Tricks of the Trypanosomes: Outsmarting the Human Immune System

For those who don't already know, I'm currently taking a class titled "molecular genetics". This may not be shocking to those who know that I'm a biologist, but those who know me well know that I'm an *organismal* biologist. I learned the basics of genetics as an undergrad, but I've done zero work on the molecular level. To say the least I've been a bit nervous about taking this course. Luckily, I've got an excellent professor and a better memory for the subject than I typically give myself credit for. I feel like I'm understanding things fairly well, but I'm not going to let myself get too confident just yet. Last week I was reading one of the chapters and to my utter delight my eyes scanned the word "trypanosome". What? Go back! Read that again! And there it was again, trypanosome...I was intrigued.

The chapter I was reading was about RNA splicing and it turns out that these little guys have a unique way of splicing. Before I get too far into the depths of this amazing phenomenon and the broader implications of this ability, let's review a bit for those who need it. First of all, DNA holds the blueprints for creating things like proteins. Protein codes need to find their way from the nucleus to the ribosomes, which are the little factories that make the proteins. Enter messenger RNA or "mRNA", the Kinkos and FedEx of the cell. First, a RNA polymerase (think of this as the Xerox) binds to a gene with the help of transcription factors to specific locations known as the promoter sequences. The polymerase then makes a complementary copy of whatever gene it is bound to in the form of an mRNA. This mRNA is not an exact copy of the gene, think of it more like a negative of a photo that can be used to create proteins later. As the mRNA forms according to the DNA template, it copies more than strictly what is needed to make the desired proteins. The initial mRNA strand (called a "pre-mRNA") contains both sequences relative to protein coding ("exons") and sequences that do not code for proteins ("introns"). Obviously, the mRNA doesn't need the introns and mRNA is kind of a no-nonsense sort of guy, so he needs to get rid of them. How do you get rid of sequences that are interspersed throughout a strip of sequences? You cut and paste, of course! Through a series of complex chemical and enzymatic reactions, the pre-mRNAs are cut apart or "spliced" and then the relevant bits of sequences are fused together to form the mRNA. This mRNA will then be prepared for transport out of the nucleus, through the cytoplasm, and into the ribosomes, where they will be utilized for the creation of proteins needed for the cell.

There are many more details to the process described in the previous paragraph...believe me, I had to know the mechanisms for a test a few days ago....but that should be all that you need to know to follow the next bit.

Let's look back at splicing. In most eukaryotes, the pre-mRNA can either be spliced to create a particular protein, or in some cases can be spliced in different ways to create multiple different types of proteins (we call this last ability "alternative splicing"). Trypanosomes do things differently. In a very Frankensteiny fashion that I find sort of appropriate for the parasite with links to zombie legends, trypanosomes actually splice different pre-mRNAs together to form totally new proteins. This process is known as "trans-splicing" and was actually discovered in Trypanosoma brucei They do this as part of a way to trick their hosts' immune systems. You see, trypanosomes wear this strange little coat made of VSG (variable surface glycoproteins). To WAY over-simplify this, the surface of coat changes depending on the proteins produced after trans-splicing.

Interestingly, trans-splicing has now also been found to occur within both Drosophila melanogaster and Caenorhabditis elegans, which are genetic model organisms (a fruit fly and a nematode respectively), and within two other types of parasites. The other parasites include schistosomes (blood flukes) and Ascaris lumbricoides (giant intestinal roundworms or "maw worms"). As in trypanosomes, these parasites utilize trans-splicing to help them evade their hosts' immune systems.

Because these parasites have the ability to create seemingly infinite combinations of exons to devise new and different proteins, it is rather difficult to develop vaccines. Vaccines work by giving you a little bit of exposure to disease agents so that your body can develop antibodies to fight off these little infections. By doing so, your body now knows how to fight the same infection if it comes back for another go at you. With your newly acquired arsenal, you are able to vanquish these invaders should they ever attempt a hostile corporeal-take-over. Your body's ability to mobilize effective antibodies is dependent on being able to recognize a returning pathogen as being such. If the same pathogen that you've previously encountered comes in wearing a different coat, then the antibodies you've already made against that pathogen may not recognize that there is a threat they are capable of suppressing. Thus, if an organism is able to continuously change its surface protein structures using trans-splicing, it can successfully outsmart the human immune system repeatedly. Especially since we can't get a handle on how to induce our bodies to make antibodies that can see through the parasites' clever disguises. (*Side Note*: They do have a handle on how to vaccinate against schistosomes now, but on-going human trials have yet to reveal just how effective this new SM-14 vaccine is.)

A model of the VSG

When it comes to trypanosomes, the human body isn't totally defenseless without a vaccine. I was ecstatic to learn that human bodies produce natural trypanolytic factors, which do exactly what they sound like they do. These bad boys kill trypanosomes.The machinery behind this is fascinating, but to get to the point on this blogpost, I'll spare you the details for now. So, now we have trypanosomes changing their VSG coats to fool our antibodies into being docile and we have the rogue trypanolytic factors who come in kicking like Chuck Norris to save the day. These factors protect us from a number of species of trypanosomes, though a few manage to step up their game.

African Sleeping Sickness (a.k.a. African trypanosomiasis) in humans is caused by one of two subspecies of Trypanosoma brucei: T. b. gambiense and T. b. rhodesiense. These parasites have evolved two distinct ways of resisting our otherwise super-awesome trypanolytic factors. Let's start with T. b. gambiense. This one causes about 97% of human cases of African Sleeping Sickness. Essentially, a mutation in the genes of this species allows for resistance to a major component of the trypanolytic immune response. The cool part is that this mutation is thought to have evolved alongside another parasitic protist, Plasmodium, which causes malaria. The malarial parasite does lots of amazing things to red blood cells, but for the sake of staying on topic, let's just mention that it causes them to burst. When red blood cells burst, they release lots of free haem, which gets bound by haptoglobin. The haptoglobin carries the lost haem out of the body. Haptoglobin is utilized during the trypanolytic defense and without it the Chuck Norris-like factors are much less efficient. Thus, malaria may have given T. b. gambiense the evolutionary hand up it needed to develop a resistance to some of our trypanolytic factors.

Resistance in T. b. rhodesiense is different. Rather than relying on a mutation for resistance to trypanolytic factors, this parasite makes its very own anti-trypanolytic. This parasite creates a serum resistance associated protein (SRA) that binds to the trypanolytic factor in such a way that it renders the most important one completely useless.

Moral of the Story
A common theme that parasitologists encounter when looking at host-parasite interactions is what we call the "Red Queen Hypothesis". This reference comes from Lewis Carroll's Through the Looking Glass...a line from the Red Queen about doing lots of running to stay in the same place. This hypothesis describes how organisms evolve not only in response to pressure affecting reproductive success, but also in response to merely surviving the advances of their enemies. The evolutionary arms race between host and parasite typically escalates on the basis of survival and not reproduction. We see this demonstrated between humans and trypanosomes as each party develops ways of outwitting the other.

Now that we have seen a glimpse of this raging battle, we can start to ask more questions....such as how does Trypanosoma cruzi, the agent that causes Chagas' disease, evade our trypanolytic factors? How do schistosomes and ascarids use trans-splicing to evade host immune responses? and What benefit does trans-splicing serve to non-parasitic animals like fruit flies and soil nematodes? (Or do these extra genes come in handy for defense against their own parasites?...plot-twist!) So continues the circle of life as a researcher...find lots of answers to the simple question of "Why do trypanosomes need to trans-splice their RNA?" and draw more additional questions than what you draw of conclusions. Keep running parasites, we parasitophiles will be sure to eagerly stand beside and watch you do so.

Just thought I'd leave this here for you...
because I love you...
and because it truly has no mana cost plus haste.