Salmonellae interactions with host processes

Abstract

Salmonellae invasion and intracellular replication within host cells result in a range of diseases, including gastroenteritis, bacteraemia, enteric fever and focal infections. In recent years, considerable progress has been made in our understanding of the molecular mechanisms that salmonellae use to alter host cell physiology; through the delivery of effector proteins with specific activities and through the modulation of defence and stress response pathways. In this Review, we summarize our current knowledge of the complex interplay between bacterial and host factors that leads to inflammation, disease and, in most cases, control of the infection by its animal hosts, with a particular focus on Salmonella enterica subsp. enterica serovar Typhimurium. We also highlight gaps in our knowledge of the contributions of salmonellae and the host to disease pathogenesis, and we suggest future avenues for further study.

Figure 1. Host pathways manipulated by Salmonella during epithelial cell invasion

Following contact with host cells, Salmonella spp. translocate effectors via the Salmonella pathogenicity island 1 (SPI-1) type III secretion system (T3SS) to mediate invasion. On the SCV and potentially at the plasma membrane, SopB recruits Rab5 and the PI3K Vps34 to generate PI(3)P. SopB can also recruit AnxA2 to the membrane, which functions as a platform for actin rearrangements, and SopB also activates Rho-kinase dependent actin rearrangements. SopE and SopE2 activate host Rho GTPases to promote bacterial internalization by actin reorganization. Activation of Rac1 (conversion from Rac1-GDP to Rac1-GTP) by SopE in particular promotes the recruitment of factors to the host cell membrane (such as WAVE regulatory complex (WRC) and N-WASP), which promotes actin rearrangements at the host cell membrane, proximal to extracellular Salmonella spp.. The host Arf1 GEF, ARNO, is activated by binding to Arf6 and is recruited to the plasma membrane by PI(3)P, which is possibly generated by SopB. Activation of Arf1 promotes WRC-dependent actin polymerization and bacterial uptake. WRC and N-WASP activate actin related proteins (Arp2/3) which results in localized actin polymerization to promote bacterial uptake into non-phagocytic cells. The effectors SipA and SipC promote bacterial invasion through their actin bundling functions. SipA may need to be cleaved into two domains (potentially by caspase-3) for activation. SptP promotes restoration of epithelial cell architecture after bacterial entry by reversing the activation of Rho GTPases.

Figure 2. Salmonella-induced inflammation promotes pathogen transmission

Localized inflammation in the intestinal tract is important for promoting transmission of salmonellae. Activation of Rho GTPases by the effectors SopE, SopE2 and SopB induces MAPK pathways, thereby activating NFκB and AP1, which stimulates production of the proinflammatory cytokine IL-8 to promote transepithelial migration of neutrophils into the intestinal lumen. SipA activates Caspase-3 and may be subsequently cleaved by this protease;; cleavage of SipA also promotes transepithelial migration of neutrophils. Caspase-1 is activated by FliC (a flagellin protein), PrgJ (a rod protein of the T3SS apparatus) and possibly by the effector SipB in macrophages, which may stimulate the release of IL-1β and IL-18. SopE activation of Rho GTPases can also activate Caspase-1 in epithelial cells. Intracellular LPS can activate Caspases4 and 5, resulting in the release of mature IL-1β and IL-18, which promotes the synthesis of IL-17 and IL-22 by T-cells and amplifies inflammation in the intestinal mucosa. SopE also activates the production of nitrate by host cells, which can be used by luminal Salmonella spp. for respiration. Pathogen invasion also induces the release of reactive oxygen species (ROS) and lipocalin-2. ROS converts the respiratory by-product thiosulfate (generated by the microbiota) into tetrathionate, which can be used by Salmonella spp. (but not the microbiota) for respiration. Lipocalin-2 sequesters the iron siderophore enterochelin from the microbiota, but the Salmonella spp. siderophore salmochelin escapes sequestration. Together, these changes promote outgrowth and transmission of Salmonella spp. that reside in the intestinal lumen. The MAPK inflammatory responses are dampened by the activities of SptP, SpvC, AvrA, SspH1 and GogB, but the mechanistic basis of this is not fully understood.

Figure 3. Salmonella manipulation of host membranes

After invasion, Salmonella reside in a vacuolar compartment that undergoes various surface modifications and alterations that distinguish it morphologically, forming the Salmonella-containing vacuole (SCV). In some cell types (such as epithelial cells), the SCV acquires markers of late endosomal compartments including LAMP-1 (purple), Rab7, vacuolar ATPase and cholesterol. Rab7 interacts with Rab7-interacting lysosomal protein (RILP) and the microtubular motor protein, dynein, and this complex is important for centripetal movement of the SCV in the cell early in infection. Salmonella pathogenicity island 2 (SPI-2) effectors (shown in deep red) are released across the SCV membrane into the host cell cytosol by the T3SS. These effectors distribute to different locations in the cell and are required for SCV vacuolar modifications and microtubular-based movement, including promoting the dynamic formation of endosomal tubules (ETs). Binding of SseJ to RhoA results in direct modification of the lipid content of the SCV and ETs (producing cholesterol esters), whereas SseL counteracts this activity by decreasing lipid droplet accumulation in cells. SseF and SseG promote microtubule bundling and the aggregation of endosomal vesicles;; in addition, recruit Golgi-derived vesicles to the SCV. SteC induces actin rearrangements around the SCV and can alter the position of the SCV within the cell, and SopB induces phosphorylation and activation of myosin II indirectly. SopB phosphatase activity reduces the membrane charge of the SCV by preventing the accumulation of Rabs, which are important for promoting SCV fusion to lysosomes. SifA binds to the eukaryotic SifA and kinesin interacting protein (SKIP), which activates the microtubular motor protein kinesin to mediate the extension of ETs. PipB2 is involved in recruiting kinesin to the SCV and ETs,and SifA-SKIP is suggested to bind Rab7 and Rab9. SifA binding to Rab7 may disrupt interactions between Rab7-RILP and dynein, which would result in increased peripheral movement of ETs. SifA binding to Rab9 blocks retrograde transport of mannose 6 phosphate receptor (MPR) hydrolyases and MPR to the Golgi, which blocks lysosomal fusion with the SCV. Salmonella spp. manipulate host membranes to direct the SCV towards the nucleus and then promotes SCV movement towards the cell periphery where bacteria are presumably released into the intestinal lumen to infect neighbouring cells.

Figure 4. Autophagy and inflammasome activation in response to Salmonella spp. infection

Most intracellular S. Typhimurium reside in SCVs but in some cellular models of infection, some bacteria escape into the host cytosol. Both cytosolic bacteria (shown in step 3) and damaged SCVs (shown in step 5) can be ubiquitinated, and are therefore recognized by the bridging adaptors such as NDP52, OPTN or p62, which leads to the recruitment of LC3 and autophagic clearance. In parallel, Salmonella spp. mediate an acute and transient amino acid starvation response, which reactivates mTOR, thereby inhibiting autophagy. The physiologic significance of this on bacterial clearance remains unclear, since most Salmonella spp. remain inside the SCV. Detection of specific bacterial components (such as flagellin, the T3SS rod protein PrgJ and LPS) in the host cytosol can trigger activation of inflammasome-mediated cell death via caspase-1 and -11 (shown in step 7). SPI-1 mediated detection of PrgJ and flagellin by NAIP2 and NAIP5 respectively in mice trigger NLRC4 mediated Caspase-1 activation, while citrate and potentially bacterial cardiolipin can trigger Caspase-1 activation via NLRP3 inflammasome. The detection of cytosolic LPS in the host can trigger Caspase-11 activation.