What flow cytometry assay could be used to monitor B-cell depletion during rituximab immunotherapy, if clinically requested, and is professional interpretation required for such assessment?

Rituximab is a genetically engineered chimeric antibody directed against CD20 antigen which is expressed on the surface of B-cells from pre-B cell (intermediate and late hematogones) through memory B-cell stages but not on hematopoietic stem cells, pro-B cells (early hematogones), and normal plasma cells. CD20 is expressed on more than 90% of B-cell non-Hodgkin lymphomas. The binding of rituximab to CD20 leads to B-cell lysis due to possible mechanisms: complement-dependent cytotoxicity, antibody-dependent cell-mediated cytotoxicity, and apoptosis. Currently, indications for use of rituximab include CD20-positive B-cell non-Hodgkin lymphomas and autoimmune/connective tissue diseases (e.g. rheumatoid arthritis), among others.

A quantitative flow cytometric assay may be used to monitor B-cell depletion during rituximab therapy by assessing the percentage and absolute count of CD19+ B-cells and/or CD20+ B-cells, but there are no specific guidelines of which markers are needed. The assay usually includes CD19, CD20, and other T/NK markers to confirm the B-cell lineage of the cells of interest. Percentages and absolute counts of CD19, CD20, and others are included in the report. Typically, no professional interpretation is required for a flow cytometric assay that reports and monitors percentages and absolute counts of lymphocyte subsets.

Examples:
There are three likely scenarios when doing the assay to monitor B-cell depletion during rituximab therapy.







READING MATERIALS
1. Rituximab- FDA https://www.accessdata.fda.gov/drugsatfda_docs/label/2009/103705s5299lbl.pdf
2. Trouvin AP, et al. Usefulness of monitoring of B cell depletion in rituximab-treated rheumatoid arthritis patients in order to predict clinical relapse: a prospective observational study. Clin Exp Immunol 2015, 180:11-8.
3. Jilani I, et al. Transient down-modulation of CD20 by rituximab in patients with chronic lymphocytic leukemia. Blood 2003, 102:3514-20.