Recognition of enteroinvasive Escherichia coli and Shigella flexneri by dendritic cells: distinct dendritic cell activation states

SHORT COMMUNICATIONS

Recognition of enteroinvasive Escherichia coli and Shigella flexneri by dendritic cells: distinct dendritic cell activation states

Ana Carolina Ramos MorenoI, ⁺; Karen Spadari Ferreira^II; Lucas Gonçalves Ferreira^I; Sandro Rogério de Almeida^I; Marina Baquerizo Martinez^I

^IFaculdade de Ciências Farmacêuticas, Universidade de São Paulo, São Paulo, SP, Brasil

^IIDepartamento de Doenças Infecciosas, Universidade Federal de São Paulo, São Paulo, SP, Brasil

ABSTRACT

The innate and adaptive immune responses of dendritic cells (DCs) to enteroinvasive Escherichia coli (EIEC) infection were compared with DC responses to Shigella flexneri infection. EIEC triggered DCs to produce interleukin (IL)-10, IL-12 and tumour necrosis factor (TNF)-α, whereas S. flexneri induced only the production of TNF-α. Unlike S. flexneri, EIEC strongly increased the expression of toll like receptor (TLR)-4 and TLR-5 in DCs and diminished the expression of co-stimulatory molecules that may cooperate to inhibit CD4⁺ T-lymphocyte proliferation. The inflammation elicited by EIEC seems to be related to innate immunity both because of the aforementioned results and because only EIEC were able to stimulate DC transmigration across polarised Caco-2 cell monolayers, a mechanism likely to be associated with the secretion of CC chemokine ligands (CCL)20 and TNF-α. Understanding intestinal DC biology is critical to unravelling the infection strategies of EIEC and may aid in the design of treatments for infectious diseases.

Key words: enteroinvasive Escherichia coli - Shigella flexneri - dendritic cells - innate immune response

Enteroinvasive Escherichia coli (EIEC) are closely related to Shigella spp, showing remarkable phenotypic and genotypic similarities. However, the disease induced by EIEC is generally less severe than that induced by Shigella spp (Dupont et al. 1971, 1989, Moreno et al. 2010). This difference in severity may be associated with a lower level of expression of virulence factors by EIEC in the presence of various host cells, such as macrophages and intestinal epithelial cells (Gibotti et al. 2004, Moreno et al. 2009, Bando et al. 2010).

Beyond the virulence factors expressed by certain microorganisms, bacteria are the key players in the complex interactions between invading microbes and host cells and they trigger inflammatory events in the infectious milieu (Ramos et al. 2004, Phalipon & Sansonetti 2007, Sperandio et al. 2008). The mucosa of the colon is the main site of interaction between EIEC and host cells and in this setting, mucosal dendritic cells (DCs) play a key role in transporting and processing intestinal antigens to Peyer's patches and the mesenteric lymph nodes (Niess & Reinecker 2006). Specifically, DCs are antigen-presenting cells that establish a link between the innate and adaptive immune responses. Moreover, DCs are the only antigen-presenting cells that migrate to the secondary lymph nodes and present antigens to naÏve T cells (Banchereau et al. 2000). Despite the efficient DC-mediated immune response, certain pathogens affect the biological functions of immune cells and subvert the host innate immune response by manipulating the expression of surface molecules and cytokine production, which contribute to adaptive immunity (Sperandio et al. 2008).

Very few studies have addressed how EIEC are recognised by the immune system. Therefore, the lack of information regarding EIEC biology confirms the need to further study the interactions between EIEC and DCs. We hypothesised that there is a difference in host recognition of EIEC and Shigella spp, which may explain the comparatively lower virulence of EIEC.

Here, we report the differential response of a murine intestinal DC-like population, characterised by CD11c expression, to EIEC O124:H^- vs. Shigella flexneri M90T. Because pattern recognition receptors on DCs are the likely initiators of the immune response to bacterial infections, we sought to determine how EIEC O124:H^- and S. flexneri M90T specifically initiate the toll like receptor (TLR) signalling pathway and the expression of co-stimulatory molecules. These two events lead to the maturation and polarisation of DCs and subsequently drive T-cell activation. We found that murine CD11c⁺ cells expressed large amounts of TLR-4 and TLR-5 after EIEC infection, whereas S. flexneri induced only TLR-4 expression (Fig. 1A). We hypothesised that CD11c⁺ cells respond differently to these two bacterial species because the recognition of EIEC may be dependent on the TLR-4 and TLR-5 signalling pathways, whereas other pathways may be necessary for the recognition of S. flexneri. Other EIEC surface elements may also be involved in EIEC recognition by DCs, such as OmpA, which plays an important role in the adhesion of E. coli to intestinal epithelial cells and triggers the migration of DCs across these cells (Torres et al. 2006).

Consistent with our findings, Rallabhandi et al. (2008) performed a comparative study of TLR-4 expression in HEK293T cells after activation with different sources of lipopolysaccharide (LPS) and reported that NF-ΚB reporter activity was significantly lower in response to S. flexneri 2a than to E. coli LPS. Moreover, the recognition of S. flexneri was recently linked to TLR-2 and the major outer membrane protein of S. flexneri 2a (Pore et al. 2010).

Our study found that only EIEC induced the expression of TLR-5 in CD11c⁺ cells. This phenotype was also observed in murine macrophages after infection with EIEC O124:H^-, but not when they were infected with S. flexneri M90T (data not shown). Cell-surface TLR-5 recognises flagellin, a structural protein that forms a major portion of flagellar filaments (Ramos et al. 2004). Although EIEC are typically immotile, they have been shown to move in modified motility assays conducted at reduced agar concentrations (Andrade et al. 2002). Also in accord with our results, Tominaga et al. (1994) demonstrated that the flagellin gene is cryptic in the genus Shigella. Therefore, our results suggest that future investigations should elucidate the role of the flagella in the EIEC invasion process and in EIEC survival within hosts, particularly because flagellin recognition by TLR-5 can trigger proinflammatory and adaptive immune responses. Although motility may represent an evolutionary adaptation important for bacterial survival, this characteristic could also represent a detrimental EIEC phenotype.

The detection of bacterial antigens by DCs initiates intracellular signalling that results in pathogen uptake and processing and the subsequent presentation of antigens to T cells. This presentation occurs in the context of several stimuli required for naÏve T-cell activation, including co-stimulatory signals and cytokine expression (Iwasaki & Medzhitov 2010). Therefore, to further understand EIEC biology, we analysed the phenotype acquired by CD11c⁺ cells challenged with EIEC or S. flexneri, focussing on the expression of surface molecules, cytokine profiles and T cell activation.

By flow cytometric analysis, we found that CD11c⁺ cells infected with EIEC exhibited lower expression of the co-stimulatory molecules CD86 and CD40 compared with CD11c⁺ cells infected with S. flexneri. Additionally, the expression of major histocompatibility complex II was not altered between EIEC-infected cells and S. flexneri-infected cells and only S. flexneri induced low CD80 expression (Fig. 1A). During EIEC infection, CD11c⁺ cells also produced higher concentrations of interleukin (IL)-10, IL-12p70 and tumour necrosis factor (TNF)-α (Fig. 1B). Perhaps due to their phenotype, EIEC-infected CD11c⁺ cells were unable to activate naÏve CD4⁺ T-cell proliferation (Fig. 1C). The profile of cells infected with S. flexneri was the inverse, exhibiting significant expression of the co-stimulatory molecules CD40, CD80 and CD86 and a greater ability to stimulate T-cell proliferation. Cytokine production, in turn, was only significantly higher in infected cells than in control cells for TNF-α. These data suggest that DC-like cells infected with S. flexneri have the ability to induce an effective adaptive immune response due to their expression of CD40, CD80 and CD86. However, the inflammation elicited by EIEC may have direct effects on innate immunity during early infection because this strain was able to stimulate the DC-like cells to produce IL-10 and IL-12p70, in addition to expressing TLR-4 and TLR-5. The hyporesponsiveness of cells infected with S. flexneri was also reflected in the production of TNF-α, IL-6, IL-1β and IL-10 by peripheral blood mononuclear cells. Consistent with our data, cytokine levels were found to be significantly lower in response to S. flexneri LPS/lipid than to E. coli LPS/lipid (Rallabhandi et al. 2008). Recently, Bando et al. (2010) demonstrated that murine macrophages infected with EIEC or S. flexneri produce IL-10 and TNF-α, although there is no difference between the levels of cytokines released in response to these two species.

Different antigens and immunisation conditions may lead to responses that vary both quantitatively and qualitatively. In accord with our findings, Sperandio et al. (2008), using a human intestinal infection model, observed that S. flexneri down-regulated innate immunity genes, such as the main DC chemoattractant ccl20. Such down-regulation led to the compromised recruitment of DCs to the lamina propria of infected tissues. In a similar cell-line model (Fig. 2A), we observed that S. flexneri failed to induce DC transmigration across polarised intestinal cell monolayers (Fig. 2B). EIEC, in contrast, were able to induce DC transmigration to the apical side of a transwell, likely resulting in a higher concentration of CC chemokine ligands (CCL)20 released on the basolateral side (Fig. 2C). Interestingly, flagellin triggers the transient expression of CCL20 in intestinal epithelial cells (Sierro et al. 2001) and induces the constitutive expression of ccl20 in CCR6-producing DCs associated with the Peyer's patch epithelium (Cook et al. 2000). It may be possible to expand this analogy to include the TLR-5-mediated epithelial "education" of DCs, imprinting intestinal mucosal responses to EIEC infection. Our data suggest that intestinal DCs play an important role in the clearance of EIEC infection and that they likely are not as efficient during S. flexneri infection.

Clearly, S. flexneri adaptively exploits and evades potentially harmful responses of the immune system (Phalipon & Sansonetti 2007) and this phenomenon could be a key underlying cause of the differential pathogenicity of EIEC and S. flexneri. However, host-pathogen interactions have two components: the virulence and pathogenesis of the bacterium and the inflammatory response of the host's immune system.

In conclusion, we believe that during the inflammation mediated by EIEC-infected DCs, the innate immune response and cell migration are preserved, but the DCs fail to activate naÏve T lymphocytes. As a result, the DCs are not fully functioning when they are infected with EIEC. These findings, together with the high production of TNF-α and the potential expression of flagellin by EIEC and of TLR-5 by CD11c⁺ cells upon EIEC infection, may represent important differences between EIEC and S. flexneri pathogenicity. Future studies regarding DC function will be needed to further characterise EIEC pathogenicity.

Received 24 March 2011

Accepted 1 December 2011

Financial support: FAPESP, CNPq

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Publication Dates

• Publication in this collection
  03 Feb 2012
• Date of issue
  Feb 2012

History

• Received
  24 Mar 2011
• Accepted
  01 Dec 2011