Evaluation of the protection conferred by a naturally attenuated Neospora caninum isolate against congenital and cerebral neosporosis in mice

Working towards the development of vaccines and chemotherapeutics against neosporosis-With all of its ups and downs-Looking ahead

Neospora caninum is an apicomplexan and obligatory intracellular parasite, which is the leading cause of reproductive failure in cattle and affects other farm and domestic animals, but also induces neuromuscular disease in dogs of all ages. In cattle, neosporosis is an important health problem, and has a considerable economic impact. To date there is no protective vaccine or chemotherapeutic treatment on the market. Immuno-prophylaxis has long been considered as the best control measure. Proteins involved in host cell interaction and invasion, as well as antigens mediating inflammatory responses have been the most frequently assessed vaccine targets. However, despite considerable efforts no effective vaccine has been introduced to the market to date. The development of effective compounds to limit the effects of vertical transmission of N. caninum tachyzoites has emerged as an alternative or addition to vaccination, provided suitable targets and safe and efficacious drugs can be identified. Additionally, the combination of both treatment strategies might be interesting to further increase protectivity against N. caninum infections and to decrease the duration of treatment and the risk of potential drug resistance. Well-established and standardized animal infection models are key factors for the evaluation of promising vaccine and compound candidates. The vast majority of experimental animal experiments concerning neosporosis have been performed in mice, although in recent years the numbers of experimental studies in cattle and sheep have increased. In this review, we discuss the recent findings concerning the progress in drug and vaccine development against N. caninum infections in mice and ruminants.

A Listeria monocytogenes-Based Vaccine Formulation Reduces Vertical Transmission and Leads to Enhanced Pup Survival in a Pregnant Neosporosis Mouse Model

The apicomplexan parasite Neospora caninum is the worldwide leading cause of abortion and stillbirth in cattle. An attenuated mutant Listeria monocytogenes strain (Lm3Dx) was engineered by deleting the virulence genes actA, inlA, and inlB in order to avoid systemic infection and to target the vector to antigen-presenting cells (APCs). Insertion of sag1, coding for the major surface protein NcSAG1 of N. caninum, yielded the vaccine strain Lm3Dx_NcSAG1. The efficacy of Lm3Dx_NcSAG1 was assessed by inoculating 1 × 10⁵, 1 × 10⁶, or 1 × 10⁷ CFU of Lm3Dx_NcSAG1 into female BALB/c mice by intramuscular injection three times at two-week intervals, and subsequent challenge with 1 × 10⁵N. caninum tachyzoites of the highly virulent NcSpain-7 strain on day 7 of pregnancy. Dose-dependent protective effects were seen, with a postnatal offspring survival rate of 67% in the group treated with 1 × 10⁷ CFU of Lm3Dx_NcSAG1 compared to 5% survival in the non-vaccinated control group. At euthanasia (25 days post-partum), IgG antibody titers were significantly decreased in the groups receiving the two higher doses and cytokines recall responses in splenocyte culture supernatants (IFN-γ, IL-4, and IL-10) were increased in the vaccinated groups. Thus, Lm3Dx_NcSAG1 induces immune-protective effects associated with a balanced Th1/Th2 response in a pregnant neosporosis mouse model and should be further assessed in ruminant models.

Vaccination with Neospora GRA6 Interrupts the Vertical Transmission and Partially Protects Dams and Offspring against Neospora caninum Infection in Mice

Vaccination is the mainstay of preventative measures for numerous infectious diseases. Neospora caninum infection induces storms of abortion in pregnant cows and ewes, resulting in drastic economic losses because of fetal losses and culling of the dams. Herein, we evaluated the potential of recombinant protein of N. caninum dense granule protein 6 fused with glutathione-S-transferase (NcGRA6+GST) as a vaccine candidate against neosporosis in a pregnant mouse model. The protective efficacy was investigated by subcutaneous inoculation of BALB/c mice with recombinant NcGRA6+GST (25 pmol), and GST alone (25 pmol) or phosphate-buffered saline (PBS) as the controls. This study revealed the partial ability of NcGRA6+GST to protect the dams and offspring from N. caninum infection during the critical period of pregnancy. This ability was revealed by higher survival rate and lower parasite burden in brains of offspring of the NcGRA6+GST-immunized group in comparison with the control groups. In addition, mouse dams from NcGRA6+GST-immunized groups exhibited lower clinical score and minimum alteration in body weight in comparison with PBS or GST groups after challenge with N. caninum tachyzoites. Taken together, our results suggest the efficacy of recombinant NcGRA6 for interrupting the vertical transmission of N. caninum in mice by reducing the severity of infections in dams and offspring.

The Impact of BKI-1294 Therapy in Mice Infected With the Apicomplexan Parasite Neospora caninum and Re-infected During Pregnancy

Exposure of Neospora caninum tachyzoites to BKI-1294 in vitro results in the formation of long-lived multinucleated complexes (MNCs). However, in vivo treatment of BALB/c mice with BKI-1294 shortly after N. caninum infection during pregnancy was safe and profoundly reduced pup mortality and vertical transmission. We hypothesized that the formation of MNCs could trigger immune responses that contribute to BKI efficacy in vivo. In this study, mice were first vaccinated with a sublethal dose of N. caninum tachyzoites and were treated with BKI-1294. We then investigated the effects of these treatments after mating and re-infection during pregnancy. Effects on fertility, pup survival, vertical transmission, and parasite load in dams were evaluated. Cytokines in sera or splenocyte culture supernatants were assessed by either ELISA or the Luminex™ 200 system, and humoral immune responses against tachyzoite and MNC antigens were compared by ELISA, Western blotting and immunoproteomics. Our results showed that BKI-1294 treatment of live-vaccinated mice reduced the cerebral parasite load in the dams, but resulted in higher neonatal pup mortality and vertical transmission. In live-vaccinated mice, cytokine levels, most notably IFN-y, IL-10, and IL-12, were consistently lower in BKI-1294 treated animals compared to non-treated mice. In addition, comparative Western blotting identified two protein bands in MNC extracts that were only recognized by sera of live-vaccinated mice treated with BKI-1294, and were not found in tachyzoite extracts. We conclude that treatment of live-vaccinated mice with BKI-1294 influenced the cellular and humoral immune responses against infection, affected the safety of the live-vaccine, and decreased protection against re-infection and vertical transmission during pregnancy.

The evaluation of attenuated Neospora caninum by long-term passages on murine macrophage cell line in prevention of vertical transmission in mice

To date, there is no effective vaccine to prevent abortion or vertical transmission associated with neosporosis in cattle. In the present study, the efficacy of a live experimental vaccine of Neospora caninum attenuated (NCa) by long-term serial passages on a murine macrophage cell line was evaluated in the prevention of vertical transmission and abortion in the mouse model. Forty non-pregnant mice were randomly divided into four equal groups including non-immunized/challenged (injected with PBS); positive control (inoculated with un-attenuated NC-1 tachyzoites); immunized/challenged (inoculated with NCa attenuated strain) and immunized/non-challenged or vaccinated (inoculated with NCa) groups. Following pregnancy synchronization, both the immunized and control mice were challenged with virulent live NC-1 tachyzoites (2.5 × 10⁶) in the mid-pregnancy stage. The number of abortions and post-natal pup mortalities was recorded. Serological, molecular, and histopathologic examinations were employed to evaluate the efficacy of the vaccine and the vertical transmission rates. Results indicated that the live attenuated N. caninum strain (NCa) could significantly reduce the risk of abnormal parturitions and fetal mortality in the vaccinated group (20 %) compared to the non-immunized/challenged group (80 %). Also, the NCa strain reduced the lesion score in the brain of the offspring (0.3 vs 1.9) compared to the non-immunized/challenged group (P < 0.05). The molecular assay showed a decrease in the parasite DNA detection rates from 83 % and 77 % in the non-immunized/challenged group to 27 % and 0 % in the vaccine group in the brain and liver tissues, respectively. While in the immunized/non-challenged group no parasite DNA was detected in the brain tissue samples of the pups. Serological analyses showed that NCa strain was able to stimulate the humoral immunity and create effective protection against neosporosis with a moderate systemic IFN-γ response. In conclusion, the NCa strain could significantly (P < 0.05) reduce the risk of vertical transmission and proved to be a safe vaccine while conferring significant levels of protection in the laboratory mice.

A model for chronic bovine besnoitiosis: Parasite stage and inoculation route are key factors

In this work, an experimental model for chronic besnoitiosis in bovine was developed and characterized. Using a previously established calf model, two new variables (parasite stage and inoculation route) were combined and used. Twelve Holstein Friesian 3-month-old male calves were randomly divided into four groups of three animals each. Bradyzoites were obtained from a chronically infected bull and used for inoculation via three different inoculation routes. Three groups were inoculated with 10⁶ bradyzoites by intravenous (G1), subcutaneous (G2) and intradermal (G3) routes, and a non-infected control group (G4) was inoculated with PBS. The trial lasted for 90 days and included daily clinical monitoring as well as weekly skin biopsies and blood sampling. Sera were obtained to analyse both cellular and humoral responses. Once the calves were euthanized, tissues from the skin, eyes, respiratory and reproductive tracts, among others, were collected to study presence of the parasite. Clinically, the infection was classified as mild to moderate for the acute stage since all infected calves showed lymphadenopathy from four days post-infection (pi) and fever from one week pi until 24 days pi. However, the most relevant results were achieved during the chronic stage that was classified as moderate to severe. In fact, pathognomonic conjunctival cysts were observed in all infected calves from 40 days pi onwards and were more abundant in G3. Moreover, one calf from this group developed skin lesions (49 days pi). The microscopic tissue cysts and Besnoitia DNA were detected primarily in skin, reproductive tract and respiratory tissue samples, and parasite load was higher in G3. In conclusion, the parasite stage (bradyzoite) and the inoculation route are key factors that influence the outcome of an infection. In particular, the intradermal route led to more severe clinical signs of the chronic phase in the inoculated calves.

Immunization with a cocktail of antigens fused with OprI reduces Neospora caninum vertical transmission and postnatal mortality in mice

OprI is an outer membrane lipoprotein from Pseudomonas aeruginosa, and when fused to a recombinant antigen, will exert adjuvant properties by engaging Toll-like receptor 2, leading to dendritic cell activation. Previous studies have shown that the Neospora caninum (Nc) antigens NcPDI, NcROP2 and NcROP40 are implicated in host cell interactions and are promising vaccine candidates. In two independent experiments, the efficacy of a polyvalent vaccine formulation composed of OprI-NcPDI, OprI-NcROP2 and OprI-NcROP40 (collectively named O-Ags) was assessed in non-pregnant and pregnant Balb/c mouse models challenged with tachyzoites of the high-virulence isolate Nc-Spain7. Parameters that were investigated were clinical signs, fertility, parasite burden in adult mice, humoral and cellular immune responses at different time-points prior to and after challenge infection, vertical transmission and post-natal survival of offspring mice, all to explore potential correlations with efficacy. Vaccination of mice with O-Ags induced a mixed Th1/Th2 immune response in adult mice and led to significantly increased protection against cerebral infection. Vaccination with O-Ags also resulted in reduced vertical transmission, and postnatal disease in offspring was significantly inhibited at a rate not observed in mice infected with a high-virulence isolate to date. However, O-Ags mixed with TLR ligands targeting TLR3 and TLR7, which are known to induce clear Th1-biased responses, or vaccination with OprI fused to the non-N. caninum antigen ovalbumin (OprI-OVA) did not confer protection.

Neospora caninum: Chronic and congenital infection in consecutive pregnancies of mice

Neospora caninum, the causative agent of bovine neosporosis is the major cause of abortion in cattle worldwide. The principal route of transmission is via in utero infection of the offspring. Congenitally-infected dams remain persistently infected for life and might undergo abortions in consecutive pregnancies. In the present study, the effect of N. caninum in chronic and congenital infection was examined. CD1 mice were infected intra-peritoneally with live tachyzoites of the NcIs491 isolate, while non-infected mice served as a control. There were no clinical signs of infection observed following inoculation, but high titers of specific anti- N. caninum antibodies were detected. A month after infection, when chronic-infection was established, mice were mated. Fertility, litter size and mortality rate were monitored within two generations of four consecutive pregnancies. During a nine months period of the study all females maintained high level of antibodies, while the non- infected control mice remained seronegative. There was no difference in the fertility rate of the dams, or in the litter size of infected and control mice. Mortality of offspring of the first and second generations of the infected dams was observed within the two first weeks of life. The vertical transmission was analyzed by PCR assay of offspring brains. PCR positive results were found in all 13 litters of the first generation tested during four consecutive pregnancies. The rate of vertical transmission slightly decreased in successive pregnancies being 74.2%, 59.5%, 48.1% and 40% for the first to fourth pregnancies respectively. In the second generation 21 out of 28 litters were found positive and the overall rate of vertical transmission was 28.5%. In chronically and congenitally infected dams N. caninum infection was maintained during all successive pregnancies for about 9 months. The results show that CD-1 outbred mice are a suitable model for studying chronic and congenital neosporosis.

Approaches for the vaccination and treatment of Neospora caninum infections in mice and ruminant models

Neospora caninum is a leading cause of abortion in cattle, and is thus an important veterinary health problem of high economic significance. Vaccination has been considered a viable strategy to prevent bovine neosporosis. Different approaches have been investigated, and to date the most promising results have been achieved with live-attenuated vaccines. Subunit vaccines have also been studied, and most of them represented components that are functionally involved in (i) the physical interaction between the parasite and its host cell during invasion or (ii) tachyzoite-to-bradyzoite stage conversion. Drugs have been considered as an option to limit the effects of vertical transmission of N. caninum. Promising results with a small panel of compounds in small laboratory animal models indicate the potential value of a chemotherapeutical approach for the prevention of neosporosis in ruminants. For both, vaccines and drugs, the key for success in preventing vertical transmission lies in the application of bioactive compounds that limit parasite proliferation and dissemination, without endangering the developing fetus not only during an exogenous acute infection but also during recrudescence of a chronic infection. In this review, the current status of vaccine and drug development is presented and novel strategies against neosporosis are discussed.

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.

Effect of vaccination of cattle with the low virulence Nc-Spain 1H isolate of Neospora caninum against a heterologous challenge in early and mid-gestation

Live vaccines have emerged as one of the most potentially cost-effective measures for the control of bovine neosporosis. Previous studies have shown that Nc-Spain 1H is a naturally attenuated isolate of Neospora caninum and that immunisation with live Nc-Spain 1H tachyzoites generated a protective immune response in mice. The aim of this study was to evaluate the safety and efficacy of immunisation in cattle. N. caninum-seronegative heifers were immunised subcutaneously twice with 10⁷ live Nc-Spain 1H tachyzoites prior to artificial insemination. No adverse reactions or negative effects on reproductive parameters were recorded following immunisation. In immunised and non-challenged heifers, no foetal deaths were observed, and none of the calves was congenitally infected. The efficacy against N. caninum-associated foetal death and vertical transmission was determined after challenge with high doses of the Nc-1 isolate at 70 and 135 days of gestation, respectively. After the challenge in early gestation, the immunisation induced a protection of 50% against foetal death. In addition, the microsatellite analysis performed in PCR-positive tissue samples from foetuses that died after challenge infection showed that the profiles corresponded to the challenge isolate Nc-1. A degree of protection against vertical transmission was observed after challenge at mid-gestation; calves from immunised heifers showed significantly lower pre-colostral Neospora-specific antibody titres than calves from the non-immunised/challenge group (P < 0.05). Strong antibody and interferon gamma responses were induced in the immunised heifers. This study indicates that the immunisation before pregnancy with the Nc-Spain 1H vaccine isolate appeared to be safe and reduced the occurrence of N. caninum-associated abortion and vertical transmission in experimentally infected cattle. In light of these encouraging results, the next step for testing this live attenuated candidate should be the assessment of its efficacy and safety in naturally infected cattle.