Clinical trials are currently in progress to evaluate the therapeutic efficacy of recombinant human TRAIL and fully humanized monoclonal antibodies that target TRAIL-R1 (DR4, mapatumumab) and TRAIL-R2 (DR5, lexatumumab). Although these proteins are cytotoxic to sensitive tumor cells, many tumors are resistant and require a sensitization stimulus. We have reported that rituximab inhibits anti-apoptotic survival pathways such as NF-κB activity and sensitizes tumor cells to both chemotherapeutic drugs and Fas-L- induced apoptosis (

Vega et al.,
J. Immunol
). Sensitization to TRAIL is also regulated by NF-κB. Thus, we hypothesize that rituximab-induced inhibition of NF-κB also sensitizes B-NHL cells to TRAIL apoptosis. Treatment of Ramos, Daudi, 2F7 and Raji with rituximab sensitized the cells to TRAIL apoptosis in a concentration dependant feature. Sensitization to TRAIL by Rituximab involved the type II mitochondrial apoptosis pathway. Examination of the mechanism of rituximab sensitization to TRAIL revealed that inhibition of NF-κB was associated with inhibition of the DR5 transcription repressor Yin Yang 1 (YY1) and up regulation of DR5 (Vega et al., Blood 108(11):2387, ASH Annual Meeting Abstract 2006). There was no up regulation of DR4 by rituximab. The direct role of YY1 in the rituximab induced sensitization to TRAIL was corroborated by YY1siRNA. These findings suggested that one mechanism of Rituximab sensitization to TRAIL involved up regulation of DR5 expression. However, since TRAIL binds to both DR4 and DR5, the role of DR4 in rituximab induced sensitization to TRAIL needed further clarification. This was addressed by the use of the specific fully humanized monoclonal antibodies directed against DR4 (TRAIL-R1) (HGS-ETR1, mapatumumab) and DR5 (TRAIL-R2) (HGS-ETR2, lexatumumab) provided by Human Genome Sciences. Treatment of Ramos with rituximab sensitized the cells to both HGS-ETR1 and HGS-ETR2 induced apoptosis in a concentration dependant fashion. The sensitization by these antibodies was comparable to that achieved with TRAIL. These findings demonstrate that rituximab sensitizes B-NHL cells to both TRAIL and DR4/DR5 agonist-antibodies. The studies also suggest that, while the up regulation of DR5 expression by rituximab via inhibition of YY1 was critical for TRAIL apoptosis, up regulation of DR4 by rituximab was not required for sensitization with HGS-ETR1 monoclonal antibody. Further, these results suggest that rituximab may affect cell signaling induced by both DR4 and DR5. The molecular mechanism by which rituximab sensitizes B-NHL cells to TRAIL and both HGS-ETR1 and HGS-ETR2 will be presented.

Author notes

Disclosure: No relevant conflicts of interest to declare.