Naturally acquired IgG antibodies against the C-terminal part of Plasmodium falciparum sporozoite threonine-asparagine-rich protein in a low endemic area

Plasmodium falciparum sporozoite threonine-asparagine-rich protein (STARP), a 78-kDa surface protein, is considered a potential vaccine candidate. The C-terminal part of STARP has been evolved under positive selection, suggesting the presence of immunodominant epitopes. However, little is known about the immune responses against STARP among individuals upon natural malaria exposure. In this study, we have cloned and expressed in Escherichia coli the recombinant C-terminal part of STARP spanning 118 amino acids in order to examine the humoral immune response against this protein. Blood samples were randomly collected from 74 individuals living in a malaria endemic area of Thailand who were acutely infected with P. falciparum (n = 54) and with Plasmodium vivax (n = 20). Malaria-negative blood samples were also obtained from 27 individuals living in the same endemic area who had experienced prior infection with P. falciparum 6 months to 1 year before sample collection and 20 healthy subjects without history of malaria exposure. Western blot analysis revealed that IgG antibodies against this recombinant peptide were found in 23 of 54 serum samples (42.6%) from P. falciparum-infected individuals. All serum samples from P. vivax-infected cases, non-infected individuals, and those who experienced prior infection with P. falciparum gave negative results, indicating that naturally acquired IgG antibodies against the C-terminal part of STARP are species-specific and short-lived. Provided that antibodies against STARP could confer protection, it is likely that malaria vaccine derived from the C-terminal part of STARP could probably be boosted upon natural exposure to P. falciparum.