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Infection and Immunity, February 2006, p. 953-960, Vol. 74, No. 2
0019-9567/06/$08.00+0     doi:10.1128/IAI.74.2.953-960.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

The Protozoan Neospora caninum Directly Triggers Bovine NK Cells To Produce Gamma Interferon and To Kill Infected Fibroblasts

Preben Boysen,1 Siv Klevar,2 Ingrid Olsen,2 and Anne K. Storset1*

Department of Food Safety and Infection Biology, Norwegian School of Veterinary Science, P.O. Box 8146 Dep., N-0033 Oslo, Norway,1 Department of Animal Health, National Veterinary Institute, P.O. Box 8156 Dep., N-0033 Oslo, Norway2

Received 29 June 2005/ Returned for modification 17 August 2005/ Accepted 5 November 2005

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-{gamma}) 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-{gamma} 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-{gamma} response was measured in cocultures of NK cells and N. caninum-infected autologous fibroblasts. However, no NK cell-derived IFN-{gamma} 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.

* Corresponding author. Mailing address: Department of Food Safety and Infection Biology, Norwegian School of Veterinary Science, P.O. Box 8146 Dep., N-0033 Oslo, Norway. Phone: 47 22964776. Fax: 47 22964818. E-mail: anne.storset{at}veths.no.

Editor: J. F. Urban, Jr.

Infection and Immunity, February 2006, p. 953-960, Vol. 74, No. 2
0019-9567/06/$08.00+0     doi:10.1128/IAI.74.2.953-960.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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