Mechanisms of cross hyporesponsiveness to Toll-like receptor bacterial ligands in intestinal epithelial cells.
OBJECTIVE
Despite the ability to participate in immune responses and the continuous presence of bacteria and bacterial products, functional responses of intestinal epithelial cells (IEC) seem to be muted. Previously, tolerance to Toll-like receptors (TLRs) ligands has been described in monocytic cells. However, mechanisms in the intestine are unknown.


METHODS
The effect of purified lipopolysaccharide (LPS) and lipoteichoic acid (LTA) on expression and function of TLRs in intestinal epithelial cells (Colo205, SW480, T84) was assessed by Northern and Western blot and FACS analysis, kinase activity assays, immunohistochemistry, and ELISA.


RESULTS
Expression of TLRs except 10 was detected in primary IEC and TLR1-10 in the cultured cells. Short-term stimulation with LPS or LTA activated proinflammatory signaling cascades in IEC, including phosphorylation of IRAK and MAP kinases and increased IL-8 secretion, whereas prolonged incubation resulted in a state of hyporesponsiveness with no reactivation of the cells by a second challenge with either substance detected. The cells remained responsive to tumor necrosis factor (TNF). Hyporesponsive cells showed no alteration in expression of TLR or signaling molecules but revealed a decrease in TLR surface expression and IRAK activity. Toll-interacting protein (Tollip) mRNA and protein expression were increased in hyporesponsive cells, and overexpression of Tollip in IEC resulted in a significantly decreased proinflammatory response.


CONCLUSIONS
Continuous presence of specific bacterial components results in a status of hyporesponsiveness in otherwise reactive IEC. Down-regulation of TLR surface expression and up-regulation of inhibitory Tollip with decreased phosphorylation of IRAK might all contribute to this hyporesponsiveness.
Otte
Jan-Michel
JM
Gastrointestinal Unit, Center for the Study of Inflammatory Bowel Disease, Massachusetts General Hospital, Boston, USA.
Cario
Elke
E
Podolsky
Daniel K
DK
D001419:Bacteria / Q000378:metabolism; D001422:Bacterial Adhesion; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism; D045744:Cell Line, Tumor; D003110:Colonic Neoplasms; D015536:Down-Regulation; D004847:Epithelial Cells / Q000378:metabolism* / Q000382:microbiology*; D015870:Gene Expression; D006801:Humans; D053592:Interleukin-1 Receptor-Associated Kinases; D016209:Interleukin-8 / Q000235:genetics / Q000378:metabolism; D007413:Intestinal Mucosa / Q000166:cytology*; D047908:Intracellular Signaling Peptides and Proteins; D008024:Ligands; D008070:Lipopolysaccharides / Q000494:pharmacology; D008562:Membrane Glycoproteins / Q000235:genetics* / Q000378:metabolism*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000235:genetics* / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D013682:Teichoic Acids / Q000494:pharmacology; D051194:Toll-Like Receptor 1; D051193:Toll-Like Receptors
D002352:Carrier Proteins; D016209:Interleukin-8; D047908:Intracellular Signaling Peptides and Proteins; D008024:Ligands; D008070:Lipopolysaccharides; D008562:Membrane Glycoproteins; D011956:Receptors, Cell Surface; C410602:TOLLIP protein, human; D013682:Teichoic Acids; D051194:Toll-Like Receptor 1; D051193:Toll-Like Receptors; C009900:lipoteichoic acid; D011494:Protein Kinases; D053592:Interleukin-1 Receptor-Associated Kinases; D020928:Mitogen-Activated Protein Kinases