 Patients who respond to the initial dose, as defined by a decrease in the UC disease activity index UCDAI by greater than or equal to 3 points at Day 29 compared to baseline, will be offered the option of receiving up to 3 additional doses of MDX-1100 at their originally assigned dose level. Re-dosing will be permitted at the time of disease flare which is defined as a worsening of the modified UCDAI of greater than or equal to 2 compared to the prior nadir. In order to obtain PK data, the first re-dose may not be administered until Day 43 and all subsequent doses may be great than or equal to 28 days apart. A response in the repeat dosing phase will be defined as a decrease in the mUCDAI of greater than or equal to 2 as compared to the previous baseline mUCDAI. For the second infusion, the comparison will use the mUCDAI calculated at Day 29 or subsequent nadir. 