Extra credit opportunity – see Bi1 website

The Immune System

A complex system that is responsible for distinguishing us from everything foreign to us, and for protecting us against infections and foreign substances. The immune system works to seek and kill invaders.

www.als.net/als101/glossary.asp

Which, in this list, is not a part of the immune system?

1) Mucous

2) Skin

3) Thymus

4) All of above are parts of the immune system.

5) None of the above are parts of the immune system.

Clicker question

Clicker question

Which of the following disorders result from immune system malfunctions?

1) Allergies2) Diabetes3) Arthritis4) Multiple sclerosis5) All of the above6) None of the above

Overview of your immune system

• First line of defense: Physical barriers that viruses, bacteria must cross– skin covers ~2 m2

– Mucous membranes that line digestive, respiratory, reproductive tracts cover ~400 m2

• Second line of defense: Innate immune system (germline-encoded receptors -- no adaptation to specific pathogens)– Macrophages (Greek for “big eater”), neutrophils, natural killer (NK)

cells– Cytokines -- hormone-like proteins that mediate inflammation,

Complement proteins

• Third line of defense (vertebrates only): Adaptive immune system (adapts to defend against specific pathogens using variable receptors)– B cells make antibodies that vary -- can make an antibody specific for

any new antigen– T cells mediate cellular responses using variable receptors (T cell

receptors; TCRs)

Immune cells and other blood cells made in bone marrow -- all are descendents of self-renewing stem

cells

p. 4 “How the Immune System Works” by Lauren Sompayrac

Make variable antibodies

Membrane-bound variable T cell receptors

Kill cells that are missing self proteins

Phagocyte

Phagocyte

Note these are adult stem cells, NOT embryonic stem

cells.

Innate Immune System

• Second line of defense -- works against invaders that breach physical barriers of skin and mucosa

• “Innate” because shared by all animals (vertebrates and invertebrates)

Figure 2-1Three phases of an initial immune response

First two phases rely on recognition by germline-encoded receptors of the innate immune system.Third phase uses variable antigen-specific receptors produced as a result of gene segment rearrangements (not germline encoded).

The innate immune system responds more quickly than adaptive immune

system.Why is a quick response important?

• Starting with one bacterium that doubles every thirty minutes --> 100 x 1012 bacteria in one day

• 100 x 1012 bacteria equivalent to ~100 liters of a dense culture

• Total volume of blood in human ~5 liters• VERY important to check a bacterial infection

quickly!

Clicker question

Fruit flies can be infected by _____. They use ______ immune responses to clear infections.

1) Bacteria and viruses; innate and adaptive2) Bacteria; innate3) Viruses; innate and adaptive4) Viruses; adaptive5) Bacteria and viruses; innate6) Bacteria and viruses; adaptive

Three components of the innate immune system

• Phagocytes (cells) (e.g., macrophages)• Complement proteins• Natural killer (NK) cells

Innate immune receptors recognize features common to many pathogens. Receptors are employed by all cells of a given cell type. Response does not lead to immunological memory.

Adaptive immune receptors are antigen specific. Antigen receptors of adaptive immune system are clonally distributed on individual lymphocytes. Response can lead to immunological memory.

Macrophages can engulf and digest bacteria

p. 4 “How the Immune System Works” by Lauren Sompayrac

Macrophage about to eat a bacterium

http://www.biochemweb.org/neutrophil.shtml

Clicker question

Which component of bacteria serves as a very potent stimulant of the innate immune system?

1) Ribosomes2) Proteins3) Carbohydrates4) DNA5) RNA6) Plasmids

Clicker question

Carbohydrates on viruses strongly activate the innate immune system.

1) True2) False

Complement system• Ancient system (found in invertebrates such as

sea urchins)• ~20 different proteins that work together to

destroy invaders and recruit immune cells• Activated three different ways

– “Classical” pathway: by antibodies bound to pathogen (vertebrates only)

– “Alternative” pathway: by bacterial surfaces– Lectin activation pathway: by binding of mannose-

binding lectin (MBL) to yeast, bacteria, parasites or viruses (e.g., HIV)

• Activation of complement system is tightly regulated because end results can be dangerous

Figure 2-18

The three pathways of complement activation converge

Combination of adaptive and innate immune

responsesInnate immunity Innate immunity

Figure 2-11

Binding of mannose-binding lectin, a plasma protein, initiates lectin

pathway of complement activation.

MBL discriminates self carbohydrates from

non-self carbohydrates by recognition of a particular pattern of sugar residues

Figure 2-35 part 3 of 3One of the end results of complement activation -- the membrane attack complex kills a cell

Electron micrographs of ~100 Å diameter membrane attack complex channels

Laboratory uses of complement:isolate one population of cells by

killing off another population

Example:

Have mixture CD4 T-cells and CD8 T-cells

Want only CD8 T-cells

Add anti-CD4 antibody to mixture of T cells. It binds. Now add complement, and CD4 T-cells will be killed, leaving you with CD8 T-cells only.

Clicker questionIf you sequenced the receptors and proteins of the innate immune system from identical twins, they would be identical, regardless of differences in immunological experience.

If you sequenced the receptors and proteins of the adaptive immune system from identical twins, they would be identical, regardless of differences in immunological experience.

1) True, True2) False, False3) True, False4) False, True

Clicker questionWhich type of pathogen is easier for the innate immune system to deal with: bacteria or viruses?

1) Bacteria

2) Viruses

So far, we’ve talked only about active recognition of features of pathogens…

But pathogens have also developed ways to remove some of the cell’s critical proteins, often so that they can escape detection by the immune system.

For example, in the adaptive immune system, T lymphocytes (T cells) recognize viral fragments (peptides) bound to MHC proteins.

It’s hard for a virus to hide out in a cell if the cell surface MHC proteins contain viral fragments that can be recognized by T cells.

What’s a virus to do? Get rid of the host MHC proteins!

For every strategy a virus comes up with, the immune system (usually) has an

answer…

Natural killer cells recognize cells that do not express adequate levels

of MHC proteins on their surface.

They respond to “missing self”.

How could you design a system that responds to the absence of a critical

surface protein?A trick used by many viruses is to down-regulate MHC proteins -- MHC proteins

normally alert the immune system to the presence of a virus.

1) Create a receptor that activates a killer cell if it can’t find its target protein.

2) Create a killer cell with two receptors: one inhibitory (binds to MHC protein) and one activating. Inhibitory signal overrides activating signal, so killer cell does nothing unless it finds a cell with no MHC proteins.

Clicker question

Natural killer (NK) cells

• Can kill tumor cells, virus-infected cells, bacteria, parasites, fungi in tissues

• Identify targets based on “missing self” – Two types of NK receptors: inhibitory and activating– If inhibitory receptor recognizes a self protein (a class I MHC

molecule) on a target cell, the NK cell is turned OFF even if activating receptor binds a ligand on the same target cell

– If activating receptor binds a ligand, but inhibitory receptor does not (target cell has down-regulated class I MHC proteins), NK cells kill

– Many virally-infected cells and tumor cells down-regulate expression of class I MHC molecules (NK cells important for preventing cancers)

Extra slides

Not discussed in lecture

Figure 2-19

Overview of complement activation• C3 proteins cleaved into C3a and C3b.

• C3b reacts with amino (NH3) or hydroxyl (OH) groups often found on surfaces of invaders.

• Activated C3b neutralized by H2O if doesn’t react with NH3 or OH group within ~60 µsec. (Important for regulation!)

• Complement protein B binds to cell surface-bound C3b, then protein D cleaves B to yield C3bBb.

• C3bBb (a “convertase”) cleaves other C3 proteins to C3b to increase number of C3b molecules bound to the invader.

• C3bBb also cleaves C5. C5b combines with C6, C7, C8 and C9 to make a “membrane attack complex” (MAC). C9 forms a channel that opens a hole in invader membrane to lyse it.

• Many of the “a” fragments (e.g., C5a) are peptide mediators of inflammation.

The immune cells we’re talking about are called white blood cells.

This means they are in the ____.

How do they get to a site of infection?

Figure 2-44Inactive neutrophils are swept along by blood at ~1000 µm/sec. Activated macrophages produce alarm cytokines (IL-1 and TNF; interleukin-1 and tumor necrosis factor) that turn on selectins (sugar-binding proteins) on endothelial cells. Now neutrophils roll along endothelium, making and breaking contacts with endothelial cells on walls of veins.

Upon activation by inflammatory signals (e.g., complement protein C5a -- more later), binding proteins on the neutrophil (integrin) and on endothelial cells (ICAM) are induced, resulting in tighter binding, arrested rolling, and squeezing of neutrophil through endothelium to the site of infection.

Neutrophils leave the blood and migrate to sites of infection

Figure 2-44Watch “The Inner Life of the Cell”(link on Bi1 website)

Main points: If nothing is wrong, neutrophils travel at high speed through the blood.If there is an infection somewhere, macrophages produce alarm molecules that cause proteins on the endothelial cells* to be expressed.

These new proteins bind weakly to sugars on the neutrophils, causing the neutrophils to slow down, but not stop (they roll along the endothelial surface). By moving slowly, they have a chance to find the infection site.When the neutrophil comes to a site near the infection, it stops because inflammatory signals at the infection site induce proteins on it and the nearby endothelial cells that bind tightly to each other. The neutrophil now squeezes through the endothelium to get to the site of infection.

*endothelial cells: cells lining blood vessels

Neutrophils leave the blood and migrate to sites of infection

Leukocyte rolling and migration through blood vessel walls