Review of innate cell TLR activation by Pseudomonas aeruginosa to explain lung inflammation during cystic fibrosis, and the prospects of preventing infection. TLRs function in innate immunity by detecting conserved structures present in bacteria, viruses, and fungi. Although TLRs do not necessarily distinguish pathogenic organisms from commensals, in the context of compromised innate immunity and combined with pathogensˈ effector molecules, TLRs drive the host response to the organism. This review will discuss the evidence and role(s) of TLRs in the response to the opportunistic bacterial pathogen, Pseudomonas aeruginosa, as it relates to respiratory infection and CF, in which innate immune mechanisms are indeed compromised. Outer membrane lipoproteins, LPS, flagellin, and nucleic acids all serve as ligands for TLR2, -4, -5, and -9, respectively. These TLRs and their respective downstream effector molecules have proven critical to the host response to P. aeruginosa, although the protective effects of TLRs may be impaired and in some cases, enhanced in the CF patient, contributing to the particular susceptibility of individuals with this disease to P. aeruginosa infection.

The healthy gut is achieved and maintained through a balanced relationship between the mucosal immune system, microbial communities resident in the lumen, and the intestinal epithelium. The intestinal epithelium plays an exceptionally important role in harmonizing the interaction between the host immunity and the luminal residents, as this selectively permeable barrier separates but also allows interchange between the 2 environments. Interleukin (IL)-10 has been well established to play an important role in maintaining gut homeostasis by imparting diverse effects on a variety of cell types in this relationship. In the intestine, the source and the target of IL-10 include leukocytes and epithelial cells. Given that both the epithelium and IL-10 are essential players in supporting homeostasis, we discuss the relationship between these 2 factors, focusing on epithelial sources of IL-10 and the effects of IL-10 on the intestinal epithelium. Insight into this relationship reveals an important aspect of the innate immune function of intestinal epithelial cells.

It is well understood that multiple antimicrobial peptides (AMPs) are constitutively deployed by the epithelium to bolster the innate defenses along the entire length of the intestines. In addition to this constitutive/homeostatic production, AMPs may be inducible and levels changed during disease. In contrast to this level of knowledge on AMP sources and roles in the intestines, our understanding of the complement cascade in the healthy and diseased intestines is rudimentary. Epithelial cells make many complement proteins and there is compelling evidence that complement becomes activated in the lumen. With the common goal of defending the host against microbes, the opportunities for cross-talk between these two processes is great, both in terms of actions on the target microbes but also on regulating the synthesis and secretion of the alternate family of molecules. This possibility is beginning to become apparent with the finding that colonic epithelial cells possess anaphylatoxin receptors. There still remains much to be learned about the possible points of collaboration between AMPs and complement, for example whether there is reciprocal control over expression in the intestinal mucosa in homeostasis and restoring the balance following infection and inflammation.

We reported earlier that IL-1β, an NF-κB-regulated cytokine, was made by intestinal epithelial cells during detachment-induced apoptosis (anoikis) and that IL-1 was antiapoptotic for detached cells. Since surviving anoikis is a prerequisite for cancer progression and metastases, we are further exploring the link between anoikis and cytokines. Here we determined that multiple genes are expressed following detachment including a number of NF-κB-regulated products and therefore aimed to determine whether NF-κB signalling plays any role in regulating apoptosis. Using Western blotting, we detected that IκBα becomes phosphorylated immediately following detachment and that levels of phospho-IκBα peaked within 20 min. Phosphorylation of IκBα was followed by Rel A (p65) nuclear translocation. Increased NF-κB activity following detachment was confirmed using the detection of NF-κB-promoted luciferase gene expression delivered by adenovirus infection. Infection of cells with adenovirus expressing a super-repressor IκBα protein and pharmacological inhibitors of NF-κB resulted in the failure to phosphorylate IκBα, a more rapid activation of caspases and earlier apoptosis. We also detected that IκB kinase α (IKKα) and not IKKβ became phosphorylated following detachment. Since IKKα is activated by NF-κB-inducing kinase (NIK), we overexpressed native NIK using an adenovirus vector that resulted in enhanced phospho-IκBα and nuclear p65 in detached cells compared to control detached cells but did not result in a significantly greater number of cells surviving to 24 h. We conclude that detachment directly activates NF-κB, which, in addition to launching an inflammatory cytokine wave, contributes to a delay in apoptosis in intestinal epithelial cells.