This study will test the hypotheses that therapeutic modulation of T-cell function by teplizumab is well tolerated in subjects with moderate or more severe psoriasis, and that this treatment ameliorates the immunopathology of psoriasis. This study will evaluate the safety, pharmacokinetics (PK), and pharmacodynamics (PD) of subcutaneous (SC) administration for 6 days. Once the SC maximum tolerated dose is identified, this dose will be administered to a cohort of subject by intravenous for comparison of PK and PD.