DNA Immunization with the Cysteine-Rich Interdomain Region 1 of the Plasmodium falciparum Variant Antigen Elicits Limited Cross-Reactive Antibody Responses

Abstract

The variant surface antigens of Plasmodium falciparum are an important component of naturally acquired immunity and an important vaccine target. However, these proteins appear to elicit primarily variant-specific antibodies. We tested if naked DNA immunization can elicit more cross-reactive antibody responses and allow simultaneous immunization with several variant constructs. Mice immunized with plasmid DNA expressing variant cysteine-rich interdomain region 1 (CIDR1) domains of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) developed antibodies that were reactive to the corresponding PfEMP1s as measured by an enzyme-linked immunosorbent assay, flow cytometry, and agglutination of parasitized erythrocytes (PEs). We observed some cross-reactive immune responses; for example, sera from mice immunized with one domain agglutinated PEs of various lines and recognized heterologous domains expressed on the surface of Chinese hamster ovary (CHO) cells. We found no significant antigenic competition when animals were immunized with a mixture of plasmids or immunized sequentially with individual constructs. Moreover, mixed or sequential immunizations resulted in greater cross-reactive agglutination responses than immunization with a single domain. Recombinant protein (Sc y179) immunization after priming with DNA (prime-boost regimen) increased antibody titers to the homologous domain substantially but seemed to diminish the cross-reactive responses somewhat. The titer of agglutinating antibodies was previously shown to correlate with protection. Surprisingly, the agglutination titers of sera from DNA immunization were high, similar to those of pooled human hyperimmune sera. These sera also appeared to give limited low-titer variant transcending agglutination. Thus, DNA immunization appears to be a very useful tool for developing variant antigen vaccines.The Plasmodium falciparum variant antigens play an important role in the host-parasite interaction. This family of proteins is involved in parasite adhesion and sequestration and in immune evasion by antigenic variation. These proteins, designated P. falciparum erythrocyte membrane protein 1 (PfEMP1), contribute directly to the virulence and pathogenesis of falciparum malaria (5,6,7,30,33). Among the pathogenic properties of PfEMP1 are formation of rosettes with uninfected erythrocytes, bridging of clumps of infected erythrocytes through platelets, and involvement in placental malaria and cerebral malaria by mature parasitized erythrocyte (PE) adhesion in these (and other) organs (5-7, 13, 17, 25, 35, 36, 45).Antibodies to PfEMP1 are a major component of protective immunity, particularly during early childhood (9-12) and pregnancy (7,19,37,44). This immune response correlates with protection from clinical episodes with parasites expressing previously experienced PfEMP1s but may not protect against unrecognized variants (8,10,24,37,44). These properties contribute to the establishment of chronic infection.We recently demonstrated that immunization with the minim...

CD4 T Cells from Malaria-Nonexposed Individuals Respond to the CD36-Binding Domain ofPlasmodium falciparumErythrocyte Membrane Protein-1 via an MHC Class II-TCR-Independent Pathway

“…However, the fact that CD4 T cell and IFN-␥ responses observed in a large proportion of malaria-exposed individuals reacted with a single variant of CIDR-1␣ suggests that there might be cross-reactivity between CIDR-1␣ domains from different PfEMP-1 molecules. Similar cross-reactivity between CIDR-1␣ variants has been observed in studies where rodents were immunized simultaneously with several CIDR-1␣ variants (25,42,43). Together, these observations are encouraging in that it may be possible to overcome the immense antigenic diversity of PfEMP-1 in a CIDR-1␣-based vaccine.…”

Limited Cross-Reactivity among Domains of the Plasmodium falciparum Clone 3D7 Erythrocyte Membrane Protein 1 Family

“…This could be explained by a higher antigenicity and the presence of cross-reactive antibody epitopes in the CIDR domains. CIDR domains have previously been shown to induce cross-reactive antibodies in mice following immunization with plasmid DNA expressing CIDR domains (2,11), and such antibodies have been suggested to modify disease progression in vaccine experiments where Aotus nancymai monkeys were challenged with a heterologous parasite line (21).…”

Polymorphism in the Plasmodium falciparum erythrocyte-binding ligand JESEBL/EBA-181 alters its receptor specificity

“…Transfection efficiency was assayed by an immunofluorescence assay as described below. Binding of untreated and enzyme-treated erythrocytes to untransfected CHO-K1 cells, and to cells expressing CIDR1 in pRE4 (29), a PfEMP1 domain for endothelial cytoadherence, was tested as a negative control. For the immunofluorescence assay, transfected cells were washed in PBS and fixed in 3.7% formaldehyde for 5 min at room temperature.…”