The innate immune system (and complement system) Flashcards
The innate immune system is comprised of two lines of defence, which?
The innate immune system consists of barriers (both physiological and chemical) and cellular responses (when the barrier is breached).
The anatomical barriers make up the first line of defence. Name three key barrier organs/tissues and explain how they protect against pathogens.
Epithelial layers of the skin: physical barrier hindering pathogen entry, production of antimicrobial proteins like psoriasin.
Mucosal tissues (e.g., gastrointestinal, respiratory, and urogenital tracts): Mucus entrapment and preventing pathogen from reaching epithelium, cilia sweeping outwards (airway), low ph (urogenital) and overall production of antimicrobial like lacroferrin that limits growth of bacteria and fungi, disrupts microbial membranes and limits effectivity of some viruses.
Glandular tissues (e.g., salivary, lacrimal, and mammary glands): Flushing by secretions and mucus outwards and containing antimicrobial peptides and proteins like lysozymesthat cleaves glycosidic bonds in peptidoglycan of bacterial cells walls, leading to lysis.
What are the general characteristics of antimicrobial peptides?
The general characteristics of antimicrobial peptides are:
- They’re ancient! Very well conserved.
- Less than 100 aa long
- Positively charged and have both hydrophilic and hydrophobic regions
- Interact with acidic phospholipids in lipid bilayers and forms pores wich disrupt membranes of microbes
- The peptides enter the microbes
and exert toxic effects
For example defensins and histatins.
Which cells are involved in the cellular response of the innate immune system?
Mainly the cells of the myeloid lineage: Neutrophils, Eosinophils, Basophils, Mast cells, Monocytes, Macrophages, Dendritic cells and the innate lymphoid cells (ILCs): mainly NK cells.
How does the innate cells recognize pathogens?
Via pattern recognition receptors, that bind PAMPs, motifs present on/in groups of pathogens: bacteria, fungi, parasites and viruses.
Upon PRR-PAMP interaction, signalling pathways are activated, contributing to innate/inflammatory responses
Which cells express pattern recognition receptors?
Basically all cells express at least some PRRs. They are of course expressed by all the cells in the innate imunnity but also in cells in tissues that are more exposed to infection than others, like epithelial cells, mucosal and glandular tissues and vascular endothelial cells that line the blood vessels to name a few. Cytosolic sensors of viral nucleic acids are expressed by most if not all cells in the body, important given that most cell types are susceptible to infection with viruses.
There are several families of innate receptors, which and what are their function in brief?
– Toll like receptors (TLRs): recognize a wide variety of PAMPs and are present both on plasma membrane and intracellular membranes. They induce production of antimicrobials, antivirals,
and cytokines and the inflammatory response.
– C-type lectin receptors (CLRs): A very heterogenous group of plasma membrane PRRs (not as similar as TLRs). CLRs are expressed on many different cell types including like APCs, neutrophils and subsets of B and T cells. CLRs recognize mainly cell wall components such as sugars/polysaccharides of bacteria/fungi but also carbohydrate components of viruses, parasites and some allergens. Most CLRs initiate signalling through dimerization activated phosphorylation of tyrosine residues in their cytoplasmic domains (ITAMs) or associated signalling chains and downstream effects are similar to those of TLRs.
– RIG-I-like receptors (RLRs): Members of this family bind viral dsRNA in the cytosol with their helicase domain, assembles into a tetramer and associate with proteins on mitochondria to trigger signalling. The signalling pathway lead to the activation of NF-κB and IRF3 and IRF7 - antiviral responses.
– NOD-like receptors (NLRs): Large and diverse family of cytosolic PRRs recognising intracellular PAMPs (like breakdown products of bacteria) and DAMPs (sensing stress). Upon ligand binding they dimerize and recruit RIP2 which signals and have the same downstream effects as endosomal TLRs. Some NLRs (like ND1 and NOD2) can also trigger autophagy and some can form the inflammasone.
Remember that cells can have several PRRs, so these processes can happen simultaneously depending on the pathogen.
How does TLRs recognize PAMPs?
TLRs share a structural element called leucine-rich repeats (LRRs) which are used to recognize and bind PAMPs. Upon PAMP binding, they are induced to dimerize (ligand-induced TLR dimerization) and their cytoplasmic regions convey the signal intracellularly.
Give three examples of TLRs and what they recognize.
- TLR1 recognizes triacyl lipopeptides on mycobacteria and gram-negative bacteria
- TLR4 recognizes LPS in gram negative bacteria (plasma membrane)
TLR2 recognizes peptidoglycans on gram-positive bacteria
- TLR6 recognizes diacyl lipopolypeptides on gram positive bacteria and mycobacteria (plasma membrane)
- TLR3 recognizes viral dsRNA
- TLR7/8 recognizes viral ssRNA
- TLR9 recognizes CpG unmethylated dinucleotides on bacterial DNA and Dinucleotides in some herpesviruses.
The downstream signalling of all TLRs share a key transcription factor, which and what does it do?
An important example of a shared downstream signalling component for all TLRs is the transcription factor NF-κB, which is of key importance for activating the expression of many innate and inflammatory genes.
Some pathways and components are activated only by some TLRs – specifically induced expression of genes that are important in fighting that particular type of microbe. Give one example.
One example is the expression of potent antiviral type I interferons, IFN-α and IFN-β, induced by pathways downstream of the TLRs that bind viral components.
There are two major pathways used in TLR signalling, which?
The MyD88 adaptor pathway and the TRIF pathway.
- The MyD88 pathway is used by all plasma membrane TLRs but also by some endosomal TLRs (TLR7 for example). This pathway is particularly important in defence mechanisms against extracellular pathogens.
- The TRIF pathway is commonly used by endosomal TLRs and activate transcription factors that lead to increased production of antiviral interferons.
Note: TLR4 is unique among TLRs in its ability to activate both signalling pathways, but it does so sequentially. When in the plasma membrane, it can signal through MyD88 and then TLR4 is only able to initiate the TRIF pathway from the endosomal compartment following endocytosis.
Explain the MyD88 adaptor pathway of TLR signaling in short.
MyD88 is an adaptor protein that bind the cytosolic TIR elements of dimerized TLRs. Upon TLR dimerization, MyD88 binds and initiates signalling through recruiting the IRAK1 and IRAK4 kinases, which phosphorylates components of the TAK1 complex. TAK1 phosphorylation activated MAP kinase pathways, which activate transcription factors such as AP-1 and the TAK1 phosphorylation also initates a series of events
leading to the degradation of the NFkB-inhibitor IkBNF-κB –> which releases NF-κB so it can go into the nucleus and act.
Both NF-κB and AP-1 are essential for activating key antimicrobial proteins and peptides as well as proinflammatory cytokines and chemokines that are of key importance in the innate immune response.
Explain the TRIF pathway of TLR signalling in short.
Upon endosomal TLR dimerization, the TRIF adaptor binds and recruits and activates TRAF3, which generates a scaffold that recruits and activates a kinase complex called the IKK complex which incudes TBK1 (TANK- binding kinase 1). TBK1 phosphorylates and activates IRF3 and IRF7, each of which dimerizes and enters the nucleus, inducing the transcription of the IFN-α and IFN-β genes which are crucial in the antiviral response.
Later in the response, TRIF also activates TRAF6, leading to NF-κB activation.
Does the endosomal TLRs use only one pathway for downstream signaling?
Some only use one, like TLR3 which uses TRIF, but some endosomal TLRs like TLR7, TLR8 and TLR9 all use both the MyD88 and the TRIF pathway simultaneously. In the endosome, MyD88- associated IRAK1 directly phosphorylates IRF7. This allows IRF7 dimerization, activation, nuclear translocation, and induction of IFN gene expression.
