Investigating the role of the actin cytoskeleton in regulating TLR-mediated B cell responses during inflammatory challenges

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B cells are a major regulator in maintaining immune homeostasis. Correct B cell activation is ensured by specific recognition of antigen by the B cell receptor (BCR) together with co-stimulation provided by activated T cells. Cytokine signalling or the engagement of Toll-like receptors (TLR) also influence B cell function. Aberrant B cell activation might lead to the development of autoimmune diseases, as seen in Wiskott-Aldrich Syndrome (WAS). WAS is an immunodeficiency associated with high levels of IgA and IgE antibodies, increased susceptibility to infections and heightened risk of immune-mediated disorders, including colitis. Cause of disease are genetic mutations leading to defective expression of the WAS protein (WASP) or the WASP interacting protein (WIP), both regulators of the actin cytoskeleton. B cells deficient for WASP or WIP are hyper-proliferative to both BCR and TLR signalling, as we and others recently demonstrated. With the here described project, we wish to understand and further elucidate the role of those actin regulators in TLR signalling in the context of inflammation. Preliminary findings for this grant application show that in response to TLR4 stimulation in vitro, B cells deficient in WIP (WIP KO) secrete high amounts of the pro-inflammatory cytokines IL-6 and TNF-a. Concomitantly, WIP KO B cells show a diminished expression of the plasma cell specific transcription factor (TF) IRF4 and also of the TF T-bet, which might influence antibody and cytokine production. Pro-inflammatory cytokines can become self-destructive in the acute setting of sepsis or in response to the commensal microbiota. Our preliminary results indeed show an increased B cell activation and IgA production in the gut of non-immunised WIP KO mice, and an early onset of colitis in chimeras in which only B cells lack WIP. Thus, a defective differentiation into plasma cells together with an increased IL-6 and TNF-a production by B cells might ultimately enhance the autoimmune phenotype in WAS. We therefore hypothesize that in B cells WIP is involved in the modulation of signalling pathways regulating TF expression, antibody production and cytokine secretion after TLR4 engagement thereby influencing immune homeostasis in the gut. To test this hypothesis, we plan to (1) analyse the role of the actin regulators WIP and WASP in TLR4 signal induction, endocytosis, cytokine and antibody production of B cells in vitro and in vivo, (2) analyse the underlying molecular mechanism determining TF expression and B cell fate decision and (3) analyse the in vivo impact of the imbalanced cytokine production, plasma cell differentiation and IgA antibody secretion of WIP KO B cells during sepsis and the onset of colitis. By investigating these aims, we will gain further understanding of autoimmune diseases like WAS which may be useful for the development of novel treatment options, not only in WAS but also other immune-related disorders, like inflammatory bowel disease.