Commensal gut flora and brain autoimmunity: a love or hate affair?

Multiple sclerosis (MS) and other chronic inflammatory autoimmune diseases represent major public health challenges in industrialised Western society. MS results from an autoimmune attack against myelin structures by self-reactive lymphocytes, which are normal components of the healthy immune repertoire. The nature of the triggers that convert the innocuous self-reactive lymphocytes into an autoaggressive phenotype is poorly understood. In the past, it was primarily suspected that pathogenic infections trigger MS. However, so far, none of the incriminated pathogenic microbes were firmly associated with the disease. A growing body of evidence in animal models of MS implicates the gut microbiota in the induction of central nervous system (CNS) autoimmunity. The mammalian gut harbors a diverse population of microbial organisms which are essential for our well being. There is an increasing understanding that the gut microbiota not only modulates the local immune functions but also affects the systemic immune system. We are only just beginning to understand the nature of the interactions of the gut microbiota with the host's immune system especially in the context of autoimmune diseases. This review will address the influence of intestinal microbiota on immune homeostasis and on the development of autoimmune responses at sites distal to the intestine with a particular emphasis placed on a discussion about CNS autoimmunity.