Intracluster recombination and var transcription switches in the antigenic variation of Plasmodium falciparum

Symposium (U) I Parasirology

58

M-6. Antigenic <-M6- 1

Variation and Antigenic

Inrernarional47

(Suppl.) (1998) 49-63

Polymorphism

SELECTION M RECOMBINATION AND POLYMORPHIC PLASMODIUM FALCIPARUM ANTIGEN GENES IN NATURAL POPULATIONS

s-\lfi-3

ANTIGENIC VARIATION AND SURFACE GLYCOPROTEIN GENE SWITCHING OF PNEUMOCYSTIS

CARlNIl

Conway D’, Roper C*,

Oduola A**‘, Birtks R*, Atnot D***, Cirobusch M*, Curtis C*, Wamasuriya G*,

Kremsne.r P****, Greenwood B* * Department of Infectious

and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, UK; ** College of Medicine, University of Ibadan, Ibadan, Nigeria; *** ICAPB, University of Edinburgh, Edinburgh, UK, **** Albert Schweizer Hospital, Lambarene, Gabon.

The timctions of most Pfusrnodiurn Jdciparum antigens are unknown. Molecular population genetic analyses of multiple polytnoiphic sites in antigen genes can allow patterns of natural selection on the antigens to be detected, and should guide the development and testing of hypotheses about function and immunological relevance of particular antigen domains. New data from a study of polymorphisms in P. _faicipurum genes in 547 isolates from six African populations is presented, which indicate that frequent meiotic recombination occurs within genes and that selection operates on particular domains. In the mspl gene, linkage disequilibrium(measured by the rr and D’ indices) declines rapidly with increasing molecular map distance between sequence sites (within less than 1kb), except in the population with the lowest endemicity and the lowest proportion of mixed allele infections, This has major implications for mapping of sequence sites under selection, as different sites within the same gene segregate independently as Mendelian units. Analysis of Fsr indices across all six populations for 10 polymorphic sites in the mspl gene indicates that one particular site is under balancing selection, suggesting the existence of allele-specific immunity to this part of the protein. MOLECULAR

S-M6-2

AND IMMUNOLOGIC IN P. FALCIPARUM

DIVERSITY ANTIGEN AltafA.

ASPECTS

CANDIDATE

GENES

Lal

National

Center

for

Infectious

Diseases,

Centers

for

Disease

Dtseaes,

Control

and

Prevention. Atlanta, GA 30341 PoIymorphism

IS an Inevitable

component

especially in multistage mfectmus organwns origin

maintenance,

and

selection

of

of a complex

co”sequence of the interaction between the wite

distinct

variants

IS a

and host (both vector and

human host’s immune system). Over the last several years several tnvestigators in many different countries have mvestigated the genetic dwersity of malana vaccine candidate

antigens.

Many

of these studies have employed

approach. While these studws provtde importat

information,

cross-sectional

but tt IS a snap sba

of a much larger ptcture. We and others have employed longitudinal study design for e+ami”ing the “atwe and extent of genetic dtversity m malanal antigen genes. Our

study

destgn

Immunologic.

and

epidemiololpc

of

delineating

also

combines

entomologtc

epidemtologic,

tnvestigations

clinical,

aimed

at:

parasitologr.

1)

studying

Ihe

P. falciparum malaria m an area of hrgh transmtssion; 2)

chamctenstics

of

parasites; and 3) detetmtning

naturally

acquwul

immuntty

against

the drvemtty of vacane antigen gems.

malarta

Ustng this

stody design. we have examtned the diversity I” several vacctne antlge” genes. The more extensive studkes have bee” done in the case ol the circumsporozoite (CS) protein and the major surface protein-l the tdentdication of new genotypes.

(MSP-I).

These studies have led to

Bawd w CIDSS-over events, we have been

able to predict the presence of new alleles of MSP- I antigen.

The presence of

some of these new alleles was conf~tmed by us and others in natural parasite populations.

Complementing

study destgn also allows

our mdecular

biologtc studies, the longttudmal

us to Investigate mtmune responses (El and T-cell)

against these candtdate vaccine anttgens.

For instance. I” the case of T-cell

studies. we are able to demonstrate how polymwphtsm CS protan

affects antigeluc recogmtmn.

The major cell surface glycoprotein (MSG) of carinii plays a crucial role in pathobiology of P. carinii infection, host-parasite interaction and host defense by T cells, alveolar macrophages or humoral immunity. Genes encoding MSGs are repeated, highly polymorphic, distributed among all of the 14-15 chromosomes, and are expressed from a unique expression site termed UCS (upstream conserved sequence). The discovery of a UCS expression site within the telomere region suggests that MSG genes or batteries of them are translocated to the UCS expression site by means of reciprocal recombination or gene conversion. Part of silent MSG repertoires are also localized in the telomere regions. These features are similar to DNA rearrangement of the Variant Surface Glycoprotein (VSG) genes of Trypanosoma Pneumocysris

brucei.

Subtilisin-like protease genes are often localized in the MSG gene clusters. Their protein sequences are highly polymorphic and contain a C-terminal anchoring sequence in the cell membrane, suggesting that these surface polymorphic proteases are involved in pathobiology or antigenic variation of P. curinii.

INTRACLUSTER RECOMBINATION AND var TRANSCRIPTION SWITCHES IN THE ANTIGENIC VARIATION OF PLASMODlUM FALCIPARUM

Deitsch K, del Pinal A, Wellems T Malaria Genetics Section, Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, Bethesda, MD 20892-0425

organtan,

such as malaria parasite. in which

antigemcally

J&,, Nakamura, Y. IMSUT Institute of Medical Science, University of Tokyo, Tokyo 108, Japan

S-M6-J

OF VACCINE

Molecular Vaccine Section, Immunology Branch. Dtwsto” of Parwttc

m

in the CTL epltopes of the

In the case of the MSP-lantige”,

have bee” able to examine the role of different isotypes in chmcal protection. have also bee” able to Investigate the cross-reactivtty of the vanant epitopes.

we WC

Plasmodium falciparum infections are persistent, recurrent and characterized by vanable patterns of disease that,only slowly come under control of the immune system. These features correlate with the antigenic character and cytoadherence properties of individual parasitized erythrocytes, which display members of a family of variable 200,000-350,000 M, surface proteins (REMPI molecules). Spontaneous switches in these surface molecules allow evasion of host immunity. Over the course of a single infection, these switches produce different antigenic forms, allowing recurrent waves of parasitemia to develop ahead of antibody recognition and immune attack. The PfEMPl variant surface proteins are encoded by an extremely diverse family of genes (var). The size and diversity of this family is evident both in the varcomplement of each parasite (50-150 genes) and in the diversity of vargenes among different parasites. The mechanisms sustaining this diversity and controlling transcription switches are thus central to pathogenesis. To investigate these mechanisms, we have examined a cluster of vargenes on chromosome 12 that showed spontaneous recombination and transcriptton switch events. Switches in this cluster were not associated with sequence changes in promoter regions. A selected varpromoter was therefore used to drive a luciferase promoter in transfected episomes. The promoter was found to be actrve regardless of the transcriptional state of the endogenous promoter. An eprgenetic mechanism that depends on local structure of chromatin and its associated proteins may therefore be a major system responsible for the control of var gene transcription.