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IMMUNOLOGY AND
MEDICAL
MICROBIOLOGY
ELSEVIER
FEMS Immunology
and Medical Microbiology
15 (1996) 61-72
Review article
Vaccines against protozoa1 diseases of veterinary importance
A.W.C.A. Comelissen
a, Th.P.M. Schetters b,*
a Utrecht University, Faculty of Veterinary Medicine, Institute of Infectious Diseases and Immunology, Department of Parasitology and
Tropical Veterinary Medicine, PO Box 80.165, 3508 TD Utrecht, The Netherlands
’ Intervet International B.V., Department of Parasitology, P.O. Box 31, 5830 AA Boxmeer. The Netherlands
Received
14 March 1996; revised 24 April 1996; accepted 2 May 1996
Abstract
Protozoan parasites are important
animal and human pathogens. At present, most of these infections are controlled by
chemotherapy. In addition, vaccines are available for some of these diseases. There is, however, still an urgent need for the
development of vaccines against protozoal diseases, since the current array of available vaccines is very limited. This review
describes the different approaches that have been taken to develop such vaccines and discusses the difficulties that hampered
vaccine development. Many of the problems are related to the complex life cycle of these parasites and the virtual lack of
mass in vitro culture systems. We also give an overview of the commercial and non-commercial vaccines that do exist at
present. Finally, we describe the future directions of this interesting field. New techniques and strategies include parasite
cultivation methods and recombinant-DNA
techniques, such as vector vaccines and DNA-vaccines.
Moreover, these
approaches are complemented by the development of sophisticated adjuvants; the coupling of immunoprotective
molecules
to entities with adjuvant activity or the use of cytokines, e.g. IL-12 Through these innovations new vaccines against
protozoa1 diseases will become available in the near future.
Keywords: Anaplasma; Bab,esia; Eimeria; Leishmunia; Protozoa;
1. Introduction
The most important animal and human pathogens
of parasitic protozoa belong to the orders Eucoccidia
Babesia,
Theileria, Plasmodium,
Cryp(genera:
tosporidium, Eimeria and Toxoplasma) and Kinetoplastida (genera: Trypanosoma and Leishmania). At
present, most of the infections
and/or
diseases
caused by these protozoans are controlled by chemotherapy. These drugs are very active, but may lose
* Corresponding
author.
0928.8244/96/$32.00
Copyright
PIZ SO928-8244(96)00040-5
0 1996 Federation
of European
Toxoplasma; Trypanosoma; Vaccine
their effectiveness due to the rapid development and
spread of resistance.
Although
the question
of
whether drugs or vaccines are likely to be the most
suitable control method is still subject to debate, it is
generally accepted that both approaches will be complementary in the prevention and treatment of protozoan infections [l-3].
In this review, we shall describe some of the
principal strategies used to develop vaccines against
protozoan parasites of veterinary importance (Section 2) and discuss some of the difficulties that have
been encountered (Section 3). We will illustrate some
of the underlying principles in the case of malaria
Microbiological
Societies. Published
by Elsevier Science B.V.
62
A. W.C.A. Comelissen,
Th.P.M. Schetters/ FEMS Immunology
parasites, since this will allow us to stress some
features common to all parasitic diseases. Finally we
will present an overview of presently available vaccines (Section 4).
2. Vaccine development
Much of the research to develop protozoan vaccines has followed approaches taken for viral- and
bacterial diseases. The classical strategy, where the
whole organism is used for developing a vaccine, has
also been applied for protozoan parasites. Different
methods have been used, which are based on the
biology of the parasite and the pathology that it
induces. Low dose infection(s) have been used for
vaccination purposes [4]. This is especially true for
parasites that only temporarily parasitise their host
and where the pathology is directly related to the
number of infective organisms. Repeated, so-called
trickle infections of short duration lead in many
cases to immunity against the parasite [5]. In particular cases a single infection can induce immunity. A
major draw-back of this approach is that it requires
intensive surveillance.
Furthermore,
standardisation
of these procedures is difficult, as the parasitic load,
which is a balance between the time period a parasite
proliferates
and the timing and dosage of the
chemotherapeutic
agent, can only be estimated [61.
Leishmanisation
in man with Leishmania major is
one example of this technique [7].
Selection or induction of parasite lines with reduced virulence (attenuation)
is one of the most
successful strategies for the development of conventional vaccines. Attenuation is defined as a procedure
for reproducibly obtaining a parasite line of reduced
virulence. The biology behind attenuation
is not
clearly understood. Attenuation can be achieved by
very different approaches including irradiation, treatment with chemicals, passage through intermediate
hosts of the same or of different species, passage
through splenectomised
intermediate hosts, and in
vitro cultivation [S]. Sometimes combinations of these
techniques have been used [9]. In general, attenuation appears to be more a matter of chance than the
result of a goal-directed approach. A special form of
attenuation has been the selection for Eimeria lines
with reduced life cycles (precociousness;
Section 3).
and Medical Microbiologv
IS
f 1996)
61-72
Major drawbacks of live vaccines reside in the
fact that they are living organisms with the potential
to adapt to new situations. Reversal to virulence is
frequently found in attenuated parasite lines, especially after passage through a vector. In addition, live
vaccines might spread in the environment
which in
certain situations is an unwanted side-effect. These
problems are overcome when killed organisms are
used in vaccines. Parasite lines used in such vaccines
can be virulent or attenuated, both of which could
also be genetically modified. These so-called killed
vaccines, however, are usually poorly immunogenic,
unless a proper adjuvant is added. Adjuvants have to
be carefully selected not to induce adverse reactions.
For that reason Freund’s adjuvants are not used in
product formulations.
In some cases effective immune responses can be induced when using liposomes or immunostimulating
complexes (ISCOMS)
to incorporate the relevant antigen(s) [lo- 121.
Conventional
subunit vaccines contain (partially)
purified fractions of parasite material. Importantly,
not only somatic parasite antigens but also molecules
excreted from or secreted by the parasite are used in
subunit vaccine preparations
[13]. As with killed
vaccines, an adjuvant is required to stimulate an
immune response in the recipient.
Vector vaccines contain genetically modified microorganisms
that produce the relevant parasite
molecule (or critical part thereof). An important
aspect of these vaccines is that one can target the
antigen delivery making use of the biology of the
vector itself. Vectors employed for vaccines against
gut-dwelling parasites usually are intestinal microorganisms like Salmoneh
species. Since vector vaccines are live vaccines, no adjuvant is required.
During the last decade there has been wide-spread
use of recombinant-DNA
technology for producing
vaccines. Parasite molecules to be used in a vaccine
can be produced by genetically modified microorganisms, usually bacteria. For this type of vaccine an
adjuvant is usually required. In protozoology no such
vaccines have yet been produced with satisfying
efficacy. Using molecular biological techniques it is
possible to modify the genetic material of protozoa.
Recently, it has been shown that it is possible to
replace a whole gene in organisms such as Leishmania, Tqpanosoma,
Toxoplasma and Plasmodium
[ 14-171. Gene replacement may lead to organisms of
A. W.C.A. Comelissen, Th.P.M. Schetters/ FEMS Immunology and Medical Microbiology I5 (1996) 61-72
low virulence which can subsequently
be used as
vaccine strains. The so-called ‘knock-out’ parasite
lines of L. major appear to fulfil these requirements
b31.
3. The life cycle and antigen complexity
As discussed above., vaccine development for protozoan parasites does not basically differ from strategies used for the development of viral- and bacterial
vaccines. However, the conventional
strategies of
attenuation, inactivation or development of subunit
vaccines cannot readily be applied due to the fact
that most protozoan parasites cannot be mass-cultured in vitro.
A second difficulty can be found in the protozoan
life cycle. The life cycle of representatives
of the
Eucoccidia and Kinetoplastida are complex and comprise different developmental
stages, each with its
own set of specific antigens. Many molecular biological studies have shown that genes encoding such
antigens differ, for instance in the number of variable
repetitive regions, thereby generating antigenic diversity. The host is thus faced with an extensive
number of proteins. This implies that criteria for
selecting specific prote:ins to be included in vaccines
have to be set. These criteria should relate to the
biology of the parasite, the immunological
interaction between parasite and host, and vaccination studies.
3.1. Sexual reproduction
and genotypic
variation
A third important factor in vaccine development
is the absence or presence of a sexual process.
Eucoccidians have a life-cycle in which asexual multiplication by post-meiotic sporogony and/or different types of merogony alternate with sexual reproduction [ 191. DNA measurements by direct fluorometry of individual Feulgen-stained
stages of, for instance, T. gondii [20] and P. falciparum [21] have
shown that meiosis occurs within the zygote. In
Eucoccidians,
this process is best studied in Plasmodium. Here it has been demonstrated that meisosis
follows the normal eukaryotic pattern and can lead to
three forms of genetic recombination:
(i) independent assortment of genes on different chromosomes,
63
(ii) crossing-over events between genes on the same
chromosome, and (iii) intragenic recombination,
in
which novel alleles of a gene are generated (reviewed by Walliker [22]).
These processes occur frequently under natural
condition and can generate novel genotypes, and thus
heterogenous populations [22]. This has been demonstrated for example for several antigens that are
potential human vaccine candidates. The two merozoite surface proteins (MSPl and MSP2) contain
segments of tandemly repeated sequences, which
vary both in length and sequence in different alleles.
These alleles can readily be identified by the fragment size in a PCR protocol with MSPl- or MSP2specific primers. Babiker et al. [23] demonstrated the
presence of 17 different alleles of h4SPl and 23
alleles of MSP2 in 53 isolates from malaria patients
taken on a single occasion. This confirmed a previous study of Conway and McBride [24], who showed
that variant epitope-specific monoclonals recognized
36 and 8 alleles respectively of 344 culture isolates
from a hyperendemic area in the Gambia.
This situation might be different in Kinetoplastida. In this order sexual reproduction, meiosis, and
genetic recombination
has only been well documented in African trypanosomes [25,26]. This process is apparently absent in T. cruzi, the causative
agent of Chagas’ disease. Here, a sexual process and
crossings between gametes do not exist, or do not
result in reassortment of genes on different chromosomes, crossing-over
events between genes, or the
generation of novel alleles by intragenic recombination. Hence, these parasite populations have a clonal
structure: the progeny is generated by mitotic divisions and are therefore identical to their .parents
[27,28]. Diversification
of clonal populations
is
achieved by random mutations. Individual isolates of
different geographical regions can, therefore, differ
dramatically in their pathology and biological and
biochemical characteristics.
3.2. Antigenic
variation
Protozoan parasites cannot only generate diversity
by genotypic variation. These parasites can also augment variability
by replacing antigens which are
exposed to the host immune system. This process is
known as antigenic variation. It is well understood at
64
A. W.C.A. Cornelissen, Th.P.M. Schetters/ FEMS Immunology and Medical Microbiology 15 (1996) 61-72
the molecular level in the African trypanosome T.
brucei [29,30], and was recently described for the
human malaria parasite P. falciparum [31-331 and
B. bovis [34]. A detailed description of this process
falls outside the scope of this review. However, one
has to keep in mind that this process adds to the
repertoire of strategy that parasites use in order to
escape from the immune response of their host.
4. General aspects of vaccines
In the natural situation the host is often able to
mount an effective immune response against protozoan parasites. However, a long-lasting effective immunity can only be maintained when the host remains a carrier. This is termed premunition [35]. It is
either acquired as the result of a persistent latent
infection or by frequent reinfections of the host. The
successful generation of protective protozoan vaccines is based on this principle. Vaccines either limit
the proliferation of the parasite and/or protect the
host against the pathology of the infections. Both
result, however, in the prevention of clinical disease.
Extensively research efforts have led to the development of various protozoan vaccines at the experimental level and some of them have shown an
acceptable level of efficacy [36]. Whether or not the
estimated efficacy is enough to warrant commercialisation depends on the specific situation [37]. In some
cases vaccines are being produced by governmental
institutions. This is especially the case in markets
that are not easily accessible for international companies. Australia and New Zealand have very strict
regulations on the importation of extraneous organisms. Consequently,
a live vaccine has to be produced within these countries. The costs involved in
setting up production facilities in each of these countries has prevented international
companies from
manufacturing, for instance, a bovine babesiosis vaccine. This vaccine is therefore only produced at the
Tick Fever Research Centre of the Queensland Department of Primary Industries. An additional reason
for private companies not to manufacture a promising vaccine is the insufficient
size of a particular
market. Companies will lose their interest when the
development and registration costs are too high in
relation with the anticipated market share [37].
4.1. Commercial
vaccines
4. I, 1. Coccidiosis (Eimeria species)
Although coccidiosis affects many other species
than the chicken, commercialisation
of a vaccine
against coccidiosis was restricted for use in the fowl
industry (chicken, turkey). The reason for this is that
economic losses due to coccidiosis in these branches
of the industry render the use of a vaccine worthwhile, mainly because of the large number of animals involved. The currently available vaccines all
contain live oocysts of one or more of the Eimeria
species affecting fowl (Table 1).
To restrict pathology due to vaccination different
strategies are used: administration
of a low number
of virulent live oocysts; use of oocysts derived from
Eimeria strains selected for reduced virulence; use of
chemotherapeutic
agents after administration of virulent live oocysts. As indicated in Section 2 (see
above), reduced virulence is sometimes achieved
after culture of the parasite in chicken embryo’s [38],
or by selecting for parasite strains with short life
cycles (development
to oocysts), which is called
precociousness [39]. The vaccines that are marketed
at present, serve the broiler-breeder
market and the
egg-laying
market. Although some manufacturers
present their vaccine for use on the broiler market, it
is doubtful if the number of oocysts required annually could be readily produced. This is especially the
case for vaccines based on strains with reduced
virulence which show reduced oocyst shedding.
4.1.2. Toxoplasmosis (Toxoplasma gondii)
Toxoplasmosis has gained renewed interest mainly
due to the fact that this infection is a common cause
of death in AIDS patients: up to 50% of these
patients die due to toxoplasmosis encephalitis [40].
For veterinary use a vaccine against toxoplasmosis
for use in sheep, pigs and cats is desirable. Interestingly, in each of these animals the purpose is different: for sheep a vaccine ought to reduce Toxoplasma-induced
abortion, for pigs reduction of tissue
cysts is aimed at, and for use in cats the vaccine aims
at reducing the oocyst output, which eventually would
reduce zoonotic infection (Table 3). At present a
commercial
vaccine
against
Toxoplasma-induced
abortion in ovines is available [41] (Table 3). The
vaccine is based on a live attenuated strain of T.
X b
live attenuated
live attenuated
live attenuated
live virulent
Biopharm, Czech Republic
Biopharm, Czech Republic
Elanco Products Co.
a Virulent parasites (low dose).
b Precocious strains.
’ Parasites attenuated by passage through chicken embryos.
d Virulent parasites controlled by ionophores.
X ’
X ’
X a
live virulent
Livacox D
Livacox T
Vat-M
X a
live virulent
E. tenella
against coccidiosis
X a
Type
vaccines
live virulent
available
Sterwin Laboratories,
Millsboro, USA
Immucox c
Vetech Laboratories Inc.,
Rockwood, Canada
Immucox c2
Vetech Laboratories Inc.,
Rockwood, Canada
Paracox (TM) Pitmar-Moore Europe Ltd.
Manufacturer
Trademark
Coccivac D
of commercially
Table 1
Composition
E. maxim
Xa
Xa
Xa
x b (two
strains)
E. aceruulina
Xa
Xa
Xa
Xb
in fowl
x b
Xa
Xa
X=
E. necatrix
Xb
Xa
Xa
E. brunetti
Xb
X”
E. mitis
Xb
XB
XB
E. praecox
[731
Xa
Xa
[801
178,791
[78,791
[75-771
[741
[741
Reference
E. miuati
E. hagani
66
A. W.C.A. Comelissen, Th.P.M. Schetters/ FEMS Immunology and Medical Microbiology 15 (1996) 61-72
go&ii designated S48. This parasite strain has one
additional advantage: it cannot form tissue cysts [42].
Toxoplasma tissue cysts are resting stages of the
parasite that are found in muscle i.e. meat. This stage
is infective for man. The vaccine strain cannot form
these cysts and is thus safe for use in animals used
for human consumption. For use in cats another T.
go&ii strain might become available in the near
future (Table 2). This vaccine can induce immunity
in cats. However, the strain used can not form the
male gametes. It can thus not complete the phase of
sexual reproduction in cats and as a result cats do not
excrete oocysts with their faeces. Sporulated oocysts
are also infective stages, and are the second source
for acquiring a Toxoplasma infection.
against anaplasmosis are described here because the
disease is transmitted by the same vector as the
Babesiu parasites. Hence, a realistic vaccine against
tropical bovine babesiosis
should protect against
anaplasmosis as well [37]. Two commercially available products have been developed. One is based on
live attenuated forms of A. marginale (Anaviv; Table
3). Attenuation was achieved by serial passages in
splenectomised animals and irradiation 1471. Importantly, the shelf life of this product could be considerably prolonged by lyophilisation.
Efficacy was
demonstrated in a vaccination-challenge
experiment
using the homologous (Florida) strain for challenge
[47]. No data are available concerning heterologous
(field) challenges.
The second product (Anaplaz;
Table 3) is based on killed organisms of a virulent
Anuplusma strain. This vaccine induces partial protection against heterologous challenge infection [48].
4.1.3. Babesiosis (Babesia canis)
Babesiosis in dogs is prevalent in mediterranean
areas. In certain districts of France the prevalence
within the dog population is approximately 15% [43].
A vaccine was developed for use in European canine
babesiosis (Pirodog), which is based on the same
principle as for tropical babesiosis in bovines [44].
The vaccine consists of concentrated supematants of
B. canis cultures, adjuvanted with saponin. Hence it
is a non-live subunit vaccine (Table 3). Under certain
circumstances
the vaccine is effective up to 88%
[45]. However, other data indicated that vaccine
efficacy was about 25% [43]. In the latter study
concurrent Babesia infections occurred in vaccinated
animals, indicating that antigenic diversity of the
parasite should be considered as a cause for vaccination failures. This was corroborated by results from
laboratory challenges in which it was shown that
vaccination of dogs with culture-derived
supernatant
antigen protected animals against a homologous
challenge only [46].
4.2. Non-commercial
vaccines
4.2.1. Theileriosis (Theileria species)
For the control of the different diseases caused by
Theileria species live vaccines were developed that
afford significant protection against theileriosis in
the field situation (reviewed by Pipano [49]). Attenuation of Theileria species in cell culture has been
achieved for T. annulatu (Table 4). The vaccine
consists of schizonts grown in tissue culture, and has
been used widely and successfully [50]. Similarly, T.
hirci was adapted and used to vaccinate sheep and
goats [51]. For T. parva, the causative agent of East
Coast Fever (ECF) vaccination results were poor,
although adaptation to in vitro growth could be
obtained [6]. However, chemotherapeutically
controlled infections of T. parua are capable of protecting cattle from ECF effectively, provided there is
careful monitoring of animals during the vaccination
period. Recent developments
focus on the use of
recombinant subunit vaccines [52,53].
4.1.4. Anaplasmosis (Anaplasma marginale)
Although species of Anaplasma belong to the
Rickettsiae and thus not to the Protozoa, vaccines
Table 2
Different strains of Toxoplasma gondii, and their potential use as vaccine
Strain
Attenuation
Target animal
Inoculum
Purpose
Reference
S48
T-263
TS-4
in vivo passage
chemically induced
chemically induced
sheep/goats
cats
tachyzoite
tissue cyst
tachyzoite
reduce abortion
reduce oocyst output
reduce tissue cyst number
[411
pigs
1811
Manufacturer
Mycofarm UK Ltd.AgVax Dev. New Zealand
ProtaTek Int. Inc., St. Paul, USA
Fort Dodge Laboratories, Fort Dodge, USA
Rhone-Merieux Lyon, France
Toxovax
Anaviv
Anaplaz
Pirodog
against toxoplasmosis,
live attenuated
live attenuated
killed virulent organisms
subunit culture supematant
Type
vaccines)
Trademark
vaccines (other than coccidiosis
available
Table 3
Commercially
and babesiosis
Toxoplasma gondii
Anaplasma marginale
Anaplasma marginale
Babesia canis
Species
anaplasmosis
sheep
cattle
cattle
dogs
Target animal
effect
abortion
parasitaemia
clinical disease
clinical disease
Protective
reduces
reduces
reduces
reduces
t41,821
[g31
[84]
[45]
Reference
68
Table 4
Vaccines
A. W.C.A. Cornelissen, Th.P.M. Schetters/ FEMS Immunology and Medical Microbiology I5 (1996) 61-72
against protozoa1 diseases, distributed
Species
Type
Theileria annulata
Theileria hirci
Theileriu parva (East Coast Fever)
Babesia bouis
Babesia bigemina (strain G)
Babesia diuergens
live
live
live
live
live
live
Leishmania
killed
species
by non-commercial
organisations
cell culture attenuated
cell culture attenuated
virulent chemotherapy controlled
attenuated in vivo passage
attenuated in vivo passage
attenuated in vivo passage
4.2.2. Babesiosis (Babesia bovis, Babesia bigemina,
Babesia diverge&
As referred to above, in Australia a vaccine against
tropical babesiosis (caused by B. bouis) has been
used for decades [54]. The parasite is attenuated by
rapid passages in splenectomised calves which leads
to parasite strains of less virulence [55]. Annually
over 800000 doses are used by Australian stockowners [56]. Similar vaccine preparations of B. bouis
strains have been used in South America 1571. Unlike
in the case of B. bouis, rapid passage of B. bigemina
in splenectomised
calves did not reliably lead to
attenuated parasites of less virulence. However, a
strain designated G strain was obtained after a series
of passages through ticks, splenectomised
animals
and non-splenectomised
animals [58]. This strain is
still being used for vaccine production in Australia.
Since 1975 a live attenuated vaccine against B.
divergens has been used in Sweden. The vaccine is
produced in experimentally
infected splenectomised
calves by the National Veterinary Institute (NVI). In
one survey of 5682 calves, vaccine-related
disease
was observed in three animals, and subsequent infection observed in eight animals (S.M. Taylor, personal communication).
Currently 45000 doses are
used per year. The vaccine is also used in endemic
areas in Finland. In Austria the Federal Institute for
Control of Infectious Diseases in Animals has produced a live attenuated vaccine against B. diuergens.
The vaccine was used until 1988. Due to insufficient
acquired protection the vaccine was replaced by a
formalinised whole blood dead vaccine (S.M. Taylor, personal communication).
No data are available
as to the efficacy of the new vaccine.
Parasite stage
Target animal
Reference
schizont
schizont
sporozoite
merozoite
merozite
merozoite
cattle
sheep, goats
cattle
cattle
cattle
cattle
L49.501
[501
promastigotes
humans
t61
[561
[561
SM. Taylor,
personal communication
L601
4.2.3. Leishmaniosis (Leishmania species)
Although not distributed on a wide scale, a vaccine against dermal leishmaniosis in South America
should be mentioned. The vaccine was developed
based on the observation of Gomes that intravenous
injection of killed promastigotes
in patients with
active American dermal leishmaniasis
led to a regression of the lesions [59]. During subsequent studies a vaccine prepared from killed and sonicated
promastigotes of five Brazilian strains of Leishmania was developed, and used during an epidemic of
American dermal leishmaniosis in Brazil. The difference between the infection rate of vaccinated and
control subjects was statistically significant (1.7% in
the vaccinates and 8.9% in the controls). Side-effects
following immunization
were minimal [60]. This
killed leishmanial vaccine will be standardized as far
as possible and produced commercially
in Brazil
(Biobras-Bioquimica
do Brasil, S.A.; [61]). Although
this vaccine is being developed for use in humans it
is envisaged that a spin-off product for use in animals will become available.
5. Future directions
The use of live vaccines will be continued as
these are the most potent inducers of protective
immunity. Concurrently,
studying the life cycle of
protozoan parasites and the underlying mechanisms
of the host-parasite interaction will further improve
our understanding
at the molecular level, and open
new perspectives in the conception of alternative
approaches in vaccine development.
As there is a
A. W.C.A. Comelissen, Th.P.M. Schetters/ FEMS Immunology and Medical Microbiology 15 (1996) 61-72
renewed interest in anti-disease vaccines the identification of parasite derived molecules that trigger
pathological reactions in the host will acquire more
attention [62]. This will allow the cloning of these
genes and the subsequent development of attenuated
vaccines by site-directed genetic engineering. More
specific anti-disease
vaccines are those aimed at
inducing anti-toxin immunity. The available subunit
vaccines against babesiosis are based on parasite
exoantigens [4-4]. These vaccines do not aim to control parasite proliferation, but they diminish the development of the pathological reactions that are usually evoked in the ho:% Identification
of analogous
molecules in other protozoan parasites might open
ways for vaccine development.
Current research activities aimed at the description of defined antigens (and associated genes) as
vaccine candidates is combined with the analysis of
suitable antigen delivery systems. These will be chosen on the required site of antigen delivery, e.g.
Salmonella-based
vectors for antigen delivery at the
gut mucosa, and will include microbiological vectors
such as viruses, bacteria and even protozoa [63].
Problems associated with antigenic diversity of the
parasite are being tackled using molecular biological
techniques. Synthetic vaccines that combine helper T
cell epitopes derived from common antigens (like
BCG) and B cell epitopes known to be present on a
range of variant parasite strains are being developed
[@I.
The current array of available vaccines is clearly
limited. Optimism about the prospects for novel
vaccines appears justified in the light of the development of revolutionary
adjuvant systems. These include the inclusion
of sequences
that code for
molecules with adjuvant activity in vector vaccines
and, in the case of synthetic vaccines, the coupling
of protective epitopes to carrier molecules with adjuvant activity (e.g. coupling of antigens to E. coli
lipoprotein [65]). New adjuvant formulations that can
be combined with soluble antigen and still induce
cellular immunity in animals after immunization
are
becoming available [66,67]. More sophisticated compounds with adjuvant activity are the cytokines, interleukin 12 being the most promising. It functions in
the helper T cell pathway, and directs responses
towards Thl cell activity which is associated with
the cellular arm of immunity [68,69].
69
Additional new approaches, such as nucleic acid
vaccines, will no doubt be used to generate new
vaccines against protozoa1 diseases. Nucleic acid
vaccines can stimulate protective cytotoxic T-cell
responses as well as T helper cell- and humoral
immunity, without the intrinsic risks associated with
live vaccines [70,71]. Sedegah et al. recently showed
that mice immunized with plasmid DNA encoding
the P. yoellii circumsporozoite
protein, could be
protected against infection [72]. These results indicate that commercial DNA vaccines against protozoan parasites may become available in the near
future.
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Cerami, A. and Warren, KS. (1994) Drugs. Parasitol. Today
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