Structural aspects of molecular recognition in the immune system. Part II: Pattern recognition receptors (IUPAC Technical Report)

Abstract The vertebrate immune system uses pattern recognition receptors (PRRs) to detect a large variety of molecular signatures (pathogen-associated molecular patterns, PAMPs) from a broad range of different invading pathogens. The PAMPs range in size from relatively small molecules, to others of intermediate size such as bacterial lipopolysaccharide, lipopeptides, and oligosaccharides, to macromolecules such as viral DNA, RNA, and pathogen-derived proteins such as flagellin. Underlying this functional diversity of PRRs is a surprisingly small number of structurally distinct protein folds that include leucine-rich repeats in Toll-like receptors (TLRs) and NOD-like receptors (NLRs), the DExH box helicase domain in RIG-like receptors (RLRs), and C-type lectin domains (CTLDs) in the C-type lectins. Following PAMP recognition by the PRRs, downstream signaling pathways activate the innate immune system to respond to invading pathogenic organisms. The resulting stimulatory response is also vital for a balanced adaptive immune response to the pathogen, mediated by circulating antibodies and/or cytotoxic T cells. However, an aberrant stimulation of the innate immune system can also lead to excessive inflammatory and toxic stress responses. Exciting opportunities are now arising for the design of small synthetic molecules that bind to PRRs and influence downstream signaling pathways. Such molecules can be useful tools to modulate immune responses, for example, as adjuvants to stimulate adaptive immune responses to a vaccine, or as therapeutic agents to dampen aberrant immune responses, such as inflammation. The design of agonists or antagonists of PRRs can now benefit from a surge in knowledge of the 3D structures of PRRs, many in complexes with their natural ligands. This review article describes recent progress in structural studies of PRRs (TLRs, NLRs, CTLs, and RLRs), which is required for an understanding of how they specifically recognize structurally diverse “foreign” PAMPs amongst a background of other “self” molecules, sometimes closely related in structure, that are present in the human body.

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Innate immunity and the pneumococcus

Microbiology ◽

10.1099/mic.0.28551-0 ◽

2006 ◽

Vol 152 (2) ◽

pp. 285-293 ◽

Cited By ~ 52

Author(s):

Gavin K. Paterson ◽

Tim J. Mitchell

Keyword(s):

Innate Immunity ◽

Immune System ◽

Streptococcus Pneumoniae ◽

Immune Responses ◽

Innate Immune System ◽

Innate Immune ◽

First Line ◽

Adaptive Immune Responses ◽

Adaptive Immune ◽

Made In

The innate immune system provides a non-specific first line of defence against microbes and is crucial both in the development and effector stages of subsequent adaptive immune responses. Consistent with its importance, study of the innate immune system is a broad and fast-moving field. Here we provide an overview of the recent key advances made in this area with relation to the important pathogen Streptococcus pneumoniae (the pneumococcus).

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Antibody equivalent molecules of the innate immune system: parallels between innate and adaptive immune proteins

Innate Immunity ◽

10.1177/1753425910370498 ◽

2010 ◽

Vol 16 (3) ◽

pp. 131-137 ◽

Cited By ~ 19

Author(s):

Nades Palaniyar

Keyword(s):

Pattern Recognition ◽

Immune System ◽

Innate Immune System ◽

Adaptive Immune System ◽

Surfactant Protein ◽

Innate Immune ◽

Future Research ◽

Surfactant Protein D ◽

Adaptive Immune ◽

Carbohydrate Arrays

Soluble pattern-recognition innate immune proteins functionally resemble the antibodies of the adaptive immune system. Two major families of such proteins are ficolins and collectins or collagenous lectins (e.g. mannose-binding lectin [MBL], surfactant proteins [SP-A and SP-D] and conglutinin). In general, subunits of ficolins and collectins recognize the carbohydrate arrays of their targets via globular trimeric carbohydrate-recognition domains (CRDs) whereas IgG, IgM and other antibody isotypes recognize proteins via dimeric antigen-binding domains (Fab). Considering the structure and functions of these proteins, ficolins and MBL are analogous to molecules with the complement activating functions of C1q and the target recognition ability of IgG. Although the structure of SP-A is similar to MBL, it does not activate the complement system. Surfactant protein-D and conglutinin could be considered as the collagenous non-complement activating giant IgMs of the innate immune system. Proteins such as peptidoglycan-recognition proteins, pentraxins and agglutinin gp-340/DMBT1 are also pattern-recognition proteins. These proteins may be considered as different isotypes of antibody-like molecules. Proteins such as defensins, cathelicidins and lactoferrins directly or indirectly alter microbes or microbial growth. These proteins may not be considered as antibodies of the innate immune system. Hence, ficolins and collectins could be considered as specialized ‘antibodies of the innate immune system’ instead of ‘ante-antibody’ innate immune molecules. The discovery, structure, functions and future research directions of many of these soluble proteins and receptors such as Toll-like and NOD-like receptors are discussed in this special issue of Innate Immunity.

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The innate immune system

Oxford Textbook of Medicine ◽

10.1093/med/9780198746690.003.0038 ◽

2020 ◽

pp. 307-314

Author(s):

Paul Bowness

Keyword(s):

Immune System ◽

Immune Responses ◽

Innate Immune System ◽

Cell Types ◽

Innate Immune ◽

Microbial Pathogens ◽

Innate Immune Responses ◽

Adaptive Immune ◽

System P ◽

Different Cell Types

The innate immune system comprises evolutionarily ancient mechanisms that mediate first-line responses against microbial pathogens, and are also important in priming and execution of adaptive immune responses, and in defence against tumours. These responses, which recognize microbial non-self, damaged self, and absent self, are characterized by rapidity of action and they involve various different cell types, cell-associated receptors, and soluble factors. Previously thought to lack plasticity or memory, certain innate immune responses have recently been shown to be capable of ‘learning’ or ‘training’. Most cells of the innate immune system are derived from the myeloid precursors in the bone marrow. These include monocytes and their derivatives—macrophages and dendritic cells, blood granulocytes (neutrophils, basophils, and eosinophils), and tissue mast cells.

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PRRs in pathogen recognition

Open Life Sciences ◽

10.2478/s11535-006-0024-4 ◽

2006 ◽

Vol 1 (3) ◽

pp. 299-313 ◽

Cited By ~ 2

Author(s):

Satoshi Uematsu ◽

Shizuo Akira

Keyword(s):

Pattern Recognition ◽

Immune Responses ◽

Pattern Recognition Receptors ◽

Innate Immune ◽

Innate Immune Responses ◽

Pathogen Recognition ◽

First Line ◽

Adaptive Immune Responses ◽

Adaptive Immune ◽

Wide Range

AbstractThe innate immune system provides the first line of host defense against invading microorganisms before the development of adaptive immune responses. Innate immune responses are initiated by germline-encoded pattern recognition receptors (PRRs), which recognize specific structures of microorganisms. Toll-like receptors (TLRs) are pattern-recognition receptors that sense a wide range of microorganisms, including bacteria, fungi, protozoa and viruses. TLRs exist either on the cell surface or in the lysosome/endosome compartment and induce innate immune responses. Recently, cytoplasmic PRRs have been identified which detect pathogens that have invaded the cytosol. This review focuses on the pathogen recognition of PRRs in innate immunity.

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Tweaking Innate Immunity: The Promise of Innate Immunologicals as Anti-Infectives

Canadian Journal of Infectious Diseases and Medical Microbiology ◽

10.1155/2006/195957 ◽

2006 ◽

Vol 17 (5) ◽

pp. 307-314 ◽

Cited By ~ 8

Author(s):

Kenneth L Rosenthal

Keyword(s):

Innate Immunity ◽

Immune System ◽

Immune Responses ◽

Innate Immune System ◽

Immune Defense ◽

Innate Immune ◽

Parasitic Infections ◽

Type I ◽

Adaptive Immune Responses ◽

Adaptive Immune

New and exciting insights into the importance of the innate immune system are revolutionizing our understanding of immune defense against infections, pathogenesis, and the treatment and prevention of infectious diseases. The innate immune system uses multiple families of germline-encoded pattern recognition receptors (PRRs) to detect infection and trigger a variety of antimicrobial defense mechanisms. PRRs are evolutionarily highly conserved and serve to detect infection by recognizing pathogen-associated molecular patterns that are unique to microorganisms and essential for their survival. Toll-like receptors (TLRs) are transmembrane signalling receptors that activate gene expression programs that result in the production of proinflammatory cytokines and chemokines, type I interferons and antimicrobial factors. Furthermore, TLR activation facilitates and guides activation of adaptive immune responses through the activation of dendritic cells. TLRs are localized on the cell surface and in endosomal/lysosomal compartments, where they detect bacterial and viral infections. In contrast, nucleotide-binding oligomerization domain proteins and RNA helicases are located in the cell cytoplasm, where they serve as intracellular PRRs to detect cytoplasmic infections, particularly viruses. Due to their ability to enhance innate immune responses, novel strategies to use ligands, synthetic agonists or antagonists of PRRs (also known as 'innate immunologicals') can be used as stand-alone agents to provide immediate protection or treatment against bacterial, viral or parasitic infections. Furthermore, the newly appreciated importance of innate immunity in initiating and shaping adaptive immune responses is contributing to our understanding of vaccine adjuvants and promises to lead to improved next-generation vaccines.

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Pattern Recognition Receptors in Cancer Progression and Metastasis

Cancer Growth and Metastasis ◽

10.4137/cgm.s24314 ◽

2015 ◽

Vol 8 ◽

pp. CGM.S24314 ◽

Cited By ~ 35

Author(s):

Sanjay Pandey ◽

Saurabh Singh ◽

Vandana Anang ◽

Anant N. Bhatt ◽

K. Natarajan ◽

...

Keyword(s):

Pattern Recognition ◽

Immune System ◽

Tumor Cell ◽

Cancer Progression ◽

Innate Immune System ◽

Pattern Recognition Receptors ◽

Innate Immune ◽

Preventive Strategy ◽

Chronic Activation

The innate immune system is an integral component of the inflammatory response to pathophysiological stimuli. Toll-like receptors (TLRs) and inflammasomes are the major sensors and pattern recognition receptors (PRRs) of the innate immune system that activate stimulus (signal)-specific proinflammatory responses. Chronic activation of PRRs has been found to be associated with the aggressiveness of various cancers and poor prognosis. Involvement of PRRs was earlier considered to be limited to infection- and injury-driven carcinogenesis, where they are activated by pathogenic ligands. With the recognition of damage-associated molecular patterns (DAMPs) as ligands of PRRs, the role of PRRs in carcinogenesis has also been implicated in other non-pathogen-driven neoplasms. Dying (apoptotic or necrotic) cells shed a plethora of DAMPs causing persistent activation of PRRs, leading to chronic inflammation and carcinogenesis. Such chronic activation of TLRs promotes tumor cell proliferation and enhances tumor cell invasion and metastasis by regulating pro-inflammatory cytokines, metalloproteinases, and integrins. Due to the decisive role of PRRs in carcinogenesis, targeting PRRs appears to be an effective cancer-preventive strategy. This review provides a brief account on the association of PRRs with various cancers and their role in carcinogenesis.

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Innate Immune Responses to Influenza Virus Infections in the Upper Respiratory Tract

Viruses ◽

10.3390/v13102090 ◽

2021 ◽

Vol 13 (10) ◽

pp. 2090

Author(s):

Edin J. Mifsud ◽

Miku Kuba ◽

Ian G. Barr

Keyword(s):

Influenza Virus ◽

Immune System ◽

Epithelial Cells ◽

Immune Responses ◽

Respiratory Tract ◽

Innate Immune System ◽

Innate Immune ◽

Upper Respiratory Tract ◽

Adaptive Immune ◽

Upper Respiratory

The innate immune system is the host’s first line of immune defence against any invading pathogen. To establish an infection in a human host the influenza virus must replicate in epithelial cells of the upper respiratory tract. However, there are several innate immune mechanisms in place to stop the virus from reaching epithelial cells. In addition to limiting viral replication and dissemination, the innate immune system also activates the adaptive immune system leading to viral clearance, enabling the respiratory system to return to normal homeostasis. However, an overzealous innate immune system or adaptive immune response can be associated with immunopathology and aid secondary bacterial infections of the lower respiratory tract leading to pneumonia. In this review, we discuss the mechanisms utilised by the innate immune system to limit influenza virus replication and the damage caused by influenza viruses on the respiratory tissues and how these very same protective immune responses can cause immunopathology.

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