Bone under Attack
The inflammatory cytokine IL-17 promotes rheumatoid arthritis (RA) development, and IL-17 neutralization may be clinically beneficial for patients with RA. In murine collagen-induced arthritis, IL-17 can be produced by CD4+ T cells (Th17 cells) or γδ T cells. To better understand the mechanism of IL-17 activity in arthritis, Pöllinger et al. (p. 2602) studied the relative involvement of these two T cell types in disease pathogenesis. Treatment of arthritic mice with anti–IL-17A established the pathogenic role of this cytokine in osteoclast-mediated bone destruction, leading the authors to address the source of IL-17 production in arthritic joints. Similar numbers of IL-17–producing CD4+ and γδ T cells were found in the joints, and both cell types demonstrated a similar potential to induce osteoclastogenesis in vitro. However, cell depletion experiments revealed that CD4+ T cells, but not γδ T cells, were required for arthritis progression and osteoclast activity. In both arthritic mice and RA patients, CD4+ T cells were found in close association with osteoclasts in the joints, whereas γδ T cells were more diffusely dispersed. This CD4+ T cell/osteoclast association occurred in an Ag-specific manner, and it was suggested that IL-23 production by osteoclasts may establish a positive feedback loop that augments Th17-mediated osteoclastogenesis. In refining our understanding of the mechanisms responsible for bone destruction in RA, these data have broad clinical implications.
The Liver's View of LPS
Host recognition of LPS usually involves its binding to CD14 and is important in both the clearance of Gram-negative bacterial infections and the development of sepsis. Recently, it has been suggested that responses to LPS can occur in the absence of CD14 in some tissues. To address the involvement of CD14 in liver inflammation, McAvoy et al. (p. 2592) analyzed its role in neutrophil migration within the liver during endotoxemia. Surprisingly, neutrophil infiltration into the liver and trafficking within the liver microvasculature occurred independently of CD14 in response to systemic LPS administration. This CD14-independent neutrophil recruitment was not observed in other organs, and trafficking in the liver was not altered by the presence of soluble CD14 or of CD14 expression on any cell type. Although CD14 was not required for neutrophil adhesion within liver sinusoids, it was found to be important for neutrophil activation and for systemic inflammation in response to LPS treatment. Thus, although the liver produces soluble CD14 as an acute-phase protein, it uses a CD14-independent mechanism to recognize LPS and recruit neutrophils and thus may have a unique role in antibacterial immunity.
Transcriptionally Tying Up TLR3
Despite the importance of TLRs in immunity, the mechanisms regulating TLR gene transcription are not yet clear. TLR3 plays a critical role in antiviral immunity and the activation of myeloid dendritic cells (DCs). To analyze the regulation of the gene encoding TLR3, Fragale et al. (p. 1951) assessed the relative contributions of IFN regulatory factor (IRF) transcription factors in TLR3 expression. IRF-1 and IRF-2 synergistically stimulated the expression of human TLR3, and IRF-1 recruited CBP/p300 as a coactivator. Conversely, IRF-8 inhibited IRF-1–mediated TLR3 expression and recruited histone deacetylase 3 as a co-repressor. IRF-8 was found to be activated by Src homology 2 domain-containing protein tyrosine phosphatase (SHP2), which catalyzed its dephosphorylation and allowed its binding to the TLR3 promoter. During monocyte-derived DC differentiation, TLR3 expression increased when IRF-8 was phosphorylated and sharply declined upon its SHP2-mediated dephosphorylation. Supporting this mechanism of IRF-8 negative regulatory activity, inhibition of SHP2 promoted TLR3 expression and DC maturation. Taken together, these data define an antagonistic mechanism of IRF-1– and IRF-8–mediated transcriptional control of TLR3 expression, which may have relevance to future strategies to modulate TLR function in inflammation and infection.
Don't Skimp on Selenium
Dietary selenium enhances immune function, mainly via its incorporation into selenoproteins that are often involved in redox regulation. Selenoprotein K (Sel K) is a small selenoprotein that lacks defined redox motifs and has no known function. Verma et al. (p. 2127) developed Sel K-deficient mice to determine its potential role in the immune response. In wild-type mice, dietary selenium augmented expression of Sel K, which was localized to endoplasmic reticulum membranes and expressed at relatively high levels in lymphoid organs. Although Sel K-deficient mice had no apparent defects in immune system development, T cells, neutrophils, and macrophages from these mice demonstrated impaired receptor-mediated calcium flux compared with wild-type mice. Corresponding with this impairment, activities depending on calcium flux, including T cell proliferation, T cell and neutrophil chemokine receptor-mediated migration, and macrophage FcγR-mediated oxidative burst, were reduced in knockout mice. Sel K-deficient mice were also more susceptible to West Nile Virus infection than controls, as evidenced by decreased survival, increased viral titers and neuroinflammation, and reduced serum cytokine production. This demonstration of Sel K's involvement in immunity supports the importance of dietary selenium for proper immune function.
Stress Signal as Adjuvant
Adjuvants enhance immune responses to vaccination but can also cause unwanted inflammation. Pattern recognition receptors are targets of many adjuvants and can recognize both pathogen-derived molecules and “danger” signals produced by stressed cells. Andris et al. (p. 2245) hypothesized that ATP depletion as a result of metabolic stress could augment immunity and thus be useful in vaccine design. When coadministered with a T cell-dependent Ag, the ATP synthase inhibitor oligomycin augmented Ag-specific IgG production; however, it did not affect the response to a T cell-independent Ag. This oligomycin-mediated immune activation required neither caspase 1-dependent inflammasomes nor IL-6 production. Because oligomycin is highly toxic, the authors next investigated the adjuvant activity of the ATP-depleting nucleoside AICAR. Like oligomycin, AICAR enhanced humoral immunity to a T cell-dependent Ag. This effect required CD4+ T cells but did not involve the induction of inflammation or dendritic cell activation. Mechanistic analysis revealed that AICAR induced CD4+ T cell proliferation and the production of IL-4 and IL-21. This study suggests that metabolic stress may serve as a “danger” signal, and molecules that reduce intracellular ATP may augment vaccine-induced immunity without stimulating inflammation.
Adenosine Acts out in Sepsis
In sepsis, excessive inflammation and cardiovascular dysfunction lead to a high mortality rate. Immunosuppression via A2B adenosine receptors (A2BRs) can reduce tissue damage during inflammation. Belikoff et al. (p. 2444) hypothesized that this immunosuppression could exacerbate sepsis by preventing bacterial clearance and augmenting immune dysfunction, and that interference with A2BR activity might be beneficial in septic patients. In a cecal ligation and puncture (CLP) model of sepsis in outbred mice, use of an A2BR antagonist increased survival and bacterial clearance while inhibiting inflammatory cytokine and chemokine production compared with control treatment. A2BR-deficient mice also demonstrated reduced CLP-induced mortality compared with wild-type mice, and increases in survival were associated with improvements in cardiovascular function. These A2BR knockout mice had significantly enhanced bacterial clearance that was linked to improved phagocytic activity and cytokine production by macrophages. Finally, administration of the A2BR antagonist to mice predicted to die following CLP provided protection against septic lethality. These data suggest that immunosuppressive activities of A2BR impair bacterial clearance and play a pathogenic role in sepsis, and inhibition of this receptor may be useful in limiting septic mortality.
What a Difference a C Makes
The transcription factor NF-κB induces the expression of a wide variety of inflammatory mediators and is consequently important in the development of septic shock. Thair et al. (p. 2321) hypothesized that genetic variation in components of NF-κB signaling pathways could affect mortality in this inflammatory syndrome. In a cohort of septic shock patients, analysis of 59 single nucleotide polymorphisms (SNPs) in 19 genes involved in NF-κB signaling identified a SNP in NF-κB–inducing kinase (NIK) as significantly associated with mortality. Patients with the CC genotype of this SNP had a significantly higher risk of mortality than patients with CT or TT genotypes, and this association was replicated in a second large cohort of septic shock patients. In both cohorts, this CC genotype was also associated with a higher incidence of organ dysfunction. In lymphoblastoid cell lines, the CC genotype of the NIK SNP was associated with decreased expression of the chemokine CXCL10 at both the mRNA and protein levels, and this association was also observed in septic shock patients. These data identify a genetic marker that may be predictive of septic shock severity and provide a starting point to explore the involvement of NIK signaling in septic shock development.
A Powerful Point Mutation
E3 ubiquitin ligases such as Cbl-b regulate the balance between T cell activation and tolerance through an unidentified mechanism. To determine whether the catalytic ubiquitylating ligase activity of Cbl-b is involved in this control of T cell responses, Paolino et al. (p. 2138) analyzed mice carrying a point mutation in the Cbl-b RING finger domain. This mutation had no apparent effect on Cbl-b expression or on T cell development and differentiation. However, T cells from the mutant mice were hyperresponsive to TCR stimulation and did not require CD28 costimulation for high levels of proliferation and cytokine production. In a model of Ag-specific CD8+ T cell tolerance, the Cbl-b E3 ligase activity was found to be necessary for tolerance induction, as mutant mice died as a result of excessive T cell activation. The mutant mice also developed spontaneous systemic autoimmune responses involving inflammatory infiltration of multiple organs and the production of serum autoantibodies. Interestingly, the CD8+ T cell hyperactivation observed in these mice also lent them the ability to spontaneously reject tumors that were lethal to wild-type mice. Overall, the phenotype of the mutant mice was very similar to that of Cbl-b knockout mice, indicating that the catalytic activity of Cbl-b was the crucial factor responsible for this molecule's regulation of T cell responses. This understanding of Cbl-b activity may be applicable to the therapeutic modulation of T cell activation.
Summaries written by Jennifer Hartt Meyers, Ph.D.