BCMA Versus CD138 for Identifying Multiple Myeloma on Flow Cytometry

Introduction: Flow cytometry is an important tool in detecting multiple myeloma (MM), especially in cases of minimal residual disease (MRD). The reliability and stability of CD138, a marker used in many institutions on antibody panels to detect MM, has been questioned in recent studies, spurring the search for alternative markers. In this study, our aim was to compare BCMA (CD269) to CD138 in its ability to detect MM cells in flow cytometric analysis, in order to determine whether or not BCMA should be included in standard MM flow cytometry panels.

Methods: In this retrospective study, 25 consecutive cases of multiple myeloma diagnosed on bone marrow biopsy (including cases of initial diagnosis, relapse, and MRD) were identified from our institution's CoPath database from November 2015 through September 2016. These cases included 21 patients. Four patients had two flow cytometry cases included; in these patients, the second bone marrow aspirate was obtained at least six weeks after the first.

Cases were re-gated and analyzed (Kaluza Analysis program) with an 8-color flow cytometry panel containing the following markers: CD138, BCMA, CD45, CD19, CD3, CD14, CD64, GLY. All cases analyzed had also been run with a panel which included kappa/lambda to confirm the monoclonality of the plasma cell population identified. The same back-gating and Boolean gating strategy was used on all 25 cases. Boolean gates were used to exclude red cells and monocytes from the gated MM cells (i.e., cells expressing CD64, CD14, and GLY). BCMA and CD138 expression was compared on this gated population of MM cells. Data for coefficient of variation (CV) and geometric mean of both BCMA and CD138 expression were obtained from the Kaluza program for each case.

Results: In our 25 cases, gating with BCMA expression was able to detect the same population of MM cells as CD138. In 7/25 (28%) cases, BCMA detected MM cells which were not detected by CD138. In 2/7 cases, the MM cells detected were CD138-negative, but BCMA-positive; both were cases of MRD. In 5/7 cases, the MM cells showed two populations, one positive for both BCMA and CD138, and the other only positive for BCMA (CD138-negative); four of these cases were MRD. The CV in 21/25 cases was larger for CD138 than BCMA. The average CV for BCMA was 62.12 (range 39.19-107.06), while the average CV for CD138 was 121.46 (range 35.86-264.34). In comparing the CVs for BCMA and CD138 across all cases, the difference was statistically significant (p = 0.00003). In comparing the geometric mean (mean fluorescence intensity) for BCMA and CD138, BCMA tended to stain more dimly than CD138, but was in a more consistent range, while CD138 showed a large variation in its staining intensity. The average geometric mean for BCMA was 3.71 (range 1.63-10.17), while the average for CD138 was 82.94 (range 0.44-639.39).

Discussion: Our study shows that BCMA not only detects as many MM cells as CD138, but also has a lower coefficient of variation (p = 0.00003). The lower amount of variability in BCMA staining on MM cells makes this marker easier to use technically in terms of ease of gating around a more tightly clustered population of cells. The 7/25 cases which demonstrated CD138 negativity in all or part of the MM cell population detected also support previous studies which suggested that CD138 is a less reliable marker. 6/7 of these cases were MRD, showing that BCMA may be especially helpful in cases of MRD. Adding BCMA to MM antibody panels would help to ensure that these populations of CD138-negative MM cells would not be missed.

In addition to its usefulness in diagnosis and monitoring of MM (especially MRD), new therapeutic options for MM are a reason to include BCMA in the more commonly used flow cytometry panels. For example, immunotherapies against CD38 necessitate the use of other markers when monitoring patients who have been treated with these agents. BCMA is also currently being targeted in chimeric antigen receptor (CAR) T-cell therapies for MM in clinical trials, another reason to routinely analyze MM cells for BCMA.

Conclusion: Our study shows BCMA to be a more reliable marker for detecting MM cells than CD138, and supports a recommendation that BCMA be routinely used on flow cytometry panels to detect MM cells.