The Last word in Malaria Prevention - a Myth or a Reality?
The Big Question: When a natural malaria infection does not confer
lasting immunity,
will a successful vaccine ever be available? |
(With Dr.
P.P. Venugopalan, Former Professor and Head,
Dept. of Community Medicine, Kasturba Medical College, Mangalore)
Malaria is one of the
commonest infectious diseases in the tropics, affecting more than 200 million people and
killing more than 3 million every year. A safe and effective vaccine would have been the
easiest way to control this disease, but even after decades of search, that vaccine is
still elusive. The complex life cycle of the parasite involving human and vector
mosquitoes as well as its allelic diversity and antigenic variations makes the development
and implementation of effective malaria control intervention problematic. It is now
becoming evident that multi-intervention approach may be the most appropriate way of
combating malaria in view of the increasing resistance of the parasite to antimalarial
drugs as well as vector mosquitoes to insecticides. Malaria vaccines will therefore play a
major role in future malaria interventions. New malaria vaccine candidates will require
testing in malaria endemic countries. Sufficient sites for testing potential malaria
vaccines must be prepared.
Significant progress has
been made in the development of the malaria vaccine during the last 20 years. An ideal
malaria vaccine is one that would prevent the infection at the first instance and if this
is not possible, should decrease the intensity of infection and should be successful in
preventing malaria transmission. As there is no single antigen that is able to provide
cent percent protection, answer lies in a mixture of antigens synthetically put together.
Safety, efficacy, storage, adjuvant, frequency of administration, dose and mode of
administrations etc. are other areas of concern. Ninety percent of the 300-500 million
clinical cases of malaria per year worldwide occur in Africa. Thus, research must be
directed toward the 1 million African children under 5 years of age who die every year of
malaria. Given the characteristics of the normal immune response to malaria
(relatively short-lived and not completely effective), it is understandable that the main
goal is to try to increase the host's natural immunity. The best candidates for designing
a malaria vaccine are the proteins required for parasite survival, those with low mutation
rates and conserved epitopes. Because these proteins play an important role in multiple or
alternative steps during the invasion process, they should be the targets against which a
protective immune response should be elicited. The interaction between the malaria
parasite and its host is complex. It is therefore crucial to define new ways of improving
the immune response - such as directly modifying the chemical structure of epitopes or
using new adjuvants or DNA immunization techniques - to produce novel vaccines against
this disease.
The most well identified difficulty in
vaccine research is the rapid alteration in its antigenic determinants. Another area
requiring attention is the human genetic polymorphism which is associated with resistance
to malaria as in the cases of a and b thalassemia, haemoglobin C, G6PD deficiency,
haemoglobin S and haemoglobin E, ABO blood groups (Fya and Fyb) etc.
Vaccines based on a single antigen have a
limited role to play in malaria because not all people respond to the same antigen.
Further plasmodia can create antigen variants to elude immune system surveillance. So
efforts have been directed to develop multi-stage, multi-component vaccine, incorporating
multi-antigenic sequences from different asexual and sexual stages of plasmodia. About
nine different malaria antigens have been identified, which may not be the end of the
road. The Indo-US researchers have combined the coding sequences for these key portions,
called epitopes into one synthetic gene as the basis for the new vaccine named CDC\NII
Malvac-1. The monkey trials will be followed by human trials.
Vaccines have been found
successful in simian malaria caused by isolated merozoites of P. knowlesi.
The following can be
listed as reasons for incomplete protection against malaria:
- Polymorphism and clonal variation in
antigens of plasmodium
- Parasite induced immunosuppression
- Intracellular parasites
- Lack of MS proteins on infected RBCs.
The following are the
difficulties in vaccine research:
- Problems in vaccine production including
not being able to grow the parasite in large quantities
- Difficulty of evaluation
- Parasites’ ingenious ways of avoiding
hosts’ immune response
- Complexity of conducting clinical and
field trials
- Mutation of the parasites
- Antigenic variations e.g. MSA-I has 8
variants, MSA-2 has 10 and CSP has 6 variants (Orissa)
- Multiple antigens, specific to species and
stage
The tissue stage or the exoerythrocytic
(EE) stage of the malaria parasite, for many years remained the most neglected form mainly
because of its inaccessibility being located in the liver. The advent of in vitro
techniques resulting in the successful cultivation of these forms in primary hepatocyte
cultures and a variety of cell lines has greatly augmented research on these stages and
have provided unique in vitro systems which can be used as primary screens for candidate
chemotherapeutic and immunoprophylactic agents and have facilitated better understanding
of the sporozoite-hepatocyte interactions. Sensitive and specific nucleic acid probes (DNA
and ribosomal RNA) have been developed to quantify EE stages in infected livers. Efforts
to establish SCID mouse as a model for cultivation of EE stages of human malaria parasites
have been encouraging. The earlier assumptions that these tissue stages are free from
immune attack have been proven wrong and the hepatic phase itself now appears to be
essential for the induction of protection against the pre-erythrocytic stages. Liver stage
specific antigens have been identified in recent years. Despite its intracellular
position, this 'hidden' form has been found to constitute a target for antibodies,
cytokines, and cytotoxic T cells.
Classification of
Malaria Vaccines
Stage of Plasmodium |
Antigens |
Salient features |
| Pre-erythnocytic |
Irradiated sporozoites, Circum Sporozoite Protein (CSP) or
peptides, Liver stage Antigens -1 (LSA-1) |
Stage/species specific; antibody blocks infection of liver; large
immunising dose required; can abort an infection |
| Merozoite and Erythrocytes |
Erythrocyte Binding Antigen (EBA-175), Merozoite Surface Antigen
1&2 (MSA-1&2); Ring Infected Erythrocyte Surface Antigen (RESA); Serine Repeat
Antigen (SERA); Rhoptry Associated Protein (RAP); Histidine Rich Protein (HRP); Apical
Membrane Antigen-1 (APM-1) |
Specific for species and stage; Cannot abort an
infection; Prevents invasion of erythrocytes, thus reducing severity of infection |
| Gametocytes & gametes |
Pfs 25, 48/45k, Pfs 230 |
Prevents infection of mosquitoes; antibody to this antigen
prevents either fertilization or maturation of gametocytes, zygotes or ookinetes; is of
use in endemic areas but not suited for travelers; antibody blocks transmission cycle |
| Combined vaccine (cocktail) |
SPf 66 (based on pre-erythrocytic and asexual blood stage proteins
of Pf) |
Based on incorporation of antigens from different stages into one
vaccine to produce an immune response, blocking all stages of the parasite development |
Examples of malarial vaccines:
C.S.P. Vaccine: Kenyan study
concluded that CSP vaccine induced antisporozoite antibody is not protective.
Encouraging results have been reported
with a CSP-HBs Ag Hybrid Vaccine (U.S. Army and SKB).
NYVAC - Pf. 7: This vaccine blocks
transmission of the parasite from vertebrate host to mosquitoes. The highly attenuated
NYVAC vaccinia virus strain has been utilized to develop a multiantigen, multistage
vaccine candidate for malaria. Genes encoding seven Plasmodium falciparum
antigens derived from the sporozoite (circumsporozoite protein and sporozoite surface
protein 2), liver (liver stage antigen 1), blood (merozoite surface protein 1, serine
repeat antigen, and apical membrane antigen 1), and sexual (25-kDa sexual-stage antigen)
stages of the parasite life cycle were inserted into a single NYVAC genome to generate
NYVAC-Pf7. Each of the seven antigens was expressed in NYVAC-Pf7-infected culture cells,
and the genotypic and phenotypic stability of the recombinant virus was demonstrated. When
inoculated into rhesus monkeys, NYVAC-Pf7 was safe and well tolerated. Antibodies that
recognize sporozoites, liver, blood, and sexual stages of P. falciparum were
elicited. Specific antibody responses against four of the P. falciparum antigens
(circumsporozoite protein, sporozoite surface protein 2, merozoite surface protein 1, and
25-kDa sexual-stage antigen) were characterized. The results demonstrate that NYVAC-Pf7 is
an appropriate candidate vaccine for further evaluation in human clinical trials.
Recombinant Vaccine: Against
P. vivax blood stage infection, a recombinant C- terminal fragment of MSP-1 in
block co-polymer adjuvant with T- helper epitopes, the yeast expressed P2 P30 PV20019
recombinant vaccine offers partial protection in Saimiri monkeys.
Combination of malarial
antigens with immune boosting adjuvants and hepatitis B surface antigens have been
reported. Liver stage vaccine may hold the key to reduce relapse/ re-infections in malaria
prone individuals.
Gamete Vaccine:
When the antibodies are taken up by the mosquitoes, gametes escaping the RBCs will be
neutralised, thus preventing fertilisation and reducing transmission.
DNA Vaccine: Based
on a synthetic gene, made by adding 21 epitopes of 9 different antigens present in P.
faciparum. Epitopes are small regions in proteins, which are recognised by immune cells.
This has been developed by CDC with National Institute of Immunology. Example - Pf
155/RESA (Ring Infected Erythrocyte Surface Antigen). It is based on the priniciple as
elucidated in the figure below:
| Gene
from an Immunogen |
|
Instead
of administering a protein, genetic information for the protein is injected. The host is
thus capable to produce introduced gene products in sufficient quantities, as both humoral
and cellular responses are induced. Easier and cheaper to produce and do not need
refrigeration and can be genetically manipulated. |
| Insert
the gene into the Expression Plasmid |
|
| Transform
Bacterial cells, grow bacteria, purify plasmid DNA |
|
| Immunize
with Immunogen-expressing plasmid |
|
|
|
Naked DNA vaccine is
capable of developing 'killer' cytotoxic T Lymphocyte (CTL) responses which are dose
related. Stimulation of an immune response through the introduction of foreign genes
resulting in the production of foreign genes and foreign protein is the basis of DNA
vaccine.
CDC/NII MLVAC-1: It
is a candidate vaccine that codes for nine different antigens that the plasmodium
expresses during its development is liver, blood and circulation in the hosts. The vaccine
stood the challenge on rabbit trial.
Patorraya Vaccine
(Cocktail vaccine): Although knowledge of the parasite's biology is incomplete, research
has allowed insight into some of the mechanisms that the parasite uses to evade host
immunity. This is the basis for adopting an ''antigenic cocktail'' approach toward
obtaining a synthetic or recombinant subunit vaccine such as the synthetic Colombian
Malaria vaccine SPf 66. SPf 66 consists of 3 peptide epitopes from 3 blood stage proteins
(35 KD, 55KD,83 KD) intercalated with NANP sequence. This vaccine is designed to block the
parasite at its later merozoite form, when it emerges from initial incubation in liver.
The vaccine stimulates production of antibodies, which would prevent the parasite from
infecting RBCs. During the development of Spf 66, field trials under both low and high
malaria endemicity areas in Latin America and Africa have been carried out, at a dosage of
1 mg for children < 5 years and 2 mg for adults over deltoid on days zero, 30 and 180
days. The results from these studies showed a protective efficacy ranging between 38.8 and
60.2% against Plasmodium falciparum malaria. In Tanzania, the efficacy has been 31% in
children (1-5 yrs old), while protective efficacy in Gambia was 8% (in infants 6-11months
old).
SPf 66 with QS - 21
adjuvant is also undergoing trials.
Conclusion: Sporozoite vaccines
containing CSP, generated by recombinant DNA technology, combined with potent T-cell
epitopes for higher immunogenecity seem to inhibit early liver-stages of the parasite and
may be compared to causal prophylaxis. Blood stage antigens combined with Freund's
adjuvant or other immunoboosters may generate effects resembling chloroquine
chemoprophylaxis. Merozoite vaccines get longer time to interact with their target than
the transiently appearing sporozoites in case of sporozoite vaccines. Gamete vaccines hold
promise for future where in antibodies when taken up by the mosquitoes will neutralise the
gametes escaping RBCs, thus preventing fertilization. This will reduce transmission and a
reduction in nosocomial infection. The cocktail vaccine is the most promising one.
|
