TLR signaling at the intestinal epithelial interface.
The intestinal epithelium provides a critical interface between lumenal bacteria and the mucosal immune system. Whereas normal commensal flora do not trigger acute inflammation, pathogenic bacteria trigger a potent inflammatory response. Our studies emanate from the hypothesis that the intestinal epithelium is normally hyporesponsive to commensal pathogen-associated molecular patterns (PAMPs) such as LPS. Our data demonstrate that normal human colonic epithelial cells and lamina propria cells express low levels of TLR4 and its co-receptor MD-2. This expression pattern is mirrored by intestinal epithelial cell (IEC) lines. Co-expression of TLR4 and MD-2 is necessary and sufficient for LPS responsiveness in IEC. Moreover, LPS sensing occurs along the basolateral membrane of polarized IEC in culture. Expression of MD-2 is regulated by IFN-gamma. Cloning of the MD-2 promoter demonstrates that promoter activity is increased by IFN-gamma and blocked by the STAT inhibitor SOCS3. We conclude from our studies that the intestinal epithelium down-regulates expression of TLR4 and MD-2 and is LPS unresponsive. The Th1 cytokine IFN-gamma up-regulates expression of MD-2 in a STAT-dependent fashion. The results of our studies have important implications for understanding human inflammatory bowel diseases.
Abreu
Maria T
MT
Inflammatory Bowel Disease Center, Division of Gastroenterology, Department of Medicine, Cedars-Sinai Medical Center, Los Angeles, California 90048, USA. maria.abreu@cshs.org
Thomas
Lisa S
LS
Arnold
Elizabeth T
ET
Lukasek
Katie
K
Michelsen
Kathrin S
KS
Arditi
Moshe
M
D000954:Antigens, Surface / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D018938:Caco-2 Cells; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D007371:Interferon-gamma / Q000276:immunology; D007413:Intestinal Mucosa / Q000187:drug effects / Q000276:immunology / Q000378:metabolism*; D008070:Lipopolysaccharides / Q000276:immunology / Q000494:pharmacology; D051218:Lymphocyte Antigen 96; D008562:Membrane Glycoproteins / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D012097:Repressor Proteins / Q000494:pharmacology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction; D000071223:Suppressor of Cytokine Signaling 3 Protein; D050826:Suppressor of Cytokine Signaling Proteins; D051197:Toll-Like Receptor 4; D051193:Toll-Like Receptors; D014157:Transcription Factors / Q000494:pharmacology
D000954:Antigens, Surface; C495377:LY96 protein, human; D008070:Lipopolysaccharides; D051218:Lymphocyte Antigen 96; D008562:Membrane Glycoproteins; D011956:Receptors, Cell Surface; D012097:Repressor Proteins; C106644:SOCS3 protein, human; D000071223:Suppressor of Cytokine Signaling 3 Protein; D050826:Suppressor of Cytokine Signaling Proteins; C495345:TLR4 protein, human; D051197:Toll-Like Receptor 4; D051193:Toll-Like Receptors; D014157:Transcription Factors; D007371:Interferon-gamma