Mem Inst Oswaldo Cruz, Rio de Janeiro, 102(3) June 2007
Comparative recognition by human IgG antibodies of recombinant proteins representing three asexual erythrocytic stage vaccine candidates of Plasmodium vivax
IDepartamento de Análises Clínicas e Toxicológicas, Faculdade de Ciências Farmacêuticas, Universidade de São Paulo, Av. Prof. Lineu Prestes, 580, Cidade Universitária, 05508-900 São Paulo, SP, Brasil
IIDepartamento de Patologia, Centro de Ciências Biológicas, Universidade Federal do Pará, Belém, PA, Brasil
IIIMalaria Research Group, International Centre for Genetic Engineering and Biotechnology, New Delhi, India
IVDepartamento de Microbiologia, Imunologia e Parasitologia, Escola Paulista de Medicina, Universidade Federal de São Paulo, São Paulo, SP, Brasil, 04023-062
In previous immuno-epidemiological studies of the naturally acquired antibody responses to merozoite surface protein-1 (MSP-1) of Plasmodium vivax, we had evidence that the responses to distinct erythrocytic stage antigens could be differentially regulated. The present study was designed to compare the antibody response to three asexual erythrocytic stage antigens vaccine candidates of P. vivax. Recombinant proteins representing the 19 kDa C-terminal region of MSP-1(PvMSP19), apical membrane antigen n-1 ectodomain (PvAMA-1), and the region II of duffy binding protein (PvDBP-RII) were compared in their ability to bind to IgG antibodies of serum samples collected from 220 individuals from the state of Pará, in the North of Brazil. During patent infection with P. vivax, the frequency of individuals with IgG antibodies to PvMSP119, PvAMA-1, and PvDBP-RII were 95, 72.7, and 44.5% respectively. Although the frequency of responders to PvDBP-RII was lower, this frequency increased in individuals following multiple malarial infections. Individually, the specific antibody levels did not decline significantly nine months after treatment, except to PvMSP119. Our results further confirm a complex regulation of the immune response to distinct blood stage antigens. The reason for that is presently unknown but it may contribute to the high risk of re-infection in individuals living in the endemic areas.
Plasmodium vivax is the second most prevalent specie that causes malaria in humans (Mendis et al. 2001). In Brazil, P. vivax was responsible by approximately 75% of 597,907 cases reported in 2005 (Se-cretaria de Vigilância em Saúde, Ministério da Saúde). In spite of its high prevalence in Brazil as in many other parts of the world, the immunological mechanisms operating in individuals exposed to P. vivax have been very poorly explored.
In recent years, we have carried out several immunological studies using malaria recombinant proteins corresponding to the P. vivax merozoite surface protein 1 (MSP-1) and apical membrane antigen 1 (AMA-1) (Soares et al. 1997, 1999, Rodrigues et al. 2003, 2005). These immuno-epidemiological studies on naturally acquired immunity to merozoite proteins are of particular importance as these MSP-1 and AMA-1 are being intensively studied as a candidate for development of a vaccine against malaria (Good 2005). So far, we found that a high frequency of individuals from the Brazilian Amazon area, where P. vivax is endemic, had IgG antibodies to recombinant proteins based on 19 kDa C-terminal region of MSP-1 (PvMSP119) (Soares et al. 1997, 1999, Rodrigues et al. 2003) and the AMA-1 ectodomain (PvAMA-1) (Rodrigues et al. 2005, Oliveira et al. 2006).
As a continuation of our immuno-epidemiological studies in Brazil, in the present study, we compared the IgG antibody response to recombinant proteins based on three asexual erythrocytic stage antigens vaccine candidates of P. vivax, including the region II of the duffy binding protein (PvDBP-RII).
MATERIALS AND METHODS
Human serum samples - Serum samples were collected during the period from 1995 to 1999 from 220 patients living in endemic areas for malaria in the state of Pará, in the North of Brazil. At the time of blood collection, these subjects were positive for P. vivax by conventional thick blood smears microscopic examination. Details on the study area, patient age and diagnosis have been reported previously (Soares et al. 1997, 1999, Rodrigues et al. 2003, 2005, Oliveira et al. 2006). Only individuals whom precise information on the number of P. vivax malaria episodes was available were used to establish a correlation between the number of malarias and the frequency of responders to recombinant proteins. A second group of individuals (n = 33) donated blood nine months after being treated for malaria. Those individuals did not report subsequent malaria infections in that period. A third group of individuals was healthy adult volunteers selected among blood donors from the city of São Paulo, state of São Paulo, in the Southeastern of Brazil (n = 26). Malaria is not present in this part of the country and these individuals had never had malaria or traveled to malaria endemic areas. The Ethics Committee of the University of São Paulo has approved our studies.
PvMSP119- The recombinant protein PvMSP119 represents the 19 kDa C-terminal region of the P. vivax MSP-1. This protein was expressed in Escherichia coli and purified as publish elsewhere (Cunha et al. 2001).
PvAMA-1 - The recombinant representing the PvAMA-1 ectodomain was made as already reported (Rodrigues et al. 2005).
PvDBP-RII - The recombinant protein PvDBP-RII represents region II of P. vivax DBP. This protein was expressed in E. coli and purified as previously described (Singh et al. 2001).
ELISA for detection of human IgG antibodies - Human IgG antibodies against PvMSP119 , PvAMA-1, and PvDBP-RII were detected by ELISA as described (Rodrigues et al. 2003, 2005). The results were expressed as index of reactivity (IR = OD492 values of test sample divided by the value of the cutoff). Cutoff points were set at three standard deviations above the mean OD492 of sera from 26 individuals, unexposed to malaria, from the city of São Paulo. Values of IR > 1.0 were considered as positive.
Statistical analysis - Differences between proportions of responders were analyzed using the Chi-square test. Comparison of antibody level (IR) of independent samples was performed using one-way analysis of variance (ANOVA). Wilcoxon Signed Rank test was used to compare dependent samples.
Comparison of IgG antibody response of individuals during patent P. vivax infection to recombinant proteins representing MSP119, AMA-1, and DBP-RII of P. vivax - Initially, we compared the IgG antibody response of individuals during acute infection to the recombinant proteins representing PvMSP119, PvAMA-1, and PvDBP-RII. The frequency of individuals with IgG antibodies to all three proteins during patent infection was only 37.7%. No response to any antigen was observed in only 2.7% of individuals. Fig. 1 shows the frequency of responders against each recombinant protein. The results showed that the frequency of individuals that presented IgG antibodies to recombinant proteins PvMSP119, PvAMA-1, and PvDBP-RII were 95, 72.7, and 44.5%, respectively. These data confirmed that all three proteins are immunogenic during natural infections. The prevalence of antibodies against PvMSP119 was significantly higher than the prevalence against PvAMA-1 (Chi-Square test, P < 0.0001). The prevalence for both were higher than those observed for PvDBP-RII (Chi-Square test, P < 0.0001).
When we compared the IR values from individual serum samples, we observed that values obtained for PvMSP119 were higher than all other groups (Table I). The IR values to PvDBP-RII were lower than those observed to PvAMA-1 (One Way ANOVA, P < 0.0001).
We then determined whether there was a correlation between the frequency of IgG antibodies and episodes of P. vivax infection. For this purpose, the sera of the 213 individuals were separated in two groups: (i) primary-infected, individuals with no previous malaria episodes (n = 145) and (ii) individuals with one or more previous malaria episodes (n = 68). We found that the frequency of responders to PvMSP119 did not change significantly when we divided them in primary and multiple-infected (Chi-Square test, P > 0.05) confirming data previously obtained by our group that showed that antibody response against PvMSP119 was established after a single exposure to malaria (Rodrigues et al. 2003, 2005). In contrast, the frequency of serum samples containing IgG against PvAMA-1 and PvDBP-RII was significantly lower in group primary-infected when compared with the group multi-infected individuals (Chi-Square test, P < 0.0001), suggesting that the serocon-version to these proteins occurs after multiple exposures to malaria infection (Fig. 2). However, no difference was observed between the frequency of positive sera during the second/third or more malaria episodes, showing that specific IgG responses to both antigens developed after few malaria episodes (data not shown).
Longevity of the IgG antibody response to recombinant proteins representing MSP119, AMA-1 and DBP-RII of P. vivax - In earlier studies, we observed that the IgG antibody titers to recombinant proteins representing the N- and C-terminal regions of P. vivax MSP-1 declined significantly after treatment (Soares et al. 1999). Using serum samples collected during patent infection and nine months after treatment (paired groups, n = 33), we compared the frequency of responders and the antibody levels (estimated by OD492) to the different recombinant proteins. The proportions of individuals with antibodies specific to PvAMA-1 and PvDBP-RII were not significant during the infection or after treatment (Chi-Square test, P < 0.05). The low frequency of responders to PvDBP-RII can be explained by the fact that only primary infected individuals constitute this group. In contrast, the proportions of individuals with antibodies specific to PvMSP119 were statistically higher during infection than nine months after treatment (Chi-Square test, P < 0.0001).
When the antibody levels of individuals who recognized each recombinant protein were estimated (Table II), we found that these levels were not significant during the infection and after treatment, except to PvMSP119 (Wilcoxon Signed Ranks test, P < 0.0001).
In the present study we compared the recognition by human IgG antibodies of recombinant proteins representing three asexual erythrocytic stage vaccine candidates of P. vivax (MSP119, AMA-1, and DBP-RII). Initially, we confirmed that the frequency of individuals who presented IgG antibodies to PvMSP119 and PvAMA-1 during patent infection was high and comparable to the frequency observed in our recent studies (Rodrigues et al. 2003, 2005). On the other hand, the frequency of responders to DBP-RII was significantly lower. One possible explanation for this fact could be the requirement of repeated exposures to parasite antigens in order to promote seroconvertion. Eventually, repeated P. vivax infections could significantly increase the frequency of responders, indicating that most individuals may become responders depending on the degree of exposure. To answer that question, we divided the individuals that informed the number of previous malaria episodes in two groups: primary-infected and individuals with one or more previous malaria episodes. Our analysis showed that the frequency of responders to PvAMA-1 and PvDBP-RII was higher in the multiple-infected. This data suggests that multiple infections provide a boost to the production of specific antibodies confirming recent observations obtained in other studies in distinct endemic areas of Brazilian Amazon (Ceravolo et al. 2005, Tran et al. 2005). In the case of PvMSP119, specific IgG responses developed faster after a single exposition to malaria infection, also confirming previous observations of distinct groups that PvMSP119 is highly immunogenic during natural human infections (Park et al. 2001, Lim et al. 2002, Rodrigues et al. 2003, Morais et al. 2005, Wickramarachchi et al. 2007).
We also evaluated the persistence of antibody response to the different recombinant proteins in individuals exposed to P. vivax. We compared the presence of specific antibodies and their levels in a period of nine months following P. vivax infection in the absence of reinfection. During acute infection, eleven individuals (33.3%) displayed antibodies to all three proteins. After treatment, all those individuals persisted serologically positive to at least one antigen. When evaluated each individual recombinant protein, in the majority of individuals, specific IgG antibodies did not diminish significantly in that period, except to PvMSP119. This observation confirms our earlier description that during acute infection, there was a significant higher frequency of responders who had higher IgG antibody titers to PvMSP119 when compared to individuals few months after treatment for malaria (Soares et al. 1997, 1999). The significant decay of the number of positive sera and antibody levels to PvMSP119 contrasted with the fact that the frequency of responders to PvAMA-1, increased slightly after treatment. The vast majority of the individuals (85.7%) that became serologically negative to PvMSP119 maintained detectable antibodies to PvAMA-1. Similar results were recently obtained in a distinct endemic area of Brazilian Amazon which showed that anti-PvAMA-1 antibodies persisted in > 40% of those subjects exposed to focal malaria eight months before (Morais et al. 2006).
The reason why the antibody production is differentially sustained to each recombinant antigen after treatment is currently unknown and deserves to be investigated further in the future. Effective treatment of malaria infections has been shown to lead to rapid decline in antibody concentrations. In spite of that, memory B cells can persist in the absence of antigen and can be rapidly reactivated on reinfection (reviewed by Struik & Riley, 2004).
In summary, our results show that all three vaccine candidates are immunogenic molecules during natural malaria infections and these recombinant proteins may be useful tools to perform immuno-epidemiological studies in malaria endemic areas and to further studies towards the vaccine potential of these proteins.
To Vanessa Carvalho Tavares for technical assistance.
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