Identification of a conserved region of Plasmodium falciparum MSP3 targeted by biologically active antibodies to improve vaccine design

Merozoite surface protein 3 (MSP3) is a target of antibody-dependent cellular inhibition (ADCI), a protective mechanism against Plasmodium falciparum malaria. From the C-terminal half of the molecule, 6 overlapping peptides were chosen to characterize human immune responses. Each peptide defined at least 1 non-cross-reactive B cell epitope. Distinct patterns of antibody responses, by level and IgG subclass distribution, were observed in inhabitants of a malaria-endemic area. Antibodies affinity purified toward each peptide differed in their functional capacity to mediate parasite killing in ADCI assays: 3 of 6 overlapping peptides had a major inhibitory effect on parasite growth. This result was confirmed by the passive transfer of anti-MSP3 antibodies in vivo in a P. falciparum mouse model. T helper cell epitopes were identified in each peptide. Antigenicity and functional assays identified a 70-amino acid conserved domain of MSP3 as a target of biologically active antibodies to be included in future vaccine constructs based on MSP3.

Schistosomiasis vaccines

While there are many challenges in vaccine development, none is greater than that of developing vaccines against large metazoan parasites such as schistosomes, the parasitic worms that are responsible for schistosomiasis. Initial optimism stemming from the identification of the first vaccine candidate antigens that gave protection in animals has been dashed by the failure, as yet, of any of the vaccine candidate antigens to enter Phase III clinical trials. Now, despite an improved understanding of the biology of the parasites and of the immune responses they stimulate in naturally exposed populations, the vaccine effort is stalled. The control effort has switched heavily in favour of the wider use of conventional chemotherapy with praziquantel, which is now affordable by all but the poorest countries. Disagreements among researchers in the schistosome field as to whether or not a vaccine is needed have not helped convince funding agencies that schistosomiasis vaccines, rather than drugs, should be a priority. With the schistosome genome projects at an advanced stage plus the power of the proteomics, perhaps it is still too early to call time on schistosome vaccine development.

Identification of a compensatory mutant (lpg2-REV) of Leishmania major able to survive as amastigotes within macrophages without LPG2-dependent glycoconjugates and its significance to virulence and immunization strategies

Different Leishmania species rely to different extents on abundant glycoconjugates, such as lipophosphoglycan (LPG) and related molecules, in mammalian infections. Previously, we showed that Leishmania major deletion mutants lacking the Golgi GDP-mannose transporter LPG2, which is required for assembly of the dominant phosphoglycan (PG) repeats of LPG, were unable to survive in macrophages. These lpg2- mutants, however, retained the ability to generate asymptomatic, persistent infections in mice. In contrast, Ilg and colleagues showed that Leishmania mexicana LPG2 mutants retained virulence for mice. Here we identified a partial revertant population of the L. major lpg2- mutants (designated lpg2(-)REV) that had regained the ability to replicate in macrophages and induce disease pathology through a compensatory change. Like the lpg2 parent, the lpg2(-)REV revertant was unable to synthesize LPG2-dependent PGs in the promastigote stage and thus remained highly attenuated in the ability to induce infection. However, after considerable delay lpg2(-)REV revertant-infected mice exhibited lesions, and amastigotes isolated from these lesions were able to replicate within macrophages despite the fact that they were unable to synthesize PGs. Thus, in some respects, the lpg2(-)REV amastigotes resemble L. mexicana amastigotes. Future studies of the gene(s) responsible may shed light on the mechanisms employed by L. major to survive in the absence of LPG2-dependent glycoconjugates and may also improve the potential of the lpg2- L. major line to serve as a live parasite vaccine by overcoming its tendency to revert toward virulence.

The EhADH112 recombinant polypeptide inhibits cell destruction and liver abscess formation by Entamoeba histolytica trophozoites

The Entamoeba histolytica EhCPADH complex, formed by a cysteine proteinase (EhCP112) and an adhesin (EhADH112), is involved in adherence, phagocytosis and cytolysis. This makes this complex an attractive candidate as a vaccine against amoebiasis. Here, we produced the recombinant polypeptide EhADH243, which includes the adherence epitope detected by a monoclonal antibody against the EhCPADH complex. EhADH243 was purified, and the effect of the polypeptide on in vitro and in vivo virulence was studied. Antibodies against EhADH243 reacted with the EhCPADH complex and with the recombinant polypeptide. EhADH243 and antibodies against this polypeptide inhibited adherence, phagocytosis and destruction of cell monolayers by live trophozoites, but had little effect on cell monolayer destruction by trophozoite extracts. EhADH243 recognized a 97 kDa protein in the MDCK membrane fraction that could be a putative receptor for E. histolytica trophozoites. Hamsters immunized with EhADH243 developed humoral response against EhCPADH, and animals were partially protected from amoebic liver abscess.

Vaccines and immunotherapies for the prevention of infectious diseases having cutaneous manifestations

Although the development of antimicrobial drugs has advanced rapidly in the past several years, such agents act against only certain groups of microbes and are associated with increasing rates of resistance. These limitations of treatment force physicians to continue to rely on prevention, which is more effective and cost-effective than therapy. From the use of the smallpox vaccine by Jenner in the 1700s to the current concerns about biologic warfare, the technology for vaccine development has seen numerous advances. The currently available vaccines for viral illnesses include Dryvax for smallpox; the combination measles, mumps, and rubella vaccine; inactivated vaccine for hepatitis A; plasma-derived vaccine for hepatitis B; and the live attenuated Oka strain vaccine for varicella zoster. Vaccines available against bacterial illnesses include those for anthrax, Haemophilus influenzae, and Neisseria meningitidis. Currently in development for both prophylactic and therapeutic purposes are vaccines for HIV, herpes simplex virus, and human papillomavirus. Other vaccines being investigated for prevention are those for cytomegalovirus, respiratory syncytial virus, parainfluenza virus, hepatitis C, and dengue fever, among many others. Fungal and protozoan diseases are also subjects of vaccine research. Among immunoglobulins approved for prophylactic and therapeutic use are those against cytomegalovirus, hepatitis A and B, measles, rabies, and tetanus. With this progress, it is hoped that effective vaccines soon will be developed for many more infectious diseases with cutaneous manifestations.

Differential diagnosis of five avian Eimeria species by polymerase chain reaction using primers derived from the internal transcribed spacer 1 (ITS-1) sequence

Chicken coccidia are protozoan parasites of the genus Eimeria. They cause economical losses in the poultry industry globally. The various species can be distinguished on the basis of the morphology of the oocysts and parasitic site in intestine, but these criteria sometimes are unreliable. Therefore, a species-specific polymerase chain reaction (PCR) was developed. Based on variable sequence regions, specific primers were constructed for the differentiation of five Eimeria species (Eimeria acervulina, E. brunette, E. maxima, E. necatrix, and E. tenella). PCR products were amplified from coccidian vaccine (coccivac-D and coccivac-B) and E. tenella and were subsequently sequenced. Similarities of the five species sequences between the vaccines and Genbank were 94-100%. Analysis of the E. tenella internal transcribed spacer 1 (ITS-1) partial sequence from Taiwan and from Genbank indicated that the similarity was 99.6%. The PCR sensitivity test of E. tenella in Taiwan is 50 oocysts. The five sets of primers will not amplify any non-specific bands of the chicken genome or its intestinal contents. Therefore, the five sets of specifically designed primers are guaranteed to be useful for differential diagnosis of avian coccidiosis caused by Eimeria spp.

Evaluation of immune responses in horses immunized using a killed Sarcocystis neurona vaccine

Clinically normal horses developed cellular immunity to Sarcocystis neurona following IM vaccination with a commercial killed S. neurona vaccine, as indicated by the development of measurable anti-S. neurona IgG antibodies and additional intradermal skin testing. Large-scale independent assessments of the vaccine's performance and safety are in progress under field conditions. The next step in the evaluation of this vaccine would be to attempt experimental challenge after a reproducible reliable equine model of S. neurona encephalitis has been established that allows for reisolation of the pathogen after challenge.

Vaccination against congenital toxoplasmosis

Different types of Toxoplasma gondii vaccines were evaluated using parasitological and histopathological means to induce immunity in Swiss pregnant mice and their pups against the challenge with virulent RH strain. Immunization was performed before mating by using live cyst vaccines (LCV), LCV-IL-2 combination, irradiated cyst vaccine (ICV) and ICV-IL-2 combination. It was demonstrated that pre-immunization with the current vaccines offered significant protection of both dams and pups. The highest level of protection was noticed in mice which received LCV-IL-2, followed by ICV-IL-2, then LCV and the least protection was elicited in dams immunized with ICV alone. The results threw light on the possibility of applying such vaccines not only in mice but also in other mammalian hosts including human.

Induction of crossreactive antibodies against the Plasmodium falciparum variant protein

The variant antigen Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), present on the surface of P. falciparum-parasitized erythrocytes (PE), plays a central role in naturally acquired immunity, although antibodies to PfEMP1 are predominantly variant specific. To overcome this major limitation for vaccine development, we immunized mice with three cysteine-rich interdomain 1 (CIDR1) domains of PfEMP1 that have the critical function of binding the PE to CD36 on endothelium and thus preventing spleen-dependent killing of the parasite. The immunizations consisted of different combinations of three CIDR1 encoded by DNA followed by recombinant protein boost. Immunizations with a single variant in a prime-boost regimen induced no or low cross-reactivity toward heterologous CIDR1; however, a broad range of crossreactivity was detected in mice that were immunized with all three variants simultaneously. The induced crossreactivity suggests that an anti-PfEMP1 vaccine may be possible.

A new method for the experimental production of necrotic enteritis and its use for studies on the relationships between necrotic enteritis, coccidiosis and anticoccidial vaccination of chickens

A new method for the experimental production of necrotic enteritis in chickens is described. The main features are the use of a diet high in wheat and fish meal content; oral administration of a non-lethal inoculum of the coccidium Eimeria maxima followed 6 days later by the bacterium Clostridium perfringens type A per cloaca, so that the bacterial inoculum is deposited at the time and place when and where the intestinal coccidial lesions are maximal; grading of coccidial and clostridial lesions in individual birds sampled during the 14 days following the coccidial infection. The new method was used to examine the relationship between clostridial and coccidial infections. Frank coccidiosis, caused by virulent E. maxima, exacerbated the lesions of necrotic enteritis and other clinical effects due to a subsequent challenge with virulent C. perfringens type A. Immunization with a live, pentavalent, attenuated anticoccidial vaccine (Paracox-5) protected against a severe challenge with heterologous E. maxima. Furthermore, vaccination with Paracox-5, by virtue of its protection against clinical coccidiosis due to the E. maxima challenge, indirectly protected birds against a subsequent challenge with virulent C. perfringens. The results are reconciled with previous field observations on concomitant coccidiosis and necrotic enteritis in chicken flocks.

The Houghton strain of Eimeria tenella: a review of the type strain selected for genome sequencing

A landmark decision was taken in 2002 by the Biotechnology and Biological Sciences Research Council (BBSRC) in the UK to fund the genome sequencing of an eimerian (apicomplexan) parasite from the fowl. The project is a joint collaboration between the Institute for Animal Health (IAH) at Compton, UK and the Wellcome Trust Sanger Institute, Cambridge, UK (see http://www.sanger.ac.uk/Projects/E_tenella/ and http://www.iah.bbsrc.ac.uk/eimeria/). The species chosen, Eimeria tenella, is the best known member of the genus and the Houghton (H) strain is one of the most widely studied. The H strain was isolated in 1949 in the UK. It was maintained initially at the Houghton Poultry Research Station (HPRS), Houghton, and, following closure of that laboratory in 1992, thereafter at the IAH in Compton. At various times the parasite has been provided to other institutions and research groups carrying out coccidiosis research in the fowl. The H strain has been utilised in many fundamental studies on the eimerian life cycle and its relationship with the host. It has also been used to investigate the nature of drug resistance, and to derive attenuated lines-one of which, a "precocious line", is a component of a multivalent live attenuated coccidiosis vaccine (Paracox vaccine, Schering-Plough Animal Health). In this article, some immunological, epidemiological, genetic, and chemotherapeutic investigations with the H strain are reviewed, a summary is provided of the biological characteristics of the parasite and some of the core methods used to prepare purified extracellular life cycle stages for experimental studies or passage are given.

Recombinant bovine herpesvirus-1 expressing p23 protein of Cryptosporidium parvum induces neutralizing antibodies in rabbits

In order to develop a vaccine against cryptosporidiosis in cattle, we constructed a recombinant bovine herpesvirus-1 (BHV-1) expressing an immunodominant surface protein, p23, of Cryptosporidium parvum sporozoites. In the recombinant virus, the p23 gene under the control of a CAG promoter and a gene coding for an enhanced green fluorescent protein were integrated into the gG gene of BHV-1. Despite a low frequency of homologous recombination, cloning of the recombinants was easy because of the specific fluorescence of the plaques formed by recombinants. These plaques were among the plaques of the nonfluorescent parental virus. All clones selected for fluorescence also contained the p23 gene. In MDBK cells infected with the recombinant BHV-1, the antibody against the p23 protein recognized the p23 protein as an approximately 23-kDa specific band in Western blotting analysis. Rabbits immunized with the recombinant produced IgG against the p23 protein. It was also demonstrated that the sera of immunized rabbits reduced infection of C. parvum sporozoites in HCT-8 cells. The serum of an immunized rabbit reduced infection compared with the normal rabbit serum control. These results indicate that the recombinant BHV-1 induces neutralizing antibodies in rabbits.

Leishmania (Leishmania) amazonensis-induced cutaneous leishmaniasis in the primate Cebus apella: a model for vaccine trials

A primate model of leishmaniasis was developed with the objective of future vaccine testing. Lesion development and immunological parameters were studied upon primary and secondary infections. Seven Cebus apella were injected subcutaneously with 2 x 10(6) Leishmania (Leishmania) amazonensis promastigotes. Erythematous nodules appeared 19-29 days p.i., which disappeared 100 days p.i. Four months later, six of the monkeys were challenged with the same inoculum; three of them developed erythematous nodules after 7 days p.i., with ulcer formation in two of these subjects. The lesions were short-lived and all were cured 40 days post challenge. Anti-Leishmania IgG antibodies were detected and they increased after the challenge infection. Leishmania antigen-induced lymphoproliferation was found 1 month post-primary infection, which coincided with IFN-gamma production and lesion development. It decreased to control levels afterwards, but at the time of the challenge dose, it was significantly above the initial level. After the challenge infection, it first increased then decreased sharply at 40 days post-challenge, coinciding with the healing of the lesion. It increased again to a higher level at 60 days post-challenge. Leishmania (Leishmania) amazonensis-infection in C. apella did not induce complete protection against a secondary infection with a homologous parasite although specific antibody production and lymphoproliferation with IFN-gamma production were observed. This fact indicates that vaccine has to be better than infection in the induction of protective immunity, and raises a question on in vitro parameters that should be considered as a counterpart of expected protection induced by vaccine candidate. In addition, we conclude that this is a useful primate model for the evaluation of candidate vaccines.

Frontiers in research on parasitic protozoa

Frontiers in research on parasitic protozoa was the theme of the Autumn Symposium of the British Section of the Society of Protozoologists, held 2 September 2002, in London, UK.

Vaccination against Leishmania infections

Leishmaniasis, that affects millions of people worldwide, is an infectious disease caused by the protozoan parasite Leishmania. Incidence of the condition appears to be increasing in several parts of the world. Of the three main presentations of the disease, i.e. cutaneous, mucocutaneous and visceral, only the first one tends to heal spontaneously, while the other two are considered fatal if left to run their natural course. Recovery from leishmaniasis, whether spontaneous or drug-induced, is usually accompanied by solid immunity against reinfection, which provides a rationale for attempting to design vaccines against the disease. This review presents an outline of the main immunological features of Leishmania infections and of the mechanisms thought to operate in recovery from the disease. It describes various experimental approaches to vaccination in man and animal models, including the use of virulent and avirulent organisms, of dead parasites and extracts thereof, and of purified parasite proteins. Assays using novel technologies, such as the direct injection of DNAs encoding parasite proteins, or the inoculation of viral or bacterial vectors expressing such molecules, as well as recent experiments aimed at inducing an immune response against saliva of the insect vector, are also reviewed. Observations made during the course of these studies have reinforced the notion that vaccination against leishmaniasis is indeed feasible. However, in spite of intensive efforts by many groups and many reports of success in man and in animal models, a consensus is yet to emerge as to what constitutes the best approach to vaccination against leishmaniasis.

Prospects for a human Toxoplasma vaccine

Human toxoplasmosis is usually benign, but may occasionally lead to severe or lethal damages when combined with immunosuppressive states or when transmitted to the fetus during pregnancy. Only a vaccine could prevent these harmful effects. The oral route is the natural portal of entry of T. gondii. A protective immune response at the mucosal level is required to kill the parasite as soon as it penetrates the intestinal barrier thus preventing toxoplasma from invading the host and settling into tissues. The probable major roles played by both CD8 T cells and antibodies, specially IgA, suggest that the best strategy would be to stimulate both the cellular and humoral arms of the mucosal immune system. Mucosal dendritic cells have been shown to induce good protection against oral toxoplasma challenge. Our hypothesis is that an acceptable and effective human vaccine would have to carry the optimized synthetic vaccine (subunit, DNA or replicon) plus an appropriate adjuvant and to target the mucosal dendritic cells by means of an inert delivery system such as polymer microparticles, which can be endocytosed by M cells of the gut or nasal-associated lymphoid tissues.

Comparison of an attenuated anticoccidial vaccine and an anticoccidial drug programme in commercial broiler chickens in Italy

A live attenuated anticoccidial vaccine (Paracox) was compared with a nicarbazin-monensin anticoccidial drug shuttle programme in three crops of Italian broilers, comprising a total of 290,405 chickens. All birds received the antibiotic growth promoter avilamycin. No coccidiosis was evident during the trials, but the occurrence of oocysts in the litter demonstrated that a natural challenge was present. Vaccinated birds consistently performed at least as well as those treated with the anticoccidial drug shuttle. The final mean weights of vaccinated birds were significantly greater (P < 0.001) than those of birds treated with anticoccidial drugs, both for females at 36/37 days and males at 56 days. Feed conversion ratios, total mortality including culls, the proportion of rejects at the processing plant, and the moisture content of the litter were not significantly different between the two control methods. Growth curves showed that there was no post-vaccinal growth check in the vaccinated birds and no intolerance of the anticoccidial drug treatment. There was no overall seasonal effect, regardless of treatment, on the performance of the three crops reared from November 1997 to July 1998. These findings suggest that the use of this vaccine is likely to enable the replacement of anticoccidial drug shuttle programmes in broilers reared under conditions similar to those used in these Italian flocks.

Invasion, and short- and long-term survival of Babesia divergens (Phylum Apicomplexa) cultures in non-bovine sera and erythrocytes

In order to explore the feasibility of producing a Babesia divergens live vaccine free of bovine material contaminants the parasite's ability to grow in human, sheep and horse erythrocytes and serum and serum-free medium was investigated. B. divergens was successfully maintained in bovine erythrocytes overlaid with serum-free HL-1 medium. Supplementation of the culture medium with bovine or sheep serum improved parasite growth (monitored by measuring parasitaemia and uptake of tritiated hypoxanthine) whereas horse and human sera reduced parasite growth. As assessed by Giemsa's stained and FITC-labelled blood smears, the parasite invaded all erythrocyte types. Polyparasitism was less common in sheep and horse erythrocytes than in bovine and human erythrocytes. Accole stages were observed in bovine, human and sheep but not in horse erythrocytes. Proliferation following invasion was higher in human but lower in horse and sheep erythrocytes compared with bovine erythrocytes. Long-term cultures of B. divergens reached similar peak parasitaemias in human, sheep and bovine erythrocytes. Attempts to establish long-term cultures in horse erythrocytes failed. These results suggest that B. divergens is not host specific at the level of host cell attachment and invasion. Instead, parasite survival appears to be decided once the organism has gained access into the cell.

Molecular characterization of an acetylcholinesterase implicated in the regulation of glucose scavenging by the parasite Schistosoma

Acetylcholinesterase (AChE) present on the surface of the trematode blood fluke Schistosoma has been implicated in the regulation of glucose scavenging from the host blood. Determination of the molecular structure and functional characteristics of this molecule is a crucial first step in understanding the novel function for AChE and in evaluating the potential of schistosome AChE as a target of new parasite control methods. We have determined the primary structure of acetylcholinesterase from Schistosoma haematobium. Immunolocalization studies confirmed that the enzyme was present on the parasite surface as well as in the muscle. The derived amino acid sequence possesses features common to acetylcholinesterases: the catalytic triad, six cysteines that form three intramolecular disulphide bonds, and aromatic residues lining the catalytic gorge. An unusual feature is that the fully processed native enzyme exists as a glycoinositol phospholipid (GPI)-anchored dimer, but the sequence of the C?terminus does not conform to the current consensus for GPI modification. The enzyme expressed in Xenopus oocytes showed conventional substrate specificity and sensitivity to established inhibitors of AChE, although it is relatively insensitive to the peripheral site inhibitor propidium iodide. Distinctions between host and parasite AChEs will allow the rational design of schistosome-specific drugs and vaccines.

CpG oligodeoxynucleotides as vaccine adjuvants in primates

Synthetic oligodeoxynucleotides (ODN) containing unmethylated CpG motifs act as immune adjuvants in mice, boosting the humoral and cellular response to coadministered Ags. CpG ODN that stimulate human PBMC are only weakly active in mice. Thus, alternative animal models are needed to monitor the activity and safety of "human" CpG ODN in vivo. This work demonstrates that rhesus macaques recognize and respond to the same CpG motifs that trigger human immune cells. Coadministering CpG ODN with heat-killed Leishmania vaccine provided significantly increased protection of macaques against cutaneous Leishmania infection. These findings indicate that rhesus macaques provide a useful model for studying the in vivo activity of human CpG motifs, and that ODN expressing these motifs act as strong immune adjuvants.

How do host immune responses affect nematode infections?

Host immune responses limit, and in some instances eliminate, nematode infections. There is considerable interest in enhancing these natural processes by the use of antinematode vaccines to achieve control of infection or disease. How nematodes are damaged is unclear. Worms might be damaged directly by effector cells and molecules of the immune system. Alternatively, they might be damaged by the physiological stress of their efforts to resist attack. Separating these possibilities could have important implications for approaches to the control of nematode infections and the disease that they cause.

Efficacy of a combination febantel-praziquantel-pyrantel product, with or without vaccination with a commercial Giardia vaccine, for treatment of dogs with naturally occurring giardiasis

OBJECTIVE

To determine efficacy of treatment with a combination febantel-praziquantel-pyrantel product, with or without vaccination with a commercial Giardia vaccine, in dogs with naturally occurring giardiasis.

DESIGN

Prospective trial.

ANIMALS

16 Beagles naturally infected with Giardia duodenalis.

PROCEDURES

During phase 1, 6 dogs were treated with the parasiticide for 3 days (4 were also vaccinated). Four weeks later, all 6 dogs were treated with the parasiticide again for 5 days and were bathed and moved to clean cages after the last treatment (phase 2). Nine dogs were treated with the parasiticide for 3 (n = 4) or 5 (5) days and bathed and moved to clean cages after the last treatment (phase 3). Fecal samples were collected twice weekly for 24 days after treatment and tested for cysts with a quantitative zinc sulfate flotation technique and for Giardia antigen with an immunoassay.

RESULTS

Dogs in phase 1 were all shedding cysts again by day 24. In phase 2, only 1 dog shed cysts after treatment, and shedding was transient (day 17). In phase 3, neither cysts nor antigen was detected in fecal samples from 2 of 4 dogs treated for 3 days and 4 of 5 dogs treated for 5 days. In 18 of 57 (31.6%) fecal samples, cysts were seen, but results of the immunoassay were negative.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that when a combination febantel-praziquantel-pyrantel product is used to treat dogs with giardiasis, bathing and changing the environment after treatment may be more important in preventing recurrence than duration of treatment.

Immunity following use of Australian tick fever vaccine: a review of the evidence

OBJECTIVE

To review the evidence available on the degree and duration of immunity provided by Australian tick fever vaccines against Babesia bovis, B. bigemina and Anaplasma marginale infections in Australia and overseas.

BACKGROUND

Vaccines containing attenuated strains of B bovis and B bigemina as well as A. centrale grown in splenectomised calves have been used in Australia since 1964 to immunise cattle against tick fever. About 800,000 doses of vaccine are supplied annually and much of the evidence for protection is field evidence rather than conventional immunological measures or pen trials.

CONCLUSIONS

Immunity to Babesia bovis and B. bigemina--A single inoculation generally provides sound, long-lasting protection both in Australia and overseas. No evidence was found of a loss of immunity with time. Vaccine failures to B. bovis do occur, but are uncommon and evidently caused by a number of factors, including immune responsiveness of the vaccinated animals, and immunogenicity of the vaccine strain. Immunity to Anaplasma marginale--The vaccine containing A. centrale provides partial, variable protection against A. marginale. Protection against challenge in Australia is adequate in most cases to prevent disease and use of the vaccine in this country appears to be justified. Protection against antigenically diverse, highly virulent stocks of A. marginale in other countries is, at times, clearly inadequate and better vaccines are required in situations where the challenge is severe.

The African Green Monkey model for cutaneous and visceral leishmaniasis

Non-human primates are valuable models for biomedical research because of their similarities to human anatomy, immunology and physiology. Leishmaniasis, a disease caused by protozoan parasites, has a worldwide distribution and results in high morbidity and mortality. Availability of a non-human primate model of leishmaniasis would facilitate the study of different aspects of this disease and would accelerate the development of vaccines and new drugs. In this article, some interesting features of the vervet monkey (African Green monkey) model of human cutaneous and visceral leishmaniasis are discussed.

Genetical and electron microscopical studies on Cryptosporidia

The present work was to clarify whether C. parvum oocysts have different strains in human patients by using different staining, electron microscopical and genetical techniques. A trial to induce a vaccine against Cryptosporidial infection in mice was carried out using killed autoclaved Cryptosporidial oocysts. The results obtained were satisfactory. Two genotypes of C. parvum, viz: human and zoonotic genotypes were detected and described. The killed vaccine used orally gave a promising results.

Host responses against the fish parasitizing ciliate Ichthyophthirius multifiliis

Recent studies have shown that fish are able to mount protective immune responses against various parasites. One of the best characterized parasite-host system in this context is the ciliate Ichthyophthirius multifiliis (Ich) parasitizing a range of freshwater fishes. Both specific and non-specific host defence mechanisms are responsible for the protection of fish against challenge infections with this ciliate. The specific humoral components comprise at least specific antibodies. The non-specific humoral elements included are the alternative complement pathway and probably lectins. Cellular factors involved in the specific response are B-cells and putative T-cells. The non-specific effector cells recognized are various leukocytes. In addition, goblet-cells and mast cells (EGC-cells) may have a function. The NCC-cell (suggested analogue to NK-cells in mammals) seems to play a role in the non-specific response. This well documented protective response in freshwater fishes against Ich has urged the development of anti-parasitic vaccines. Indeed, such products based on formalin killed parasites have been developed and found to offer the vaccinated host a satisfactory protection. However, the collection of parasites for vaccine production is extremely laborious. It involves keeping infected fish due to the fact that in vitro propagation of the parasite is still insufficiently developed. Gaining knowledge of amino acid sequences and its encoding DNA-sequences for the protective antigens (i-antigens) in the parasite was a major breakthrough. That achievement made it possible to produce a recombinant protein in E. coli and preliminary results indicated a certain protection of fish vaccinated with this product. Recent work has shown that the free-living and easily cultivated ciliate Tetrahymena can be transformed and express the i-antigen. This path seems to be promising for future development of vaccines against Ich. A novel approach in fish is the development of DNA-vaccines. Successful DNA-vaccination trials have been conducted in fish against viral infections and the technology also makes it possible to develop a DNA-vaccine against Ich. Other approaches to immuno-protection against Ich have been the use of heterologous vaccines. Thus, both bath and injection vaccination using live or killed (un-transformed) Tetrahymena has been reported to offer treated fish a certain level of protection. Such protection could be explained by non-specific reactions and the efficacy and duration of this vaccination type should be further evaluated.

An update on antileishmanial vaccine candidates and prospects for a canine Leishmania vaccine

Dogs are the domestic reservoir for Leishmania infantum, the parasite causing zoonotic visceral leishmaniasis (VL) in both the Old and New Worlds. Since the available methods for canine leishmaniasis treatment and control have limited efficacy, the development of a canine Leishmania vaccine is highly desirable. Mechanisms of antileishmanial immune responses in murine, human, and canine infections are briefly presented. Vaccine candidates, including live or killed parasites, Leishmania purified fractions, defined recombinant parasite antigens, live recombinant bacteria expressing Leishmania antigens and antigen-encoding DNA plasmids, are reviewed. Finally, some practical requirements for the evaluation of vaccine candidates in dogs are indicated.

[Sleeping sickness, a reemerging sickness]

Human African Trypanosomiasis (THA) has reappeared in most intertropical countries of Black Africa and an estimated 400,000 new cases are reported every year. Genetic tests which now make possible the differentiation of morphologically similar trypanosome subspecies showed that a large variety of game and domestic animals act as reservoir hosts of Trypanosoma brucei gambiense, thus making it even more difficult to fight the disease. The detection of cases and their treatment are absolutely necessary in preventing the spread of the disease. This can only be carried out with mobile medical teams which seek out patients. This detection currently calls on techniques which isolate trypanosomes and serological techniques. The collection of data with regards to different geographical positions, makes it possible to determine exactly which regions are affected, possibly affected or sound. Although the diagnosis of the hemo-lymphatic stage can be determined without any great problem nowadays, the point at which the trypanosome invades CNS tissues and the ways in which it occurs are as yet unknown, even though the role of nitric oxyde and cytokines is better understood. Antibodies, anti-neurofilaments and anti-galactocerebrosides when found in the cerebro-spinal fluid are characteristic of the nervous stage of the disease. This condition is really sleeping sickness, not only hypersomnia. The more seriously the patient is affected the shorter the sleep-wake cycles are during the nycthemeron. These early disorders can be quickly reversed thanks to therapy, which to day uses melarsoprol, an arsenical drug, which cannot be of great promise as it is very toxic. Current research into nitro-imidazole derivatives (particularly megazol) seems a promising therapy as they were effective in vitro and in vivo, in rodents and primates.

WorldLeish II: impressions from a 'new leishmaniac'

The second International Congress on Leishmaniasis organized by the Hellenic Pasteur Institute, Turkish Society for Parasitology and the University of Crete was held 20-24 May 2001 in Crete, Greece. The participants focused attention on leishmanial infections world-wide covering aspects of vector biology, host immunology, and in particular, canine leishmaniasis.

Heterologous immunity revisited

Heterologous immunity, or protection by one invading organism against another across phylogenetic divides, has been recognised for decades. It was initially thought to operate largely through enhancement of phagocytosis, but this explanation became untenable when it was realised it worked extremely well against intraerythrocytic protozoa and killed them while they were free in the circulation. Clearly a soluble mediator was called for. This review summarises the logic that arose from this observation, which led to a wider appreciation of the roles of pro-inflammatory cytokines, and then nitric oxide, in the host's response against invaders, as well as the ability of these mediators to harm the host itself if they are generated too enthusiastically. This has led to a discernable pattern across heterologous immunity as a whole, and its lessons influence a range of areas, including vaccine development.

Vaccination against tropical theileriosis

Theileria annulata, the cause of tropical theileriosis is propagated in cattle with stage-to-stage transmission by Hyalomma ticks. Three stages in the life cycle of the parasite--tick-derived sporozoites, intramononuclear schizonts, and erythrocytic merozoites--infect cattle. When cattle are inoculated with schizont-infected cells, the parasite is transferred from the donor cell to the recipient. The main pathological damage in cattle is induced by the schizont stage. Each development stage of T. annulata elicits a specific immune response. Schizont-infected lymphoid cells can be grown indefinitely in culture and prolonged cultivation results in loss of virulence. Blood-derived schizonts induce stronger immunity than culture-derived schizonts, which suggests that restrictions on the parasite population or antigenic variation occur during prolonged cultivation. The duration of immunity following sporozoite or schizont infections has not yet been determined, but does not appear to be lifelong. The attenuated, culture-derived anti-theileria vaccine proved to be safe and effective in prevention of field theileriosis in large enzootic areas.

Characterization of attenuated Theileria annulata vaccines from Spain and the Sudan

Theileriosis caused by Theileria annulata can be effectively prevented by vaccination with attenuated, cultured schizonts. Although these attenuated vaccines have been applied for a long time, not much is known about the fate of the vaccine strain in the field. Here, two experimental Spanish vaccine strains originating in Cádiz and Cáceres, and one Sudanese strain are studied to address the development of a carrier status and the infectivity for Hyalomma ticks. Moreover, the heterogeneity of the merozoite surface protein, Tams1, was analyzed in search for an attenuation marker. Using the sensitive reverse line blot (RLB) hybridization, the development of a low level carrier status was demonstrated in the Cáceres and Sudanese line vaccinated calves. Although no signal was detected in the Cádiz line vaccinated calves, seroconversion against the schizont stage was observed, as it was in all other calves. The experimental transmission of T. annulata by Hyalomma ticks to naïve calves was unsuccessful for all cell line inoculated calves. Tams1 heterogeneity indicated a clonal selection of parasites during the process of attenuation, but the Tams1 sequence itself has no connection with the attenuation status. In conclusion, a carrier status develops in attenuated schizont culture vaccinated calves, but is not infective for Hyalomma ticks. Based on these data, the risk for spread of the vaccine strains in the field may be very low.

Vaccines against Theileria parva

Bovine theileriosis caused by Theileria parva continues to be a major economic problem in many parts of Eastern, Southern, and Central Africa. Due to the unsustainable nature of the present control method--using toxic acaricides to kill ticks--alternative control methods are being sought. Live vaccines are being used in many countries in the region. These vaccines are based on the infective sporozoite stage of the parasite. Sporozoites are inoculated in cattle with simultaneous administration of a long-acting formulation of oxytetracycline. These vaccines are poorly adopted in the region, mainly because of problems associated with the use of live parasites. An experimental recombinant vaccine based on a sporozoite surface antigen (p67) has been developed. Immunization with this antigen induces neutralizing antibodies and, under laboratory conditions, this technique protects approximately 70% of the immunized cattle to a defined needle challenge. The efficacy of the vaccine is currently being evaluated under field challenge in Kenya. Since a vaccine based on a single antigen may not be sustainable under field conditions, a search for schizont antigens that induce protective cell-mediated immune responses continues. It is expected that the ultimate vaccine against theileriosis will incorporate a mixture of several antigens derived from both sporozoite and schizont stages, contributing to robust immunity.

Culture-derived parasites in vaccination of cattle against tick-borne diseases

The major economically important tick-borne diseases of cattle are theileriosis, babesiosis, anaplasmosis, and cowdriosis. Culture-derived attenuated schizonts of Theileria annulata have proved to be safe for all types of cattle and they protect against tick-borne theileriosis. T. parva was also successfully grown in vitro; however, inoculation of cattle with allogeneic schizont-infected cells resulted in rejection and destruction of the parasites together with the host cells. The number of schizont-infected cells needed for immunization is greater than for T. annulata theileriosis. Culture-propagated Babesia bovis and B. bigemina were used for large scale vaccination in the field. An avirulent population of Babesia spp. was obtained by in vitro cloning; inoculation of cattle did not induce clinical babesiosis, but produced specific antibodies. Culture-derived exoantigens of Babesia spp. proved to be completely safe for cattle, however, they conferred less protection than live parasites. Cell-cultured Cowdria ruminantium was highly infective for susceptible animals but, attenuated in vitro, could offer a potential source for vaccination. Anaplasma marginale, successfully grown in tick cell culture, may be developed for vaccines. Factors that should be considered in the developing of vaccines against tick-borne diseases include: the protective immune response to the pathogenic parasite developmental stages, virulence, immunological strain differences, and antigenic variations in cattle and in culture.

Vaccine strategies against schistosomiasis: from concepts to clinical trials

Schistosomiasis, the second major parasitic disease in the world after malaria, affects 200 million people. Vaccine strategies represent an essential component of the control of this chronic debilitating disease where the deposition of millions of eggs in the tissues is the main cause of pathology. Research developed in our laboratory over the last 20 years has led to the identification of novel effector mechanisms, pointing for the first time to the protective role of Th2 responses and of IgE antibodies now supported by seven studies in human populations. The identification and molecular cloning of a target antigen, a glutathione S-transferase (GST), has made it possible to demonstrate its vaccine potential in several animal species (rodents, cattle, primates) and to establish consistently the capacity of vaccination to reduce female worm fecundity and egg viability through the production of neutralizing antibodies (IgA and IgG). Following promising preclinical studies, clinical trials (phase I and II) have been undertaken using Schistosoma haematobium GST, Sh28GST. High titers of neutralizing antibodies were produced (IgG3 and IgA) together with Th2 cytokines, consistently with the concepts developed from experimental models. With these results we are on the way towards a feasible approach of vaccine development against a major human parasitic disease.

Autoclaved Leishmania major vaccine for prevention of visceral leishmaniasis: a randomised, double-blind, BCG-controlled trial in Sudan

BACKGROUND

Visceral leishmaniasis is a major cause of morbidity and mortality in the Sudan. Drug treatment is expensive, and drug resistance is becoming increasingly common. Safe, effective, and cheap vaccines are needed. We report the results of a vaccine trial against human visceral leishmaniasis.

METHODS

We undertook a double-blind randomised trial to test the safety and efficacy of an autoclaved Leishmania major (ALM) promastigote vaccine (1 mg per dose). Of 5093 volunteers screened, 2306 had negative leishmanin skin tests and reciprocal titres of less than 6400 in the direct agglutination test. They were randomly assigned two doses of ALM mixed with BCG or BCG alone. Volunteers were followed up for 2 years. The primary endpoint was clinical visceral leishmaniasis or post-kala-azar dermal leishmaniasis. Analyses were by intention to treat.

FINDINGS

Side-effects were confined to the injection site. The cumulative frequency of visceral leishmaniasis at 2 years did not differ significantly between the group assigned ALM plus BCG and that assigned BCG alone (133/1155 [11.5%] vs 141/1151 [12.3%], p=0.6). The vaccine efficacy was 6% (95% CI -18 to 25). The proportion of individuals showing leishmanin skin conversion was significantly higher in the ALM plus BCG group than in the BCG alone group throughout follow-up (303 [30%] vs 72 [7%] at 42 days). Individuals whose leishmanin test converted after vaccination (induration > or =5 mm) had a significantly lower frequency of visceral leishmaniasis than non-responders (27/375 [7.2%] vs 210/1660 [12.7%], p=0.003).

INTERPRETATION

We found no evidence that two doses of ALM plus BCG offered significant protective immunity against visceral leishmaniasis compared with BCG alone. Leishmanin skin conversion with an induration of 5 mm or more in either group was associated with protection from the disease.

Visceral leishmaniasis control: a public health perspective

Visceral leishmaniasis (VL), also known as kala-azar, is a vector-borne disease caused by a protozoan of the Leishmania donovani complex. A phlebotomine sandfly transmits the parasite from person to person or via an animal reservoir. VL is a severe, debilitating disease, characterized by prolonged fever, splenomegaly, hypergammaglobulinaemia and pancytopenia. Patients become gradually ill over a period of a few months, and nearly always die if untreated. Case-fatality ratios are high even in treated patients. Worldwide an estimated 500,000 VL cases occur each year. This study reviews clinical, epidemiological and public health aspects of the disease and shows how critical adequate case detection is for the success of VL control. Examination of the issue of VL diagnosis with respect to the global challenges in VL control leads to the observation that a sound diagnostic-therapeutic algorithm for the health services in endemic areas is badly needed. Serological tests could be an alternative to parasitological diagnosis and the direct agglutination test (DAT) was found to fulfil many criteria for a 'field test', including cost effectiveness. Although research needs on vaccine and better drugs continue to be high on the agenda, a VL test-treatment strategy based on currently available highly sensitive serological tests, such as the DAT, should be introduced in the health services in endemic areas.

Functional analysis of proteins involved in Plasmodium falciparum merozoite invasion of red blood cells

Plasmodium falciparum causes the most lethal form of malaria in humans and is responsible for over two million deaths per year. The development of a vaccine against this parasite is an urgent priority and potential protein targets include those on the surface of the asexual merozoite stage, the form that invades the host erythrocyte. The development of methods to transfect P. falciparum has enabled the construction of gain-of-function and loss-of-function mutants and provided new strategies to analyse the role of parasite proteins. In this review, we describe the use of this technology to examine the role of merozoite antigens in erythrocyte invasion and to address their potential as vaccine candidates.

Vaccines against the avian enteropathogens Eimeria, Cryptosporidium and Salmonella

The worldwide poultry industry provides a substantial proportion of the nutritional requirement of the human population. To keep pace with the increasing demand for the high-quality, low-cost protein source that poultry provides, intensive rearing practices have been developed within the past few decades. For example, chickens are housed routinely in crowded environments under adverse conditions, and genetic strains have been selected for rapid growth, high protein-to-fat content and superior egg-laying characteristics. A major negative consequence of these practices has been an increase in the incidence of diseases. Enteric diseases in particular have emerged as a major problem threatening the future viability of the poultry industry. A variety of methods have been used to combat avian diseases in the commercial setting, including improved farm management practices, the use of antibiotic drugs, the selection of disease-resistant strains of chickens, and the manipulation of the chicken's immune system. In the latter category, the development of vaccines against the major avian diseases has become a priority in the poultry industry. This review will highlight recent progress in vaccine development against three major avian enteric pathogens: Eimeria, Cryptosporidium and Salmonella.

An experimental vaccine providing heterologous protection for Leishmania species in murine model

Five groups each of 20 BALB/C mice were immunized by heat killed L. donovani promastigotes. Two groups served as control. Each group of mice was subdivided into 2 equal subgroups and challenged with either L. donovani or L. major. The results revealed a high cross reactivity paralleled by a high cross protection between L. donovani and L. major. Heat killed L. donovani promastigotes super-passed other tested SDS-PAGE fractionated and eluted L. d. fractions identified by immune BALB/c mice in providing protection against both L. donovani and L. major. The percentage of protection (reduction in parasite burden) offered by each vaccine was as follows: 80.21%, 36.11%, 9.29%, 33.44% and 17.48% respectively against challenge infection with L. donovani and 3.012 x 10(4), 1.095 x 10(4), 6.054 x 10(2), 2.409 x 10(4), and 3.34 x 10(2), respectively against challenge infection with L. major.

Loss of matrix metalloproteinase 9 activity in Theileria annulata-attenuated cells is at the transcriptional level and is associated with differentially expressed AP-1 species

The schizont stage of the protozoan parasite Theileria annulata reversibly transforms bovine monocytes into an immortalised and metastatic state. We have been studying T. annulata induction of host matrix metalloproteinases (MMP) which are involved in parasite dissemination and pathogenesis. We have observed that prolonged in vitro culture of T. annulata-infected cell lines results in their attenuation and this process is associated with alterations in both host and parasite gene expression. In particular, a loss in bovine MMP expression in later passage cultures suggests that these parasite-induced MMPs are virulence factors. As a means to further our understanding of the attenuation process we examine in detail the parasite-induced differential expression of one particular bovine proteinase, MMP9, in non-attenuated (p58) and attenuated (p158) passage levels of the Ode vaccine line. We show here that MMP9 expression is regulated at the transcriptional level and we suggest that a particular parasite-induced AP-1 recognition transcription factor present in the Ode non-attenuated line may have a role to play in the expression of this host gene.

Development of vaccines against human parasitic diseases: tools, current status and perspectives

Vaccines against malaria, leishmaniasis and schistosomiasis are in the most advanced stages of development of all vaccines for human parasitic diseases. Despite the remarkable progress made in identifying and producing protective antigens, at present there are no generally accepted vaccines against parasitic diseases. Vaccines for malaria and leishmaniasis have been taken to clinical trials while vaccines for schistosomiasis are in Phase I/II trials. This review will focus on the most promising antigenic preparations, emphasising the tools, present status and perspectives for development of vaccines against malaria, leishmaniasis and schistosomiasis.

Schistosome population genetic structure: when clumping worms is not just splitting hairs

Schistosomiasis is a major public health problem, affecting over 200 million people worldwide. Although Schistosoma mansoni has been studied rigorously in an attempt to provide a vaccine based on a number of candidate antigens, there has been a lack of complementary effort to determine the range and distribution of variation in representative molecules throughout natural populations. Here, Jason Curtis and Dennis Minchella highlight current (and suggest future) research efforts aimed at assessing genetic variation in schistosome populations, and call for a more robust consideration of schistosome population genetics.

BCG expressing LCR1 of Leishmania chagasi induces protective immunity in susceptible mice

Cellular immune responses are required for protective immunity against Leishmania chagasi. Immunization strategies using live intracellular bacteria (e.g., bacille-Calmette Guerin strain of Mycobacterium bovis) expressing recombinant antigens can induce cellular immune responses to these antigens. Previous studies demonstrated that the L. chagasi antigen LCR1 stimulates IFN-gamma production from T cells of infected BALB/c mice, and immunization with recombinant LCR1 partially protects against L. chagasi infection. To determine whether live bacteria could enhance the immunization potential of LCR1, we engineered BCG expressing LCR1 (BCG-LCR1). Subcutaneous immunization with BCG-LCR1, but not with BCG containing plasmid only (BCG-pMV261), elicited better protective immunity against L. chagasi infection than LCR1 protein alone. BCG-LCR1 administered intraperitoneally did not protect. Splenocytes from mice immunized s.c. with either BCG-LCR1 or BCG-pMV261 and then infected with L. chagasi promastigotes had increased antigen-induced IFN-gamma and reduced IL-10 production compared to splenocytes of control mice. We propose that BCG-LCR1 promotes a Th1-type protective immune response, and it may be a useful component of a Leishmania vaccine.