Western blot analysis was performed as described previously [17] . Briefly, in duplicate, cell lysates of hMPVinfected Vero cells were separated on a 12% polyacrylamide Tris-HCl Gel (Bio-Rad), transferred to a Hybond-P polyvinylidene difluoride membrane (Amersham Biosciences) and immunostained with either hamster Ab # 121-1017-133 (MedImmune) directed to hMPV F, mouse Ab directed to GFP (Roche Molecular Biochemicals), or mouse Ab directed to actin (Chemicon MAB #1501). Bands were visualized by incubation with horseradish peroxidase-conjugated anti-hamster Mab or anti-mouse Mab, developed with chemiluminescence substrate (Amersham Biosciences), and exposed to Biomax MR film (Kodak). No Longer an Innocent Bystander: Epithelial Toll-Like Receptor Signaling in the Development of Mucosal Inflammation Diseases of mucosal inflammation represent important causes of morbidity and mortality, and have led to intense research efforts to understand the factors that lead to their development. It is well accepted that a breakdown of the normally impermeant epithelial barrier of the intestine, the lung, and the kidney is associated with the development of inflammatory disease in these organs, yet significant controversy exists as to how this breakdown actually occurs, and how such a breakdown may lead to inflammation. In this regard, much work has focused upon the role of the epithelium as an “innocent bystander,” a target of a leukocyte-mediated inflammatory cascade that leads to its destruction in the mucosal inflammatory process. However, recent evidence from a variety of laboratories indicates that the epithelium is not merely a passive component in the steps that lead to mucosal inflammation, but is a central participant in the process. In addressing this controversy, we and others have determined that epithelial cells express Toll-like receptors (TLRs) of the innate immune system, and that activation of TLRs by endogenous and exogenous ligands may play a central role in determining the balance between a state of “mucosal homeostasis,” as is required for optimal organ function, and “mucosal injury,” leading to mucosal inflammation and barrier breakdown. In particular, activation of TLRs within intestinal epithelial cells leads to the development of cellular injury and impairment in mucosal repair in the pathogenesis of intestinal inflammation, while activation of TLRs in the lung and kidney may participate in the development of pneumonitis and nephritis respectively. Recent work in support of these concepts is extensively reviewed, while essential areas of further study that are required to determine the significance of epithelial TLR signaling during states of health and disease are outlined. Standing at the interface between the host and the environment, mucosal-lined surfaces represent the first line of defense against potential pathogens. This defensive role is particularly relevant to the mucosa of the gastrointestinal tract, the pulmonary system, and the urinary tract, each of which is particularly susceptible to the development of inflammatory diseases due to their role as a barrier that must not only protect, but also serve the physiological function of each of the organ systems. In the case of the gastrointestinal tract, mucosal inflammation is manifest as inflammatory bowel disease (including Crohn's disease and ulcerative colitis) (1) (2) (3) or necrotizing enterocolitis (NEC), a leading cause of death in preterm infants (4) . In the case of the pulmonary system, mucosal inflammation may be manifest as pneumonitis, pneumonia, or asthma (5-7), acute and chronic pulmonary conditions that have a high degree of morbidity and potential mortality. And in the case of the urinary tract, mucosal inflammation may be manifest as interstitial nephritis, cystitis, and urethritis (8) (9) (10) , causes of significant morbidity in patients of all ages. To elucidate the pathogenesis of mucosal inflammatory diseases, research over the past several decades has focused on the role of the immune system in their developmentin particular the relationship between mucosal lymphocytes, macrophages, and neutrophils, and the effects of their cellular by-products on mucosal integrity and function (11) (12) (13) . However, recent work has shed light upon the important role that the epithelia itself may play as a primary regulator of the immune response in the development of mucosal inflammation. No longer an innocent bystander, the epithelial-lined mucosa at each of these sites has been shown to possess all of the required armamentarium to allow an effective response to invading challenges, and to lead the battle to neutralize potential microbial threats (14) (15) (16) . Not only is the epithelium able to respond to potentially dangerous microbial products, it also may sense endogenous molecules that are released during conditions of stress, hypoxia, or injury-so-called danger molecules that may play a critical role in the development of mucosal inflammation (17) (18) (19) . In order, therefore, to understand the pathogenesis of mucosal inflammation and to assist in the rational design of anti-inflammatory strategies, it is necessary to define the receptors and signaling pathways that mediate the inflam-matory response with respect to the epithelium itself. The innate immune system consists of a series of receptors and their associated signaling molecules that is present both on leukocytes and epithelial cells through the body, and which initiates an immune response by responding directly to pre-formed ligands ( [20] provides a recent review). The innate immune system lies in contradistinction to the adaptive immune system, a set of cellular and molecular interactions that must first "learn" how to deal with a potential pathogen, and then respond through the release of antibodies or other cellular derived products. The innate immune system includes pattern recognition receptors such as the TLRs, the NOD-like receptors (NLRs), the RIG-like receptors (RLRs), and C-type lectins, and their role in inflammatory signaling in leukocytes has been extensively reviewed (21) (22) (23) 186) . Relatively few reports have focused on the ability of the innate immune system to signal within the epithelium, although emerging evidence from a variety of laboratories including our own indicates that innate immune signaling within the epithelium plays a critical role in the pathogenesis of mucosal inflammation. The current review will focus on the TLR family, which has been shown to play a critical role in the response of epithelial cells to bacterial and endogenous ligands in the pathogenesis of various mucosal inflammatory diseases (Table 1) .