Accumulation of PMNs at inflamed sites is a common outcome of colonization of mucosal surfaces by pathogenic bacteria. Intestinal epithelial cells respond by releasing cytokines and distinctive PMN-specific chemoattractants that—in combination with numerous adhesion molecules—recruit PMNs from the blood stream and direct their movement through endothelial and epithelial barriers to the luminal surface. Recruitment of PMNs is the first line of response of the host immune system to bacterial infection, geared toward destruction of invading pathogens. However, the nonspecific neutrophil effectors can cause collateral damage, thus potentially contributing to disease pathology. Moreover, several pathogens have evolved strategies to resist neutrophil killing or even benefit from eliciting inflammation.
In line with other enteric pathogens, previous studies have shown that EAEC infection of polarized intestinal epithelial cells triggers mitogen-activated protein kinase signaling cascades that lead to nuclear factor kappa-B (NFκB) activation, which in turn stimulates the release of an array of pro-inflammatory cytokines, including the potent PMN chemokine interleukin (IL)-8.11,12
Thus, basolaterally released IL-8 likely plays a major role in recruiting PMNs to the subepithelial space in response to EAEC infection.
A recent study from our group shows that EAEC-induced migration of PMNs across the epithelium requires apical secretion of a second, lipid-based PMN chemoattractant (). Specifically, EAEC infection of polarized T84 colonic epithelial cells triggers calcium-independent phospholipase A2
)-mediated release of arachidonic acid from the cell membrane. Through enzymatic action of 12-lipoxygenase (12-LOX), arachidonic acid is then metabolized into hepoxilin A3
), a member of the eicosanoid class of lipids with potent PMN chemoattractant properties. EAEC infection also triggers an increase in expression of the apically located membrane ATP-binding cassette (ABC) transporter multidrug resistance-associated protein 2 (MRP2), which subsequently functions as an efflux pump for the vectoral release of HXA3
to the apical surface. Secreted HXA3
then forms a chemotactic gradiant through the tight junctional complex, thus directing paracellular transit of PMNs across the epithelial monolayer to the luminal surface13
Figure 1. Model of EAEC-induced PMN transepithelial migration. AAF-mediated binding of EAEC to the surface of the intestinal epithelium triggers basolateral release of pro-inflammatory cytokines, e.g., IL-8, thus recruiting PMNs to the subepithelial (more ...)
Increasing evidence suggests that 12-LOX-mediated apical release of HXA3
to promote PMN transepithelial migration is a conserved mechanism by which the intestinal epithelium responds to intruding inflammatory pathogens, including Salmonella enterica
serovar Typhimurium (S.
Typhimurium), Shigella flexneri
, Campylobacter species and EAEC.13-15
Yet, the upstream events by which these pathogens elicit inflammation are very much distinct, reflecting their discrete strategies for promoting infection. As an example, S.
Typhimurium and S. flexneri
rely on effector proteins, translocated into the host cells by type III secretion systems (T3SS), to interact with host cells, leading to invasion of the epithelium and triggering and manipulation of innate immune responses.16,17
While E. coli
pathotypes such as enteropathogenic E. coli
and enterohemorrhagic E. coli
also employ T3SS-dependent infection strategies, this does not appear to be the case for EAEC. Moreover, unlike S. flexneri
. Typhimurium, EAEC strains generally do not invade the epithelium, and instead remain anchored in the intestinal mucosa.18
Our recent work shows that EAEC-induced PMN transmigration requires only binding of the bacteria to the apical epithelial surface, an event facilitated by aggregative adherence fimbriae (AAF), the principal adhesins of EAEC.19
The pro-inflammatory properties appear to be conserved among different variants of these adhesins as all four AAF subtypes identified thus far promote PMN transepithelial migration.19
The AAF subunits are phylogenetically related to those of the Afa/Dr family of E. coli
adhesins, all of which employ the chaperone-usher pathway for fimbrial assembly.20
Notably, other members of this family, such as the F1845 adhesin and Dr hemagglutinin of diffusely adhering E. coli
(DAEC), have also been shown to promote PMN transepithelial migration,6
thus inferring a common strategy of F1845/Dr/AAF-mediated inflammatory responses among these two E. coli