Immune recognition refers the process that immune system recognizes antigenic substances, especially recognition of oneself and non-self. The recognition of foreign substances such as pathogen associated pattern molecules and antigens by immune cells is the basis for the body's immune response to foreign substances. The immune response in your body includes two types: innate immune response and adaptive immune response. Both of them must be triggered by immune recognition. But there are many differences between the immune recognition of innate and adaptive immune response. Their recognition features are shown on the table 1.
Table 1. Recognition features of innate immunity and adaptive immunity
Recognition features | Innate immunity | Adaptive immunity |
---|---|---|
Identify antigen types and target molecular structure | Usually identify the components of microorganisms and their products (PAMP), and DAMP | Can recognize both microbial molecular antigens and non-microbial antigens |
Specificity of recognition | Pan-specific | Highly specific (recognize different kinds of microorganisms and different epitopes of microorganisms) |
Recognize Receptor Gene | Code in germline | Rearrangement during ontogeny (BCR somatic gene mutation) |
Recognition receptor distribution | Non-cloning | Cloning |
The fundamental principle of immune recognition is the interaction between biomolecules, or molecular recognition, including pattern recognition receptors (PPRs) and pathogen associated molecular patterns (PAMPs), cytokines and cytokine receptors, The mutual recognition between lymphocyte antigen receptors (TCR, BCR) and specific antigen epitopes (or MHC-antigen peptide complexes) as well as between antigens and antibodies. Here, we focus on the pattern recognition receptors.
Pattern Recognition Receptors (PRRs) are a class of receptors that can directly recognize the specific molecular structures on the surface of pathogens, apoptotic host cells, and damaged senescent cells [1]. PRRs play a crucial in the initiation of innate immune response by detecting potential harmful pathogens. The PRRs are divided into four families, including toll-like receptors (TLR), nucleotide-binding oligomerization domain-like receptors (NLR), C-type lectin receptors (CLR) and RIG-1 like receptors (RLR).
These receptors are mainly present at the antigen presenting cells surface to recognize extracellular pathogens, in the endosomes where they sense intracellular invaders such as viruses and finally in the cytoplasm. But they are also found in other immune and non-immune cells. These receptors recognize conserved molecular structures of pathogens. These motifs called pathogen- or microbe-associated molecular patterns (PAMPs or MAMPs) are usually specific to the microorganism and essential for its viability. PAMPs that have been identified so far are proteins, nucleic acids or glycans.
Innate immune system is highly conserved among species from plants to mammals and offers the first line of host defense against invading pathogens. Upon microbial infection, innate immune system recognizes the invasion of microbes by pattern-recognition receptors (PRRs). The figure 1 shows that three classes of PRR families (TLRs, RLRs and NLRs) sense microbial products or endogenous danger signals to trigger downstream signaling pathways, resulting in the production of type I IFN, proinflammatory cytokines and the activation of caspase-1.
Figure 1. Pattern-recognition receptor (PRR) signaling pathways.
* This picture is derived from reference 2.
As you see, the three PRR families differ in their ligand recognition, signal transduction and sub-cellular localization. Toll-like receptors (TLRs) recognize endogenous and exogenous danger signals, consist of an extracellular domain containing leucine-rich repeats (LRRs) for ligand binding and a cytoplasmic Toll/IL-1 receptor domain that links to adapter proteins and complex signaling pathways. Nucleotide oligomerization domain (NOD)-like receptors are a family of 22 proteins that contain LRRs for potential ligand binding, a NOD, and a caspase activation and recruitment domain (CARD), Pyrin domain, or a baculovirus inhibitor of apoptosis repeat (BIR) domain for initiation of signaling. Retinoic acid-inducible gene (RIG)-like receptors consist of two N-terminal CARDs for signaling and an RNA helicase domain [3].
PRRs are widely expressed in a variety of tumor tissues, such as colon cancer, lung cancer, breast cancer and gastric cancer [4] [5]. The activation of PRRs on the surface of tumor cells can induce the expression of a large number of cytokines, chemokines, hormones, and vascular-promoting factors, which is one of the important factors to induce the formation of tumor inflammatory microenvironment and promote the development of tumor. At the same time, the activation of PRRs on immune cells can induce antigen-presenting cells including DCs, tumor-associated macrophages, and B cells to activate tumor-specific T cell responses or enhance the antitumor effects of phagocytes. These also indicate that the role of PRRs in immunotherapy against tumors is very important and might represent a new strategy for patients with tumors. In addition, accumulating studies also demonstrated that PRRs play an important role in respiratory diseases, nervous disease, digestive diseases and more.
*Click the target to see all the related research reagents of them.
References
[1] Danyang Li and Minghua Wu. Pattern recognition receptors in health and diseases [J]. Signal Transduct Target Ther. 2021; 6: 291.
[2] Sujeong Hong, Sangjun Park and Je-Wook Yu. Pyrin Domain (PYD)-containing Inflammasome in Innate Immunity [J]. Journal of Bacteriology and Virology. 2011. Vol. 41, No. 3 p.133 – 146.
[3] Douglas R.McDonaldOferLevy. 3 - Innate immunity. Clinical Immunology (Fourth Edition). 2013, Pages 35-46.
[4] Żeromski J, et al. Significance and role of pattern recognition receptors in malignancy [J]. Arch. Immunol. Ther. Exp. 2019; 67: 133–141.
[5] Hopcraft SE, Damania B. Tumour viruses and innate immunity [J]. Philos. Trans. R. Soc. Lond. B Biol. Sci. 2017; 372: 20160267.
Related Diseases
Relate Signaling Pathways