Echinococcus granulosus: Different cytokine profiles are induced by single versus multiple experimental infections in dogs

Abstract

Modulation of host responses is an important strategy by which parasites ensure successful establishment and persistence. Host counteraction against this modulation may be required for the host to develop resistance to infection. In this pilot study, experimental infection of dogs with Echinococcus granulosus induced a strong polarization of the cytokine response towards a Th2 phenotype. Consecutive rounds of infection and cure induced resistance to infection resulting in a dramatically lower parasite burden. Repeatedly-infected resistant dogs also lost immune polarization and developed a balanced Th1/Th2 response. No major differences were observed in the production of regulatory cytokines (IL-10, TGF-β) between dogs with high parasite load and dogs with only few intestinal parasites. These results suggest that E. granulosus-driven immunomodulation contributes to successful infection in the definitive host. This information might be relevant for the development of more effective vaccines against this stage of the parasite.

Introduction

Echinococcus granulosus, the causative agent of cystic echinococcosis, is still endemic in many countries on all continents and continues to be an important cause of morbidity and mortality. Echinococcosis is an emerging zoonotic disease (new areas of infection are frequently reported), and also a re-emerging problem in some regions. (Eckert and Deplazes, 2004, Moro and Schantz, 2006, Schantz, 2006). The life cycle of E. granulosus involves two mammalian hosts. The definitive hosts are primarily dogs which harbor adult worms in their small intestines. Natural intermediate hosts, particularly sheep and cattle, become infected after ingestion of eggs released in the faeces of infected dogs. Humans are accidental intermediate hosts of this parasite (Thompson, 1995).

E. granulosus presents different immunological relationships with its hosts. Great effort has been invested to understand the immunobiology of the parasite in the intermediate host (Baz et al., 2006, Siracusano et al., 2008, Zhang et al., 2003). However, there is scarce information about the immunologic response in the definitive host, likely because of the difficulties of working with dogs, the scarcity of tools to evaluate immune responses in them, and the lack of an appropriate alternative experimental model for the establishment of infections with the tapeworm stage.

The immune system of dogs responds to a primary infection with E. granulosus, but the responses appear weak and ineffective suggesting immunomodulation by the parasite. The scoleces of worms successfully installed in dogs’ intestines adhere strongly to crypts and establish intimate contact with the gut mucosae (Howell and Smyth, 1995, Thompson, 1995). Dogs develop serum IgG and local IgA responses to infection and show parasite-specific proliferation of Peyer’s patch cells. There is also preliminary evidence for a role of IgE in the protection against infection (Deplazes et al., 1994, Jenkins and Rickard, 1986, Moreno et al., 2004). Moreover, peripheral lymphocytes from dogs experimentally infected with E. granulosus display an enhanced proliferative response to the mitogen Concanavalin A (ConA), and dogs with the highest responses had significantly fewer parasites than less reactive dogs (Al-Khalidi and Barriga, 1986). Notwithstanding, the dynamics of cellular recruitment and the profile of the inflammatory response in the gut of dogs upon infection have not yet been characterized.

Here we report that E. granulosus primary infection modifies the cytokine profile of dog cells in response to a mitogenic (ConA) stimulation. Dogs experimentally infected with E. granulosus showed a strong polarization towards a Th2 phenotype as analyzed by mRNA (messenger RNA) levels of several cytokines in cells isolated from Peyer’s patches and spleen. We also show that the cytokine responses in dogs that undergo successive rounds of infection and cure, resulting in dramatically decreased tapeworm establishment, are similar to the cytokine profile of non-infected dogs.