Antigen switching in malaria
Grade level(s):High School (9-12), Grade 9, Grade 10, Grade 11, Grade 12
Topic:Genetics, Immune system
The parasite that causes malaria changes its cell surface proteins to hide from the host's immune system.
Before: gene, protein
During: malaria, antigen
What you need:
Handouts: exercise description and worksheet (in Student_Handout.pdf); "Grab bags" containing slips of paper representing infected red blood cells.
Students can work as individuals, pairs, or small groups; groups of ~3 seem to work well.
Total time: 50 minutes to 1 hour
- Introduction: 10-15 min
- Activity: 30-40 min
- Wrap-up/Closing activity: 10-15min
In this activity, students will model how the parasitic malaria protist Plasmodium falciparum evades the host immune response through a phenomenon called antigen switching. Specifically, slips of paper representing malaria-infected red blood cells will be used to demonstrate how random changes in the expression of Plasmodium proteins that display on the surface of human red blood cells helps the parasite avoid destruction by the host immune system. Students start with a single infected red blood cell with a specific surface marker protein, and from there will simulate the spread of infection through multiple generations of infection (each generation consisting of a parasite infecting a red blood cell, dividing and multiplying inside the red blood cell, then bursting to release new parasites that go on to infect new red blood cells). Student will find that the parasite occasionally changes the type of surface marker protein expressed over several generations. When the immune system begins destroying infected cells displaying the original surface protein, cells that have switched to expressing a different protein survive and continue to divide.
Students need a basic understanding of the following:
1. Cell structure and the components of a cell (in this lesson, students will need to learn about red blood cells and the fact that in humans, they are unique in that they lack organelles, nuclei, and DNA).
2. The concept of parasitic relationships (here they will learn about a unicellular parasitic protist called plasmodium falciparum)
3. Genes and DNA are hereditary materials
4. Proteins as the products of genes and DNA and as functional molecules in various processes
5. The human immune system and how the body recognizes foreign molecules (Antigens) using antibodies to trigger a defensive response
Students will be able to explain how antigenic switching allows the malaria parasite to evade the host immune system.
Malaria, a widespread disease throughout tropical regions of the world, affects 500 million people per year and kills 1 million, mostly young children and pregnant women. The disease can be caused by several species of the protozoan parasite Plasmodium, which is transmitted by mosquitoes and infects human red blood cells. Most of the fatal cases of malaria are caused by Plasmodium falciparum, which has developed several tricks for evading the human immune system.
The genome of P. falciparum contains a family of about 60 related genes, called the var (variable) genes. The proteins encoded by these genes are exported to the plasma membrane of the infected red blood cell. Different var gene products can bind to proteins found on human blood vessel walls or normal red blood cells. Binding of infected blood cells to blood vessel walls (sequestration) prevents the infected cell from being filtered out and destroyed by the spleen. If the var gene product binds specifically to proteins found in the blood vessels of particular organs, it can cause additional complications including cerebral malaria (binding of infected blood cells to blood vessels in the brain, leading to coma and death) or placental malaria (binding of infected blood cells to blood vessels in the placenta, which can be life-threatening to both mother and baby). Binding of the var gene product to proteins found on uninfected red blood cells (rosetting) may act as a kind of camouflage, hiding the infected cell from immune system cells in the bloodstream, including macrophages, B cells, and T cells.
The location of the var gene product on the outer surface of infected red blood cells does leave it vulnerable to recognition by the immune system as an antigen, or foreign substance, that should be attacked. The parasite evades this recognition by the immune system through antigenic switching. Of the approximately 60 var genes present in each parasite, only one is transcribed into mRNA and translated into protein at any given time. The parasite can switch expression from one var gene to another at a low rate (around 1% per generation). When the immune response destroys parasites expressing the predominant var gene in a population, the few parasites that have switched expression to another var gene are able to survive and reproduce.
1. Print and copy the student handouts (Student_Handout.pdf).
2. Print sheets of cell images (Cell_images.pdf); cut to separate individual cells (there are 3 types of infected cell: blue, green, and purple).
3. Divide cells among grab bags; each group will need the following three bags:
a. Blue: 32 blue cells, 2 green cells, 1 purple cell
b. Green: 32 green cells, 2 purple cells, 1 blue cell
c. Purple: 32 purple cells, 2 blue cells, 1 green cell
Note on materials: each bag needs 35 infected cells, therefore you need to print Cell_images.pdf three times for every two sets of bags that your class needs. For example, if you have 36 students working in pairs, you'll need 18 sets of bags (54 bags total) and you'll need to print Cell_images.pdf 27 times.
Lesson Implementation / Outline
1. Assess students' knowledge of malaria by brainstorming and making a list on the board.
2. Fill in missing basic background: Disease caused by protozoan parasite, found in tropical regions, transmitted by mosquito, infects red blood cells. (refer to the Content Background for Instructor and the weblinks for background on malaria)
3. Introduce the var genes and the concept of antigen switching.
1. Divide students into small groups (2-4 students/group)
2. Give each group three grab bags, one of each var type, and a starting parasite (it's probably simplest if every group starts with the same var type; for example, every group gets a slip of paper with an infected red blood cell displaying the blue triangular var protein).
3. Hand out worksheets; have students note their starting parasite as generation 1.
4. Each group should take the grab bag corresponding to its starting parasite, put their starting slip of paper in the bag, and pull out two new parasites. Record these new parasites on the worksheet as generation 2.
Special Note: At this point it may be helpful to explain that the process of putting one infected cell into the bag and taking out two newly infected cells is a simplification of how P. falciparum infects new red blood cells within the host. Inside red blood cells, the parasite multiplies and periodically breaks out, lysing the red blood cell, to invade other fresh red blood cells. Typically 15-32 new parasites are released from a singly infected red blood cell. Once a new blood cell is infected it is possible that the parasite will switch the var type gene being expressed.
5. Repeat: return each of the two new parasites to the grab bag(s) corresponding to their var type(s), and draw two new parasites from each of those bags (total of 4 new parasites). Record on worksheet as generation 3.
6. Repeat once more (total of 8 parasites), record as generation 4.
7. At generation 4, the instructor plays the part of the immune system by removing any parasites with the original var type from each group's collection.
8. Continue as before through 3 additional generations (if any groups lost all of their parasites after generation 4, that infection was successfully destroyed by the immune system, so there will be no additional generations).
Ask students to explain, based on the activity they just carried out, how antigenic variation can be useful to the parasite. (Allows P. falciparum to evade the host immune system)
Summarize basic malaria biology and how antigen switching helps the parasite survive and multiply.
Extensions and Reflections
The basic idea is applicable to any type of natural selection (for instance, development of drug resistance), though in that case, the variation comes through genetic mutations rather than switches in gene expression.