Antigenicity and Immunogenicity of <em>Plasmodium vivax</em> Merozoite Surface Protein-3

<div><p>A recent clinical trial in African children demonstrated the potential utility of merozoite surface protein (MSP)-3 as a vaccine against <em>Plasmodium falciparum</em> malaria. The present study evaluated the use of <em>Plasmodium vivax</em> MSP-3 (PvMSP-3) as a target antigen in vaccine formulations against malaria caused by <em>P. vivax</em>. Recombinant proteins representing MSP-3α and MSP-3β of <em>P. vivax</em> were expressed as soluble histidine-tagged bacterial fusions. Antigenicity during natural infection was evaluated by detecting specific antibodies using sera from individuals living in endemic areas of Brazil. A large proportion of infected individuals presented IgG antibodies to PvMSP-3α (68.2%) and at least 1 recombinant protein representing PvMSP-3β (79.1%). In spite of the large responder frequency, reactivity to both antigens was significantly lower than was observed for the immunodominant epitope present on the 19-kDa C-terminal region of PvMSP-1. Immunogenicity of the recombinant proteins was studied in mice in the absence or presence of different adjuvant formulations. PvMSP-3β, but not PvMSP-3α, induced a TLR4-independent humoral immune response in the absence of any adjuvant formulation. The immunogenicity of the recombinant antigens were also tested in formulations containing different adjuvants (Alum, <em>Salmonella enterica</em> flagellin, CpG, Quil A,TiterMax® and incomplete Freunds adjuvant) and combinations of two adjuvants (Alum plus flagellin, and CpG plus flagellin). Recombinant PvMSP-3α and PvMSP-3β elicited higher antibody titers capable of recognizing <em>P. vivax-</em>infected erythrocytes harvested from malaria patients. Our results confirm that <em>P. vivax</em> MSP-3 antigens are immunogenic during natural infection, and the corresponding recombinant proteins may be useful in elucidating their vaccine potential.</p>