A DNA vaccine protects mice against the rickettsial agent Cowdria ruminantium

A glycosylated recombinant subunit candidate vaccine consisting of Ehrlichia ruminantium major antigenic protein1 induces specific humoral and Th1 type cell responses in sheep

Heartwater, or cowdriosis, is a tick-borne disease of domestic and wild ruminants that is endemic in the Caribbean and sub-Saharan Africa. The disease is caused by an intracellular pathogen, Ehrlichia ruminantium and may be fatal within days of the onset of clinical signs with mortality rates of up to 90% in susceptible hosts. Due to the presence of competent tick vectors in North America, there is substantial risk of introduction of heartwater with potentially devastating consequences to the domestic livestock industry. There is currently no reliable or safe vaccine for use globally. To develop a protective DIVA (differentiate infected from vaccinated animals) subunit vaccine for heartwater, we targeted the E. ruminantium immunodominant major antigenic protein1 (MAP1) with the hypothesis that MAP1 is a glycosylated protein and glycans contained in the antigenic protein are important epitope determinants. Using a eukaryotic recombinant baculovirus expression system, we expressed and characterized, for the first time, a glycoform profile of MAP1 of two Caribbean E. ruminantium isolates, Antigua and Gardel. We have shown that the 37–38 kDa protein corresponded to a glycosylated form of the MAP1 protein, whereas the 31–32 kDa molecular weight band represented the non-glycosylated form of the protein frequently reported in scientific literature. Three groups of sheep (n = 3–6) were vaccinated with increasing doses of a bivalent (Antigua and Gardel MAP1) rMAP1 vaccine cocktail formulation with montanide ISA25 as an adjuvant. The glycosylated recombinant subunit vaccine induced E. ruminantium-specific humoral and Th1 type T cell responses, which are critical for controlling intracellular pathogens, including E. ruminantium, in infected hosts. These results provide an important basis for development of a subunit vaccine as a novel strategy to protect susceptible livestock against heartwater in non-endemic and endemic areas.

Immunization with Ehrlichia P28 outer membrane proteins confers protection in a mouse model of ehrlichiosis

The obligately intracellular bacterium Ehrlichia chaffeensis that resides in mononuclear phagocytes is the etiologic agent of human monocytotropic ehrlichiosis (HME). HME is an emerging and often life-threatening, tick-transmitted infectious disease in the United States. Effective primary immune responses against Ehrlichia infection involve generation of Ehrlichia-specific gamma interferon (IFN-γ)-producing CD4(+) T cells and cytotoxic CD8(+) T cells, activation of macrophages by IFN-γ, and production of Ehrlichia-specific antibodies of the Th1 isotype. Currently, there are no vaccines available against HME. We evaluated the ability of 28-kDa outer membrane proteins (P28-OMP-1) of the closely related Ehrlichia muris to stimulate long-term protective memory T and B cell responses and confer protection in mice. The spleens of mice vaccinated with E. muris P28-9, P28-12, P28-19, or a mixture of these three P28 proteins (P28s) using a DNA prime-protein boost regimen and challenged with E. muris had significantly lower bacterial loads than the spleens of mock-vaccinated mice. Mice immunized with P28-9, P28-12, P28-19, or the mixture induced Ehrlichia-specific CD4(+) Th1 cells. Interestingly, mice immunized with P28-14, orthologs of which in E. chaffeensis and E. canis are primarily expressed in tick cells, failed to lower the ehrlichial burden in the spleen. Immunization with the recombinant P28-19 protein alone also significantly decreased the bacterial load in the spleen and liver compared to those of the controls. Our study reports, for the first time, the protective roles of the Ehrlichia P28-9 and P28-12 proteins in addition to confirming previous reports of the protective ability of P28-19. Partial protection induced by immunization with P28-9, P28-12, and P28-19 against Ehrlichia was associated with the generation of Ehrlichia-specific cell-mediated and humoral immune responses.

Animal research ethics in Africa: an overview

With research projects that use animals on the increase worldwide and in Africa in particular, animal research ethics should continue to be reviewed to improve the welfare of animals used in research. The welfare of animals used in research has gained attention globally that has led to the development of guidelines and in some instances national laws governing animal experimentation. Although there may not be empirical data on the existence or adequacy of national and/or institutional policies and guidelines on the use of animals in research in Africa, most African countries are not yet at the same level as developed countries. Consequently, some researchers based at institutions in developed countries may be tempted to 'export' their research activities to collaborating African institutions where ethical and legal frameworks on use of animals may be less stringent than in the developed countries. An appreciation of the intrinsic value of animals should be enough driving force for human beings to strive to promote humane treatment of animals. This paper gives an overview of uses and potential abuses of animals in research for the benefit of stakeholders such as researchers, research institutions, Ethics Review Committees (ERCs) and policy makers and ends with suggestions on possible ways of ensuring humane treatment of animals used in research in Africa in particular and globally in general.

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.

Diversity of Ehrlichia ruminantium major antigenic protein 1-2 in field isolates and infected sheep

Proteins expressed from the map1 multigene family of Ehrlichia ruminantium are strongly recognized by immune T and B cells from infected animals or from animals that were infected and have recovered from heartwater disease (although still remaining infected carriers). Analogous multigene clusters also encode the immunodominant outer membrane proteins (OMPs) in other ehrlichial species. Recombinant protein analogs of the expressed genes and DNA vaccines based on the multigene clusters have been shown to induce protective immunity, although this was less effective in heterologous challenge situations, where the challenge strain major antigenic protein 1 (MAP1) sequence differed from the vaccine strain MAP1. Recent data for several ehrlichial species show differential expression of the OMPs in mammalian versus tick cell cultures and dominant expression of individual family members in each type of culture system. However, many genes in the clusters appear to be complete and functional and to generate mRNA transcripts. Recent data also suggest that there may be a low level of protein expression from many members of the multigene family, despite primary high-level expression from an individual member. A continuing puzzle, therefore, is the biological roles of the different members of these OMP multigene families. Complete genome sequences are now available for two geographically divergent strains of E. ruminantium (Caribbean and South Africa strains). Comparison of these sequences revealed amino acid sequence diversity in MAP1 (89% identity), which is known to confer protection in a mouse model and to be the multigene family member primarily expressed in mammalian cells. Surprisingly, however, the greatest sequence diversity (79% identity) was in the less-studied map1-2 gene. We investigated here whether this map1-2 diversity was a general feature of E. ruminantium in different cultured African strains and in organisms from infected sheep. Comparison of MAP1-2s revealed amino acid identities of 75 to 100% (mean of 86%), compared to 84 to 100% (mean of 89%) for MAP1s. Interestingly, MAP1-2s varied independently of MAP1s such that E. ruminantium strains with similar MAP1s had diverse MAP1-2s and vice versa. Different MAP1-2s were found in individual infected sheep. Different regions of a protein may be subjected to different evolutionary forces because of recombination and/or selection, which results in those regions not agreeing with a phylogeny deduced from the whole molecule. This appears to be true for both MAP1 and MAP1-2, where statistical likelihood methods detect heterogeneous evolutionary rates for segments of both molecules. Sera from infected cattle recognized a MAP1-2 variable-region peptide in enzyme-linked immunosorbent assay, but less strongly and consistently than a MAP1 peptide (MAP1B). Heterologous protective immunity may depend on recognition of a complex set of varying OMP epitopes.

Immunotherapy of malignant brain tumors

Despite aggressive multi-modality therapy including surgery, radiation, and chemotherapy, the prognosis for patients with malignant primary brain tumors remains very poor. Moreover, the non-specific nature of conventional therapy for brain tumors often results in incapacitating damage to surrounding normal brain and systemic tissues. Thus, there is an urgent need for the development of therapeutic strategies that precisely target tumor cells while minimizing collateral damage to neighboring eloquent cerebral cortex. The rationale for using the immune system to target brain tumors is based on the premise that the inherent specificity of immunologic reactivity could meet the clear need for more specific and precise therapy. The success of this modality is dependent on our ability to understand the mechanisms of immune regulation within the central nervous system (CNS), as well as counter the broad defects in host cell-mediated immunity that malignant gliomas are known to elicit. Recent advances in our understanding of tumor-induced and host-mediated immunosuppressive mechanisms, the development of effective strategies to combat these suppressive effects, and a better understanding of how to deliver immunologic effector molecules more efficiently to CNS tumors have all facilitated significant progress toward the realization of true clinical benefit from immunotherapeutic treatment of malignant gliomas.

Defining the immune response to Ehrlichia species using murine models

Pathogenic bacteria belonging to the family Anaplasmataceae include species of the genera Ehrlichia and Anaplasma. Ehrlichia chaffeensis, first known as the causative agent of human monocytic ehrlichiosis, also infects several vertebrate hosts including white-tailed deer, dogs, coyotes and goats. E. chaffeensis is transmitted from the bite of an infected hard tick, such as Amblyomma americanum. E. chaffeensis and other tick-transmitted pathogens have adapted to both the tick and vertebrate host cell environments. Although E. chaffeensis persists in both vertebrate and tick hosts for long periods of time, little is known about that process. Immunological studies will be valuable in assessing how the pathogen persists in nature in both vertebrate and invertebrate hosts. Understanding the host immune response to the pathogen originating from dual host backgrounds is also important to develop effective methods of diagnosis, control and treatment. In this paper, we provide our perspective of the current understanding of the immune response against E. chaffeensis in relation to other related Anaplasmataceae pathogens.

Protection against heartwater by DNA immunisation with four Ehrlichia ruminantium open reading frames

We have reported previously that a recombinant DNA vaccine consisting of four Ehrlichia ruminantium (Welgevonden) open reading frames (ORFs) known as the 1H12 cocktail provided protection against a virulent E. ruminantium (Welgevonden) needle challenge in sheep. In this study, we have investigated the vaccine effectiveness of two other cocktails of E. ruminantium (Welgevonden) ORFs, as well as single ORFs from the 1H12 cocktail, to protect sheep against a virulent needle challenge with the homologous strain. Each individual 1H12 ORF provided protection, but all the animals vaccinated with the other cocktails succumbed to the challenge.

Evaluation of E. ruminantium Genes in DBA/2 Mice as Potential DNA Vaccine Candidates for Control of Heartwater

Heartwater caused by the rickettsia Ehrlichia ruminantium (E. ruminantium) is an acute and fatal tick-borne disease of domestic and some wild ruminants. A user-friendly vaccine does not exist. We selected and tested nine genes of E. ruminantium for protection against challenge in a DBA/2 mouse model, in order to identify candidate genes for incorporation into a recombinant vaccine. Of the nine DNA vaccine constructs tested, four DNA constructs 14HWORF1/VR1012, 14HWORF2/VR1012, 27HWORF1/VR1012, and HSP58/VR1012 were not protective and were excluded from the study. The remaining five DNA constructs-MAP2/ VR1012, 1HWORF3/ VR1012, 4HWORF1/ VR1012, 18HWORF1/ VR1012, and 3GDORF3/ VR1012-offered partial protection against lethal challenge demonstrated by reduced mortalities compared to control groups. Protection was augmented when DNA primed mice were boosted with a respective homologous recombinant protein. Protection in these five groups was associated with the induction of cell-mediated or T helper 1 (Th1) type of immune responses characterized by the production of large amounts of interferon-gamma and interleukin-2 in in vitro proliferation assays using E. ruminantium antigens for stimulation. These responses were enhanced when the DNA-vaccinated DBA/2 mice were boosted with specific homologous recombinant protein vaccination. In a preliminary follow-up study, protection conferred by DNA vaccination with individual gene constructs was not enhanced when the protective constructs were administered in combination (including the map-1 gene of E. ruminantium). Further evaluation of these and other untested DNA constructs is necessary to optimize their expression in vivo in the presence of molecular adjuvants, such as the IFN-gamma gene, GM-CSF gene, IL-12 gene, and CpG motifs to fully evaluate their protective value.

Mechanisms of immunity to Ehrlichia muris: a model of monocytotropic ehrlichiosis

Ehrlichia species can cause life-threatening infections or chronic persistent infections. Mechanisms of protective immunity were examined in an Ehrlichia muris mouse model of monocytotropic ehrlichiosis. C57BL/6 mice possessed strong genetic resistance to E. muris of an undetermined mechanism. CD8 T lymphocytes were particularly important, as revealed by 81% fatalities for E. muris-infected, major histocompatibility complex class I gene knockout mice compared with no deaths for wild-type C3H mice. Moreover, 80% of C3H mice depleted of CD8 and CD4 cells died of E. muris infection compared with only 44% of CD4 cell-depleted mice. CD8 T lymphocytes were demonstrated for the first time in an Ehrlichia infection to exhibit cytotoxic T-lymphocyte activity against Ehrlichia-infected target cells. Both gamma interferon and tumor necrosis factor were shown to play synergistic roles in protective immunity in vivo for the first time, as demonstrated by 75% fatalities when both cytokines were neutralized compared with minimal mortality when they were depleted separately. Passive transfer of antibodies, but not Fab fragments, to E. muris protected C3H/SCID mice against lethal infection. The mechanism of increased susceptibility (22% lethality) of C57BL/6 major histocompatibility complex class II gene knockout mice and CD4 cell-depleted C3H mice (i.e., through a gamma interferon or antibody mechanism), as well as the more important role of CD8 T lymphocytes (in the form of cytotoxic T-lymphocyte activity and/or gamma interferon production), remains to be elucidated. Protective immunity against monocytotropic E. muris is mediated by a combination of CD8 and CD4 T lymphocytes, gamma interferon, tumor necrosis factor alpha, and antibodies.

Analysis of Ehrlichia ruminantium-specific T1/T2 responses during vaccination with a protective killed vaccine and challenge of goats

Ehrlichia ruminantium is an obligate intracellular bacterium that causes heartwater in ruminants and for which T-cell-mediated immunity is believed to play an important role in protection. To better characterize protective cellular immunity, E. ruminantium-specific IFN-gamma and IL-4 recall responses in major T-cell subsets were analysed by flow cytometry during immunization of goats with a killed vaccine and following a virulent challenge. The killed vaccine elicited both CD8+ and CD4+ subsets to produce cytoplasmic IFN-gamma in the absence of IL-4, thus indicating a biased T1 response. The relative capacity of CD8+ T-cells to produce IFN-gamma was significantly higher than CD4+ T-cells but the final contribution of both subsets was comparable. Circulating ER-specific CD4 and CD8 effectors substantially decreased in numbers after the booster injection and could not be detected in most animals during challenge, which warrants further investigation in immune compartments other than blood. Since IFN-gamma inhibits the growth of the pathogen in target cells, the information provided in this study on E. ruminantium-specific T1 responses will be valuable to develop cellular tools for the identification of potential protective antigens.

Development of improved vaccines for heartwater

Heartwater is a tick-borne disease of ruminants which causes major economic losses for domestic livestock owners throughout sub-Saharan Africa and the Caribbean. It is caused by the intracellular rickettsia Ehrlichia (formerly Cowdria) ruminantium and the only commercially available vaccination procedure is over 50 years old. It involves infecting animals with cryopreserved sheep blood containing virulent E. ruminantium organisms, followed by antibiotic treatment when fever develops. Experimental attenuated, inactivated, and nucleic acid vaccine procedures have been investigated over the last half-century, but none of them has yet been particularly successful. We have developed two new experimental vaccines, a live attenuated vaccine and a nucleic acid vaccine. The attenuated vaccine was developed by continuous passage of E. ruminantium organisms of the virulent Welgevonden isolate in a continuous canine macrophage-monocyte cell line. After more than 50 passages the cultures produced no disease when inoculated into mice or sheep, and the inoculated animals were 100% immune to a subsequent lethal homologous needle challenge. The nucleic acid vaccine is based on four E. ruminantium genes from a genetic locus involved in nutrient transport. A cocktail of all four genes, cloned in a DNA vaccine vector and used to immunize sheep, engendered 100% protection against a subsequent lethal needle challenge with the homologous isolate and with each of five different virulent heterologous isolates. Sheep immunized with this cocktail were also exposed to a field challenge in a heartwater-endemic area and few animals survived. This suggests that the local E. ruminantium genotypes were different from any which were administered by needle challenge, or that needle challenge is not a good model for tick challenge in the field.

Kinetics of antibody response to Ehrlichia canis immunoreactive proteins

Immunoreactive proteins of Ehrlichia canis and Ehrlichia chaffeensis that have been characterized include a family of 28-kDa major outer membrane proteins (p28) and two large antigenically divergent surface glycoprotein orthologs. We previously demonstrated that recombinant E. canis p28 and the 140- and 200-kDa glycoproteins gp140 and gp200, respectively, react strongly with serum antibodies from suspect canine ehrlichiosis cases that were positive for E. canis by immunofluorescent antibody test and in various phases of acute or chronic infection (J. Clin. Microbiol. 39:315-322, 2001). The kinetics of the antibody response to these potentially important vaccine and immunodiagnostic candidates is not known. Acute-phase serum antibody responses to whole-cell E. canis lysates and recombinant p28, gp140, and gp200 were monitored for 6 weeks in dogs experimentally infected with E. canis. Irrespective of the inoculation route, a T-helper 1-type response was elicited to E. canis antigens consisting of immunoglobulin G2 antibodies exclusively in both acute and convalescent phases in most dogs. Analysis of immuoreactive antigens for peak intensity and relative quantity identified major immunoreactive E. canis antigens recognized early in the infection as the 19-, 37-, 75-, and 140-kDa proteins. Later in infection, additional major immunoreactive E. canis proteins were identified, including the 28-, 47-, and 95-kDa proteins and the recently identified 200-kDa glycoprotein. All dogs had developed antibody against the recombinant gp140, gp200, and p28 in the convalescent phase. Immunoreactivity and antibody response kinetics suggest that major immunoreactive proteins identified are immunodominant, but early recognition suggests increased dominance by some antigens.

Preparation of Ehrlichia ruminantium challenge material for quantifiable and reproducible challenge in mice and sheep

The causative agent of heartwater, Ehrlichia ruminantium, is a tick-transmitted pathogen that infects bovine endothelial cells. Due to the obligate intracellular nature of this organism obtaining pure material in sufficient quantities for challenge studies is difficult. A murine model is frequently used to study potential vaccine candidates but giving reproducible challenges in this model for heartwater has always been problematic. We have therefore performed a series of experiments to optimize the parameters governing the reproducibility of challenge material. Two cryoprotectants were compared for the preparation of challenge material, buffered lactose peptone (BLP) and sucrose-potassium-glutamate (SPG). In addition two sources of virulent E. ruminantium were used, infected bovine endothelial cultures and infected mouse spleen homogenates. We also examined practical parameters affecting the reproducibility of challenge experiments: the time it takes to deliver the challenge material, the length of time a mouse remains immune to E. ruminantium challenge, and the effect of a given challenge dose. Finally, we performed a pilot study to determine whether mice could be used to titrate challenge material to be used for experiments in sheep. We found that: (a) E. ruminantium-infected mouse spleen homogenate provides more reproducible challenges than tissue culture material; (b) SPG is a better cryoprotectant than BLP; (c) challenge material should be used within 20min of thawing; (d) it is not essential to use syngeneic material for murine challenge experiments; (e) Balb/c mice are more sensitive to E. ruminantium challenge than C57BL/6J mice; (f) mice immunized by infection and treatment for use as positive immune controls should be challenged within 3 months of immunization; and (g) mice should be challenged with a dose not exceeding 10 LD(50)s.

Transcriptional analysis of the major antigenic protein 1 multigene family of Cowdria ruminantium

The major antigenic protein 1 (MAP1) of the tick-borne rickettsial pathogen Cowdria ruminantium is encoded by a multigene family containing conserved and variable genes. The part of a locus containing the map1 multigene family that was characterized contained three homologous, but non-identical map1 genes, designated map1-2, map1-1, and map1. Reverse transcriptase-polymerase chain reaction was used to study the transcriptional activity of these genes in isolates of C. ruminantium grown in bovine endothelial cells, in two different tick cell lines, and in Amblyomma variegatum ticks. The map1 gene was always transcribed, whereas transcription of map1-2 was not detected under any of the tested conditions. The map1-1 gene transcript was detected in A. variegatum ticks, but was not found in virulent C. ruminantium Senegal grown in bovine endothelial cells at 30 or 37 degrees C. Interestingly, transcripts of map1-1 were also found in different passages of the in vitro attenuated Senegal isolate grown in bovine endothelial cells, as well as in the Gardel isolate grown in two tick cell lines. When transcribed, map1-1 was present on a polycistronic messenger together with map1.

Immunisation of cattle against heartwater by infection with Cowdria ruminantium elicits T lymphocytes that recognise major antigenic proteins 1 and 2 of the agent

There is growing evidence that immunity of cattle to Cowdria ruminantium infection is mediated by T lymphocytes. C. ruminantium antigens that stimulate these responses are therefore of considerable importance to the development of a sub-unit vaccine against the disease. We have examined T cell responses against recombinant analogues of the surface-exposed C. ruminantium major antigen 1 (MAP1) a 28.8 kDa protein and MAP2 (21 kDa) antigen in cattle immunised by infection and treatment. Vigorous and sustained proliferative responses to both antigens were observed in peripheral blood mononuclear cells from immune cattle. MAP1-specific responses were predominantly restricted to cluster of differentiation four antigen positive T cells (CD4+ T cells). Reverse transcription polymerase chain reaction (RT-PCR) analysis of cytokine expression by T cell lines derived from this population revealed strong expression of interferon gamma (IFN-gamma), interferon alpha (IFN-alpha), tumour necrosis factor alpha (TNF-alpha), tumour necrosis factor beta (TNF-beta), interleukin-2 receptor alpha (IL-2Ralpha) transcripts, and weak expression of IL-2 and IL-4. Supernatants from these T cell cultures contained IFN-gamma protein. CD4+ T cell clones specific for MAP1 were generated. Two of these clones proliferated in the presence of autologous infected endothelial cells. In contrast, the response to MAP2 was characterised largely by proliferation of gamma delta (gammadelta) T cells. RT-PCR analysis of cytokine expression by T cell lines which were dominated by gammadelta T cells revealed expression of IFN-gamma, TNF-alpha, TNF-beta, IL-2Ralpha transcripts. Supernatants of these T cell cultures also contained IFN-gamma protein. Our findings indicate that immunisation of cattle by infection with C. ruminantium results in generation of MAP1- and MAP2-specific T cell responses that may play a role in protection against the pathogen.

DNA vaccination with map1 gene followed by protein boost augments protection against challenge with Cowdria ruminantium, the agent of heartwater

A DNA vaccine encoding the immunodominant MAP1 protein of Cowdria ruminantium (Crystal Springs (CS) strain) was shown to partially protect DBA/2 mice against homologous lethal challenge. To enhance the protective capacity of this DNA vaccine, the effects of length of interval between vaccinations and of prime-boost regimes were investigated. Increasing the interval between vaccinations from 2 to 12 weeks did not result in better protection (P=0.900). However, boosting DNA vaccine-primed mice with recombinant MAP1 protein significantly augmented protection on homologous challenge in various trials from 13-27 to 53-67% (P<0.050). The augmented protection by the prime-boost regimen correlated with augmented T(H1) type immune responses that were induced by the DNA vaccine. These responses were characterized by production of IFN-gamma, IL-2 and anti-MAP1 antibodies of predominantly IgG2a isotype, and were critical for protection against C. ruminantium infection. Cytokine analyses were done at 48h after in vitro stimulation of splenocytes with C. ruminantium or control antigens. In contrast, splenocytes of DNA vector control mice produced no cytokines and these mice were fully susceptible to challenge. In addition, DBA/2 mice immunized with the recombinant MAP1 protein without DNA vaccine priming produced non-protective T(H2) type immune responses which were characterized by production of IL-4, IL-5, IL-10 and IgG1 anti-MAP1 antibodies. A second DNA vaccine containing map1 gene from the Mbizi strain of C. ruminantium also delivered by a prime-boost regime, conferred less protection against heterologous challenge. Hence, in developing DNA vaccines against heartwater that contain map1 gene, a prime-boost regimen should be adopted and gene sequence heterogeneity of field isolates should also be considered.

New technology for improved vaccine safety and efficacy

Nearly all of the 2000 vaccines presently licensed by the US Department of Agriculture for veterinary use in the United States are conventional vaccines containing either killed or modified live whole bacteria or viruses. Recent advances in molecular biology, immunology, microbiology, and genetics and in understanding microbial pathogenesis have led to the development of a wide variety of new approaches for developing safer and more effective vaccines. This article briefly describes these new technologies and their potential advantages and disadvantages as compared with conventional killed and modified live vaccines.

A subset of Cowdria ruminantium genes important for immune recognition and protection

Cowdria ruminantium causes the tick-borne rickettsial disease of heartwater, which is devastating to livestock production in sub-Saharan Africa. Current diagnosis and control methods are inadequate. We have identified and sequenced a subset of genes encoding recombinant antigens recognized by antibody and peripheral blood mononuclear cells from immune ruminants. The identified genes include many with significant similarity to those of Rickettsia prowazekii, genes predicted to encode different outer membrane proteins and lipoproteins and a gene containing an unusual tandem repeat structure. Evidence is presented for immune protection by recombinant antigens in a mouse model of C. ruminantium infection. These data identify new recombinant antigens for evaluation in vaccines and diagnostic tests to control heartwater.

An efficacious recombinant subunit vaccine against the salmonid rickettsial pathogen Piscirickettsia salmonis

Piscirickettsia salmonis is the aetiological agent of salmonid rickettsial septicaemia, an economically devastating rickettsial disease of farmed salmonids. Infected salmonids respond poorly to antibiotic treatment and no effective vaccine is available for the control of P. salmonis. Bacterin preparations of P. salmonis were found to elicit a dose-dependent response in coho salmon (Oncorhynchus kisutch), which varied from inadequate protection to exacerbation of the disease. However, an outer surface lipoprotein of P. salmonis, OspA, recombinantly produced in Escherichia coli elicited a high level of protection in vaccinated coho salmon with a relative percent survival as high as 59% for this single antigen. In an effort to further improve the efficacy of the OspA recombinant vaccine, T cell epitopes (TCE's) from tetanus toxin and measles virus fusion protein, that are universally immunogenic in mammalian immune systems, were incorporated tandemly into an OspA fusion protein. Addition of these TCE's dramatically enhanced the efficacy of the OspA vaccine, reflected by a three-fold increase in vaccine efficacy. These results represent a highly effective monovalent recombinant subunit vaccine for a rickettsia-like pathogen, P. salmonis, and for the first time demonstrate the immunostimulatory effect of mammalian TCE's in the salmonid immune model. These results may also be particularly pertinent to salmonid aquaculture in which the various subspecies are outbred and of heterologous haplotypes.

Anaplasmosis control. Past, present, and future

Control methods for anaplasmosis have not changed markedly during the past 50 years and include arthropod control, chemoprophylaxsis, vaccination, and maintenance of an Anaplasma-free herd. Control measures implemented vary with geographic location, and depend on availability, cost, and the feasibility of application. Vaccination has been an effective means of preventing outbreaks of anaplasmosis, but these vaccines, both live and inactivated, are dependent on bovine blood as the source of infection or antigen. Blood-derived vaccines are difficult to standardize and bear the risk of transmitting other bovine pathogens inapparent at the time of blood collection. Extensive purification is required to remove bovine cell membranes, which may cause side effects. Most importantly, geographic isolates of A. marginale are often not cross-protective. Development of a tick cell culture system for A. marginale shows promise as a source of antigen for development of an improved inactivated vaccine in the near future that is free from bovine pathogens. Development of an antigenically defined molecular vaccine appears to be a realistic goal, although further research is required to determine epitopes involved in both humoral and cellular immunity, to define antigenic variation during cyclic rickettsemia, and to develop effective delivery systems for optimization of the immune response.

Highly conserved regions of the immunodominant major surface protein 2 of the genogroup II ehrlichial pathogen Anaplasma marginale are rich in naturally derived CD4+ T lymphocyte epitopes that elicit strong recall responses

Genogroup II ehrlichia, including the agent of human granulocytic ehrlichiosis, Ehrlichia phagocytophila, and the bovine pathogen Anaplasma marginale, express a markedly immunodominant outer membrane protein designated major surface protein 2 (MSP2). MSP2 is encoded by a multigene family, resulting in the expression of variant B cell epitopes. MSP2 variants are sequentially expressed in the repeated cycles of rickettsemia that characterize persistent A. marginale infection and control of each rickettsemic cycle is associated with development of a variant-specific IgG response. Importantly, these persistent rickettsemic cycles are controlled at levels 100-1000 times lower than those responsible for clinical disease during acute infection. Control of rickettsemia during persistence could result from an anamnestic Th lymphocyte response to conserved regions of MSP2 that enhances the primary Ab response against newly emergent variants. Comparison of MSP2 variants reveals conserved N and C termini flanking the central, surface-exposed hypervariable region that represents the variant B lymphocyte epitopes. We demonstrate MSP2-specific CD4(+) T lymphocyte recognition of epitopes common to several strains of A. marginale and the related pathogen A. ovis. Furthermore, T lymphocyte lines from three individuals identified six to nine overlapping peptides representing a minimum of four to seven dominant or subdominant epitopes in these conserved N and C termini. Immunodominant peptides induced high levels of IFN-gamma, a cytokine associated with protection against ehrlichia and needed for rapid generation of variant-specific IgG2. The presented data support the potential importance of a strong Th lymphocyte response to invariant MSP2 epitopes in controlling rickettsemia during persistent infection to subclinical levels.

CD8(+) T cell knockout mice are less susceptible to Cowdria ruminantium infection than athymic, CD4(+) T cell knockout, and normal C57BL/6 mice

The role of T cells in immunity to Cowdria ruminantium was investigated by studying the responses to infection of normal, athymic, CD4(+) T cell knock out (KO) and CD8(+) T cell KO C57BL/6 mice. Normal C57BL/6 mice could be immunized by infection and treatment, and immunity was adoptively transferable from immune to naive mice by splenocytes. Following infection, athymic mice died sooner than normal mice (P=0.0017), and could not be immunized by infection and treatment. CD4(+) T cell KO mice were as susceptible to infection as normal mice and could be immunized by infection and treatment. In contrast, CD8(+) T cell KO mice were less susceptible than normal and CD4(+) T cell KO mice and 43% self-cured, while those that died did so after a prolonged incubation period. Antibody responses to C. ruminantium were CD4(+) T cell dependent, because responses were detected in immune normal and CD8(+) T cell KO mice but not in immune CD4(+) KO mice (P=0.005). Since CD8(+) T cell KO mice were less susceptible to infection, and since CD4(+) T cell KO mice could be immunized, it can be concluded that immunity to C. ruminantium can be mediated by both CD4(+) and CD8(+) T cells.

Monoclonal antibody binding to a surface-exposed epitope on Cowdria ruminantium that is conserved among eight strains

Monoclonal antibodies (MAb) binding to Cowdria ruminantium elementary bodies (EB) were identified by enzyme-linked immunosorbent assay, and surface binding of one MAb (446.15) to intact EB was determined by immunofluorescence, immunogold labeling, and transmission electron microscopy. MAb 446.15 bound an antigen of approximately 43 kDa in immunoblots of eight geographically distinct strains. The MAb did not react with Ehrlichia canis antigens or uninfected bovine endothelial cell lysate and may be useful in diagnostic assays and vaccine development.

A polarized Th1 type immune response to Cowdria ruminantium infection is detected in immune DBA/2 mice

Immune responses to Cowdria ruminantium, an intracellular organism that causes heartwater in domestic ruminants, were characterized in a DBA/2 mouse model. Immunity induced by infection and treatment was adoptively transferable by splenocytes and could be abrogated by in vivo depletion of T cells but not by inhibition of nitric oxide synthase using NG-monomethyl-L-arginine. IgG2a and IgG2b C. ruminantium-specific responses were detected in immune mice. Culture supernatants of splenocytes from immune DBA/2 mice, which were stimulated with crude C. ruminantium antigens or recombinant major antigenic proteins 1 or 2, contained significant levels of interferon (IFN)-gamma and interleukin (IL)-6, but insignificant levels of IL-1alpha, IL-2, IL-4, IL-5, IL-10, IL-12, tumor necrosis factor-alpha (TNF), and nitric oxide. A similar response was detected during primary infection, although IFN-gamma levels decreased significantly during clinical illness and then increased following natural or antibiotic-aided recovery. These data support the conclusion that protective immunity to C. ruminantium in DBA/2 mice is mediated by T cells and is associated with a polarized T helper 1 type of immune response. This murine model could be utilized to screen for protective C. ruminantium antigens that provoke Th1 type immune responses and for evaluation of these antigens in recombinant vaccines against heartwater.

Sequence and expression analysis of a surface antigen gene family of the rickettsia Anaplasma marginale

The tick-borne rickettsial organism, Anaplasma marginale, causes a disease in cattle of world-wide economic significance. This disease, anaplasmosis, is characterized by severe hemolytic anemia, high levels of rickettsemia and, often, death in animals over 3years of age. Animals that survive acute infection remain carriers, with continuous sub-microscopic cycles of rickettsemia that can persist for the lifetime of the animal. In the search for potential recombinant immunogens, it was discovered that several surface proteins of A. marginale encode polymorphic multigene families. Despite the small size of the genome (approx. 1250kb), these surface antigen gene families comprise greater than 2% of the genome. We present here a mapping, sequencing and expression analysis of five complete or partial genes encoding MSP1b in a Florida strain of A. marginale. Two genes are complete; they encode mRNA that is translated into polypeptide products. Three genes are incomplete and appear to be derived from the complete genes by a series of segmental intragenic recombinations. In two of the incomplete genes, 5' sequence in the incomplete genes is 3' sequence in the complete genes. Recombination within these gene families may generate diversity in surface antigens through combinatorial rearrangements. This could contribute to persistence in the chronic infections caused by A. marginale and related rickettsiae.

Identification of Cowdria ruminantium antigens that stimulate proliferation of lymphocytes from cattle immunized by infection and treatment or with inactivated organisms

Cowdria ruminantium is an obligate intracellular pathogen that causes heartwater in ruminants. Several findings suggest that T cells play an important role in protection against the disease. In order to identify which proteins are involved in T-cell immunity, C. ruminantium proteins were fractionated by continuous-flow electrophoresis and tested for their ability to stimulate lymphocyte proliferation in vitro. C. ruminantium-infected endothelial cell lysates were fractionated at between 11 and 38 kDa and 50 and 168 kDa on 15 and 7% acrylamide gels, respectively. In an attempt to stimulate the natural infective process, peripheral blood mononuclear cells (PBMC) were obtained from two cattle rendered immune by infection and treatment and assayed in proliferation assays with fractionated proteins. In a parallel study, four cattle were immunized with inactivated C. ruminantium to determine whether their lymphocytes also responded to fractionated proteins. Proliferation assays after immunization by infection and treatment detected no C. ruminantium-specific proliferation in vitro after one vaccination. Proliferation was observed, however, between 1 and 4 weeks after challenge. This was followed by a period of no detectable response, after which the response reappeared. PBMC from animals immunized with inactivated organisms proliferated specifically in response to antigen soon after the first immunization. Only C. ruminantium proteins with low molecular masses of 11, 12, 14 to 17, and 19 to 23 kDa induced proliferative responses by lymphocytes from all six animals. These protein fractions may have potential as vaccine antigens.

Interleukin-12 as an adjuvant promotes immunoglobulin G and type 1 cytokine recall responses to major surface protein 2 of the ehrlichial pathogen Anaplasma marginale

Anaplasma marginale is a tick-transmitted pathogen of cattle closely related to the human ehrlichiae, Ehrlichia chaffeensis and the agent of human granulocytic ehrlichiosis (HGE). These pathogens have in common a structurally conserved outer membrane protein (OMP) designated the major surface protein 2 (MSP-2) in A. marginale and HGE and OMP-1 in E. chaffeensis. Protective immunity against ehrlichial pathogens is believed to require induction of gamma interferon (IFN-gamma) and opsonizing immunoglobulin (Ig) subclasses directed against OMP epitopes that, in concert, activate macrophages for phagocytosis and killing. Because interleukin-12 (IL-12) acts as an adjuvant for protein immunization to induce IFN-gamma and protective immunity against intracellular pathogens, we hypothesized that as an adjuvant with MSP-2, IL-12 would augment type 1 recall responses to A. marginale. IL-12 was coadsorbed with MSP-2 to alum and shown to significantly enhance IFN-gamma production by lymph node cells (LNC) and LNC-derived CD4(+) T-cell lines from immunized calves following recall stimulation with A. marginale. LNC proliferation and IL-2 production were also enhanced in IL-12-treated calves. Elevated recall proliferative responses by peripheral blood mononuclear cells were still evident 9 months after immunization. Serum IgG levels were consistently increased in IL-12 immunized calves, predominantly due to higher IgG1 responses. The results support the use of IL-12 coadsorbed with OMP of ehrlichial pathogens in alum to amplify both antibody and type-1 cytokine responses important for protective immunity.

Bovine CD4+ T-cells lines reactive with soluble and membrane antigens of Cowdria ruminantium

Cowdria-specific CD4+ T-cell lines generated from immunised cattle respond to both soluble and membrane proteins of the agent. Furthermore, the lines produced the Cowdria-inhibitory cytokine IFN-gamma in response to soluble antigens fractionated by gel filtration and FPLC. Activity eluted as a single peak around fraction 15 for all T-cell lines tested. This fraction induced the highest production of IFN-gamma by the lines and was shown by SDS-polyacrylamide gel electrophoresis and silver staining analysis to contain less than 10 different bands ranging from 22 to 32 kDa. Given their high sensitivity and specificity, these short-term CD4+ T-cell lines will be valuable tools for the identification of Cowdria antigens for incorporation in a subunit vaccine.

Biased immunoglobulin G1 isotype responses induced in cattle with DNA expressing msp1a of Anaplasma marginale

Immunization with the native major surface protein 1 (MSP1) (a heterodimer containing disulfide and noncovalently bonded polypeptides designated MSP1a and MSP1b) of the erythrocytic stage of Anaplasma marginale conferred protection against homologous challenge (G. H. Palmer, A. F. Barbet, W. C. Davis, and T. C. McGuire, Science 231:1299-1302, 1986). The MSP1a polypeptide possesses a conserved neutralization-sensitive epitope. In the present study, the immune response to DNA-mediated immunization using msp1a was studied. The plasmid pVCL/MSP1a, which encodes the complete msp1a gene of A. marginale under the control of human cytomegalovirus immediate-early enhancer/promoter and intron A, was constructed. The immune responses elicited by immunization with pVCL/MSP1a into cardiotoxin-induced regenerating muscle were evaluated in mice and cattle. Antibody reactive with native MSP1a was detected in pooled sera of immunized BALB/c mice 3 weeks following primary immunization. Two calves seronegative for A. marginale were immunized four times, at weeks 0, 3, 7, and 13, with pVCL/MSP1a. By 8 weeks, both calves responded to MSP1a with an antibody titer of 1:100, which peaked at 1:1,600 and 1:800 by 16 weeks after the initial immunization. Interestingly, immunoblotting with anti-immunoglobulin G1 (anti-IgG1) and anti-IgG2 specific monoclonal antibodies revealed a restricted IgG1 anti-MSP1a response in both animals. T-lymphocyte lines, established after the fourth immunization, proliferated specifically against A. marginale homogenate and purified MSP1 in a dose-dependent manner. These data provide a basis for an immunization strategy to direct bovine immune responses by using DNA vaccine vectors containing single or multiple genes encoding major surface proteins of A. marginale.

Molecular basis for vaccine development against the ehrlichial pathogen Anaplasma marginale

Anaplasma marginale is a tick-transmitted ehrlichial pathogen causing severe morbidity and mortality in livestock on six continents. Development of safe effective vaccines would be greatly facilitated by identification of the protective immune mechanisms and by understanding how the pathogen evades immune effectors to establish persistent infection. In this article, Guy Palmer and colleagues review recent progress in identifying how defined epitopes induce protective immunity and the role of antigenic variation in these epitopes as a mechanism of persistence.

Immune responses to Cowdria ruminantium infections

Understanding the basis of protective immunity to Cowdria ruminantium will facilitate the development of an effective subunit vaccine against heartwater in ruminants and contribute to a better definition of protective immune mechanisms to obligate intracellular pathogens in general. Until recently, immunological studies of heartwater in ruminants concentrated solely on antibody responses. Since 1995, the mechanisms underlying cell-mediated immunity of heartwater have been analysed. Progress achieved in these areas is discussed here by Philippe Totté and colleagues, with special emphasis on ruminants, the natural hosts of C. ruminantium.

Vaccine strategies for Cowdria ruminantium infections and their application to other ehrlichial infections

Suman Mahan and co-authors review the strategies applied to develop improved vaccines for Cowdria ruminantium infections (heartwater). Inactivated vaccines using cell-cultured C. ruminantium organisms combined with an adjuvant are capable of protecting goats, sheep and cattle against lethal C. ruminantium challenge. Immune responses induced with this vaccine, or after recovery from infection, target outer membrane proteins of C. ruminantium, in particular the major antigenic protein 1 (MAP-1). Genetic immunizations with the gene encoding MAP-1 induce protective T helper cell type 1 responses against lethal challenge in a mouse model. Similarly, homologues of MAP-1 in other phylogenetically and antigenically related ehrlichial agents such as Anaplasma marginale and Ehrlichia chaffeensis are also targets of protective responses. Given the antigenic similarities between the related ehrlichial agents, common strategies of vaccine development could be applied against these agents that cause infections of importance in animals and humans.

The map1 gene of Cowdria ruminantium is a member of a multigene family containing both conserved and variable genes

Heartwater is an economically important disease of ruminants caused by the tick-transmitted rickettsia Cowdria ruminantium. The disease is present in Africa and the Caribbean and there is a risk of spread to the Americas, particularly because of a clinically asymptomatic carrier state in infected livestock and imported wild animals. The causative agent is closely related taxonomically to the human and animal pathogens Ehrlichia chaffeensis and Ehrlichia canis. A dominant immune response of infected animals or people is directed against variable outer membrane proteins of these agents known, in E. chaffeensis and E. canis, to be encoded by polymorphic multigene families. We demonstrate, by sequence analysis, that map1 encoding the major outer membrane protein of C. ruminantium is also encoded by a polymorphic multigene family. Two members of the gene family are located in tandem in the genome. The upstream member, orf2, is conserved, encoding only 2 amino acid substitutions among six different rickettsial strains from diverse locations in Africa and the Caribbean. In contrast, the downstream member, map1, contains variable and conserved regions between strains. Interestingly, orf2 is more closely related in sequence to omp1b of E. chaffeensis than to map1 of C. ruminantium. The regions that differ among orf2, map1, and omp1b correspond to previously identified variable sequences in outer membrane protein genes of E. chaffeensis and E. canis. These data suggest that diversity in these outer membrane proteins may arise by recombination among gene family members and offer a potential mechanism for persistence of infection in carrier animals.

Molecular characterization of a 28 kDa surface antigen gene family of the tribe Ehrlichiae

Antisera against different Ehrlichiae recognize an immunodominant, cross-reacting approximately 28 kDa surface antigen defined as the MAP1 in Cowdria ruminantium. These antigens are considered valuable in developing serodiagnostic tests and recombinant vaccines for Ehrlichiae infections. To evaluate the relationship in three closely related Ehrlichiae, Ehrlichia chaffeensis, Ehrlichia canis, and C. ruminantium, the structure of the 28 kDa antigen genes was analyzed. We describe the cloning and characterization of DNA encoding genes homologous to MAP1 from E. chaffeensis and E. canis. The cloned segment of E. chaffeensis contains one expressed and four transcriptionally silent tandemly arranged, nonidentical genes; the E. canis locus consists of two nonidentical genes. Comparative analysis of these genes revealed the presence of four conserved regions separated by three highly variable regions. B-cell epitope analysis identified three major cross-reacting epitopes that map to the variable regions. Location of the epitopes at the variable regions and the presence of multigene family with only one expressed copy suggest a mechanism of immune evasion in these Ehrlichiae.