Evaluation of cellular and humoral systemic immune response against Giardia duodenalis infection in cattle.

Giardia duodenalis causes diarrhoea in humans and a wide range of mammals, including cattle. In cattle, the infection often has a chronic character. Infected calves may excrete cysts for several months, suggesting that Giardia is able to suppress and evade the immune response. In this study six calves were infected with G. duodenalis assemblage A and E and housed in an environment that allowed reinfection. Cyst excretion was monitored twice a week and blood was collected every 2 weeks, until decreasing cyst counts indicated the development of protective immunity. The kinetics of the circulating memory cells and serum antibodies were followed up throughout this period. Cyst excretion started 1 week post-infection and remained high until week 14. Low cyst counts from week 15 p.i. onwards indicated that the calves had developed immunity. From week 5 p.i. significant proliferation of CD4(+) αβ T-cells was observed after in vitro stimulation with G. duodenalis antigen. Characterisation of the proliferating CD4(+) T-cells using real time qPCR showed that at the peak of antigen driven PBMC proliferation the majority of cells were CD4(+) T-cells expressing IL-17 and to a lesser extent FoxP3. The cell proliferation was strongly reduced after plastic adhesion of the PBMC, suggesting a role for antigen-presenting cells. Failure to restore proliferation of depleted PBMC with Giardia-stimulated monocyte-derived dendritic cells (MoDC) and unchanged proliferation after depletion of CD21(+) B-cells showed that other antigen-presenting cells than MoDC and B-cells were important for T-cell proliferation. Analysis of the antibody response indicated that serum IgG1 and IgA levels against G. duodenalis assemblage A and E increased from week 11 post-infection. From the start of the antibody response, all trophozoites stained positive in an immunofluorescence assay with serum antibodies, indicating that a broad repertoire of antibodies was produced against all variant-specific surface proteins. Further research is necessary to determine which effector T-cell subset produces IL-17 and which cells play a role in antigen presentation.