Adenosine and Inflammation
Recently, the purine adenosine (Ado) has been implicated as an endogenous mediator of inflammation. In fact, several studies have indicated that Ado acts as an anti-inflammatory molecule, although this has been complicated by the fact that four different Ado receptor subtypes exist and have different effects. One of these subtypes, adenosine A2B receptor (AA2BR), has been implicated in anti-inflammatory responses and inhibition of NF-κB inflammatory signaling cascades. Frick et al. (p. 4957) found that the predominant adenosine receptor subtype expressed on intestinal epithelial cells is AA2BR and wanted to test its role in intestinal inflammation and colitis. Using the model of dextran sodium sulfate (DSS)-induced colitis, it was found that mice lacking the AA2BR (Aa2br−/−) developed a more severe form of colitis compared with wild-type mice. Disease severity was determined as a combination of weight loss, colonic shortening, and systematized scoring of fecal and other disease-related changes. Enteral delivery of an AA2BR inhibitor, PSB1115, caused AA2BR-sufficient animals to develop a disease similar to that found in Aa2br−/− mice. In vitro analysis of human intestinal epithelial cells revealed that Ado analogues caused an induction in IL-10 mRNA and protein expression. This induction could be blocked with the use of the inhibitor PSB1115, indicating that the role of AA2BR was similar in mice and human intestinal epithelia. The data demonstrated that AA2BR regulated inflammation in DSS-induced colitis, and additional experiments confirmed that it did so in an IL-10-dependent fashion. Thus, AA2BR and Ado appear to modulate inflammatory responses in the intestine, and AA2BR acts as a protective mechanism expressed by intestinal epithelia.
MicroRNAs and Asthma
MicroRNAs (miRNAs), small 19- to 25-nt single-stranded molecules, specifically silence target genes by binding to their 3′-UTRs and causing translational inhibition or mRNA degradation. Recent work has hinted that miRNAs may play a role in the pathogenesis of asthma. Lu et al. (p. 4994) used a microarray approach to identify 21 genes that were differentially regulated in a mouse model of allergic airway inflammation. This model consisted of transgenic mice with doxycycline-induced, lung-specific IL-13 expression that developed asthma, but findings were also confirmed in two other models of allergen-induced asthma and in mice that had lung-specific expression of IL-4. MicroRNA 21 (miR-21) was overexpressed in induced-IL-13 mice and miR-1 was underexpressed when compared with controls. IL-13 caused the expression of miR-21 in an IL-13 receptor α I-dependent manner but allergen-induced miR-21 expression was induced through another pathway. In this case, miR-21 expression was independent of IL-13 receptor α 1 and STAT6. Using an approach that predicted miR-21 targets, the authors identified the IL-12p35 gene as a target and confirmed that miR-21 bound to the 3′-UTR of a reporter construct containing the IL-12p35 3′-UTR. The ability of miR-21 to bind and inhibit expression of the IL-12p35 reporter was dose dependent and confirmed with mutational analysis. These data demonstrate a particular expression pattern of miRNA in asthma and indicate that miR-21 targets IL-12, an important molecule in developing Th responses.
Lipocalin 2 belongs to a family of antimicrobial proteins and sequesters the siderophore enterobactin, an iron-binding bacterial survival factor, to consequently control bacterial infections. The role that lipocalin plays in mucosal immunity has remained relatively unclear, though gene expression analyses of tissues from Klebsiella pneumoniae infection or IL-17 stimulation have shown an up-regulation of lipocalin 2 expression. Chan et al. (p. 4947) demonstrated that lipocalin 2 was required to control Klebsiella pneumoniae lung infection in mice. In addition, the rapid induction of this antimicrobial peptide is TLR4-dependent upon Klebsiella pneumoniae infection. The cytokines IL-1β and IL-17 were also capable of inducing lipocalin 2, with mucosally administered IL-1β being able to rescue a defect in the induction of this molecule and infection resistance in TLR4-deficient mice. As would be expected, mice deficient in lipocalin 2 had impaired defenses against bacterial infection of the lung and were susceptible to bacterial sepsis. However, when lipocalin 2 was reconstituted in the lung specifically by administering recombinant protein, mice were now able to control and clear bacterial infection. Thus, the authors concluded that lipocalin 2 plays an integral role in controlling Klebsiella pneumoniae infection in the lung and is a critical part of mucosal host defense.
Subverting Parasite Evasion
To combat infection by the protozoan parasite Leishmania major, the host must mount an effective CD4+ T cell response that causes secretion of IFN-γ. BALB/c mice are susceptible to L. major because they respond to the parasite with a Th2 response characterized by IL-4 and IL-10 production. The CD4+ T cells in susceptible BALB/c mice respond to the immunodominant epitope Leishmania homologue of activated receptor for c-kinase (LACK) 158–173 and do not afford protection from the parasite. DM is a non-classical MHC class II molecule that edits the APC epitope repertoire and influences which epitopes are predominantly expressed by the APC. Raising the question of what would happen to leishmanial infection if the activity of DM in susceptible BALB/c mice was subverted, Kamala and Nanda (p. 4882) infected BALB/c DM-deficient mice with L. major. BALB/c mice lacking DM were resistant to infection by L. major, unlike wild-type BALB/c mice. In addition, the DM-deficient mice did not present the LACK 158–173 epitope on the surface of infected APCs. Instead, APCs from DM-deficient animals displayed a much wider range of parasite-derived epitopes and mediated protection against infection through production of IFN-γ. Thus, by subverting the ability of DM to edit epitopes in BALB/c mice, the authors were able to protect this previously susceptible mouse strain against parasite infection. These results identify DM as a host susceptibility gene for L. major infections.
Autophagy and the Art of Fine Dining
Autophagy is an evolutionarily conserved process by which cells remove defective organelles and protein aggregates. In mammals, autophagy also plays a role in controlling intracellular pathogens. To identify novel molecules in human autophagy, Huett et al. (p. 4917) used a bioinformatics approach based on the core yeast autophagy proteins. Fourteen mammalian genes were identified by predicted interaction with the yeast autophagy proteins, and several of these are known to play a role in membrane organization. The authors centered on FNBP1L, otherwise known as Toca-1, which was predicted to interact with autophagy-related 3 homologue (ATG3). This interaction was confirmed biochemically. FNBP1L has been implicated in neuronal membrane rearrangement events and Cdc42-dependent actin polymerization. These effects are achieved through three functional domains of the protein: an N-terminal FCH-BAR (FER/CIP4 homology domain)/F-BAR domain (2), a SH3 (Src homology 3) domain, and the HR1 (protein kinase C-related kinase homology region 1) domain. Through RNA interference studies, FNBP1L was determined to be necessary for autophagy of the intracellular Salmonella enterica serovar Typhimurium and for control of bacterial infection. However, FNBP1L was not necessary for autophagic processes that were stimulated with rapamycin or serum starvation, suggesting separate pathways. Using bioinformatics and biochemical approaches, the authors identified and characterized a novel mammalian molecule that plays a role in autophagy of intracellular bacteria. This work also demonstrates that different autophagic processes require separate component molecules.
Dissecting the Importance of CD3
The CD3γ, δ, ε, and ζ molecules are all associated with the pre-TCR. However, what role each subunit plays in receptor-stimulated signal transduction has been somewhat unclear due to redundant mechanisms. Brodeur et al. (p. 4844) have determined that the intracytoplasmic (IC) domain of CD3ε is necessary for early thymocyte maturation. In CD3ε-deficient mice, reintroduction of CD3ε lacking the IC domain did nothing to restore the defect in thymocyte development from double negative (DN) to double positive (DP). Within the IC domain, the membrane-proximal basic amino acid-rich sequence (BRS) is necessary for thymocytes to progress from the DN stage to DP. The proline-rich sequence (PRS) found in the CD3ε IC is necessary for the thymocytes to efficiently proliferate. Within the context of the pre-TCR, the ligand-independent recruitment of the signaling molecule Nck to the CD3ε PRS was suggested to be important for thymocyte progression from the DN3 to DN4 stage of thymocyte development. These data identify distinct roles for the motifs of the IC domain of CD3ε and help discern the roles that the CD3 subunits play in thymocyte development and proliferation.
ACTIVatINg A Treg
Induced T regulatory cells (iTreg) are created by the TGF-β mediated conversion of CD4+CD25− T cells into CD4+CD25+Foxp3-expressing cells. Activin A, one of a family of dimeric proteins belonging to the TGF-β superfamily, is released during inflammation. Huber et al. (p. 4633) examined the effects of activin A on inducing CD4+CD25− T cell conversion to iTreg. Activin A was able to effect conversion to iTreg in a dose-dependent manner and synergized with TGF-β to induce iTreg from CD4+CD25− T cells in vitro. Both the activation of Smad2 and p38 MAP kinase by activin A in CD4+CD25− T cells and the blockage of iTreg induction in cells expressing a dominant negative form of the TGF-β type II receptor indicated that a competent TGF-β signaling pathway was necessary for the function of activin A. In transgenic mice that overexpressed activin A under control of the keratin14 promoter, which is expressed in the thymic cortical epithelium, there was an increase in the numbers of peripheral Treg, although the central Treg number found in the thymus was comparable to that of wild-type mice. Thus, the data demonstrate that that activin A plays a role in the TGF-β-mediated development of iTreg, particularly in expanding the peripheral population of this important immunoregulatory cell type.
Dectin, IL-17, and Fungus
Invasive fungal infections caused by Aspergillus fumigatus are a constant risk for those with immunosuppression. Werner et al. (p. 4938) examined what role the β-glucan/pattern recognition receptor Dectin-1 played in host defense against this invasive fungus. Dectin-1 deficient mice (Dectin-1−/−) that were not pharmacologically immunosuppressed were less resistant to intratracheal fungal challenge than wild-type mice. Dectin-1−/− mice demonstrated >80% mortality within 5 days due to compromised respiratory function. The secretion of IL-1α, IL-1β, TNF-α, CCL3, CCL4, and CXCL1 were all decreased in Dectin-1−/− A. fumigatus-challenged mice. As a result, fungal growth was not controlled in the lung due to insufficient neutrophil recruitment. Both alveolar macrophage proinflammatory cytokine secretion and neutrophil reactive oxygen responses to the fungus were impaired in Dectin-1−/− mice. Furthermore, the production of IL-17 after fungal challenge was dependent on Dectin-1; neutralization of IL-17 caused a defect in A. fumigatus clearance. Thus, Dectin-1 plays an important and necessary role in host defense against fungal infections.
Summaries written by Kira R. Gantt, Ph.D.