Potent Malaria Transmission-Blocking Antibody Responses Elicited by Plasmodium falciparum Pfs25 Expressed in Escherichia coli after Successful Protein Refolding

Abstract

ABSTRACTProduction of Pfs25, aPlasmodium falciparumtransmission-blocking vaccine target antigen, in functional conformation with the potential to elicit effective immunogenicity still remains a major challenge. In the current study, codon-harmonized recombinant Pfs25 (CHrPfs25) was expressed inEscherichia coli, and purified protein after simple oxidative refolding steps retained reduction-sensitive conformational epitopes of transmission-blocking monoclonal antibodies. CHrPfs25 formulated in several adjuvants elicited strong immunogenicity in preclinical studies in mice. Antibodies elicited after immunization recognized native Pfs25 on the surface of live gametes ofP. falciparumand demonstrated complete malaria transmission-blocking activity. The transmission-blocking efficacy was 100% even after a 1:128 dilution of sera from immunized mice in the complete Freund's adjuvant and Montanide ISA51 groups and after a 1:16 dilution of sera from mice in the alum group. The blocking was mediated by antibodies; purified IgG at concentrations as low as 31.25 μg/ml exhibited 100% transmission blocking in membrane feeding assays employing two different species of mosquitoes,Anopheles gambiaeandAnopheles stephensi. This study provides the first evidence for successful expression of biologically functional rPfs25 inE. coli. The extremely potent malaria transmission-blocking activity of antibodies elicited by immunization with purified protein provides strong support for further evaluation ofE. coli-derived CHrPfs25 as a malaria transmission-blocking vaccine in human clinical trials.