CD137/4-1BB Is Inducibly Expressed On CLL B Cells through CD40 Signaling.

Abstract 1266

Poster Board I-288

CD137/4-1BB, a TNFR family member is expressed on activated T cells as a co-stimulator. It transduces the signal for cell survival and differentiation and plays a crucial role in CD8 cytotoxic T cells. Recently, an increasing number of reports have indicated its important role on tumor immunity, and the studies of immunotherapy targeting CD137 are on going. Therefore, it is important to know the expression of CD137 and CD137L on malignant cells for the establishment of immunotherapy. As a first step, we examined CD137 expression on PBMCs from healthy donors and CLL patients. When PBMCs from healthy donors were stimulated with PMA and ionomycin, CD137 expression was induced not only on T cells but also on activated B cells. However, when PBMCs from CLL patients were stimulated in the same way, we could not detect CD137 induction on CLL B cells. Since more than 90% of lymphocytes in the patients were CLL B cells, it is conceivable that activated T cells were required to induce CD137 on B cells. To test this hypothesis we next co-cultured CLL cells with T cells activated with anti-CD3/CD28 antibody-coated beads. In this co-culture CD137 was induced on CLL B cells, and this induction was diminished by anti-CD154 blocking antibody. Furthermore, CD137 was inducibly expressed on CLL B cells after co-culture with HeLa cells transfected with CD154 gene. The induction of CD137 mRNA was also clearly detected by RT-PCR after this stimulation. This CD137 induction was more significantly observed on CLL B cells (n=14, MFIR 11.5±6.9) as compared with B cells from healthy donors (n=4, MFIR 3.7±0.6, p=0.001) or non-CLL B cell malignancies (n=9, MFIR 4.2±3.43, p=0.003). Stimulation of CD137 expressed on BJAB transfectants by co-culture with CD137L-transfected CHO cells induced a conspicuous nuclear translocation of p52, a non-canonical NF-κB factor. In agreement with this activation, the expression of survival factor BCL-XL was upregulated. Consequently, the CD137 signal augmented the survival of CD154 stimulated CLL B cells in vitro. CD40 ligation can induce anti-CLL immunity and reduce CLL cells clinically. These data suggest that the inducibly expressed CD137 may diminish the effectiveness. It is possible that the fine adjustment of these co-stimulators can lead more effective immunotherapy.