Continuing our investigation of the innate immune system, one of the most important factors to
consider during an immune response is how quickly host cells can respond to an infectious threat.
Bacterial doubling times can range from the order of hours to minutes, meaning that they can rapidly
overtake host tissues if they aren’t contained quickly enough. As we’ve discussed before,
the immune system needs to be tightly regulated so that the mechanisms which fight pathogens
don’t start harming host tissues, but it can take hours to sense a threat, and then transcribe,
translate, and secrete effector proteins that are best-suited to fight that threat. In order
to counter pathogens, a cell needs a molecular mechanism that allows it to take decisive action
and warn other cells quickly. One way to do this is through the inflammasome, which is a large,
multiprotein complex that senses threats and initiates inflammation.
Let’s first describe the inflammasome in general terms, and then we’ll learn about
some of the most well-studied inflammasome sensors. Of all the known inflammasomes,
there are three major components they all have in common. Those are one, a sensor,
two, a CARD domain, and three, inflammatory caspases. As you might guess from its name,
the sensor senses a wide range of pathogenic or damage-associated signals. Immunologists
are still learning how sensing of these signals triggers a response, since the sensor doesn’t
necessarily need to bind the ligand in order for it to be sensed, which makes it different from a
pattern recognition receptor. The CARD domain can either be built directly into the sensor,
or it can be found in the adaptor protein, ASC, that many inflammasomes use. The CARD domain
is essential for the inflammatory caspases to bind to the inflammasome complex. Inflammatory
caspases, usually caspase-1 or caspases 4 and 5 in humans, are what kick off the signaling cascade.
They are zymogens, meaning they need to be cleaved to be activated, but once they are
activated, they are responsible for all of the effector functions of the inflammasome,
which we’ll talk about in a moment. Before the inflammasome can assemble,
it first needs to be primed. The priming step transcriptionally upregulates the sensor
component, as well as IL-1β, which is one of the downstream cytokine effectors of the inflammasome.
This is the first signal in a two-signal system, which as we’ve discussed is quite common in the
immune system. Priming occurs when pattern recognition receptors sense PAMPs or DAMPs.
The next step is sensing, which occurs when the sensor component senses additional PAMP or DAMP
signals. Remember that the inflammasome sensor doesn’t necessarily need to bind directly to these
signals. We’ll get into specifics when we examine the individual inflammasomes, but some of these
signals include bacterial lipopolysaccharide, viral DNA, bacterial toxins, extracellular ATP,
silica particles, reactive oxygen species, and changes in intracellular potassium levels.
Once the sensor is activated, the inflammasome begins to assemble. Multiple copies of the active
sensor gather, and each of them bind to the ASC adaptor if necessary. Remember that the
ASC adaptor has a CARD domain that allows pro-caspsase-1 to bind. The fully assembled
inflammasome has multiple subunits of each of the components, meaning that when it’s ready,
it can execute downstream steps quickly. The way pro-caspase-1 binds to the inflammasome
enables autocatalytic cleavage of pro-caspase-1, meaning that it cleaves and activates itself.
Activated caspase-1 can then cleave pro- IL-1β and pro-IL-18. The mature forms of these proteins,
IL-1β and IL-18, are then potent stimulators of inflammation in other cells. IL-1β signals
through the IL-1 receptor, which signals similarly to a TLR, in that it has a TIR domain
and activates NF-kB through MyD88, also inducing transcription of several pro-inflammatory genes,
including the cytokines IL-6 and TNF-a. IL-18 can induce vascular components of inflammation,
such as increased expression of cell adhesion proteins with production of chemokines. It
can also induce IFN-y, which is an important cytokine for dealing with viral infections.
Caspase-1 also cleaves a host protein called gasdermin-D. Cleaved gasdermin D moves to the cell
membrane and forms pores, causing the cell to die in an inflammatory form of programmed cell death
called “pyroptosis.” Pyroptosis is an effective way to kill intracellular pathogens, and it can
also recruit other immune cells to the site of damage. Current research is adding more nuance
to these pathways, and a better understanding of what kinds of signals lead cells to fully undergo
pyroptosis. It is currently understood that IL-1β and IL-18 are released through gasdermin-D
pores. Inflammasomes have been most widely studied in myeloid cells like macrophages and dendritic
cells, but there has been growing appreciation for inflammasome activation in epithelial cells,
which line the surfaces of the body that interface with the outside world.
Now that we’ve discussed the general principles of inflammasome activity,
let’s talk about a few specific inflammasomes. Many of these inflammasomes belong to the NLR
family of pattern recognition receptors that we discussed in the previous tutorial.
NLRP1 seems to be found in more diverse cell types than some of the other inflammasomes,
as it can be found in adaptive immune cells and even non-immune cells, in addition to
innate immune cells. The NLRP1 inflammasome is activated when the NLRP1 sensor is cleaved by
Bacillus anthracis lethal toxin, which is one of the components of anthrax toxin. Certain alleles
of NLRP1 in rodents can also respond to infection with the protozoan parasite Toxoplasma gondii.
NLRP3 is one of the best-studied inflammasomes, and is primarily expressed in myeloid cells like
macrophages and dendritic cells. It is also known as NALP3 or cryopyrin. NLRP3 can be activated in
response to a huge range of bacterial, fungal, and viral infections, as well as many DAMPs. Some of
these DAMPs include extracellular ATP, monosodium urate, cholesterol crystals, and amyloid beta,
which is the peptide that makes up amyloid plaques in the brains of patients with
Alzheimer’s disease. NLRP3 also senses several environmental irritants, like silica and asbestos.
NLRC4 is another inflammasome that is primarily expressed in immune cells and tissues.
Unlike the other NLRPs, NLRC4 contains an endogenous CARD domain, meaning that it can
directly bind procaspase-1. NLRC4 is involved in sensing several different bacterial infections,
including Salmonella, Legionella, Shigella, and Pseudomonas. NLRC4 has been shown to be
activated in response to flagellin, the protein that makes up bacterial flagella.
NLRP6, also known as NALP6, is a more recently discovered member of the NLR inflammasome family.
It is unique among inflammasomes in that it can activate the transcription factor
NF-kB, similar to a pattern recognition receptor. NLRP6 senses bacterial infections such as Listeria
and Staphylococcus aureus, as well as some viral infections. NLRP6 is highly expressed in
the intestinal epithelium, and may be involved in controlling the composition of the gut microbiota.
In addition to the NLR family inflammasomes, there are some inflammasome sensors that sense
double-stranded DNA. The AIM2 inflammasome senses cytosolic double-stranded DNA, meaning that it
is especially important for sensing viral and intracellular bacterial infections. Unlike some
sensors, which can distinguish between microbial and host DNA, AIM2 can be activated by either,
as long as it is in the cytosol. Remember that host DNA should be safely contained in the nucleus
under normal conditions, meaning that it will be shielded from activating AIM2. This is similar
to a recently discovered inflammasome called IFI16, which also senses double-stranded DNA.
There is also what is called the non-canonical inflammasome. In the non-canonical inflammasome,
direct cytosolic sensing of intracellular LPS by caspase-11 in mice, or caspases 4 and 5 in humans,
can lead to assembly of a macromolecular complex that cleaves gasdermin-D, enabling pyroptosis.
In some cases, non-canonical inflammasome activation leads to canonical caspase activation
that can then cleave pro-IL-18 and pro-IL-1β. Inflammasomes are an exciting phenomenon,
and their study represents a relatively new branch of immunology. There has been
quite a bit of recent research focusing on their importance in controlling various infections, as
well as their ability to perpetuate inflammation and disease when they aren’t well-regulated.
And with this topic covered, let’s continue working our way through the innate immune system.