HC3: Innate and adaptive immune responses & key cytokines

Barriers

There are 3 barriers that form the body's defense mechanism:

• Mechanical barriers
• Chemical barriers
• Microbiological barriers

If the barrier is breached, the innate immune response kicks in. It recognizes that a pathogen has invaded the body and tries to innate it. If the innate immune system cannot destroy the pathogen immediately, it will continue with the induced response. It attracts more lymphocytes to the area of infection → inflammation. When this still isn't enough to destroy the pathogen, the adaptive immune system is activated. B- and T-cells are activated to combat the pathogens. If this doesn't work, the infection becomes chronic or can lead to death.

Innate versus adaptive immune response

Different aspects of the immune system are deployed depending on the type of infection and the location:

Components

Innate immunity has different components divided into 3 categories:

1. Barriers
2. Soluble proteins
   • • Cytokines
     • Complement system
     • Defensins (antimicrobial peptides)
     • Inter-related soluble protein systems
     • Lipid inflammatory mediators
3. Cells
   • NK-cell
   • Monocyte, macrophage, dendritic cell
   • Neutrophil
   • Eosinophil
   • Basophil
   • Mast cell

5 stages

An innate immune response has 5 stages:

1. Recognition of infection or damage
   1. The immune system distinguishes "self" from "non-self": macrophage receptors recognize the cell-surface carbohydrates of bacterial cells but not those of human cells
   2. The macrophage expresses several receptors specific for bacterial constituents: pattern recognition receptors (PRR) are activated by pathogen-associated molecular patterns (PAMPs)
      • For example Toll-like receptors
2. Recruitment of cells and soluble proteins
3. Elimination of the microbe
   • Binding of the pathogen to phagocytic receptors on macrophages induces its engulfment and degradation
   • Binding of pathogen components to pathogen recognition receptors on macrophages induces the synthesis of inflammatory cytokines
4. Resolution of inflammation, repair and return to homeostasis
5. Induction of adaptive immunity if necessary

Acute inflammatory response:

Cytokines cause the acute inflammatory response. There are many types of cytokines with many different functions, but they all result in inflammation at the infected place:

• IL-1band TNF-a: induce blood vessels to become more permeable, enabling effector cells and fluid containing soluble effector molecules to enter the infected tissue
• IL-6: induces fat and muscle cells to metabolize, make heat and raise the temperature in the infected tissue
• CKCL8: recruits neutrophils from the blood and guides the to the infected tissue
• IL-12: recruits and activates NK-cells that in turn secrete cytokines that strengthen the macrophages response to infection

Many things happen in the immune inflammatory response:

1. A surface wound introduces bacteria → resident effector cells are activated to secrete cytokines
2. Vasodilation and increased vascular permeability allow fluid, protein and inflammatory cells to leave the blood and enter the tissue
3. The infected tissue becomes inflamed → redness, heat, swelling and pain

Neutrophils

A neutrophil plays an important role in the innate immune system. It is one of the cells that is recruited to the place of infection. A neutrophil has different receptors for many bacteria or fungus components. One of the things a neutrophil can do to eliminate the pathogen is phagocytosis.

Phagocytosis:

When a bacterium binds to a neutrophil, the following happens:

1. A bacterium is engulfed by a neutrophil
2. The phagosome fuses with azurophilic and specific granules
3. The pH of the phagosome rises → the antimicrobial response is activated → the bacterium is killed by toxic granules in the neutrophil
4. The pH of the phagosome decreases → fusion with the lysosomes allows acid hydrolases to degrade the bacterium completely
5. The neutrophil dies by apoptosis and is phagocytosed by a macrophage

Recruitment of leukocytes to sites of infection:

Leukocytes (like neutrophils and monocytes) are recruited from the blood to the sites of infection by binding to adhesion molecules and by chemo-attractants produced in response to the infection:

1. When a macrophage is activated by binding a pathogen, it produces chemokines such as CKCL8
2. The chemokine binds to a leukocyte, which is rolling across the endothelium of a blood vessel
3. The leukocyte tightly binds to the specific cells
4. Diapedesis: the leukocyte migrates through the endothelium
5. The leukocyte migrates to the site of infection

Netosis:

Neutrophils can "catch" pathogens by throwing out their DNA-content which forms extracellular traps. They can throw out a web made of chromatin and certain enzymes to trap and digest the pathogen.

Cytokines

Cytokines not only cause the acute inflammatory response at the place where the infection is happening, but they also have systemic effects:

• Act on fat and muscle: increase the metabolism and raise the body temperature
• Act on the hypothalamus: increase the body temperature as a whole → fever → decreases replication of the pathogen
• Act on bone marrow: induces neutrophil mobilization → phagocytosis
• Act on the liver: acute-phase proteins are made → activation of the complement system

When inflammation takes place, more cells are recruited towards the site of infection, the vessels are dilated, more soluble proteins are recruited. Eventually the pathogens will be phagocytosed. In case of a systemic infection, like blood poisoning (sepsis), the immune cells are recruited to the whole body. This leads to a severe inflammatory response, which can lead to:

• A systemic edema which can lower the blood flow a lot
• Loss of too many proteins
• Damaged organs
• A state of shock or death

In most cases of sepsis, the neutrophil levels are also high.

Complement system

The complement system is a crucial mediator functioning in both innate and adaptive immune systems. It has 3 main functions:

• Activation of inflammation
• Splitting of microbes
• Lysis of target cells

It is a quick-reacting system of plasma proteins which can immediately mark the pathogen to be destroyed.

The complement system looks like this:

1. 3 pathways can activate the complement system
   • Alternative pathway
     • Coats the pathogen surface with the complement system's activated proteins → C3 starts cleaving more often
     • First to act
   • Lectin pathway
     • When there is lots of pathogen invasion and the liver is systemically activated
     • The liver produces mannose-binding lectin, which binds to the pathogen surface → the pathogens are coded for recognition
     • Second to act
   • Classical pathway
     • A C-reactive protein or antibody binds to a specific antigen on the pathogen surface
     • Third to act because there has to be an antigen on the pathogen
2. Activation of inflammation → C3 convertase is activated → the complement system is activated
3. Cleavage: C3-protein → C3a and C3b
   • C3b is covalently bound to the surface components of the pathogen
4. Various processes take place
   • C3a recruits inflammatory cells → the inflammation is enhanced
   • C3b binds to pathogens → uptake and killing by phagocytes is facilitated
     1. Complement activation leads to deposition of C3b on the bacterial cell surface
     2. CR1 on the macrophage binds C3b on the bacterium
     3. Endocytosis of the bacterium by the macrophage
     4. Macrophage membranes fuse, creating a membrane-bound vesicle → the phagosome
     5. Lysosomes fuse with the phagosomes forming the phagolysosome
5. C3 and C3b activate C5 → perforation of pathogen cell membranes
6. C5 splits into C5a and C5b
7. C5b attracts C6 and C7 → this complex enters the membrane of the pathogen
8. C8 and various copies of C9 join the complex → the C5b-9 MAC (membrane attack complex)
9. C9 perforates the membrane → cell lysis
10. The pathogen dies

The system is very complex. All of these proteins have very different functions which play roles in different diseases.

Neisseria meningitis:

People deficient with any of the C5-C9 components needed for MAC, tend to suffer from a form of meningitis caused by neisseria meningitidis ("meningokokken"). Meningitis is characterized by headaches and severe skin rash.

Natural Killer cells

There is also another way the innate immune system can kill pathogens → Natural Killer cells (NK-cells). NK-cells mainly play a role in viral infections:

1. A viral infection of cells triggers the interferon response
2. The infected cell produces type I interferons (IFN-aand IFN-b) → interferon response
   • Resistance to viral replication in all cells is induced
   • Expression of ligands for receptors on NK cells is increased
   • NK cells are activated to kill the virus-infected cells

Types of interferon:

• Type I interferon: IFN-a and IFN-b
• Type II interferon: IFN-g

NK-effector functions:

NK-cells have multiple functions:

• Kill directly
  1. A viral infection leads to interferons being produced by the viral cells
  2. NK-cells proliferate
  3. NK-cells differentiate into cytotoxic effector cells
  4. Effector NK-cells kill virus-infected cells by inducing them to undergo apoptosis
• Source of cytokines
  • For example IF-g
• Antibody dependent cytotoxicity: the adaptive immune response is enhanced
  1. The antibody binds antigens on the surface of the target cell
  2. Fc receptors on NK-cells recognize the bound antibody
  3. Cross-linking of Fc receptors signals the NK cell to kill the target cell
  4. The target cell dies by apoptosis

In conclusion there are 3 ways the immune system can kill pathogens:

• Through NK-cells
• Through the complement system
• Through phagocytosis

Healing and repair

Stage D is made up out of resolution, repair and healing:

• Macrophages: can engulf the neutrophils that died and can get rid of the microbes
• Fibroblasts: play a role in generating scar tissue
• Angiogenesis: for the formation of blood vessels to the site of infection to regenerate the tissue

Induction of the adaptive immune system

If the innate immune system is not able to completely contain the infection, the adaptive immune system is activated. Stage E is made up out of induction of adaptive immune response. The dendritic cell plays an important role:

1. Dendritic cells take up bacterial antigens in the skin and then move to enter a draining lymphatic vessel
   • Dendritic cells are messengers between the innate and adaptive immune system
2. In the lymph node, the dendritic cell can present the antigen → settles in the T-zone