Exploiting host immunity: the Salmonella paradigm

Abstract

Pathogens have evolved clever strategies to evade and in some cases exploit the attacks of an activated immune system. Salmonella enterica is one such pathogen, exploiting multiple aspects of host defense to promote its replication in the host. Here we review recent findings on the mechanisms by which Salmonella establishes systemic and chronic infection, including strategies involving manipulation of innate immune signaling and inflammatory forms of cell death, as well as immune evasion by establishing residency in M2 macrophages. We also examine recent evidence showing that the oxidative environment and the high levels of antimicrobial proteins produced in response to localized Salmonella gastrointestinal infection enable the pathogen to successfully outcompete the resident gut microbiota.

Keywords: Salmonella; immune evasion; immunity; inflammation; mucosal immunity.

Figure 1. Salmonella takes advantage of innate immune responses inside the macrophage

PAMPs from Salmonella are recognized by TLRs, signaling from which induces an array of responses, including acidification of the phagosome. This drop in pH triggers Salmonella to secrete effectors, which in turn modify the phagosome to generate a replicative compartment known as the Salmonella-Containing Vacuole (SCV). Salmonella can also activate NLRs, including NLRC4 (activated by flagellin) and NLRP3 (unknown ligand). Activation of NLRC4 and NLRP3 lead to processing of pro- Caspase-1 into its active form, followed by cleavage of pro-IL-1β and pro-IL-18 into their active forms and by the induction of pyroptosis. In addition to NLRC4 and NLRP3, NLRP6 and NLRP12 are also activated by Salmonella, albeit trough unknown ligands. These latter NLRs inhibit phosphorylation of IκBα and ERK, thus preventing nuclear translocation of NF-κB and ERK, respectively. This subsequently diminishes the production of proinflammatory mediators and results in the inefficient clearance of phagocytized bacteria. During Salmonella infection, type I IFN signaling is activated through the production of type I IFNs, leading to the association between IFNAR and RIP1. This in turn induces the formation of the RIP1–RIP3 complex, resulting in necroptosis of macrophages. The reduction in proinflammatory mediators and induction of macrophage death by necroptosis diminishes the immune system’s ability to control the replication and spread of Salmonella within the host. Activities promoting the growth or survival of Salmonella are depicted with red lines and boxes.

Figure 2. Salmonella uses M2 macrophages to establish a chronic infection

PPARδ is upregulated in CD301⁺ macrophages during Salmonella infection. Following activation by fatty acids present in engulfed apoptotic cells, PPAR receptors translocate to the nucleus where they induce the M2 macrophage phenotype (ARG1⁺, YM1⁺, CD206⁺). M2 macrophages produce high levels of anti-inflammatory cytokines such as IL-10, and low levels of proinflammatory cytokines such as TNFα and IL-6. In addition, activation of PPARδ increases intracellular glucose availability and enhances Salmonella replication in macrophages and in mice, whereas this pathogen fails to persist in Pparδ null mice. The anti-inflammatory environment and metabolic state present in M2 macrophages allow Salmonella to establish a chronic infection. Abbreviations: SCV, Salmonella containing vacuole.

Figure 3. Salmonella exploits intestinal antimicrobial responses

During colonization of the gastrointestinal tract, Salmonella invades and escapes from epithelial cells, gaining access to the lamina propria. There, Salmonella invades/is taken up by macrophages (MΦ) and dendritic cells (DC), and resides inside the Salmonella containing vacuole (SCV). Once infected, these phagocytes produce IL-1β, IL-18 (see Fig. 1) and IL-23. IL-23 signals to various cells types (e.g. Th17 cells) to produce IL-17 and IL-22. Production of these cytokines is further increased by IL-6, which is released in greater abundance by epithelial cells following Salmonella-mediated PPARγ downregulation. Epithelial cells also express receptors for IL-17 and IL-22, and ligand binding induces production of antimicrobial proteins such as Lipocalin-2 (LCN2; iron starvation) and calprotectin (CP; zinc and manganese starvation). Contrary to the majority of the resident microbiota, Salmonella has evolved to evade the detrimental effects of these antimicrobial proteins, producing an additional iron-scavenging siderophore that cannot be bound by LCN2, and expressing a high affinity zinc transporter to overcome zinc sequestration by CP. Epithelial cells also produce CXC chemokines following IL-17 and IL-22 stimulation, leading to the recruitment of polymorphonuclear cells (PMN; primarily neutrophils) to the site of infection, which also produce the aforementioned antimicrobial proteins. PMNs phagocytose Salmonella and kill it with the help of reactive oxygen (ROS) and nitrogen species (RNS), among other mechanisms. IL-17 and IL-22 together with IFN-γ also induce iNOS in epithelial cells, an enzyme involved in the production of NO (an RNS). Whereas Salmonella is resistant to moderate levels of ROS and RNS, these responses transform the gut into an oxidative, inhospitable environment for many members of the anaerobic microbiota. Moreover, this oxidative environment provides Salmonella with additional electron acceptors, for example: (1) Hydrogen sulfide (H₂S), produced by the microbiota, is converted into (2) thiosulfate by epithelial cells; ROS oxidize thiosulfate to (3) tetrathionate. Salmonella’s ability to anaerobically respire tetrathionate allows it to utilize non-fermentable carbon sources (C-source) such as ethanolamine, which is generated from the membranes of dead enterocytes. Together, these resistance mechanisms and metabolic properties give Salmonella a competitive advantage over the gut microbiota, allowing it to exploit host inflammation and grow to high numbers in the gastrointestinal lumen.