Receptors Of The Innate Immune System

Pattern recognition is the cornerstone of innate immunity, enabling rapid detection of pathogens and initiation of defense mechanisms. This recognition relies heavily on receptors of the innate immune system, specialized molecules that recognize conserved microbial components known as pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs). These receptors trigger a cascade of intracellular signaling events, leading to the production of cytokines, chemokines, and other effector molecules that coordinate the inflammatory response and activate adaptive immunity.

Introduction to Innate Immune Receptors

Innate immune receptors are germline-encoded, meaning they are inherited and do not require rearrangement or somatic mutation like the receptors of the adaptive immune system (T cell receptors and B cell receptors). This allows for immediate recognition of threats without prior exposure. These receptors are expressed on a variety of immune cells, including macrophages, dendritic cells, neutrophils, natural killer (NK) cells, and even some non-immune cells like epithelial cells.

The ability of innate immune receptors to discriminate between self and non-self is crucial to prevent autoimmunity. While they primarily recognize PAMPs derived from pathogens, they can also detect DAMPs, which are released by damaged or stressed cells. This allows the innate immune system to respond not only to infection but also to tissue injury or cellular stress.

Key Features of Innate Immune Receptors:

• Germline-encoded: Ensures rapid and immediate response.
• Broad specificity: Recognize conserved molecular patterns.
• Expressed on various cell types: Enable widespread surveillance.
• Discrimination between self and non-self: Prevents autoimmunity.
• Activation of downstream signaling pathways: Initiate inflammation and adaptive immunity.

Major Classes of Innate Immune Receptors

Several families of receptors contribute to the innate immune response, each recognizing a distinct set of ligands and initiating specific signaling cascades. Key classes include:

1. Toll-like Receptors (TLRs)
2. NOD-like Receptors (NLRs)
3. RIG-I-like Receptors (RLRs)
4. C-type Lectin Receptors (CLRs)
5. DNA Sensors

Let's explore each of these in detail.

1. Toll-like Receptors (TLRs)

TLRs are arguably the most well-characterized family of innate immune receptors. Discovered in Drosophila melanogaster, where they were found to be essential for antifungal immunity, TLRs are transmembrane proteins that recognize a wide range of PAMPs derived from bacteria, viruses, fungi, and parasites.

Structure and Localization:

TLRs are type I transmembrane proteins characterized by an extracellular leucine-rich repeat (LRR) domain responsible for ligand recognition and an intracellular Toll/IL-1 receptor (TIR) domain involved in signaling.

TLRs are strategically localized to different cellular compartments:

• Cell surface TLRs (TLR1, TLR2, TLR4, TLR5, TLR6, TLR10): Primarily recognize microbial membrane components, such as lipids, lipoproteins, and proteins.
• Endosomal TLRs (TLR3, TLR7, TLR8, TLR9): Recognize nucleic acids derived from viruses and bacteria that have been internalized into endosomes.

Ligand Specificity:

Each TLR recognizes a distinct set of PAMPs:

• TLR1:TLR2 heterodimer: Triacyl lipoproteins (bacteria, mycobacteria)
• TLR2:TLR6 heterodimer: Diacyl lipoproteins (bacteria, mycobacteria)
• TLR3: Double-stranded RNA (dsRNA) (viruses)
• TLR4: Lipopolysaccharide (LPS) (Gram-negative bacteria)
• TLR5: Flagellin (bacteria)
• TLR7: Single-stranded RNA (ssRNA) (viruses)
• TLR8: Single-stranded RNA (ssRNA) (viruses)
• TLR9: CpG DNA (bacteria, viruses)
• TLR10: Ligand still being investigated (potentially involved in immune regulation)

Signaling Pathways:

Upon ligand binding, TLRs recruit adaptor proteins containing a TIR domain, such as MyD88, TRIF, TIRAP, and TRAM. These adaptor proteins initiate downstream signaling cascades, leading to the activation of transcription factors like NF-κB and IRFs (Interferon Regulatory Factors).

• MyD88-dependent pathway: Activation of NF-κB, leading to the production of pro-inflammatory cytokines such as TNF-α, IL-1β, and IL-6.
• TRIF-dependent pathway: Activation of IRFs, leading to the production of type I interferons (IFN-α/β), which are crucial for antiviral immunity.

Role in Immunity and Disease:

TLRs play a critical role in initiating inflammatory responses and bridging innate and adaptive immunity. They contribute to:

• Antimicrobial defense: Activation of macrophages, dendritic cells, and neutrophils, leading to phagocytosis, cytokine production, and antigen presentation.
• Adaptive immune responses: Upregulation of costimulatory molecules on antigen-presenting cells (APCs), enhancing T cell activation.
• Autoimmune diseases: Aberrant TLR activation can contribute to chronic inflammation and autoimmunity. For example, TLR7 and TLR9 have been implicated in systemic lupus erythematosus (SLE).
• Inflammatory diseases: TLRs are involved in the pathogenesis of inflammatory bowel disease (IBD), atherosclerosis, and other chronic inflammatory conditions.

2. NOD-like Receptors (NLRs)

NLRs are intracellular sensors that detect PAMPs and DAMPs in the cytoplasm. Unlike TLRs, which are transmembrane receptors, NLRs are cytosolic proteins. They play a critical role in regulating inflammation, apoptosis, and autophagy.

Structure and Classification:

NLRs share a common modular structure:

• LRR domain: Responsible for ligand recognition.
• NACHT domain: Involved in self-oligomerization and activation.
• N-terminal effector domain: Mediates downstream signaling. This domain varies among different NLRs and determines their specific function. Common effector domains include:
  • CARD (Caspase recruitment domain): Found in NLRP1, NLRC4, and others. Mediates interaction with caspases.
  • PYD (Pyrin domain): Found in NLRP3. Mediates interaction with the adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD).
  • BIR (Baculovirus inhibitor of apoptosis repeat): Found in NAIPs.

Based on their N-terminal effector domain, NLRs are classified into several subfamilies, including:

• NLRPs (NLR family, pyrin domain-containing): NLRP1, NLRP3, NLRP6, NLRP7, NLRP12, etc.
• NLRCs (NLR family, CARD domain-containing): NLRC4, NLRC5, etc.
• NLRBs (NLR family, BIR domain-containing): NAIPs (NLR apoptosis inhibitory proteins)

Ligand Specificity:

NLRs recognize a diverse range of ligands, including:

• NLRP3: Recognizes a wide array of stimuli, including:
  • PAMPs: Bacterial toxins (e.g., nigericin), viral RNA
  • DAMPs: ATP, uric acid crystals, cholesterol crystals, amyloid-beta
  • Environmental irritants: Silica, asbestos
• NLRC4: Recognizes bacterial flagellin and components of the bacterial type III secretion system (T3SS).
• NLRP1: Recognizes bacterial muramyl dipeptide (MDP) and other bacterial products.
• NAIPs: Recognize bacterial flagellin and components of the bacterial T3SS.

Signaling Pathways:

Many NLRs, particularly NLRP3 and NLRC4, activate the inflammasome, a multi-protein complex that leads to the activation of caspase-1. Caspase-1 then cleaves pro-IL-1β and pro-IL-18 into their mature, bioactive forms, IL-1β and IL-18, respectively. These cytokines are potent pro-inflammatory mediators that play a key role in host defense and inflammation.

• NLRP3 inflammasome activation:
  1. Recognition of PAMPs or DAMPs by NLRP3.
  2. Oligomerization of NLRP3, recruitment of ASC, and activation of caspase-1.
  3. Caspase-1 cleavage of pro-IL-1β and pro-IL-18.
  4. Secretion of IL-1β and IL-18, leading to inflammation.

Role in Immunity and Disease:

NLRs play a crucial role in:

• Antimicrobial defense: Activation of the inflammasome, leading to the release of IL-1β and IL-18, which enhance macrophage activation, neutrophil recruitment, and adaptive immune responses.
• Autoimmune diseases: Dysregulation of NLRP3 inflammasome activation has been implicated in autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, and type 1 diabetes.
• Inflammatory diseases: NLRP3 inflammasome activation contributes to the pathogenesis of gout, atherosclerosis, and Alzheimer's disease.
• Cancer: The role of NLRs in cancer is complex and context-dependent. In some cases, NLR activation can promote anti-tumor immunity, while in others, it can contribute to tumor development and progression.

3. RIG-I-like Receptors (RLRs)

RLRs are cytoplasmic sensors that detect viral RNA. They are essential for initiating antiviral immunity by inducing the production of type I interferons (IFN-α/β) and other antiviral cytokines.

Structure and Classification:

The RLR family consists of three members:

• RIG-I (Retinoic acid-inducible gene I): Recognizes short double-stranded RNA (dsRNA) with a 5'-triphosphate.
• MDA5 (Melanoma differentiation-associated gene 5): Recognizes long dsRNA.
• LGP2 (Laboratory of Genetics and Physiology 2): Structurally similar to RIG-I and MDA5 but lacks the CARD domain. It can modulate RLR signaling, but its exact role is still being investigated.

RLRs share a common domain structure:

• CARD (Caspase activation and recruitment domain): Involved in downstream signaling.
• Helicase domain: Involved in RNA binding and unwinding.
• RD (Repressor domain): Regulates RLR activity.

Ligand Specificity:

• RIG-I: Recognizes short dsRNA with a 5'-triphosphate, which is commonly found in viral RNA. It can also recognize RNA containing non-canonical base pairings.
• MDA5: Recognizes long dsRNA, such as that produced during viral replication.

Signaling Pathways:

Upon binding to viral RNA, RIG-I and MDA5 undergo a conformational change that exposes their CARD domains. These CARD domains then interact with the CARD domain of the adaptor protein MAVS (Mitochondrial antiviral signaling protein), which is located on the outer mitochondrial membrane.

MAVS activation triggers downstream signaling cascades, leading to the activation of transcription factors IRF3 and NF-κB. IRF3 translocates to the nucleus and induces the expression of type I interferons (IFN-α/β), while NF-κB induces the expression of pro-inflammatory cytokines.

• RLR signaling pathway:
  1. Recognition of viral RNA by RIG-I or MDA5.
  2. Interaction of RLR CARD domain with MAVS on the mitochondrial membrane.
  3. Activation of IRF3 and NF-κB.
  4. Production of type I interferons (IFN-α/β) and pro-inflammatory cytokines.

Role in Immunity and Disease:

RLRs play a crucial role in:

• Antiviral immunity: Induction of type I interferons, which inhibit viral replication, activate NK cells, and enhance the adaptive immune response.
• Autoimmune diseases: Aberrant RLR activation has been implicated in autoimmune diseases such as SLE and Sjögren's syndrome.
• Cancer: RLR signaling can promote anti-tumor immunity by inducing the expression of type I interferons and activating NK cells.

4. C-type Lectin Receptors (CLRs)

CLRs are a diverse family of pattern recognition receptors that bind to carbohydrates in a calcium-dependent manner. They are expressed on a variety of immune cells, including dendritic cells, macrophages, and neutrophils. CLRs recognize a wide range of glycans present on the surface of bacteria, fungi, viruses, and parasites, as well as endogenous glycans expressed by host cells.

Structure and Classification:

CLRs are type II transmembrane proteins characterized by an extracellular C-type lectin domain (CTLD) that binds to carbohydrates. Based on their structure and function, CLRs are classified into several subfamilies, including:

• Mannose receptor (MR, CD206): Recognizes mannose, fucose, and N-acetylglucosamine.
• Dectin-1: Recognizes β-glucans, which are major components of fungal cell walls.
• Dectin-2: Recognizes α-mannans, which are found on the surface of some fungi and bacteria.
• DC-SIGN (Dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin, CD209): Recognizes mannose and fucose.
• CLEC9A: Recognizes F-actin exposed by necrotic cells.

Ligand Specificity:

CLRs recognize a diverse range of carbohydrates, including:

• Mannose: Found on the surface of many bacteria, fungi, and viruses.
• Fucose: Found on the surface of bacteria and parasites.
• β-glucans: Major components of fungal cell walls.
• α-mannans: Found on the surface of some fungi and bacteria.

Signaling Pathways:

CLR signaling pathways vary depending on the specific receptor and the cell type. Some CLRs, such as Dectin-1, contain an immunoreceptor tyrosine-based activation motif (ITAM) in their cytoplasmic domain, which triggers downstream signaling cascades involving Syk kinase. Other CLRs, such as DC-SIGN, lack an ITAM but can still activate signaling pathways through interactions with other receptors or adaptor proteins.

• Dectin-1 signaling pathway:
  1. Recognition of β-glucans by Dectin-1.
  2. Activation of Syk kinase.
  3. Activation of transcription factors NF-κB and AP-1.
  4. Production of cytokines such as TNF-α, IL-1β, and IL-10.

Role in Immunity and Disease:

CLRs play a crucial role in:

• Antimicrobial defense: Enhancement of phagocytosis, activation of macrophages and dendritic cells, and induction of cytokine production.
• Adaptive immune responses: Modulation of T cell activation and differentiation.
• Autoimmune diseases: Aberrant CLR activation has been implicated in autoimmune diseases such as rheumatoid arthritis and SLE.
• Cancer: The role of CLRs in cancer is complex and context-dependent. In some cases, CLR activation can promote anti-tumor immunity, while in others, it can contribute to tumor development and progression.

5. DNA Sensors

Innate immune cells possess intracellular DNA sensors that detect foreign or misplaced DNA in the cytoplasm. These sensors play a critical role in responding to viral and bacterial infections, as well as detecting cellular damage.

Key DNA Sensors:

• cGAS (cyclic GMP-AMP synthase): cGAS is a cytosolic DNA sensor that recognizes double-stranded DNA (dsDNA). Upon binding to DNA, cGAS catalyzes the synthesis of cyclic GMP-AMP (cGAMP), a second messenger molecule.
• AIM2 (Absent in melanoma 2): AIM2 is an inflammasome sensor that recognizes dsDNA in the cytoplasm. Upon binding to DNA, AIM2 recruits ASC and caspase-1, leading to the activation of the inflammasome and the release of IL-1β and IL-18.
• IFI16 (Interferon-gamma-inducible protein 16): IFI16 is a DNA sensor that can be located in both the nucleus and the cytoplasm. It recognizes dsDNA and can activate both the inflammasome and the STING pathway.

Signaling Pathways:

• cGAS-STING Pathway:
  1. cGAS recognizes dsDNA in the cytoplasm.
  2. cGAS synthesizes cGAMP.
  3. cGAMP binds to STING (Stimulator of interferon genes), an ER-resident protein.
  4. STING translocates from the ER to the Golgi apparatus.
  5. STING activates TBK1 (TANK-binding kinase 1).
  6. TBK1 phosphorylates IRF3, leading to its translocation to the nucleus.
  7. IRF3 induces the expression of type I interferons (IFN-α/β).

Role in Immunity and Disease:

DNA sensors play a crucial role in:

• Antiviral immunity: Detection of viral DNA and induction of type I interferons, which inhibit viral replication and activate NK cells.
• Antibacterial immunity: Detection of bacterial DNA and activation of the inflammasome, leading to the release of IL-1β and IL-18.
• Autoimmune diseases: Aberrant activation of DNA sensors has been implicated in autoimmune diseases such as SLE and Aicardi-Goutières syndrome.
• Cancer: The role of DNA sensors in cancer is complex and context-dependent. In some cases, DNA sensor activation can promote anti-tumor immunity, while in others, it can contribute to tumor development and progression.

Conclusion

Innate immune receptors are essential for the rapid and effective recognition of pathogens and tissue damage. They initiate signaling cascades that activate inflammatory responses, promote phagocytosis, and bridge innate and adaptive immunity. Dysregulation of innate immune receptor signaling can contribute to a variety of diseases, including autoimmune disorders, inflammatory conditions, and cancer. A deeper understanding of these receptors and their signaling pathways is crucial for developing novel therapeutic strategies to treat and prevent these diseases.