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Title: The role of Wiskott-Aldrich Syndrome protein-mediated actin dynamics in controlling type-I IFN production in plasmacytoid dendritic cells
Authors: Prete, Francesca
Supervisore/Tutore: Benvenuti, Federica
Issue Date: 24-Apr-2012
Publisher: Università degli studi di Trieste
Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency characterized by recurrent infections, and a marked predisposition to develop autoimmune phenomena. The disease is caused by mutations in WASp, a key regulator of actin polymerization expressed only in hematopoietic cells. A general impairment of hematopoietic cell functions contributes to the pathogenesis of the disease. Neutrophils, B cells, T cells and DCs deficient for WASp were all shown to have impaired homing ability, a cellular function that strictly depends on spatio-temporal regulation of actin polymerization. WASp null T cells present severe impairment in coupling TCR stimulation to proliferation and fail to organize signaling molecules within the immunological synapse (Badour et al., 2004; Dupre et al., 2002; Sims et al., 2007). B cells intrinsic defects include altered B cell receptor clustering, defective homeostasis of mature B cells and a specific reduction in the expression of the complement receptor (Park et al., 2005; Simon et al., 1992; Westerberg et al., 2001). WASp null DCs fail to assemble podosomes and display late migration from the periphery to lymph nodes (de Noronha et al., 2005). Moreover, WASp expression in DCs is critical to organize the dynamic cytoskeletal changes that facilitate DC-T cell interaction during antigen presentation (Pulecio et al., 2008; Bouma et al., 2011). Together, these cellular alterations provided clues to understand the reduced response to pathogens and the immunodeficiency of WAS patients.
However, the mechanisms by which perturbation of actin dynamics promote autoimmune phenomena are less clear.
Autoimmune complications occur in 40-72% of children with severe WAS phenotype. The most common autoimmune features that develop in WAS patients include hemolytic anemia, vasculitis, renal disease, and arthritis (Dupuis-Girod et al., 2003; Sullivan et al., 1994; Humblet-baron et al., 2007). Impairment of T and B cell tolerance have been reported in WAS patients and in Was-/- mice, but the exact cellular mechanisms that link loss of WASp function to autoimmunity have not been fully elucidated yet (Becker-Herman et al., 2011; Recher et al., 2012; Marangoni et al., 2007; Maillard et al., 2007; Humblet-Baron et al., 2007).
It is increasingly recognized that excessive activation of pDCs and elevated type-I interferon (IFN) levels are pathogenic in several human autoimmune diseases such as SLE, psoriasis, Sjogren’s syndrome. Since WAS autoimmune manifestations partially overlap with those of type-I IFN diseases, we hypothesized that pDCs/IFN-α axis may have a role in WAS-associated autoimmunity.
In the first part of results we present the analysis of the pDC compartment in a mouse model of the disease, a mouse knock-out for WASp (WKO mice). We show that pDCs from WKO animals are chronically activated, secrete type-I IFN constitutively and become refractory to further stimulation in vivo. By depleting WASp expression or by interfering with actin dynamics in pDCs we prove that WASp-mediated actin dynamics control the activation of the TLR9/IFN-α pathway in a cell autonomous fashion.

Based on the results of previous section, we speculated that WASp may regulate IFN-α production by controlling the correct organization of the endocytic pathway. It is well known, in fact, that type-I IFN production relies on spatio-temporal regulation of TLR9 signaling in the endocytic pathway, at the same time, it is largely established that many steps of the endocytosis are regulated by actin regulatory proteins of the WASp family. Therefore, we proceed presenting a series of experiments exploring the role of WASp in mediating signaling downstream TLR9 activation in pDCs. Tracking of TLR9 agonist in pDCs show that WASp controls cellular architecture and early endosomes size leading to accumulation of large aggregates of TLR9 agonist in WASp-deficient cells.

To demonstrate a link between the alteration in the pDCs/IFN-α axis found in WKO mice and the pathogenesis of WAS autoimmunity, we first analyzed the presence in patients of clinical aspects known to be associated with type-I IFN diseases. We report that WAS patients display a moderate type-I interferon signature, indicating an exposure to elevated levels of this cytokine. Finally, we moved back to the mouse model of WAS to start investigating the presence of alterations of innate/adaptive immunity classically attributed to unrestrained type-I IFN production. In line with the well known inducing effects of systemic type-I IFNs, our preliminary data illustrate a generally more activated phenotype of immunocytes in WKO mice.

In sum, our work provide 1) the first demonstration of an altered pDCs/IFN-α axis in WAS, 2) the existence of a cell intrinsic mechanism of increased pDCs activation in WASp-null pDC, and 3) a new role for actin in restraining excessive activation of TLR9 in pDCs.
These observations add a new layer of complexity to our understanding of the pathophysiology of WAS, and raise important considerations about the treatment of patients with autoimmune phenomena.
Ciclo di dottorato: XXIV Ciclo
Keywords: WASp, pDCs, Type-I-IFN, autoimmunity
Language: en
Type: Doctoral Thesis
NBN: urn:nbn:it:units-4469
Appears in Collections:Scienze biologiche

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