Peripheral blood plasmablasts, a potential confounder in <scp>chronic lymphocytic leukemia measureable residual disease</scp> analysis

Measurable residual disease (MRD) testing for chronic lymphocytic leukemia (CLL) is often done on peripheral blood (PB) since the concordance of results with bone marrow is high and testing is less invasive. When analyzing CLL MRD data, one must be aware of small, normal populations in the PB that may be mistaken for residual CLL cells. As part of our CLL MRD assay validation, PB samples were collected from 10 healthy donors and a 2-tube CLL MRD flow cytometry panel was stained for each donor using markers CD19, CD20, BAFF-R, kappa, lambda, CD5, CD200, CD23, CD38, CD81, ROR1, CD79b, CD43, and CD45. Additional markers were utilized to exclude T-cells, NK-cells, and myeloid cells from the analysis. Samples were acquired on the Navios EX flow cytometer, and the data were analyzed using FCS Express software. Once the test was implemented, CLL PB patient samples were monitored. All 10 PBs from healthy donors contained small populations of cells present in the lymphocyte gate which mimicked CLL cells in their expression of CD45, CD19, CD20, CD43 (both positive, although CLL cells showed dimmer positive expression), CD79b, and level of surface light chains, immunophenotypically compatible with plasmablasts. Clinical implementation of the CLL assay revealed 12 out of 77 (16%) CLL PB patient samples demonstrating a small population of plasmablasts. Plasmablasts normally exist in peripheral blood at levels detectable by flow cytometry MRD assays and may be a potential confounder in the identification of MRD in CLL.