cytokine release is stimulated by an unknown ligand.47 Consistent after S. typhimurium infection.57 Interestingly, IL-18 release in the with these results, NLRP3 and NLRC4 were shown to play mucosa is independent of flagellin recognition since flagellin- redundant roles in vivo since mice deficient in both NLRP3 and deficient S. typhimurium induce comparable levels of IL-18 and NLRC4 had higher bacterial loads in MLN, spleen and liver, IFNc 57. The nature of the ligand and the receptor inducing similar to Caspase-1-deficient mice, while mice deficient in only Caspase-1 activation remains unknown thus far. Another major one NLR were comparable to wild-type mice.47 inflammatory pathway in the mucosa is controlled by the release Although in vitro infection models have established that of IL-23. The levels of IL-17 and IL-22 increase dramatically Salmonella flagellin is a potent activator of innate immune sensors within 2–5 h post-infection in bovine or avian ligated loops.22,23 such as TLR5 and NLRC4, the importance of this recognition for Such an early amplification of host inflammatory responses could the different phases of a Salmonella infection remains unclear involve the stimulation of antigen-experienced T cells by IL-23, because Salmonellae downregulate flagellin expression in vivo.48,49 inducing the release of IL-17 and IL-22. Consistently, IL-232/2 For example, Cummings et al. showed that FliC expression was mice do not produce IL-17 and IL-22 in response to S. typhi- anatomically restricted in mice infected orally with S. typhimur- murium infections.17 In vitro, IL-23 is mainly produced by macro- ium, and fliC is expressed in the PP, but not in the mesenteric phages and dendritic cells, yet its source during S. typhimurium lymph nodes and spleen.49 In addition, flagellin-deficient infections remains unknown. In addition to T cells, Innate Salmonellae have been shown to have comparable virulence to Lymphoid Cells (ILCs), which include natural killer cells (NK wild-type bacteria.50,51 This downregulation of flagellin expression cells) and lymphoid tissue-inducer cells (LTi cells), could be appears to be an important virulence mechanism, since the growth potential sources of IL-22.58,59 In particular CD4+ LTi cells play of Salmonellae constitutively overexpressing flagellin are highly a critical role in the development of lymphoid tissues and in attenuated in a mouse model of infection.52 Growth restriction inflammatory responses. CD4+ LTi cells have recently been is most likely mediated by the NLRC4-Caspase-1 axis, since in shown to be a major source of IL-22 production in response to contrast to wild-type Salmonella, Salmonella overexpressing infections with C. rodentium.60 This response was dependent on flagellin are de©tected by2NL0RC41,a 2PRR Lthataindnuces dtheeILs-23 aBndwias oindusced cin ireseponnsetocLyemph.oid toxin (LT), a assembly of Caspase-1 activating inflammasome complexes in the member of the Tumor Necrosis factor (TNF) core family that is presence of flagellin in the cytoplasm.40 Activation of Caspase-1 expressed mainly by lymphocytes, including T, B, NK and LTi was suggested to induce pyroptosis of infected host macrophages, cells.61,62 However, a role of LTi cells in S. typhimurium infections release clearance.40 of the Since pathogen NLRC4 and plays subsequent a Dreduondant neutrophil nrole owithtNLdRP3i mediated sthe remains tIL-r23irebceptuorontteheir to be demonstrated. .surface, Distinct including T cell populations Th17 cells, express cdT in activating Caspase-1 in vivo, flagellin sensing by NLRC4 is cells and NK T cells.17,63-65 The combination of all of these likely to be important for host defense early in the infection, most cytokines induces a strong inflammatory environment in the likely in the GI tract where the bacteria still express flagellin. intestine, which is characterized by an increased production of However, during the systemic phase, NLRP3 recognizes intra- anti-microbial peptides (IL-22/23 axis) and a recruitment of cellular Salmonellae and mediates inflammasome-dependent neutrophils by the IL-17/23 axis, leading to diarrhea and the defense. deprivation of nutrients, which affects commensals as well as the Mucosal inflammation during S. typhimurium infections. A pathogen (Fig. 2C). hallmark of S. typhimurium infections is an acute, mucosal Neutrophil influx during S. typhimurium infections. The inflammation in the gut. This inflammation is a stereotypic host activation of innate immune responses in the mucosa and the response triggered by the detection of bacterial PAMPs within resulting inflammation serves as an activator of further innate the mucosa by PRRs and is induced in response to Salmonella immune responses. One of these is the recruitment of neutro- penetrating the intestinal epithelium and surviving in tissue phils to the mucosa, which is mostly mediated via the IL-23/ macrophages. Invasion and intracellular replication requires the IL-17 axis (Fig. 2). The production of IL-17 stimulates the coordinated action of the two virulence-associated T3SSs called secretion of CXC chemokines by intestinal epithelial cells and SPI-1 and SPI-2. Consistently, both SPI-1 and SPI-2 were shown granulopoiesis in the bone marrow by inducing the production to contribute to intestinal inflammation after oral infections of of G-CSF (granulocyte colony-stimulating factor).66 Consistently, calves or streptomycin pretreated mice.53-55 In vitro infections IL-17-deficient mice have a severe defect in the recruitment of of cultured macrophages led to the identification of a variety of neutrophils to the intestinal mucosa.22 IL-1β production could mechanisms by which the host can recognize the presence of also contribute to neutrophil recruitment, since Caspase-1- S. typhimurium (see above). Activation of these PRRs leads to the deficient mice exhibit reduced levels of CXC chemokines in the expression and secretion of key cytokines such as IL-18 and IL-23 cecal mucosa early after S. typhimurium infection.57 The recruit- (Fig. 2A), which amplify the inflammatory response by paracrine ment of neutrophils is crucial to prevent the dissemination of signaling mechanisms, inducing the massive secretion of IFNc, Salmonella from the gut, since neutropenia increases the risk of IL-22 and IL-17 by mucosa-resident T cells (Fig. 2B). In parti- systemic infections.67,68 Although S. typhimurium is mainly con- cular, IL-18, which is released in an inflammasome dependent sidered an intracellular pathogen, the role of neutrophils in manner, is critical to induce T cells to secreted IFNc release in containing the infection is likely to involve ingestion and killing vivo.56 Consistently, Caspase-1-deficient mice are deficient for of extracellular bacteria.69 Salmonella is susceptible to neutrophil- gamma interferon expression in the murine cecum early (12 h) mediated killing when it exits epithelial cells and transits to www.landesbioscience.com Gut Microbes 67