A prospective iterative trial of proteasome inhibitor (PI)-based therapy for reducing HLAantibody (Ab) levels was conducted in five phases differing in bortezomib dosing density and plasmapheresis timing. Phases included 1 or 2 bortezomib cycles (1.3mg/m2,6-8 doses), one rituximab dose and plasmapheresis. HLA Abs were measured by solid phase and flow cytometry (FCM) assays. Immunodominant Ab (iAb) was defined as highest HLA Ab level. Forty-four patients received 52 desensitization courses (7 patients enrolled in multiple phases): Phase 1 (n=20), Phase 2 (n=12), Phase 3 (n=10), Phase 4 (n=5), Phase 5 (n=5).