# Innate Immunity - Pattern Recognition and Inflammasomes

## Introduction and Evolutionary Context

### The First Line of Defense

Innate immunity represents the most phylogenetically ancient arm of the immune system, with conserved elements traceable across both invertebrate and vertebrate species spanning hundreds of millions of years of evolution. Unlike the adaptive immune system, which requires days to weeks to mount a primary response, the innate immune system provides immediate defense within minutes to hours of encountering a pathogen, and it does so without requiring any prior sensitization or immunologic memory. This rapid response is accomplished through the use of germline-encoded pattern recognition receptors that enable the innate immune system to distinguish self from non-self with remarkable precision and efficiency.

Beyond its role as a first responder, the innate immune system serves as a critical bridge to adaptive immunity. Through antigen presentation by dendritic cells and macrophages, and through the elaboration of cytokines that shape the character of the adaptive response, innate immune cells determine whether the subsequent T and B cell response will be polarized toward a Th1, Th2, Th17, or regulatory phenotype. In this way, the innate immune system functions not merely as a holding action, but as the essential director of the overall immune response.

### Key Concepts

The innate immune system relies on the recognition of two broad categories of molecular signatures. Pathogen-associated molecular patterns (PAMPs) are conserved structures found across broad classes of microorganisms and include molecules such as lipopolysaccharide (LPS) from gram-negative bacteria, flagellin from motile bacteria, double-stranded RNA from replicating viruses, and unmethylated CpG DNA motifs characteristic of bacterial and viral genomes. Damage-associated molecular patterns (DAMPs) represent an equally important category, consisting of endogenous molecules released from stressed, injured, or necrotic host cells. These include extracellular ATP, the nuclear protein HMGB1, uric acid crystals, and mitochondrial DNA. The recognition of DAMPs enables the innate immune system to respond not only to infection but to sterile tissue injury as well.

The receptors that detect these molecular patterns, collectively termed pattern recognition receptors (PRRs), are fundamentally different from the antigen receptors of the adaptive immune system. PRRs are non-clonal, meaning that every cell of a given type expresses the same set of receptors, and they do not undergo the somatic recombination that generates the extraordinary diversity of T cell receptors and B cell receptors. This means the innate immune system relies on a finite repertoire of receptors to detect a vast array of threats, a strategy that has proven remarkably effective across evolutionary time.

## Pattern Recognition Receptors (PRRs)

### Toll-Like Receptors (TLRs)

The Toll-like receptors constitute the best-characterized family of pattern recognition receptors in humans. Ten functional TLRs have been identified in the human genome (TLR1 through TLR10), while additional family members (TLR11 through TLR13) have been identified in mice but are absent in humans. The TLRs are type I transmembrane glycoproteins characterized by extracellular leucine-rich repeat (LRR) domains that mediate ligand recognition, and an intracellular Toll/IL-1 receptor (TIR) domain that initiates downstream signaling.

The TLRs are strategically distributed between the cell surface and endosomal compartments, reflecting the distinct locations where different microbial products are encountered. The cell surface TLRs include TLR1, TLR2, TLR4, TLR5, TLR6, and TLR10. Among these, TLR4 occupies a position of particular importance as the receptor for bacterial LPS. TLR4 requires the co-receptors MD-2 and CD14 for efficient LPS recognition, and it is unique among the TLRs in its ability to signal through both of the major TLR signaling pathways. TLR2 forms heterodimers with either TLR1 (recognizing triacyl lipopeptides from bacteria) or TLR6 (recognizing diacyl lipopeptides), thereby expanding the range of microbial lipid structures that can be detected. TLR5 serves as the receptor for bacterial flagellin, the structural protein of the flagellum.

| TLR | Location | Ligand | Adaptor Pathway | Clinical Relevance |
|-----|----------|--------|-----------------|-------------------|
| TLR1/2 | Cell surface | Triacyl lipopeptides | MyD88 | Bacterial recognition |
| TLR2/6 | Cell surface | Diacyl lipopeptides | MyD88 | Bacterial recognition |
| TLR3 | Endosome | dsRNA | TRIF only | HSV encephalitis (deficiency) |
| TLR4 | Cell surface | LPS (with MD-2, CD14) | MyD88 + TRIF | Gram-negative sepsis |
| TLR5 | Cell surface | Flagellin | MyD88 | Motile bacteria |
| TLR7 | Endosome | ssRNA | MyD88 | Viral detection; imiquimod target |
| TLR8 | Endosome | ssRNA | MyD88 | Viral detection |
| TLR9 | Endosome | Unmethylated CpG DNA | MyD88 | Bacterial/viral DNA; CpG adjuvants |
| TLR10 | Cell surface | Unknown | MyD88 | Function incompletely characterized |

The endosomal TLRs, which include TLR3, TLR7, TLR8, and TLR9, are positioned within intracellular vesicles where they detect nucleic acid structures that become accessible after microbial degradation within the endolysosomal pathway. TLR3 detects double-stranded RNA, a replication intermediate of many viruses, and is notable for signaling exclusively through the TRIF adaptor rather than MyD88. TLR7 and TLR8 recognize single-stranded RNA, with TLR7 also serving as the target of the synthetic immune response modifier imiquimod. TLR9 detects unmethylated CpG DNA motifs, which are abundant in microbial genomes but rare and methylated in mammalian DNA.

The intracellular signaling pathways downstream of TLR engagement converge on two major adaptors. The MyD88-dependent pathway is utilized by all TLRs except TLR3 and leads to the activation of the transcription factors NF-kB and AP-1, driving the expression of pro-inflammatory cytokines such as TNF-alpha, IL-6, and IL-1beta. The TRIF-dependent (MyD88-independent) pathway is used by TLR3 and, uniquely, also by TLR4, and leads to the activation of IRF3, which induces the production of type I interferons (IFN-alpha and IFN-beta), critical for antiviral defense.

<image>A detailed diagram showing all 10 human Toll-like receptors arranged by location (cell surface vs endosomal). Each TLR is shown as a transmembrane protein with leucine-rich repeat ectodomains. Arrows indicate their respective ligands (LPS for TLR4, flagellin for TLR5, CpG DNA for TLR9, etc.). Intracellular signaling cascades are shown branching into MyD88-dependent and TRIF-dependent pathways, converging on NF-kB, AP-1, and IRF3/7 transcription factors. Color-coded: blue for cell surface TLRs, red for endosomal TLRs.</image>

### NOD-Like Receptors (NLRs)

The NOD-like receptors are a family of cytoplasmic sensors comprising more than 20 members in humans. These intracellular sentinels detect microbial products that have gained access to the cytosol, either through active bacterial secretion systems, membrane damage, or phagocytic escape. Two members, NOD1 and NOD2, function primarily as sensors of peptidoglycan fragments. NOD1, also known as CARD4, detects gamma-D-glutamyl-meso-diaminopimelic acid (iE-DAP), a component found predominantly in gram-negative bacterial peptidoglycan. NOD2, also known as CARD15, detects muramyl dipeptide (MDP), a peptidoglycan motif common to virtually all bacteria. Mutations in NOD2 have significant clinical relevance: loss-of-function mutations (R702W, G908R, and the frameshift L1007fs) are strongly associated with Crohn's disease, while gain-of-function mutations cause Blau syndrome, a granulomatous disorder affecting the skin, joints, and eyes.

NLRP3 stands as the best-characterized of the inflammasome-forming NLR proteins and will be discussed in detail in the inflammasome section below. NLRC4, also known as IPAF, serves as a sensor for cytosolic flagellin and components of the bacterial type III secretion system, functioning through upstream NAIP sensor proteins that confer specificity for different bacterial structures.

### RIG-I-Like Receptors (RLRs)

The RIG-I-like receptors are a family of cytoplasmic RNA sensors that serve as critical first responders to viral infection. The family includes three members: RIG-I (retinoic acid-inducible gene I), MDA5 (melanoma differentiation-associated protein 5), and LGP2. RIG-I preferentially detects short double-stranded RNA molecules bearing a 5'-triphosphate cap, a molecular signature characteristic of viral replication intermediates, and is particularly important for sensing influenza and Sendai viruses. MDA5, by contrast, detects long double-stranded RNA and is essential for sensing picornaviruses, including encephalomyocarditis virus. Both RIG-I and MDA5 signal through the mitochondrial adaptor protein MAVS (also known as IPS-1), which recruits downstream signaling molecules to activate IRF3/7 and NF-kB, culminating in the production of type I interferons and pro-inflammatory cytokines.

From a clinical perspective, gain-of-function mutations in MDA5 (encoded by the IFIH1 gene) are associated with the type I interferonopathies Aicardi-Goutieres syndrome and Singleton-Merten syndrome, conditions characterized by constitutive interferon production leading to neurologic, dermatologic, and vascular pathology.

### C-Type Lectin Receptors (CLRs)

The C-type lectin receptors represent a family of pattern recognition receptors with particular importance in antifungal immunity. Dectin-1 recognizes beta-glucan, a major structural component of fungal cell walls, and signals through the Syk kinase and CARD9 adaptor to activate NF-kB and induce pro-inflammatory cytokine production. Dectin-2 recognizes high-mannose structures on fungal surfaces, while Mincle detects both damaged host cells and mycobacterial trehalose dimycolate (cord factor). The importance of this pathway is underscored by the clinical phenotype of CARD9 deficiency, which confers profound susceptibility to invasive fungal infections, particularly with Candida species and dermatophytes, while leaving other aspects of immunity relatively intact.

### Cytosolic DNA Sensors

The cGAS-STING pathway represents the principal mechanism by which the innate immune system detects cytosolic double-stranded DNA, a molecular danger signal that arises during infection with DNA viruses, intracellular bacterial pathogens, and in certain autoimmune and autoinflammatory conditions. The enzyme cGAS (cyclic GMP-AMP synthase) binds cytosolic dsDNA in a sequence-independent manner and catalyzes the synthesis of the second messenger cGAMP (cyclic GMP-AMP), which in turn activates the endoplasmic reticulum-resident adaptor protein STING (stimulator of interferon genes). Activated STING recruits TBK1 kinase and the transcription factor IRF3, driving potent type I interferon production.

The clinical significance of this pathway is exemplified by SAVI (STING-associated vasculopathy with onset in infancy), a condition caused by gain-of-function mutations in STING that lead to constitutive type I interferon production. SAVI manifests with interstitial lung disease, cutaneous vasculopathy with acral ulceration, and systemic inflammation, and it represents one of the prototypic type I interferonopathies.

## The Inflammasome

### Structure and Assembly

Inflammasomes are multiprotein oligomeric complexes that assemble in the cytoplasm in response to a variety of danger signals. The canonical inflammasome consists of three core components: a sensor protein (typically an NLR family member or the AIM2 protein), the adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD, also known as PYCARD), and the effector cysteine protease caspase-1. Upon activation, the sensor protein oligomerizes and recruits ASC through homotypic pyrin domain (PYD) interactions. ASC then polymerizes into large filamentous assemblies that coalesce into a single perinuclear aggregate approximately one micrometer in diameter, visible by immunofluorescence microscopy as the characteristic "ASC speck." This speck serves as a platform for the recruitment and proximity-induced auto-activation of pro-caspase-1.

### NLRP3 Inflammasome

The NLRP3 inflammasome is the most clinically relevant and extensively studied of the inflammasome complexes. Its activation follows a tightly regulated two-signal model. Signal 1, termed the priming signal, is typically provided by TLR engagement or other NF-kB-activating stimuli. Priming accomplishes two essential functions: it upregulates the transcription of NLRP3 itself (which is expressed at low basal levels insufficient for inflammasome formation) and it induces the transcription of pro-IL-1beta, the inactive precursor of the mature cytokine. Signal 2, the activation signal, can be provided by a remarkably diverse array of stimuli, including potassium efflux through ion channels such as the P2X7 receptor (activated by extracellular ATP), the generation of mitochondrial reactive oxygen species, lysosomal destabilization with release of cathepsins into the cytosol, and calcium flux. The breadth of activating stimuli has led to the hypothesis that NLRP3 does not detect these signals directly but rather responds to a common cellular perturbation, such as potassium efflux or mitochondrial dysfunction, that they collectively induce.

The list of known NLRP3 activators is extensive and includes ATP, the bacterial toxin nigericin, the vaccine adjuvant alum, silica crystals, cholesterol crystals, monosodium urate crystals (the causative agent of gout), and amyloid-beta peptide (implicated in Alzheimer's disease). A critical co-factor for NLRP3 inflammasome assembly is NEK7 (NIMA-related kinase 7), a cell cycle-associated kinase that directly binds the NLRP3 leucine-rich repeat domain and is required for NLRP3 oligomerization.

<image>A step-by-step illustration of NLRP3 inflammasome assembly and activation. Panel A shows Signal 1 (priming): a TLR on the cell surface recognizing LPS, with downstream NF-kB activation leading to transcription of NLRP3, pro-IL-1beta, and pro-IL-18. Panel B shows Signal 2 (activation): potassium efflux through P2X7 receptor (triggered by extracellular ATP), mitochondrial ROS generation, and lysosomal rupture releasing cathepsins. Panel C shows the assembled inflammasome complex with NLRP3 oligomer, ASC filaments forming a speck, and recruited pro-caspase-1. Panel D shows active caspase-1 cleaving pro-IL-1beta and pro-IL-18 into mature forms, and gasdermin D into its pore-forming N-terminal fragment. Medical illustration style with labeled molecular components.</image>

### Other Inflammasomes

Beyond NLRP3, several other inflammasome complexes have been characterized. The NLRC4 inflammasome is activated by cytosolic flagellin and type III secretion system (T3SS) components through a family of upstream NAIP (NLR family apoptosis inhibitory protein) sensors. In humans, NAIP5 and NAIP6 detect flagellin, NAIP1 detects the T3SS needle protein, and NAIP2 detects the T3SS inner rod protein. Gain-of-function mutations in NLRC4 cause autoinflammation with infantile enterocolitis (AIFEC), a severe inflammatory condition presenting in the first weeks of life with recurrent fevers, enterocolitis, and macrophage activation syndrome.

The AIM2 (absent in melanoma 2) inflammasome is unique in that it does not belong to the NLR family. Instead, AIM2 uses a HIN-200 domain to directly bind cytosolic double-stranded DNA, making it an important sensor of DNA viruses and intracellular bacteria that escape the phagosome. AIM2 recruits ASC through its pyrin domain and activates caspase-1 through the same downstream pathway as NLR-based inflammasomes.

The pyrin inflammasome represents a particularly elegant sensing mechanism. Rather than detecting a specific microbial product, pyrin monitors the activity of Rho GTPases, which are commonly targeted and inactivated by bacterial toxins. When Rho GTPase modification is detected, pyrin is activated and assembles an inflammasome complex. Mutations in the MEFV gene encoding pyrin cause Familial Mediterranean Fever (FMF), the most common monogenic autoinflammatory disease, characterized by recurrent episodes of serositis (peritonitis, pleuritis, pericarditis), fever, and risk of secondary AA amyloidosis.

### Downstream Effectors

The effector protease caspase-1, historically known as IL-1beta-converting enzyme (ICE), is the central enzymatic output of all canonical inflammasomes. Active caspase-1 cleaves two critical pro-inflammatory cytokine precursors: pro-IL-1beta (31 kDa) to its mature biologically active form (17 kDa), and pro-IL-18 to mature IL-18. IL-1beta is a potent pyrogen and pro-inflammatory mediator, while IL-18 promotes IFN-gamma production and thus bridges innate and adaptive immunity.

In addition to cytokine processing, caspase-1 cleaves gasdermin D (GSDMD), a critical executioner of inflammatory cell death. The N-terminal fragment of gasdermin D oligomerizes and inserts into the plasma membrane, forming pores of 10 to 20 nanometers in diameter. These pores serve two essential functions: they provide a conduit for the release of mature IL-1beta and IL-18, which lack conventional signal peptides for secretory pathway export, and at sufficient density they mediate pyroptosis, a form of inflammatory programmed cell death characterized by cell swelling, membrane rupture, and release of pro-inflammatory intracellular contents.

| Feature | Pyroptosis | Apoptosis | Necroptosis |
|---------|-----------|-----------|-------------|
| Key mediators | Caspase-1, -4, -5, -11 | Caspase-3, -7 (executioner); Caspase-8, -9 (initiator) | RIPK1, RIPK3, MLKL |
| Pore-forming protein | Gasdermin D | None | MLKL |
| Inflammatory? | Yes (highly) | No (immunologically silent) | Yes |
| Membrane integrity | Disrupted (pores then lysis) | Maintained (cleared by phagocytes) | Disrupted (lysis) |
| Cytokine release | IL-1beta, IL-18 | None | DAMPs |

It is important to distinguish pyroptosis from other forms of programmed cell death. Pyroptosis is dependent on inflammatory caspases (caspase-1, -4, -5, and -11) and the pore-forming activity of gasdermin D, and it is inherently pro-inflammatory. Apoptosis, by contrast, is mediated by executioner caspases (caspase-3, -7) activated through either intrinsic (caspase-9) or extrinsic (caspase-8) pathways, and it is generally immunologically silent, with the dying cell maintaining membrane integrity and being cleared by phagocytes without releasing inflammatory mediators. Necroptosis is a third distinct pathway of regulated cell death, dependent on the kinases RIPK1 and RIPK3 and the pseudokinase MLKL, which forms pores in the plasma membrane leading to inflammatory cell lysis.

## Cryopyrin-Associated Periodic Syndromes (CAPS)

### Spectrum of Disease

The cryopyrin-associated periodic syndromes represent a clinical spectrum of autoinflammatory diseases caused by gain-of-function mutations in NLRP3 (previously designated CIAS1). These mutations result in constitutive or easily triggered NLRP3 inflammasome activation with overproduction of IL-1beta. The CAPS spectrum encompasses three conditions of increasing severity, all inherited in an autosomal dominant pattern.

| CAPS Subtype | Severity | Key Features | Onset | Distinguishing Complications |
|-------------|----------|--------------|-------|------------------------------|
| FCAS | Mildest | Cold-triggered urticarial rash, fever, arthralgias | First 6 months of life | Episodes resolve within 24 hours |
| MWS | Intermediate | Urticarial rash, sensorineural hearing loss | Childhood | Secondary AA amyloidosis |
| NOMID/CINCA | Most severe | Continuous inflammation, chronic aseptic meningitis, destructive arthropathy | Birth | Intellectual disability, epiphyseal overgrowth |

Familial Cold Autoinflammatory Syndrome (FCAS) represents the mildest form and is characterized by cold-triggered episodes of urticarial rash, fever, and arthralgias, typically with onset in the first six months of life. Episodes occur within one to two hours of cold exposure and resolve spontaneously within 24 hours. Muckle-Wells Syndrome (MWS) occupies the middle of the severity spectrum and features urticarial rash, progressive sensorineural hearing loss, and a significant risk of developing secondary AA amyloidosis with renal deposition over time. NOMID/CINCA (Neonatal-Onset Multisystem Inflammatory Disease, also known as Chronic Infantile Neurologic Cutaneous and Articular syndrome) represents the most severe end of the spectrum. NOMID is characterized by continuous systemic inflammation from birth, chronic aseptic meningitis with papilledema and potential for intellectual disability, destructive arthropathy with epiphyseal overgrowth, and persistent urticarial rash. Notably, approximately 40 percent of NOMID patients are somatic mosaics, meaning the NLRP3 mutation is present in only a fraction of cells and may not be detected by standard Sanger sequencing, requiring more sensitive techniques such as next-generation sequencing or droplet digital PCR.

### Treatment with IL-1 Blockade

The recognition that excessive IL-1beta production drives the entire spectrum of CAPS has led to transformative therapeutic advances with IL-1 targeted therapies. Anakinra, a recombinant human IL-1 receptor antagonist administered at 100 mg subcutaneously daily, produces a dramatic response within hours in CAPS patients, with resolution of fever, rash, and inflammatory markers. Canakinumab, a fully human monoclonal antibody directed against IL-1beta, offers the convenience of subcutaneous injection every eight weeks at a dose of 150 mg and has received FDA approval for CAPS. Rilonacept, a dimeric fusion protein that acts as an IL-1 trap by binding both IL-1alpha and IL-1beta, is administered at 160 mg subcutaneously weekly and is also FDA-approved for CAPS. All three agents have demonstrated sustained efficacy in long-term follow-up studies, and early initiation of IL-1 blockade can prevent or stabilize organ damage, including hearing loss and amyloidosis.

| Agent | Mechanism | Dose/Route | Frequency | Target |
|-------|-----------|------------|-----------|--------|
| Anakinra | IL-1 receptor antagonist | 100 mg SC | Daily | IL-1alpha and IL-1beta (receptor level) |
| Canakinumab | Anti-IL-1beta monoclonal antibody | 150 mg SC | Every 8 weeks | IL-1beta only |
| Rilonacept | IL-1 trap (dimeric fusion protein) | 160 mg SC | Weekly | IL-1alpha and IL-1beta |

## Clinical Applications in Allergy/Immunology

### Innate Immunity in Allergic Disease

The innate immune system plays a critical and increasingly recognized role in the initiation and amplification of allergic inflammation. Epithelial cells at mucosal surfaces produce a trio of alarmins, TSLP (thymic stromal lymphopoietin), IL-25, and IL-33, in response to allergen exposure, epithelial damage, proteases, and other environmental insults. These alarmins serve as a bridge between innate and type 2 adaptive immunity by activating dendritic cells toward a Th2-promoting phenotype and directly stimulating group 2 innate lymphoid cells (ILC2s). ILC2s, upon activation by IL-33, produce copious amounts of IL-5 and IL-13, driving eosinophilic inflammation and mucus production independent of adaptive immune recognition. This ILC2-driven pathway explains why innate immune cells can initiate and sustain type 2 inflammation even in the absence of antigen-specific T cell responses.

The NLRP3 inflammasome has been implicated in allergic sensitization through its activation by environmental adjuvants such as diesel exhaust particles, silica, and the vaccine adjuvant alum. By promoting IL-1beta release, these activators may lower the threshold for allergic sensitization and amplify the subsequent Th2 response. Conversely, TLR agonists are being exploited therapeutically as adjuvants in allergen immunotherapy. CpG oligonucleotides (TLR9 agonists) conjugated to allergens promote a Th1-skewing immune response, while monophosphoryl lipid A (MPL, a TLR4 agonist) is incorporated into adjuvanted allergy vaccines to enhance tolerogenic immune responses.

### Innate Immune Defects

Defects in innate immune signaling pathways cause a distinct spectrum of immunodeficiency syndromes. MyD88 deficiency, an autosomal recessive condition, leads to impaired signaling downstream of most TLRs and the IL-1 receptor, resulting in a surprisingly narrow susceptibility to invasive pyogenic bacterial infections, particularly with Streptococcus pneumoniae and Staphylococcus aureus. IRAK-4 deficiency phenocopies MyD88 deficiency because IRAK-4 is the kinase immediately downstream of MyD88 in the TLR signaling cascade. Both conditions are notable for causing severe invasive pneumococcal disease in childhood that tends to improve with age as adaptive immunity matures and compensates. TLR3 deficiency presents with a very specific phenotype of herpes simplex encephalitis, reflecting the critical role of TLR3-mediated type I interferon production in controlling HSV-1 in the central nervous system.

STING gain-of-function mutations cause SAVI (STING-associated vasculopathy with onset in infancy), characterized by interstitial lung disease with pulmonary fibrosis, cutaneous vasculopathy with acral ulceration and necrosis, and an elevated type I interferon gene signature. SAVI represents a prototypic type I interferonopathy and is treated with JAK inhibitors to suppress the downstream effects of constitutive interferon signaling.

| Defect | Inheritance | Pathway Affected | Susceptibility | Distinguishing Feature |
|--------|-------------|-----------------|----------------|----------------------|
| MyD88 deficiency | Autosomal recessive | TLR/IL-1R signaling | Invasive pyogenic bacteria (S. pneumoniae, S. aureus) | Improves with age |
| IRAK-4 deficiency | Autosomal recessive | TLR/IL-1R signaling (kinase downstream of MyD88) | Invasive pyogenic bacteria | Phenocopies MyD88 deficiency |
| TLR3 deficiency | Autosomal recessive | TRIF pathway / type I IFN | Herpes simplex encephalitis | Narrow susceptibility to HSV-1 CNS infection |
| CARD9 deficiency | Autosomal recessive | CLR/Syk signaling | Invasive fungal infections (Candida, dermatophytes) | Selective antifungal immune defect |
| STING GOF (SAVI) | Autosomal dominant | cGAS-STING / type I IFN | Autoinflammation (not infection) | ILD, cutaneous vasculopathy; treated with JAK inhibitors |

<image>A clinical photograph-style illustration showing the characteristic features of cryopyrin-associated periodic syndromes across the CAPS spectrum. Three panels arranged left to right showing increasing severity: FCAS (cold-induced urticarial rash on extremities), MWS (urticarial rash plus depiction of sensorineural hearing loss with audiogram), and NOMID/CINCA (frontal bossing, joint deformity with epiphyseal overgrowth, and fundoscopic view showing papilledema). Below each panel, a timeline showing age of onset and key clinical milestones.</image>

### Trained Immunity

The concept of trained immunity has fundamentally challenged the classical paradigm that innate immune cells are incapable of memory. Trained immunity refers to the epigenetic reprogramming of innate immune cells, particularly monocytes and macrophages, following an initial inflammatory stimulus that results in an enhanced response to subsequent, even unrelated, challenges. The molecular basis involves stable histone modifications, particularly the activating mark H3K4me3 (trimethylation of histone H3 at lysine 4) at the promoters of inflammatory genes such as TNF-alpha and IL-6.

BCG vaccination has emerged as the most studied inducer of trained immunity. Epidemiologic data suggesting that BCG reduces all-cause mortality in neonates beyond what would be expected from tuberculosis prevention alone has been corroborated by mechanistic studies demonstrating that BCG induces trained immunity through metabolic rewiring of monocytes. This reprogramming involves a shift from oxidative phosphorylation to aerobic glycolysis (a Warburg-like effect), accumulation of the metabolite fumarate (which inhibits histone demethylases, stabilizing H3K4me3 marks), and activation of the mevalonate pathway. These findings have clinical relevance for understanding the heterologous protective effects of BCG against non-mycobacterial infections and have spurred ongoing research into the therapeutic potential of trained immunity in infectious disease and cancer.

## Key Clinical Pearls

- TLR4 is unique among TLRs in signaling through both MyD88 and TRIF pathways
- TLR3 is the only TLR that signals exclusively through TRIF (not MyD88)
- NLRP3 inflammasome requires two signals: priming (NF-kB) and activation (K+ efflux, ROS, etc.)
- NOD2 loss-of-function mutations cause Crohn's disease; gain-of-function mutations cause Blau syndrome
- MyD88 and IRAK-4 deficiency present with invasive pyogenic infections that improve with age
- CAPS is a spectrum (FCAS-MWS-NOMID) all caused by NLRP3 gain-of-function and all respond to IL-1 blockade
- Gasdermin D pore formation is the final common pathway for pyroptosis and IL-1beta/IL-18 secretion

<image>A comprehensive flowchart/algorithm for the clinical evaluation of a patient with suspected autoinflammatory disease involving inflammasome pathways. Starting node: "Recurrent fevers + systemic inflammation (elevated CRP/SAA)." Decision branches include: cold-triggered episodes (consider FCAS/CAPS), associated hearing loss (MWS), periorbital edema and meningitis (NOMID), peritonitis/serositis (FMF - pyrin inflammasome), and macrophage activation (NLRC4 GOF). Each branch leads to specific genetic testing panels and first-line treatments (anakinra, canakinumab, colchicine). Color-coded boxes for diagnosis, testing, and treatment.</image>

## References
1. Takeuchi O, Akira S. Pattern recognition receptors and inflammation. *Cell*. 2010;140(6):805-820.
2. Broz P, Dixit VM. Inflammasomes: mechanism of assembly, regulation and signalling. *Nat Rev Immunol*. 2016;16(7):407-420.
3. Hoffman HM, Broderick L. The role of the inflammasome in patients with autoinflammatory diseases. *J Allergy Clin Immunol*. 2016;138(1):3-14.
4. Netea MG, et al. Trained immunity: a program of innate immune memory in health and disease. *Science*. 2016;352(6284):aaf1098.
5. de Jesus AA, Canna SW, Liu Y, Goldbach-Mansky R. Molecular mechanisms in genetically defined autoinflammatory diseases: disorders of amplified danger signaling. *Annu Rev Immunol*. 2015;33:823-874.