Differential activation of dendritic cell subsets by Schistosoma mansoni
Webb, Lauren Michelle
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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.