Experimental infection and protection against ... - TI Pharma

A saturation of antibody avidity and concentration induced by malaria vaccine
candidate Apical Membrane Antigen 1
Introduction
The functional activity of antibodies depends on epitope specificity, titre and
affinity. The avidity of antibodies defines the sum of the individual affinities
between the antibody binding sites and single epitopes [1, 2] and is an
important parameter defining antibody quality. Both concentration and avidity
of antibodies should be addressed in vaccines that depend on humoral
responses to convey protection. One candidate immunogen is Plasmodium
falciparum Apical Membrane Antigen 1 (PfAMA1). Located at the surface of
blood-stage malaria parasites, AMA1 plays an essential role in the process of red
blood cell invasion [3-5]. Functional antibody binding has been shown to prevent
parasite invasion [6], protecting mice and monkeys against parasite
multiplication and disease [7]. The full-length ectodomain recombinant PfAMA1
(amino acids 25-545) of the Plasmodium falciparum FVO strain has been
expressed under cGCP in Pichia pastoris [8]. The product was formulated with
Alhydrogel, Montanide ISA 720 and AS02A and tested for safety and
immunogenicity in humans [9]. We investigated the concentration and avidity of
antibodies induced by recombinant AMA1 in rabbits and human immunizations
studies by Surface Plasmon Resonance. We correlated these data with the
capacity of sera to functionally inhibit parasite growth in vitro.
Materials and Methods
Antigens
Six different recombinant PfAMA1 constructs were prepared as described
previously [10]. Pf3mH comprises the prosequence, subdomain I and subdomain
II, Pf8mH comprises subdomain II only, Pf9mH comprises subdomain II and
subdomain III and Pf10mH comprises subdomain III only, covering amino-acid
residues 25-442, 303-442, 303-544 and 419-544 of the FVO AMA1 antigen
respectively (Figure 1). The Pf4mH construct comprises the prosequence,
subdomain I, subdomain II and subdomain III, representing residues 25 to 544
from the Pf FVO strain as described by Kocken et al. [11]. Construct Pf11.0
comprises amino-acid residues 25-545, but lacks the additional Myc-His tag
(Figure 1). Based on rabbit immunization studies, the latter construct was used
for a human phase I trial [9].
83