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Background. Recombinant T-cell receptor ligand 1000 (RTL1000) is a single-chain protein construct containing the outer two domains of HLA-DR2 linked to myelin-oligodendrocyte-glycoprotein- (MOG-) 35–55 peptide. Analogues of RTL1000 induce T-cell tolerance, reverse clinical and histological disease, and promote repair in experimental autoimmune encephalomyelitis (EAE) in DR2 transgenic, C57BL/6, and SJL/J mice. Objective. Determining the maximum tolerated dose, safety, and tolerability of RTL1000 in multiple sclerosis (MS) subjects. Methods. This was a multicenter, Phase I dose-escalation study in HLA-DR2+ MS subjects. Consecutive cohorts received RTL1000 doses of 2, 6, 20, 60, 200, and 100mg, respectively. Subjects within each cohort randomly received a single intravenous infusion of RTL1000 or placebo at a 4:2 ratio. Safety monitoring included clinical, laboratory, and brain magnetic resonance imaging (MRI) evaluations. Results. Thirty-four subjects completed the protocol. All subjects tolerated the 2–60mg doses of RTL1000. Doses ≥100mg caused hypotension and diarrhea in 3 of 4 subjects, leading to discontinuation of further enrollment. Conclusions. The maximum tolerated dose of RTL1000 in MS subjects is 60mg, comparable to effective RTL doses in EAE. RTL1000 is a novel approach for MS treatment that may induce immunoregulation without immunosuppression and promote neural repair.