical value and broad application prospects of plant polysaccharides make them an important direction for the development of new drugs. A variety of plant polysaccharides have been used as drugs or as adjuvants to enhance drug efficacy or reduce side effects. As research progresses, the immunomodulatory mechanisms of active plant polysaccharides will be explained more clearly, and more active plant polysaccharides will be developed and utilized.
All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication. MY and HL wrote the manuscript together. YZ edited the manuscript. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The role of salinity in seed maturation of the euhalophyte Suaeda salsa. Plant Biosyst. For topics on particular articles, maintain the dialogue through the usual channels with your editor.
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Year International Collaboration Show this widget in your own website. Begell House. The great advances in immunology in recent years make this field one of the most rapidly growing in biological sciences. This remarkable growth is stimulated by the influx of investigators from other disciplines such as biochemistry, genetics, molecular biology, and by an increased number of investigators who came to immunology through the more traditional routes of microbiology and various medical disciplines.
Sampson , Roni T. Falk , Catherine Schairer , Steven C. Moore , Barbara J. Fuhrman , Cher M. Dallal , Douglas C. Bauer , Joanne F. Brinton , Mitchell H. Gail , Regina G. Ziegler , Xia Xu , Robert N. Hoover and Gretchen L. Therapeutics, Targets, and Chemical Biology.
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