Protective Epitopes of the Plasmodium falciparum SERA5 Malaria Vaccine Reside in Intrinsically Unstructured N-Terminal Repetitive Sequences

Abstract

The malaria vaccine candidate antigen, SE36, is based on the N-terminal 47 kDa domain of Plasmodium falciparum serine repeat antigen 5 (SERA5). In epidemiological studies, we have previously shown the inhibitory effects of SE36 specific antibodies on in vitro parasite growth and the negative correlation between antibody level and malaria symptoms. A phase 1 b trial of the BK-SE36 vaccine in Uganda elicited 72% protective efficacy against symptomatic malaria in children aged 6–20 years during the follow-up period 130–365 days post–second vaccination. Here, we performed epitope mapping with synthetic peptides covering the whole sequence of SE36 to identify and map dominant epitopes in Ugandan adult serum presumed to have clinical immunity to P. falciparum malaria. High titer sera from the Ugandan adults predominantly reacted with peptides corresponding to two successive N-terminal regions of SERA5 containing octamer repeats and serine rich sequences, regions of SERA5 that were previously reported to have limited polymorphism. Affinity purified antibodies specifically recognizing the octamer repeats and serine rich sequences exhibited a high antibody-dependent cellular inhibition (ADCI) activity that inhibited parasite growth. Furthermore, protein structure predictions and structural analysis of SE36 using spectroscopic methods indicated that N-terminal regions possessing inhibitory epitopes are intrinsically unstructured. Collectively, these results suggest that strict tertiary structure of SE36 epitopes is not required to elicit protective antibodies in naturally immune Ugandan adults.

Preclinical Studies on a New Vaccine Formulation of BK-SE36, a Malaria Vaccine Candidate

“…SERA5 may likely overcome challenges with regards to extensive polymorphism 43) and strict structural requirement for protective epitopes 16) . Further studies on the phase 1b clinical trial in Uganda suggest that BK-SE36 does not show allele-specific efficacy (Arisue et al, in preparation) and protective efficacy may not be influenced by African HLA II haplotype (Tougan et al, unpublished).…”

“…The N-terminal part of the protein correlates with reduced parasite density 12) and absence of clinical symptoms in infected children/adults 13) -15) . Moreover, the N-terminal repetitive sequence regions were identified as immunodominant IgG epitopes in Ugandan malaria-immune volunteers and the physicochemical properties of these protective epitopes suggest that they are intrinsically unstructured and, thus, a strict tertiary structure of SE36 epitopes is not required to elicit protective antibodies 16) .…”

The malaria parasite Plasmodium falciparum in red blood cells selectively takes up serum proteins that affect host pathogenicity

“…Mouse anti-SE36 and rabbit anti-EXP2/3C sera were prepared accordingly to previously described protocols [21,22]. Anti-protein S polyclonal antibody (pAb) (16910-1-AP), anti-prothrombin pAb (24295-1-AP), antivitronectin pAb (15833-1-AP), anti-alpha-2-HS-glycoprotein pAb (16571-1-AP), anti-alpha-2-macroglobulin pAb (13545-1-AP), anti-band 3 pAb (18566-1-AP), antifibrinogen beta chain pAb (16747-1-AP), and rabbit control IgG (30000-0-AP) were obtained from Proteintech (Rosemont, IL, USA).…”

Untitled

“…Although there are effective antimalarial medicines such as artemisinin [2] and newly developed vaccines such as BK-SE36 [3], the eradication of malaria needs further basic research and development. In this paper strategic to enter into human blood cells [1].…”