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Immune System Part I: The Evolution of the Immune System1Immune System and Innate ImmunityThe human body has over 100 trillion bacteria living on it or within it.An immune system protects an organism from pathogens and other foreign material.The innate immune system provides the first line of defense byactivating a small variety of immune cells to produce molecules that stop the invading pathogens and its resulting diseaseproviding physical barriers to stop a pathogen

2Point out that most bacteria living on humans are harmless and can even be beneficial. There are approximately five pounds of bacteria living in or on the human body.

The innate immune system was once called the non-specific immune system and there has been an explosion of knowledge in this area.

Graphic http://www.bluebananadesigns.com/illusMacrophageMed.html

Adaptive Immune SystemThe innate immune system is immediate and utilized by many multicellular organisms.In contrast, an adaptive immune system produces antibodies specific to a pathogen or foreign particle after the pathogen activates B- and T-lymphocytes. The adaptive immune response is slower than the innate immune response and it may take days to become effective. Present only in jawed vertebrates

3Graphic http://www.bluebananadesigns.com/illusMacrophageMed.html

The adaptive immune system used to be called the specific immune system. (emphasis on adaptive)The innate immune system is thought to be an ancient conserved biochemical pathway when compared to the adaptive immune system. Ask the students what that last fact tells them. Good student answers will address the fact that the innate immune system is thought to constitute an evolutionarily older defense strategy, and is the dominant immune system found in plants, fungi, insects, and in primitive multicellular organisms. (Students may not know that last part!)Homeostasis and the Immune SystemPathogens invade organisms because they are seeking:a source of food or waterprotectiona place for reproduction

Defense mechanisms evolved to rid the organism of pathogens and return the organism back to its original state.

4It may be useful to go over some terminology with the students:

Homeostasis -- the internal regulation of a system so that its properties remain constant. Homeostasis includes regulation of such things as pH, water, glucose, pathogens, temperature, etc.

Disease -- any abnormal condition in an organism that interferes with its vital physiological processes, caused by pathogenic microorganisms, parasites, unfavorable environmental, genetic, or nutritional factors.

Infectious disease -- a disease that is caused by pathogenic microorganisms that have gained entrance to the body and then multiply and potentially transfer to other organisms.

Pathogen -- an infectious organism or particle that can cause disease in another organism. Pathogens may be lethal.

Immune system -- a defense system that works against invading pathogens and resulting diseases.

The immune system is a method of maintaining homeostasis in an organism. The immune system defends the body against invading pathogens. Point out that the immune system used to be considered a system found only in vertebrates and was not associated with other organisms. It is now recognized that many organisms including sea anemone, fruit flies, and plants also have immune systems.

In this unit, there will be constant references to the evolution of the immune system, cell communication, and homeostasis.

Graphic http://www.bluebananadesigns.com/illusMacrophageMed.html

Evolution of the Immune System

Ancestral amoebas performed phagocytosis to obtain food.It would be advantageous for these ancestral amoeba to be able to distinguish between self and non-self when ingesting other cells.The interaction between receptors and signature molecules could provide this distinction mechanism.5Point out to students that amoebas and immune system phagocytes are very similar. They both move with pseudopods (students may or may not have this prior knowledge and be familiar with amoebas). Both of these cells utilize phagocytosis and destroy other cells.

The ancestor to the amoeba lived in a freshwater environment. Phagocytosis allowed it to ingest other cells and extract nutrients from them. If the ancestral amoeba could not distinguish between other amoeba cells and foreign cells (other than amoeba), it is possible that amoeba cells could have completely disappeared.

It was advantageous for amoebas to distinguish between self and non-self cells. The best way to do this is by way of receptors and signature molecules.

Signature molecules are identifying molecules that are unique to a particular group of organisms. For example, cellulose is a signature molecule for plants, chitin is a signature molecule for certain fungi, muramic acid or murein is a signature molecule for bacteria (not Archaea), and double-stranded (DS) RNA is a signature molecule for certain RNA viruses. This list is not exhaustive. Signature molecules would have allowed the amoeba to distinguish between another amoeba and a foreign cell or potential food. Graphic-http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artsep01/amoeba.html

6General Cell Communication

Propose a cell communication mechanism that ancestral amoeba may have used to distinguish between foreign cells and other amoeba cells.Graphic-CampbellPoint out that the immune system is involved in cell communication. There are three features of cell communication as shown in the diagram.

Reception occurs between a membrane receptor and a signaling molecule or, in this case, a signature molecule on the pathogen.Once reception occurs, the receptor activates a signal transduction pathway.The final result of the signal transduction pathway is some sort of cellular response. In the case of ancestral amoeba, it is assumed that the response was phagocytosis. Explain to students how this mechanism could also be used by multicellular organisms to defend the body against pathogens.

Q: Propose a cell communication mechanism that ancestral amoeba may have used to distinguish between foreign cells and other amoeba cells.

A: The ancestral amoeba probably had several receptors for different signal or signature molecules of foreign cells. If a receptor was activated, it then engaged a transduction pathway that resulted in phagocytosis of the attached cell. If a cell did not attach, it may have been another amoeba and would be ignored. 67General Cell Communication

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78Cell Communication and Immune System

Example of receptors and various signature molecules or PAMPs.Explain how this part of the immune system is an example of cell communication.List all the observations you can make about this diagram.

Graphic Campbell

Point out the following:

The receptor is unique and will attach only to signal molecules. The signal molecules or signature molecules for a group of pathogens are also called pathogen-associated molecular patterns (PAMPs).

For example only one receptor might be needed for bacteria because bacteria have cell walls made of murein or muramic acid. If a bacterium with the cell wall made of murein attaches to the receptor, it is recognized as a foreign object.

Once the PAMP attached to the receptor, then a certain transduction pathway was activated resulting in a response that would aid in eliminating the pathogen.

TLR stands for Toll-like-receptor. These are similar to the Toll receptor found in Drosophila. The Toll receptor was discovered by Christiane Nsslein-Volhard of the Max Planck Institute in Tbingen in 1985 in looking at Drosophila larva. Her comment about an odd looking larva was Das war ja toll meaning That was weird. The gene responsible for the oddity was coined the Toll gene and the gene product was called the Toll receptor. While important in the development of larva, later it was discovered this receptor was important in the innate immune response.

Many proteins have been identified that are similar in structure and function to the Toll receptor and are hence called Toll-like-receptors or TLR.

For example:TLR4 receptor for a lipopolysaccharide found in gram-negative bacteriaTLR5 receptor for protein flagellin found in bacteriaTLR3 receptor DS RNA found in certain RNA virusesTLR9 receptor for unmethylated DNA found in DNA viruses and bacteria

These are all molecules that would NOT be found in the host organism and are foreign to the host organism.

Answer to the diagram explanation. The diagram is an example of cell communication. The parts are the following:Stimulus-PAMP such as flagellin or ds RNAReceptors-TLR receptors unique for a certain foreign PAMPTransduction Pathway- promoting or aiding in an immune response such as the inflammatory responseResponse-Inflammatory response

Observations may includeReceptors are proteins.Receptors are found in the membrane.Receptors are also found in the vesicles but inserted in the membrane.Receptors activate a transduction pathway.There are different type of stimuli and each one has a unique receptor.8Evolution and Immunity

List any similarities that you observe among the 3 pathways. If this pathway was once in an ancestral cell, explain why the differences exist between the three organisms.Toll-like receptor(TLR) pathways comparisonfor plants, invertebrates and vertebrates

9There is DNA evidence that Toll pathway existed prior to the divergence of plants and animals. While the previous slide showed the inflammatory response was activated in that particular cell, there are various types of immune responses that can occur. For example, one transduction pathway can result in the activation of transcription factors that activate genes that synthesize players participating in the immune response.

Point out to the students that the fact that these pathways and receptors are similar to one another, suggests an evolutionary relationship between plants, invertebrates, and vertebrates.

Students need to know:There are receptors, and these receptors attach to a PAMP (signature molecules found on the pathogen and not the host). Once the PAMP is attached it activates some sort of transduction pathway. The response of the transduction pathway is associated with aiding an immune response. For example, it may result in the production of transcription factors. The transcription factors will activate genes involved in an immune response. They need to know this is considered to be an ancient pathway that has been conserved and passed down through time.Differences exist due to the accumulation of various mutations. Accumulated mutations somehow enhanced the function of the proteins. Mutations that were detrimental were probably not passed on. Students do not need to know the actual names or pathways of an individual type of cell.

Students do not need to know the names of the proteins or pathways shown in the diagram.

Graphic- http://www.nature.com/nri/journal/v1/n2/fig_tab/nri1101-135a_F1.htmlPlant Defenses

Plants, like animals, must defend against pathogens and herbivores with: physical barriers secondary metabolitesimmune responses10Emphasize that plants, unlike animals, cannot escape potential consumers. Unlike many animals (echinoderms and flatworms are some notable exceptions), plants have the ability to regenerate or replace parts.

Ask students to suggest strategies for defense. Possible suggestions follow:An epidermis with a waxy cuticle making it more difficult for bacteria and fungi to penetrate into the inner body of the plant. Thorns and spines that discourage herbivores from eating parts of a plant. Secondary metabolites that can make herbivores ill or poison them.An immune response that includes apoptosis (cell suicide) of infected cells (also called the hypersensitivity response) and release of chemical signals to protect the rest of the healthy plant. Plants Physical DefensesPathogens can invade plants. Plantsphysical defenses include:hairs that insects cannot penetratethorns or spines that large herbivores avoid waxy cuticles that bacteria and fungi cannot penetrate

11Q: Ask the students to come up with any other physical adaptations that might prevent plant injury or disease.A: leaf shedding (protects against disease and pests), autumn leaf coloration (warning signal for insects that migrate feed on the plant), thigmonastic movements as in the mimosa leaf (leaf closes as predator touches it), etc.

Q: Ask the students how pathogens might enter a plant. A: Pathogens might enter the plant via stomates or lenticels or wounds on a plant.

Q: Ask the students the purpose of wound dressing when trimming a tree. A: It is to prevent pathogens from entering the plant.

Graphic-http://bluejaybarrens.blogspot.com/2009/11/wild-basil.html

http://www.veggiegardeningtips.com/plant-disease-prevention-in-the-vegetable-garden/Plant Defenses Against HerbivoryHerbivores harm plants by eating them. Defenses against herbivory include production of secondary metabolites.

12Graphic- http://arthurevans.wordpress.com/page/2/

Point out the following-Many plants have indeterminate growth. They continue to grow. If the plant is damaged, it simply regrows the part that is damage. Some herbivores however, can completely destroy a plant.

A secondary metabolite is a metabolite that is not necessary for the plant to live, reproduce or function. It has some other use or function.

Production of secondary metabolites can be a defense against herbivory. Secondary metabolites can

Act on the nervous system of insects and mollusks or mammalsMimic the natural hormones of insects so that the larvae do not develop correctly.Damage the digestive tracts of herbivoresSome are fungicides.

Examples of effects of secondary metabolites1. Canavanine made by a plant is similar to the amino acid arginine.

Secondary metabolite is a metabolite that is not necessary for the plant to live, reproduce or function. Defense and Secondary Metabolites

13Emphasize the effects of secondary metabolites:

Canavanine is made by a plant. Canavanine is similar to the amino acid arginine. The tRNA for arginine cannot distinguish between arginine and canavanine, thus when proteins are synthesized, canavanine is often substituted for arginine. Canavanine has different properties, and therefore the structure of proteins containing canavanine changes. These proteins are no longer functional and this leads to developmental abnormalities. The insect feeding on the plant dies.Caffeine can over stimulate herbivores.Morphine can sedate herbivores. Certain plant oils, like peppermint or lemon, can repel insects with their odors. Plant Defenses Against Herbivory

14This table lists various secondary plant metabolites involved in plant defense. It is not important that students know this list of secondary metabolites, but it is wise to emphasize the role column as illustrative examples. There are over 10,000 identified metabolites!

Students need to know that secondary metabolites are not necessary for the existence of the plant, but can function in chemical defense mechanisms.

Some secondary metabolites have BENEFITS for humans: Morphine- pain killer; from the poppy plantSalicylic acid- (aspirin) antiflammatory and headache reliever; found in willow treesQuinine- antimalarial drug; found in the bark of the cinchona tree.Taxol-anticancer drug; found in the berries of the Pacific yewGenistein- birth control mimics estrogen; found in soybeansCaffeine- increases metabolism; found in coffee bean and tea leaves (and a favorite among AP Biology teachers!)

Plant Defenses Against Herbivory

15Graphic- Hillis

This table lists some various secondary metabolites of plants that are used in defense of the plant. Students do not have to know the list of secondary metabolites. There are over 10,000 identified metabolites.

Students need to know that secondary metabolites are not necessary for the existence of the plant but can function in the chemical defense of a plant.

Some secondary metabolites have benefits for humans Morphine- pain killer from the poppy plantSalicylic acid- (aspirin) antiflammatory and headache reliever found in willow treesQuinine- antimalarial drug found in the bark quinineTaxol-anticancer drug found in the berries of the Pacific yewGenistein- birth control mimics estrogen found in soybeansCaffeine- increased metabolism found in coffee bean and tea leaves

Coevolution of Herbivores and PlantsCertain herbivores and plants have coevolved with one another.

This butterfly can breakdown the secondary metabolite produced by the passion vine and use a nitrogen by-product in protein metabolism.

16Let students propose may have happened. Prod them along an evolutionary pathway

While this metabolite may have been harmful at first, there may have been a butterfly with a mutation that allowed it to sequester the metabolite, break it down, and utilize the by-products.

Graphic- http://www.harkphoto.com/hsara2.jpgInduced Defense Response

Certain pathogens will produce molecules that produce the hypersensitive response (HR).This results in localized death of plant cells very quickly.

17Graphic- Campbell

Avirulent pathogens are pathogens that cause disease but do not kill the plant. In certain plants an interesting defense response or induced response can be observed with the infection from certain avirulent pathogens. Point out that most plant pathogens are avirulent. Discuss the advantage of an avirluent pathogen as opposed to a virulent pathogen. If the pathogen kills the host, then it is eliminating its potential food source.

Certain avirulent pathogens produce a molecule(s) that is recognized by a protein (R) found in the host plant. The R protein is coded for by the R gene. The pathogen molecule that is produced is called the avirulent protein (avr). The avirulent protein is coded for by the avirulent gene . It is avirulent protein that interacts with R protein. Once the pathogen molecule combines with the R protein, it causes a very strong defense response.

Plants without the R protein may die (not always). The gene-for-gene hypothesis describes how both the plant pathogen and the plant have genes that produce molecules that interact with one another in the defense response. This includes the R protein and avr protein interactions

The results of the R-protein activation include hypersensitivity response and the systemic acquired response.

Hypersensitivity response (HR)- Once the pathogen molecule attaches to the R-protein a transduction pathway is activated. This includes

Sealing off the infected area.Initiate the production of molecules which have fungicidal and bactericidal properties.Other pathways are activated that produce hydrogen peroxide and nitric oxide which kills the infected and surrounding cells. Before the diseased cells die, they produce a signal, methysalicylic acid which travels to the rest of the plant inducing the systemic acquired resistance (SAR)

Systemic Acquired Resistance

Methyl salicylate from the HR is spread to rest of the plant.Once converted to salicylic acid, a transduction pathway is activated causing systemic acquired resistance (SAR) for the entire plant that lasts for several days. 18SAR is caused by the signal molecule, salicylic acid (acetylsalicylic acid is found in aspirin). Salicylic acid causes the production of pathogen-related proteins (PR proteins). These proteins protect the whole plant from invaders for several days. It is not permanent immunity like the adaptive immune system.

Point out that natives used to chew willow bark to ease pain like a tooth ache or muscles ache. Acetylsalicylic acid is the active ingredient in aspirin. Historical note: Aspirin was first isolated by Felix Hoffmann, a chemist with the German company Bayer in 1897.Immunity and Invertebrates -- Physical BarriersExamples of innate immunity in Drosophila:First line of defense is the exoskeleton made of chitin. Chitin also lines the digestive tract. The enzyme lysozyme found in the gut breaks down the cell walls of bacteria.

19Emphasize that having the gut lined with chitin confers a physical barrier to large pathogens like bacteria. Lysozyme is the enzyme that is found in tears, saliva, and milk. Lysozyme breaks down the cell walls of bacterial cells.

Hemocytes phagocytize microbes like macrophages and amoebas.

Graphic-http://rice.bio.indiana.edu:7082/images/Drosophilidae/Drosophila_melanogaster_m/index.htmlInnate ImmunityIf the pathogen escapes the digestive tract, it will be in a fluid called hemolymph. Moving throughout the hemolymph are hemocytes.Some hemocytes can destroy pathogens by phagocytosis.

20Emphasize:Insects are invertebrates like mollusks and echinoderms. They also have an open circulatory system with fluid flowing through large cavities. The fluid is called hemolymph with cells (hemocytes) circulating through the cavities. These hemocytes have the ability to do phagocytosis. They engulf microbes as a part of the immune system.

Arthropods are the most successful animal phylum. They are found virtually in every niche. They are exposed to a great variety of pathogens. It is not surprising that invertebrates have an immune system to defend themselves. Graphic Campbell

Innate ImmunitySome hemocytes have Toll receptors that produce antimicrobial peptides which inactivate or kill fungal or bacteria by disrupting plasma membranes. Each Toll receptor identifies certain signature molecules as a pathogen or foreign material.

21Emphasize:

The first invertebrate immune pathway was discovered in Drosophila . The discovery occurred while studying a receptor/transduction pathway important in larval development. The receptor being studied in development was also important in activating an immune response by producing antimicrobial peptides. They are called peptides and not proteins because they contain only 18-45 amino acids or so. Proteins have, on average, 400 amino acids.

The receptor is called the Toll receptor as mentioned on an earlier slide.

Jules Hoffman was awarded the 2011 Nobel Prize in physiology or medicine for his work in discovering the evolutionary relationship of the innate immune system in animals and other organisms.

Graphic http://www.els.net/WileyCDA/ElsArticle/refId-a0021845.htmlToll Receptor

The Toll receptor recognizes signature molecules or PAMPs.This activates signal transduction pathways, often producing transcription factors.These transcription factors activate genes needed to synthesize anti-microbial peptides for destruction or deactivation of pathogens.22Emphasize:Over 10 Toll receptors have been identified in Drosphila. Each different TOLL binds with signature molecules (PAMPs) unique to a group of pathogens. This allows a few receptors to respond to a great number of pathogens. For example all Gram-negative bacteria or all fungi.The response is immediate.The response is not long lasting like the adaptive immune response.

Q: Ask students to identify structures or processes that confirm is an example of cell communication.A: There is a signal (PAMP)There is a receptor (Toll)There is transduction pathwayThere is a response (immune response)

Graphic http://www.els.net/WileyCDA/ElsArticle/refId-a0021845.htmlEvidenceThese flies were engineered so that the immune cells expressed a green fluorescent protein when the innate immune system was activated. The top fly was inoculated with bacteria and the one below was stabbed with a sterile needle (no injection of bacteria). The top flys immune system was activated while the bottom flys was not.

23Q: Why was it important to stab a fly with sterile needle?

A: It is used as a control to make sure that simply the act of stabbing the fly did NOT cause the innate immune response to be activated.

Graphic http:Campbell

Created by:

Carol LeiblScience Content DirectorNational Math and Science

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