Lecture 2: Molecules and Cells of The Immune System

Stages of the Immune Response

• 1. Prevention – barriers

• 2. Awareness-recognition – barrier cells are alerted, with mechanisms present to recognise an infection

• 3. Immediate Response: innate immune cells

Initiation of Phagocytosis

• Phagocytes/ innate cells recognise specific molecules on/in targets:

  • Pathogens: Recognise conserved features like LPS, cell wall, flagella

  • Damaged cells: Recognise damage signals like exposed sugars, cell products, and intracellular ATP

  • Healthy cells: Not recognised by phagocytes (no phagocytosis)

Mannose Receptors

• Part of the C-type lectin receptor family

• Bind to carbohydrate structures (mannose) found on e.g. yeast, parasites, bacteria

Dectin-1/ B-Glucan Receptors

• Part of the C-type Lectin Receptors

• Binds b-glucan structures

• Involved in anti-fungal responses

Scavenger Receptors

• Bind low density lipoproteins, sialic acid

• Found on bacteria and yeast

Lipid Receptors

• Regulate how cells metabolise fats and sugars

• This metabolic control is especially important for macrophages

• Metabolism influences which pathogens macrophages respond to

• Lipid receptors direct immune function through the metabolic path

CR3 and CR4

• Complement receptors

• Recognise consevered features e.g. LPS, liphosphoglycan, bcateria and yeast

Nobel Prize in Physiology and Medicine 2011

• Awarded to Bruce A. Beutler and Jules A. Hoffmann

• “for their discoveries concerning the activation of innate immunity"

• Jules Hoffman: Discovered Toll gene in Drosophila – discovered innate receptors that recognised bacteria

• Bruce Beutler: Discovered Toll Like Receptors in mammals

Toll Like Receptors

• Family of Pattern Recognition Receptors.

• Recognise:

  • highly conserved structures on pathogens called Pathogen Associated Molecular Patterns (PAMPs)

  • Danger Associated Molecular Patterns (DAMPs) from damaged cells e.g. uric acid, Heat Shock Protein

• Located on the cell surface or intracellularly

• Form dimers (homo- or heterodimers) that determine pathogen specificity

• Once activated, they recruit intracellular molecules, resulting in a signalling cascade with the net result of gene transcription of specific cytokines that are important in infection responses and events e.g. phagocytosis

Examples of TLRs

• TLR5 – homodimer that recognises bacteria flagella

• TLR9 recognises CPG DNA

• TLR3 – located intracellularly and recognises doubles stranded RNA – important in virus recognition

• TLR7 and 8 recognise single-stranded RNA

Retinoic Acid Inducible Gene (RIG)-Like Receptor (RLR)

• A pattern recognition family

• Includes RIG-1 and cytosolic receptors that are involved in viral recognition

C-Type Lectin Receptors (CLRs)

• A family of pattern recognition receptors

• Major role in the recognition of yeast, some bacteria and parasitic worms

Nucleotide Binding Domain (NOD)-Like Receptors

• E.g. NOD2, NLRP3, NLRP11, Cytosolic receptors

• Recognise intracellular substances e.g. viruses and bacteria

  • some are more evolved to recognise damage

Damage Receptors

• Detect damage caused by pathogens to initiate phagocytosis

• Do not respond to harmless/ beneficial microbes (e.g. microbiome)

• Key receptors include:

  • NLRs (e.g. NLRP1, NLRP3, NLRC4) – sense cellular stress/damage

  • RAGE – binds to damage markers like calprotectin, a product of neutorphils (inflammation marker in stool/serum)

  • P2X7R – binds extracellular ATP, a danger signal

Phagocyte Receptors

• Cells that act to recognise generic features of pathogens, infections and feature of damaged cells

Phagocytosis

• = eating

• 1. Attachment via PRRs

• 2. Ingestion – cells change their structure to enfold and engulf the pathogen

• 3, Killing – pathogen taken in via the phagocytic vacuole where there are enzymes present to degrade and destroy it

• 4. Degradation

Cytokines

• Soluble hormone like molecules of the immune (and other) systems e.g. Epidermal growth factor

  • any one _________ can act on a cell to make a whole range of _________

Families of Cytokines and Their Main Function

• Interleukins (1-41): Diverse functions in immune response

• Interferons: Specialised for anti-viral response

• Colony Stimulating Factors: Promote differentiation and generation of new immune cells (Haematopoiesis)

• Tumor Necrosis Factor (TNF): Involved in inflammation, targeted in autoimmune disease therapy

• Chemokines: Small cytokines involved in chemotaxis (cell movement)

Types of Cytokine Action

• Will act on other cells that have a receptor for it via:

  • Endocrine: target is located far away – reached via the circulation (rare)

  • Paracrine: act on nearby cells

  • Autocrine: act on the cell that produces it, if it has the receptor present

Inflammasome

• Part of the damage-sensing response

• Triggered when receptors activate a signaling cascade to form the inflammasome

• PRRs-NLRs (e.g., NLRP1, NLRP3, NLRC4) initiate the inflammasome platform, allowing signalling molecules to come together

• Functions:

  • Initiates a specialised type of programmed cell death

  • Enables secretion of active cytokines (IL-1β, IL-18) by cleaving and releasing them from their pro-forms

Drivers of Inflammation

• Cytokines and the recruitment of immune cells

Local Inflammatory Response to Infection

• PRRs on macrophages recognise infection and trigger cytokine production

• Cytokines cause:

  • Changes in vasculature: vessel widening and increased permeability

  • Recruitment of immune cells to the site of damage through leaky blood vessels

• Chemokines mediate recruitment, bringing additional immune cells

• Neutrophils: Major first responders to infection, most common innate immune WBC

• Monocytes: Recruited and can later differentiate into macrophages or dendritic cells

• Results in oedema: Swelling due to plasma fluid and protein accumulation in response to cytokine and inflammatory product generation

• Mast cells may also be recruited

Mast Cells

• Granulocyte

• Innate cells that contain granules that are filled with pre-formed chemical mediators that can be rapidly deployed

Resting vs Activated Granulocyte

• Resting: nucleus, cytoplasm containing granules

• Activated: contents of granules are released - able to carry out transcription and make a range of new products

Mast Cell Products:

• Two types

  • Pre-formed granules - released immediately

    • Histamine

    • Heparin (anticoagulant)

    • Enzymes e.g. tryptase, chymase – important in effector response

  • Effector substances - synthesed later - delayed release

    • Prostaglandins

    • Leukotrienes

    • Cytokines

Systemic Inflammatory Response

• Mast cells release and synthesise various products upon recruitment

• If the local response is high enough, it can trigger a ______ response

  • Cytokines can act endocrinically, affecting distant tissues (e.g., the brain)

  • This can give rise to symptoms like loss of appetite and fever due to immune system changes

• The liver may be implicated and releases IL-6, contributing to the systemic response

• Acute phase proteins (ancient part of the innate immune system) are activated, playing a role in immune defence with various effector functions

Acute Phase Proteins

• Fibrinogen: Involved in clotting

• Haptoglobulin: Binds iron

• Complement C3: Cleaved to form C3a and C3b, which act as opsonins

• Mannose Binding Lectin (MBL): An opsonin that triggers the complement system

• Serum Amyloid: Inhibits fever and platelet activation

• C-reactive protein (CRP): Binds phosphorylcholine, acts as an opsonin, and can trigger complement

• Surfactant proteins SP-A and SP-D: Act as opsonins

• All these proteins are synthesised by the liver and play a key role in amplifying the innate immune response

Opsonin

• Something that will bind to a molecule on a pathogen and acts as an ‘eat me’ signal for macrophages and other signals

Opsonins of the Innate Immune System

Complement

• A collection of soluble proteins important in innate defence

  • Circulates in inactive form in plasma

  • Activated by infection or immune cells, initiating a cascade that result in the release and activation of prodcuts that contribute to inflammation or will act as opsinins

Function of Complement

• Target lysis (destruction of pathogens)

• Chemotaxis (e.g., C3b directs immune cells)

• Activation of mast cells (e.g., C3a, C5a)

• Clearance of immune complexes (via C3b)

• Amplifies the immune response by helping to kill cells, activate mast cells, guide immune cells, and aid in immune resolution and repair

Complement Pathways

• Classical Pathway: Triggered by antibody or CRP

• Alternative Pathway: Triggered by C3b

• Lectin Pathway: Initiated by MBL

Interferons

• Big cytokine family

• Inflammatory cytokine – involved in the virus responses

• Inhibit viral replication within infected cell

• Bind to cells expressing interferon receptors – render them resistant to infection

• Activate macrophages and natural killer cells

Sources of Interferons

• Produced by virus-infected cells

• Also made by other immune cells

Actions of Interferons

• Can act autocrine (on the same cell) or paracrine (on nearby cells)

• Trigger an antiviral state in surrounding cells – a shielding effect - renders cells resistant to infection

• Help kill viruses inside infected cells; renders neighbouring cells resistant to infection

• Enhance macrophage function

• Secreted IFNs bind to IFN receptors and initiate antiviral defence (e.g. blocking infection)

Natural Killer Cells

• Innate cells with a simillar progenitor to lymphocytes (simillar apperance)

• They can release lytic granules that kill some virus-infected cells

• They are an important early source of interferons – crucial in controlling an early viral infection

  • Function was inhibited by Sars-Cov2

Natural Killer Cells Mechanism of Action

• Different mechanisms for recognising infected cells

• Detect changes via receptor recognition or detection of missing normal 'self' receptors (e.g. MHC I loss)

• Once an infected cell is recognised, NK cells bind rapidly and can kill the cell within

Innate Cells

• Granulocytes (Inc. Mast Cells)

• Eosinophils (common)

• Basophils

• Lymphoid Like cells (Inc. NK cells)

• Innate lymphoid cells

• Phagocytes/ APC (inc. macrophages, monocytes and neutrophils)

• Dendritic Cells - direct adaptive immunity

Eosinophils

• Common cells

• Involved in

  • anti-pathogen responses

  • roles in immune cell recruitment – amplification

  • Obesity

Basophils

• Rare in circulation

• Involved in

  • anti-pathogen response

  • may support the generation of adaptive immunity

Innate Lympoid Cells

• Part of the lymphoid lineage

• 3 subtypes defined by cytokine profile/function ILCs1-3

  • ILCs1 has a similar function to NK cells

  • ILCs 2: helminth infections and allergens,

  • ILC3s: responses against extracellular microbes and tissue homeostasis

• Can support gut immune responses

• Critical role in barrier function

Innate Immune System (Function)

• Provides vital early responses to infection

• Innate cells work together to limit the spread of infection

• Without an effective innate response, infections are more likely to become uncontrolled

• Deals with the threat and communicates with other aspects of the immune system (adaptive immunity and lymphcytes)

Innate Immune System

• Immediate

• Varied methods of attack

• Not very specific; generic

• Helps direct adaptive response

• “Memory”- imprinting; crude memory where if the same type of flu is encountered after a few years it will result in a more powerful response

• neutrophils, macrophages,

• natural killer cells

Adapative Immune System

• Later in the immune response

•  via B and T-cells

• Targeted and SPECIFIC killing

• Has Memory ‘forever’

• B lymphocytes,

• T lymphocytes

Innate and Adaptive Immmunity

• Immediate response of phagocytes/granulocytes – within a few hrs, followed by the NK response

• As the innate response beings to peak to control the virus and stop it from getting ‘too high,’ the adaptive response will be activated and will take over

• Generation of new effector cells and molecules

• Specific immune system (Adaptive immunity)