The increased expression of genes induced by type I interferon (IFN) is characteristic of viral infections and systemic lupus erythematosus (SLE). We showed that mitochondrial antiviral signaling (MAVS) protein, which normally forms a complex with retinoic acid gene I (RIG-I)–like helicases during viral infection, was activated by oxidative stress independently of RIG-I helicases. We found that chemically generated oxidative stress stimulated the formation of MAVS oligomers, which led to mitochondrial hyperpolarization and decreased adenosine triphosphate production and spare respiratory capacity, responses that were not observed in similarly treated cells lacking MAVS. Peripheral blood lymphocytes of SLE patients also showed spontaneous MAVS oligomerization that correlated with the increased secretion of type I IFN and mitochondrial oxidative stress. Furthermore, inhibition of mitochondrial reactive oxygen species (ROS) by the mitochondria-targeted antioxidant MitoQ prevented MAVS oligomerization and type I IFN production. ROS-dependent MAVS oligomerization and type I IFN production were reduced in cells expressing the MAVS-C79F variant, which occurs in 30% of sub-Saharan Africans and is linked with reduced type I IFN secretion and milder disease in SLE patients. Patients expressing the MAVS-C79F variant also had reduced amounts of oligomerized MAVS in their plasma compared to healthy controls. Together, our findings suggest that oxidative stress–induced MAVS oligomerization in SLE patients may contribute to the type I IFN signature that is characteristic of this syndrome.

Humans and mice lacking functional caspase-8 in T cells manifest a profound immunodeficiency syndrome due to defective T cell antigen receptor (TCR)-induced NF-B signaling and proliferation. It is unknown how caspase-8 is activated following T cell stimulation, and what is the caspase-8 substrate(s) that isActivation of caspases has traditionally been associated with oligomerization of death receptors and induction of cell death (1). It has been determined more recently that caspase activity is also required for the induction of proliferation by primary naïve human and murine T cells (2-4). Inhibition of caspase activity by the pan-caspase blocker, Z-VAD-fmk, 4 or the caspase-8-specific blocker, IETD-fmk, greatly reduced production of IL-2 and proliferation following TCR ligation (2, 3). It was subsequently shown that a non-functional mutation in the caspase-8 gene in humans (5, 6) or the deletion of caspase-8 in murine T cells (7) resulted in an immunodeficiency syndrome characterized by markedly decreased production of IL-2 and proliferative capacity of T cells. It remained uncertain from these studies how caspase activation was linked to T cell antigen receptor (TCR) ligation, what were the regulatory protein(s) and substrate(s) of caspase activity, and whether this required the presence of the death receptor Fas (CD95/APO1). Equally puzzling was how the activation of caspase-8 might be limited to avoid induction of apoptosis. We recently observed that following T cell activation, cleavage occurs of certain known caspase substrates, such as c-FLIP-Long form (c-FLIP L ) and RIP1, although another caspase-8 substrate associated with cell death, Bid, was not cleaved (4). This suggested that active caspase-8 may become sequestered in a specific site within T cells after activation.We considered the possibility of c-FLIP L as both an activator of caspase-8 and an early caspase-8 substrate after T cell activation. c-FLIP L is homologous to caspase-8 in containing two death effector domains, but bears a mutation in the caspase domain, which renders it enzymatically inactive (8). Following ligation of Fas, c-FLIP L is co-recruited with caspase-8 to the death-inducing signal complex (1,8). In addition, c-FLIP L is able to directly heterodimerize with and activate caspase-8 in its full-length form, independently of Fas (9 -11). c-FLIP L , furthermore, contains a caspase cleavage site at Asp 376 that yields processed p43 FLIP (12, 13). As such, c-FLIP L may be an early caspase-8 substrate during T cell activation. c-FLIP L also associates with RIP1 and TRAF2 to promote activation of NF-B (14). Mice overexpressing c-FLIP L in the T cell compartment manifest augmented IL-2, and enhanced proliferation (15, 16). Collectively, c-FLIP L may provide an important link in our understanding of how TCR stimulation activates caspase-8, what is the caspase-8 substrate(s), and how caspase activity may link to NF-B activation.

Caspase-8, a cysteine-protease, initiates apoptosis when activated via death receptors. Caspase-8 is also essential for initiating T lymphocyte proliferation following T cell antigen receptor (TCR) signaling. Given these disparate functions of caspase-8, we sought to determine whether this represented only a difference in the magnitude of caspase-8 activation, or different intracellular locations of active caspase-8. We demonstrate by high-resolution multi-color confocal laser scanning microscopy an aggregation of active caspase-8 within membrane lipid rafts in T cells stimulated with anti-CD3. This suggests that following TCR stimulation active caspase-8 physically interacts with lipid raft proteins, possibly to form a signaling platform. In contrast, Fas stimulation of T cells resulted in a much more profound activation of caspase-8 that was exclusively cytosolic. These confocal microscopic findings were confirmed using discontinuous sucrose gradient ultracentrifugation to isolate lipid raft versus cytosolic components. This sequestration model of caspase-8 activation was further supported by the observation that a classic caspase-8 substrate, BID, was not cleaved in CD3 stimulated T cells, but was cleaved after Fas engagement. Our data support a model that the location of active caspase-8 may profoundly influence its functional capacity as a regulator of either cell cycling or cell death.

Background: c-FLIP L is a regulator of caspase-8 activity in T lymphocytes. Results: Caspase-8 activity is lost upon deletion of c-FLIP L . p43FLIP rescues caspase-8 activity through Raf1, TRAF2, and RIPK1 association, augmenting ERK and NF-B pathways. Conclusion:The FLIP L cleavage product p43FLIP promotes activation of pathways involved with T cell growth. Significance: This study provides new insight into the regulation of caspase-8 activity by c-FLIP.

Sexual bias is a hallmark in various diseases. This review evaluates sexual dimorphism in clinical and experimental coxsackievirus B3 (CVB3) myocarditis, and how sex bias in the experimental disease changes with increased age. Coxsackieviruses are major causes of viral myocarditis, an inflammation of the heart muscle, which is more frequent and severe in men than women. Young male mice infected with CVB3 develop heart-specific autoimmunity and severe myocarditis. Females infected during estrus (high estradiol) develop T-regulatory cells and when infected during diestrus (low estradiol) develop autoimmunity similar to males. During estrus, protection depends on estrogen receptor alpha (ERα), which promotes type I interferon, activation of natural killer/natural killer T cells and suppressor cell responses. Estrogen receptor beta has opposing effects to ERα and supports pro-inflammatory immunity. However, the sexual dimorphism of the disease is significantly ameliorated in aged animals when old females become as susceptible as males. This correlates to a selective loss of the ERα that is required for immunosuppression. Therefore, sex-associated hormones control susceptibility in the virus-mediated disease, but their impact can alter with the age and physiological stage of the individual.