An article published in the journal
Cell
earlier this year has provided new insight into the
development of an effective vaccine targeting the
malaria parasite at the critical blood stage of
infection.
Using single B-cell sorting, they isolated and
identified 17 distinct monoclonal antibodies (mAbs)
from patients that had been immunized against the
P. falciparum protein PfRH5 in a Phase Ia
clinical trial (Fig. 6, Fig. 7A). PfRH5 is a highly
conserved protein that plays an essential role for
the binding of basigin to the host erythrocyte.
Aotus monkeys that were vaccinated with
full-length PfRH5 were shown to be protected against
P. falciparum infection, and pAbs in the
serum generated by these monkeys were able to
inhibit P. falciparum growth
in vitro.
Figure 6: mAbs were isolated, sequenced,
recombinantly expressed, and characterized from
human volunteers that had been immunized with a
PfRH5-based vaccine.
Figure from Alanine et al. (2019) [article]
To assess whether these anti-PfRH5 mAbs could
prevent merozoite entry into erythrocytes, the
research group assayed for
in vitro growth inhibition activity (GIA)
against the P. falciparum 3D7 clone. They
found that the two most potent neutralizing mAbs
(nAbs), R5.016 and R5.004, yielded EC50 values
similar to the most potent known anti-merozoite mAbs
generated in mice (Fig 7B). Further investigation
involving epitope-binning and structure-based
analysis of PfRH5 led researchers to trace the
binding site of these two nAbs to the internal
disordered loop of the protein.
Figure 7: mAbs were grouped according to
epitope bin (A), and the corresponding GIA of
each antibody group was measured (B). The mAbs
with the most potent neutralizing activity were
R5.016 (red bin), and R5.004 (blue bin).
Selected panels from Figure 3, Alanine
et al. (2019) [article]
Scientists determined that a truncated PfRH5
construct, PfRH5∆NL, bore all the relevant epitopes
required to elicit these neutralizing mAbs within
humans. They confirmed this by measuring the ability
of full length PfRH5 and PfRH5∆NL to reverse the GIA
of polyclonal IgG from the sera of a vaccinated
humans (D84 IgG; Fig. 8A). In addition, by using
GenScript's custom rabbit polyclonal antibody
service, they were able to conclude that
immunization with either the full length PfRH5 or
PfRH5∆NL could elicit antibodies of comparable
potency (Fig. 8B, 8C). Subsequently, using X-ray
crystallography, they were able to conclusively
identify the epitopes for R5.016 and R5.004 binding
to PfRH5.
Figure 8: Immunization with PfRH5∆NL can elicit
relevant anti-PfRH5 nAbs. (A) Full length PfRH5
(PfRH5FL) and PfRH5∆NL reverse GIA of polyclonal
D84 IgG from seven immunized humans. (B) GIA
measurements and (C) corresponding EC50
measurements demonstrating that rabbits
immunized with either PfRH5FL or PfRH5∆NL
yielded polyclonal IgGs of comparable
potency.
Selected panels from Figure 3, Alanine
et al. (2019) [article]
The researchers then highlighted the importance of
evaluating the usefulness of using nAbs in
combination. When testing mAb pairs, they found that
in some cases, despite targeting distinct epitopes,
the mAbs would actually compete, antagonize and
ultimately diminish the overall neutralization
activity than if the mAb was used on its own.
Excitingly, they additionally identified a class of
antibodies that could potentiate the neutralizing
effect of other PfRH5-binding mAbs and other
merozoite proteins despite possessing no
neutralizing capacity itself. This novel
non-neutralizing antibody, R5.011, was discovered
slow down the invasion process of PfRH5, extending
the time window for neutralizing antibodies to reach
target epitopes on PfRH5.
By meticulously characterizing antibody activity,
and using evidence from structural biology,
researchers have identified a new, revolutionary
strategy to inform and evaluate malaria vaccine
design. Perhaps a vaccine which is able to elicit
nAbs and non-neutralizing mAbs against a blood-stage
malaria protein may be the key to specific and long
lasting immunity against the disease.
Figure 9: Structure overlay depicting
potent nAbs R5.016 (red) and R5.004
(blue), and non-neutralizing antibody
R5.011 (green) bound to PfRH5∆NL
(A).
Selected panel from Figure 6, Alanine
et al. (2019) [
article]
Figure 10: Graphical depiction
summarizing research findings in this
paper. Neutralizing antibodies (blue)
that inhibit merozoite (MZ) entry into
red blood cells (RBC), as well as a
class of non-neutralizing antibodies
(green) that potentiates nAb activity,
have been identified (top panel). Each
class of mAbs bind distinct epitopes on
PfRH5 (bottom left panel). The
non-neutralizing antibody discovered
likely operates by slowing down the
invasion of MZ into RBCs, affording more
time for nAbs to reach the target
epitope on PfRH5 (bottom right
panel).
Figure from Alanine et al. (2019)
[article]
*Figures in this section were gratefully
obtained from the Cell publication in
discussion: Alanine et. al. (2019) under the
Creative Commons Attribution 4.0 International
(CC BY 4.0).
Images represented have been extracted from
their larger, original figures and relabeled for
clarity in this post, with no other
modifications made.
Malarial vaccines have been in development for the
past 50 years with still no licensed product
confirmed to date. Today, there are quite a number
of malarial vaccine candidates in clinical
development, including the blood stage PfRH5-based
vaccine described in the study above. In addition to
RTS,S and the PfRH5-based vaccine described above,
Sanaria ® also has a vaccine developed
based on the whole P. falciparum sporozoite
that has been irradiated to render it
non-infectious. Ongoing studies in Western Kenya
show that the Sanaria® vaccine appears to
be safe, well-tolerated, and confers protection
against malaria.
