Optimization of aPlasmodium falciparumcircumsporozoite protein repeat vaccine using the tobacco mosaic virus platform

Abstract

Plasmodium falciparum vaccine RTS,S/AS01 is based on the major NPNA repeat and the C-terminal region of the circumsporozoite protein (CSP). RTS,S-induced NPNA-specific antibody titer and avidity have been associated with high-level protection in naïve subjects, but efficacy and longevity in target populations is relatively low. In an effort to improve upon RTS,S, a minimal repeat-only, epitope-focused, protective, malaria vaccine was designed. Repeat antigen copy number and flexibility was optimized using the tobacco mosaic virus (TMV) display platform. Comparing antigenicity of TMV displaying 3 to 20 copies of NPNA revealed that low copy number can reduce the abundance of low-affinity monoclonal antibody (mAb) epitopes while retaining high-affinity mAb epitopes. TMV presentation improved titer and avidity of repeat-specific Abs compared to a nearly full-length protein vaccine (FL-CSP). NPNAx5 antigen displayed as a loop on the TMV particle was found to be most optimal and its efficacy could be further augmented by combination with a human-use adjuvant ALFQ that contains immune-stimulators. These data were confirmed in rhesus macaques where a low dose of TMV-NPNAx5 elicited Abs that persisted at functional levels for up to 11 mo. We show here a complex association between NPNA copy number, flexibility, antigenicity, immunogenicity, and efficacy of CSP-based vaccines. We hypothesize that designing minimal epitope CSP vaccines could confer better and more durable protection against malaria. Preclinical data presented here supports the evaluation of TMV-NPNAx5/ALFQ in human trials.

Design of Alphavirus Virus-Like Particles Presenting Circumsporozoite Junctional Epitopes That Elicit Protection against Malaria

“…As the CHIK-VLP immunogens developed here displayed PfCSP regions distinct from the RTS,S and R21 immunogens (displaying the major repeat and C-terminal domains), they could be potentially be co-administered to increase the epitope breadth of responses against PfCSP [ 8 ]. Alternatively, the immunogen sequence from R21 or other promising PfCSP-based vaccines, such as the tobacco mosaic virus platform [ 52 ], could be redesigned to incorporate the junctional regions described here, which could expand the breadth of responses. Finally, heterologous prime-boost regimens focusing first on junctional epitopes, then on major or minor repeat regions, may further mature cross-reactivity that has been associated with PfCSP-neutralizing responses [ 26 , 31 , 34 , 50 ].…”

Pre-Erythrocytic Vaccines against Malaria

“…The development of better, more potent adjuvants may also help offer stronger immune responses that are sustained longer. The structural data emerging from CSP-specific antibodies could also provide key insights into improving the CSP-based vaccines [ 43 ]. The advent of novel vaccination approaches and platforms may help develop better CSP-based antimalarial vaccines in the future [ 44 , 45 ].…”

“…The R21 design incorporates a higher proportion of Pf CSP C-terminus bound to HBsAg N-terminus without the three-fold molar excess of HBsAg found in RTS, S/AS01 [ 20 , 46 ]. In this way, more CSP antigen is displayed on the vaccine particle surface, mimicking the higher CSP epitope concentration on the Plasmodium sporozoite surface [ 43 ]. This enhanced B cell activation led to stronger anti-CSP humoral immune responses [ 47 ].…”

In vitro and in vivo inhibition of malaria parasite infection by monoclonal antibodies against Plasmodium falciparum circumsporozoite protein (CSP)

“…Immunization via mosquito bite with radiation-attenuated sporozoites (IMRAS vaccine) 11 , cryopreserved irradiated sporozoites (PfSPZ vaccine) 12 or sporozoites delivered under chloroquine prophylaxis (PfSPZ-C vaccine) 13 also elicit protection against CHMI that is in part mediated by inhibitory CSP antibodies. Highly protective polyclonal and monoclonal antibodies have been mapped to the central repeats and junctional sequence, but not to the N- or C-terminal regions of CSP 2 , 14 – 16 .…”