Giardiasis remains among the most common parasitic causes of food- and water-borne diarrhoea disease worldwide and often presents with diverse clinical manifestations ranging from self-resolved asymptomatic infection to acute diarrhoea, nausea, vomiting and malabsorption. Host protective immunity to the parasite relies preferentially on intestinal IgA production and on pro-inflammatory Th17 responses. However, whether additional immune mechanisms like regulatory T cells influence the efficiency of Th17 responses during giardiasis remains unexplored. Furthermore, small intestinal eosinophils are known to provide support for IgA class switching in PP and IgA+ plasma cells in siLP and in addition, they have been shown to constrain intestinal Th17 responses. Whether eosinophils are an essential component of host immunity in terms of support for IgA and constrains for Th17 responses has also not been investigated so far. Therefore this study focused on investigating intestinal Treg/Th17 balance in inbred mouse strains with known differences in susceptibility to G. muris and in their propensity for Th17 responses. In addition, eosinophil-deficient dblGATA-1 mice were investigated for their induction and maintenance of IgA and Th17 responses during the acute and chronic stages of infection. Aims: 1. To ascertain whether inbred mouse strains of different genetic backgrounds, known to display differences in their control of Giardia infection. 2. To establish whether Giardia infection leads to differences in Treg responses or an imbalance in Treg/Th17 ratios in these hosts and how that correlates with efficient control of infection. 3. To determine whether small intestinal eosinophils play a role in the induction and maintenance of intestinal IgA and Th17 responses during G. muris infection. 4. To establish whether murine eosinophils can directly recognise and respond to Giardia trophozoite antigens It was found that while acute G. muris infection does not lead to a significant upregulation in IL-17A secreting cells in PP or siLP in either mouse strain, it does induce higher proliferation, metabolic activity and retention of Th17 cells in C57BL/6 mice, but not in BALB/c. BALB/c mice were found to harbour constitutively lower frequencies of Th17 cells and IL-17A+ T cells, however they display significantly higher frequencies of a subset of RORgt+ Tregs than C57BL/6 mice and this coincides with their higher cyst shedding rates and lower Th17 cell activity, and resulted in a shift in RORgt+ Treg:Th17 ratios in response to infection. Furthermore, C57BL/6 mice displayed a trend for unregulated IgA-producing cells in siLP in response to infection and harboured significantly higher serum IgA and IgG2b antibodies than BALB/c mice. Furthermore, eosinophil-deficient dblGATA-1 mice mice and littermate control mice infected with G. muris showed no decreases in their propensity for IgA class switching in PP or Th17 cell responses during the chronic stages of infection. This work demonstrates that poor control of G. muris infection in inbred mice coincides with low Th17 proliferation, metabolic activity and IL-17A production, as well as with constitutively higher frequencies of RORgt+ Tregs and with a significant shift in RORgt+ Treg:Th17 ratios, suggesting that Tregs potentially play an important role in the maintenance of host protective immunity during giardiasis. Furthermore, results here indicate that small intestinal eosinophils are potentially redundant with respect to their involvement in the induction and maintenance of intestinal IgA and Th17 responses during giardiasis.
