TRAIL Receptor 1 and TRAIL Receptor 2 Monoclonal Antibodies Induce Apoptosis and Increase Susceptibility to Cytostatic Agents in Mantle Cell Lymphoma.

Background: TRAIL is a member of the TNF-family of cytokines, which induces apoptosis in various solid tumors by activation of the death receptors DR4 and DR5. Previous studies demonstrated that human agonistic antibodies to TRAIL receptors TRAIL R1 (ETR1) and TRAIL R2 (ETR2) activate the apoptosis pathway via caspase 8. Also, preliminary data suggest an efficacy of the antibodies in human lymphoma cell lines; however, little is known about the effects of ETR1 and 2 in mantle cell lymphoma (MCL), a distinct lymphoma subtype with an especially poor clinical outcome.

Methods: 4 MCL cell lines (HBL2, GRANTA 519, Jeko-1, NCEB-1) and two lymphatic control cell lines (Jurkat, Karpas 422) exposed to different doses of ETR1 and ETR2 were studied for inhibition of proliferation (cell count) and metabolism (WST-1 assay); cell apoptosis was quantified by flow cytometry (Annexin V staining). In addition, all cell lines were also exposed to combinations of ETR1 and various cytostatic compounds (cytarabine, fludarabine or mitoxantrone) to explore potential synergism.

Results: Inhibition of proliferation and induction of apoptosis was achieved in all cell lines in a dose and time dependent manner, but susceptibility varied strongly between the MCL cell lines. With the notable exception of NCEB-1, ETR1 inhibited cell proliferation more effectively than ETR2. IC₅₀ values after 24 hours exposure of ETR1 was 3,26 μg/ml (HBL-2) and 1,09 μg/ml (Jeko-1), whereas no IC₅₀ was reached in the remaining MCL cell lines. Accordingly, HGS-ETR1 at a concentration of 2μg/ml induced apoptosis in 9% (Granta 519) to 63% (Jeko-1) of cells after 24 hours, respectively. In contrast, IC₅₀ values of ETR 2 were only reached in Jeko-1 (0,85μg/ml). Preliminary data indicate a synergistic effect of HGS-ETR1 in combination with conventional cytostatic agents, confirmatory experiments will be presented at the meeting.

Conclusions: Our data indicate that both, ETR1 and ETR2, induce apoptosis in MCL cell lines; however, ETR1 seems to be more effective. Interestingly, combination experiments suggest that induction of apoptosis by TRAIL receptor antibodies may overcome chemotherapy resistance. These results form the rationale for combined approaches in future clinical trials.