Plasma cCD20 sequesters rituximab.

(A) Mixing study results demonstrating the capability of plasma cCD20 to compete for rituximab and prevent it from binding to CLL cells. The top row shows an analysis of CD20 on the surface of CLL cells before and after rituximab was added alone or along with plasma. The lower row shows detection of the rituximab on the surface of CLL cells using the PE-labeled anti-Fc fragment of the mouse Ig. (B) Mixing study results using Raji cells and plasma from a patient with CLL and plasma from a healthy control individual. There is greater inhibition of rituximab binding to the Raji cells using plasma from the CLL patient with high cCD20 compared with plasma from the healthy control individual.