© 2004 Blackwell Publishing Ltd 531 Parasite Immunology , 2003, 25, 531–543 Blackwell Publishing, Ltd. ORIGINAL ARTICLE Antigenic cross-reactivity in Plasmodium Antigenic cross-reactivity between different alleles of the Plasmodium falciparum merozoite surface protein 2 INGRID FELGER, SYLVIA STEIGER, CHRISTOPH HATZ, TOM SMITH & HANS-PETER BECK Swiss Tropical Institute, Basel, Switzerland SUMMARY The polymorphic domain of the gene encoding Plasmodium falciparum merozoite surface protein 2 (MSP2) was PCR amplified from blood of malaria patients, genotyped, and 19 distinct fragments were cloned and expressed in E. coli . The reactivity of naturally occurring antibodies against this panel of recombinant MSP2 antigens was tested using 67 homo- logous or heterologous sera from a serum bank of travel clinic patients. Sera from semi-immune individuals strongly recog- nized almost all recombinant antigens. Sera from primary infected patients either did not react at all (9 sera), or reacted weakly against varying numbers of antigens (39 sera). The antigens that showed reactions were mostly of the allelic family corresponding to the infecting clone, but in very few cases also of the alternative allelic family. Single clone infections and repeated samples from the same individual were analysed in greater detail. Thus, we were able to quantify cross-reactivity induced by a single P. falciparum infection. Within the two allelic families of MSP2, cross-reactivity was observed between some but not all alleles of the same family, whereas antibodies cross-reactive between variants belonging to differ- ent allelic families were detected in only a few cases. Keywords MSP2, recombinant antigens, polymorphic domains, cross-reactivity, primary exposure INTRODUCTION The merozoite surface protein 2 (MSP2) is a polymorphic anti- gen located on the surface of merozoites, the stages of Plas- modium falciparum that invade red blood cells. In areas highly endemic for malaria, the proportion of individuals seropositive for recombinantly expressed full length MSP2 variants in ELISAs increases with age (1). The overall antibody prevalence (IgG) in a Papua New Guinean population including all ages was 90%. The high proportion of individuals recognizing recom- binant MSP2 demonstrates that MSP2 is highly antigenic in natural infections. Sero-epidemiological studies have found a negative correlation between antibody levels of some variants of MSP2 and malaria morbidity (1–3). Laboratory and animal studies indicated that immune responses against MSP2 induced protective immune responses (reviewed in (4)), this being the rationale for the development of an MSP2 vaccine. More recently, the almost full length 3D7-MSP2 has been part of a subunit vaccine against malaria that was tested in a phase 2b field trial in Papua New Guinea (PNG) (5). The effects of this Combination B malaria vaccine on subsequent msp2 genotypes suggested that the MSP2 component accounted, at least in part, for the observed functional activity. MSP2 is a highly polymorphic surface antigen, of which more than 150 different alleles have been described so far (6 –9). The different alleles of MSP2 share the N- and C-terminal con- stant domains which flank a central variable region. The con- served domains were found to be poorly antigenic. In humans (4) and in monkeys (10) the C- and N-terminal domains were recognized only weakly or not at all. In ELISAs the C-terminal constant domain was only recognized by 36% of adult immune sera and with significantly lower OD values than the full length molecule or the dimorphic regions (11). The variable part of MSP2 consists of both a repetitive domain of allele-specific length and a dimorphic domain of either 64 amino acids or about 125 amino acids in length, determining the FC27- or 3D7-allelic family, respectively. Over extended stretches, the dimorphic domains of MSP2 are totally conserved but in parts show considerable micro- heterogeneity, and in particular the 3D7 family-specific region Correspondence: Dr Ingrid Felger, Swiss Tropical Institute, Socinstrasse 57, P.O. Box, CH-4002 Basel, Switzerland (e-mail: ingrid.felger@unibas.ch). Received: 2 June 2003 Accepted for publication: 10 December 2003