Expression and Function of B7.2 and B7-H2 Molecules on Myeloma Cells

(INTRODUCTION) The B7 family molecules, which play an important role in the immune response by costimulating or coinhibiting T cells via antigen–T-cell receptor interactions, are expressed not only on professional antigen-presenting cells but also on some tumor cells. We previously reported that the expression of B7.2 and B7-H2 on leukemic cells was associated with poor prognosis in acute myeloid leukemia. In this study, we investigated whether functional B7.2 and B7-H2 molecules are expressed on myeloma cells and, if so, whether these molecules are associated with pathophysiology in multiple myeloma (MM).

(METHODS AND RESULTS)

1. Using flow cytometry (FCM), we examined the expression of B7.2 and B7-H2 molecules on fresh plasma cells in bone marrow samples from 10 normal individuals, 21 monoclonal gammopathy of undetermined significance (MGUS) patients, and 40 MM patients. The percentages of B7.2⁺ cells in plasma cells were much higher in MM patients than in MGUS patients or in normal individuals (MM vs. MGUS, P = 0.0128; MM vs. normals, P = 0.0053). When MM patients were divided into two groups, those in whom more than 40% of myeloma cells expressed B7.2 (n = 20, called B7.2^high+ MM patients in this study) showed significantly lower levels of hemoglobin and platelets and higher levels of M-protein compared with other MM patients (B7.2^−/low MM patients in this study, n = 20). Meanwhile, B7-H2 expression on plasma cells was clearly documented only in 4 patients with intractable disease: 1 plasma cell leukemia, 1 progressive disease, and 2 chemotherapy-resistant MM. Notably, the expression of B7.2 and B7-H2 on myeloma cells was augmented at the refractory stage compared with expression at the initial diagnosis in 2 patients examined. All of the above findings support the concept that these molecules are associated with disease progression in MM.

2. Based on the above data, we speculated that B7.2 or B7-H2 expression on myeloma cells was associated with their proliferative potential. When KMS-27 cells (a human myeloma cell line [HMCL]), part of which expressed these B7 family molecules, were analyzed in FCM, B7.2⁺ and B7-H2⁺ cells had significantly fewer G0/G1-phase cells and more G2/M-phase cells compared with B7.2⁻ and B7-H2⁻ cells, respectively. Consistent with these results, when four cell populations (B7.2⁺, B7.2⁻, B7-H2⁺, and B7-H2⁻ cells) were isolated and examined, B7.2⁺ and B7-H2⁺ KMS-27 cells proliferated more rapidly in liquid cultures and formed more colonies in semisolid cultures compared with B7.2⁻ and B7-H2⁻ KMS- 27 cells, respectively. The same growth advantage of myeloma cells expressing B7.2 and B7-H2 molecules was also documented in all other HMCLs examined. When B7.2 and B7-H2 expression was induced on HMCL RPMI8226 cells by cultivation with tumor necrosis factor-a, the cell cycle was clearly stimulated. Furthermore, when 293T cells were transfected with either B7.2 or B7-H2 gene or Mock, B7.2 or B7-H2 gene induction induced cell cycle activation.

3. Finally, we examined whether B7.2 and B7-H2 molecules on myeloma cells affect T cell immunology. In the mixed lymphocyte-myeloma reaction using KMS-27 cells and CD4⁺ T cells, an antagonistic monoclonal antibody (mAb) against B7.2 decreased the production of interleukin (IL)-10 as well as that of interferon (IFN)-g and IL-2. Meanwhile, an antagonistic mAb against inducible costimulator (ICOS), which blocks the B7-H2-ICOS pathways, decreased the production of IL-10 and IFN-g but not that of IL-2. The finding that both B7.2 and B7-H2 molecules on myeloma cells enhanced IL-10 production is particularly interesting because IL-10 not only reduces the antitumor immune response in general but also is a growth factor for myeloma cells.

(CONCLUSIONS) Our results show that functional B7.2 and B7-H2 molecules are expressed on myeloma cells and that these molecules may be associated with a proliferative advantage of MM cells and therefore contribute to the pathophysiology of MM.