Constitutively High Expression of CD137L on Primary MM Cells Could be Inhibited By Chemotherapy but Re-Emerged after Desease Relapse

Objective: Multiple myeloma (MM) is a kind of malignant monoclonal plasma cell disorder. CD137L molecules are important member of TNFsuperfamily. Under physiological conditions, CD137L express on the surface of various active APCs and participate in keeping the balance of immune system. Under pathological conditions, CD137L could express on the surface of various tumor cells. For example, in hematologic malignancies, about 35% of AML with FAB type M4 or M5, and most CLL, B-cell lymphoma cells express CD137L. CD137L molecules play an important role in the maturation process of B cells. Active CD137L can promote proliferation, differentiation and immune response of B cells. But when B cells transform to plasma cells, the majority of B antigens will be lost, such as CD19, CD20, HLA-DR. CD137L are also lost in the process of transformation. Our previous studies showed that there is no expression of CD137L and CD137 molecular on normal plasma cells. But CD137L were highly expressed on MM cell lines U-266, KMS-11, My-5, LP-1 and 8266. CD137L signaling promoted U266 cell proliferation remarkably, which could be blocked by Bortizomib. CD137L signaling also pushed MM cells in the G1 phase to enter the S phase. To understand the expression of CD137L and CD137 on MM primary cells，We examined the bone marrow specimens of MM patients, then analyzed the relationship between the expression of CD137L and the clinical characteristics of patients.

Methods: Flow cytometry was used to detect the CD137L and CD137 expression. Markers of normal plasma cell were CD45+CD38+CD138+CD19+CD56-, Markers of abnormal plasma cell were CD45-/CD45lowCD38++CD138+CD19-CD56+. bone marrow specimens from 127 cases of MM patients treated in the First Affiliated Hospital of Soochow University from 2012 to 2013 and the normal control from 10 volunteers were collected with the consent of patients and volunteers. 45 patients were newly diagnosed, 72 patients were checked after treatment, 10 patients were relapsed and refractory; 68 patients were males, 59 patients were females; The median age was 58(36-73); 3 pts were D-S staging I, 14 pts II, 59 pts III; 25 pts were ISS staging I, 65 pts II, 35 pts III.

Results: Normal plasma cells didn’t express CD137L and CD137 molecular .CD137L molecular but not CD137 were expressed on primary MM cells in all 45 de novo patients. The median level was 46.7 (3.6–96.7)%, significantly higher than that on normal plasma cells (P <0.05). A retrospective analysis had been made to find no correlation between CD137L expression and patients’ age, gender, type, DS stage, ISS stage , LDH, creatinine, serum calcium, AKP, M protein and extramedullary plasmacytoma. The CD137L expression of MM cells from 12 de novo patients treated with PAD were decreased gradually with the increase of treatment courses and improvement of efficacy. The expression of CD137L on MM cells in 79 patients grouped in CR (n = 12), PR (n = 51), recurrent (n = 16) were 4.5 (0–18)%，10 (−056)% (P<0.05), 30.5 (5.3–95)% (P = 0.00) respectively.

Conclusions: CD137L molecular without CD137 were constitutively highly expressed on MM cell lines and primary MM cells, but hardly on normal plasma cells. There was no relationship between the expression of CD137L with patients’ clinical and biological characteristics. But our data showed that constitutively high expression of CD137L molecular on primary MM cells could be inhibited by chemotherapy but re-emerged after desease relapse.