Quantification of B-cell maturation antigen, a target for novel chimeric antigen receptor T-cell therapy in Myeloma.

B-cell maturation antigen (BCMA) is expressed by normal and malignant plasma cells and is targeted via anti-BCMA chimeric antigen receptor T-cell therapy (BCMA CAR T-cell therapy) in plasma cell myeloma (PCM) patients. Surface BCMA expression is required for CAR T-cell binding and killing. We determined the incidence and intensity of expression of BCMA in bone marrow PCM cells using flow cytometry (FC) and immunohistochemistry (IHC). PCM BCMA expression was assessed by FC in 70 patients and in 43 concurrent specimens by IHC. BCMA expression was detected in 94% of patients. FC could assess BCMA expression in all specimens and expression was quantifiable (QuantiBRITE system, BD Biosciences, San Jose, CA) in 89% of cases. Expression was highly variable and could be numerically classified into dim, moderate or bright levels of expression. In the 43 specimens assessed successfully by both IHC and FC, FC showed higher positivity rate (97%) than IHC (72%), indicating that FC is more useful than IHC in detection of BCMA (p = 0.002; McNemar's test). We conclude that FC is more sensitive than IHC and can be used to objectively quantify BCMA expression by myeloma cells. IHC is primarily useful when there is significant infiltration of the bone marrow by myeloma and is less sensitive with low numbers of myeloma cells. Furthermore, the ability of FC to differentiate between normal and abnormal plasma cells and to quantify BCMA on these cells, makes it a useful and sensitive tool in screening patients for CAR T-cell therapy and for follow-up post therapy.