The ProtozoanNeospora caninumDirectly Triggers Bovine NK Cells To Produce Gamma Interferon and To Kill Infected Fibroblasts

Abstract

Natural killer (NK) cells are considered to be key players in the early innate responses to protozoan infections, primarily indirectly by producing gamma interferon (IFN-␥) in response to cytokines, like interleukin 12 (IL-12). We demonstrate that live, as well as heat-inactivated, tachyzoites of Neospora caninum, a Toxoplasma-like protozoan, directly trigger production of IFN-␥ from purified, IL-2-activated bovine NK cells. This response occurred independently of IL-12 but was increased by the addition of the cytokine. A similar IFN-␥ response was measured in cocultures of NK cells and N. caninum-infected autologous fibroblasts. However, no NK cell-derived IFN-␥ response was detected when cells were cultured with soluble antigens from the organism, indicating that intact tachyzoites or nonsoluble components are necessary for NK cell triggering. Furthermore, N. caninum-infected autologous fibroblasts had increased susceptibility to NK cell cytotoxicity compared to uninfected fibroblasts. This cytotoxicity was largely mediated by a perforin-mediated mechanism. The activating receptor NKp46 was involved in cytotoxicity against fibroblasts but could not explain the increased cytotoxicity against infected targets. Interestingly, N. caninum tachyzoites were able to infect cultured NK cells, in which tachyzoites proliferated inside parasitophorous vacuoles. Together, these findings underscore the role of NK cells as primary responders during a protozoan infection, describe intracellular protozoan infection of NK cells in vitro for the first time, and represent the first functional study of purified bovine NK cells in response to infection.

Direct Binding of Human NK Cell Natural Cytotoxicity Receptor NKp44 to the Surfaces of Mycobacteria and Other Bacteria

“…It has also been reported that the infective forms of Leishmania species (live promastigotes) are able to directly activate NK cells to secrete IFN-␥ in the absence of accessory cells (7,24). Finally, evidence for a direct contact between bovine NK cells and the protozoan Neospora caninum has recently been reported (9).…”

Roles of CD122+ Cells in Resistance against Neospora caninum Infection in a Murine Model

“…A former study indicated that N. caninum directly become a trigger for the production of IFN- from purified, IL-2-activated bovine NK cells which killed infected fibroblasts [5]. Furthermore, NK cells have been shown to act as early responders in experimental infection with N. caninum in calves [19].…”

“…Generally, innate cells, such as natural killer (NK) cells and NKT cells, play essential roles as primary effector cells at the interface between the host and parasite until the establishment of adaptive immunity [16]. Previously, it has been shown that N. caninum becomes a trigger to produce IFN- in bovine NK cells [5,19]. However, the role of NK or NKT cells in the defense against N. caninum has not been well clarified.…”

Indoleamine 2,3-Dioxygenase Is Involved in Defense againstNeospora caninumin Human and Bovine Cells

“…For example, Pfefferkorn found that the IFN-␥-inducible tryptophan-degrading enzyme IDO is capable of blocking the growth of T. gondii (36). Proliferation of N. caninum in bovine fibroblasts was analyzed in a coculture system consisting of N. caninum-infected fibroblasts and bovine NK cells (6). Boysen et al found that N. caninuminfected fibroblasts induce IFN-␥ production in NK cells, which inhibit N. caninum growth by killing infected fibroblasts.…”