Naturally acquired inhibitory antibodies to Plasmodium vivax Duffy binding protein are short-lived and allele-specific following a single malaria infection.

The Duffy binding protein of Plasmodium vivax (DBP) is a critical adhesion ligand that participates in merozoite invasion of human Duffy-positive erythrocytes. A small outbreak of P. vivax malaria, in a village located in a non-malarious area of Brazil, offered us an opportunity to investigate the DBP immune responses among individuals who had their first and brief exposure to malaria. Thirty-three individuals participated in the five cross-sectional surveys, 15 with confirmed P. vivax infection while residing in the outbreak area (cases) and 18 who had not experienced malaria (non-cases). In the present study, we found that only 20% (three of 15) of the individuals who experienced their first P. vivax infection developed an antibody response to DBP; a secondary boosting can be achieved with a recurrent P. vivax infection. DNA sequences from primary/recurrent P. vivax samples identified a single dbp allele among the samples from the outbreak area. To investigate inhibitory antibodies to the ligand domain of the DBP (cysteine-rich region II, DBP(II)), we performed in vitro assays with mammalian cells expressing DBP(II) sequences which were homologous or not to those from the outbreak isolate. In non-immune individuals, the results of a 12-month follow-up period provided evidence that naturally acquired inhibitory antibodies to DBP(II) are short-lived and biased towards a specific allele.

Longevity of naturally acquired antibody responses to the N- and C-terminal regions of Plasmodium vivax merozoite surface protein 1.

In an earlier study, we found that individuals with patent infection had significantly higher IgG antibody titers to the 19-kD C-terminal region of Plasmodium vivax merozoite surface protein 1 (PvMSP1) than individuals treated for malaria 1-4 months earlier. These results suggested that the antibody levels decreased rapidly following treatment. The present study was designed to determine the persistence of antibody response to the N- and C-terminal regions of PvMSP1 after infection with P. vivax in individuals from the city of Bélem in northern Brazil. Our results demonstrated that the vast majority of individuals had a significant decrease in antibody titers to the C-terminal region of PvMSP1 in a period of two months following treatment. Among responders to the C-terminal region, 44.4% became serologically negative and 44.4% had their antibody titers reduced by an average of 13-fold. Only 11.2% of the individuals had their antibody titers maintained or slightly increased during that period. A decrease in the antibody response to the recombinant protein representing the N-terminal region of PvMSP1 was also noted; however, it was not as dramatic. The rapid decrease in the antibody levels to the C-terminal region of PvMSP1 might contribute to the high risk of reinfection in these individuals.

Antibody response to the N and C-terminal regions of the Plasmodium vivax Merozoite Surface Protein 1 in individuals living in an area of exclusive transmission of P. vivax malaria in the north of Brazil.

Recently, we found that a recombinant protein based on the 19 kDa C-terminal region of the Plasmodium vivax Merozoite Surface Protein 1 (PvMSP1(19)) was recognized by a large proportion of individuals naturally infected. The present study was designed to determine the prevalence of antibody to PvMSP1(19) in individuals from the village of Cotijuba, northern Brazil, where only P. vivax transmission occurs. Immuno-epidemiological studies on the prevalence of antibody to the C-terminus of PvMSP1 are of particular importance as this region of MSP1 is being intensively studied as a prime candidate for development of a vaccine against malaria. We evaluated the antibody response to PvMSP1(19), and compared it to the N-terminal region of PvMSP1 and to blood stage antigens. The total frequencies of individuals with IgG to blood stages, PvMSP1(19) or the N-terminal region of PvMSP1 were 76.6, 42.3 and 29.8%, respectively. The frequency of responders to PvMSP1(19) did not increase with age. However, the frequency of responders to this recombinant protein was significantly higher (77.4%) in individuals with a recent ( < 6 months) history of malaria, when compared to subjects whose last malaria attack occurred more than 6 months before (43.9%), or to individuals without a past history of symptomatic malaria (6.25%). These results confirm earlier studies by demonstrating that the PvMSP1(19) is highly immunogenic in individuals recently exposed to P. vivax malaria.

Malaria vaccine: roadblocks and possible solutions.

Malaria remains the most prevalent and devastating parasitic disease worldwide. Vaccination is considered to be an approach that will complement other strategies for prevention and control of the disease in the future. In the last 10 years, intense studies aimed at the development of a malaria vaccine have provided important knowledge of the nature of the host immunological mechanisms of protection and their respective target antigens. It became well established that protective immune responses can be generated against the distinct stages of Plasmodium. However, in general, protective immune responses are directed at stage-specific antigens. The elucidation of the primary structure of these antigens made possible the generation of synthetic and recombinant proteins that are being extensively used in experimental immunizations against the infection. Today, several epitopes of limited polymorphism have been described and protective immunity can be generated by immunization with them. These epitopes are being tested as primary candidates for a subunit vaccine against malaria. Here we critically review the major roadblocks for the development of a malaria vaccine and provide some insight on how these problems are being solved.

Colorimetric leukocyte adherence inhibition assay: a new auxiliary technique for the diagnosis of schistosomiasis.

We describe a modification of the leukocyte adherence inhibition assay (LAI) in which we propose the use of 3-(4,5-dimethylthiazole-2-yl)-2-5-diphenyltetrazolium bromide (MTT) dye which is taken up and reduced by mitochondria. The method was tested by screening peripheral blood leukocytes from Schistosoma mansoni-infected patients. Peripheral blood leukocytes from patients (N = 21) but not from the blood of normal subjects (N = 10) failed to adhere to glass in the presence of soluble adult worm antigenic preparation (SWAP). The non-adherence index (NAI) values for schistosomiasis patients were in the range of 11.0 to 72.3 (mean +/- SEM = 29.3 +/- 4.3), whereas the values for normal subjects were -56.0 to +2.0 (-25.9 +/- 7.6) and those for treated patients -59.6 to +4.0 (-19.3 +/- 5.8). Our results show that the colorimetric LAI assay can be used as an auxiliary test for the diagnosis of schistosomiasis.

Immunogenicity assay of the Leishmune vaccine against canine visceral leishmaniasis in Brazil.

Leishmune is the industrialized version of the FML-saponin vaccine which has been shown to develop 92-95% protection in vaccinated dogs and 76-80% vaccine efficacy against field canine visceral leishmaniasis (CVL) in Brazil. Leishmune has been proven to be safe and tolerable and a transmission-blocking vaccine which renders vaccinated dogs non-infectious to sand fly vectors. In the present investigation, 550 healthy seronegative dogs of endemic and epidemic areas of Brazil were monitored for Leishmune-induced immunogenicity during a 2-year trial. Another group of 588 untreated exposed dogs was also studied in parallel. Both groups were seronegative on day 0. The strong immunogenicity induced by Leishmune vaccine was demonstrated by the 98% of FML-seroconversion, increase in absorbencies, the 82.7% DTH positive reactions and increase in skin test size diameters, the average increase in CD8+ total lymphocytes population in blood (27.1%), expected for QS21 saponin-containing vaccine, the sustained proportions of CD4+ T cells, and the average increased proportions of CD21+ B lymphocytes (42.3%). The Leishmune-induced protection against CVL is demonstrated by the results: 98.8% asymptomatic dogs (at the end of first year) and 99% healthy survivors (at the end of the second year) among vaccinated dogs, compared to the 79.4% asymptomatic and 61% survivor dogs (p<0.001) monitored in the untreated exposed cohort. In spite of the low vaccine coverage, it was possible to detect a 66.1% (p<0.005) reduction in Belo Horizonte and an 80.2% (p<0.005) reduction in Araçatuba of the incidence of CVL among vaccinated dogs, when compared to the global incidence of CVL of each town, respectively. Our preliminary results support the potential use of Leishmune to prevent CVL epidemics.

Immunogenic properties of the Plasmodium vivax vaccine candidate MSP1(19) expressed as a secreted non-glycosylated polypeptide from Pichia pastoris.

The 19 kDa C-terminal region of the merozoite surface protein 1 (MSP1(19)) is one of the most promising vaccine candidates against the erythrocytic forms of malaria. In the present study, a gene encoding the Plasmodium vivax MSP1(19) epitope (PvMSP1(19)) and the Pan-Allelic DR epitope (PADRE) was expressed in the methylotrophic yeast Pichia pastoris. A non-glycosylated form of the recombinant protein rPvMSP1(19)-PADRE was purified from culture supernatants. This recombinant protein maintains its antigenicity, being recognized by a very high percentage (85.6%) of sera from Brazilian individuals naturally exposed to P. vivax. The antibody immune response elicited by rPvMSP1(19)-PADRE was compared in C57BL/6 mice immunized with different adjuvant formulations. After 3 immunizing doses, antibody titres induced in the presence of the adjuvants monophosphoryl lipid A, trehalose dicorynomycolate and cell wall skeleton or alum plus CpG ODN 1826 were as high as titres generated by Complete Freund's Adjuvant. Based on these immunological studies, we concluded that rPvMSP1(19)-PADRE deserves further evaluation in pre-clinical immunizations against P. vivax in non-human primates.

Comparison of the immunogenic properties of recombinant proteins representing the Plasmodium vivax vaccine candidate MSP1(19) expressed in distinct bacterial vectors.

The 19kDa C-terminal region of the merozoite surface protein 1 (MSP1(19)) is one of the most promising vaccine candidates against the erythrocytic forms of malaria. In the present study, we used three different Escherichia coli expression vectors to generate five recombinant proteins representing the MSP1(19) of Plasmodium vivax. These proteins were compared for reactivity with a panel of sera from individuals naturally exposed to P. vivax and for their immunogenicity in mice. Among the proteins studied, MSP1(19) expressed by the vector pET (His(6)-MSP1(19)) was better recognized by the antibodies of several individuals exposed to P. vivax. The addition of the T-cell Pan-allelic DR epitope (PADRE) did not alter the recognition of this recombinant protein by human antibodies. Although recombinant proteins were immunogenic to mice, immunization with MSP1(19) expressed by the pET or pGEX vectors induced significantly higher antibody titers than a protein produced by the pMAL vector. The antibody immune response elicited by His(6)-MSP1(19) containing the PADRE epitope was compared using different adjuvant formulations. After only two immunizing doses, antibody titers induced in the presence of the adjuvants TiterMax, MPL/TDM/CWS or alum plus CpG ODN 1826 were as high as titers generated by complete Freund's adjuvant. We concluded that, among the bacterial recombinant proteins, MSP1(19) expressed by the vector pET should be selected for further evaluation in pre-clinical immunizations against P. vivax.

Acquired immune responses to the N- and C-terminal regions of Plasmodium vivax merozoite surface protein 1 in individuals exposed to malaria.

In this study, we evaluated the naturally acquired immune response to Plasmodium vivax merozoite surface protein 1 (PvMSP1) in individuals with recent clinical episodes of malaria from the state of Para, Brazil. Ten recombinant proteins representing the first 682 amino acids (aa) of the N-terminal region and one representing the final 111 aa of the C-terminal region were expressed in Escherichia coli as glutathione S-transferase fusion proteins. Both of these regions have been suggested as candidates for development of a vaccine against Plasmodium sp. The total frequencies of individuals with antibodies and cellular immune responses to PvMSP1 were high (83.8 and 75%, respectively). The recombinant proteins representing the N- and C-terminal regions were recognized by 51.4 and 64.1% of sera, respectively. The frequency of responders to the C-terminal region increased according to the number of previous malaria episodes, reaching 83.3% after four episodes. Cellular immune response was measured by in vitro proliferation and gamma interferon production. Peripheral blood mononuclear cells of 75 and 47.2% of individuals proliferated in response to stimulation by the N- and C-terminal regions, respectively. Also, we found that one protein representing the N terminus and a second representing the C terminus of PvMSP1 stimulated 54.5% of individuals to secrete gamma interferon. We concluded that PvMSP1 is immunogenic to a large proportion of individuals exposed to malaria. Our results also suggested that the C-terminal region of PvMSP1 containing the two epidermal growth factor-like domains is particularly immunogenic to antibodies and T cells during natural infection in humans.

Comparison of antibody and protective immune responses against Trypanosoma cruzi infection elicited by immunization with a parasite antigen delivered as naked DNA or recombinant protein.

Immunization with naked DNA represents an attractive strategy for development of vaccines against a variety of infections including those caused by protozoan parasites. Recently, we have described that immunization with a plasmid containing the trans-sialidase (TS) gene induced protective immunity against Trypanosoma cruzi infection in BALB/c mice. The present study was aimed at examining and comparing the effectiveness of immunization using either plasmid or recombinant delivered antigens in a mouse strain highly susceptible to infection (A/Sn). Two plasmids were generated containing the coding region for the catalytic domain of TS. TS gene was inserted into pcDNA3 vector with or without the coding region for TS signal peptide. These two plasmids were found to be equally immunogenic at inducing antibodies to TS or inhibition of T. cruzi infection. A third plasmid, in which the TS gene was inserted into the vector VR1012, was as immunogenic as the two others. Immunization with a TS recombinant protein in alum generated a significantly higher antibody response as measured by ELISA or inhibition of TS enzymatic activity. Most relevant, this immunization reduced the mortality due to acute infection.

Antigenic properties of the merozoite surface protein 1 gene of Plasmodium vivax.

Plasmodium vivax is responsible for an approximate 35 million yearly human cases of malaria. Unfortunately, due to the low mortality rate associated with it and the difficulties of continuously in vitro culturing of this parasite, vaccine development against this human malaria has been largely neglected. In here, the antigenic properties of the merozoite surface protein 1 gene of P. vivax (PvMSP-1), were studied. Thus, seven recombinant bacterial plasmids coding different regions of the PvMSP-1 protein were constructed and used to immunize BALB/c mice. The results demonstrated that a plasmid encoding the entire N-terminus comprising 682 amino acids and a plasmid encoding the C-terminus including the two juxtaposed epidermal growth factor (EGF)-like domains fused to the Hepatitis B surface antigen, were antigenic. Moreover, the elicited immune responses were similar to those reported for these same PvMSP-1 regions in natural human infections.

A Plasmodium vivax vaccine candidate displays limited allele polymorphism, which does not restrict recognition by antibodies.

BACKGROUND: The 19 kDa C-terminal region of the merozoite surface protein 1 (MSP1(19)) has been suggested as candidate for part of a subunit vaccine against malaria. A major concern in vaccine development is the polymorphism observed in different plasmodial strains. The present study examined the extension and immunological relevance of the allelic polymorphism of the MSP1(19) from Plasmodium vivax, a major human malaria parasite. MATERIALS AND METHODS: We cloned and sequenced 88 gene fragments representing the MSP1(19) from 28 Brazilian isolates of P. vivax. Subsequently, we evaluated the reactivity of rabbit polyclonal antibodies, a monoclonal antibody, and a panel of 80 human sera to bacterial and yeast recombinant proteins representing the two allelic forms of P. vivax MSP1(19) described thus far. RESULTS: We observed that DNA sequences encoding MSP1(19) were not as variable as the equivalent region of other species of Plasmodium, being conserved among Brazilian isolates of P. vivax. Also, we found that antibodies are directed mainly to conserved epitopes present in both allelic forms of the protein. CONCLUSIONS: Our findings suggest that the use of MSP1(19) as part of a subunit vaccine against P. vivax might be greatly facilitated by the limited genetic polymorphism and predominant recognition of conserved epitopes by antibodies.

Malaria vaccine: roadblocks and possible solutions

Malaria remains the most prevalent and devastating parasitic disease worldwide. Vaccination is considered to be an approach that will complement other strategies for prevention and control of the disease in the future. In the last 10 years, intense studies aimed at the development of a malaria vaccine have provided important knowledge of the nature of the host immunological mechanisms of protection and their respective target antigens. It became well established that protective immune responses can be generated against the distinct stages of Plasmodium. However, in general, protective immune responses are directed at stage-specific antigens. The elucidation of the primary structure of these antigens made possible the generation of synthetic and recombinant proteins that are being extensively used in experimental immunizations against the infection. Today, several epitopes of limited polymorphism have been described and protective immunity can be generated by immunization with them. These epitopes are being tested as primary candidates for a subunit vaccine against malaria. Here we critically review the major roadblocks for the development of a malaria vaccine and provide some insight on how these problems are being solved.

Colorimetric leukocyte adherence inhibition assay: a new auxiliary technique for the diagnosis of Schistosomiasis

We describe a modification of the leukocyte adherence inhibition assay (LAI) in which we propose the use of 3-(4,5-dimethylthiazole-2-yl)-2-5-diphenyltetrazolium bromide (MTT) dye which is taken up and reduced by mitochondria. The method was tested by screening peripheral blood leukocytes from Schistosoma mansoni-infected patients. Peripheral blood leuckocytes from patients (N=2) but not from the blood of normal subjects (N=10) failed to adhere to glass in the presence od soluble adult worm antigenic preparation (SWAP). The non-adherence index (NAI) values for schistosomiasis patients were in the range of 11.0 to 72.3 (mean ñ SEM = 29.3 ñ 4.3), whereas the values for normal subjects were -56.0 to +2.0(-25.9 ñ 7.6) and those for treated patients -59.6 to +4.0 (-19.3 ñ 5.8). Our results show that the colorimetric LAI assay can be used as an auxiliary test for the diagnosis of schistosomiasis