Protective human immunity as a vaccine discovery tool for falciparum malaria

Large screen approaches to identify novel malaria vaccine candidates

Until recently, malaria vaccine development efforts have focused almost exclusively on a handful of well characterized Plasmodium falciparum antigens. Despite dedicated work by many researchers on different continents spanning more than half a century, a successful malaria vaccine remains elusive. Sequencing of the P. falciparum genome has revealed more than five thousand genes, providing the foundation for systematic approaches to discover candidate vaccine antigens. We are taking advantage of this wealth of information to discover new antigens that may be more effective vaccine targets. Herein, we describe different approaches to large-scale screening of the P. falciparum genome to identify targets of either antibody responses or T cell responses using human specimens collected in Controlled Human Malaria Infections (CHMI) or under conditions of natural exposure in the field. These genome, proteome and transcriptome based approaches offer enormous potential for the development of an efficacious malaria vaccine.

S. mansoni Trapping in Lungs Contributes to Resistance to Reinfection

Worm transplantation studies show that physiological and reproductive status of the worm is influenced by the microenvironment of the host and critical for vaccine design. Worm migration studies in rats with ⁷⁵Se-methionine labeled cercariae demonstrated that resistance to reinfection (R/R) requires a host immune response resulting in worm death. In permissive hosts, inflammation due to anti eggs immunity leads to host death, whereas in non-permissive hosts this is not the case due to reduced egg burdens. Eggs-induced pathology and inflammatory debris resulting from immune attack on worms are important for vaccine design. Protective immune responses are perhaps induced when naïve hosts are vaccinated with either schistosome-derived molecules or attenuated cercariae as suggested by the induction of protective anti-parasite antibodies and monoclonals. However, these immunological strategies rarely produce 85–90% R/R as is achievable by portal-caval shunting. Alternatively, induction of anti-schistosoma immunity may induce portacaval shunting, seems highly unlikely although not yet tested. Differential screening with sera from twice-infected rats, protective (F2x) from Fisher vs. non-protective (W2x) from Wistar–Furth rats, was used to identify candidate vaccine antigens.

Polymorphism study of rhoptry associated membrane antigen (RAMA) gene of Plasmodium falciparum—A putative vaccine candidate

The antigenic diversity of Plasmodium falciparum is one of the major obstacles to antimalarial vaccine development. Thus, it becomes obvious to search for a protein that is fairly conserved and is necessary for the parasite to survive in the host cell. Rhoptry associated membrane antigen (RAMA) plays a potential role in parasite biology and invasion. The C-terminal end of RAMA is shown to have protective immune responses and binds to the erythrocyte membrane. In this work, we have studied the polymorphism of RAMA gene in the C-terminal end from 1525 to 3196 nucleotide (nt) in 230 samples. The presence of few variants suggests RAMA to be under a balancing selection. The above criterion of restricted antigenic diversity and a protective immune response towards the C-terminal end makes RAMA a strong vaccine candidate.

Antibodies elicited in adults by a primary Plasmodium falciparum blood-stage infection recognize different epitopes compared with immune individuals

Background

Asexual stage antibody responses following initial Plasmodium falciparum infections in previously healthy adults may inform vaccine development, yet these have not been as intensively studied as they have in populations from malaria-endemic areas.

Methods

Serum samples were collected over a six-month period from twenty travellers having returned with falciparum malaria. Fourteen of these were malaria-naïve and six had a past history of one to two episodes of malaria. Antibodies to seven asexual stage P. falciparum antigens were measured by ELISA. Invasion inhibitory antibody responses to the 19kDa fragment of merozoite surface protein 1 (MSP1₁₉) were determined.

Results

Short-lived antibody responses were found in the majority of the subjects. While MSP1₁₉ antibodies were most common, MSP1 block 2 antibodies were significantly less frequent and recognized conserved domains. Antibodies to MSP2 cross-reacted to the dimorphic allelic families and anti-MSP2 isotypes were not IgG3 skewed as shown previously. MSP1₁₉ invasion inhibiting antibodies were present in 9/20 patients. A past history of malaria did not influence the frequency of these short-lived, functional antibodies (p = 0.2, 2-tailed Fisher's exact test).

Conclusion

Adults infected with P. falciparum for the first time, develop relatively short-lived immune responses that, in the case of MSP1₁₉, are functional. Antibodies to the polymorphic antigens studied were particularly directed to allelic family specific, non-repetitive and conserved determinants and were not IgG subclass skewed. These responses are substantially different to those found in malaria immune individuals.