Role of Toll-like receptors in gastrointestinal malignancies

Mounting evidence supports the tenet that innate immune responses to luminal microbes participate in the development of gastrointestinal malignancies. The gastrointestinal tract is relatively unique in that it has evolved in the presence of diverse enteric microflora. Intestinal flora is required to develop a normal adaptive immune response in the periphery. With the characterization of the innate immune system, we have begun to understand the adaptations the intestine has made to the microbiota. The interaction between the microbiota and the intestinal mucosa through Toll-like receptors (TLRs) is required to maintain intestinal homeostasis. In particular, intestinal epithelial cells and lamina propria mononuclear cells such as antigen-presenting cells and T cells must respond to breaches in the mucosal barrier by activating TLR-dependent pathways that result in increased epithelial proliferation, wound healing and recruitment of acute inflammatory cells. In the setting of chronic inflammation such as Helicobacter pylori (H. pylori) infection in the stomach or idiopathic inflammatory bowel disease, the process of repair may eventually result in carcinogenesis. The following review highlights human and animal data that support a role for innate immune responses and TLRs specifically in promoting gastrointestinal malignancies. Candidate pathways linking TLRs to gastrointestinal malignancies include activation of nuclear factor-kappaB and cyclooxygenase-2. Studying the link between innate immune signaling and gastrointestinal malignancies offers the possibility to identify novel ways to both prevent and treat gastrointestinal cancer.