A heterologous prime/boost immunisation strategy protects against virulent E. ruminantium Welgevonden needle challenge but not against tick challenge

Screening for immune biomarkers associated with infection or protection against Ehrlichia ruminantium by RNA-sequencing analysis

Heartwater is one of the most economically important tick-borne fatal diseases of livestock. The disease is caused by the bacteria Ehrlichia ruminantium transmitted by Amblyomma ticks. Although there is evidence that interferon-gamma controls E. ruminantium growth and that cellular immune responses are protective, an effective recombinant vaccine for this disease is lacking. Analyses of markers associated with infection as well as protection will lead to a better understanding of the E. ruminantium immune response and corresponding pathways induced in sheep peripheral blood mononuclear cells (PBMC) will assist in development of such a vaccine. In this study, Biomarkers of infection (BMI) were identified as uniquely expressed genes during primary infection and biomarkers of protection (BMP) associated with immune to heartwater were identified post challenge. Sheep were experimentally infected and challenged with E. ruminantium infected ticks. The immune phenotypic and transcriptome profile of their PBMC were compared to their own naïve PBMC collected before infection. The study revealed 305 differentially expressed genes (DEGs) as BMI, of these 17 were upregulated at all three time-points investigated. These DEGs, form part of the bacterial invasion of epithelial cells Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway, and others detected from day 1 post infection and are considered predictive markers for early heartwater infection in ruminants. Similarly, a total of 332 DEGs were identified as BMP, of these 100 were upregulated and 75 were downregulated at all three time-points investigated. However, at D1PC most DEGs were downregulated (n = 1312) that correlated with a reduction in the % CD4 and CD8 T cells detected with flow cytometry. KEGG pathway analyses showed complete down regulation of T cell specific pathways possibly due to homing of immune cells to the site of infection after acquired immunity developed. At D4PC, expression levels of most of these downregulated genes increased and by D6PC they were upregulated. This indicates that the sampling time-point for biomarker analyses is important when results for acquired immune responses are inferred. This data identified DEGs that could be considered as biomarkers of protective immunity that can be used for identification of vaccine antigens and provides a strong foundation to further development of heartwater recombinant vaccines.

Th1 and Th2 epitopes of Cowdria polymorphic gene 1 of Ehrlichia ruminantium

Cowdria polymorphic gene 1 (cpg1, Erum2510, ERUM_RS01380) has been shown to induce 30% and 100% protection in sheep immunised by deoxyribonucleic acid (DNA) prime combined with DNA boost and DNA prime combined with protein boost, respectively, against heartwater infection via needle challenge. To localise its antigenic regions for inclusion in a multi-epitope DNA vaccine against heartwater, Erum2510 was cleaved into five overlapping subfragments. These subfragments were expressed individually in an Escherichia coli host expression system and evaluated for their ability to induce proliferative responses, Th1 and Th2 cytokines (interferon gamma [IFN-γ] and interleukin 4 [IL-4]) via enzyme-linked immunospot (ELISpot), quantitative real time polymerase chain reaction (qRT-PCR) and flow cytometry. Recombinant (r)proteins 3 and 4 were shown to induce immunodominant Th1 and Th2 immune responses characterised by the secretion of effector cytokines IFN-γ and IL-4 in addition to differential messenger ribonucleic acid (mRNA) expression of tumour necrosis factor (TNF), IL-2, IL-1, IL-18, IL-10, transforming growth factor (TGF), granulocyte-macrophage colony-stimulating factor (GM-CSF) and inducible nitric oxide synthase (iNOS). Thirty-seven overlapping synthetic peptides (16 mer) spanning the lengths of these immunodominant rproteins were synthesised and assayed. A peptide pool comprising p9 and p10 derived from rprotein 3 induced a Th1-biased immune response. A peptide pool comprising p28 and p29 derived from rprotein 4 induced a mixed Th1 and Th2 immune response characterised by secretion of IFN-γ and differential mRNA expression of IL-1, IL-2, IL-10, IL-12, iNOS, TGF, TNF and GM-CSF. Only one of the peptides (p29) induced secretion of IL-4. Phenotypic analysis showed significant activation of cluster of differentiation 8+ (CD8+), cluster of differentiation 4+ (CD4+) and B+ lymphocyte populations. Findings suggest that Erum2510 rproteins and synthetic peptides can induce both cellular and humoral immune responses, thereby implicating their importance in protection against heartwater.Contribution: This study will facilitate the design of an effective multi-epitope DNA vaccine against heartwater that will contribute to control this economically important disease in sub-Saharan Africa and beyond.

Transcriptome analysis of Ehrlichia ruminantium in the ruminant host at the tick bite site and in the tick vector salivary glands

Heartwater is a non-contagious tick-borne disease of domestic and wild ruminants. Data regarding the complex processes involved during pathogen-vector-host interaction during Ehrlichia ruminantium infection is lacking and could be improved with knowledge associated with gene expression changes in both the pathogen and the host. Thus, in the current study, we aimed to identify E. ruminantium genes that are up-regulated when the pathogen enters the host and before the disease is established. Identification of such genes/proteins may aid in future vaccine development strategies against heartwater. RNA-sequencing was used to identify E. ruminantium genes that were exclusively expressed at the tick bite site in sheep skin biopsies (SB) and in adult tick salivary glands (SG). RNA was extracted from pooled samples of the SB or SG collected at different time points during tick attachment and prior to disease manifestation. Ribosomal RNA (rRNA) was removed and the samples were sequenced. Several E. ruminantium genes were highly expressed in all the samples while others were exclusively expressed in each. It was concluded that E. ruminantium genes that were exclusively expressed in the SB or both SB and SG when compared to the transcriptome datasets from bovine elementary bodies (BovEBs) from cell culture may be considered as early antigenic targets of host immunity. In silico immunogenic epitope prediction analysis and preliminary characterization of selected genes in vitro using ELIspot assay showed that they could possibly be ideal targets for future vaccine development against heartwater, however, further epitope characterization is still required.

Safety and efficacy of an attenuated heartwater (Ehrlichia ruminantium) vaccine administered by the intramuscular route in cattle, sheep and Angora goats

Heartwater is an economically important tick-borne disease of ruminants in Africa. The current commercial vaccine uses live Ehrlichia ruminantium from blood of infected sheep, requires antibiotic treatment during infection, needs to be administered intravenously and does not protect against all South African isolates. An attenuated tissue culture vaccine not requiring antibiotic treatment and effective against different field strains in small groups of goats and sheep was reported previously. The objective of the present study was to test safety and efficacy of this vaccine administered by intramuscular (i.m.) inoculation in larger groups of sheep, Angora goats and cattle. Animals were vaccinated via intravenous (i.v.) and i.m. routes and received E. ruminantium homologous challenge by feeding of infected ticks or by i.v. inoculation of infected blood. For vaccine titration in sheep and goats, the optimum safe and efficacious dose was determined using 2 ml equivalent of 10²-10⁵ culture-derived live elementary bodies (EBs). Similarly, the vaccine was titrated in cattle using 5 ml containing 10⁵-10⁷ EBs. Seventy percent of i.v. vaccinated and 9.7% of i.m. vaccinated Angora goats receiving 10⁵ EBs, developed severe reactions to vaccination and were treated. These treated animals and the remaining 90.3% of i.m.- vaccinated goats showed 100% protection against i.v. or tick challenge. Sheep and Angora goats vaccinated i.m. with 10⁴ EBs had no vaccination reactions and were fully protected against i.v. or tick challenge. Similarly, vaccinated cattle (dose 10⁶ EBs) did not react to vaccine inoculation and were fully protected against i.v. or tick homologous challenge. Control non-vaccinated animals reacted severely to challenge and required oxytetracycline treatment. This successfully demonstrated that Angora goats, sheep and cattle can be safely vaccinated with the attenuated E. ruminantium Welgevonden vaccine via the i.m. route, with no clinical reactions to vaccination and 100% protection against virulent i.v. and homologous tick challenge.

Genetic diversity of Ehrlichia ruminantium field strains from selected farms in South Africa

Heartwater is a tick-borne disease caused by the intracellular rickettsial parasite Ehrlichia ruminantium and transmitted by Amblyomma hebraeum ticks. Heartwater is problematic in endemic areas because it causes high mortality in ruminants and leads to economic losses that threaten productivity and food security. This may indicate that there is augmented genetic diversity in the field, which may result in isolates that are more virulent than the Ball3 and Welgevonden isolates. The genetic diversity of E. ruminantium was investigated in this study, focussing on the pCS20 gene region and four polymorphic open reading frames (ORFs) identified by subtractive hybridisation. The 16S ribosomal ribonucleic acid gene confirmed E. ruminantium in brain, blood and tick genomic deoxyribonucleic acid samples (n = 3792) collected from 122 farms that were randomly selected from seven provinces of South Africa where heartwater is endemic. The conserved E. ruminantium pCS20 quantitative polymerase chain reaction (qPCR) assay was used to scan all collected field samples. A total of 433 samples tested positive with the qPCR using the pCS20 gene region, of which 167 were sequenced. The known stocks and field samples were analysed, and phylogenetic trees were generated from consensus sequences. A total of 25 new clades were identified; of these, nine isolates from infected blood could be propagated in cell cultures. These clades were not geographically confined to a certain area but were distributed amongst heartwater-endemic areas in South Africa. Thus, the knowledge of strain diversity of E. ruminantium is essential for control of heartwater and provides a basis for further vaccine development.

A multi-epitope DNA vaccine co-administered with monophosphoryl lipid A adjuvant provides protection against tick transmitted Ehrlichia ruminantium in sheep

Previously, a heartwater experimental DNA vaccine provided 100% protection following laboratory challenge with Ehrlichia ruminantium administered by needle but not against an E. ruminantium tick challenge in the field. A multi-epitope DNA vaccine incorporating both CD4⁺ and CD8⁺ cytotoxic T lymphocytes epitopes could provide a better alternative. In this study, we investigated the use of multi-epitope DNA vaccines against an E. ruminantium experimental tick challenge in sheep. The multi-epitope DNA vaccines were delivered via the intramuscular route and intradermal route using the gene gun in the presence of monophosphoryl lipid A (MPL) adjuvant, which was either applied topically to the gene gun inoculation site or co-administered with the vaccine via the intramuscular route. Initially two constructs namely, pSignal plus and pLamp were tested with MPL applied topically only and no protection was obtained in this formulation. However, when pLamp was co-administered with MPL via the intramuscular route in addition to topical application, its protective efficiency improved to protect 60% of the sheep against tick challenge. In this formulation, the vaccine induced enhanced activation of memory T cell responses both before and after challenge with variations amongst the different sheep possibly due to their different genetic backgrounds. In conclusion, this study showed that a heartwater multi-epitope DNA vaccine, co-administered with MPL adjuvant can protect sheep following a laboratory E. ruminantium tick challenge.

Tick saliva and its role in pathogen transmission

Tick saliva is a complex mixture of peptidic and non-peptidic molecules that aid engorgement. The composition of tick saliva changes as feeding progresses and the tick counters the dynamic host response. Ixodid ticks such as Ixodes ricinus, the most important tick species in Europe, transmit numerous pathogens that cause debilitating diseases, e.g. Lyme borreliosis and tick-borne encephalitis. Tick-borne pathogens are transmitted in tick saliva during blood feeding; however, saliva is not simply a medium enabling pathogen transfer. Instead, tick-borne pathogens exploit saliva-induced modulation of host responses to promote their transmission and infection, so-called saliva-assisted transmission (SAT). Characterization of the saliva factors that facilitate SAT is an active area of current research. Besides providing new insights into how tick-borne pathogens survive in nature, the research is opening new avenues for vaccine development.

Innate immune transcriptomic evaluation of PBMC isolated from sheep after infection with E. ruminantium Welgevonden strain

Heartwater is a tick-borne non-infectious fatal disease of wild and domestic ruminants caused by the bacterium Ehrlichia ruminantium, transmitted by Amblyomma ticks. Although there is evidence that interferon-gamma (IFN-γ) controls E. ruminantium growth and that cellular immune responses could be protective, an effective recombinant vaccine for this disease is lacking. An overall analysis of which immune pathways are up- or down-regulated in sheep peripheral blood mononuclear cells is expected to lead to a better understanding of the global immune response of sheep to E. ruminantium infection. Therefore, a systems biology oriented approach following the infection with E. ruminantium was investigated from peripheral blood mononuclear cells to aid recombinant vaccine development. In this study, heartwater naïve sheep were infected and challenged by allowing E. ruminantium infected ticks to feed on them. After primary infection, all the animals were treated with antibiotic during the resulting febrile response. Blood was collected daily for E. ruminantium detection by qPCR (pCS20 assay). The pCS20 assay only detected the pathogen in the blood one day prior to and during the febrile stage of infection confirming infection of the sheep. IFN-γ real-time PCR indicated that this cytokine was expressed at specific time points: post infection, during the febrile stage of the disease and after challenge. These were used as a guide to select samples for transcriptome sequencing. This paper focuses on transcripts that are associated with innate activating pathways that were identified to be up- and down-regulated after primary infection and the subsequent challenge. These included the CD14 monocyte marker, toll-like receptor (TLR), nod-like receptor, chemokine, cytosolic and cytokine-cytokine interaction receptor pathways. In particular, TLR4, TLR9 and CD14 were activated together with DNA detection pathways, suggesting that vaccine formulations may be improved if CpG motifs and lipopolysaccharides are included. This data indicates that innate immune activation, perhaps by using adjuvants, should be an important component for consideration during future heartwater recombinant vaccine development.

Transcriptomic analysis of Ehrlichia ruminantium during the developmental stages in bovine and tick cell culture

The use of bioinformatics tools to search for possible vaccine candidates has been successful in recent years. In an attempt to search for additional vaccine candidates or improve the current heartwater vaccine design, a genome-wide transcriptional profile of E. ruminantium (Welgevonden strain) replicating in bovine endothelial cells (BA886) and Ixodes scapularis embryonic tick cells (IDE8) was performed. The RNA was collected from the infective extracellular form, the elementary bodies (EBs) and vegetative intracellular form, reticulate bodies (RBs) and was used for transcriptome sequencing. Several genes previously implicated with adhesion, attachment and pathogenicity were exclusively up-regulated in the EBs from bovine and tick cells. Similarly, genes involved in adaptation or survival of E. ruminantium in the host cells were up-regulated in the RBs from bovine cells. Thus, it was concluded that those genes expressed in the EBs might be important for infection of mammalian and tick host cells and these may be targets for both cell and humoral mediated immune responses. Alternatively, those exclusively expressed in the RBs may be important for survival in the host cells. Exported or secreted proteins exclusively expressed at this stage are ideal targets for the stimulation of cytotoxic T-lymphocyte (CTL) immune responses in the host.

Ehrlichioses: An Important One Health Opportunity

Ehrlichioses are caused by obligately intracellular bacteria that are maintained subclinically in a persistently infected vertebrate host and a tick vector. The most severe life-threatening illnesses, such as human monocytotropic ehrlichiosis and heartwater, occur in incidental hosts. Ehrlichia have a developmental cycle involving an infectious, nonreplicating, dense core cell and a noninfectious, replicating reticulate cell. Ehrlichiae secrete proteins that bind to host cytoplasmic proteins and nuclear chromatin, manipulating the host cell environment to their advantage. Severe disease in immunocompetent hosts is mediated in large part by immunologic and inflammatory mechanisms, including overproduction of tumor necrosis factor α (TNF-α), which is produced by CD8 T lymphocytes, and interleukin-10 (IL-10). Immune components that contribute to control of ehrlichial infection include CD4 and CD8 T cells, natural killer (NK) cells, interferon-γ (IFN-γ), IL-12, and antibodies. Some immune components, such as TNF-α, perforin, and CD8 T cells, play both pathogenic and protective roles. In contrast with the immunocompetent host, which may die with few detectable organisms owing to the overly strong immune response, immunodeficient hosts die with overwhelming infection and large quantities of organisms in the tissues. Vaccine development is challenging because of antigenic diversity of E. ruminantium, the necessity of avoiding an immunopathologic response, and incomplete knowledge of the protective antigens.

Cellular immune responses induced <i>in vitro</i> by <i>Ehrlichia ruminantium</i> secreted proteins and identification of vaccine candidate peptides

Secreted proteins are reported to induce cell-mediated immunity characterised by the production of interferon-gamma (IFN)-γ. In this study three open reading frames (ORFs) (Erum8060, Erum7760, Erum5000) encoding secreted proteins were selected from the Ehrlichia ruminantium (Welgevonden) genome sequence using bioinformatics tools to determine whether they induce a cellular immune response in vitro with mononuclear cells from needle and tick infected animals. The whole recombinant protein of the three ORFs as well as four adjacent fragments of the Erum5000 protein (Erum5000A, Erum5000B, Erum5000C, Erum5000D) were successfully expressed in a bacterial expression system which was confirmed by immunoblots using anti-His antibodies and sheep sera. These recombinant proteins were assayed with immune sheep and cattle peripheral blood mononuclear cells (PBMCs), spleen and lymph node (LN) cells to determine whether they induce recall cellular immune responses in vitro. Significant proliferative responses and IFN-γ production were evident for all recombinant proteins, especially Erum5000A, in both ruminant species tested. Thus overlapping peptides spanning Erum5000A were synthesised and peptides that induce proliferation of memory CD4⁺ and CD8⁺ T cells and production of IFN-γ were identified. These results illustrate that a Th1 type immune response was elicited and these recombinant proteins and peptides may therefore be promising candidates for development of a heartwater vaccine.

Vaccination with an Attenuated Mutant of Ehrlichia chaffeensis Induces Pathogen-Specific CD4+ T Cell Immunity and Protection from Tick-Transmitted Wild-Type Challenge in the Canine Host

Ehrlichia chaffeensis is a tick-borne rickettsial pathogen and the causative agent of human monocytic ehrlichiosis. Transmitted by the Amblyomma americanum tick, E. chaffeensis also causes disease in several other vertebrate species including white-tailed deer and dogs. We have recently described the generation of an attenuated mutant strain of E. chaffeensis, with a mutation in the Ech_0660 gene, which is able to confer protection from secondary, intravenous-administered, wild-type E. chaffeensis infection in dogs. Here, we extend our previous results, demonstrating that vaccination with the Ech_0660 mutant protects dogs from physiologic, tick-transmitted, secondary challenge with wild-type E. chaffeensis; and describing, for the first time, the cellular and humoral immune responses induced by Ech_0660 mutant vaccination and wild-type E. chaffeensis infection in the canine host. Both vaccination and infection induced a rise in E. chaffeensis-specific antibody titers and a significant Th1 response in peripheral blood as measured by E. chaffeensis antigen-dependent CD4⁺ T cell proliferation and IFNγ production. Further, we describe for the first time significant IL-17 production by peripheral blood leukocytes from both Ech_0660 mutant vaccinated animals and control animals infected with wild-type E. chaffeensis, suggesting a previously unrecognized role for IL-17 and Th17 cells in the immune response to rickettsial pathogens. Our results are a critical first step towards defining the role of the immune system in vaccine-induced protection from E. chaffeensis infection in an incidental host; and confirm the potential of the attenuated mutant clone, Ech_0660, to be used as a vaccine candidate for protection against tick-transmitted E. chaffeensis infection.

A Tick Vector Transmission Model of Monocytotropic Ehrlichiosis

BACKGROUND

Ehrlichioses are emerging, tick-borne diseases distributed worldwide. Previously established animal models use needle inoculation as a mode of infection; however, there is limited representation of natural transmission in artificially inoculated models compared with transmission by the tick vector. The objective of this study was to develop a tick vector transmission animal model of ehrlichial infection using a human pathogen, Ehrlichia muris-like agent (EMLA).

METHODS

Ixodes scapularis larvae were fed on EMLA-infected mice, and after molting, infected nymphs were used to infest naive animals.

RESULTS

Ehrlichiae were acquired by 90%-100% of feeding larvae. The majority of animals fed upon by infected nymphs developed sublethal infection with 27% lethality. Bacteria disseminated to all tissues tested with greatest bacterial loads in lungs, but also spleen, lymph nodes, liver, kidneys, brain, and bone marrow. Numerous foci of cellular infiltration, mitoses, and hepatocellular death were observed in liver. Mice infected by tick transmission developed higher antiehrlichial antibody levels than needle-inoculated animals. Tick-feeding-site reactions were observed, but there was no observed difference between animals infested with infected or uninfected ticks.

CONCLUSIONS

For the first time we were able to develop a tick transmission model with an Ehrlichia that is pathogenic for humans.

A user-friendly and scalable process to prepare a ready-to-use inactivated vaccine: the example of heartwater in ruminants under tropical conditions

The use of cheap and thermoresistant vaccines in poor tropical countries for the control of animal diseases is a key issue. Our work aimed at designing and validating a process for the large-scale production of a ready-to-use inactivated vaccine for ruminants. Our model was heartwater caused by the obligate intracellular bacterium Ehrlichia ruminantium (ER). The conventional inactivated vaccine against heartwater (based on whole bacteria inactivated with sodium azide) is prepared immediately before injection, using a syringe-extrusion method with Montanide ISA50. This is a fastidious time-consuming process and it limits the number of vaccine doses available. To overcome these issues, we tested three different techniques (syringe, vortex and homogenizer) and three Montanide ISA adjuvants (50, 70 and 70M). High-speed homogenizer was the optimal method to emulsify ER antigens with both ISA70 and 70M adjuvants. The emulsions displayed a good homogeneity (particle size below 1 μm and low phase separation), conductivity below 10 μS/cm and low antigen degradation at 4 °C for up to 1 year. The efficacy of the different formulations was then evaluated during vaccination trials on goats. The inactivated ER antigens emulsified with ISA70 and ISA70M in a homogenizer resulted in 80% and 100% survival rates, respectively. A cold-chain rupture assay using ISA70M+ER was performed to mimic possible field conditions exposing the vaccine at 37 °C for 4 days before delivery. Surprisingly, the animal survival rate was still high (80%). We also observed that the MAP-1B antibody response was very similar between animals vaccinated with ISA70+ER and ISA70M+ER emulsions, suggesting a more homogenous antigen distribution and presentation in these emulsions. Our work demonstrated that the combination of ISA70 or ISA70M and homogenizer is optimal for the production of an effective ready-to-use inactivated vaccine against heartwater, which could easily be produced on an industrial scale.

Development of multiple-locus variable-number tandem-repeat analysis for rapid genotyping ofEhrlichia ruminantiumand its application to infectedAmblyomma variegatumcollected in heartwater endemic areas in Uganda

The rickettsial bacteriumEhrlichia ruminantiumis the causative agent of heartwater, a serious tick-borne disease in ruminants. The genetic diversity of organisms in the field will have implications for cross-protective capacities of any vaccine developed, and for an effective vaccine design strategy proper genotyping and understanding of existing genetic diversity in the field is necessary. We searched for variable-number tandem-repeat (VNTR) loci for use in a multi-locus VNTR analysis (MLVA). Sequencing analysis of 30 potential VNTRs using a panel of 17 reference strains from geographically diverse origins identified 12 VNTRs with allelic profiles differing between strains. Application of MLVA to 38E. ruminantium-infectedAmblyomma variegatumcollected from indigenous cattle in 6 different districts of Uganda identified 21 MLVA types. The discriminatory power of MLVA was greater than that ofmap1PCR-restriction fragment length polymorphism analysis, with which only 6 genotypes were obtained. The high discriminatory power as well as cost- effective performance of MLVA provide the potential for this technique to be applied in the future with respect to optimizing vaccine trials by identifying local strain diversity, and also raise the possibility of exploring the association betweenE. ruminantiumgenotypes and phenotypes such as pathological outcome in the ruminant host.

Preparation and in vitro characterisation of Ehrlichia ruminantium plasmid DNA and proteins encapsulated into and DNA adsorbed onto biodegradable microparticles

Four E. ruminantium 1H12 open reading frames and their proteins known to protect sheep against heartwater needle challenge were encapsulated into, or adsorbed onto poly(d,l-lactide-co-glycolide) microparticles. Microspheres with smooth surface and smaller than 5 μm diameters were produced, with high adsorption and encapsulation efficiencies. Gel electrophoresis showed that neither encapsulation nor adsorption affected the stability of the DNA or proteins. Cationic microparticles released ∼40% of plasmid DNA on day 1 while PLGA 50:50-COOH microparticles co-encapsulating plasmid DNA and polyvinyl alcohol only started to release from days 12-28. Recombinant proteins were released from PLGA 85:15 and homopolymer R 203 S microparticles in a biphasic manner with a high initial burst release (∼45-80%). In contrast, PLGA 50:50 microparticles had low (15-65%) initial burst release followed by (25-80%) release by days (days 28-42). A cocktail of these microparticles could therefore be used as single-dose auto-booster vaccine.

A novel candidate HIV vaccine vector based on the replication deficient Capripoxvirus, Lumpy skin disease virus (LSDV)

BACKGROUND

The Capripoxvirus, Lumpy skin disease virus (LSDV) has a restricted host-range and is being investigated as a novel HIV-1 vaccine vector. LSDV does not complete its replication cycle in non-ruminant hosts.

METHODS

The safety of LSDV was tested at doses of 104 and 106 plaque forming units in two strains of immunocompromised mice, namely RAG mice and CD4 T cell knockout mice. LSDV expressing HIV-1 subtype C Gag, reverse transcriptase (RT), Tat and Nef as a polyprotein (Grttn), (rLSDV-grttn), was constructed. The immunogenicity of rLSDV-grttn was tested in homologous prime-boost regimens as well as heterologous prime-boost regimes in combination with a DNA vaccine (pVRC-grttn) or modified vaccinia Ankara vaccine (rMVA-grttn) both expressing Grttn.

RESULTS

Safety was demonstrated in two strains of immunocompromised mice.In the immunogenicity experiments mice developed high magnitudes of HIV-specific cells producing IFN-gamma and IL-2. A comparison of rLSDV-grttn and rMVA-grttn to boost a DNA vaccine (pVRC-grttn) indicated a DNA prime and rLSDV-grttn boost induced a 2 fold (p < 0.01) lower cumulative frequency of Gag- and RT-specific IFN-γ CD8 and CD4 cells than a boost with rMVA-grttn. However, the HIV-specific cells induced by the DNA vaccine prime rLSDV-grttn boost produced greater than 3 fold (p < 0.01) more IFN- gamma than the HIV-specific cells induced by the DNA vaccine prime rMVA-grttn boost. A boost of HIV-specific CD4 cells producing IL-2 was only achieved with the DNA vaccine prime and rLSDV-grttn boost. Heterologous prime-boost combinations of rLSDV-grttn and rMVA-grttn induced similar cumulative frequencies of IFN- gamma producing Gag- and RT-specific CD8 and CD4 cells. A significant difference (p < 0.01) between the regimens was the higher capacity (2.1 fold) of Gag-and RT-specific CD4 cells to produce IFN-γ with a rMVA-grttn prime - rLSDV-grttn boost. This regimen also induced a 1.5 fold higher (p < 0.05) frequency of Gag- and RT-specific CD4 cells producing IL-2.

CONCLUSIONS

LSDV was demonstrated to be non-pathogenic in immunocompromised mice. The rLSDV-grttn vaccine was immunogenic in mice particularly in prime-boost regimens. The data suggests that this novel vaccine may be useful for enhancing, in particular, HIV-specific CD4 IFN- gamma and IL-2 responses induced by a priming vaccine.

Progress and obstacles in vaccine development for the ehrlichioses

Ehrlichia are tick-borne obligately intracellular bacteria that cause significant diseases in veterinary natural hosts, including livestock and companion animals, and are now considered important zoonotic pathogens in humans. Vaccines are needed for these veterinary and zoonotic human pathogens, but many obstacles exist that have impeded their development. These obstacles include understanding genetic and antigenic variability, influence of the host on the pathogen phenotype and immunogenicity, identification of the ehrlichial antigens that stimulate protective immunity and those that elicit immunopathology, development of animal models that faithfully reflect the immune responses of the hosts and understanding molecular host-pathogen interactions involved in immune evasion or that may be blocked by the host immune response. We review the obstacles and progress in addressing barriers associated with vaccine development to protect livestock, companion animals and humans against these host defense-evasive and cell function-manipulative, vector-transmitted pathogens.

Natural history of Ehrlichia ruminantium

Ehrlichia ruminantium is an obligately intracellular proteobacterium which causes a disease known as heartwater or cowdriosis in some wild, and all domestic, ruminants. The organism is transmitted by ticks of the genus Amblyomma, and it is of serious economic importance wherever the natural vectors occur, an area which includes all of sub-Saharan Africa, and several islands in the Caribbean. The disease was first recognized in South Africa in the 19th century, where its tick-borne nature was determined in 1900, but the organism itself was not demonstrated until 1925, when it was recognized to be a rickettsia, initially named Rickettsia ruminantium. It was thus the first species of what are now known as Ehrlichia to be discovered, and most of the early work to elucidate the nature of the organisms, and its reservoirs and vectors, was performed in South Africa. The next milestone was the development, in 1945, of an infection and treatment regimen to immunize livestock, and this is still the only commercially available "vaccine" against the disease. Then in 1985, after fruitless attempts over many years, the organism was propagated reliably in tissue culture, opening the way for the first application of the newly developed techniques of molecular genetics. From 1990 onwards the pace of heartwater research accelerated rapidly, with notable advances in phylogeny, diagnosis, epidemiology, immunology, and vaccine development. The complete genome sequence was published in 2005, and during the last two years a new understanding has arisen of the remarkable genetic variability of the organism and new experimental vaccines have been developed. Despite all this the goal of producing an effective vaccine against the disease in the field still remains frustratingly just beyond reach. This article summarises our current understanding of the nature of E. ruminantium, at a time when the prospects for the development of an effective vaccine against the organism seem better than at any time since its discovery 83 years ago.

Antibiotic resistance free plasmid DNA expressing LACK protein leads towards a protective Th1 response against Leishmania infantum infection

Canine visceral leishmaniasis is a serious public health concern in the Mediterranean basin since dogs are the main Leishmania infantum reservoir. However, there is not a vaccination method in veterinary use in this area, and therefore the development of a vaccine against this parasite is essential for the possible control of the disease. Previous reports have shown the efficacy of heterologous prime-boost vaccination with the pCIneo plasmid and the poxvirus VV (both Western Reserve and MVA strains) expressing L. infantum LACK antigen against canine leishmaniasis. As pCIneo-LACK plasmid contains antibiotic resistance genes, its use as a profilactic method is not recommended. Hence, the antibiotic resistance gene free pORT-LACK plasmid is a more suitable tool for its use as a vaccine. Here we report the protective and immunostimulatory effect of the prime-boost pORT-LACK/MVA-LACK vaccination tested in a canine experimental model. Vaccination induced a reduction in clinical signs and in parasite burden in the liver, an induction of the Leishmania-specific T cell activation, as well as an increase of the expression of Th1 type cytokines in PBMC and target organs.