Integrative neuroimmunology of the digestive tract

The epithelium of the gastrointestinal tract is continuously exposed to the external environment containing food antigens, microbes and other pathogens. Immunologic and nonimmunologic mechanisms contribute to the neutralization and elimination of these foreign antigens. The immune system of the intestine is the most extensive in the organism and involves diffuse populations of immune cells, lymphoid aggregates and intraepithelial lymphocytes. On the other hand, the functions of the digestive tract contribute to the overall host defense (mucus secretion, gastric acid secretion, water and electrolyte secretion and peristaltism). These functions are regulated by intrinsic and extrinsic nervous systems. It is currently recognized that the physiological and pathological responses of the intestine require an integrate neuroimmune network. Such neuroimmune regulation is based on anatomical and biochemical supports. Indeed, there are membrane-to-membrane contacts between axonal varicosities and the immune cells. Specific receptors for neurotransmitters such as substance P, vasoactive intestinal polypeptide and somatostatin have been identified in many immune cells. Nerve profile change has been described under pathological conditions such as parasitic infections and acute phase of inflammation. In addition to supporting the growth and survival of several populations of nerves the classical nerve growth factor (NGF) has been shown to affect an immune cell population by inducing mast cell hyperplasia. Furthermore the NGF can induce mast cell degranulation, acting directly on mast cell membrane NGF receptors or indirectly by NGF-mediated release of substance P by peripheral extrinsic or intrinsic nerves. Moreover, non-immune cells such as epithelial and smooth muscle cells can produce immunologic messengers under pathological conditions such as infectious diseases or inflammation. Besides the local regulation of gut functions, neuroimmune control can be exerted at extra-intestinal sites. During physiological and pathological conditions, gastrointestinal secretions and motor events are strongly regulated by the central nervous system. Moreover, infectious agents can induce cytokine and particularly interleukin-1 release by the brain astrocytes and microglial cells which have been shown to play a pivotal role in fever induction and modifications of the gastrointestinal functions. Visceral afferent fibers play a pivotal role in 'cross-communication' between central sites and immune response. Recent studies evoke, more specifically, the role of vagus as a key modulatory participant in the close relationship between the extraintestinal nerves and the immune system. Future work in this field will clarify the role of the different participants in the intimate communication between the gastrointestinal tract, immune system and central nervous system.