Discordant expression of CD20 by flow cytometry and immunohistochemistry in a patient responding to rituximab: an unusual mechanism.

Immunohistochemistry using the L26 antibody recognizes an intracellular domain of CD20, whereas the L27 antibody used for surface CD20 staining by flow cytometry (FC) recognizes an extracellular domain and would be expected to be a better predictor of response to rituximab. We present a 75-year-old man who was initially treated for CD20- diffuse large B-cell lymphoma based on FC and, at relapse, still had CD20- disease by FC but CD20+ disease by immunohistochemistry. The patient responded to rituximab alone. On further study, it was shown that the malignant B cells, but not normal B cells, expressed the L27 surface binding site only within the intracellular domain. Therefore, it appears that the rituximab binding site is distinct from the surface binding site, and when there is a disparity between the methods to detect CD20 expression, consideration should be given to include rituximab in the treatment plan.