Innate Immunity and Pattern Recognition Receptors

Innate Immunity and Pattern Recognition Receptors

Dr. Rebecca Coll

• Email: r.coll@qub.ac.uk

Learning Outcomes

• Distinguish between the Innate and Adaptive Immune Systems

• Understand which immune cells and soluble mediators are involved in the Innate Immune system

• Understand the temporal kinetics of the immune response

• Provide a general view of the innate immune system including key cell types involved

• Identify the main pattern recognition receptor families and cite at least one ligand for each of them

• Know the key adaptor proteins and the main signalling pathways activated

• Describe the key outcomes of PRR signalling

The Immune System in a Nutshell

• Immune Response Components:

  • Recognition of the pathogen

  • Mounting an appropriate reaction against the pathogen to eliminate it

• Types of Immune Response:

  • Natural or Innate (non-adaptive) immunity

  • Acquired or Specific (adaptive) immunity

The Innate Immune System

• Characteristics:

  • Universal and evolutionary conserved mechanism of host defence

  • First line of defence, predating adaptive response

  • Present in all multicellular organisms

  • Utilizes receptors and effectors for general protection

  • Distinguishes self from non-self

  • Defects are rare and almost always lethal

  • Example: Toll mutant Drosophila cannot mount an effective defence to bacteria or fungi, leading to death

  • Nobel Prize awarded to Jules Hoffman in 2011 for contributions to the understanding of innate immunity

Kinetics of the Innate and Adaptive Responses

• Infection Phases:

  • Divided into three phases:

  1. Activation of the Innate Immune System (Phase 1)

  2. Continued Innate Immune Response (Phase 2)

  3. Adaptive Immunity (Phase 3) which results in clonal expansion of antigen-specific lymphocytes

• Response Characteristics:

  • Innate Response:

  • Rapid

  • Less specific

  • First line of defence

  • Adaptive Response:

  • Slow

  • Highly specific

  • Enhanced by repeated exposure

Components of the Innate Immune System

1. Physical Barriers:

   • Skin

   • Cough reflex

   • Ciliary movement in respiratory tract

   • Gastric pH as a barrier to pathogens

2. Soluble Factors:

   • Complement system

   • Surfactant

   • Antimicrobial peptides

   • Cytokines

3. Cellular Components:

   • Neutrophils

   • Monocytes

   • Macrophages (Mϕ)

   • Dendritic Cells (DCs)

   • Epithelial Cells

Natural Physical, Chemical, and Mechanical Barriers

• Skin: Stratified epithelium provides a strong physical barrier.

• Gut: Single cell layer of columnar epithelium with protective mechanisms.

• Lungs: Pseudostratified columnar epithelium in upper airways and single cell layer in lower airways.

• Tears and Mucus: Protection from pathogens through mechanisms such as tears, nasal cilia, and the low pH in stomach.

• Chemical Factors:

  • Fatty acids, enzymes like pepsin and lysozyme present in tears and saliva.

  • Antimicrobial peptides like defensins and cathelicidin.

Innate Immunity – Key Cell Types

• Neutrophils:

  • Type of granulocyte

  • Contains cytoplasmic granules

  • Polymorphonuclear

  • Engages in phagocytosis

  • Short lifespan (hours)

  • Crucial for clearing bacterial infections

• Monocytes / Macrophages:

  • Monocytes circulate as precursors to macrophages

  • Differentiated into tissue-specific macrophages, e.g., alveolar macrophages, Kupffer cells, microglia, etc.

  • Functions include phagocytosis, antigen presentation, activation of T cells

• Dendritic Cells:

  • Primarily found in lymphoid tissue

  • Serve as professional APCs and are potent stimulators of T-cell responses

Distribution of Immune Cells in the Human Body

• Total immune cells: Approximately $1.8 imes 10^{12}$

  • Locations:

  • Lungs: 95 ext{ extit{% CI}} ext{ } 1.5-2.3 imes 10^{12}

  • Blood: $4 imes 10^{10}$

  • Liver: $5 imes 10^{10}$

  • GI tract: $5 imes 10^{10}$

  • Lymphatic system: $7 imes 10^{11}$

  • Bone marrow: $7 imes 10^{11}$

  • Skin: $8 imes 10^{10}$

How Does the Innate Immune System ‘Sense’ Infection?

• The concept proposed by Charles Janeway in 1989 suggests that pathogen sensing is mediated by germline-encoded pattern recognition receptors (PRRs) that detect conserved products of microbial biosynthetic pathways

Pathogen Associated Molecular Patterns (PAMPs)

• PRRs Definition:

  • Recognize conserved molecular structures from pathogens known as PAMPs.

  • PAMPs are specific to microorganisms and essential for their viability or virulence.

PRRs Trigger Cell Defense

• Outcomes of PRR activation include:

  • Production of cytokines, chemokines, and interferons

  • Engaging in phagocytosis

  • Inducing autophagy

  • Cell death pathways (apoptosis, pyroptosis, necrosis)

  • Antigen presentation and upregulation of co-stimulatory molecules

  • Critical for initiating the adaptive immune response

Membrane Associated PRRs – Toll-Like Receptors (TLRs)

• Overview:

  • 11 members in mammals that recognize diverse ligands from bacteria, viruses, fungi, protozoa, and host.

• PAMP Recognition:

  • TLR Family recognizes various PAMPs:

  • TLR1/2: Lipopeptides

  • TLR2/6: Lipopeptides

  • TLR4: Lipopolysaccharide (LPS)

  • TLR5: Flagellin

  • TLR9: CpG DNA

TLR Signalling Mechanism

• Upon ligand binding, TLRs dimerize (homo or hetero), triggering intracellular kinase signalling cascades.

• Key kinases activated include IRAKs, TAK1, MAPKs, and IKKs which lead to the activation of transcription factors such as NFκB, AP1, and CREB.

• Resulting transcription factor activity promotes the production of pro-inflammatory cytokines (e.g., IL-1, IL-6, TNF).

TLR4 and Gram-Negative Bacteria Sensing

• TLR4 is essential for sensing gram-negative bacteria through LPS.

• Studies revealed that macrophages deficient in TLR4 cannot produce TNF or nitric oxide in response to LPS.

• An important figure in this research is Prof. Shizuo Akira, awarded the Nobel Prize in 2011.

Endosomal Membrane TLRs

• TLRs in endosomes activate transcription factors IRF3/7, enhancing the production of type I interferons.

• Additionally, they can activate TRAF6, triggering MAPK and NFκB pathways.

TLR Signalling via Adapter Proteins

• Key Adapter Proteins:

  • MyD88: common adapter for TLRs, except TLR3

  • Mal/TIRAP: a bridging adapter for MyD88

  • Endosomal TLR4 utilizes TRAM to connect to TRIF (also used by TLR3).

MyD88 Functionality in Bacterial Sensing

• Mice deficient in MyD88 are resistant to LPS shock and cannot produce IL-6, TNF, or IL-1 in response to LPS.

  • The functional understanding of MyD88 highlights its critical role in innate immunity.

TLR – Key Signalling Pathways

• Major pathways activated by TLR stimulation include:

  • NFκB Pathway: Leads to pro-inflammatory responses

  • MAPK Pathway: Activates AP1 and CREB for increased cytokine production

  • IRF Pathway: Activates the transcription of interferon genes

TLR – Key Signalling Outcomes

• Outcomes upon TLR engagement include:

  • Induction of pro-inflammatory cytokines such as IL-1, IL-6, TNF, IL-12

  • Production of anti-inflammatory cytokines (e.g., IL-10)

  • Release of chemokines attracting immune cells

  • Synthesis of anti-microbial molecules (defensins)

  • Enhanced antigen presentation through CD80/86 and CD40 co-stimulatory molecules.

Membrane Associated PRRs – C-type Lectin Receptors (CLRs)

• Functionality:

  • CLRs recognize carbohydrates from various sources including fungi and bacteria.

• Signalling activates transcription factors such as NFκB, AP1, and NFAT.

CLR Signalling Outcomes

• Outcomes from CLR activation consist of:

  • Inducing pro-inflammatory cytokines (e.g., IL-6, IL-12)

  • Augmenting anti-inflammatory cytokine production (e.g., IL-10)

  • Chemokines recruitment to infection sites

  • Synthesis of anti-microbial agents (defensins)

Cytosolic PRRs - Nod-like Receptors (NLRs)

• Key Sensors:

  • NOD1 and NOD2 act as cytosolic sensors for bacterial peptidoglycan and changes in the cell cytoskeleton.

• Signalling Mechanism:

  • RIP2 is the key adaptor for NOD signalling, activating NFκB and MAPK pathways.

Key Outcomes from NOD1 and NOD2 Activation

• Induced elements include:

  • Production of pro-inflammatory cytokines (e.g., IL-6, TNF)

  • Anti-inflammatory cytokines production (e.g., IL-10)

  • Release of chemokines attracting immune cells

  • Synthesis of anti-microbial molecules (defensins)

Cytosolic PRRs - RIG-I-like Receptors (RLRs)

• Receptors Identified:

  • Include RIG-I, MDA5, and LGP2, primarily sensing viral RNA.

• Importance of Cytosolic Movement:

  • They shuttled to mitochondria for signalling to occur.

RLR Signalling Mechanism

• Critical Adaptor:

  • IPS-1 serves as the key adaptor leading to activation of NFκB, MAPK, and IRF pathways.

Cytosolic PRRs – DNA Sensors

• Examples of DNA Sensors:

  • DAI, IFI16, DDX41, DNA-PK, and AIM2.

• Functionality:

  • Detect DNA from pathogens and host, initiating IRF and NFκB signalling.

DNA Sensor Signalling

• Universal Sensor Role:

  • cGAS acts as the universal DNA sensor, generating cyclic GMP-AMP (cGAMP) from ATP and GTP.

• Importance of cGAMP:

  • cGAMP is recognized by STING, crucial for the activation of IRF pathway.

• STING Trafficking:

  • Moves from ER to Golgi to mediate downstream signalling.

Primary Immunodeficiencies of Cytosolic PRRs

• Influence of Genetic Function:

  • Gain of function or loss of function can lead to autoinflammation and severe immune dysfunction.

Innate and Adaptive Immune Systems Synergy

• Connection:

  • There is close synergy between innate and adaptive systems

  • Adaptive responses enhance innate responses efficiency.

  • The innate response lays groundwork for activating adaptive immunity.

Further Reading

• Immunobiology, 9th Edition, by Janeway (Chapters 1/2/3)

• Kuby Immunology (Chapter 4 or 5 on innate immunity)

• Articles available on Canvas

• Contact Email: r.coll@qub.ac.uk