Text S1 Folding of His 6 -proHNP1B in redox buffer system. In a previous study, Wu et al. demonstrated efficient folding of proHNP1 in vitro using reduced/oxidized glutathione to mediate thiol-disulfide exchange [29] . Employing this approach, 10 mg of HPLC purified His 6 -proHNP1 was dissolved in 500 ml 50 mM Tris HCl pH 7.5, 0.8 M urea and 100 mM NaCl (TUN buffer) with or without 2 mM reduced and 0.4 mM oxidized glutathione and incubated at 37uC for 4 h at which point the reaction was stopped by addition of 50 ml acetic acid. Reaction products were purified by RP-HPLC on a Vydac analytical C18 column (4.6 mm6250 mm) using a 20-to-55% (1% per min; 1 ml/min flow rate) linear gradient of ACN in 0.1% TFA acetonitrile. Peak fractions were collected and analyzed by AU-PAGE. In the absence of glutathione, the retention time during RP-HPLC and electrophoretic pattern was unchanged compared to the respective starting materials (Fig. S1A and B ). After incubation with glutathione, the behavior of proHNP1A was unchanged on C18 RP-HPLC (Fig. S1C) and on AU PAGE (Fig.  S1C, lane 5) . However, treatment of proHNP1B with glutathione generated a new species that behaved like proHNP1A on both C18 RP-HPLC and AU-PAGE (Fig. S1C) . These data indicate that proHNP1A and proHNP1B are folded and misfolded forms of His 6 -proHNP1respectively and that proHNP1B can be folded in the presence of reduced/oxidized glutathione. (DOC) Foxp3(+) Regulatory T Cells Control Persistence of Viral CNS Infection We earlier established a model of a persistent viral CNS infection using two week old immunologically normal (genetically unmodified) mice and recombinant measles virus (MV). Using this model infection we investigated the role of regulatory T cells (Tregs) as regulators of the immune response in the brain, and assessed whether the persistent CNS infection can be modulated by manipulation of Tregs in the periphery. CD4(+) CD25(+) Foxp3(+) Tregs were expanded or depleted during the persistent phase of the CNS infection, and the consequences for the virus-specific immune response and the extent of persistent infection were analyzed. Virus-specific CD8(+) T cells predominantly recognising the H-2D(b)-presented viral hemagglutinin epitope MV-H(22–30) (RIVINREHL) were quantified in the brain by pentamer staining. Expansion of Tregs after intraperitoneal (i.p.) application of the superagonistic anti-CD28 antibody D665 inducing transient immunosuppression caused increased virus replication and spread in the CNS. In contrast, depletion of Tregs using diphtheria toxin (DT) in DEREG (depletion of regulatory T cells)-mice induced an increase of virus-specific CD8(+) effector T cells in the brain and caused a reduction of the persistent infection. These data indicate that manipulation of Tregs in the periphery can be utilized to regulate virus persistence in the CNS. The role of CD4 + CD25 + regulatory T cells (Tregs) in autoimmune and pathogen-induced immune responses has been studied intensively during recent years. An important tool was provided by the discovery of the transcription factor Foxp3 (forkhead box P3) as a marker for Tregs and their suppressive activity [1, 2, 3, 4] . During acute viral infections depletion of Tregs was found to prevent the development of exhausted T cells and to improve the immune response. In addition, transient depletion of Tregs in several persistent viral infections led to reactivation of virus-specific T cells and reduction of the virus load [5, 6, 7, 8, 9] . The important protective role of Tregs against an overshooting immune response in the CNS became obvious in animal models of stroke and experimental autoimmune encephalitis [10, 11] , and human immunodeficiency virus-1 (HIV-1)-associated neuro-degeneration, where they reduce astrogliosis and microglia-mediated inflammation [12] . Interestingly, some viruses even developed the strategy to support the expansion of Tregs in order to suppress anti-viral cytotoxic T cell (CTL) responses and to limit viral immunopathogenesis [13, 14, 15, 16, 17, 18] . Defects in regulation of numbers or the activity of Tregs are also involved in a number of human autoimmune diseases such as type 1 diabetes, rheumatoid arthritis, and multiple sclerosis [19] .