Reactive oxygen and nitrogen intermediates and products from polyamine degradation are Babesiacidal in vitro

Pathogenesis of Anemia in Canine Babesiosis: Possible Contribution of Pro-Inflammatory Cytokines and Chemokines-A Review

Canine babesiosis is a tick-borne protozoan disease caused by intraerythrocytic parasites of the genus Babesia. The infection may lead to anemia in infected dogs. However, anemia is not directly caused by the pathogen. The parasite's developmental stages only have a marginal role in contributing to a decreased red blood cell (RBC) count. The main cause of anemia in affected dogs is the immune response to the infection. This response includes antibody production, erythrophagocytosis, oxidative damage of RBCs, complement activation, and antibody-dependent cellular cytotoxicity. Moreover, both infected and uninfected erythrocytes are retained in the spleen and sequestered in micro-vessels. All these actions are driven by pro-inflammatory cytokines and chemokines, especially IFN-γ, TNF-α, IL-6, and IL-8. Additionally, imbalance between the actions of pro- and anti-inflammatory cytokines plays a role in patho-mechanisms leading to anemia in canine babesiosis. This article is a review of the studies on the pathogenesis of anemia in canine babesiosis and related diseases, such as bovine or murine babesiosis and human or murine malaria, and the role of pro-inflammatory cytokines and chemokines in the mechanisms leading to anemia in infected dogs.

A vaccine for human babesiosis: prospects and feasibility

Babesiosis is a tick-borne disease caused by intraerythrocytic Babesia parasites. It is a well-known illness in companion animals and livestock, resulting in substantial economic losses in the cattle industry. Babesiosis is also recognized as an emerging zoonosis of humans in many countries worldwide. There is no vaccine against human babesiosis. Currently, preventive measures are focused on vector avoidance. Although not always effective, treatment includes antimicrobial therapy and exchange transfusion. In this review, we discuss the host's immune response to the parasite, vaccines being used to prevent babesiosis in animals, and lessons from malaria vaccine development efforts to inform the development of a human babesiosis vaccine. An effective human vaccine would be a significant advance towards curtailing this rapidly emerging disease.

Immunization of Cattle With Recombinant Structural Ectodomains I and II of Babesia bovis Apical Membrane Antigen 1 [BbAMA-1(I/II)] Induces Strong Th1 Immune Response

Both strong innate and adaptive immune responses are an important component of protection against intraerythrocytic protozoan parasites. Resistance to bovine babesiosis is associated with interferon (IFN)-γ mediated responses. CD4⁺ T cells and macrophages have been identified as major effector cells mediating the clearance of pathogens. Previously, the apical membrane antigen 1 (AMA-1) was found to significantly induce the immune response inhibiting B. bovis merozoite growth and invasion. However, a detailed characterization of both humoral and cellular immune responses against the structure of B. bovis AMA-1 (BbAMA-1) has not yet been established. Herein, the present study aimed to express the recombinant BbAMA-1 domain I+II protein [rBbAMA-1(I/II)], which is the most predominant immune response region, and to characterize its immune response. As a result, cattle vaccinated with BbAMA-1(I/II) significantly developed high titters of total immunoglobulin (Ig) G antibodies and a high ratio of IgG2/IgG1 when compared to control groups. Interestingly, the BbAMA-1(I/II)-based formulations produced in our study could elicit CD4⁺ T cells and CD8⁺ T cells producing IFN-γ and tumor necrosis factor (TNF)-α. Collectively, the results indicate that immunization of cattle with BbAMA-1(I/II) could induce strong Th1 cell responses. In support of this, we observed the up-regulation of Th1 cytokine mRNA transcripts, including IFN-γ, TNF-α, Interleukin (IL)-2 and IL-12, in contrast to down regulation of IL-4, IL-6 and IL-10, which would be indicative of a Th2 cytokine response. Moreover, the up-regulation of inducible nitric oxide synthase (iNOS) was observed. In conclusion, this is the first report on the in-depth immunological characterization of the response to BbAMA-1. According to our results, BbAMA-1 is recognized as a potential candidate vaccine against B. bovis infection. As evidenced by the Th1 cell response, it could potentially provide protective immunity. However, further challenge-exposure with virulent B. bovis strain in immunized cattle would be needed to determine its protective efficacy.

The Cross-Species Immunity During Acute Babesia Co-Infection in Mice

Babesiosis causes high morbidity and mortality in immunocompromised individuals. An earlier study suggested that lethal Babesia rodhaini infection in murine can be evaded by Babesia microti primary infection via activated macrophage-based immune response during the chronic stage of infection. However, whether the same immune dynamics occur during acute B. microti co-infection is not known. Hence, we used the mouse model to investigate the host immunity during simultaneous acute disease caused by two Babesia species of different pathogenicity. Results showed that B. microti primary infection attenuated parasitemia and conferred immunity in challenge-infected mice as early as day 4 post-primary infection. Likewise, acute Babesia co-infection undermined the splenic immune response, characterized by the significant decrease in splenic B and T cells leading to the reduction in antibody levels and decline in humoral immunity. Interestingly, increased macrophage and natural killer splenic cell populations were observed, depicting their subtle role in the protection. Pro-inflammatory cytokines (i.e. IFN-γ, TNF-α) were downregulated, while the anti-inflammatory cytokine IL-10 was upregulated in mouse sera during the acute phase of Babesia co-infection. Herein, the major cytokines implicated in the lethality caused by B. rodhaini infection were IFN- γ and IL-10. Surprisingly, significant differences in the levels of serum IFN- γ and IL-10 between co-infected survival groups (day 4 and 6 challenge) indicated that even a two-day delay in challenge infection was crucial for the resulting pathology. Additionally, oxidative stress in the form of reactive oxygen species contributed to the severity of pathology during acute babesiosis. Histopathological examination of the spleen showed that the erosion of the marginal zone was more pronounced during B. rodhaini infection, while the loss of cellularity of the marginal zone was less evident during co-infection. Future research warrants investigation of the roles of various immune cell subtypes in the mechanism involved in the protection of Babesia co-infected hosts.

Harnessing Mycobacterium bovis BCG Trained Immunity to Control Human and Bovine Babesiosis

Babesiosis is a disease caused by tickborne hemoprotozoan apicomplexan parasites of the genus Babesia that negatively impacts public health and food security worldwide. Development of effective and sustainable vaccines against babesiosis is currently hindered in part by the absence of definitive host correlates of protection. Despite that, studies in Babesia microti and Babesia bovis, major causative agents of human and bovine babesiosis, respectively, suggest that early activation of innate immune responses is crucial for vertebrates to survive acute infection. Trained immunity (TI) is defined as the development of memory in vertebrate innate immune cells, allowing more efficient responses to subsequent specific and non-specific challenges. Considering that Mycobacterium bovis bacillus Calmette-Guerin (BCG), a widely used anti-tuberculosis attenuated vaccine, induces strong TI pro-inflammatory responses, we hypothesize that BCG TI may protect vertebrates against acute babesiosis. This premise is supported by early investigations demonstrating that BCG inoculation protects mice against experimental B. microti infection and recent observations that BCG vaccination decreases the severity of malaria in children infected with Plasmodium falciparum, a Babesia-related parasite. We also discuss the potential use of TI in conjunction with recombinant BCG vaccines expressing Babesia immunogens. In conclusion, by concentrating on human and bovine babesiosis, herein we intend to raise awareness of BCG TI as a strategy to efficiently control Babesia infection.

Prevalence of tick-borne haemoparasites and their perceived co-occurrences with viral outbreaks of FMD and LSD and their associated factors

Species of Theileria, Babesia, and Anaplasma are Tick-borne pathogens (TBPs) that are prevalent throughout the world, particularly in the tropical and subtropical regions. Associated diseases of Theileriosis, Babesiosis, and Anaplasmosis, respectively, represents a major threat to livestock production in many countries. TBPs have a high prevalence in different geographical locations in Egypt. Foot and mouth disease (FMD) and Lumpy skin disease (LSD) are considered endemic bovine viral diseases in Egypt. Our clinical observations during the epidemics of LSD and FMD viruses showed higher prevalence rates for the TBPs. To investigate this correlation, a total of 670 samples from cattle and buffalo were collected during the summers of 2017 and 2018 distributed throughout ranches and smallholders in two geographical locations in Egypt. Two farms with a recent clinical outbreak of LSD with a total of 270 animals, while the other location included three farms with a recent FMD outbreak with a combined 400 cattle. Examined animals were classified mainly according to age, gender, species, breed (native versus crossbred), and the presence of ticks. Whole blood samples were collected for TBPs and viral (LSD and FMD) examinations, while tissue specimens were collected for detection of FMD and LSD viruses by real-time PCR.

Our results confirmed significantly higher prevalence rates for the TBPs in LSD-positive than LSD-negative animals, while no significant difference could be detected for the prevalence rate of the TBPs in the FMD positive and negative groups. The prevalence of Babesia and Theileria was significantly (P < 0.05) higher in cross-breeds than native cattle. Infections with Anaplasma and co-infections with Babesia-Anaplasma and Theileria-Anaplasma were significantly higher in native than cross-breeds cattle. The intensity of parasitic infection (parasitemia) has a significant difference in the positive groups for the two viruses compared to the negative groups. These results collectively confirming the enhancing role of LSD on the prevalence rate of the haemoprotozoal infections leading to more serious outcomes to the livestock infections, and therefore the control of haemoprotozoal infections should be implemented as a part of viral epidemics control.

Babesia microti Confers Macrophage-Based Cross-Protective Immunity Against Murine Malaria

Malaria and babesiosis, the two primary intraerythrocytic protozoan diseases of humans, have been reported in multiple cases of co-infection in endemic regions. As the geographic range and incidence of arthropod-borne infectious diseases is being affected by climate change, co-infection cases with Plasmodium and Babesia are likely to increase. The two parasites have been used in experimental settings, where prior infection with Babesia microti has been shown to protect against fatal malarial infections in mice and primates. However, the immunological mechanisms behind such phenomena of cross-protection remain unknown. Here, we investigated the effect of a primary B. microti infection on the outcome of a lethal P. chabaudi challenge infection using a murine model. Simultaneous infection with both pathogens led to high mortality rates in immunocompetent BALB/c mice, similar to control mice infected with P. chabaudi alone. On the other hand, mice with various stages of B. microti primary infection were thoroughly immune to a subsequent P. chabaudi challenge. Protected mice exhibited decreased levels of serum antibodies and pro-inflammatory cytokines during early stages of challenge infection. Mice repeatedly immunized with dead B. microti quickly succumbed to P. chabaudi infection, despite induction of high antibody responses. Notably, cross-protection was observed in mice lacking functional B and T lymphocytes. When the role of other innate immune effector cells was examined, NK cell-depleted mice with chronic B. microti infection were also found to be protected against P. chabaudi. Conversely, in vivo macrophage depletion rendered the mice vulnerable to P. chabaudi. The above results show that the mechanism of cross-protection conferred by B. microti against P. chabaudi is innate immunity-based, and suggest that it relies predominantly upon the function of macrophages. Further research is needed for elucidating the malaria-suppressing effects of babesiosis, with a vision toward development of novel tools to control malaria.

Interplay between Attenuation- and Virulence-Factors of Babesia bovis and Their Contribution to the Establishment of Persistent Infections in Cattle

Bovine babesiosis is an acute and persistent tick-borne global disease caused mainly by the intraerythrocytic apicomplexan parasites Babesia bovis and B. bigemina. B. bovis infected erythrocytes sequester in blood capillaries of the host (cytoadhesion), causing malaria-like neurological signs. Cytoadhesion and antigenic variation in B. bovis are linked to the expression of members of the Variant Erythrocyte Surface Antigen (VESA) gene family. Animals that survive acute B. bovis infection and those vaccinated with attenuated strains remain persistently infected, suggesting that B. bovis parasites use immune escape mechanisms. However, attenuated B. bovis parasites do not cause neurological signs in vaccinated animals, indicating that virulence or attenuation factors play roles in modulating parasite virulence phenotypes. Artificial overexpression of the SBP2t11 protein, a defined attenuation factor, was associated with reduced cytoadhesion, suggesting a role for this protein as a key modulator of virulence in the parasite. Hereby, we propose a model that might be functional in the modulation of B. bovis virulence and persistence that relies on the interplay among SBP2t, VESA proteins, cytoadhesion, and the immune responses of the host. Elucidation of mechanisms used by the parasite to establish persistent infection will likely contribute to the design of new methods for the control of bovine babesiosis.

Cholinesterases as markers of the inflammatory process associated oxidative stress in cattle infected by Babesia bigemina

The objective of this study was to assess the influence of an asymptomatic experimental infection by Babesia bigemina on cholinesterase's as markers of the inflammatory process and biomarkers of oxidative imbalance. For this purpose, eight naive animals were used, as follows: four as controls or uninfected; and four infected with an attenuated strain of B. bigemina. Blood samples were collected on days 0, 7 and 11 post-inoculation (PI). Parasitemia was determined by blood smear evaluation, showing that the infection by B. bigemina resulted in mean 0.725 and 0.025% on day 7 and 11 PI, respectively, as well as mild anemia. The activities of acetylcholinesterase, butyrylcholinesterase and catalase were lower, while levels of thiobarbituric acid reactive substances and superoxide dismutase activity were higher in infected animals, when compared with the control group. This attenuated strain of B. bigemina induced an oxidative stress condition, as well as it reduces the cholinesterasés activity in infected and asymptomatic cattle. Therefore, this decrease of cholinesterase in infection by B. bigemina purpose is to inhibit inflammation, for thereby increasing acetylcholine levels, potent anti-inflammatory molecules.

Macrophages are the determinant of resistance to and outcome of nonlethal Babesia microti infection in mice

In the present study, we examined the contributions of macrophages to the outcome of infection with Babesia microti, the etiological agent of human and rodent babesiosis, in BALB/c mice. Mice were treated with clodronate liposome at different times during the course of B. microti infection in order to deplete the macrophages. Notably, a depletion of host macrophages at the early and acute phases of infection caused a significant elevation of parasitemia associated with remarkable mortality in the mice. The depletion of macrophages at the resolving and latent phases of infection resulted in an immediate and temporal exacerbation of parasitemia coupled with mortality in mice. Reconstituting clodronate liposome-treated mice at the acute phase of infection with macrophages from naive mice resulted in a slight reduction in parasitemia with improved survival compared to that of mice that received the drug alone. These results indicate that macrophages play a crucial role in the control of and resistance to B. microti infection in mice. Moreover, analyses of host immune responses revealed that macrophage-depleted mice diminished their production of Th1 cell cytokines, including gamma interferon (IFN-γ) and tumor necrosis factor alpha (TNF-α). Furthermore, depletion of macrophages at different times exaggerated the pathogenesis of the infection in deficient IFN-γ(-/-) and severe combined immunodeficiency (SCID) mice. Collectively, our data provide important clues about the role of macrophages in the resistance and control of B. microti and imply that the severity of the infection in immunocompromised patients might be due to impairment of macrophage function.

Vaccines against bovine babesiosis: where we are now and possible roads ahead

SUMMARY Bovine babesiosis caused by the tick-transmitted haemoprotozoans Babesia bovis, Babesia bigemina and Babesia divergens commonly results in substantial cattle morbidity and mortality in vast world areas. Although existing live vaccines confer protection, they have considerable disadvantages. Therefore, particularly in countries where large numbers of cattle are at risk, important research is directed towards improved vaccination strategies. Here a comprehensive overview of currently used live vaccines and of the status quo of experimental vaccine trials is presented. In addition, pertinent research fields potentially contributing to the development of novel non-live and/or live vaccines are discussed, including parasite antigens involved in host cell invasion and in pathogen-tick interactions, as well as the protective immunity against infection. The mining of available parasite genomes is continuously enlarging the array of potential vaccine candidates and, additionally, the recent development of a transfection tool for Babesia can significantly contribute to vaccine design. However, the complication and high cost of vaccination trials hinder Babesia vaccine research, and have so far seriously limited the systematic examination of antigen candidates and prevented an in-depth testing of formulations using different immunomodulators and antigen delivery systems.

Babesia bovis:lipids from virulent S2P and attenuated R1A strains trigger differential signalling and inflammatory responses in bovine macrophages

The intra-erythrocytic protozoanBabesia bovisis an economically important pathogen that causes an acute and often fatal infection in adult cattle. Babesiosis limitation depends on the early activation of macrophages, essential cells of the host innate immunity, which can generate an inflammatory response mediated by cytokines and nitric oxide (NO). Herein, we demonstrate in bovine macrophages that lipids fromB. bovisattenuated R1A strain (LA) produced a stronger NO release, an early TNFαmRNA induction and 2-fold higher IL-12p35 mRNA levels compared to the lipids of virulent S2P strain (LV). Neither LAnor LVinduced anti-inflammatory IL-10. Regarding signalling pathways, we here report that LAinduced a significant phosphorylation of p38 and extracellular signal-regulated kinases 1 and 2 (ERK1/2) whereas LVonly induced a reduced activation of ERK1/2. Besides, NF-κB was activated by LAand LV, but LAproduced an early degradation of the inhibitor IκB. Interestingly, LVand the majority of its lipid fractions, exerted a significant inhibition of concanavalin A-induced peripheral blood mononuclear cell proliferation with respect to LAand its corresponding lipid fractions. In addition, we determined that animals infected with R1A developed a higher increase in IgM anti-phosphatidylcholine than those inoculated with S2P. Collectively, S2P lipids generated a decreased inflammatory response contributing to the evasion of innate immunity. Moreover, since R1A lipids induced a pro-inflammatory profile, we propose these molecules as good candidates for immunoprophylactic strategies against babesiosis.

Macrophages are critical for cross-protective immunity conferred by Babesia microti against Babesia rodhaini infection in mice

Although primary infection of mice with Babesia microti has been shown to protect mice against subsequent lethal infection by Babesia rodhaini, the mechanism behind the cross-protection is unknown. To unravel this mechanism, we investigated the influence of primary infection of mice with nonlethal B. microti using different time courses on the outcome of subsequent lethal B. rodhaini infection. Simultaneous infections of mice with these parasites resulted in rapid increases in parasitemia, with 100% mortality in BALB/c mice, as observed with control mice infected with B. rodhaini alone. In contrast, mice with acute, resolving, and chronic-phase B. microti infections were completely protected against B. rodhaini, resulting in low parasitemia and no mortalities. Mice immunized with dead B. microti were not protected from B. rodhaini infection, although high antibody responses were induced. Interestingly, the protected mice had significantly decreased levels of antibody response, cytokines (including gamma interferon [IFN-γ], interleukin-2 [IL-2], IL-8, IL-10, and IL-12), and nitric oxide levels after infection with B. rodhaini. SCID mice and IFN-γ-deficient mice with chronic B. microti infections demonstrated protective responses comparable to those of immunocompetent mice. Likewise, in vivo NK cell depletion did not significantly impair the protective responses. Conversely, macrophage depletion resulted in increased susceptibility to B. rodhaini infection associated with changes in their antibody and cytokines profiles, indicating that macrophages contribute to the protection against this challenge infection. We conclude that future development of vaccines against Babesia should include a strategy that enhances the appropriate activation of macrophages.

Dynamics of bovine spleen cell populations during the acute response to Babesia bovis infection: an immunohistological study

The spleen is a critical organ in defence against haemoparasitic diseases like babesiosis. Many in vitro and ex vivo studies have identified splenic cells working in concert to activate mechanisms required for successful resolution of infection. The techniques used in those studies, however, remove cells from the anatomical context in which cell interaction and trafficking take place. In this study, an immunohistological approach was used to monitor the splenic distribution of defined cells during the acute response of naïve calves to Babesia bovis infection. Splenomegaly was characterized by disproportionate hyperplasia of large versus small leucocytes and altered distribution of several cell types thought to be important in mounting an effective immune response. In particular, the results suggest that the initial crosstalk between NK cells and immature dendritic cells occurs within the marginal zone and that immature dendritic cells are first redirected to encounter pathogens as they enter the spleen and then mature as they process antigen and migrate to T-cell-rich areas. The results of this study are remarkably similar to those observed in a mouse model of malarial infection, suggesting these dynamic events may be central to the acute response of naïve animals to haemoparasitic infection.

The bovine spleen: interactions among splenic cell populations in the innate immunologic control of hemoparasitic infections

Over the past several years, innate immunity has been recognized as having an important role as a front-line defense mechanism and as an integral part of the adaptive immune response. Innate immunity in cattle exposed to hemoparasites is spleen-dependent and age-related. In this review, we discuss general aspects of innate immunity and the cells involved in this aspect of the response to infection. We also provide examples of specific splenic regulatory and effector mechanisms involved in the response to Babesia bovis, an important tick-borne hemoparasitic disease of cattle. Evidence for the regulatory and effector role of bovine splenic monocytes and DC both in directing a type-1 response through interaction with splenic NK cells and γδT-cells will be presented.

Differential response of splenic monocytes and DC from cattle to microbial stimulation with Mycobacterium bovis BCG and Babesia bovis merozoites

Both bovine peripheral blood monocyte-derived dendritic cells (DC) and myeloid DC from afferent lymph have been described, but resident DC from other bovine tissues have not been fully characterized. The spleen as a secondary lymphoid organ is central to the innate and acquired immune response to various diseases particularly hemoprotozoan infections like babesiosis. Therefore, we developed methods to demonstrate the presence of myeloid DC from the spleen of cattle and have partially characterized a DC population as well as another myeloid cell population with monocyte characteristics. The phenotypic profile of each population was CD13+CD172a+/-CD14-CD11a-CD11b+/-CD11c+ and CD172a+CD13+/-CD14+CD11a-CD11b+/-CD11c+, respectively. The CD13+ population was found exclusively in the spleen whereas the CD172a+ population was present at the same percentage in the spleen and peripheral blood. CD13+ cells developed a typical veiled appearance when in culture for 96 h. The two cell populations differed in their ability to produce nitric oxide and had a different pattern of cytokine mRNA when stimulated with Mycobacterium bovis BCG or Babesia bovis merozoites. The data demonstrate the presence of a myeloid splenic DC with attributes consistent with an immature status.

Babesiosis of cattle

Tick fever or cattle fever (babesiosis) is economically the most important arthropod-borne disease of cattle worldwide with vast areas of Australia, Africa, South and Central America and the United States continuously under threat. Tick fever was the first disease for which transmission by an arthropod to a mammal was implicated at the turn of the twentieth century and is the first disease to be eradicated from a continent (North America). This review describes the biology of Babesia spp. in the host and the tick, the scale of the problem to the cattle industry, the various components of control programmes, epidemiology, pathogenesis, immunity, vaccination and future research. The emphasis is on Babesia bovis and Babesia bigemina.

Prospects for recombinant vaccines against Babesia bovis and related parasites

Babesial parasites infect cattle in tropical and temperate regions of the world and cause significant morbidity and mortality. Discovery of protective antigens that could be used in a killed vaccine has been slow and to date there are few promising vaccine candidates for cattle Babesia. This review describes mechanisms of protective innate and adaptive immune responses to babesial parasites and different strategies to identify potentially protective protein antigens of B. bovis, B. bigemina, and B. divergens. Successful parasites often cause persistent infection, and this paper also discusses how B. bovis evades and regulates the immune response to promote survival of parasite and host. Development of successful non-living recombinant vaccines will depend on increased understanding of protective immune mechanisms and availability of parasite genomes.

Immune control of Babesia bovis infection

Babesia bovis causes an acute and often fatal infection in adult cattle, which if resolved, leads to a state of persistent infection in otherwise clinically healthy cattle. Persistently infected cattle are generally resistant to reinfection with related parasite strains, and this resistance in the face of infection is termed concomitant immunity. Young animals are generally more resistant than adults to B. bovis infection, which is dependent on the spleen. Despite the discovery of B. bovis over a century ago, there are still no safe and effective vaccines that protect cattle against this most virulent of babesial pathogens. Immunodominant antigens identified by serological reactivity and dominant T-cell antigens have failed to protect cattle against challenge. This review describes the innate and acquired immune mechanisms that define resistance in young calves and correlate with the development of concomitant immunity in older cattle following recovery from clinical disease. The first sections will discuss the innate immune responses by peripheral blood- and spleen-derived macrophages in cattle induced by B. bovis merozoites and their products that limit parasite replication, and comparison of natural killer cell responses in the spleens of young (resistant) and adult (susceptible) cattle. Later sections will describe a proteomic approach to discover novel antigens, especially those recognized by immune CD4+ T lymphocytes. Because immunodominant antigens have failed to stimulate protective immunity, identification of subdominant antigens may prove to be important for effective vaccines. Identification of CD4+ T-cell immunogenic proteins and their epitopes, together with the MHC class II restricting elements, now makes possible the development of MHC class II tetramers and application of this technology to both quantify antigen-specific lymphocytes during infection and discover novel antigenic epitopes. Finally, with the imminent completion of the B. bovis genome-sequencing project, strategies using combined genomic and proteomic approaches to identify novel vaccine candidates will be reviewed. The availability of an annotated B. bovis genome will, for the first time, enable identification of non-immunodominant proteins that may stimulate protective immunity.

Possible mechanisms underlying age-related resistance to bovine babesiosis

Calves infected with the tick-borne parasites Babesia spp. do not develop severe clinical babesiosis. Instead they display persistent low parasitaemias without any apparent ill-effects. This age-related resistance not only benefits the host, but also furthers parasite transmission. Both calves and adult animals respond with a Th I immune response to primary infection. Here we hypothesize that the difference in the outcome of infection may at least partly be explained by the localization and timing of the inflammatory response: in calves NO production occurs early and appears to be concentrated in the spleen. On the other hand, there is evidence that a delayed and systemic inflammatory response occurs in adult animals that is ineffectual and probably contributes to the pathogenesis. An improved understanding of the possible mechanisms that underlie this phenomenon may lead to new approaches for the treatment and immune prophylaxis of the disease.

The treatment of mice with Lactobacillus casei induces protection against Babesia microti infection

In this study, we report that administration of Lactobacillus casei confers protection to mice against the intracellular protozoan Babesia microti. Mice treated with L. casei orally or intraperitoneally were inoculated 7 days later with an infectious dose of B. microti. Mice treated with lactobacilli showed significant reduction in the percentage of parasitized erythrocytes (PPE) compared to untreated mice. When mice were inoculated intraperitoneally with L. casei 3 or 0 days before challenge with B. microti, the PPE was significantly lower compared to untreated mice and there were no differences between treated mice and mice immune to B. microti infection. When mice treated with live or dead L. casei were compared to mice inoculated with Freund Complete Adjuvant before a B. microti infection, a significant reduction of PPE was observed. These results show the protective effect of L. casei administered to mice against a B. microti infection and suggest that it might act by stimulating the innate immune system.

The innate immune response in calves to Boophilus microplus tick transmitted Babesia bovis involves type-1 cytokine induction and NK-like cells in the spleen

The innate immune response to Babesia bovis infection in cattle is age-related, spleen-dependent and, in stabilate inoculated calves, has type-1 characteristics, including the early induction of IL-12 and IFN-gamma. In this study with three calves, parameters of innate immunity were followed for 2 weeks after tick transmission of B. bovis. Each calf survived the acute disease episode without drug intervention, and responded with increased levels of plasma interferon-gamma and type-1 cytokine expression, monocyte/macrophage activation, and CD8+ cellular proliferation in the spleen. The proliferating CD8+ population consisted primarily of NK-like cells, and the expansion occurred in parallel with an increase in IL-15 mRNA expression in the spleen.

Nitric oxide metabolites in naturally occurring canine babesiosis

Babesiosis, caused by the virulent haemoprotozoan parasite Babesia canis rossi, is an important disease of dogs in South Africa. The nitric oxide metabolites, nitrate and nitrite (collectively termed reactive nitrogen intermediates or RNIs) were measured in admission sera from dogs in a babesiosis-endemic area. Five groups were prospectively studied: mild uncomplicated (n=9), severe uncomplicated (severe anaemia) (n=10) and complicated babesiosis (n=11); and two groups of healthy aparasitaemic dogs: endemic controls from the study area (n=10) and experimental dogs kept in tick-free conditions (n=10). Four measures of RNI production were studied: (i) serum RNI; (ii) serum RNI/creatinine ratio; (iii) fractional clearance of RNI (FC(RNI)); (iv) fractional excretion of RNI (FE(RNI)). Marked elevations of serum RNI occurred in only two dogs, both in the severe uncomplicated group. The highest concentration (log value 5.29 micromol/l) was in a dog that died, but concentrations in the other four dogs that died were unremarkable (0, 0.34, 1.66 and 2.64 micromol/l). Age, appetite and free serum haemoglobin were significant covariates for measures of RNI production. There were no significant differences among the babesiosis groups for serum RNI. Adjustment for creatinine had minor effects on the results. All babesiosis groups had significantly higher serum RNI and RNI/creatinine than the tick-free control group, but did not differ from the endemic controls except for the severe uncomplicated group, which had higher RNI/creatinine. The complicated group had significantly lower FC(RNI) and FE(RNI) than all other groups, except for the tick-free control group, which had similar FE(RNI). The results indicate that, in an endemic area, measures of RNI production are unlikely to be useful indicators of severity or outcome in canine babesiosis.

Molecular approaches to elucidating innate and acquired immune responses to Babesia bovis, a protozoan parasite that causes persistent infection

For many vector-transmitted protozoal parasites, immunological control of acute infection leads to a state of persistent infection during which parasitemias may cycle unnoticed in infected but otherwise clinically healthy animals. Achieving persistent infection is a strategy that favors parasitism, since both host and, therefore, parasite survive, and endemically infected animal populations provide a reservoir of parasites continually available for subsequent transmission. Examples of the major economically important protozoan pathogens that cause persistent infection in mammals include the related Theileria and Babesia parasites as well as Trypanosoma species. Control of acute infection and maintenance of clinical immunity against subsequent infection are determined by the interplay of innate and acquired immune responses. This review will focus on approaches taken to gain an understanding of the molecular basis for innate and acquired immunity against the hemoprotozoan parasite of cattle, Babesia bovis. Knowledge of mechanisms used by the parasite to survive within infected cattle from acute to persistent infection combined with definition of the correlates of protective immunity in cattle should be applicable to designing effective vaccines.

A novel 20-kilodalton protein conserved in Babesia bovis and B. bigemina stimulates memory CD4(+) T lymphocyte responses in B. bovis-immune cattle

Acquired immunity against the hemoprotozoan parasite Babesia bovis is believed to depend on activation of antigen-specific CD4(+) T lymphocytes and IFN-gamma production. A strategy was employed to identify potentially protective antigens from B. bovis based on memory CD4(+) T lymphocyte recognition of fractionated merozoite proteins. Fractions of merozoites separated by continuous flow electrophoresis (CFE) that contained proteins of approximately 20 kDa were shown previously to stimulate memory CD4(+) lymphocyte responses in B. bovis-immune cattle with different MHC class II haplotypes. Expression library screening with rabbit antiserum raised against an immunostimulatory 20-kDa CFE fraction identified a 20-kDa protein (Bbo20) that contains a B lymphocyte epitope conserved in geographically distant B. bovis strains. An homologous 20-kDa protein that has 86.4% identity with Bbo20 and contains the conserved B cell epitope was identified in B. bigemina (Bbg20). Southern blot analysis indicated that both Babesia proteins are encoded by a single gene. Antibody against recombinant Bbo20 protein identified the antigen in CFE fractions shown previously to stimulate memory T lymphocyte responses in immune cattle. To verify Bbo20 as an immunostimulatory T lymphocyte antigen, CD4(+) T cell lines were propagated from B. bovis-immune cattle with merozoite antigen and shown to proliferate significantly against recombinant Bbo20 protein. Furthermore, Bbo20-specific CD4(+) T cell clones proliferated in response to several B. bovis strains and produced IFN-gamma. BLAST analysis revealed significant similarity of the Bbo20 and Bbg20 amino acid sequences with the hsp20/alpha-crystallin family.

The age-related immunity in cattle to Babesia bovis infection involves the rapid induction of interleukin-12, interferon-gamma and inducible nitric oxide synthase mRNA expression in the spleen

Young calves possess a strong innate immunity against Babesia bovis infection that lasts for approximately 6 months after birth and is abrogated with the removal of the spleen. This immunity is characterized as cellular involving a soluble mediator. Nitric oxide has been implicated by virtue of its babesiacidal affects in vitro, but questioned to be as effective in vivo, due to its ability to downregulate type-1 immunity. Spleen cells were obtained from 4-month-old calves and adult steers and processed for monitoring cytokine and inducible nitric oxide synthase (iNOS) mRNA expression during the response to initial B. bovis infection. The data provided evidence of a transient role for nitric oxide in innate immunity, characterized by brief iNOS induction in the spleen of calves that was not detectable in the spleens of adults. The iNOS message followed the early induction of interleukin (IL)-12 and interferon (IFN)-gamma message in calves. The induction of IL-12 and IFN-gamma message in adults was delayed until IL-10 message was induced. Transformation growth factor-beta mRNA expression levels were greater in spleen cells from adults early in infection and then declined, whereas expression levels increased in spleen cells from calves later in the infection process. Together, the data support the concept of 'first come, first serve' cytokine influence over cellular activities, the importance of a type-1 response in the control of an initial infection and the need for tight regulation in order to prevent pathology associated with over production of nitric oxide and inflammatory cytokines.

Antigenic variation in babesiosis:is there more than one ˈwhyˈ?

Many intraerythrocytic hemoparasites survive the host immune system through rapid antigenic variation. Among babesial parasites antigenic variation has been demonstrated convincingly only for Babesia bovis and Babesia rodhaini. The molecular basis for antigenic variation in babesial parasites and its possible connection with cytoadherence and sequestration are discussed.

DNA from protozoan parasites Babesia bovis, Trypanosoma cruzi, and T. brucei is mitogenic for B lymphocytes and stimulates macrophage expression of interleukin-12, tumor necrosis factor alpha, and nitric oxide

The activation of innate immune responses by genomic DNA from bacteria and several nonvertebrate organisms represents a novel mechanism of pathogen recognition. We recently demonstrated the CpG-dependent mitogenic activity of DNA from the protozoan parasite Babesia bovis for bovine B lymphocytes (W. C. Brown, D. M. Estes, S. E. Chantler, K. A. Kegerreis, and C. E. Suarez, Infect. Immun. 66:5423-5432, 1998). However, activation of macrophages by DNA from protozoan parasites has not been demonstrated. The present study was therefore conducted to determine whether DNA from the protozan parasites B. bovis, Trypanosoma cruzi, and T. brucei activates macrophages to secrete inflammatory mediators associated with protective immunity. DNA from Escherichia coli and all three parasites stimulated B-lymphocyte proliferation and increased macrophage production of interleukin-12 (IL-12), tumor necrosis factor alpha (TNF-alpha), and nitric oxide (NO). Regulation of IL-12 and NO production occurred at the level of transcription. The amounts of IL-12, TNF-alpha, and NO induced by E. coli and protozoal DNA were strongly correlated (r2 > 0.9) with the frequency of CG dinucleotides in the genome, and immunostimulation by DNA occurred in the order E. coli > or = T. cruzi > T. brucei > B. bovis. Induction of inflammatory mediators by E. coli, T. brucei, and B. bovis DNA was dependent on the presence of unmethylated CpG dinucleotides. However, at high concentrations, E. coli and T. cruzi DNA-mediated macrophage activation was not inhibited following methylation. The recognition of protozoal DNA by B lymphocytes and macrophages may provide an important innate defense mechanism to control parasite replication and promote persistent infection.

Elevated anti-parasitic activity in peripheral blood monocytes and neutrophils of cattle infected with Babesia bovis

The innate immune response to bovine Babesia bovis infection in vivo has not previously been established. We used assays measuring phagocytosis and oxidative burst to investigate the immune response because they are indicative of the innate antimicrobial capacity of monocytes and neutrophils. Monocyte and neutrophil phagocytosis is thought to be non-specific in nature and so the phagocytosis of either opsonised Zymosan or Escherichia coli was used to indicate the non-specific phagocytic capacity of monocytes and neutrophils ex vivo. The kinetics of both phagocytic and oxidative burst activity in monocytes and neutrophils were followed twice weekly from pre-inoculation (day 0) through to 31 days after inoculation. Peripheral blood monocytes were found to display a pronounced oxidative burst, but a suppressed capacity to phagocytose during a primary infection. On the other hand, neutrophils exhibited an increased phagocytic capacity and reduced oxidative activity during a primary infection. These findings identified considerable antimicrobial activity evident in peripheral blood monocytes and neutrophils from cattle exposed to B. bovis as a primary exposure. This elevated antimicrobial activity was coincident with the time that parasite numbers peaked in the circulation and occurred prior to parasite clearance. These results suggest that peripheral blood monocytes and neutrophils are active mediators in the innate immune response to a primary B. bovis.

Babesia bovis-stimulated macrophages express interleukin-1beta, interleukin-12, tumor necrosis factor alpha, and nitric oxide and inhibit parasite replication in vitro

The tick-transmitted hemoparasite Babesia bovis causes an acute infection that results in persistence and immunity against challenge infection in cattle that control the initial parasitemia. Resolution of acute infection with this protozoal pathogen is believed to be dependent on products of activated macrophages (Mphi), including inflammatory cytokines and nitric oxide (NO) and its derivatives. B. bovis stimulates inducible nitric oxide synthase (iNOS) and production of NO in bovine Mphi, and chemical donors of NO inhibit the growth of B. bovis in vitro. However, the induction of inflammatory cytokines in Mphi by babesial parasites has not been described, and the antiparasitic activity of NO produced by B. bovis-stimulated Mphi has not been definitively demonstrated. We report that monocyte-derived Mphi activated by B. bovis expressed enhanced levels of inflammatory cytokines interleukin-1beta (IL-1beta), IL-12, and tumor necrosis factor alpha that are important for stimulating innate and acquired immunity against protozoal pathogens. Furthermore, a lipid fraction of B. bovis-infected erythrocytes stimulated iNOS expression and NO production by Mphi. Cocultures of Mphi and B. bovis-infected erythrocytes either in contact or physically separated resulted in reduced parasite viability. However, NO produced by bovine Mphi in response to B. bovis-infected erythrocytes was only partially responsible for parasite growth inhibition, suggesting that additional factors contribute to the inhibition of B. bovis replication. These findings demonstrate that B. bovis induces an innate immune response that is capable of controlling parasite replication and that could potentially result in host survival and parasite persistence.

Babesiosis

Babesiosis is an emerging, tick-transmitted, zoonotic disease caused by hematotropic parasites of the genus Babesia. Babesial parasites (and those of the closely related genus Theileria) are some of the most ubiquitous and widespread blood parasites in the world, second only to the trypanosomes, and consequently have considerable worldwide economic, medical, and veterinary impact. The parasites are intraerythrocytic and are commonly called piroplasms due to the pear-shaped forms found within infected red blood cells. The piroplasms are transmitted by ixodid ticks and are capable of infecting a wide variety of vertebrate hosts which are competent in maintaining the transmission cycle. Studies involving animal hosts other than humans have contributed significantly to our understanding of the disease process, including possible pathogenic mechanisms of the parasite and immunological responses of the host. To date, there are several species of Babesia that can infect humans, Babesia microti being the most prevalent. Infections with Babesia species generally follow regional distributions; cases in the United States are caused primarily by B. microti, whereas cases in Europe are usually caused by Babesia divergens. The spectrum of disease manifestation is broad, ranging from a silent infection to a fulminant, malaria-like disease, resulting in severe hemolysis and occasionally in death. Recent advances have resulted in the development of several diagnostic tests which have increased the level of sensitivity in detection, thereby facilitating diagnosis, expediting appropriate patient management, and resulting in a more accurate epidemiological description.

Babesiosis

Babesiosis is an emerging, tick-transmitted, zoonotic disease caused by hematotropic parasites of the genus Babesia. Babesial parasites (and those of the closely related genus Theileria) are some of the most ubiquitous and widespread blood parasites in the world, second only to the trypanosomes, and consequently have considerable worldwide economic, medical, and veterinary impact. The parasites are intraerythrocytic and are commonly called piroplasms due to the pear-shaped forms found within infected red blood cells. The piroplasms are transmitted by ixodid ticks and are capable of infecting a wide variety of vertebrate hosts which are competent in maintaining the transmission cycle. Studies involving animal hosts other than humans have contributed significantly to our understanding of the disease process, including possible pathogenic mechanisms of the parasite and immunological responses of the host. To date, there are several species of Babesia that can infect humans, Babesia microti being the most prevalent. Infections with Babesia species generally follow regional distributions; cases in the United States are caused primarily by B. microti, whereas cases in Europe are usually caused by Babesia divergens. The spectrum of disease manifestation is broad, ranging from a silent infection to a fulminant, malaria-like disease, resulting in severe hemolysis and occasionally in death. Recent advances have resulted in the development of several diagnostic tests which have increased the level of sensitivity in detection, thereby facilitating diagnosis, expediting appropriate patient management, and resulting in a more accurate epidemiological description.

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.

Modulation of host immune responses by protozoal DNA

The pathology caused by acute Babesia bovis infection is similar to that seen in severe human malaria caused by Plasmodium falciparum infection, which is related to dysregulated production of inflammatory cytokines and nitric oxide (NO). We have observed induction of NO, inducible nitric oxide synthase (iNOS) and inflammatory cytokines in macrophages by B. bovis. Furthermore, proliferation of lymphocytes from individuals never exposed to certain protozoal pathogens can be induced by crude protozoal parasite extracts. We have repeatedly observed stimulation of naive PBMC from cattle to antigenic extracts of Babesia bovis. Based on recent studies demonstrating the mitogenicity of bacterial and other non-vertebrate DNAs for murine B cells and macrophages, the mitogenic properties of B. bovis DNA were examined. B. bovis and E. coli DNAs induced proliferation of PBMC and purified B cells from non-exposed cattle. Stimulatory activity was reduced by DNase treatment and methylation with CpG methylase, indicating the presence of stimulatory non-methylated CpG motifs in the B. bovis genome. B. bovis and E. coli DNAs enhanced IgG secretion by cultured B cells, stimulating IgG1 and more strongly, IgG2. Several hexameric CpG immunostimulatory sequences (ISS) active for murine B cells were identified in an 11 kb fragment of B. bovis DNA. An oligodeoxyribonucleotide containing one of these (AACGTT), located in the rhoptry associated protein-1 (rap-1) open reading frame, stimulated B cell proliferation. These studies identify a potential mechanism by which protozoal parasites may modulate host immune responses, leading to consequences such as hypergammaglobulinemia and splenomegaly. These results also support the use of ISS as vaccine adjuvants to enhance Type 1 immune responses in cattle.

Designing blood-stage vaccines against Babesia bovis and B. bigemina

The tick-transmitted apicomplexan parasites Babesia bovis and B. bigemina cause significant disease in cattle in many tropical and temperate areas of the world. These parasites present a challenge for vaccine development, and yet provide a system for studying the pathogenesis, mechanisms of protective immunity and regulation of host immune responses associated with intraerythrocytic protozoan parasites in a non-rodent species. In this article, Wendy Brown and Guy Palmer review strategies for identifying candidate vaccine antigens of B. bovis and B. bigemina and for priming immune responses to evoke strain crossprotective immunity.

Stimulation of nitric oxide production in macrophages by Babesia bovis

Gamma interferon (IFN-gamma)-activated macrophages are believed to play a key role in resistance to Babesia bovis through parasite suppression by macrophage secretory products. However, relatively little is known about interactions between this intraerythrocytic parasite and the macrophages of its bovine host. In this study, we examined the in vitro effect of intact and fractionated B. bovis merozoites on bovine macrophage nitric oxide (NO) production. In the presence of IFN-gamma, B. bovis merozoites stimulated NO production, as indicated by the presence of increased L-arginine-dependent nitrite (NO2-) levels in culture supernatants of macrophages isolated from several cattle. The merozoite crude membrane (CM) fraction stimulated greater production of NO, in a dose-dependent manner, than did the merozoite homogenate or the soluble, cytosolic high-speed supernatant fraction. Stimulation of NO production by CM was enhanced by as little as 1 U of IFN-gamma per ml of culture medium. Upregulation of inducible NO synthase mRNA in bovine macrophages by either B. bovis-parasitized erythrocytes and IFN-gamma or CM was also observed. B. bovis-specific T-helper lymphocyte culture supernatants, all of which contained IFN-gamma, were also found to induce L-arginine-dependent NO2- production. Supernatants that induced the highest levels of NO also contained biologically active TNF. These results show that B. bovis merozoites and antigen-stimulated B. bovis-immune T cells can induce the production of NO, a molecule implicated in both protection and pathologic changes associated with hemoprotozoan parasite infections.

Amelioration of virulent Babesia bovis infection in calves by administration of the nitric oxide synthase inhibitor aminoguanidine

Calves undergoing initial infection with a virulent strain of the haemoprotozoan parasite Babesia bovis were treated with aminoguanidine (AG), an inhibitor of the inducible form of nitric oxide synthase (iNOS). The mean maximum parasitaemia of the AG treated calves was significantly lower than that of the control cattle. In addition, the febrile response and decrease in packed cell volume (PCV) observed during acute infection were significantly ameliorated in the AG treated cattle relative to the controls. However, AG had no effect on the multiplication of B. bovis in the microaerophilous stationary-phase (MASP) in-vitro culture system. These results provide evidence of a role for nitric oxide (NO) produced in response to acute infection in the pathology of bovine babesiosis.

Babesia bovis immunity. In vitro and in vivo evidence for IL-10 regulation of IFN-gamma and iNOS

IL-10 has been shown to have profound immunoregulatory attributes and in the bovine appears to downregulate both Th1- and Th2-like responses. Using RT-PCR, we demonstrate IL-10 in vitro down-regulation of mRNA expression of iNOS, the cytokines involved in nitric oxide signal transduction initiation (IFN-gamma and TNF-alpha), and other mononuclear phagocyte associate cytokines. In addition, using RT-PCR with peripheral blood leukocytes and spleen leukocytes, the Griess reaction, and a killing assay, we provide evidence for the importance of iNOS in a successful immune response to B. bovis infection and for high and persistent IL-10 mRNA expression when the immune response is unsuccessful. We also provide evidence that antibody developed early after an initial infection appears to lack protective attributes (neutralizing and opsonic). Together, the data suggests that IL-10 and IFN-gamma are critical molecules involved in the response to this intraerythrocytic protozoan infection.

The role of IL-10 in iNOS and cytokine mRNA expression during in vitro differentiation of bovine mononuclear phagocytes

In the study reported here, we used RT-PCR with primers specific for interleukin-1 (IL-1), IL-6, IL-10, tumor necrosis factor-alpha (TNF-alpha), and nitric oxide synthase (iNOS) to assess the cytokine mRNA expression associated with bovine blood monocytes during their differentiation to macrophages cultured on plastic (1 week). In addition, we used RT-PCR to assess the contribution of gammadelta T cells as a source of interferon-gamma (IFN-gamma), the induction signal for iNOS. Further, we evaluated cytocentrifuge preparations from the cultures for the production of IL-10 using specific antibody. We previously demonstrated that iNOS can be induced in cultured bovine monocytes in response to IFN-gamma and TNF-alpha but lose this capability in a short period of time. However, we demonstrate here that iNOS induction from monocytes cultured with IFN-gamma secreting gammadelta T cells is prolonged, suggesting that this source of IFN-gamma primes the monocytes before exogenous stimulation. Based on mRNA expression, placement of monocytes in culture resulted in activation, followed by quiescence. By 6 days in culture, the iNOS message was reduced below the basal level. In addition, the TNF-alpha message was substantially reduced, and IL-1 and IL-6 messages were reduced below detectable levels. This correlated with an increase in IL-10 message. Downregulation of these same cytokine messages as well as IFN-gamma message occurred within a 20-h period when IL-10 was added exogenously to cultures of total leukocytes. At the same time, there was an increase in the number of IL-10-positive cells and an increase in the intensity of anti-IL-10 staining within adherent cells. These results provide evidence for IL-10 regulation of some bovine mononuclear phagocyte effector functions.