A Bird’s Eye View of Lipids
CD1 molecules present endogenously derived lipid Ags to T cells. The human CD1 family is comprised of five nonpolymorphic isoforms, but only two isoforms, chCD1-1 and chCD1-2, exist in chickens. chCD1-2 has been previously crystallized, and its structure suggests a binding groove that accommodates only fatty acids or single alkyl chain lipids. This distinguishes it from human CD1 proteins, which contain dual pockets (denoted A’ and F’) in their binding grooves that allow the binding of lipids possessing two alkyl chains. Dvir et al. (p. 2504) crystallized chCD1-1 and found that this CD1 protein shares many similar architectural features with mammalian CD1, including a deep hydrophobic, dual-pocket Ag-binding groove. Differences included the addition of dual clefts in the binding groove of chCD1-1 and the doughnut-shaped configuration of its A’ pocket, which, with its also unusually wide portal, may accommodate longer alkyl chains than can mammalian CD1 molecules. Finally, binding tests using natural and synthetic lipids revealed that chCD1-1 was capable of binding dual-chain lipids comparable to known ligands for mammalian CD1 proteins. With these findings, the structures of all chicken CD1 proteins have been described, and disparate lipid Ag specificities are revealed.
Managing Mixed Messages
The lipid lipoxin A4 and the protein annexin A1 appear to play positive roles in the resolution of polymorphonuclear cell-mediated inflammation. These anti-inflammatory mediators and the proinflammatory peptide serum amyloid A (SAA) are recognized by the G protein-coupled receptor formyl peptide receptor 2 (Fpr2). To investigate the apparently dualistic role of Fpr2 in the inflammatory process, Dufton et al. (p. 2611) created an fpr2-deficient mouse. In vitro, fpr2−/− macrophage chemotaxis was significantly attenuated in response to stimulation with SAA or the synthetic peptide fMLP. fpr2−/− macrophages also exhibited significantly reduced levels of ERK phosphorylation when stimulated with various synthetic ligands or the pan-Fpr–agonist peptide Ac2–26. In contrast, SAA-mediated ERK phosphorylation was retained in fpr2−/− macrophages. However, in vivo, both the stimulatory effect of SAA and the inhibitory effects rendered by annexin A1, lipoxin A4, or Ac2–26 on the IL-1β–mediated recruitment of polymorphonuclear cells were diminished in fpr2−/− mice as compared with wild-type mice. Various in vivo models of inflammation revealed a protective function for Fpr2, as fpr2−/− mice exhibited increased inflammatory responses that were longer in duration. These findings identify Fpr2 and its ligands as potential therapeutic targets for modulation of the inflammatory response.
Interfering with IFN-β
The potent cytokine IFN-β plays both pro- and anti-inflammatory roles in the innate response to retroviral CNS infections, and its expression is tightly regulated via mechanisms controlling transcription, message stability, and protein stability. In this issue, Witwer et al. (p. 2369) present evidence that IFN-β protein levels in human and pigtailed macaque macrophages are also regulated by specific endogenous microRNAs (miRNAs). Using miRNA target-predication algorithms, the authors identified four miRNAs with identical macaque and human sequences that specifically bound the IFN-β 3′ untranslated region. In vitro, IFN-β secretion by primary human macrophages stimulated with polyinosinic:polycytidylic acid was significantly reduced when cells were pretreated with the miRNAs as compared with control cells treated with a control miRNA. Effects from potential off-target consequences were unlikely, as miRNA antagonist oligonucleotide-treatment of polyinosinic:polycytidylic acid-stimulated macrophages resulted in increased IFN-β secretion. Finally, rIFN-β treatment induced the expression of three of the four miRNAs in primary macaque macrophages. Taken together, these data provide evidence for an IFN-β–induced miRNA-mediated negative feedback mechanism for control of IFN-β expression levels in primate macrophages.
IFN-β Takes a Toll-less Pathway
Like viruses, the intracellular bacterium Chlamydia induces infected macrophages to secrete IFN-β. Results from previous studies suggest that pathogen recognition receptors, including TLRs and RIG-I–like receptors (RLRs), are involved. To elucidate the role of TLR signaling pathways in Chlamydia-induced IFN-β production, Prantner et al. (p. 2551) studied IFN-β induction in Chlamydia muridarum-infected mouse macrophages. TLR4-MyD88 knockout macrophages demonstrated no deficiency in Chlamydia-induced IFN-β production. These findings were also extended to TLR2, -3, -7, and -9 and their adaptor molecules MyD88 and TRIF. Instead, activation of the cytosolic receptor nucleotide-binding oligomerization domain 1 was required for maximal IFN-β induction. Additionally, the endoplasmic reticulum-resident protein stimulator of IFN gene (STING) colocalized with the chlamydial inclusion membrane and was critical for IFN-β production. Surprisingly, STING’s role in IFN-β production occurred independently of RLR interactions. Finally, IFN-β expression required the activities of p38 MAPK and the transcription factors NF-κB and IFN regulatory factor 3. These results indicate that C. muridarum induces IFN-β production independently of TLR- and RLR-mediated pathways through activation of nucleotide-binding oligomerization domain 1 and STING signaling pathways.
Muting the Gut Response
Exposed to a preponderance of commensal microbes, intestinal epithelial cells (IECs) are usually immunologically hyporesponsive to microbial products such as LPS and flagellin. Negative regulation of TLR-mediated responses appears to maintain IEC tolerance, but the mechanistic basis is unclear. Recently, the IEC-expressed single IgG IL-1-related receptor (Sigirr) was shown to negatively regulate LPS-induced IL-1β and TLR4 signaling and to inhibit experimental colitis. In this issue, Khan et al. (p. 2305) report that Sigirr negatively regulates the global TLR-mediated innate immune responses of differentiated IECs. When IECs were exposed to IL-1β, bacterial flagellin, and various other TLR family ligands, Sigirr expression levels transiently decreased. Sigirr overexpression and gene-silencing studies revealed that TLR-mediated chemokine responses were inversely correlated with the levels of Sigirr protein expression. Immunohistochemistry analysis of colonic biopsies showed that Sigirr protein expression increased as IECs differentiated, resulting in the highest expression levels at the apex of colonic crypts. These findings define a regulatory role for Sigirr in the TLR-mediated innate immune responses of differentiated human colonic IECs and identify Sigirr as a potential therapeutic target in the treatment of intestinal inflammatory disorders.
Moving Myelomonocytic Cells
Recruitment of myelomonocytic (MM) cells to tumors correlates with a poor prognosis and is induced by tumor-derived placental growth factor (PIGF) via its binding to vascular endothelial growth factor receptor 1 (VEGFR-1). To elucidate the molecular mechanisms underlying the recruitment of MM cells, Ding et al. (p. 2593) investigated the downstream signaling pathways of PIGF-induced VEGFR-1 activation. Initial experiments revealed that PIGF stimulation of VEGFR1 on monocytic cell lines and primary MM cells activated the transcription factor NFAT1, and pretreatment with NFAT1 activation inhibitors or NFAT1-specific small interfering RNAs attenuated PIGF-induced cell migration. Comparable experiments in vivo with xenograft and syngeneic tumor models substantiated these findings. Of several potential NFAT1 target genes, TNF-α was singularly significantly upregulated by activation of the PIGF/VEGFR1/NFAT1 signaling pathway, and blockade of TNF-α expression using RNA interference or Ab-mediated neutralization inhibited MM migration. Finally, NFAT1 inhibitor-blocked MM cell migration was rescued by exogenous TNF-α treatment. These findings reveal that PIGF recruits MM cells through VEGFR1/NFAT1-mediated induction of TNF-α expression and thereby expose potential new targets for cancer therapy.
α4s Prefer β1s
The α4β7 and α4β1 integrins, coexpressed at low levels on naive T cells, arise from the noncovalent pairing of the α4 subunit with either β7 or β1 subunits. Memory CD4+ T cells singularly express either α4β7 or α4β1 at high levels. α4β1 promotes T cells to traffic to sites that express its ligand VCAM-1, such as the bone marrow, whereas α4β7 promotes trafficking to sites expressing mucosal addressin cell adhesion molecule-1, including the Peyer’s patches (PPs). In this issue, DeNucci et al. (p. 2458) report an interrelationship between α4β7 and α4β1 expression. Studies with CD4+ T cells expressing elevated or deficient levels of β1 or β7 revealed that α4 preferentially paired with β1 and that α4β7 expression levels were regulated by virtue of β1 expression levels. To investigate the effects of β1 deficiency on T cell trafficking in vivo, short-term cohoming assays and long-term mixed bone marrow chimera models were used. In both models, significantly increased numbers of β1−/− CD4+ cells homed to PPs compared with wild-type cells. Finally, in response to Listeria monocytogenes infection, Ag-specific memory CD4+ cells in β1−/− mice uniformly expressed high levels of α4β7 and exhibited enhanced localization to PPs. Thus, by regulating the stoichiometry of α4β7 and α4β1 expression, β1 expression levels control the localization/trafficking of memory CD4+ cells.
Helicobacter pylori Responds to NO
Chronic infection of the stomach by the bacterium Helicobacter pylori is causally linked to peptic ulcers and gastric cancer. Its ability to persist is confounding, as ample evidence indicates that H. pylori infection elicits a strong immune response, such as the triggering of macrophages to express the enzyme inducible NO synthase (iNOS). iNOS uses the substrate l-arginine to produce the potent antimicrobial gas NO. In this issue, Lewis et al. (p. 2572) show that H. pylori infection of macrophages also induces the expression of an iNOS antagonist, the l-arginine-metabolizing enzyme arginase II (Arg2). Cotreatment of H. pylori-stimulated macrophages with an arginase-specific inhibitor, or knockdown of Arg2 expression using small interfering RNA, revealed that Arg2 decreased NO production by specifically inhibiting iNOS translation. Furthermore, isolated H. pylori-infected primary Arg2-deficient macrophages produced more NO and had higher iNOS protein levels compared with wild-type control macrophages. Confirming previous findings, l-arginine expression was necessary for Arg2 protein expression. These data suggest that H. pylori compromises the iNOS-mediated host defense by inducing Arg2 expression, which counteracts NO production 2-fold—it competes for l-arginine as a substrate and by a presently unknown mechanism inhibits iNOS translation.
Summaries written by Meredith G. Safford, Ph.D.