Abstract. The merozoite surface protein 1 (MSP-1) is expressed in all Plasmodium species and is considered a major malaria vaccine candidate. We found that MSP-1 from Plasmodium vivax (PvMSP-1) contains a region of significant sequence homology with the 190L subunit vaccine derived from the P. falciparum MSP-1. The fragment, termed Pv200L, was expressed as a recombinant protein in Escherichia coli (rPv200L) and used to asses its immunologic relevance as a vaccine target. A cross-sectional, seroepidemiologic study conducted in Buenaventura, Colombia showed that 52.2% (95% confidence interval [CI] ‫ס‬ 39.8-64.3) of individuals previously exposed to P. vivax and 72.8% (95% CI ‫ס‬ 61.8-82.1) of P. vivax-infected patients had IgG antibodies to rPv200L. Immunization of BALB/c mice and Aotus monkeys induced IgG antibodies (titer > 10 6 ) that cross-reacted with P. vivax parasites. Immunized monkeys displayed partial protection against a challenge with P. vivax blood stages. Our results suggest that Pv200L is a new malaria vaccine subunit and deserves further testing.

Abstract. The Duffy antigen (Fy) is necessary for Plasmodium vivax invasion of human erythrocytes. Some populations have a highly prevalent Fy-negative phenotype; such persons are naturally protected from P. vivax blood infection but are expected to completely support the P. vivax pre-erythrocytic cycle, representing a valuable model for studying the immune response during these parasitic stages. We typed 214 individuals, mostly Afro-Colombians, from a P. vivax-endemic area for Fy expression and determined the antibody response to P. vivax pre-erythrocytic (sporozoites and CS) and blood-stage antigens (blood forms, P. vivax merozoite surface protein 1, and P. vivax Duffy binding protein [PvDBP]). Antibody titers to P. vivax circumsporozoite protein, P11, and N-terminal peptides and the number of responders were similar in Fy-negative and Fy-positive individuals. The number of responders to sporozoites, blood forms, and PvDBP were different between these groups. Thus, Fy-negative individuals from malaria-endemic areas can be used to study the immune response to the P. vivax liver phase without interference of the erythrocytic cycle.

Duffy antigen is the receptor used by Plasmodium vivax to invade erythrocytes. Consequently, individuals lacking Duffy antigen [Fy(-)] do not develop blood-stage infections. We hypothesized that naturally exposed Fy(-) humans may develop immune responses mainly to pre-erythrocytic stages and could be used to study acquired immunity to P. vivax and to identify liver-stage antigens. We report here that antibody and IFN-c responses to known sporozoite antigens were significantly induced by natural exposure in Fy(-) humans, whereas responses to blood-stage antigens were significantly induced in Fy(+) humans. IFN-c responses to sporozoite antigens were lower in Fy(+) than in Fy(-) humans, indicating that in Fy(+) humans blood-stage infections may have suppressed T cell responses to pre-erythrocytic stages. We evaluated the immune responses to 18 novel P. vivax homologs of P. falciparum sporozoite proteins identified from the P. vivax genome sequence. Eight proteins recalled IFN-c responses in P. vivax-exposed but not in unexposed individuals. Of these, 3 antigens elicited IFN-c responses in Fy(-) but not in Fy(+) individuals. These results suggest that differential immune responses observed in naturally exposed Fy(-) and Fy(+) individuals can be exploited to identify P. vivax stage-specific antigens.

Unfortunately, in this article (in this issue) the labels indicating the different groups in column "Donors" are missing in the published Fig. 4. The complete correct Fig. 4 is as given here: Fig. 4. IFN-c responses to P. vivax proteins in humans with different Duffy phenotypes. Frozen PBMC from Fy(-) P. vivax exposed individuals, Fy(+) P. vivax exposed individuals, or unexposed controls were assayed for IFN-c induction by ELISPOT after stimulation with autologous B or M-DC transfected with novel P. vivax proteins. As positive controls, IFN-c responses in Fy(-) individuals were detected preferentially to known P. vivax sporozoite Ag (CSP or SSP2) but not to blood stage Ag (MSP-1, AMA-1, or DBP), whereas, the responses in Fy(+) humans were detected to both sporozoite and blood stage Ag. IFN-c responses to eight novel P. vivax proteins were detected in P. vivax exposed individuals, but not in unexposed controls. IFN-c responses to three of the eight responded P. vivax Ag (PVA9, PVA20 and PVA22) were detected only in Fy(-) but not Fy(+) individuals who were exposed to P. vivax, whereas the responses to other five Ag (PVA3, PVA16, PVA18, PVA19, PVA21) were detected in both Fy (-) and Fy(+)P. vivax exposed individuals. IFN-c responses to PVA6, PVA13 and PVA17 were detected in 18 of the 19 donors tested including P. vivax exposed and unexposed controls, indicating these three Ag are cross-reactive T cell Ag; a the numbers in the black boxes indicate the number of Agspecific IFN-c-producing cells/million PBMCs; b based on proteomics analysis by Florens et al. [16