Loss of CD20 expression following treatment with rituximab (chimaeric monoclonal anti‐CD20): a retrospective cohort analysis

A retrospective analysis of CD20 expression following rituximab for B‐cell non‐Hodgkin's lymphoma demonstrated a significant change in immunophenotype in 6/25 (24%) patients with persistent bone marrow (BM) infiltration. In three out of six patients, the B cells were uniformly CD20−/CD79α+, consistent with frank loss of CD20 expression. In the remaining three cases, the BM infiltrate was predominantly (> 80%) CD20−/CD79α+. Two of the former but none of the latter three cases achieved a clinical response. In three further cases, the post‐treatment BM infiltrate was composed entirely of benign or reactive CD3+ T cells. Frank loss of CD20 was not seen in 25 post‐treatment lymph node biopsies. Immunophenotyping is therefore an important adjunct in the diagnosis of BM infiltration following rituximab.