Structural and antigenic polymorphism of the 35- to 48-kilodalton merozoite surface antigen (MSA-2) of the malaria parasite Plasmodium falciparum.

isbeing considered forthe development ofamalaria vaccine. Theantigen ispolymorphic, andspecific monoclonal antibodies differentiate five serological variants ofMSA-2among25parasite isolates. Thevariants aregrouped intotwomajor serogroups, AandB.Genes encoding twodifferent variants fromserogroup Ahavebeensequenced, andtheir DNAtogether withdeduced aminoacid sequences werecompared withsequences encoded byother alleles. The comparison shows that theserological classification reflects differences inDNAsequences anddeduced primary structure ofMSA-2variants andserogroups. Thus, theoverall homologies ofDNAandaminoacidsequences areover95%amongvariants inthesameserogroup. Incontrast, similarities between thegroup Avariants and agroup Bvariant areonly 70and64%forDNAandaminoacidsequences, respectively. Wepropose that the MSA-2protein isencoded bytwohighly divergent groups ofalleles, withlimited additional polymorphism displayed within eachgroup. Plasmodium falciparum, thecausative agent ofthemost pathogenic forms ofhumanmalaria infection, remains tobe brought undereffective control. Currently, drugtreatment is themajormethodofcontrol formalaria. However, the increasing incidence ofresistance tomostantimalarial drugs hasstimulated research aimedatcontrolling this disease by vaccination.