Whilst mast cells participate in the immune defence against the intracellular bacterium Listeria monocytogenes, there is conflicting evidence regarding the ability of L. monocytogenes to infect mast cells. It is known that the pore-forming toxin listeriolysin (LLO) is important for mast cell activation, degranulation and the release of pro-inflammatory cytokines. Mast cells, however, are a potential source of a wide range of cytokines, chemokines and other mediators including osteopontin, which contributes to the clearing of L. monocytogenes infections in vivo, although its source is unknown. We therefore aimed to resolve the controversy of mast cell infection by L. monocytogenes and investigated the extent of mediator release in response to the bacterium. In this paper we show that the infection of bone marrow-derived mast cells by L. monocytogenes is inefficient and LLO-independent. LLO, however, is required for calcium-independent mast cell degranulation as well as for the transient and selective downregulation of cell surface CD117 (c-kit) on mast cells. We demonstrate that in addition to the key pro-inflammatory cytokines TNF-α and IL-6, mast cells release a wide range of other mediators in response to L. monocytogenes. Osteopontin, IL-2, IL-4, IL-13 and granulocyte macrophage colony-stimulating factor (GM-CSF), and chemokines including CCL2, CCL3, CCL4 and CCL5 are released in a MyD88-dependent manner. The wide range of mediators released by mast cells in response to L. monocytogenes may play an important role in the recruitment and activation of a variety of immune cells in vivo. The cocktail of mediators, however, is unlikely to skew the immune response to a particular effector response. We propose that mast cells provide a hitherto unreported source of osteopontin, and may provide an important role in co-ordinating the immune response during Listeria infection.

Mast cells are important cellular constituents of epithelial-mesenchymal interactions, densely located at sites of microbial entry into the host where they are continuously exposed to products from commensals. In order to avoid excessive activation and the associated pathology, mast cell responses to TLR agonists must be tightly regulated. Here, we show that exposure in vitro to subactivating levels of the epithelial cell product, IL-33, renders mast cells insensitive to bacterial cell wall products. Mast cell responsiveness to Ag, cytoplasmic dsDNA, and TLR7/8 agonists is unaffected or enhanced by IL-33. The IL-33-induced mast cell selective tolerance requires the IL-33 receptor ST2 and peritoneal mast cells from St2 −/− mice display a constitutively activated phenotype, demonstrated by increased expression of activation markers including CD11b and CD28. IL-33 exposure neither affects the levels of TLR4, MyD88, TIRAP, IL-1R associated kinase 2 (IRAK2), or IRAK4, nor induces persistent A20 or Tollip expression, but potently causes ST2-dependent IRAK1 degradation. We show that while IRAK2 is redundant for TLR4 signaling, IRAK1 is essential for TLR4 signaling in mast cells. We suggest that IL-33 produced during homeostasis retains mast cells in an unresponsive state to bacterial cell wall products via IRAK1 degradation, thus preventing chronic inflammation and tissue destruction.Keywords: IL-33 r IRAK1 r Mast cells r ST2 r TLRs Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionMast cells are closely associated with the epithelium [1], contributing to barrier function [2]. Therefore, responses to commensal bacteria must be tightly regulated to prevent the pathology Correspondence: Prof. Silvia Bulfone-Paus e-mail: Silvia.bulfone-paus@manchester.ac.uk associated with unnecessary immune activation. The crucial role of the tissues in regulating immunity is increasingly being recognized [3] and we hypothesized that epithelial or endothelial cell products may orchestrate this regulation.IL-33 and its receptor ST2 have complex roles in the LPS response [4][5][6][7][8]. IL-33 is expressed constitutively by epithelial cells [9,10] and its expression is upregulated during inflammation [11]. Mast cells express ST2 and are activated by IL-33 to release cytokines and chemokines [12][13][14][15] and, furthermore, the fact that LPS and IL-33 share the TLR/IL1R signaling pathway C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu 980Hilary Sandig et al. Eur. J. Immunol. 2013. 43: 979-988 raises the possibility that cross-talk or cross-tolerance could occur between them. While the role of IL-33 during inflammation has been widely investigated, little is known about the function of constitutively expressed IL-33. Typically, TLR4 signaling proceeds via MyD88-dependent and independent pathways but in mast cells TLR4 ligation does not engage the MyD88-independent pathway [16]. The molecular details of the MyD88-dependent pathway in mast cel...