"Extraordinary Connections - Our Ancient Immune System"

Gregory Beekman

Extraordinary Connections – Our Ancient Immune System

G.Beekman

Humans have two immune systems: innate and adaptive. The recent discoveries concerning the innate immune system are so astonishing that you may never enjoy eating a sandwich again. This may sound surprising but, so too, are the results.

Let’s start at the beginning.

Our immune system fights off infections. Until the 19th century, it was not known for sure what caused this type of illness. It was only in 1675 that the Dutch scientist Antonie van Leeuwenhoek discovered the existence of microorganisms, or microbes. Microbes are creatures that are too small to be seen with the naked eye. Their connection with disease was not realised until the work of French scientist Louis Pasteur, who found microbes were the reason French wine was going sour and suggested they could also be the cause of infectious diseases. In 1862 he invented the process of pasteurization, which uses heat to kill microorganisms. However it was German scientist Robert Koch in 1876 that finally proved the link between microbes and disease, when he showed that the bacterium Bacillus anthracis was the cause of anthrax in cattle.

We now know that microbes are the cause of all infectious diseases around the world. Microbes include bacteria, viruses, parasitic worms and fungi. Every second of every day, our bodies are assaulted by microbes. Their effects range from completely harmless to completely deadly. Those microbes that cause disease are known as pathogens. It is estimated that one-quarter of the human population will die from a pathogenic disease. The four biggest killers of humans are:

1.Human Immunodeficiency Virus (HIV), the virus that invades our immune system and causes AIDS (Acquired Immune Deficiency Syndrome)

2.Mycobacterium tuberculosis, the bacterium that invades our lungs and causes tuberculosis

3.Plasmodium falciparum, the parasite that invades our red blood cells and causes malaria

4.Influenza A virus, the main influenza virus that invades our respiratory tract and causes ‘the flu’

Note that to cause an infection, a microbe has got to get insider our bodies. This realization led to the fundamental question of immunology: how does our body respond to an invading microbe? This was a question that many scientists were trying to answer. The Russian scientist Elie Metchnikoff made a major discovery in 1882 when he discovered a process called phagocytosis. This is where a large cell of the immune system ‘devours’ an invading pathogen. Metchnikoff called these cells phagocytes, from the Greek words phagein (‘to devour’) and kytos (‘cell’). Phagocytosis was subsequently found to occur in many other animals. Metchnikoff co-won the 1908 Nobel Prize in Physiology or Medicine for this work.

However, a major problem remained. How did our bodies identify invading pathogens? That is, how does our immune system tell ‘self’ from ‘non-self’? It would take more than a century after Metchnikoff’s prize-winning discovery of phagocytosis before that question was finally answered. What that answer revealed would become one of the most astonishing facts about all life on Earth.

In the 1990s, Jules A. Hoffmann began studying the common fruit fly (Drosophila melanogaster). He was able to create a group of mutant flies that lacked a gene known as Toll. He then infected them with the fungus Aspergillus fumagatus. As a control, he also infected a group of normal flies. After three days, most of the normal flies were still alive but all of the Toll-mutants had died.

This proved that the Toll gene played a crucial role in the anti-fungal immune response of the fruit fly: removing the Toll gene ‘switched off’ its immune system. But what does the Toll gene do? The Toll gene creates a protein, also called Toll, which sits on the surface of immune cells. Hoffmann had shown that this protein was the trigger that ‘switched on’ the immune system in fruit flies. In the 1990s, this was a sensational result.

However, as interesting as flies are, they are not human. What scientists really wanted to know was how our own immune systems worked. Around the time Hoffmann was working with fruit flies, Bruce Beutler had begun working with various mutant strains of mice. Each group of mice had the same gene missing.  Beutler infected all these different groups not with live bacteria but with a protein molecule found on the surfaces of many different bacteria. This molecule is called LPS, standing for lipopolysaccharide. It was known that this molecule caused a strong immune response that led to blood poisoning and death.

After being infected, all but one group of mice got ill and died. The group that survived did not show any sign of illness. These became known as the LPS-resistant strain. This result showed that the gene the LPS-resistant mice were missing played a crucial role in switching on the mouse immune system.

Unfortunately, the method that created the mutant mice was based on a random process. Thus, no one knew what gene these mice lacked. Bruce Beutler and his team spent five years trying to identify this missing gene. During that time, Hoffmann published his discovery about fruit flies and Toll genes but Beutler knew it would have no relevance to his own research: mammals were just too different from flies.

Imagine Beutler’s surprise when he discovered his LPS-resistant strains were missing a Toll gene! A bigger shock was to come. Beutler performed similar experiments on human cells and showed the same result: without a Toll gene, the human immune system did not respond to LPS infection.

Science had discovered something unexpected: fruit flies, mice and humans all shared the same immune-system gene! The proteins that these genes create are now known as Toll-like receptors, or TLRs. Beutler and Hoffmann were jointly awarded one-half of the 2011 Nobel Prize in Physiology or Medicine for this work (the other half went to Ralph M. Steinman for his work on the adaptive immune system, but that’s another story).

We can now incorporate Metchnikoff’s 1882 discovery of phagocytosis with the discovery of Toll-like receptors over a century later and explain how our immune system identifies self from non-self.

Imagine a rod-shaped bacterium that invades and enters our bloodstream. Already in our blood are white blood cells, the phagocytes that Metchnikoff discovered. Phagocytes are much larger than invading pathogens, and we can think of them as large footballs. On the surface of these phagocytes are Toll-like receptors (TLRs). We can imagine the TLRs as being hooks shaped like question mark symbols. Imagine two TLR hooks screwed into the surface of the phagocyte such that the hooks are touching ‘back-to-back’. These two hooks now form one receptor and the area between the tops of these two hooks creates a space with a particular shape. It is this shape that is crucially important in detecting the invading bacterium.

Let’s consider the rod-shaped bacterium again. On its surface are various molecules. One of these molecules is lipopolysaccharide (LPS). In the bloodstream, the phagocyte will eventually collide with the bacterium. The LPS molecule on the bacterium is like a ‘key’ and the space between the two TLR hooks is like a ‘lock’. When the two collide, the bacterium ‘key’ fits into the ‘lock’ of the phagocyte’s TLR. That is, the Toll-like receptor has created a space that exactly matches the shape of the lipopolysaccharide molecule.

When this lock-and-key mechanism is activated, the bacterium is bound to the phagocyte. Now the phagocyte begins the devouring process. The areas of the phagocyte around the perimeter of the bacterium begin to push outwards and surround the bacterium. It’s a bit like the phagocyte growing arms and giving the bacterium a hug. This process continues until the bacterium is completely contained within the phagocyte.

A crucial feature of the TLR hooks is the fact that these hooks have a part inside the phagocyte and a part outside. It is this feature which makes the whole process work. Once the bacterium binds to the parts of the TLR outside the phagocyte, this informs the parts of the TLR inside the phagocyte to release molecules that tell the cell what to do. These are known as signalling molecules. They eventually reach the nucleus of the phagocyte and instruct it to begin producing anti-microbial peptides (AMPs).

AMPs are chemicals which will kill the bacterium. The phagocyte will manufacture not just one but a whole range of chemicals. One of those chemicals, surprisingly, is hydrogen peroxide – what we normally call bleach. Some chemicals puncture the surface of the bacterium to allow the other chemicals entry so that they can destroy the bacterium from the inside out.

The end result is complete destruction of the bacterium. A similar process works for the other microbes that invade our bodies. The mechanism described above, of Toll-like receptors detecting a molecule on the surface of a pathogen and causing anti-microbial peptides to be released to destroy it, is part of the innate immune system. What surprised scientists in the late 1990s is that all animals on earth have an identical system!

In humans, we have ten Toll-like receptors on our immune cells. Each has a very particular shape that is used to recognise different signature molecules on pathogenic microbes. Bruce Beutler had discovered TLR-4, which is used by our bodies to identify a class of bacteria called Gram negative bacteria that all have LPS molecules on their surface. Other TLRs identify viruses, parasitic worms and fungi. Mice actually have thirteen Toll-like receptors and fruit flies have three. Each receptor detects a different molecule and thus can identify the bacteria, viruses, parasitic worms and fungi that invade their species.

Astonishingly, such a system was also found by Pamela Ronald in rice plants! Yes, even plants and trees and flowers get infected by microbes. Again, a very similar mechanism is used to detect and destroy the invading pathogens.

We now know that all creatures on earth share the same mechanism. If there was ever any doubt over the truth of Charles Darwin’s theory of evolution, then this proves beyond all measure how true his theory is.

Let’s step back in time.

Over one billion years ago, the first multi-cellular organisms began to appear in Earth’s oceans. To survive, they had to identify that something was food and could be eaten. At the same time, they had to avoid being food for something else. That is, they had to identify a threat and develop the aggression necessary to kill that threat.

These traits were so useful that they were conserved throughout evolution. These first multi-cellular organisms gave rise to all plants and animals that exist on Earth today. All of these creatures possess the ability to recognise a threat and then kill it. The way all creatures recognise the threat is with the Toll-like receptors of the innate immune system. These receptors then release signalling molecules that trigger the manufacture of anti-microbial peptides which kill the invading pathogen. This system arose over one billion years ago and is still in use today. (For completeness, note that vertebrates also possess an adaptive immune system. It is believed this arose around 420 million years ago. The adaptive immune system produces antibodies and is the system which allows us to be vaccinated - but that’s a different story.)

The innate (or ancient) immune system reveals an astonishing connection between all creatures on Earth.

Elephants, lions, dolphins, salmon, prawns, tulips, grass, trees, rice plants, mice, flies, spiders, worms, birds, starfish and even your lunch all use the same molecules as humans to identify and kill invading pathogens.

Yes, even your lunch.

Do you enjoy BLT sandwiches? BLT stands for bacon, lettuce and tomato. The bread is usually made from wheat. Just think about the ingredients next time you bite into one. The pig, the lettuce, the tomato plant, the wheat and you all share the same ancient immune system.

You are eating your far-distant cousins.

I wonder: does knowing this make it taste any better?