Rituximab 500mg/day will be given IV on days -6 and +1. At the first dose (D-6), rituximab infusion will be started at 50mg/h and escalate the infusion rate by 50mg every 30minutes to a maximum of 400mg/h. Starting the second dose (days -4, -2 and +1). IV infusion will be started at 100mg/h and escalate the infusion rate 100mg every 30minutes to a maximum of 400mg/h. Premedicate with Solumedrol 250mg IV, acetaminophen 650mg po qd and diphenhydramine 25mg 30 minutes before infusion on days -6 and +1. Premedicate acetaminophen 650mg po qd and diphenhydramine 25mg 30 minutes before infusion on days -4 and -2.