TLR signaling at the intestinal epithelial interface

Abstract

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.

Md-2

“…For instance, a human corneal epithelial cell line (HCEC) and primary alveolar epithelial cells were reported to be TLR4 positive and MD-2 negative, as judged by their ability of acquiring LPS responsiveness when recombinant MD-2 was provided in the culture medium (Jia et al, 2004;Kennedy et al, 2004). Exposure to gamma interferon, TNFa or PAMPs enhanced the expression of both molecules and affected TLR4 redistribution (Abreu et al, 2003;Monick et al, 2003). Contrasting results, supported by RT-PCR, confocal microscopy and functional data, suggest that corneal epithelial cells and corneal fibroblasts are positive for both TLR4 and MD-2 and depend on the presence of CD14 and LBP to maximally respond to LPS (Blais et al, 2005;Kumagai et al, 2005).…”

Intestinal epithelial barrier and mucosal immunity

“…Moreover, a stronger reactivity of fetal enterocytes to TLR ligands has been observed when compared with that of adult IECs [24,25]. Lower reactivity of adult IECs was shown to be caused by regulated expression of MD-2 and TLR4 [26]; furthermore, a plausible molecular explanation for neonatal hyper-reactivity has been recently formulated [27]. Lower levels of regulatory molecules of NF-kB -IkBa, IkBb and IkBe -have been observed in fetal compared with adult human IEC lines and in preweaned rats compared with the postweaning intestine.…”

Intestinal epithelial barrier and mucosal immunity

“…Upon engagement by microorganisms, epithelial cells can produce a number of proinflammatory cytokines and chemokines, including IL-8, IL-1, GM-CSF, GRO and MCP-1 [99]. Interestingly, pathogenic bacteria, but not commensals, can usually trigger such a response [100,101]. Cytokine and chemokine production by epithelial cells leads to recruitment of polymorphonuclear leukocytes, macrophages and lymphocytes and initiates inflammatory tissue responses, thereby affecting both innate and adaptive immune responses at mucosal sites [99,102].…”