Comparison of the antibody responses to Plasmodium vivax and Plasmodium falciparum antigens in residents of Mandalay, Myanmar

Exploring genetic polymorphisms among Plasmodium vivax isolates from the Thai-Myanmar borders using circumsporozoite protein (pvcsp) and ookinete surface protein (pvs25) encoding genes

Plasmodium vivax malaria cases remain high along the Thai-Myanmar and Thai-Cambodia borders. Plasmodium vivax circumsporozoite protein (pvcsp) and Plasmodium vivax ookinete surface protein (pvs25) genes are promising molecular markers of the genetic diversity of P. vivax. This study investigated the genetic diversity of pvcsp and pvs25 in P. vivax isolates collected from the Thai-Myanmar border. The DNA samples were amplified, and the genotypes were analyzed by PCR-RFLP and DNA sequencing. Pvcsp genotypes, VK210, VK247, and mixed types, were found in 203 (91.9%), 15 (6.8%), and 3 (1.3%) of the isolates, respectively. Twenty-four allelic variants were observed, of which a high prevalence of VK210E and VK247E were reported. Two pvcsp variants, VK210C and VK210M showed significantly higher parasite density (46,234 (1154-144,000) vs. 25,606 (1373-68,878), respectively). The genetic diversity of pvcsp along the Thai-Myanmar border during 2002-2015 showed dynamic changes with both positive and negative selection. The frequency and distribution of pvcsp pattern might be changed over time and might be other factors contributing to gene selection. Three amino acid substitutions of pvs25, i.e., E97Q, I130T, and Q131K, were investigated with frequencies of 10 (4.5%), 221 (100%), and 204 (92.3%) isolates, respectively. There was no association between parasite density and pvs25 polymorphisms. The frequency of pvs25 polymorphism was similar to that previously reported, with the absence of random mutation. In conclusion, the genetic variation of pvcsp was changed over times whereas the genetic diversity of pvs25 was limited; these variations would be helpful for further vaccine development against P. vivax malaria.

Naturally acquired antibodies to gametocyte antigens are associated with reduced transmission of Plasmodium vivax gametocytes to Anopheles arabiensis mosquitoes

Naturally acquired antibodies may reduce the transmission of Plasmodium gametocytes to mosquitoes. Here, we investigated associations between antibody prevalence and P. vivax infectivity to mosquitoes. A total of 368 microscopy confirmed P. vivax symptomatic patients were passively recruited from health centers in Ethiopia and supplemented with 56 observations from asymptomatic P. vivax parasite carriers. Direct membrane feeding assays (DMFA) were performed to assess mosquito infectivity; for selected feeds these experiments were also performed after replacing autologous plasma with malaria naïve control serum (n=61). The prevalence of antibodies against 6 sexual stage antigens (Pvs47, Pvs48/45, Pvs230, PvsHAP2, Pvs25 and PvCelTOS) and an array of asexual antigens was determined by ELISA and multiplexed bead-based assays. Gametocyte (ρ< 0.42; p = 0.0001) and parasite (ρ = 0.21; p = 0.0001) densities were positively associated with mosquito infection rates. Antibodies against Pvs47, Pvs230 and Pvs25 were associated with 23 and 34% reductions in mosquito infection rates (p<0.0001), respectively. Individuals who showed evidence of transmission blockade in serum-replacement DMFAs (n=8) were significantly more likely to have PvsHAP2 or Pvs47 antibodies. Further studies may demonstrate causality for the observed associations, improve our understanding of the natural transmission of P. vivax and support vaccine development.

An immunomics approach for the analysis of natural antibody responses to Plasmodium vivax infection

High throughput immunomics is a powerful platform to discover potential targets of host immunity and develop diagnostic tests for infectious diseases. We screened the sera of Plasmodium vivax-exposed individuals to profile the antibody response to blood-stage antigens of P. vivax using a P. vivax protein microarray. A total of 1936 genes encoding the P. vivax proteins were expressed, printed and screened with sera from P. vivax-exposed individuals and normal subjects. Total of 151 (7.8% of the 1936 targets) highly immunoreactive antigens were identified, including five well-characterized antigens of P. vivax (ETRAMP11.2, Pv34, SUB1, RAP2 and MSP4). Among the highly immunoreactive antigens, 5 antigens were predicted as adhesins by MAAP, and 11 antigens were predicted as merozoite invasion-related proteins based on homology with P. falciparum proteins. There are 40 proteins that have serodiagnostic potential for antibody surveillance. These novel Plasmodium antigens identified provide the clues for understanding host immune response to P. vivax infection and the development of antibody surveillance tools.

Development of malaria transmission-blocking vaccines: from concept to product

Despite decades of effort battling against malaria, the disease is still a major cause of morbidity and mortality. Transmission-blocking vaccines (TBVs) that target sexual stage parasite development could be an integral part of measures for malaria elimination. In the 1950s, Huff et al. first demonstrated the induction of transmission-blocking immunity in chickens by repeated immunizations with Plasmodium gallinaceum-infected red blood cells. Since then, significant progress has been made in identification of parasite antigens responsible for transmission-blocking activity. Recombinant technologies accelerated evaluation of these antigens as vaccine candidates, and it is possible to induce effective transmission-blocking immunity in humans both by natural infection and now by immunization with recombinant vaccines. This chapter reviews the efforts to produce TBVs, summarizes the current status and advances and discusses the remaining challenges and approaches.

Malaria vaccine: a bright prospect for elimination of malaria

Malaria remains one of the few diseases those continue to scourge human civilization despite the significant advances in disease control strategies over the last century. Malaria is responsible for more than 500 million cases and 1-3 million deaths annually. Approximately 85% of these deaths are among children, mostly in Africa, primarily due to P. falciparum. Whole cell vaccines, irradiated sporozoites and genetically attenuated sporozoites have demonstrated long lasting, sterile protection against plasmodium infection in animal and experimental clinical studies. Atypical membrane protein 1 and merozoite surface protein 1 are the two most extensively studied asexual blood stage vaccine candidates. The most promising candidate vaccine under development is RTS, S combined with AS01 adjuvant. Initial results from phase III trials of this candidate vaccine show 50% reduction of malaria in 5-17 mo aged children during the 12 mo after vaccination. WHO anticipates that the RTS,S/AS01 vaccine will be recommended for the 6-14 week age group for co-administration together with other vaccines as part of routine immunization programs in malaria endemic countries. Malaria vaccine could play an important role in elimination and eventual eradication of malaria.