Innate Immunity II

What are antimicrobial peptides (host defence)?

They are short cationic peptides with an amphipathic structure that is constitutively expressed (always on)

What are antimicrobial peptides active against?

1. Bacteria

2. Fungi

3. Enveloped viruses

What do antimicrobial peptides bind to? How?

They bind to negatively charged microbial structures and membranes via electrostatic interaction

What are 2 functions of antimicrobial peptides?

1. Disrupt microbial membrane integrity by introducing bulky hydrophobic amino acids

2. Internalized peptides will inhibit DNA/RNA/protein synthesis and activate antimicrobial enzymes

3. Provide an entryway for liquid to get into the cell and they can enter into osmotic shock and burst

Do antimicrobial peptides kill microbes quickly or does it take a while?

Rapidly (within minutes to hours)

What is an example of an antimicrobial peptide?

Defensins (alpha and beta)

What are defensins secreted by? Where are they stored?

Intestinal epithelial Paneth cells and other epithelial cells. Stored in neutrophil granules

Explain the steps that would occur for a human beta1-defensin to kill a bacterial cell

1. The positively charged end of the defensin will interact with the lipid bilayer of the microbe

2. It will insert itself into the hydrophobic part of the lipid bilayer which creates a pore

3. Osmosis takes place where water is entering where the molarity is less

4. Leads to the destruction of the microbe

Why are human membranes not targeted by defensins?

This is because our lipid bilayers are relatively neutral

What must occur for infection to be initiated?

Pathogens have to get past the epithelium (penetrate the epithelial barriers)

What are the five ways that pathogens can get past the epithelial barriers? What are the associated examples?

1. Airways (streptococcus pneumoniae)

2. Gastrointestinal tract (E.Coli)

3. Genitourinary tract (HIV) (injuries to epithelial cells)

4. Cuts in the skin (Staphylococcus aureus)

5. Mosquito bites (West Nile virus)

How do microbes attach to epithelial cells?

Through specific receptors

After penetration of epithelial barriers, what are pathogens detected by?

Innate immune components

What are the three catergories of innate immune components?

1. Soluble innate immune recognition elements

2. Phagocytic and other cells

3. PRRs

What are three examples of soluble innate immune recognition elements?

1. Mannose-binding lectin

2. C-reactive protein

3. Collectins

What are the four examples of phagocytic and other cells?

1. Neutrophils

2. Macrophages

3. Dendritic cells

4. NK cells

What is an example of a PRR? Where are they present?

Toll-like receptors (TLR). Present on innate immune cells (macrophages, mast cells, dendritic cells) that function as sentinel cells in tissues

What are PRRs used by? Why?

Phagocytic cells to try and get rid of the pathogen

What do various secretions contain?

Substances with anti-bacterial activity

What anti-bacterial activity does gastric juice contain?

Acid

What anti-bacterial activity does semen contain?

Spermine and zinc

What anti-bacterial activity does milk contain?

Antimicrobial peptides and lactoperoxidase

What anti-bacterial activity does tears, saliva, and nasal seceretions contain?

Lysozyme

What are two examples of soluble factors?

Collectins and complement

Where are soluble factors found?

In the blood and tissues

What are collectins?

They are small proteins that are able to kill certain bacteria by cell wall disruption, cause bacteria to aggregate, thereby enhancing phagocytosis and they can activate complement by the lectin pathway

Where are collectins found?

In the pulmonary surfactant in the lungs

What is complement?

It is a feed-forward cascade that enables killing spontaneously or after antibodies or mannose-binding lectin binds to target cells

Where is complement found?

In the blood and tissues

Why is there disruption of the cell wall or enhanced phagocytosis through soluble factors?

This is because soluble factors can bind to the surface of the microbe and macrophages will have receptors for these soluble factors resulting in phagocytosis. Eventually there will be activation of the complement which will lead to further destruction and formation of pores.

What happens once the microbe is taken up by the epithelial cells?

Interferon is produced which is a type of cytokine

What does interferon prevent the microbe from doing?

Prevents viral replication

What are the two types of interferons (IFN)?

1. Type I

2. Type 2

What are the key members of type I IFN?

1. IFN-a

2. IFN-B

What is the function of Type I IFNs?

Inhibit viral replication (generate an antiviral state)

What is the key member of type II IFN?

IFN-y (gamma)

What is the function of type II IFN?

Enhance killing of virus-infected cells and promote Th1 immunity

What is IFN-a and IFN-B produced by?

IFN-a: Leukocytes (immune cells)

IFN-B-fibroblasts

Can be produced by any cell in the body through

What is IFN-y produced by?

NK and T cells

When are Type I IFNs produced?

After a cell recognizes a viral component through PRRs (TLR)

What are two ways that type I IFNs can be induced?

1. Presence of viral RNA or DNA activates type I IFN production via stimulation of PRRs such as TLRs

2. Interferons can then induce antiviral responses in neighbouring cells by binding to type I IFN receptors leading to mRNA degradation and inhibition of protein synthesis making them refractory to viral growth

Why is mRNA degradation and inhibition of protein synthesis good?

This prevents the virus from replicating

What do plasmacytoid dendritic cells do?

Out of all the cell types, they produce the most type I IFNs when they come in contact with viruses

What organelle do plasmacytoid dendritic cells have a lot of?

ER because this is how the Type I IFNs are being made

What happens to Type I IFNs once they are produced by plasmacytoid dendritic cells?

They get pumped out into circulation to make surrounding cells virally resistant

Do plasmacytoid dendritic cells need a productive viral infection to up regulate genes coding for Type I IFNs?

No

What recognition mechanism for viral infection is used for plasmacytoid dendritic cells?

1. TLR7 (recognize ssRNA)

2. TLR8 (recognize ssRNA)

What are plasmacytoid dendritic cells important for?

Driving the immune response against viral infections by activating NK cells