We, thus, investigated the possibility that, because of the structural selleck chemicals llc promiscuity (further supported by the killing properties of a structurally related TCR peptide), the S20-3 peptide designed to bind the Fas receptor may also bind TNFR and trigger necrosis. We detected TNFRI expression in BJAB, Jurkat, and Daudi cells (Figure 3), and the TNFRI-blocking selleck screening library antibody significantly inhibited S20-3– and TNF-α–induced cell killing in all 3 cell lines (Figure 4B and C). On the contrary, the TNFRII-blocking antibody showed no inhibitory effect on the S20-3 cell-killing of TNFRII-positive Daudi cells (Figure 4B). This

finding is not surprising considering the fact that activation of TNFRII triggers pro-survival signaling in hematological

cancer cells [22], and activation of TNFRI is required for any death signaling from TNFRII Selleckchem P5091 due to the lack of a death domain in TNFRII [27]. Our results with FADD– and caspase-8–defective Jurkat cells are in agreement with the reports showing that under apoptosis-deficient conditions (such as non-functional caspase-8 or FADD), stimulation with FasL or TNF-α could induce cell death with morphological features of necrosis/necroptosis [21, 28, 29]. Furthermore, lack of FADD, but not of caspase-8, was shown to sensitize Jurkat cells to TNF-α–induced necrosis [30]. Smac mimetic BV6 enhanced TNF-induced cell death in leukemia cells in 2 different ways: necroptosis, when the cells were apoptosis resistant (FADD– and selleck products caspase-8–deficient), and caspase-8–dependent apoptosis in apoptosis-proficient cells [31]. We hypothesize that the different death pathways can be activated in response to

S20-3 treatment in Jurkat, Daudi, and BJAB cells, depending on the availability of and sensitivity to Fas and TNFRs. Another possibility is a cross talk between signaling events from TNF and Fas receptors, as reported by Takada et al., in which TNFRI is recruited by Fas to induce apoptosis [32]. An additional important observation is that the S20-3 peptide activity seemed to be specific to malignant cells; leukemia T cells displayed a much greater sensitivity to S20-3 than nonmalignant cells (Figure 2C). While the constitutive expression of TNF receptors was clearly demonstrated in most tumor cells, in normal peripheral lymphocytes, the expression of TNF receptors is subjected to a positive and negative regulation and can be induced by different stimuli [33, 34]. However, normal unstimulated PBMCs express very low amounts of mRNAs for TNFRII > TNFRI > Fas [35], and normal lymphocytes were shown to be resistant to stimulation with activating antibodies targeting TNFRI, TNFRII, or Fas [36]. Thus, our findings of cancer-specific killing by the S20-3 peptide are in agreement with these reports.