Functional activation of T cells by dendritic cells and macrophages exposed to the intracellular parasite Neospora caninum

Congenital Transmission of Apicomplexan Parasites: A Review

Apicomplexans are a group of pathogenic protists that cause various diseases in humans and animals that cause economic losses worldwide. These unicellular eukaryotes are characterized by having a complex life cycle and the ability to evade the immune system of their host organism. Infections caused by some of these parasites affect millions of pregnant women worldwide, leading to various adverse maternal and fetal/placental effects. Unfortunately, the exact pathogenesis of congenital apicomplexan diseases is far from being understood, including the mechanisms of how they cross the placental barrier. In this review, we highlight important aspects of the diseases caused by species of Plasmodium, Babesia, Toxoplasma, and Neospora, their infection during pregnancy, emphasizing the possible role played by the placenta in the host-pathogen interaction.

Neospora caninum glycosylphosphatidylinositols used as adjuvants modulate cellular immune responses induced in vitro by a nanoparticle-based vaccine

Neospora caninum causes abortion in ruminants, leading to important economic losses and no efficient treatment or vaccine against neosporosis is available. Considering the complexity of the strategies developed by intracellular apicomplexan parasites to escape immune system, future vaccine formulations should associate the largest panel of antigens and adjuvants able to better stimulate immune responses than natural infection. A mucosal vaccine, constituted of di-palmitoyl phosphatidyl glycerol-loaded nanoparticles (DGNP) and total extract (TE) of soluble antigens of Toxoplasma gondii, has demonstrated its efficacy, decreasing drastically the parasite burden. Here, DGNP were loaded with N. caninum TE and glycosylphosphatidylinositol (GPI) of N. caninum as Toll-like receptor (TLR) adjuvant able to induce specific cellular and humoral immune responses. Activation of TLR2 and TLR4 signalling pathway in HEK reporter cells induced by GPI was abrogated after its incorporation into DGNP. However, in murine bone marrow-derived dendritic cells, an adjuvant effect of GPI was observed with higher levels of interleukin (IL)-1β, reduced levels of IL-6, IL-12p40 and IL-10, and decreased expression of major histocompatibility complex (MHC) molecules. GPI also modulated the responses of bovine peripheral blood mononuclear cells, by increasing the production of IFN-γ and by decreasing the expression of MHC molecules. Altogether, these results suggest that GPI delivered by the DGNP might modulate cell responses through the activation of an intracellular pathway of signalisation in a TLR-independent manner. In vivo experiments are needed to confirm the potent adjuvant properties of N. caninum GPI in a vaccine strategy against neosporosis.

From Signaling Pathways to Distinct Immune Responses: Key Factors for Establishing or Combating Neospora caninum Infection in Different Susceptible Hosts

: Neospora caninum is an intracellular protozoan parasite affecting numerous animal species. It induces significant economic losses because of abortion and neonatal abnormalities in cattle. In case of infection, the parasite secretes numerous arsenals to establish a successful infection in the host cell. In the same context but for a different purpose, the host resorts to different strategies to eliminate the invading parasite. During this battle, numerous key factors from both parasite and host sides are produced and interact for the maintaining and vanishing of the infection, respectively. Although several reviews have highlighted the role of different compartments of the immune system against N. caninum infection, each one of them has mostly targeted specific points related to the immune component and animal host. Thus, in the current review, we will focus on effector molecules derived from the host cell or the parasite using a comprehensive survey method from previous reports. According to our knowledge, this is the first review that highlights and discusses immune response at the host cell-parasite molecular interface against N. caninum infection in different susceptible hosts.

Differential Responses of Bovine Monocyte-Derived Macrophages to Infection by Neospora caninum Isolates of High and Low Virulence

Neospora caninum, a protozoan parasite closely related to Toxoplasma gondii, represents one of the main causes of abortion in cattle. Macrophages (MØs) are mediators of the innate immune response against infection and likely one of the first cells encountered by the parasite during the host infection process. In this study, we investigated in vitro how high or low virulent isolates of N. caninum (Nc-Spain7 and Nc-Spain1H, respectively) interact with bovine monocyte-derived MØs and the influence of the isolate virulence on the subsequent cellular response. Both isolates actively invaded, survived and replicated in the MØs. However, Nc-Spain7 showed a higher invasion rate and a replication significantly faster, following an exponential growth model, whereas Nc-Spain1H presented a delayed replication and a lower growth rate without an exponential pattern. N. caninum infection induced a hypermigratory phenotype in bovine MØs that was characterized by enhanced motility and transmigration in vitro and was accompanied by morphological changes and abrogated extracellular matrix degradation. A significantly higher hypermotility was observed with the highly virulent isolate Nc-Spain7. Nc-Spain1H-infected MØs showed elevated reactive oxygen species (ROS) production and IL12p40 expression, which also resulted in increased IFN-γ release by lymphocytes, compared to cells infected with Nc-Spain7. Furthermore, IL-10 was upregulated in MØs infected with both isolates. Infected MØs exhibited lower expression of MHC Class II, CD86, and CD1b molecules than uninfected MØs, with non-significant differences between isolates. This work characterizes for the first time N. caninum replication in bovine monocyte-derived MØs and details isolate-dependent differences in host cell responses to the parasite.

In vitro cellular responses to Neospora caninum glycosylphosphatidylinositols depend on the host origin of antigen presenting cells

Neosporosis due to Neospora caninum causes abortions in farm animals such as cattle. No treatment and vaccine exist to fight this disease, responsible for considerable economic losses. It is thus important to better understand the immune responses occurring during the pathogenesis to control them in a global strategy against the parasite. In this context, we studied the roles of N. caninum glycosylphosphatidylinositols (GPIs), glycolipids defined as toxins in the related parasite Plasmodium falciparum. We demonstrated for the first time that GPIs could be excreted in the supernatant of N. caninum culture and trigger cell signalling through the Toll-like receptors 2 and 4. In addition, antibodies specific to N. caninum GPIs were detected in the serum of infected mice. As shown for other protozoan diseases, they could play a role in neutralizing GPIs. N. caninum GPIs were able to induce the production of tumour necrosis factor-α, interleukin(IL)-1β and IL-12 cytokines by murine macrophages and dendritic cells. Furthermore, GPIs significantly reduced expression of major histocompatibility complex (MHC) molecules of class I on murine dendritic cells. In contrast to murine cells, bovine blood mononuclear cells produced increased levels of IFN-γ and IL-10, but reduced levels of IL-12p40 in response to GPIs. On these bovine cells, GPI had the tendency to up-regulate MHC class I, but to down-regulate MHC class II. Altogether, these results suggest that N. caninum GPIs might differentially participate in the responses of antigen presenting cells induced by the whole parasite in mouse models of neosporosis and in the natural cattle host.

Neospora GRA6 possesses immune-stimulating activity and confers efficient protection against Neospora caninum infection in mice

Vaccination has the potential to be the most cost-effective control measure for reducing the economic burden of neosporosis in cattle. In this study, the immune-stimulatory effect of recombinant Neospora caninum dense granule protein 6 (NcGRA6) was confirmed via its triggering of IL-12p40 production in murine macrophages. BALB/c mice were immunized with recombinant NcGRA6 fused with glutathione S-transferase (GST) protein with or without oligomannose-coated-liposomes (OMLs) as the potential adjuvant. Specific IgG1 antibody production was observed from 21 and 35 days after the first immunization in NcGRA6+GST- and NcGRA6+GST-OML-immunized mice, respectively. However, specific IgG2a was detected 1 week after the infection, and IgG2a levels of the NcGRA6+GST- group were higher than those of the NcGRA6+GST-OML-group. Moreover, spleen cell proliferation with concomitant interferon-gamma production was detected in mice immunized with NcGRA6+GST, indicating that a significant cellular immune response was induced. Mouse survival rates against N. caninum challenge infection were 91.7% for NcGRA6+GST and 83.3% for NcGRA6+GST-OML, which were significantly higher than those of control groups (GST-OML: 25%, phosphate-buffered saline: 16.7%). This indicates that naked NcGRA6+GST induced protective immunity. Thus, our findings highlight the immune-stimulating potential of NcGRA6 and the ability to induce protective immunity against N. caninum infection in mice.

Characteristic pro-inflammatory cytokines and host defence cathelicidin peptide produced by human monocyte-derived macrophages infected withNeospora caninum

Neospora caninumis a coccidian intracellular protozoan capable of infecting a wide range of mammals, although severe disease is mostly reported in dogs and cattle. Innate defences triggered by monocytes/macrophages are key in the pathogenesis of neosporosis, as these cells are first-line defenders against intracellular infections. The aim of this study was to characterize infection and innate responses in macrophages infected withN. caninumusing a well-known cell model to study macrophage functions (human monocyte THP-1 cells). Intracellular invasion of live tachyzoites occurred as fast as 4 h (confirmed with immunofluorescence microscopy usingN. caninum-specific antibodies). Macrophages infected byN. caninumhad increased expression of pro-inflammatory cytokines (TNFα, IL-1β, IL-8, IFNγ). Interestingly,N. caninuminduced expression of host-defence peptides (cathelicidins), a mechanism of defence never reported forN. caninuminfection in macrophages. The expression of cytokines and cathelicidins in macrophages invaded byN. caninumwas mediated by mitogen-activated protein kinase (MEK 1/2). Secretion of such innate factors fromN. caninum-infected macrophages reduced parasite internalization and promoted the secretion of pro-inflammatory cytokines in naïve macrophages. We concluded that rapid invasion of macrophages byN. caninumtriggered protective innate defence mechanisms against intracellular pathogens.

Neospora caninum Activates p38 MAPK as an Evasion Mechanism against Innate Immunity

Due to the high prevalence and economic impact of neosporosis, the development of safe and effective vaccines and therapies against this parasite has been a priority in the field and is crucial to limit horizontal and vertical transmission in natural hosts. Limited data is available regarding factors that regulate the immune response against this parasite and such knowledge is essential in order to understand Neospora caninum induced pathogenesis. Mitogen-activated protein kinases (MAPKs) govern diverse cellular processes, including growth, differentiation, apoptosis, and immune-mediated responses. In that sense, our goal was to understand the role of MAPKs during the infection by N. caninum. We found that p38 phosphorylation was quickly triggered in macrophages stimulated by live tachyzoites and antigen extracts, while its chemical inhibition resulted in upregulation of IL-12p40 production and augmented B7/MHC expression. In vivo blockade of p38 resulted in an amplified production of cytokines, which preceded a reduction in latent parasite burden and enhanced survival against the infection. Additionally, the experiments indicate that the p38 activation is induced by a mechanism that depends on GPCR, PI3K and AKT signaling pathways, and that the phenomena here observed is distinct that those induced by Toxoplasma gondii’s GRA24 protein. Altogether, these results showed that N. caninum manipulates p38 phosphorylation in its favor, in order to downregulate the host’s innate immune responses. Additionally, those results infer that active interference in this signaling pathway may be useful for the development of a new therapeutic strategy against neosporosis.

Predominant role of interferon-γ in the host protective effect of CD8(+) T cells against Neospora caninum infection

It is well established that CD8⁺ T cells play an important role in protective immunity against protozoan infections. However, their role in the course of Neospora caninum infection has not been fully elucidated. Here we report that CD8-deficient mice infected with N. caninum presented higher parasitic loads in the brain and lungs and lower spleen and brain immunity-related GTPases than their wild-type counterparts. Moreover, adoptive transfer of splenic CD8⁺ T cells sorted from N. caninum-primed immunosufficient C57BL/10 ScSn mice prolonged the survival of infected IL-12-unresponsive C57BL/10 ScCr recipients. In both C57BL/6 and C57BL/10 ScSn mice CD8⁺ T cells are activated and produce interferon-γ (IFN-γ) upon challenged with N. caninum. The host protective role of IFN-γ produced by CD8⁺ T cells was confirmed in N. caninum-infected RAG2-deficient mice reconstituted with CD8⁺ T cells obtained from either IFN-γ-deficient or wild-type donors. Mice receiving IFN-γ-expressing CD8⁺ T cells presented lower parasitic burdens than counterparts having IFN-γ-deficient CD8⁺ T cells. Moreover, we observed that N. caninum-infected perforin-deficient mice presented parasitic burdens similar to those of infected wild-type controls. Altogether these results demonstrate that production of IFN-γ is a predominant protective mechanism conferred by CD8⁺ T cells in the course of neosporosis.

Immune response in the adipose tissue of lean mice infected with the protozoan parasite Neospora caninum

The adipose tissue can make important contributions to immune function. Nevertheless, only a limited number of reports have investigated in lean hosts the immune response elicited in this tissue upon infection. Previous studies suggested that the intracellular protozoan Neospora caninum might affect adipose tissue physiology. Therefore, we investigated in mice challenged with this protozoan if immune cell populations within adipose tissue of different anatomical locations could be differently affected. Early in infection, parasites were detected in the adipose tissue and by 7 days of infection increased numbers of macrophages, regulatory T (Treg) cells and T-bet(+) cells were observed in gonadal, mesenteric, omental and subcutaneous adipose tissue. Increased expression of interferon-γ was also detected in gonadal adipose tissue of infected mice. Two months after infection, parasite DNA was no longer detected in these tissues, but T helper type 1 (Th1) cell numbers remained above control levels in the infected mice. Moreover, the Th1/Treg cell ratio was higher than that of controls in the mesenteric and subcutaneous adipose tissue. Interestingly, chronically infected mice presented a marked increase of serum leptin, a molecule that plays a role in energy balance regulation as well as in promoting Th1-type immune responses. Altogether, we show that an apicomplexa parasitic infection influences immune cellular composition of adipose tissue throughout the body as well as adipokine production, still noticed at a chronic phase of infection when parasites were already cleared from that particular tissue. This strengthens the emerging view that infections can have long-term consequences for the physiology of adipose tissue.

Role of the chemokine receptor CCR5-dependent host defense system in Neospora caninum infections

Background

Neospora caninum, a Toxoplasma gondii-like obligate intracellular parasite, causes abortion in cattle and neurological signs in canines. To understand neosporosis better, studies on host cell migration and host immune responses during the early phase of infection are important. Although the C-C chemokine receptor 5 (CCR5) plays a crucial role in immune cell migration, the role played by it in protective immunity against N. caninum is poorly understood.

Methods

CCR5^−/− mice were used to investigate their sensitivity levels to N. caninum infection and their ability to activate immune cells against this parasite.

Results

Increased mortality and neurological impairment were observed in the N. caninum-infected CCR5^−/− mice. In comparison with wild-type mice, CCR5^−/− mice experienced poor migration of dendritic cells and natural killer T cells to the site of infection. Dendritic cells in an in vitro culture from CCR5^−/− mice could not be activated upon infection with N. caninum. Furthermore, higher levels of IFN-γ and CCL5 expression, which are associated with brain tissue damage, were observed in the brain tissue of CCR5^−/− mice during the acute phase of the infection, while there was no significant difference in the parasite load between the wild-type and CCR5^−/− animals. Additionally, a primary microglia culture from CCR5^−/− mice showed lower levels of IL-6 and IL-12 production against N. caninum parasites.

Conclusions

Our findings show that migration and activation of immune cells via CCR5 is required for controlling N. caninum parasites during the early phase of the infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-014-0620-5) contains supplementary material, which is available to authorized users.

Orp8 deficiency in bone marrow-derived cells reduces atherosclerotic lesion progression in LDL receptor knockout mice

Introduction

Oxysterol binding protein Related Proteins (ORPs) mediate intracellular lipid transport and homeostatic regulation. ORP8 downregulates ABCA1 expression in macrophages and cellular cholesterol efflux to apolipoprotein A-I. In line, ORP8 knockout mice display increased amounts of HDL cholesterol in blood. However, the role of macrophage ORP8 in atherosclerotic lesion development is unknown.

Methods and Results

LDL receptor knockout (KO) mice were transplanted with bone marrow (BM) from ORP8 KO mice and C57Bl/6 wild type mice. Subsequently, the animals were challenged with a high fat/high cholesterol Western-type diet to induce atherosclerosis. After 9 weeks of Western-Type diet feeding, serum levels of VLDL cholesterol were increased by 50% in ORP8 KO BM recipients compared to the wild-type recipients. However, no differences were observed in HDL cholesterol. Despite the increase in VLDL cholesterol, lesions in mice transplanted with ORP8 KO bone marrow were 20% smaller compared to WT transplanted controls. In addition, ORP8 KO transplanted mice displayed a modest increase in the percentage of macrophages in the lesion as compared to the wild-type transplanted group. ORP8 deficient macrophages displayed decreased production of pro-inflammatory factors IL-6 and TNFα, decreased expression of differentiation markers and showed a reduced capacity to form foam cells in the peritoneal cavity.

Conclusions

Deletion of ORP8 in bone marrow-derived cells, including macrophages, reduces lesion progression after 9 weeks of WTD challenge, despite increased amounts of circulating pro-atherogenic VLDL. Reduced macrophage foam cell formation and lower macrophage inflammatory potential are plausible mechanisms contributing to the observed reduction in atherosclerosis.

Macrophage depletion prior to Neospora caninum infection results in severe neosporosis in mice

We observed that murine macrophages showed greater activation and increased interleukin 6 (IL-6), IL-12p40, and interferon gamma (IFN-γ) production during Neospora caninum infection. Many macrophages migrated to the site of infection. Furthermore, macrophage-depleted mice exhibited increased sensitivity to N. caninum infection. This study indicates that macrophages are required for achieving protective immunity against N. caninum.

Vaccines against neosporosis: what can we learn from the past studies?

Neospora caninum is an intracellular apicomplexan parasite, which is a leading cause of abortion in cattle; thus neosporosis represents an important veterinary health problem and is of high economic significance. The parasite can infect cattle via trans-placental transmission from an infected cow to its fetus (vertical transmission), or through the oral route via ingestion of food or water contaminated with oocysts that were previously shed with the feces of a canid definitive host (horizontal transmission). Although vaccination was considered a rational strategy to prevent bovine neosporosis, the only commercialized vaccine (Neoguard®) produced ambiguous results with relatively low efficacy, and was recently removed from the market. Therefore, there is a need to develop an efficient vaccine capable of preventing both, the horizontal transmission through infected food or water to a naïve animal as well as the vertical transmission from infected but clinically asymptomatic dams to the fetus. Different vaccine strategies have been investigated, including the use of live attenuated vaccines, killed parasite lysates, total antigens or antigen fractions from killed parasites, and subunit vaccines. The vast majority of experimental studies were performed in mice, and to a certain extent in gerbils, but there is also a large number of investigations that were conducted in cattle and sheep. However, it is difficult to directly compare these studies due to the high variability of the parameters employed. In this review, we will summarize the recent advances made in vaccine development against N. caninum in cattle and in mice and highlight the most important factors, which are likely to influence the degree of protection mediated by vaccination.

Mucosal and systemic T cell response in mice intragastrically infected with Neospora caninum tachyzoites

The murine model has been widely used to study the host immune response to Neospora caninum. However, in most studies, the intraperitoneal route was preferentially used to establish infection. Here, C57BL/6 mice were infected with N. caninum tachyzoites by the intragastric route, as it more closely resembles the natural route of infection through the gastrointestinal tract. The elicited T-cell mediated immune response was evaluated in the intestinal epithelium and mesenteric lymph nodes (MLN). Early upon the parasitic challenge, IL-12 production by conventional and plasmacytoid dendritic cells was increased in MLN. Accordingly, increased proportions and numbers of TCRαβ+CD8+IFN-γ+ lymphocytes were detected, not only in the intestinal epithelium and MLN, but also in the spleen of the infected mice. In this organ, IFN-γ-producing TCRαβ+CD4+ T cells were also found to increase in the infected mice, however later than CD8+ T cells. Interestingly, splenic and MLN CD4+CD25+ T cells sorted from infected mice presented a suppressive activity on in vitro T cell proliferation and cytokine production above that of control counterparts. These results altogether indicate that, by producing IFN-γ, TCRαβ+CD8+ cells contribute for local and systemic host protection in the earliest days upon infection established through the gastrointestinal tract. Nevertheless, they also provide substantial evidence for a parasite-driven reinforcement of T regulatory cell function which may contribute for parasite persistence in the host and might represent an additional barrier to overcome towards effective vaccination.

Oligomannose-coated liposome-entrapped dense granule protein 7 induces protective immune response to Neospora caninum in cattle

Neospora caninum is an intracellular protozoan parasite that causes abortion in cows. Vaccination is an important strategy for control of neosporosis, and a safe and effective vaccine suitable for cattle is required. Dense granule protein 7 of N. caninum (NcGRA7) is a secretory protein with high antigenicity in hosts. We demonstrated previously that NcGRA7 entrapped in liposomes coated with mannotriose (M3-NcGRA7) could induce a parasite-specific T-helper type 1 immune response and produce humoral antibodies that resulted in increased offspring survival and decreased infection in the brains of mice dams. In the present study, the efficacy of M3-NcGRA7 as a vaccine candidate against N. caninum has been evaluated in cattle (n=12). Cattle were immunized with M3-NcGRA7 containing 50 μg (n=4) or 200 μg NcGRA7 (n=4) subcutaneously twice with a 4-week interval and all cattle including the non-immunized controls (n=4) were inoculated with 10(7) tachyzoites of Nc-1 strain 27 days after the second immunization and euthanized at 85-87 days post infection (dpi). In immunized cattle, NcGRA7-specific antibody production and IFN-γ production in PBMC was induced before challenge. At 3 dpi, body temperature and concentration of serum IFN-γ tended to be higher in control cattle than in the immunized cattle. Furthermore, the parasite load in the brain significantly decreased in cattle immunized with 50 μg M3-NcGRA7 compared with controls. These results suggest that M3-NcGRA7 can induce protective immune responses to N. caninum tachyzoites in cattle, which could lead to practical application of safe and effective subunit vaccines.

Presence of dendritic cells in chicken spleen cell preparations and their functional interaction with the parasite Toxoplasma gondii

Toxoplasmosis is a worldwide epizootic disease of mammals. Chickens, albeit being less susceptible, can be contaminated in free-range flocks and may have an important role in parasite transmission. Plastic adherence selection of chicken spleen cells enriched 8F2+ (putative chicken CD11c) MHC II+ cells of the myeloid type; however, we did not succeed to separate dendritic cells from macrophages using their feature to become loosely adherent after culture as in mammals. Still we clearly identified dendritic-like cells being morphologically distinguishable from macrophages in the KUL01 (macrophage marker) negative fraction, exhibiting responsiveness to LPS and parasite extracts by developing characteristic cellular protrusions as well as a minor phagocytic incorporation of dead parasites. Live T. gondii tachyzoites were able to invade the two different types of myeloid adherent cells, to replicate, and to induce an overall decrease in the expression of MHC II and co-stimulatory molecules, CD80 and CD40. Our data indicate that dendritic cells in addition to macrophages may have a role in hiding viable replicating T. gondii tachyzoites from the immune system and in shuttling them to different organs in the chicken as previously described for different Apicomplexa infecting mammals.

Maternal and foetal immune responses of cattle following an experimental challenge with Neospora caninum at day 70 of gestation

The immune responses of pregnant cattle and their foetuses were examined following inoculation on day 70 of gestation either intravenously (iv) (group 1) or subcutaneously (sc) (group 2) with live NC1 strain tachyzoites or with Vero cells (control) (group 3). Peripheral blood mononuclear cell (PBMC) responses to Neospora antigen and foetal viability were assessed throughout the experiment. Two animals from each group were sacrificed at 14, 28, 42 and 56 days post inoculation (pi). At post mortem, maternal lymph nodes, spleen and PBMC and when possible foetal spleen, thymus and PBMC samples were collected for analysis. Inoculation with NC1 (iv and sc) lead to foetal deaths in all group 1 dams (6/6) and in 3/6 group 2 dams from day 28pi; statistically significant (p ≤ 0.05) increases in cell-mediated immune (CMI) responses including antigen-specific cell proliferation and IFN-γ production as well as increased levels of IL-4, IL-10 and IL-12 were observed in challenged dams compared to the group 3 animals. Lymph node samples from the group 2 animals carrying live foetuses showed greater levels of cellular proliferation as well as significantly (p ≤ 0.05) higher levels of IFN-γ compared to the dams in group 2 carrying dead foetuses. Foetal spleen, thymus and PBMC samples demonstrated cellular proliferation as well as IFN-γ, IL-4, IL-10 and IL-12 production following mitogenic stimulation with Con A from day 14pi (day 84 gestation) onwards. This study shows that the generation of robust peripheral and local maternal CMI responses (lymphoproliferation, IFN-γ) may inhibit the vertical transmission of the parasite.

Infected dendritic cells facilitate systemic dissemination and transplacental passage of the obligate intracellular parasite Neospora caninum in mice

The obligate intracellular parasite Neospora caninum disseminates across the placenta and the blood-brain barrier, to reach sites where it causes severe pathology or establishes chronic persistent infections. The mechanisms used by N. caninum to breach restrictive biological barriers remain elusive. To examine the cellular basis of these processes, migration of different N. caninum isolates (Nc-1, Nc-Liverpool, Nc-SweB1 and the Spanish isolates: Nc-Spain 3H, Nc-Spain 4H, Nc-Spain 6, Nc-Spain 7 and Nc-Spain 9) was studied in an in vitro model based on a placental trophoblast-derived BeWo cell line. Here, we describe that infection of dendritic cells (DC) by N. caninum tachyzoites potentiated translocation of parasites across polarized cellular monolayers. In addition, powered by the parasite's own gliding motility, extracellular N. caninum tachyzoites were able to transmigrate across cellular monolayers. Altogether, the presented data provides evidence of two putative complementary pathways utilized by N. caninum, in an isolate-specific fashion, for passage of restrictive cellular barriers. Interestingly, adoptive transfer of tachyzoite-infected DC in mice resulted in increased parasitic loads in various organs, e.g. the central nervous system, compared to infections with free parasites. Inoculation of pregnant mice with infected DC resulted in an accentuated vertical transmission to the offspring with increased parasitic loads and neonatal mortality. These findings reveal that N. caninum exploits the natural cell trafficking pathways in the host to cross cellular barriers and disseminate to deep tissues. The findings are indicative of conserved dissemination strategies among coccidian apicomplexan parasites.

Vaccines against a Major Cause of Abortion in Cattle, Neospora caninum Infection

Simple Summary

We review the efforts to develop a vaccine against neosporosis, caused by the apicomplexan parasite Neospora caninum. Vertical transmission is the main mode of infection, and can lead to stillbirth, abortion, or birth of weak calves. We provide information on the biology of Neospora caninum and on the disease caused by this parasite, and summarize the current understanding on how the host deals with infection. We review studies on live- and subunit-vaccines, and demonstrate advantages and setbacks in the use of small laboratory animal models in investigations on a disease with high relevance in cattle.

Abstract

Neosporosis, caused by the apicomplexan parasite Neospora caninum, represents one of the economically most important causes of abortion in cattle. During pregnancy, the parasite infects the placental tissue and the fetus, which can lead to stillbirth, abortion, or birth of weak calves. Alternatively, calves are born without clinical symptoms, but they can carry over the parasite to the next generation. In addition, N. caninum causes neuromuscular disease in dogs. The economic importance of neosporosis has prompted researchers to invest in the development of measures to prevent infection of cattle by vaccination. A good vaccine must stimulate protective cellular immune responses as well as antibody responses at mucosal sites and, systemically, must activate T-helper cells to produce relevant cytokines, and must elicit specific antibodies that aid in limiting parasite proliferation, e.g., by interference with host cell invasion, activation of complement, and/or opsonization of parasites to have them killed by macrophages. Different types of vaccines have been investigated, either in bovines or in the mouse model. These include live vaccines such as naturally less virulent isolates of N. caninum, attenuated strains generated by irradiation or chemical means, or genetically modified transgenic strains. Live vaccines were shown to be very effective; however, there are serious disadvantages in terms of safety, costs of production, and stability of the final product. Subunit vaccines have been intensively studied, as they would have clear advantages such as reduced costs in production, processing and storage, increased stability and shelf life. The parasite antigens involved in adhesion and invasion of host cells, such as surface constituents, microneme-, rhoptry- and dense granule-components represent interesting targets. Subunit vaccines have been applied as bacterially expressed recombinant antigens or as DNA vaccines. Besides monovalent vaccines also polyvalent combinations of different antigens have been used, providing increased protection. Vaccines have been combined with immunostimulating carriers and, more recently, chimeric vaccines, incorporating immuno-relevant domains of several antigens into a single protein, have been developed.