The effect of rituximab on anti-platelet autoantibody levels in patients with immune thrombocytopenia.

Rituximab is an effective therapy resulting in a platelet count improvement in 60% of patients with immune thrombocytopenia (ITP). Rituximab depletes B cells; thus, a reduction in platelet autoantibody levels would be anticipated in patients who achieve a clinical response to this treatment. The objectives of this study were to determine whether rituximab was associated with a reduction in platelet autoantibody levels, and to correlate the loss of autoantibodies with the achievement of a treatment response. We performed a case-control study nested within a previous randomized controlled trial of standard therapy plus adjuvant rituximab or placebo. We measured platelet-bound anti-glycoprotein (GP) IIbIIIa and anti-GPIbIX using the antigen capture test. Of 55 evaluable patients, 25 (45%) had a detectable platelet autoantibody at baseline. Rituximab was associated with a significant reduction in anti-GPIIbIIIa levels (P = 0·02) but not anti-GPIbIX levels (P = 0·51) compared with placebo. Neither the presence of an autoantibody at baseline nor the loss of the autoantibody after treatment was associated with a response to rituximab. The subset of patients with persistent autoantibodies after treatment failed to achieve a platelet count response, suggesting that persistence of platelet autoantibodies can be a marker of disease severity.