Liu, J.Buisman-Pijlman, F.Hutchinson, M.2015-06-192015-06-192014Frontiers in Neuroscience, 2014; 8(SEP):309-1-309-151662-45481662-453Xhttp://hdl.handle.net/2440/92387Major depressive disorder (MDD) poses one of the highest disease burdens worldwide. Yet, current treatments targeting serotonergic and noradrenaline reuptake systems are insufficient to provide long-term relief from depressive symptoms in most patients, indicating the need for new treatment targets. Having the ability to influence behavior similar to depressive symptoms, as well as communicate with neuronal and neuroendocrine systems, the innate immune system is a strong candidate for MDD treatments. Given the complex nature of immune signaling, the main question becomes: What is the role of the innate immune system in MDD? The current review presents evidence that toll-like receptor 4 (TLR4), via driving both peripheral and central immune responses, can interact with serotonergic neurotransmission and cause neuroendocrine disturbances, thus integrating with widely observed hallmarks of MDD. Additionally, through describing the multi-directional communication between immune, neural and endocrine systems in stress, TLR4-related mechanisms can mediate stress-induced adaptations, which are necessary for the development of MDD. Therefore, apart from exogenous pathogenic mechanisms, TLR4 is involved in immune changes as a result of endogenous stress signals, playing an integral part in the pathophysiology, and could be a potential target for pharmacological treatments to improve current interventions for MDD.en© 2014 Liu, Buisman-Pijlman and Hutchinson. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.toll-like receptor 4TLR4HPAneuroendocrineneuroimmunologystressdepressionMDDJournal article003001096510.3389/fnins.2014.003090003465351000022-s2.0-84908670733138642Liu, J. [0000-0003-1887-0218]Buisman-Pijlman, F. [0000-0003-2099-6450]Hutchinson, M. [0000-0003-2154-5950]