Redefining an epitope of a malaria vaccine candidate, with antibodies against the N-terminal MSA-2 antigen of Plasmodium harboring non-natural peptide bonds

The quest for novel vaccine candidates against malaria and other infectious diseases often involves innovative strategies. One such approach focuses on selecting non-polymorphic peptides derived from key antigens of pathogens. These peptides are then strategically modified to elicit a robust protective immune response. This method is particularly promising for combating diseases like malaria, where genetic variability among pathogens poses significant challenges.

The MSA-2(21-40) peptide, known for its highly conserved primary structure across malaria species, plays a critical role in the red blood cell (RBC) binding ability of *Plasmodium falciparum*. Given its importance, researchers designed structurally defined probes using non-natural peptide-bond isosteres. These probes were created to mimic the natural structure of the peptide while enhancing its stability and immunogenic properties. The focus was on modifying the (30)FIN(32)-binding motif, a region crucial for the parasite’s interaction with host cells.

Two peptide mimetics, referred to as reduced amide pseudopeptides, were synthesized. In these mimetics, natural amide bonds within the (30)FIN(32) motif were replaced with ψ-[CH2-NH] methylene amide isostere bonds. Specifically, one bond was introduced between the F-I pair, and another between the I-N pair, resulting in compounds coded as ψ-128 and ψ-130. These modifications aimed to enhance the structural integrity and antigenic potential of the peptides, making them more effective as immunogens.

The pseudopeptides were used to generate poly- and monoclonal antibodies in animal models, including Aotus monkeys and BALB/c mice. Through controlled in vitro immunization experiments, IgM isotype cell clones derived from reactive mice were induced to switch to IgG subclasses. This process resulted in the production of mature immunoglobulins that recognized a novel epitope within the MSA-2(25-32) antigen. Additionally, these antibodies cross-reacted with two polypeptides from rodent malaria species, highlighting their broad applicability.

Functional assays were conducted to evaluate the antibodies’ efficacy against malaria. The results demonstrated their high effectiveness in controlling infection in vitro. Furthermore, in vivo studies revealed a significant neutralizing capacity against rodent malaria infections. These findings underscore the potential of site-directed peptide mimetics to disrupt the biological development of *Plasmodium* parasites, offering a promising avenue for future research.

The neutralizing effect of antibodies induced by these pseudopeptides highlights their value in developing immunoprophylactic strategies. By targeting conserved regions of key antigens, these tools could pave the way for vaccines that provide long-lasting protection against malaria. Such advancements are critical for controlling the spread of this devastating disease and improving global health outcomes. The success of this approach also opens doors for similar strategies against other transmissible diseases.