Innate Immune System

Immunology Lecture 2: The Innate Immune System

Objectives of Lecture 2

1. Complement System Pathways

   • Understand the alternative and lectin pathways and their contributions to innate immunity.

   • Identify key components and differences between the pathways.

2. Professional Phagocytes

   • Characteristics and functions of macrophages and neutrophils.

   • Mechanisms of how these cells reach infection sites and their roles in innate immunity.

   • Importance of dendritic cells (DCs) in both innate and adaptive immunity.

3. Sentinel Cells Recognition

   • Mechanisms by which sentinel cells detect invaders.

4. Role of Interferon and NK Cells

   • Understand the functions of interferon and natural killer (NK) cells in viral infections.

   • Recognize the cooperative nature of innate immunity.

Complement System

Overview

• Definition: A collection of about 20 proteins in serum that combat invaders.

• Activation Pathways:

  • Classical Pathway: Triggered by antibodies.

  • Alternative Pathway: Evolved before the classical pathway.

  • Lectin Pathway: May be the most significant.

Activation Process

• C3 Protein: Spontaneously cleaved into C3a and C3b.

• C3b Binding: Binds to hydroxyl and amino groups on foreign cells.

• 

• Addition: Complement protein B is attached to C3B, and protein D clips off B to make C3bBb (C3 Convertase)

  

• Amplification: C3 convertase (C3bBb) formation leads to more C3 cleavage.

  • Creates more C3b, which joins with more B to become C3 convertases

  

• Membrane Attack Complex (MAC): C3 Convertase can cleave complement protein C5 into C5a and C5b

  • C5b binds to the bacterial cell, and recruits C6,C7,C8,C9.

  • This forms pore called a membrane attack complex (MAC) that can assemble in exposed membranes on the outer surface of an organism

  • 

    The organism undergoes lysis and dies.

Lectin Pathway

Mannose is a sugar found on bacteria, fungi, viruses, and parasites. It is not found on local human cells, making this pathway more specialized

• Mannose-Binding Lectin (MBL): Binds to mannose on pathogens, activating MASPs to generate C3 convertase.

• MASPs = Mannose-binding lectin associate serine proteases

  • Generate C3 convertase (C2aC4b)

  • C3 Convertase cleaves C3, creating C3b.

  • What follows is the exact same process as the alternative pathway.

    

Functions of Complement

• Opsonization: Modified C3b (iC3b) aids in marking pathogens for phagocytosis.

• Chemoattractants: C3a and C5a recruit phagocytes to infection sites.

Professional Phagocytes

Macrophages

• Location: Reside in tissues (skin, lungs, intestines).

• States:

  • Resting: Garbage collectors.

    • Most of life span is spent as garbage collectors.

  • Primed: Can present antigens.

    • Contain major histocompatibility complex II (MHC II) - signals helper T-cells.

    • Interferon gamma (IFN - gamma) from Natural Killer cells (NK) or dendritic cells stimulates the macrophage

  • Hyperactivated: Focus on killing invaders.

    

Neutrophils

• Location: Found in the bloodstream, short-lived as they are toxic and can damage tissues

• Functions:

  • Kill invaders and die by apoptosis.

    • They do not present antigens

  • Produce cytokines to recruit other immune cells.

  • Can form neutrophil extracellular traps (NETs) to trap pathogens.

Recruitment Mechanism

Neutrophils exit the bloodstream through a regulated process involving selectins and integrins, ensuring controlled tissue damage.

Steps:

1. Neutrophils are lined with SLIG (selectin ligand). When inflamed tissue, IL-1 (interleukin) and TNF (tumor necrosis factor) are produced, blood vessels produce SEL (selectin)

2. SLIG binds to selectin, slowing down the neutrophils in the blood stream. Process takes about 6 hours

3. Once bound to the cell, the neutrophil expresses INT.

   • Chemoattractants C5a and LPS (lipopolysaccharide) are signalled, which means the neutrophil will express INT

4. INT binds to ICAM (intercellular adhesion molecule) and holds the neutrophil in place

5. The neutrophil leaves the blood stream, following the “scent” of f-met and C5a

   • formyl methionine (f-met) is expressed on bacteria.

3 types of molecules must be expressed before the neutrophil leaves the blood stream: interleukins (IL-1), TNF, LPS, and Complement proteins (C5a). This prevents neutrophils from leaving without infection and destroying normal tissues.

Sentinel Cells

Recognition Mechanisms

• Pattern Recognition Receptors (PRRs): Detect pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs).

  • DAMPS are released from dying cels

• Toll-like Receptors (TLRs): Key PRRs that recognize specific PAMPs, such as LPS from Gram-negative bacteria.

  • As stated, TLRs are a type of PRR

  • TLRs can be on the cell of macrophages of dendritic cells (DCs)

    • DCs have TLRS and recognize PAMPS

Importance of PRRs

• Recognize general, shared characteristics of many invaders of the same class

  • TLR4 recognizes LPS on all gram-negative bacteria

  • LPS is required for gram negative bacteria, it is a conserved trait

Interferon and NK Cells

Interferon Response

• Type 1 Interferons (α and β): Produced by plasmacytoid dendritic cells (pDCs) and macrophages, upon viral detection, inducing antiviral protein production in neighboring cells.

  • IFN-alpha or beta Bind to the receptors of neighboring cells, and induce the production of antiviral proteins

  • Uninfected warned cells continuing being themselves, but infected warned cells commited suicide via apoptosis (controlled cell death)

NK Cells

• Characteristics: Short-lived lymphocytes that respond to infected cells.

• On call from the blood like neutrophils, and use a similar mechanism to leave the blood stream.

• In tissues, they divide to boost their numbers.

• Functions:

  1. Receive cytokine signals from other immune cells (like neutrophils) or infected cells (cells signalling IF-α or IF- β), or bacteria (secreting LPS). They then Secrete cytokines (e.g., IFN-γ) to stimulate macrophages.

  2. Induce apoptosis in infected cells via Fas/Fas ligand or injecting granzyme using perforin proteins.

How do NK cells identify target cells?

• NK cells have no T-Cell receptors (TCRs), but two other receptors on target cells signal NK to act on that cell:

  • MHC I - inhibitory signal, tells the NK cell not to kill. This signal is found on all human cells

  • Surface proteins/carbohydrates - expressed when a cell is stressed (when infected) signals the NK cell to kill

    

Some viruses prevent MHC I from being expressed, twarts the CTLs (killer T cells), but it does not thwart an NK cell, as the NK cell still sees the protein/carb signal.

Cooperative Efforts in Innate Immunity

• Collaboration: Macrophages alert neutrophils to exit the bloodstream.

• Cytokine Signaling: NK cells produce IFN-γ after being stimulated by LPS from gram negative bacteria, which activates macrophages, creating a feedback loop for enhanced immune response.

  • IFN-γ hyperactivates the macrophage, which then makes TNF (tumor necrosis factor), which signals itself to make cytokine IL-12

• Continuing the Cycle: TNF and IL-12 stimulate NK cells to produce more IFN-γ (stimulating more macrophages), and to produce the IL-2 receptor

  • IL-2 binds to NK cells, making them proliferate to create more cells.

• Complement protein signalling - iC3b on target organisms stimulate macrophages to create complement proteins C3, B, D, which are used to start the alternative pathway.

  

  Resource Management: The innate response is proportional to the infection size, ensuring efficient resource use.

  • Viruses can hide inside cells, making them more of a challenge for the innate immune response

Summary of Innate Immunity

• Neutrophils and Dendritic Cells: Key players in the innate immune response, with distinct roles in pathogen recognition and response.

• Overall Function: The innate immune system acts as the first line of defense, utilizing various cells and proteins to combat infections effectively