GPI-anchored proteins have been found on the surfaces of extracellular merozoites or apical organelle membranes [
7]. Most GPI-anchored merozoite proteins are refractory to genetic deletion, suggesting that they play important roles in blood-stage development [
19]. Thirty predicted GPI-anchored proteins have been identified in
P. vivax [
11]. The well-characterized GPI-anchored protein PvMSP1 has been selected as a malaria vaccine candidate for its immunogenic properties in a large proportion of individuals exposed to malaria [
20,
21]. The PvMSP1 paralog is a GPI-anchored erythrocyte binding ligand [
16] that induces a specific cellular immune response conferring protection against
P. vivax [
22]. Recently, a GPI-anchored micronemal antigen, PvGAMA, has been shown to bind human erythrocytes independently of their Duffy antigen status [
23]. Furthermore, the GPI motif of these antigens is thought to be an important factor in inducing proinflammatory responses [
24]. In this study, we measured the response frequency to rPv32 in 96 patients with a
P. vivax mono-infection from an endemic area in the ROK and found that nearly 42.7% of this population had antibodies against Pv32 (Table
1). These data reconfirmed a large number of serum samples as reliable data from previous preliminary findings of Pv32 antigenicity [
9]. However, two false positives have been detected in 96 non-exposed samples that may have cross-reacted with some other proteins from healthy individuals. Antibodies are essential for acquired human immunity to malaria. Antibodies are associated with patient age, exposure, active infection and antigens. The immunogenic activity of Pv32 may be because of a parasite surface protein that was frequently exposed to the host immune system. In this study, antibody titers against Pv32 showed median levels of antibody titers, not higher titers compared to other GPI-anchored antigens, such as PvMSP1, PvMSP1P, and PvRAMA from Korean patients [
9,
14,
19]. Second, the low endemicity of vivax malaria in ROK also may be related with the low antibody responses from the low frequency of exposure to infective bites in field sites. Thus, functional analysis of Pv32 remains to be investigated regarding whether the anti-Pv32 antibody could inhibit parasite invasion or even be a protective antibody in human patients. Accordingly,
P. knowlesi could be alternatively used for functional study due to the difficulty of
P. vivax culture.
This study first described the identification and characterization of a GPI-anchored and cysteine-rich Pv32 as a merozoite surface protein. The characteristics of Pv32 were identified from the conserved gene sequence, the protein’s expression toward the schizont stage and its localization and the broad recognition presented by the sera from individuals infected with P. vivax. These data suggest that Pv32 could be a good potential vaccine candidate. Further immunogenicity and protection-inducing ability studies are thus needed in the Aotus experimental model to confirm the potential of Pv32-based vaccine against P. vivax malaria. As one of the few candidates with minimal polymorphism, it may potentially provide sustained protection against this antigenic variant.