Discussion

Monocytes are pivotal due to the links they form between the innate and adaptive immune response and are one of the first immune cells encountered by intra-cellular parasites during infection. Our previous data confirmed that neonatal monocytes have a higher level of secretion of IL-1β and TNF-α in response to LPS, IFN-γ and Alum, than adult derived monocytes. Here, we attempted to resolve if this age related difference was maintained in the context of in-vitro infection with Neospora caninum infection.

N. caninum (NCLiv-1) was maintained in VERO cell lines and purified CFSE labelled parasites used to infect naïve CD14⁺cells which were purified by magnetic cell separation. The number of parasitized monocytes was determined after infection or co-culture with autologous NK-cells culture. CD80 expression was determined as a marker of cellular activation, by flow cytometry. These results reveal a greater reduction of parasitaemia in neonates with higher levels of IL-1β and IL-6 during N. caninum infection compared to adult cattle.  Neonatal NK-cells also display enhanced cytotoxic activity, measured through perforin and granzyme production after co-culture with N. caninum infected monocytes. Complementary gene array analysis was also performed which suggests that during infection, neonates have a greater magnitude of response and a more complex network of upregulated genes are altered. Overall our comparisons show that there is a fundamental difference in the steady-state and in the response to intracellular parasite infection in neonatal monocyte led inflammatory responses.