Clinical Utility of Slam Family Member CD229 in Identifying Tumor Cells and High-Risk Disease Markers, CD86 (B7-2) and CD126 (IL-6 receptor), Using Flow Cytometric Analysis in Multiple Myeloma

Introduction: The immunophenotypic analysis of plasma cells using flow cytometry (FCM) is useful for the diagnosis for multiple myeloma (MM) and the detection of MM cells after treatment. CD138 is a well-known surface marker identifying plasma cells, although decreased CD138 expression on plasma cells is frequently observed in MM patients with relapsed or progressive disease. Thus, more stable MM antigens are needed. The SLAM family molecule CD229 has recently been reported to be specifically overexpressed on MM cells including their clonogenic precursors, suggesting that it may be a good marker to identify MM cells. Furthermore, some surface antigens, i.e., costimulatory molecule CD86 (B7-2), its receptor CD28, IL-6 receptor (CD126), insulin-like growth factor-1 (CD221), and coinhibitory molecule B7-H1 (PD-L1, CD274), were reported to be associated with poor prognosis. This study aimed to validate the potential of CD229 as a new MM cell marker and the efficacy of immunophenotypes on MM cells as prognostic markers in a multicenter study.

Patients & Methods: Two-hundred thirteen patients comprising 144 newly diagnosed (18 asymptomatic and 126 symptomatic) MM patients, 25 refractory/relapsed MM patients, and 44 monoclonal gammopathy of undetermined significance (MGUS) patients were enrolled. Immunophenotyping of plasma cells in bone marrow was performed with standard 3-color FCM, in which plasma cells were gated by CD38-highly positive cells and 9 parameters, i.e., expression of MM antigens (CD138, CD229) and prognosis-related antigens previously reported (CD28, CD45, CD56, CD86, CD126, CD221, CD274) on MM cells, were analyzed. Expression levels of prognosis-related antigens were compared between patients in high-risk categories, i.e., International Scoring System (ISS) stage II/III, high-risk chromosomal abnormalities [t(4:14), t(14:16), del17p] by FISH, high-risk group stratified by the International Myeloma Working Group (IMWG) [ISS stage II/II and t(4:14) and/or del17p], high serum lactase dehydrogenase (LDH) level, or revised ISS stage III, and other patients. The revised ISS is a powerful new tool to predict outcome in MM patients treated with novel agents based on 3 factors: serum LDH level, ISS stage, and high-risk chromosomal abnormalities [Oliva S, et al. EHA2014, Abstract #S1289]. Differences between continuous variables were evaluated using the Mann-Whitney U-test.

Results: 1) CD229 was expressed on almost all MM cells, even on low CD138-expressing MM cells. MM cells from ISS stage II/III patients expressed significantly lower levels of CD138 than those from ISS stage I patients (P = 0.0004). Furthermore, although CD138 expression levels in symptomatic MM patients were lower than those in patients with asymptomatic MM and MGUS and the expression was decreased in refractory/relapsed MM patients, CD229 expression levels on plasma cells were similar in MGUS and MM patients. 2) Newly diagnosed MM patients with high-risk chromosomal abnormalities or in the IMWG high-risk group had higher levels of CD86 and CD126 compared with other patients (P = 0.0056 and 0.0248 in high-risk chromosomal abnormalities, P = 0.0221 and 0.0396 in IMWG high-risk group, respectively). Furthermore, patients in revised ISS stage III had higher levels of CD126 compared with others (P = 0.0646). 3) Although the serum IL-6 level was significantly higher in ISS stage II/III patients, in patients with high LDH levels, and in revised ISS stage III patients compared with other groups, there was no correlation between serum IL-6 levels and expression levels of the IL-6 receptor CD126 on MM cells.

Conclusion: CD229 has the potential to become a new marker for the identification of MM cells and target for immunotherapy. High expression levels of CD86 and CD126 were associated with high-risk patients, and CD126 may be associated with survival in patients treated with novel agents. FCM analysis may be useful for predicting prognosis as well as detecting malignant clones. Further studies are in progress to clarify the significance of those molecules on MM cells.