Evidence for an Innate Immune Response in the Immature Human Intestine: Toll-Like Receptors on Fetal Enterocytes

Abstract

The intestinal epithelium is an active participant in the mucosal immune response against luminal pathogens. Microorganisms and their cell wall products, i.e. lipopolysaccharide (LPS), can stimulate the enterocyte to produce an innate immune response with the increased production of IL-8 via an activation of the transcription factor NFB. The innate response mechanism, however, has not been understood until the recent description of a family of human toll-like receptors (hTLR) on immune cells that interact with LPS and modulate the IL-8 response via an intracellular signal transduction pathway similar to that of the IL-1 receptor family. Accordingly, in this study we have sought to determine the constitutive and regulated expression of hTLR on a nonmalignant human fetal primary small intestinal cell line (H4 cells) and on small intestinal samples of ileum from human fetuses (age 18 -21 wk). Specimens were examined by reversetranscription PCR, Western blot analysis, and immunofluorescence for hTLR2 and hTLR4 mRNA and protein and to determine whether their expression was regulated by LPS or by an endogenous inflammatory stimulus, IL-1␤. hTLR2 and hTLR4were expressed constitutively on H4 cells and on human fetal small intestinal enterocytes, predominantly on the basolateral surface of crypt enterocytes. Inflammatory stimuli appeared to regulate hTLR transcription (IL-1␤ increased both hTLR2 and hTLR4 whereas LPS decreased hTLR4) and possibly translation (qualitative observations). The presence of hTLR on human fetal enterocyte suggests a mechanism for the innate immune response to pathogens and could provide the basis for further study of the accentuated inflammatory response in age-dependent gastrointestinal diseases such as necrotizing enterocolitis. The mucosal surface of the gastrointestinal tract interfaces with the complex environment of the gut lumen, which contain an extensive variety of both commensal and pathogenic microbes. To contain this bacterial antigenic load, the mucosal epithelium has evolved features allowing it to function as an active participant in mucosal immune responses as well as a physical barrier to the uptake of noxious substances (1). When microbes interact with the gut epithelium, the gut can react by evoking both an immediate and innate as well as a delayed, adaptive immune response to contain their penetration. This interaction is termed "microbial-epithelial crosstalk" (2, 3). The innate immune response represents a genetically conserved immune response across species against pathogens and their cellular products, termed PAMP, which are recognized by PRR on immune cells (macrophages, monocytes, etc.) (4). This innate response can evoke either phagocytosis, a release of antibacterial peptides (defensins), or can stimulate secretion of inflammatory cytokines tumor

Human Intestinal Epithelial Cells Are Broadly Unresponsive to Toll-Like Receptor 2-Dependent Bacterial Ligands: Implications for Host-Microbial Interactions in the Gut

“…We and others have previously demonstrated that IEC express very low levels of TLR4 and its coreceptor MD-2 and are unresponsive to LPS (73,83,89). Recent studies demonstrate that intestinal macrophages are also poorly LPS responsive and lack CD14 (90).…”

Soluble Forms of Toll-Like Receptor (TLR)2 Capable of Modulating TLR2 Signaling Are Present in Human Plasma and Breast Milk

“…We reasoned that, due to the expression of TLR4 and TLR2 by fetal enterocytes and the latter's hypersensitivity to LPS (36,37) as well as the high levels of sCD14 in milk (16,17), the milk sCD14-mediated microbial recognition in the neonatal gut must be regulated to avoid excessive local inflammation.…”

Necrotizing Enterocolitis (NEC): A Devastating Disease of Prematurity

“…A large body of data suggests that imbalance between Toll like receptors 4 (TLR) and TLR 9 is responsible for development of the disease. TLR4 activation leads to apoptosis and effects enterocyte proliferation and migration [21]. Some studies have implicated the role of Paneth cells (PCs) in NEC.…”