This study will evaluate the long-term safety and lymphocyte repletion profile of psoriasis patients who have completed participation in a previous study and who received treatment with MEDI-507. Patients must have had an ALC > 30% lower than baseline (defined as baseline prior to any MEDI-507 exposure); or an absolute CD4 count of < 250 cells/uL. This study will also evaluate the development and durability of anti-MEDI-507 antibodies.