Dendritic cells (DCs) play an essential role in bridging the innate and adaptive immune system, activating T cell responses against invading pathogens. It has been shown definitively that DCs fulfil the vital role of activating Th2 responses in the liver and spleen during infection with the parasitic helminth Schistosoma mansoni. However, DCs are an incredibly heterogeneous cell type, with diverse subsets displaying different phenotypes and functions in specific tissues in the body. Moreover, relatively little is known about how DCs become activated and stimulate T cells in response to Th2-associated parasitic helminths. This thesis addresses how distinct DC subsets function in response to schistosomes, both in vitro and in vivo. The primary DC differentiation factor, Flt3-L, generates DC subsets in vitro that are analogous to the subsets resident in the lymphoid organs in the steady-state: CD24+ conventional DCs (cDCs, CD8α+ equivalents), CD11b+ cDCs and plasmacytoid DCs (pDCs). These different DC subsets displayed distinct responses to the strongly Th2- polarising soluble egg antigens (SEA) from S. mansoni. pDCs are unlikely to play a role in priming the Th2 response against SEA, although pDCs upregulated surface expression of MHC II and co-stimulatory molecules, these markers were expressed only at very low-levels, and pDCs failed to migrate to the draining lymph node (dLN) following adoptive transfer. In contrast, cDCs migrated efficiently to the T cell zone of the dLN. CD11b+ and CD24+ cDCs also significantly upregulated expression of the surface markers associated with T cell priming in response to SEA, however, this was a muted surface phenotype when compared to the classical activation elicited by a bacterial stimulus. The DC subsets produced very little cytokine in response to SEA stimulation, with the exception of Type I Interferons (IFN-I), which were uniquely secreted by CD24+ cDCs. The Toll-like receptor (TLR) adaptor proteins, TRIF and MyD88, were revealed to have contrasting roles in the control of SEA-specific IFN-I production. TRIF was essential for this response, whilst MyD88 acted as a negative regulator. TLRs are not the only receptors involved in this response however, as the C-type lectin CD205 was also required for optimal IFN-I production by SEA-stimulated cells. IFN-I proved critical to the ability of Flt3L-generated cDCs (FL-cDCs) to polarise responses following adoptive transfer, as IFN-I receptor-deficient (Ifnar1-/-) cells failed to prime an SEA-specific Th2 response in the dLN. Ifnar1-/- cells were almost completely unresponsive to SEA stimulation, failing to upregulate co-stimulatory molecules on their surface or to produce IFN-I. However Ifnar1-/- FL-cDCs displayed no deficiency in their ability to initiate T cell proliferation or IL-4-dependent Th2 polarisation in vitro. As T cell priming was abrogated in vivo only, this suggests that optimal cDC migration may be abrogated in the absence of the IFN-I receptor, although this is yet to be demonstrated definitively. The importance of IFN-I responsiveness for optimal Th2 induction during helminth infection was also assessed. Th2 responses were normal in the liver of S. mansoni Ifnar1-/- mice; however, IL-4 and IL-13 levels in the mesenteric LN (MLN) were drastically reduced. It was found that Th2 induction in the MLN was also ablated in mice infected with the gastrointestinal helminth Heligomosoides polygyrus. This suggests that there is a selective dependency on IFN-I for the activation of Th2 responses in lymphoid organs. The small intestine and the MLN provided an ideal site for further investigation of the development of the schistosome-specific immune response in peipheral tissues versus the draining lymph nodes, as this site is directly affected by parasite egg traffic during S. mansoni infection. The intestine is a unique immune environment – with a propensity towards regulation and tolerance, and a large population of innate effectors. Intestinal DCs depend on Flt3-L for their generation; however, the importance of DCs resident in the intestinal lamina propria (LP) for the initiation of Th2 inflammation in response to helminths is not yet known. Characterisation of LP DCs indicated that the activation of these cells is modulated during acute S. mansoni infection, whilst a novel model of schistosome egg deposition in the intestinal tissue illustrated that CD11c+ cells are essential for induction of the egg-specific Th2 response in both the LP and MLN following egg challenge. These data demonstrate the importance of IFN-I signalling for the development of helminth-specific immune responses, highlighting for the first time a role for this pluripotent innate effector in Th2 induction. Development of an egg challenge model in the intestine also provides an ideal setting with which to further explore the importance of IFN-I for Th2 polarisation in peripheral tissues and lymphoid organs.

Infection with the helminth parasite Schistosoma mansoni leads to chronic inflammation and Th2 mediated fibrosis, which result in severe pathology characterised by hepatosplenomegaly. Dendritic cells (DCs) are adept initiators of CD4+ T cell responses, but their fundamental importance in this regard in Th2 settings remains to be demonstrated. Indeed, the role of DCs at different stages of infection with S. mansoni is also yet to be determined. In addition, the importance of the interaction of DCs with tissue factors in the tissue microenvironment on the development of Th2 response to S. mansoni antigens is an area of active research and debate. This thesis is comprises of four studies. The first study tackles the involvement and importance of DCs in the induction and development of Th2 responses against S. mansoni using CD11c–diphtheria toxin receptor mice to deplete CD11c+ cells during the priming stage of the CD4+ Th2 response against S. mansoni. Diphtheria toxin treatment significantly depleted CD11c+ DCs from all tissues tested, with 70-80% efficacy. Even this incomplete depletion resulted in dramatically impaired CD4+ T cell production of Th2 cytokines, altering the balance of the immune response and causing a shift towards IFN-γ production. In contrast, basophil depletion using Mar-1 antibody had no measurable effect on Th2 induction in this system. These data underline the vital role that CD11c+ antigen presenting cells can play in orchestrating Th2 development against helminth infection in vivo, a response that is ordinarily balanced so as to prevent the potentially damaging production of inflammatory cytokines. The second study addresses whether the exposure of DCs to the cercarial stage of the parasite is critical for either parasite survival or the subsequent development of the Th2 immune response against later stages of infection. It was found that CD11c depletion prior to infection resulted in increased parasite survival, but did not impair the development of CD4+ T cell Th2 response later in infection. The third study asked whether DCs continue to be necessary for the maintenance of the chronic immune response during infection with S. mansoni. In contrast, depletion of CD11c+ cells during the initiation (4 to 6 weeks) or maintenance (6 to 8 weeks or 12 to 14 weeks) of Th2 response to eggs, resulted in severely impaired Th2 cytokine production. Interestingly, depletion during the later stages of infection led to dramatic weight loss and mortality, coincident with impaired CD4+ T cell responses. These data suggest that CD11c+ antigen presenting cells, in addition to being important in the early priming phase, also play a vital role in the maintenance and homeostasis of chronic CD4+ T cell responses in a Th2 infection setting, the disruption of which can have lethal consequences. The final study in this thesis aimed to establish whether the tissue factor thymic stromal lymphopoietin (TSLP) is able to enhance or modulate the Th2 responses initiated by DCs stimulated with SEA. Contrary to previous studies, it was found that BMDCs do not become phenotypically activated by TSLP, in particular, they do not up-regulate the costimulatory molecule OX40L, nor does TSLP suppress the production of IL-12p40 or IL-12p70 in response to LPS or CpG. Further, exposure to TSLP had no impact on DC cytokine production or survival. Irrespective of this unaltered profile in vitro, TSLP exposed DCs transferred in vivo induced the production of significantly more Th1 and Th2 cytokines from polyclonally restimulated splenocytes than DCs exposed to medium alone. In addition to this, TSLP altered the kinetic of the immune response induced by DCs stimulated with the soluble egg antigen (SEA) of S. mansoni. This was characterised by the antigen specific production of T cell cytokines starting more rapidly than with non-TSLP treated control DCs. The alteration in the kinetics of the immune response was not restricted to Th2 antigens and was also seen to some extent in Propionibacterium acnes stimulated DCs. This suggests a possible role for TSLP in either inducing faster DC migration or greater production of T cell chemoattractants and thus, enhancing the rate of DC interaction with T cells.