Safety Study of the Monoclonal Antibody Teplizumab (MGA031) in Subjects With Moderate or More Severe Psoriasis

Inclusion Criteria: * Chronic plaque psoriasis that has been present for more than 6 months and involves at least 10% Body Surface Area (BSA). * Baseline LS-PGA score of moderate or greater severity. * Weight \<= 125 kg (276 lb) and a BSA \<= 2.5 m\^2. Exclusion Criteria: * Clinically significant flare of psoriasis during the 12 weeks before enrollment. * Guttate, erythrodermic, palmoplantar, or pustular (von Zumbusch) psoriasis * Orally administered systemic psoriasis therapy or phototherapy within the previous 4 weeks; or had topical psoriasis treatment within the previous 2 weeks before enrollment. * Prior administration of a biologic agent/monoclonal antibody within 12 weeks before enrollment or 5 half-lives of the agent, whichever is greater. * Prior otelixizumab, OKT®3, or teplizumab. * Treatment within the last 30 days with a non-biologic drug or device that has not received regulatory approval for any indication at the time of study entry or are unwilling to forgo experimental treatment other than teplizumab during this study. * Treatment with live vaccine within 8 weeks before enrollment or during study; vaccination with an antigen or killed organism during the study, within 8 weeks before enrollment, or 8 weeks after dosing. * Evidence of active infection. * Positive IgM test for hepatitis A. * History of or positive test for hepatitis B, C, or D. * History of or positive test for HIV. * Are immunocompromised, have had recent or current serious systemic or local infection, clinical or radiological evidence of active tuberculosis, or evidence of latent TB infection. * History of chronic liver disease, peripheral vascular disease, cerebrovascular disease, cardiovascular disease, or epilepsy. * Current serious or unstable illnesses or allergies. * Clinically significant laboratory abnormalities. * Presence of serological reactivity to teplizumab (in subjects previously treated with therapeutic antibodies). * Clinically significant ECG abnormalities.