Active multiple sclerosis lesions show inflammatory changes suggestive of a combined attack by autoreactive T and B lymphocytes against brain white matter. These pathogenic immune cells derive from progenitors that are normal, innocuous components of the healthy immune repertoire but become autoaggressive upon pathological activation. The stimuli triggering this autoimmune conversion have been commonly attributed to environmental factors, in particular microbial infection. However, using the relapsing-remitting mouse model of spontaneously developing experimental autoimmune encephalomyelitis, here we show that the commensal gut flora-in the absence of pathogenic agents-is essential in triggering immune processes, leading to a relapsing-remitting autoimmune disease driven by myelin-specific CD4(+) T cells. We show further that recruitment and activation of autoantibody-producing B cells from the endogenous immune repertoire depends on availability of the target autoantigen, myelin oligodendrocyte glycoprotein (MOG), and commensal microbiota. Our observations identify a sequence of events triggering organ-specific autoimmune disease and these processes may offer novel therapeutic targets.

The tissues of the central nervous system are effectively shielded from the blood circulation by specialized vessels that are impermeable not only to cells, but also to most macromolecules circulating in the blood. Despite this seemingly absolute seclusion, central nervous system tissues are subject to immune surveillance and are vulnerable to autoimmune attacks. Using intravital two-photon imaging in a Lewis rat model of experimental autoimmune encephalomyelitis, here we present in real-time the interactive processes between effector T cells and cerebral structures from their first arrival to manifest autoimmune disease. We observed that incoming effector T cells successively scanned three planes. The T cells got arrested to leptomeningeal vessels and immediately monitored the luminal surface, crawling preferentially against the blood flow. After diapedesis, the cells continued their scan on the abluminal vascular surface and the underlying leptomeningeal (pial) membrane. There, the T cells encountered phagocytes that effectively present antigens, foreign as well as myelin proteins. These contacts stimulated the effector T cells to produce pro-inflammatory mediators, and provided a trigger to tissue invasion and the formation of inflammatory infiltrations.

Brain-derived neurotrophic factor (BDNF) has potent effects on neuronal survival and plasticity during development and after injury. In the nervous system, neurons are considered the major cellular source of BDNF. We demonstrate here that in addition, activated human T cells, B cells, and monocytes secrete bioactive BDNF in vitro. Notably, in T helper (Th)1- and Th2-type CD4+ T cell lines specific for myelin autoantigens such as myelin basic protein or myelin oligodendrocyte glycoprotein, BDNF production is increased upon antigen stimulation. The BDNF secreted by immune cells is bioactive, as it supports neuronal survival in vitro. Using anti-BDNF monoclonal antibody and polyclonal antiserum, BDNF immunoreactivity is demonstrable in inflammatory infiltrates in the brain of patients with acute disseminated encephalitis and multiple sclerosis. The results raise the possibility that in the nervous system, inflammatory infiltrates have a neuroprotective effect, which may limit the success of nonselective immunotherapies.