What Four Worms Told us About Evolution and Infectious Disease

This short video shows the agent of African eyeworm disease, Loa loa, via bright field wet mount microscopy. This video was capture by Dr. Ryan Relich at the IU School of Medicine.

Recently we published this paper in the journal PLOS One. The paper, “Differential Evolutionary Selection and Natural Evolvability Observed in ALT Proteins of Human Filarial Parasites”, had a major goal: using evolutionary biology to understand how infectious microbes change. Why care about changes in pathogens? Easy! When they change, they can do things like escape the immune protection created by either vaccines or natural infections, become resistant to current treatments, or evade detection by current lab tests. In some extreme cases, they can adapt to infect new host species (think of HIV, which adapted to humans from a closely related virus called SIV that circulates in wild primates). Studying what types of factors drive these changes may help us predict them before they happen in the future, allowing us to prepare accordingly. For our initial project in this line of work, we chose to study a family of parasitic worms called Filaria. We examined two species that cause extreme limb swelling (‘elephantiasis’) and another two that cause eye infections leading to corneal scarring and loss of eyesight (‘river blindness’). Both conditions are considered neglected tropical diseases (more on these and ways to help here), and lead to permanent disfigurement and lowered income potential.

Preventing both diseases would be most effectively done by developing vaccines. In order to develop a safe and effective vaccine, one needs to choose the right target. One of the things that makes a vaccine target ideal is that it is unlikely to change in order to escape the immune response… sounds like a job for pathogen evolutionists! We chose to look at a protein called ALT (“abundant larval transcript”) that is made by all four parasites, but no one has any idea what it does. What is known is that infected patients make strong immune responses directed at ALT, and that purified ALT protein injected into gerbils seems to protect them from becoming infected with filarial worms. We wanted to study how likely it was that ALT will change, and thus render immunity made from an ALT vaccine ineffective.

The answer required a lot of DNA sequence from numerous worms. We took many alt sequences from all four of our chosen species, determined their level of diversity, and in turn used that information to calculate what type of evolutionary selection was acting ALT. The answer was tricky, to say the least. For two of our four parasites, (Onchocerca volvulus and Brugia malayi) ALT was under purifying selection, meaning that it does not change substantially. For the other two (Loa loa and Wuchereria bancrofti), ALT was under diversifying selection, meaning that variety is strongly favored. Those two species, it would seem, are at their most fit when ALT changes frequently. Perhaps ever more strangely, favoring stability or diversity is not related to the disease they cause: one cause of elephantiasis is diversified and the other is not. The same is true of river blindness. This seemed to spell disaster for the idea of using ALT protein to vaccinate against elephantiasis OR river blindness.

Except that it didn’t! We analyzed each protein very carefully not only for where on the molecule each type of selection was acting, but which areas of each protein interact with the immune system. Those areas, termed antigenic epitopes, are the ones that truly determine protection from an infectious agent. Looking at each sequence residue by residue we found a striking pattern: each spot that the immune system interacted with to neutralize ALT and destroy the parasites was under strong purifying selection in all four species. In other words, those spots did not change, no matter which worm we were looking at. We do not know why diversity is favored for some parts of ALT in Loa loa and Wuchereria bancrofti, but we can say that it is NOT due to its escaping the immune response. From our study we are most pleased to predict that purified ALT would make a safe and effective vaccine against elephantiasis AND river blindness. We hope to have shared our view that studying diversity and evolution of infectious disease holds tremendous promise for solving old problems…and getting a jump on new ones!

#Filaria #Evolution #Loaloa #onchocercavolvulus #wuchereriabancrofti #brugiamalayi #vaccine