The development of immune responses in Balb/c mice following inoculation with attenuated or virulentNeospora caninumtachyzoites

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.

The pathogenicity of two Neospora caninum goat strains in a BALB/c mouse model

Goats are frequently described as an intermediate host for the protozoan Neospora caninum, manifesting the disease mainly by recurrent abortions with placentitis and encephalitis in fetuses. Several reports of natural and experimental infections in cattle and mice show differences in the immune response, and the outcome of the infection can be variable depending on the species affected and by the behavior of the infective strain. This study describes for the first time two Neospora caninum strains isolated from naturally infected goats from the state of Minas Gerais, Brazil. One placenta and one brain from different goats were processed for a first bioassay in gerbils (Meriones unguiculatus). Subsequently, a second bioassay was performed by inoculating the processed brain samples from gerbils into Interferon gamma (IFN-γ) knockout mice (KO mice). Tachyzoites collected from the peritoneal fluid of the KO mice were inoculated into VERO cell monolayers, where they presented a very slow growth rate. The tachyzoites were also inoculated into BALB/c mice with a dose of 10⁶ tachyzoites per animal. After a 5-week follow up, the animals infected with both of the strains developed a strong polarized Th1 response with increased serum and spleen gene expression levels of pro-inflammatory cytokines (mainly IFN-γ and TNF-α) in the first week. Tissue lesions were mild in the animals infected with both strains. Despite the strong immune response preventing an infection in the visceral organs, the parasite was able to reach the brain, causing progressive brain lesions from the second to fifth week post infection. The NC-goat1-infected mice presented with severe meningoencephalitis, but the NC-goat2-infected animals had considerable histological brain lesions only at week 5. Immunohistochemical analysis of the mouse brains revealed a different pattern of inflammatory cells compared to the naturally infected goats. A severe inflammatory infiltrate of CD3⁺ T lymphocytes was found in the NC-goat1-infected mice. A more discrete infiltrate of CD3⁺ T cells was found in the NC-goat2-infected animals. Additionally, IBA1 IHC revealed an intense microglial reaction and monocyte perivascular cuffs in the NC-goat1-infected animals and lower microglia/monocyte infiltrates in the NC-goat2-infected mice. This work contributes knowledge on the pathogenicity of new Neospora caninum strains in mice, comparable with other well-established mouse models of the disease, and demonstrates the importance of studying goats as an intermediate host of this 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.

Apicomplexan profilins in vaccine development applied to bovine neosporosis

Neospora caninum, an intracellular protozoan parasite from the phylum Apicomplexa, is the etiologic agent of neosporosis, a disease considered as a major cause of reproductive loss in cattle and neuromuscular disease in dogs. Bovine neosporosis has a great economic impact in both meat and dairy industries, related to abortion, premature culling and reduced milk yields. Although many efforts have been made to restrain bovine neosporosis, there are still no efficacious control methods. Many vaccine-development studies focused in the apicomplexan proteins involved in the adhesion and invasion of the host cell. Among these proteins, profilins have recently emerged as potential vaccine antigens or even adjuvant candidates for several diseases caused by apicomplexan parasites. Profilins bind Toll-like receptors 11 and 12 initiating MyD88 signaling, that triggers IL-12 and IFN-γ production, which may promote protection against infection. Here we summarized the state-of-the-art of novel vaccine development based on apicomplexan profilins applied as antigens or adjuvants, and delved into recent advances on N. caninum vaccines using profilin in the mouse model and in cattle.

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.

Use of a Th1 Stimulator Adjuvant for Vaccination against Neospora caninum Infection in the Pregnant Mouse Model

Vertical transmission from an infected cow to its fetus accounts for the vast majority of new Neospora caninum infections in cattle. A vaccine composed of a chimeric antigen named recNcMIC3-1-R, based on predicted immunogenic domains of the two microneme proteins NcMIC1 and NcMIC3, the rhoptry protein NcROP2, and emulsified in saponin adjuvants, significantly reduced the cerebral infection in non-pregnant BALB/c mice. Protection was associated with a mixed Th1/Th2-type cytokine response. However, the same vaccine formulation elicited a Th2-type immune response in pregnant mice and did not prevent vertical transmission or disease, neither in dams nor in offspring mice. In this study, an alternative vaccine formulation containing recNcMIC3-1-R emulsified in Freund’s incomplete adjuvant, a stimulator of the cellular immunity, was investigated. No protection against vertical transmission and cerebral infection in the pregnant mice and a very limited protective effect in the non-pregnant mice were observed. The vaccine induced a Th1-type immune response characterized by high IgG2a titres and strong IFN-γ expression, which appeared detrimental to pregnancy.

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

The parasite Neospora caninum is an important abortifacient agent in cattle worldwide. At present, the development of an effective and safe vaccine against bovine neosporosis is of great relevance. Recently, a new isolate of N. caninum (Nc-Spain 1 H) which was obtained from the brain of an asymptomatic congenitally infected calf, exhibited non-virulent behaviour in mouse and bovine infection models. The aim of this study was to determine the safety and efficacy of Nc-Spain 1 H when used as a vaccinal isolate in well-established BALB/c models of congenital and cerebral neosporosis. Mice were subcutaneously immunised twice at 3-week intervals and were challenged with 2 × 10⁶ tachyzoites of the virulent Nc-Liv isolate. After immunisation with live Nc-Spain 1 H tachyzoites, no parasitic DNA was detected in the dams’ brains before challenge and microsatellite analysis performed in PCR-positive mice showed that the profiles corresponded to the challenge isolate Nc-Liv, indicating the Nc-Spain 1 H isolate to be a safe vaccine candidate. The efficacy of the live vaccine was evaluated in the first experiment after the immunisation of mice with 5 × 10⁵ live Nc-Spain 1 H tachyzoites. This immunisation protocol significantly reduced the neonatal mortality to 2.4%, reduced the vertical transmission from 89.1% to 2.3% and completely limited the cerebral infection. These results were associated with a Th1-type immune response. In the second experiment, the effect of various immunising doses was established using ten-fold dilutions of the tachyzoites (from 5 × 10⁵ to 5 × 10). In all the cases, congenital protection rates above 60% were observed, and the mice that were immunised with the lowest dose (5 × 10) presented the highest protection rate (86%). Moreover, low immunising doses of Nc-Spain 1 H induced an IgG2a response, and high parasitic doses induced an IgG1 response. These results evidence the safety and the efficient protection that was conferred by Nc-Spain 1 H against congenital neosporosis, even when the mice were immunised with low parasitic doses.

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

Neospora caninum is an intracellular protozoan pathogen that causes abortion in cattle. We studied how the interaction between murine conventional dendritic cells or macrophages and N. caninum influences the generation of cell-mediated immunity against the parasite. We first explored the invasion and survival ability of N. caninum in dendritic cells and macrophages. We observed that protozoa rapidly invaded and proliferated into these two cell populations. We then investigated how Neospora-exposed macrophages or dendritic cells distinguish between viable and non-viable (heat-killed tachyzoites and antigenic extract) parasites. Viable tachyzoites and antigenic extract, but not killed parasites, altered the phenotype of immature dendritic cells. Dendritic cells infected with viable parasites down-regulated the expression of MHC-II, CD40, CD80 and CD86 whereas dendritic cells exposed to N. caninum antigenic extract up-regulated the expression of MHC-II and CD40 and down-regulated CD80 and CD86 expression. Moreover, only viable tachyzoites and antigenic extract induced IL-12 synthesis by dendritic cells. MHC-II expression was up-regulated and CD86 expression was down-regulated at the surface of macrophages, regardless of the parasitic form was encountered. However, IL-12 secretion by macrophages was only observed under conditions using viable and heat-killed parasite. We then analysed how macrophages and dendritic cells were involved in inducing T-cell responses. T lymphocyte IFN-γ-secretion in correlation with IL-12 production occurred after interactions between T cells and dendritic cells exposed to viable tachyzoites or antigenic extract. By contrast, for macrophages IFN-γ production was IL-12-independent and only occurred after interactions between T cells and macrophages exposed to antigenic extract. Thus, N. caninum-induced activation of murine dendritic cells, but not that of macrophages, was associated with T cell IFN-γ production after IL-12 secretion.

Protection against lethal Neospora caninum infection in mice induced by heterologous vaccination with a mic1 mic3 knockout Toxoplasma gondii strain

Neospora caninum and Toxoplasma gondii are closely related, obligate intracellular parasites infecting a wide range of vertebrate hosts and causing abortion and neonatal morbidity and mortality. Several lines of evidence suggest that cross immunity between these two pathogens could be exploited in the design of strategies for heterologous vaccination. We assessed the ability of an attenuated strain of T. gondii ("mic1-3KO strain") conferring strong protection against chronic and congenital toxoplasmosis to protect mice against lethal N. caninum infection. Mice immunized with mic1-3KO tachyzoites by the oral and intraperitoneal routes developed a strong cellular Th1 response and displayed significant protection against lethal heterologous N. caninum infection, with survival rates of 70% and 80%, respectively, whereas only 30% of the nonimmunized mice survived. We report here the acquisition of heterologous protective immunity against N. caninum following immunization with a live attenuated mic1-3KO strain of T. gondii.