Fas-stimulated generation of reactive oxygen species or exogenous oxidative stress sensitize cells to Fas-mediated apoptosis.

Inhibition of Fas-mediated apoptosis in B cell lymphomas by thiol antioxidants (glutathione and N-acetylcysteine) supported previous studies, suggesting that Fas-stimulated ROS generation may play a role in Fas-mediated apoptosis. Thus, the goal of the current study was to determine if Fas stimulation could induce ROS generation and what role, if any, it played in apoptosis. Fas crosslinking induced rapid generation of ROS (within 15 min) well before the appearance of characteristic apoptotic changes. Overexpression of catalase or superoxide dismutase suggested that Fas induced production of both superoxide anion and hydrogen peroxide. ROS generation was only observed, however, in cells that were sensitive to apoptosis and not in B cells inherently resistant to anti-Fas or in those in which resistance was induced by B cell receptor crosslinking. The exogenous addition of 250 microM hydrogen peroxide could reverse the resistant phenotype and sensitize cells to Fas-induced apoptosis. In Fas-sensitive cells, depletion of endogenous antioxidant defenses with buthionine sulfoximine increased the sensitivity to Fas-induced apoptosis, while overexpression of antioxidant enzymes and antiapoptotic proteins suggested a role for Fas-induced production of hydrogen peroxide in apoptosis. Further analysis suggested a redox-sensitive step early in Fas signaling at the level of initiator caspase (caspase-8) activation. Thus, the data suggest that the level of oxidative stress, either from exogenous sources or generated endogenously upon receptor stimulation, regulates the sensitivity to Fas-mediated apoptosis.

B cell receptor signaling mediates immediate protection from Fas-induced apoptosis upstream of caspase activation through an atypical protein kinase C isozyme and de novo protein synthesis.

Signaling through the B cell receptor (BCR) of normal splenic B cells, as well as B cell lymphoma lines, can abrogate Fas-mediated apoptosis. Using the B lymphoma line A20.2J, BCR signaling immediately inhibited Fas-induced apoptosis upstream of caspase-8 activation, as determined by Ile-Glu-Thr-Asp-(IETD)ase activity and cleavage of the caspase-8 substrate Bid. Furthermore, following overexpression of a human Fas:FLICE construct, which directly induces caspase activation in a death-inducing signaling complex-independent manner, cells could not be protected through BCR stimulation.Co-incubation with cycloheximide partially reversed protection from apoptosis and increased Fas-stimulated initiator and effector caspase activation, suggesting new protein synthesis is necessary to induce protection upstream of caspase activation. Furthermore, co-incubation with a broad-spectrum protein kinase C (PKC) inhibitor, such as bisindolylmaleimide (Bis), also partially reversed protection from apoptosis, and examination of a panel of PKC inhibitors suggested a role for atypical isozymes in protection. Bis also acted to increase initiator and effector caspase activation upon anti-IgG and anti-Fas treatment. These data suggest that BCR-induced protection is being mediated upstream of initiator caspase activation, and is partially dependent upon both PKC family members and new protein synthesis.

In vivo induction of tolerance by an Ig peptide is not affected by the deletion of FcR or a mutated IgG Fc fragment.

To induce tolerance to a variety of epitopes, we have designed a gene therapy approach in which peptides or antigens are expressed in frame on a soluble IgG fusion protein scaffold and delivered via retroviral gene therapy in B cells in vivo. Initially, tolerance to the lambda repressor cI sequence p1-102 or its immunodominant epitopes (e.g. p12-26 or p73-88) was elicited in both T cells and B cells when lipopolysaccharide (LPS) blasts are transduced and injected into naive or even primed recipients. While a role of secreted Ig fusion protein in this process is not clear, we have previously demonstrated the importance of antigen presentation on MHC class II of B cell antigen-presenting cells (APC) for tolerance induction. To further examine the role of the Ig and especially of the Fc portion of the IgG in tolerogenesis, we transduced LPS blasts from FcR gamma II(-/-), Fc gamma RI(-/-), Fc gamma RIII(-/-), FcR(-/-) or naive mice with retroviral vectors expressing IgG1-102, Delta IgG1-102 (mutated construct on position 297 of the Fc portion) or IgG12-26. When these transduced LPS blasts from FcR knockout mice were injected into normal (or knockout) syngeneic recipient mice, they induced tolerance both to the immunodominant epitopes and the full-length protein in that the antibody responses to the immunodominant epitopes were reduced. In this paper, we show that this tolerance resides at both the T and B cell level. Moreover, mutation of residue 297, which affects IgG functions including FcR binding, did not alter the tolerogenicity of the construct. These results suggest that the Fc portion of the IgG molecules is not required for humoral nor for cellular tolerance induction using the IgG-antigen tolerogens.