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Evolution of Antigenic Variation1 NNNNNNNNNNNNNNCHAPTER 15 NNNNNNNNNNNNN Evolution of Antigenic Variation1 Steven A. Frank Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697–2525, USA 15.1 INTRODUCTION processes that shape genetic variability and evolutionary change.The causes of variability and change provide the basis Infectious disease remains a major cause of morbidity and for understanding why simple vaccines work well against mortality.Consequently,great research effort has been devoted some viruses, whereas complex vaccine strategies achieve to parasites and to host immune responses that fight parasites.2 only limited success against other viruses. This has led to rapid progress in understanding the biology of The battle between host and parasite often comes down parasites, including the molecular details about how parasites to the rates at which attacker and defender molecules bind or invade hosts and escape host immune defenses.Vaccines have evade each other. The biochemical details of binding and followed, sometimes with spectacular success [45]. recognition set the rules of engagement that shape the pace, But many parasites escape host defense by varying their scale, and pattern of diversity and the nature of evolutionary antigenic molecules recognized by host immunity. Put anoth- change [45]. er way, rapid evolution of antigenic molecules all too often The first section of this chapter lists the different ways in prevents control of parasite populations. The challenge has which parasites can gain by varying their antigenic mole- been to link molecular understanding of parasite molecules cules. The parasites may, for example, extend their time of to their evolutionary change and to the antigenic variation in infection within a particular host by changing their antigenic populations of parasites. molecules to evade the specific recognition built up by the The growth of information about antigenic variation pro- host immune system. Or the parasites may vary to attack vides a special opportunity. For example, one can find in the hosts that had been exposed previously and had built up spe- literature details about how single amino acid changes in par- cific recognition against prior antigenic molecules. asite molecules allow escape from antibody binding, and how The second section focuses on the nature of binding and that escape promotes the spread of variant parasites. recognition between host and parasite molecules. I summa- Evolutionary studies no longer depend on abstractions – one rize the different ways in which parasites generate new vari- can pinpoint the physical basis for success or failure and the ants in order to escape molecular recognition. consequences for change in populations. The third section builds up the individual molecular Molecular understanding of host–parasite recognition interactions into the dynamics of a single infection within a leads to a comparative question about the forces that shape host. The parasites spread in the host, triggering immune variability. Why do some viruses escape host immunity by attack against dominant antigens.The battle within the host varying so rapidly over a few years, whereas other viruses develops through changes in population numbers – the hardly change their antigens? The answer leads to the numbers of parasites with particular antigens and the num- bers of immune cells that specifically bind to particular antigens. The fourth and fifth sections discuss how the successes 1 This chapter is a condensed version of my book, Immunology and Evolution and failures of different parasite antigens within each host of Infectious Disease, Princeton University Press, Princeton, NJ, 2002. determine the rise and fall of parasite variants over space 2I use the word parasite for all infectious agents, including viruses, bacteria, and time. The distribution of parasite variants sets the and protozoa. immune memory profiles of different hosts, which in turn Encyclopedia of Infectious Diseases: Modern Methodologies, by M.Tibayrenc Copyright © 2007 John Wiley & Sons, Inc.. 225 226 N ENCYCLOPEDIA OF INFECTIOUS DISEASES: MODERN METHODOLOGIES shape the landscape in which parasite variants succeed or The variant can spread again only after many resistant hosts fail. These coevolutionary processes determine the natural die and are replaced by young hosts without prior exposure selection of antigenic variants and the course of evolution to that antigen.Variants may, on the other hand, be main- in the parasite population. I discuss several methods that tained endemically in the host population.This requires a bal- can be used to infer the evolutionary processes that shape ance between the rate at which infections lead to host death antigenic variation. or recovery and the rate at which new susceptible hosts enter the population. The parasite population maintains as many variants as arise and do not cross-react, subject to 15.2 WHY DO PARASITES VARY? “birth–death” processes governing the stochastic origin of new variants and the loss of existing variants. In this section, I describe the benefits that antigenic variation provides to parasites.These benefits help to explain why par- 15.2.3 Infect Hosts with Genetically Variable asites vary in certain ways. Resistance Host genotype can influence susceptibility to different para- 15.2.1 Extend Length of Infection site variants. For example, MHC genotype determines the Trypanosoma brucei, the protozoan parasite responsible for host’s efficiency in presenting particular epitopes to T cells. human African trypanosomiasis (formerly sleeping sickness), From the parasite’s point of view, a particular antigenic vari- changes its dominant antigenic surface glycoprotein at a rate ant may be able to attack some host genotypes but not of 10Ϫ3 to 10Ϫ2 per cell division [138]. The trypanosome others. changes to another surface coat by altering expression Hill [55] pointed out that hepatitis B virus provides a between different genes already present in the genome. good model for studying the interaction between MHC and Infections lead to successive waves of parasitemia and clear- parasite epitopes. Preliminary reports found associations ance as novel antigenic types spread and are then checked by between MHC genotype and whether infections were specific immunity. cleared or became persistent [6,57,136].The hepatitis B virus Some viruses, such as HIV, escape immune attack by genome is very small (about 3000 base pairs, or bp), which mutating their dominant epitopes [86]. Mutational changes should allow direct study of how variation in viral epitopes to new, successful epitopes may be rare in each replication of interacts with the host’s MHC genotype.Host genotype can the virus. But the very large population size of viruses with- also affect the structure of the cellular receptors to which par- in a host means that mutations, rare in each replication, often asites attach. For example, the human CCR5 gene encodes a occur at least once in the host in each parasite generation. coreceptor required for HIV-1 to enter macrophages.A 32 bp For parasites that produce antigenic variants within hosts, deletion of this gene occurs at a frequency of 0.1 in European the infection continues until the host controls all variants, populations. This deletion prevents the virus from entering raises an immune response against a non-varying epitope, or macrophages [81,96,122].It is not clear whether minor vari- clears the parasite by nonspecific defenses. Antigenic varia- ants of cellular receptors occur sufficiently frequently to favor tion can extend the total time before clearance [36,47,89]. widespread matching variation of parasite surface antigens. Extended infection benefits the parasite by increasing the Several cases of this sort may eventually be found, but in ver- chances for transmission to new hosts. tebrate hosts genetic variation of cellular receptors may be a relatively minor cause of parasite diversity. 15.2.2 Infect Hosts with Prior Exposure Host immune memory recognizes and mounts a rapid 15.2.4 Vary Attachment Characters response against previously encountered antigens. Antigenic Parasite surface antigens often play a role in attachment and variants that differ from a host’s previous infections escape entry into host cells or attachment to particular types of host that host’s memory response. The distribution of immune tissue. Varying these attachment characters allows attack of memory profiles between hosts determines the success of different cell types or adhesion to various tissues. Such vari- each parasite variant. ability can provide the parasite with additional resources or In the simplest case, each antigenic type acts like a sepa- protection from host defenses.Protozoan parasites of the rate parasite that does not cross-react with other variants. As genus Plasmodium cause malaria in a variety of vertebrate host individuals age, they become infected by and recover hosts. Several Plasmodium species switch antigenic type [22]. from different antigenic variants. Thus, the host population Switching has been studied most extensively in Plasmodium can be classified by resistance profiles based on the past infec- falciparum [109]. Programmed mechanisms of gene expression tion and recovery of each individual [7]. choose a single gene from among many archival genetic Two extreme cases define the range of outcomes. On the copies for the P. falciparum erythrocyte membrane protein 1 one hand, each variant may occasionally spread epidemically (PfEMP1) [30].As its name
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