Protective Innate Immune Responses Against Cryptosporidium Parvum

Abstract

Cryptosporidiosis is a common infectious diarrhoeal disease of mammalian livestock and humans worldwide. The etiological organisms responsible are intestinal apicomplexans of the genus Cryptosporidium, including C. parvum, that infect intestinal epithelial cells. Immunocompromised or malnourished hosts develop severe life-threatening disease. Immunological elimination of Cryptosporidium requires CD4+ T cells and IFN-γ. Nevertheless, studies have shown innate immune responses have a significant protective role. Importantly, in T cell-deficient mice, IFN-γ is important for control of C. parvum infection. In innate immunity natural killer (NK) cells are major producers of IFN-γ and are activated by cytokines including type I IFNs but the roles of these components in immunity to Cryptosporidium infection have not been investigated. Therefore, the purpose of this project was to study the involvement of type I IFNs and NK cells in immunity to C. parvum employing in vitro and in vivo (murine) infection models. Enterocytes were shown capable of the production of type I IFNs in response to C. parvum infection. These cytokines directly inhibited parasite development in epithelial cells. Also, in neonatal SCID mice the level of infection increased after treatment with anti-type I IFN neutralising serum. A higher level of infection was observed in Rag2-/-γc-/- mice deficient in T, B and NK cells in comparison to Rag2-/- mice with a normal NK cell population and early mortality during chronic infection of adult animals was associated with the absence of NK cells. Using cultures of SCID mouse splenocytes, NK cells were the main source of IFN-γ in response to C. parvum antigen stimulation. However, IFN-γ was also found to have a protective role in Rag2-/-γc-/- mice, implying cells other than lymphocytes produce this cytokine. In conclusion, this is the first study to indicate important protective roles for type I IFNs and NK cells in innate immunity against C. parvum.