Caspase-8 functions primarily to initiate apoptosis and necrosis; however, recent studies have suggested that this enzyme may have a number of cell death–independent functions. Global or T cell–specific deletion of caspase-8 in mice results in systemic autoimmunity, suggesting that this enzyme may play a role in tolerance. Cuda et al. (p. 5548) sought to determine the role of caspase-8 in dendritic cell (DC)–mediated tolerance and demonstrated that DC-specific deletion of caspase-8 (CreCD11cCasp8fl/fl) in mice resulted in systemic lupus erythematosus–like disease and spontaneous early mortality relative to control mice. Deletion of the receptor-interacting serine-threonine kinase (RIPK)3 can reverse the caspase-8–deficient phenotype in mice; however, RIPK3 deficiency in CreCD11cCasp8fl/fl mice did not alter the effects of DC-specific caspase-8 ablation. CreCD11cCasp8fl/fl DCs showed enhanced proinflammatory cytokine production when stimulated with TLR4, 7, and 9 agonists and deletion of the TLR adaptor protein MyD88 reduced cytokine and anti-DNA Ab production and ameliorated kidney disease. CreCD11cCasp8fl/fl mice exhibited global immune cell dysregulation marked by increased CD4+ T cells and hyperactivated conventional DCs, along with increased B cell Ab production. Together, these data suggest that caspase-8 maintains tolerance by regulating MyD88 signaling in DCs and that unregulated TLR signaling in DCs promotes autoimmunity.

Much of what is known about polyomaviruses (PyV), dsDNA viruses that have pathogenic consequences in immunosuppressed hosts, comes from studies of SV40. The large T Ag (LT) of SV40, which promotes host cell transformation, is important for productive viral infection, and a better understanding of how it acts may allow for the development of effective treatments against PyV infection. SV40 LT induces IFN-stimulated genes (ISGs) in mouse embryonic fibroblasts, leading Forero et al. (p. 5933) to assess the ability of this protein to induce ISGs in human fibroblasts. LT expression in the absence of viral infection in human fibroblasts indeed resulted in ISG expression, which protected the cells against infection with a variety of viruses. ISG induction occurred through LT-mediated upregulation of IFN regulatory factor (IRF)1, which triggered IFN-β expression. Signaling through the IFNAR1, in turn, caused activation of IRF7 and IRF9 and subsequent ISG expression. The DNA damage response (DDR), which can be activated by PyV infection and is mediated by two kinases, ataxia-telangiectasia mutated (ATM) and ATM and Rad3-related (ATR), was found to be linked to LT-induced ISG expression. IRF1 upregulation and IFN-β expression required the kinase activity of ATR, but not ATM, and expression of an LT mutant unable to fully induce DDR impaired the induction of ISG and IFN-β expression. Taken together, these data indicate that innate antiviral responses can be activated in the absence of viral infection as a consequence of ATR-dependent DDR induction.

Immune suppressive regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs) are significant impediments to successful antitumor immunotherapy. Previous work showed that treatment of a murine renal adenocarcinoma (Renca) with a combination of IL-2 and agonistic anti-CD40 Ab led to potent antitumor immunity and the loss of Tregs and MDSCs. Because both Tregs and MDSCs can express Fas, a death receptor that triggers apoptosis through caspase activation, Weiss et al. (p. 5821) sought to determine whether a Fas-mediated mechanism caused the loss of these two populations. Elimination of tumor-associated Tregs and MDSCs following IL-2/αCD40 treatment was abrogated when FasL was blocked, either by a neutralizing Ab or in MRL-Fas(lpr) mice, which are deficient in functional Fas. Following IL-2/αCD40 therapy, Tregs and MDSCs upregulated active caspase levels and Tregs decreased Bcl-2 expression when compared with control cells. To determine if the Fas-mediated loss of Tregs and MDSCs was in part responsible for successful immunotherapy, MRL-Fas(lpr) Tregs were adoptively transferred into Treg-depleted wild-type mice prior to Renca challenge, and similar experiments were performed with MDSCs in the 4T1 breast cancer model. The inability of Tregs (Renca model) and MDSCs (4T1 model) to undergo Fas-mediated apoptosis abrogated the efficacy of IL-2/αCD40 therapy, as tumor sizes were similar between mice receiving IL-2/αCD40 and vehicle control therapy. Taken together, these data identify a mechanism by which Fas-mediated apoptosis can alleviate tumor-associated immune suppression and enhance tumor immunotherapy.

Invariant NKT (iNKT) cells recognize glycolipid Ags derived from self or microbial sources in the context of the nonclassical MHC molecule CD1d via their invariant TCRs. These cells can also recognize infectious agents through an indirect process thought to involve TLR stimulation of APCs that results in the presentation of endogenous Ags by CD1d. To better understand the role of TCR stimulation in iNKT cell activation, Holzapfel et al. (p. 5490) analyzed Nur77gfp transgenic mice, which upregulate GFP in response to TCR engagement. Consistent with earlier reports, MCMV infection activated iNKT cells independently of TCR stimulation in vivo, whereas infection with Streptococcus pneumoniae or Sphingomonas paucimobilis, which produce glycolipid Ags, activated iNKT cells via the TCR. Interestingly, infection of Nur77gfp mice with Salmonella typhimurium, which is not known to produce a lipid Ag, activated iNKT cells without inducing GFP upregulation in a CD1d-independent manner. Although pathogens such as S. typhimurium have been suggested to indirectly activate iNKT cells via TLR-stimulated induction of endogenous Ag presentation by CD1d, the authors could find no evidence of such a mechanism. Thus, TCR-dependent activation following infection appears to be confined to the recognition of foreign, not endogenous, Ags, and both bacterial and viral pathogens may activate iNKT cells in a TCR-independent manner.

Periodontitis is initiated by a dysbiotic microbiota that induces a damaging host inflammatory response, resulting in destruction of tooth-supporting tissues. Increases in complement activity in gingival fluid correlate with disease in periodontitis patients, and the complement receptor C5aR is important for the initiation of oral dysbiosis in mice. To further address how the complement system might contribute to periodontitis pathogenesis, Maekawa et al. (p. 6020) analyzed the involvement of the central complement component C3 in mouse and nonhuman primate models of disease. In a mouse model of Porphyromonas gingivalis–induced periodontitis, C3-deficient mice showed reductions in bone loss and inflammatory cytokine production, compared with wild-type (WT) mice. Although C3 was not required in these mice for P. gingivalis colonization or establishment of dysbiosis, it was needed for maintenance of the dysbiotic microbiota. Relative to WT controls, C3−/− mice were also protected from ligature-induced and aging-related periodontal bone loss. In cynomolgus monkeys with ligature-induced periodontitis, inhibition of C3 activation with the potent compstatin analog Cp40 significantly inhibited bone loss, inflammatory cytokine production, and osteoclastogenesis, relative to control treatment on the opposite side of each monkey’s mouth. Local inhibition of C3 activation therefore might be therapeutically useful for human periodontitis, which is often unresponsive to current treatment options.

Natural killer cells, which mediate Ab-dependent cellular cytotoxicity (ADCC), express inhibitory killer-cell Ig–like receptors (KIRs) that interact with HLA class I on target cells. This KIR/HLA interaction regulates NK cell effector function, termed ‘licensing;’ however, this interaction negatively regulates NK cell–mediated ADCC through an unclear molecular mechanism. mAbs, such as Rituximab, a chimeric anti-CD20 Ab used for the treatment of B cell lymphomas, can recruit effector cells for ADCC. Because some patients are resistant to Rituximab, mAbs such as GA101 have been glycoengineered with a modification that increases their affinity for the FcR CD16. In this issue, Terszowski et al. (p. 5618) examined whether the KIR/HLA influence on ADCC differed between conventional and glycoengineered mAbs. Incubation of HLA-deficient target cells with NK cells in the presence of Rituximab, GA101, or the non-glycoengineered parent molecule wt-GA101, showed that both GA101 and wt-GA101 activated NK cells more efficiently than Rituximab. In NK cell degranulation assays, NK cell activation decreased as the number of KIRs increased in the presence of Rituximab, GA101, and wt-GA101; however, GA101 could overcome the inhibitory signal from KIR/HLA. Thus, GA101-induced activation of NK cells was similar to Rituximab-induced activation of NK cells despite the presence of two additional HLA/KIR interactions. This enhanced activation was due to increased recruitment of licensed NK cells and to more effective degranulation, which correlated with target cell depletion. Collectively, these data demonstrate that glycoengineered mAbs activate NK cell–mediated ADCC more effectively than conventional mAbs, and suggest that modifications to increase FcR affinity and subsequent NK cell activation may overcome inhibitory signals and increase the efficacy of Ab therapy.