Mechanism of inflammasome formation and function
The synthesis of pro-inflammatory mediators begins with the action of the transcription factor NF-κB, which activates corresponding genes in the cell nucleus. However, for certain mediators such as IL-1 family cytokines (and especially IL-1β), simple synthesis is not sufficient. The cell produces them as inactive precursor molecules.
For a precursor to become functionally active, a cleavage process—processing—is required. This process is carried out by a cytosolic multi-protein complex known as the inflammasome.
The assembly and operation of the complex include the following stages:
- Cytosolic NALP receptors belonging to the NLR family recognize intracellular danger signals.
- NALP receptors, accessory factors, and a procaspase molecule combine into a single structure. Assembly is made possible by the fact that all these proteins contain a specific CARD domain.
- Inside the assembled inflammasome, the enzyme caspase-1 is activated.
- Activated caspase-1 cleaves the inhibitory segment from the IL-1β precursor, releasing the active cytokine.
Role of receptors and co-receptors in activation
Myeloid cells of the innate immune system are activated predominantly via TLR and NLR receptors. Nevertheless, other pattern recognition receptors (PRR) also play an important role. Often, they are unable to activate the cell alone, but act as potent co-receptors.
- Integrins: Being adhesion molecules, they are functionally linked to tyrosine kinases and ITAM-containing motifs. This allows them to participate in the activation of key transcription factors — NF-κB and AP-1.
- C-type lectin receptors: Most of them merely assist TLR-dependent activation. An exception and striking example of independent function is the Dectin-1 receptor.
| Characteristic | Description of the Dectin-1 receptor |
|---|---|
| Structure | Contains an ITAM sequence in its cytoplasmic portion |
| Ligand | Binds β-glucans found in the cell wall of yeast fungi |
| Effects | Triggers expression of COX-2 genes and production of cytokines (TNFα, IL-6, IL-23) |
| Value | Plays a central role in immune defense against fungal infections |
Amplification of the inflammatory response
Signaling pathways triggered by pro-inflammatory cytokines are strikingly similar to the MyD88-dependent signal transduction characteristic of TLRs.
For example, the pathways of the IL-1 receptor and TLR are nearly identical. This is because the intracellular portions of both receptors share the same TIR domain (Toll/IL-1 receptor and resistance domain). In turn, signal transduction from TNFα involves TRAF2 and TRAF3 factors, creating cross-talk with TLR and TNFR pathways.
Such structural and functional similarity leads to amplification (enhancement) of the response. When a pathogen acts on cells at the site of infection, cytokines are released. These cytokines, acting on other innate immune cells, distantly reproduce the effects of direct contact with the pathogen, but on a much larger scale. Thus, cytokines act as amplification factors for the primary response to pathogen-associated molecular patterns (PAMPs).
Non-inflammatory functions of innate immune cells
The process described above involving the inflammasome and mediator release is the classical pathway of activation (primarily characteristic of macrophages). Its main goal is to trigger inflammation and eliminate the pathogen.
However, dendritic cells and resident tissue macrophages are also capable of implementing alternative response scenarios. Their functions include:
- Maintenance of tissue homeostasis.
- Stimulation of tissue repair and regeneration.
- Strict limitation and suppression of excessive inflammatory responses.
Briefly: answers to topical questions
Precursors of which pro-inflammatory cytokines are cleaved by caspase-1?
Caspase-1 cleaves the inactive precursor molecule of IL-1 family cytokines, especially pro-IL-1β, resulting in the formation of functionally active IL-1β.
What form of programmed cell death does inflammasome activation induce?
Activation of the inflammasome and its associated caspase-1 induces pyroptosis. This is a regulated form of cell death accompanied by marked inflammation.
During pyroptosis, the following are released from the cell:
- DAMPs — endogenous molecules released upon tissue damage.
- Pro-inflammatory cytokines — interleukin-1β (IL-1β) and interleukin-18 (IL-18).
Unlike apoptosis, which proceeds without the release of cellular contents and does not cause inflammation, pyroptosis is characterized by massive mediator release and the development of an inflammatory response.
What three mandatory components make up the classical inflammasome complex?
The supramolecular inflammasome complex is formed by three main groups of components, each containing a CARD domain in its structure:
- Cytosolic receptors — intracellular pattern recognition receptors of the NLR family (e.g., NALP, NOD1, NOD2, IPAF). They are responsible for PAMP recognition.
- Procaspase — the inactive form of the enzyme (caspase-1), which is activated after complex assembly.
- Other factors — additional molecules involved in structural formation.
The assembly of these components into a single temporary structure leads to the activation of caspase-1 and subsequent processing of pro-inflammatory cytokines.
To remember the composition of the inflammasome, use the rule of three 'C's: Complex assembled by receptors, Contacts via the CARD domain, and uses Caspase-1.
Class and exam questions
Why is gene activation via NF-κB not sufficient for IL-1β?
The NF-κB factor triggers only the synthesis of the inactive precursor molecule. To gain biological activity, this molecule requires enzymatic processing (cleavage of the extra segment), which is carried out by caspase-1 within the inflammasome.
What do the structural components of the inflammasome have in common?
All elements of the complex—NALP cytosolic receptors, procaspase molecules, and accessory factors—feature a specific CARD domain in their structure, through which they bind together.
How are IL-1 and TLR receptors interconnected?
Their intracellular signaling pathways are completely identical because the cytoplasmic portion of both receptors contains a TIR domain (Toll/IL-1 receptor and resistance domain). This ensures powerful amplification of the inflammatory response.
What else to study on this topic
- Structure and functions of the TIR domain in signal transduction
- Mechanism of activation of transcription factors NF-κB and AP-1
- Role of integrins as co-receptors with ITAM motifs
- Antifungal immunity: Dectin-1 receptor and β-glucans
- Cross-talk signaling pathways of TNFα involving TRAF2 and TRAF3
Master the entire topic in the "Immunology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on 'Inflammasome' and 723 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 23 video lessons on the subject
Other topics in the "Innate Immunity" section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.