Multiple sclerosis (MS) is a serious disease of the central nervous system (CNS) involving destruction of the myelin sheaths and axons, and chronic immune activity inside the brain and spinal cord. The commonest disease course involves relapses with new neurological symptoms, followed by remission, against a background of slowly worsening disability known as progression. Several treatments are available to prevent relapses but their impact on progression is moderate and decreases with disease duration. In particular, depleting CD20-expressing cells with monoclonal antibodies such as rituximab is highly effective in the relapsing-remitting stage of the diseases. The majority of CD20-expressing cells are B cells, and the speed of the therapeutic effect suggests that peripheral, rather than CNS-resident B cells are the target. We describe two alternative hypothetical mechanisms that could explain the involvement of peripheral B cells in causing demyelinating lesions within the CNS, and propose experiments to test them. We assume that lymphocyte invasion of the CNS is initiated by T cells that encounter their cognate antigen within the CNS. The first hypothesis posits that these T cells are responding to a non-autoimmune stimulus such as a virus reactivation, and that the autoimmune component is mediated by B cells. The second hypothesis posits that the pioneer T cells are themselves autoreactive, and are activated by B cells in the periphery. 1.2 Overall ObjectiveThe objective of this project is to test two competing hypotheses to explain the mechanism of action of CD20 depletion, and to investigate whether the same mechanisms are connected to the more serious problem of ongoing progression. This will require the acquisition of much basic information about the biology of B cell trafficking, most of which will be done with animal models. We will also test some predictions of the two hypotheses in the real disease, using samples from patients with MS, and suitable controls.1.3 Specific Aims We aim to test the following hypotheses:Hypothesis 1: Relapses are caused by myelin-reactive B cells that traffic into the brain in response to local immune activity, and secrete myelin-binding antibodies.Hypothesis 2: Relapses are caused by autoreactive T cells, activated by virus-specific B cells that have co-captured self antigens, in the context of a peripheral viral infection.