Lecture 6 and 7: Cytokine Processing and Activation Flashcards
What do cytokines act as?
Intracellular messengers
What are cytokines
Structurally diverese polypeptides that function as messenger molecules
How do cytokines act as messenger molecules
- They communicate signals from one cell type to another
- Instruct the cell receiving the signal to proliferate, differentiate, secrete additional cytokines, migrate or die
How do cytokines signal
Via plasma membrane borne receptors
How do cytokines interact with their receptors
They bind with tight affinity and show high specificity
why are cytokines present at low concentrations
They have high affinity for their cognate rerceptors
What do cytokines switch on and off
Specific cellular effector funtions
what do cytokines initate
Differentiation from one cell state to another
How is cytokines switching on or off effector functions or initiating differentiation achieved
Initiating the transcription of a new cohort of genes within the cells, the products of which endow the cell with new or enhanced capabilities
What does IL-2 trigger
A transcriptional programme within T cells that enables cells to proliferate upon receipt of a signal
What does TNF induce
the trancriptional upregulation of over 50 different cytokines, chemokines, anti-bacterial proteins as well as other immune responses within responsive target cells
What does TNF trigger
Activation of neutrophils and local endothelium to upregulate integrins that facilitate extravasation of immune cells and plasma proteins (complement and acute phase) into tissues
What is one of the most important cytokine groupings
Interleukins
What does the interleukins contain
cytokines that act as communicators between leukocytes
Why are members of the interleukin family diverse
The membership is based on biology (evidence of activity on leukocytes) rather than sequence or structural homology
How many interleukins have been described to date
38
What other cytokines familes (other than interleukins) have been established based on
- support the proliferation of hematopoietic precursors (colony stimulating factors)
- cytotoxic activity towards transformed cell types (TNF)
- ability to interfere with viral replication (interferons)
What do cytokines frequently have
pleiotropic effects
What does the response of many cytokines depend on
the context in which the cytokine is delivered as well as the cell type receiving the signal
How would you describe the weight of cytokines
low weight (15-25kDa)
How are cytokines produced
In a transient manner tightly coupled to the presence of foreign material or tissue injury
What primes the mRNA of cytokines for rapid degradation
the AU rich sequences in the 3’ untranslated regions
Unlike endocrine hormones, the majority of cytokines normally act in what type of fashion
They act locally in a paracrine or even autocrine fashion
What do cytokines derived from lymphocytes rarely do
Persist in the circulation
What do the cytokines produced by non lymphoid cells such as endothelial cells and fibroblasts do once the cells are triggered by bacteria
they can release cytokines that may be detected in the bloodstream
what is septic shock
a life threatening condition that largely results from the massive overproduction of cytokines like TNF and IL-1 in response to bacteria infection
what does septic shock illustrate
the necessity to keep a tight rein on cytokine production
What can certain cytokines like TNF and Fas exist as and do
membrane anchored forms and can exert their stimulatory effects without becoming soluble
What levels are cytokine production and downstream effector function kept under control at
- transcriptional
- translational
- decoy receptors
- cytokine receptor antagonists
what do decoy receptors do
bind cytokines but do not signal
what do cytokine receptor antagoinsts do
compete with cytokines for binding to their cognate receptors
How do cytokines acti
in hierarchal cascades
what do cytokines that exert systemic powerful effects lead to
the production of numerous additional cytokines, chemokines, complement, anti-microbial peptides and other pro-inflammatory proteins
what do cytokines that have more restricted effects do
they have their activities confined to specific cell types and their effects on the production of additional cytokines and chemokines being more limited
how would you sum up cytokines acting in hierarchial cascades
some cascades act as apical or upstream regulators of many additional inflammatory factors, while others act in a more distal or downstream role
What are most cytokines regulated by
Synthesis and golgi-ER/golgi mediated secretion
Where are most proteins (including cytokines) synthesised and secreted from
-Synthesied in the endoplasmic reticulum
-Secreted from the golgi apparatus into the extracellular space
What happens to the certain cytokines that do not undergo Er to golgi secretion
They are translated into the cytoplasm and then stored either there or become imported into the nucleus
what are the certain cytokines that do not undergo ER to golgi secretion known as
DAMPs
why are DAMPs stored in the cytoplasm or important into the nucleus
because their function is to become released upon tissue damage
Why are the selection of the certain cytokines called DAMPs
as they are constituitively expressed in tissues and their release is regulated through cell death
What did Polly Matzinger advance
the danger hypothesis
what does polly matzingers danger hypothesis explain
Why cell death in a sterile setting can be a strong initiator of inflammation
what does Polly Matzingers danger hypothesis propose in response to cell death in sterile settings initiating inflammation
Tissue damage released some sort of cytokine like molecule that could initiate inflammation called danger signals
when was the danger hypothesis conceived
1994
What did Matzinger proprose that the danger would be
evolutionary conserved molecules that may have dual functions, one function inside the cell, but a cytokine like function upon release into the extracellular space due to necrosis
What do DAMPs now represent
hidden self that is revealed during necrosis
What probably represents the best candidate for DAMPs
Members of the extended IL-1 cytokine family
What does the detection of a PAMP signify
Danger and warants the initiation of an inflammatory response to coordinate the innate and adaptive immune system
What may be insufficient to warrant a full blown inflammatory response
The detection of a PAMP in isolation unless this is accompanied by evidence of tissue damage
How would the immune response to a PAMP be increased
Use of an adjuvant
What do adjuvants do
They trigger cell death at the site of injection increasing immunogenicity
What does the danger hypothesis argue as a result of the adjuvant idea
The concurrrent detection of cell death that qualifies whether a non self antigen is dangerous or not and therefore dictates whether an immune response should be elicited
Why does using tissue damage to temper the severity of an immune response make good biological sense
As ignoring non-self antigens where there is no evidence of tissue disturbance avoids mounting unecessary responses to foreign antigens that are harmless, thus saving tissues from the collateral damage caused by an unwarranted inflammatory response
How are IL-1 family members released
Via necrosis
What is one of the major features required of a DAMP
- The ability to act as cytokine like molecules upon release into the extracellular space
- The molecules should ideally be retained by viable cells and only released in circumstances where necrosis has occured
What are the IL-1 family members retained by
Viable cells that released in response to a variety of cues that share in common the ability to provoke cell death
What do signal 1 (LPS) and signal 2 (toxins, drugs etc) which activate IL-1 have in common
the ability to promote plasma membrane permeabilisation leading to necrosis
What does the introduction of LPS into the cytosol initiate
Pyroptosis
How does pyroptosis occur as a result of LPS stimulation
The non-canonical inflammasome activation leading to caspase 11 dependent processing of gasdermin D, which permeabilises plasma membranes leading to necrosis
what do mice deficient in gasdermin D do/not do
they do not undergo pyroptosis and consequently fail to release IL-1B
What does the IL-1 family consist of
The pro-inflammatory cytokines IL-1a, IL-1B, IL-18, IL-33, IL-36a, IL-36B and IL-36y as well as IL-37 and IL-38
What do the IL-1 family of cytokines possess
N-terminal prodomains of variable length
What can the activity state of the IL-1 cytokines be enhanced through
Restricted proteolysis within their N-terminal domains
What is unusual about most of the IL-1 family members
they exhibit little or no detectable biological activity as full length proteins at physiological concentrations
In their active forms, IL-1 family cytokines act as….
Potent but differential activators of diverse cell types
What can all IL-1 family members promote
The release of a diverse array of pro-inflammatory and wound healing factors form many cell types promoting the hallmarks of inflammation
What can IL-1 cytokines do to the adaptive immune system
Polarise the adaptive immune response to specific types of T cells
What have IL-18 and IL-33 been implicated in
The activation of local Tregs to induce wound healing responses in the lung and heart
What do IL-1 family members signal via
Heterodimeric plasma membrane receptors through a common signalling chain
What is the common signalling chain of the IL-1 family
IL-1RAcP
When is IL-1RAcP recruited
Upon ligand binding to the receptor
What is the motif in the IL-1 receptor family
TIR
What do IL-1alpha and B signal through
IL-R1
What does IL-18 signal through
IL-18R
What does IL-33 signal via
IL-33R (ST2)
What do the IL-36 cytokines signal through
IL-36R/IL-1R2
What may IL-37 signal through
It might act as an antagonist for the IL-18R but maybe via the orphan IL-1 family receptor SIGIRR
Why do IL-1 family cytokines require proteolytic processing for activation
IL-1 family cytokines may serve as activity recognition receptors for aberrant protease activity
What do the N-terminal prodomains serves as
Inhibitory regions to suppress the protease activity
What can the N-terminal pro-domains of IL-1 family members be cleaved by
By diverse proteases that are not normally present in the intracellular or extracellular spaces
What are the three sources of protease activity that have emerged as effectors of IL-1 family cytokine processing
- Intracellular proteases
- Extracellular proteases
- Non-self proteases
Discuss intracellular proteases
These are caspases and calpains, which are activated as a consequence of injury initiated necrosis, or pathogen-driven inflammasome activation that typically results in programmed necrosis (pyroptosis)
Discuss extracellular proteases
These include elastase, cathepsin H, mast cell chymase, cathepsin S or thrombin that are secreted by first responder cells of the immune system in response to tissue damage or pathogen detection
Discuss non-self proteases
These include Streptococcal protease SpeB, the Pseudomonas protease LasB, or the house dust mite protease Der p1, that are associated with infectious agents or allergens
Why do IL-1 family cytokines require proteolytic processing for activation
- The requirement for an activation step is simply an additional level of regulation that is imposed upon these cytokines to ensure that they are only deployed under the appropriate circumstances (maybe not the most likely)
- Cytokines have evolved specifically for the purpose of sensing abberant protease activity that is frequently associated with infectious agents or liberated cells of the immune system
What may IL-1 family cytokines serve as
Activity recognition receptors for molecular activity that betrays a severe deviation form tissue homeostasis and serves as a proxy for the detection of infection or tissue necrosis
What has been suggested about protease activity and helminths
Since helminths do not have strong PAMPs that readily permit their detection by PRRs, the protease activity may serve as a proxy for the detection of the worms by the immune system
In addition to the detection of conserved non-self molecular patterns (PAMPs), what has the immune system co-evolved to do
It has co-evolved a system to detec abberant molecular activities that perturb normal tissue and cellular homeostasis as a proxy for the detection of pathogen activity or a severe departure from tissue integrity
Why do proteases represent good proxies for infection or tissue damage
- All infectious agents contain multiple proteases that are often implicated as virulence factors
- Multiple proteases are activated upon tissue injury in the context of the coagulation and complement cascade
- Proteases are activated intracellularly or released extracellularly in the context of programmed as well as non-programmed necrosis
What are proteases associated with
Infection, injury, the detection of pathogens and pathogen responses
What did the Ruvkan lab suggest
Perturbation of key cellular machinery can also trigger pathogen responses
What has the IL-1 family cytokines evolved as
Sensors for protease activity associated with infectious agents and tissue injury that couple the detection of aberrant protease activity to the rapid initiation of inflammatory responses that coordinate wound healing as well as pathogen clearance
What further supports the idea that the IL-1 family cytkines are major initiators of inflammation
The family also contains several receptor antagonists or non-signaling decoy receptors
What can the loss of function of the IL-1R and IL-36RA lead to
Spontaneous autoinflammatory diseases that can be fatal and exacerbated through inducing tissue injury
How can inflammation be limited via the actions of the IL-1 family
Preventing the release of DAMPs through apoptosis rather than necrosis
What are some mechanisms that prevent necrosis occuring involving IL-1 DAMPs
- IL-33 are inactivated through caspase dependent proteolysis in apoptotic cells
- The pro-domain of IL-1alpha has been found to contain a nuclear localisation signal that tethers the protein within the nuclei of apoptotic cells preventing its escape
What therefore is the primary reason for apoptosis
To limit the escape of DAMPS during regulated cell death through facilitating the early removal of the cells, destroying their associated DAMPs within the phagolysosomes of the engulfing cell
What does processing and activation of IL-1 family cytokines occur by
Inflammasome mediated activation of caspase-1
What are inflammasomes assembled in response to
Noxious detection of intracellular PAMPs
Describe the NLRP3 inflammasome
- The inflammasome is activated by noxius stimuli promoting necrosis.
- This leads to potassium efflux which permits assembly and activation of the inflammasome
- This liberates the pro forms of IL-1B and IL-18
- Caspase-1 activation on inflammasomes enables the latter protease to process and activate IL-1B and IL-18 into their active forms
What do neutrophils release as part of the granule
Elastase, cathepsin G and proteinase 3
What are neutrophil proteases efficient activators of
Multiple IL-1 family cytokines
