As Many Exceptions As Rules: Immune To Evolution

Biology concepts – innate immunity, adaptive immunity, defense mechanisms, endotoxin

The Jardin des Tuileries is the setting for the

final scene of “The Happening.“ Located in

Paris between the Louvre and the Palace de

la Concorde, this garden was once a royal

promenade, but became public after the

revolution. The trees that line the walk are

chestnuts. Several species of Chestnut are

pollen sterile, meaning they don’t produce

pollen and must be cross pollinated from a

species that has pollen.

M. Night Shyamalan likes to make movies that have “hide in plain sight” twists: the psychologist is a ghost (The Sixth Sense); the villagers live in modern times (The Village); the mentor is the arch-villain (Unbreakable). In his movie, “The Happening,” mankind is under attack. Something is making us commit suicide in mass numbers. What is attacking us – or might something be defending itself from humans? If it is defensive, could be considered an immune response? If yes, then can we figure it out by deciding just who has immune responses?

Immune systems of defense can be very evolved, as in humans. Ours make use of two specific circulatory systems (blood and lymph), has organs designed to aid their generation and functions (lymph nodes, thymus, bone marrow, and spleen), and has mobile cells designed only to patrol and protect. These components function in both innate and adaptive immune cascades and webs.

Other organisms’ defense systems are not so intricately developed, but still deserve respect. Arthropods (insects, crustaceans and the like) have a highly developed innate immune system, with circulating immune cells of several types.

Mollusks (clams, octopods, and the like) also have an innate immune system with a few types of circulating immune cells. However, immune responses don’t have to be only from circulating cells. Sometimes they are proteins that kill bacteria, or merely surrounding the pathogen and keep it from the host cells. Many kinds of mollusks protect themselves by encapsulating invading parasites in a solid prison of shell-like material -we call them pearls! Any mollusk with a shell can make pearls – even snails.

Conch is a species of giant snail. It produces lovely

pearls, so pearls don’t just come from oysters. Any

shelled mollusk will react to a parasite that gets

through its shell by walling it off in layers of mother

of pearl (nacre). This is the very smooth material

that covers the inside of the shell.

Every animal has some sort of immune response built into its physiology, but supposedly only vertebrates have an adaptive immune system. Invertebrates have the older, innate system, but not the ability to adjust their recognition and response to particular pathogens like the adaptive system can. The specific, or adaptive immune system was believed to have arisen in the first of the jawed fishes (gnathostomes; gnath = jaw, and stoma = mouth), about 410 million years ago and been handed down and modified by mammals. But there are exceptions – there are always exceptions.

The Agnathans (jawless fishes, such as lampreys and hagfish) seem to have an adaptive system all their own. It has features similar to the adaptive system of jawed vertebrates, but the way that foreign antigens are recognized is completely different. The lampreys and similar organisms use a different kind of receptor molecule on immune cells. The receptors are variable, but not in the same manner as mammalian immunoglobulins. In jawed vertebrates, the antibody genes rearrange to form the basis of both circulating and receptor immunoglobulins.

This "similar but different" adaptive system would indicate that specific immune responses have sprung up at least twice in evolutionary history. I say at least twice because it is beginning to look like insects and worms may have a sort of adaptive system as well. Earthworms will reject grafts from other earthworms, and will reject a second graft faster than the first graft. So, we see that most organisms have elaborate ways to defend themselves.

This brings us back to “The Happening,” and the attack on the humans ---– it turns out that it was the trees trying to protect themselves from being overrun by mankind! Plants have defenses? Plants can sense attack and respond? Yep.

Plants don’t have immune cells, those that move around and whose job it is to protect and attack. But they do have immune defenses against pathogens, and pretty sophisticated ones at that.

This is a cartoon which shows plant immune response. First

a pathogen tries to gain entry and the plant recognizes its

surface molecules (PTI). Some pathogens survive the response

and emit effectors (ETS, effector-triggered susceptibility). The

effectors trigger ETI which increases the response proteins.

Some pathogens may survive and too much ETI and ETS

triggers the hypersensitive response. Image: Nature

444:323-329, 2006.

Plant PTI (Pattern Triggered Immunity) is similar to our innate immune system, just without the specific immune cells. In this system, plants recognize molecules that are common to microbes (MAMPS, microbe associated molecular patterns) using pattern recognizing receptors (PRRs).

This is similar to mammalian PRR systems for PAMPs (pathogen associated molecular patterns), the toll-like receptors for example. When triggered, resistance molecules and plant hormones are released to make the plant less appealing to the pathogen, or to interrupt the infection process. There are many of these resistance mechanisms, we can talk about a couple below and more in the future.

On the other hand, plant ETI (Effector Triggered Immunity) is signaled by the effector molecules released by the microbes that manage to set up shop inside plant cells or tissue. ETI is really just an increase in the amplitude of the same response molecules seen in PTI, plus another defense mechanism, called the hypersensitive response.

Some pathogens like the hypersensitive response.

Necrotrophic (necro = death, and troph = loving) fungi,

like Botrytis cinerea, or gray mold (the spots on the

leaves), must have dead tissue. They wait until some

thing else triggers the hypersensitive response, or they

trigger it themselves, and then feed of the dead plant tissue.

When a pathogen is successful at making entry into a plant at a specific site, the plant may respond by releasing oxygen and nitrogen radical compounds (those with free electrons that will attack dang near anything). This will kill the plant cell as well as the invader (hence the name “hypersensitive”), but it reduces the probability of infection by taking out everything in the area. It is a sacrifice of host cells that the plant is willing to make.

This response is much like the apoptosis (programmed cell death) that virally-infected animal cells may initiate. It is a small loss in order to protect the whole organism. Recent evidence suggests a central role for S-nitrothiols (nitric oxide linking cysteines) in both turning on and limiting the hypersensitive response by controlling the amount of NADPH oxidase, an enzyme that produces reactive species. We will see next week that this suicide mechanism is very old.

Reactive species for cell suicide is cute, but plant responses get even cuter. When threatened by some herbivorous insects, 2012 research shows that plants can call in mercenaries to help. Members of the cabbage family are troubled by the larvae (caterpillars) of the large cabbage butterfly (Pieris brassicae). When this butterfly lays its eggs on a black mustard plant, the plant sends out a chemical signal that attracts two species of wasps (Trichogramma brassicae and Cotesia glomerata).

When the male cabbage butterfly fertilizes the female

and she lays her eggs on a brussel sprout plant, the

chemicals from the male semen will trigger the plant to

make a pheromone that attracts the Trichogramma

brassicae wasp. It lays its eggs INSIDE the butterfly

eggs (yellow cones) and they feed off the butterfly eggs

as larvae. Up to 50 wasps can come out of one butterfly

egg. Image:Nina E. Fatouros.

These wasps are natural enemies of the white cabbage butterfly and will attack its eggs and larvae. Voila, the plant stops the white cabbage caterpillar from eating its leaves even before the attack begins. Most amazing, the chemical signal isn’t triggered by other, less ravenous pests, so it is a specific response. That smells like an adaptive immune response to me. While many animals can’t specify a distinct response to a particular foreign organism, it looks like many plants can. Once again, plants show us how advanced they are.

Even more in support of the idea that plants have a form of adaptive response is the discovery that they have an immune memory of sorts. In 2009, researchers at the U. of Chicago found that when attacked by a certain bacterium, Arabidopsis plants (of the mustard family, a very common plant in research) make a chemical at the site of attack called azelaic acid.

The scientists found that this compound can stimulate a faster and stronger immune response when and if the plant was ever attacked again. Azelaic acid acts by stimulating salicylic acid (a compound very similar to aspirin) production in the plant directly, and by stimulating a newly discovered protein called AZ11. The increased salicylic acid then stimulates the defense mechanism.

More recent work (2012) in the same field has identified five additional compounds from Arabidopsis that also prime immune defenses. These new compounds work by inactivating enzymes that break down salicylic acid; the plant is therefore always ready to initiate a defense. These natural chemicals may be important for agriculture in that crops could be sprayed with a primer and be ready for a quick and strong response if they are ever attacked.

Priming is important for plant immunity. Priming can

induce production of more response proteins that may

be stored in vacuoles until needed. Priming can also lead

to modification of DNA regulators, so that more response

proteins can be made over time.

An important factor in this strategy is that the primers do not affect plant growth or seed/fruit production. Many plant defense mechanisms come with an energy or growth cost, the hypersensitive response for example. The time and ATP that a plant spends on defending itself ends up costing it in growth and flower/seed/fruit production. This is important when we are talking about cash crops that feed the world’s people. The newly discovered priming agents can stoke up a plant’s immune response with no loss of growth or productivity. It’s a win-win situation for plants and people.

So animals and plants have independently developed immune responses, including adaptive memory and host cell death mechanisms. Or have they been independent?

The S-nitrosylation regulatory step in the production of reactive species is conserved (the same function, in this case mediated by the same amino acids in similar proteins) in animals, so we and they have developed a similar control – is it conserved from an ancient time before plants and animals diverged? Has the same system developed independently two time – unlikely, many orthologous systems exist, but nature is hit and miss, it rarely twice stumbles upon exactly the same way to do something. The adaptive systems developed by the jawed and jawless fishes may be an example of this. They do much the same things, but through different mechanisms. Perhaps plants and animals shared information at some point in time – horizontal gene transfer, like we talked about a long time ago?

Plants and insects can protect themselves and can adapt to different pathogens, so we have learned not to assume humans are so special. How about if we take another step along this line next week? Can bacteria protect themselves? Do they need to?

Fatouros, N., Lucas-Barbosa, D., Weldegergis, B., Pashalidou, F., van Loon, J., Dicke, M., Harvey, J., Gols, R., & Huigens, M. (2012). Plant Volatiles Induced by Herbivore Egg Deposition Affect Insects of Different Trophic Levels PLoS ONE, 7 (8) DOI: 10.1371/journal.pone.0043607

Yun, B., Feechan, A., Yin, M., Saidi, N., Le Bihan, T., Yu, M., Moore, J., Kang, J., Kwon, E., Spoel, S., Pallas, J., & Loake, G. (2011). S-nitrosylation of NADPH oxidase regulates cell death in plant immunity Nature, 264-268 DOI: 10.1038/nature10427

For more information or classroom activities, see:

Invertebrate immune systems –

http://pulse.pharmacy.arizona.edu/10th_grade/disease_epidemics/science/pathogen.html

http://www.sciencedaily.com/releases/2007/06/070621102626.htm

http://www.ncbi.nlm.nih.gov/pubmed/15199951

http://www.reefs.org/library/aquarium_net/0498/0498_1.html

http://www.fu.uu.se/jamfys/jamimm3.html

http://www.sciencedaily.com/releases/2012/01/120105111946.htm

http://www.uwosh.edu/today/7365/scientists-study-insect-immune-systems/

pearl formation –

www.yvel.com/Assets/PearlGuide.pdf

http://marinelife.about.com/od/invertebrates/f/How-Do-Pearls-Form.htm