Host-parasite interactions of Neospora caninum

The papers included in this thesis examine the host–parasite relationship in small and large animals following experimental challenges with Neospora caninum. This apicomplexan parasite is a major cause of abortion and reproductive losses in cattle worldwide. Economic and welfare issues make the development of a vaccine against the transplacental transmission of Neospora highly desirable. This thesis evaluates the host-parasite interactions in a non-pregnant mouse model examining whether the actively multiplying stage of the parasite (tachyzoite) could be attenuated through prolonged in vitro cultivation (passage) and used as a live vaccine. We show that continued maintenance of tachyzoites in tissue culture produced significantly reduced levels of morbidity and mortality in the mice following challenge, compared to mice receiving virulent parasites. Inoculation with a sub-lethal dose of tachyzoites was shown to protect against a subsequent lethal challenge of virulent parasites. Mice showing higher levels of cell mediated immunity (CMI) (antigen-specific splenocyte proliferation and interferon-γ (IFN-γ) production) had lower parasite burdens compared to mice with less pronounced CMI responses. Combined, these works show that it is possible to protect against a lethal challenge using attenuated tachyzoites and that a strong T-helper Type-1 CMI response is involved in protection and in reducing clinical disease severity. As the most commonly known route of infection with N. caninum is transplacental, from dam to foetus, we also wanted to examine the host-parasite relationship in pregnant cattle. This was done through the serial examination of the maternal and foetal immune responses of experimentally challenged cattle under controlled conditions at different stages throughout pregnancy. These works show the importance of the timing, location and magnitude of multiple components of the host immune response in determining foetal survival and also whether vertical transmission occurs. We show that both the maternal and foetal immune responses are critical in determining the clinical outcome of infection. A strong maternal CMI response was shown to aid foetal survival by reducing the numbers of parasites reaching and thus damaging the placenta. Due to the syndesmychorial nature of the ruminant placenta, any foetal responses observed are as a result of foetal infection. These experiments show that as pregnancy progresses the foetus goes from being immunologically immature and incapable of mounting a protective immune response (70 days of gestation (dg)) to becoming capable of mounting parasite-specific humoral, innate and CMI responses from around 140dg onwards. The experiments in pregnant cattle confirm the importance of parasite-specific proliferation and IFN-γ production, in reducing the magnitude of the parasite challenge reaching the maternal–foetal interface and aiding foetal survival. We also examined the immunodominant parasite peptides expressed in HPLC fractionated tachyzoite antigen, which are recognised by the cellular immune response of experimentally challenged cattle. Through LC-ESI-MS/MS, 6 Neospora proteins (including SAG1, SRS2 and GRA2) and a number of Toxoplasma gondii orthologues were identified and found to be recognised by CD4+ T-cells. These works collectively demonstrate the complexity of the host-parasite interaction in Neospora infections and show the importance of a CMI response in protection against the parasite.