The patients answering the criteria of inclusion and none inclusion (see V-2) will be randomized in 2 groups: the first group will receive a perfusion IV of rituximab to the amount of 375 mg/m2 of body surface, once per week, during 4 weeks (see VII-3); the second group will receive 4 perfusions IV of placebo according to same methods'. The evaluation (see VI-1) will be carried out at the time of the randomization, then repeated in 3 months, 6 months, 9 months and 1 year.