IMI5: Immune responses against extracellular pathogens Flashcards
Main mediators of innate immunity are?
barrier functions complement system pattern recognition receptors phagocytes granulocytes NK cells coagulation system
Where do 75% of immune cells reside?
mucosal immune system
The commensal flora are collectively known as?
The human microbiome
Bacteria can secrete antibiotics or antimicrobial peptides called ________ which can work as a first line defence against an invader
bacteriocins
Give two examples of harmless bacteria that can become pathogenic
1) The infection by the commensal microorganism Staphylococcus epidermidis after a break in the skin from a cut
2) Pseudomonas aeruginosa, which most of us can fight effectively, can infect the airways, urinary tract and wounds of vulnerable individuals and cause real harm (e.g. by causing pneumonia and in some cases sepsis)
Name 3 life-threatening bacteria that grow in the respiratory tract
Streptococcus pneumoniae, Haemophilus influenzae, Bordetella pertussis
What can Streptococcus pneumoniae, Haemophilus influenzae, Bordetella pertussislead to?
blood stream infections (when viable bacteria or fungi are found circulating in blood), pneumonia, meningitis and middle ear infections
What are the two immunologically distinct regions of the gut?
Inductive sites Effector site (makes up most of gut)
What are the inductive sites
Regions rich with naive resting immune cells: . These regions drive the education of the immune system of mucosal surfaces
What are the regions responsible for education of the immune system of mucosal surfaces called?
Peyer’s patches
What happens in Peyer’s patches?
They sample microbes in the lumen of the gut, passing them on to macrophages and B cells to promote IgA responses against these pathogens. In contrast, dendritic cells (DCs) sample other antigens to ensure that immune cells are tolerant of food, and other gut contents that are not hazardous
Where is the effector site?
the tissue underlying the epithelium (embedded within the matrix of the peyer’s patch)
What is the effector site made u of and what does it do?
It is crammed with activated effector cells: plasma cells that secrete antibodies into the mucus (usually IgA and IgM) and memory B cells, T helper (TH) cells and antigen presenting cells (APCs) - macrophages and dendritic cells.
lymphatics from peyer’s patches and villi drain into where?
Mesenteric lymph node
What is the loose connective tissue within the villi called?
lamina propria
What cells transport material across epithelial barrier via transcytosis
Microfold cells (M cells)
What do dendritic cells within Peyer’s patch do?
Extend dendrites between epithelial cells to sample antigens that are the broken down and then used for presenting to lymphocytes
What is tolerogenic activation?
Where the immune system initiates and anti-inflammatory response
With their cargo of antigen, what do the dendritic cells within the Peyer’s patch do?
Traffic to the T-cells zones. Upon encounter with T-cells, the dendritic cells convert them into regulatory T cells
Defects in the function of what cells causes inflammatory bowel disease?
regulatory T cells
Where do the regulator T cells migrate to? How?
lamina propia of the villi via the lymphatics
What do regulatory T cells do whenever they get to the lamina propia?
Secrete IL-10, which exerts an supressive reaction with the immune cells of the lamina propia and on the epithelial layer itself
What interleukin is critical in maintaining immune quiescence and preventing unnecessary inflammation
IL-10
A break down in immune homeostasis can lead to what?
gut pathology - over a log period and in an uncontrolled manner this can lead to
inflammatory bowel disease
What are the causes of inflammation in the gut?
- genetic predisposition plus
- chemical, mechanical or pathogenic barrier disruption
When bacteria influxes through the epithelium what do T-regulatory cells do? How are they activated to do this?
Down regulate IL-10 secretion, to allow an immune response to proceed.
They are activated by alarm molecules secreted by the epithelium following activation by bacteria
What is released by dendritic cells during a gut immune response?
IL-6, IL-12, IL-23
What do effector T cells do in the gut during an immune response?
They up-regulate the immune response by secreting: tumor necrosis factor, interferon gamma, IL-17
Following effector T cell arrival in the gut, what comes next?
Neutrophils
- netosis (NET)
causes collateral damage to tissues
What happens to the remaining neutrophils, once the immune response has won?
They die by apoptosis and are cleared by macrophages
How are the damaged epithelial cells replaced
By new cells from the intestinal crypts of Lieburkuhn
How do M cells act?
Passive immunity
Transport substances
stimulate production of IgA
uptake antigens viaendocytoisis, phagocytosis and transcytosis
What is the role of DCs in the Peyer’s patch?
- Sampling proteins from the gut lumen
- Presenting antigens to T cells
- Producing T regulatory cells
- Inducing tolerance to an antigen
Describe gram positive bacteria
They have a single membrane surrounded by a thick wall made of peptidoglycan, a polymer of sugars and amino-acids
Describe gram negative bacteria
have an inner membrane, a thin peptidoglycan wall and then an outer membrane, whose lipids are studded with sugars
Do gram positive or gram negative bacteria have lipopolysaccharides
Only gram-negative bacteria
What are the three key players of the complement system?
- Opsonisation
- Lysis of pathogens
- Tagging (attracting immune cells)
What are the three main initiators of complement activation, and which pathways do they initiate?
- Antibody initiates classical pathway;
- Bacterial cell wall components / PMAPs, initiate the lectin pathway
- Spontaneous hydrolysis of C3b that joins it to surfaces initiates the alternative pathway
What structure is IgA and IgM secreted in?
IgA: dimer
IgM: pentamer
In what case are antibodies required for innate immunity?
Antibodies involved in the activation of the classical complement pathway
How could low affinity antibodies activate the classical complement pathway?
IgM has 10 complementary determining regions (CDR) these can all bind with a low affinity to multiple antigens on pathogen’s surface (in a repeating pattern such as bacterial wall of virus capsid) - making its avidity 10x its affinity. Thus activating the classical complement pathway
What binds to the boundIgM molecules to initiate the classical complement pathway?
C1q
In what bacteria can the MAC components not reach the inner membrane?
Gram positive
What can the MAC components attack?
MAC is able to penetrate both of the membranes of gram-negative bacteria and those of enveloped viruses
Since gram positive bacteria cannot be attacked by MAC, how can they be targeted?
It can be efficiently opsonised by complement, and trigger the release of the anaphylotoxins that recruit and activate phagocytes
What two bacteria are able to prevent complement recognition? How?
1) Haemophilus influenzae excretes a protease enzyme able to accelerate breakdown of complement proteins;
2) Staphylococcus aureus makes capsules that make the bacterium more resistant to opsonisation by complement, and can also secrete a bacterial protein that stimulates the production of C3b-fibrinogen complexes to make an opsonised decoy
What are known to lead to susceptibility to extracellular pathogens, particularly to pyogenic bacteria, which cause purulent (better known as pus-forming) inflammation (e.g. Streptococcus pneumoniae)?
Deficiencies in early components of the alternative pathway (e.g. Factor D, Factor P or C3)
What are deficiencies in the Lectin pathway associated with?
Early childhood susceptibility to infections