Quantification of CD20 mRNA and protein levels in chronic lymphocytic leukemia suggests a post-transcriptional defect

Chronic lymphocytic leukemia is less effectively treated than other B cell malignancies with the anti-CD20 agent, rituximab, presumably due, at least in part, to low CD20 expression. CD20 expression is typically measured by flow cytometry, which may not be quantitative. This study was undertaken to measure total CD20 protein in CLL B cells using quantitative immunoblot analysis. The results demonstrated that total CD20 protein levels were consistently decreased by ∼60% in CLL B cells with low CD20 fluorescence staining. Surprisingly, real-time polymerase chain reaction analysis showed that CD20 mRNA levels were normal or close to normal, depending on the comparative B cell population, and did not correlate well with protein expression. We conclude that CD20 protein is substantially decreased in CLL due to a post-transcriptional defect.