The disease is characterized by diarrhoea and abdominal pain that normally last several days but infection can be chronic and life-threatening in immunocompromised hosts. Human illness predominantly involves two parasite species, C. hominis that is occasionally found in non-human hosts and C. parvum that infects many mammalian host species and is an important zoonotic pathogen [1]. Disease in livestock such as cattle and sheep occurs only during the neonatal period but immunocompetent humans may develop

symptoms at any age [2]. The entire DNA Damage inhibitor asexual and sexual development of Cryptosporidium takes place in epithelial cells and infection is transmitted faecal-orally by oocysts that contain four sporozoites. During host cell invasion sporozoites and merozoites do not enter the cytoplasm; instead the adjacent epithelial membrane moves to encapsulate the zoite, providing an epicellular niche for parasite development [3]. It is not known if this unusual extracytoplasmic location partially protects the parasite

from immunological attack. Parasite antigens have been shown to be expressed in the segment of host cell membrane surrounding the parasite and in the parasitophorous vacuole membrane [4]. Most of the Selleck BAY 57-1293 available knowledge of host adaptive immune responses comes from studies with mice infected with C. parvum (mice are refractory to infection with C. hominis). However, there is some understanding of mechanisms of adaptive immunity against cryptosporidia in humans and cattle. In adult mice lacking CD4+ T cells C. parvum infection is chronic and eventually causes morbidity and death [5]. For elimination of infection in humans, CD4+ T cells

are also likely to be necessary since late stage AIDS patients with low CD4+ T cell numbers commonly experience cryptosporidial infection that is chronic, spreads to extraintestinal sites (e.g. bile ducts or pancreas) and is eventually fatal [6]. The introduction of antiretroviral drugs that restore Low-density-lipoprotein receptor kinase the CD4+ T cell population has reduced the incidence of cryptosporidial infection in HIV-infected individuals [7]. Some studies with mice have suggested that CD8+ T cells or B cells may have roles in resistance but neither cell type appears to be essential for elimination of infection [5, 8, 9]. MHC Class I-dependent human CD8+ T cells cytotoxic for intestinal epithelial cells infected with C. parvum have been developed in vitro [10] but there have been no reports showing the presence of antigen-specific cytotoxic T cells in vivo. In mice, humans and cattle, development of immunity has been associated with elevated expression of the Th1 cytokines IFN-γ and IL-12 and, in mice, IL-18 [8, 11, 12]. Mice deficient in these cytokines have been shown to have increased susceptibility to infection and in some reports IFN-γ−/− mice developed fatal infections [12, 13].