Richard A. Colvin

Chemokine receptor CXCR3 and its ligands CXCL9 and CXCL10 are required for the development of murine cerebral malaria

Cerebral malaria is a significant cause of global mortality, causing an estimated two million deaths per year, mainly in children. The pathogenesis of this disease remains incompletely understood. Chemokines have been implicated in the development of cerebral malaria, and the IFN-inducible CXCR3 chemokine ligand IP-10 (CXCL10) was recently found to be the only serum biomarker that predicted cerebral malaria mortality in Ghanaian children. We show that the CXCR3 chemokine ligands IP-10 and Mig (CXCL9) were highly induced in the brains of mice with murine cerebral malaria caused by Plasmodium berghei ANKA. Mice deficient in CXCR3 were markedly protected against cerebral malaria and had far fewer T cells in the brain compared with wild-type mice. In competitive transfer experiments, CXCR3-deficient CD8+ T cells were 7-fold less efficient at migrating into the infected brains than wild-type CD8+ T cells. Adoptive transfer of wild-type CD8+ effector T cells restored susceptibility of CXCR3-deficient mice to cerebral malaria and also restored brain proinflammatory cytokine and chemokine production and recruitment of T cells, independent of CXCR3. Mice deficient in IP-10 or Mig were both partially protected against cerebral malaria mortality when infected with P. berghei ANKA. Brain immunohistochemistry revealed Mig staining of endothelial cells, whereas IP-10 staining was mainly found in neurons. These data demonstrate that CXCR3 on CD8+ T cells is required for T cell recruitment into the brain and the development of murine cerebral malaria and suggest that the CXCR3 ligands Mig and IP-10 play distinct, nonredundant roles in the pathogenesis of this disease.

Evolutionarily conserved recognition and innate immunity to fungal pathogens by the scavenger receptors SCARF1 and CD36

Receptors involved in innate immunity to fungal pathogens have not been fully elucidated. We show that the Caenorhabditis elegans receptors CED-1 and C03F11.3, and their mammalian orthologues, the scavenger receptors SCARF1 and CD36, mediate host defense against two prototypic fungal pathogens, Cryptococcus neoformans and Candida albicans. CED-1 and C03F11.1 mediated antimicrobial peptide production and were necessary for nematode survival after C. neoformans infection. SCARF1 and CD36 mediated cytokine production and were required for macrophage binding to C. neoformans, and control of the infection in mice. Binding of these pathogens to SCARF1 and CD36 was β-glucan dependent. Thus, CED-1/SCARF1 and C03F11.3/CD36 are β-glucan binding receptors and define an evolutionarily conserved pathway for the innate sensing of fungal pathogens.

Mouse CCL8, a CCR8 agonist, promotes atopic dermatitis by recruiting IL-5+ TH2 cells

Mouse CCL8 is a CC chemokine of the monocyte chemoattractant protein (MCP) family whose biological activity and receptor usage have remained elusive. Here we show that CCL8 is highly expressed in the skin, where it serves as an agonist for the chemokine receptor CCR8 but not for CCR2. This distinguishes CCL8 from all other MCP chemokines. CCL8 responsiveness defined a population of highly differentiated, CCR8-expressing inflammatory T helper type 2 (TH2) cells enriched for interleukin (IL)-5. Ccr8- and Ccl8-deficient mice had markedly less eosinophilic inflammation than wild-type or Ccr4-deficient mice in a model of chronic atopic dermatitis. Adoptive transfer studies established CCR8 as a key regulator of TH2 cell recruitment into allergen-inflamed skin. In humans, CCR8 expression also defined an IL-5–enriched TH2 cell subset. The CCL8-CCR8 chemokine axis is therefore a crucial regulator of TH2 cell homing that drives IL-5–mediated chronic allergic inflammation.

Adiponectin Inhibits the Production of CXC Receptor 3 Chemokine Ligands in Macrophages and Reduces T-Lymphocyte Recruitment in Atherogenesis

Obese individuals often have low plasma adiponectin and concomitant chronic inflammation with a predisposition to metabolic and cardiovascular diseases. The present study reports a novel antiinflammatory action of adiponectin in human monocyte-derived macrophages (M&PHgr;) suppressing T-lymphocyte accumulation in atherogenesis. RNA profiling of lipopolysaccharide-stimulated human M&PHgr; identified CXC chemokine ligands (CXCLs), such as IP-10 (interferon [IFN]-inducible protein 10) (CXCL10), I-TAC (IFN-inducible T-cell α chemoattractant) (CXCL11), and Mig (monokine induced by IFN-γ) (CXCL9), T-lymphocyte chemoattractants associated with atherogenesis, among the top 14 transcripts suppressed by adiponectin. Real-time quantitative RT-PCR and ELISA verified that adiponectin inhibited expression of these chemokines at both the mRNA and protein levels in a concentration-dependent manner. Adiponectin reduced the release by lipopolysaccharide-stimulated M&PHgr; of chemoattractant activity for CXC chemokine receptor 3–transfected (receptor for IP-10, Mig, and I-TAC) lymphocytes. Adiponectin decreased lipopolysaccharide-inducible IP-10 promoter activity in promoter-transfected THP-1 M&PHgr; but did not change IP-10 mRNA stability. In lipopolysaccharide-stimulated M&PHgr;, reduction of IFN-β by adiponectin preceded inhibition of IP-10 mRNA expression. Immunoblot and chromatin immunoprecipitation analyses demonstrated that adiponectin attenuated activation of the transcription factor IFN regulatory factor 3, involved in the MyD88-independent pathway of Toll-like receptor 4 signaling, and subsequent IFN regulatory factor 3 binding to IFN-β promoter. In vivo studies further demonstrated that apolipoprotein E/adiponectin double-deficient (apoE −/−APN−/−) mice had increased plasma IP-10 levels, accelerated T-lymphocyte accumulation in atheromata, and augmented atherogenesis compared with apoE single-deficient (apoE−/−APN+/+) mice. This study establishes that low levels of adiponectin associated with obesity, the metabolic syndrome, and diabetes favor T-lymphocyte recruitment and contribute to adaptive immune response during atherogenesis.

CXCR3 Requires Tyrosine Sulfation for Ligand Binding and a Second Extracellular Loop Arginine Residue for Ligand-Induced Chemotaxis

ABSTRACT CXCR3 is a G-protein-coupled seven-transmembrane domain chemokine receptor that plays an important role in effector T-cell and NK cell trafficking. Three gamma interferon-inducible chemokines activate CXCR3: CXCL9 (Mig), CXCL10 (IP-10), and CXCL11 (I-TAC). Here, we identify extracellular domains of CXCR3 that are required for ligand binding and activation. We found that CXCR3 is sulfated on its N terminus and that sulfation is required for binding and activation by all three ligands. We also found that the proximal 16 amino acid residues of the N terminus are required for CXCL10 and CXCL11 binding and activation but not CXCL9 activation. In addition, we found that residue R216 in the second extracellular loop is required for CXCR3-mediated chemotaxis and calcium mobilization but is not required for ligand binding or ligand-induced CXCR3 internalization. Finally, charged residues in the extracellular loops contribute to the receptor-ligand interaction. These findings demonstrate that chemokine activation of CXCR3 involves both high-affinity ligand-binding interactions with negatively charged residues in the extracellular domains of CXCR3 and a lower-affinity receptor-activating interaction in the second extracellular loop. This lower-affinity interaction is necessary to induce chemotaxis but not ligand-induced CXCR3 internalization, further suggesting that different domains of CXCR3 mediate distinct functions.

Kynurenic acid triggers firm arrest of leukocytes to vascular endothelium under flow conditions

Recent studies have demonstrated that kynurenic acid (KYNA), a compound produced endogenously by the interferon-γ-induced degradation of tryptophan by indoleamine 2,3-dioxygenase, activates the previously orphaned G protein-coupled receptor, GPR35. This receptor is expressed in immune tissues, although its potential function in immunomodulation remains to be explored. We determined that GPR35 was most highly expressed on human peripheral monocytes. In an in vitro vascular flow model, KYNA triggered the firm arrest of monocytes to both fibronectin and ICAM-1, via β1 integrin- and β2 integrin-mediated mechanisms, respectively. Incubation of monocytes with pertussis toxin prior to use in flow experiments significantly reduced the KYNA-induced monocyte adhesion, suggesting that adhesion is triggered by a Gi-mediated process. Furthermore, KYNA-triggered adhesion of monocytic cells was reduced by short hairpin RNA-mediated silencing of GPR35. Although GPR35 is expressed at slightly lower levels on neutrophils, KYNA induced firm adhesion of these cells to an ICAM-1-expressing monolayer as well. KYNA also elicited neutrophil shedding of surface L-selectin, another indicator of leukocyte activation. Taken together, these data suggest that KYNA could be an important early mediator of leukocyte recruitment.

Oligomerization of CXCL10 Is Necessary for Endothelial Cell Presentation and In Vivo Activity

The chemokine IFN-γ-inducible protein of 10 kDa (IP-10; CXCL10) plays an important role in the recruitment of activated T lymphocytes into sites of inflammation by interacting with the G protein-coupled receptor CXCR3. IP-10, like other chemokines, forms oligomers, the role of which has not yet been explored. In this study, we used a monomeric IP-10 mutant to elucidate the functional significance of oligomerization. Although monomeric IP-10 had reduced binding affinity for CXCR3 and heparin, it was able to induce in vitro chemotaxis of activated T cells with the same efficacy as wild-type IP-10. However, monomeric IP-10 was unable to induce recruitment of activated CD8+ T cells into the airways of mice after intratracheal instillation. Use of a different IP-10 mutant demonstrated that this inability was due to lack of oligomerization rather than reduced CXCR3 or heparin binding. Molecular imaging demonstrated that both wild-type and monomeric IP-10 were retained in the lung after intratracheal instillation. However, in vitro binding assays indicated that wild-type, but not monomeric, IP-10 was retained on endothelial cells and could induce transendothelial chemotaxis of activated T cells. We therefore propose that oligomerization of IP-10 is required for presentation on endothelial cells and subsequent transendothelial migration, an essential step for lymphocyte recruitment in vivo.

Synaptotagmin-mediated vesicle fusion regulates cell migration

Chemokines and other chemoattractants direct leukocyte migration and are essential for the development and delivery of immune and inflammatory responses. To probe the molecular mechanisms that underlie chemoattractant-guided migration, we did an RNA-mediated interference screen that identified several members of the synaptotagmin family of calcium-sensing vesicle-fusion proteins as mediators of cell migration: SYT7 and SYTL5 were positive regulators of chemotaxis, whereas SYT2 was a negative regulator of chemotaxis. SYT7-deficient leukocytes showed less migration in vitro and in a gout model in vivo. Chemoattractant-induced calcium-dependent lysosomal fusion was impaired in SYT7-deficient neutrophils. In a chemokine gradient, SYT7-deficient lymphocytes accumulated lysosomes in their uropods and had impaired uropod release. Our data identify a molecular pathway required for chemotaxis that links chemoattractant-induced calcium flux to exocytosis and uropod release.

Crystal Structures of Oligomeric Forms of the Ip-10/Cxcl10 Chemokine

We have determined the structure of wild-type IP-10 from three crystal forms. The crystals provide eight separate models of the IP-10 chain, all differing substantially from a monomeric IP-10 variant examined previously by NMR spectroscopy. In each crystal form, IP-10 chains form conventional beta sheet dimers, which, in turn, form a distinct tetrameric assembly. The M form tetramer is reminiscent of platelet factor 4, whereas the T and H forms feature a novel twelve-stranded beta sheet. Analytical ultracentrifugation indicates that, in free solution, IP-10 exists in a monomer-dimer equilibrium with a dissociation constant of 9 microM. We propose that the tetrameric structures may represent species promoted by the binding of glycosaminoglycans. The binding sites for several IP-10-neutralizing mAbs have also been mapped.

CXCL10 can inhibit endothelial cell proliferation independently of CXCR3.

CXCL10 (or Interferon-inducible protein of 10 kDa, IP-10) is an interferon-inducible chemokine with potent chemotactic activity on activated effector T cells and other leukocytes expressing its high affinity G protein-coupled receptor CXCR3. CXCL10 is also active on other cell types, including endothelial cells and fibroblasts. The mechanisms through which CXCL10 mediates its effects on non-leukocytes is not fully understood. In this study, we focus on the anti-proliferative effect of CXCL10 on endothelial cells, and demonstrate that CXCL10 can inhibit endothelial cell proliferation in vitro independently of CXCR3. Four main findings support this conclusion. First, primary mouse endothelial cells isolated from CXCR3-deficient mice were inhibited by CXCL10 as efficiently as wildtype endothelial cells. We also note that the proposed alternative splice form CXCR3-B, which is thought to mediate CXCL10s angiostatic activity, does not exist in mice based on published mouse CXCR3 genomic sequences as an in-frame stop codon would terminate the proposed CXCR3-B splice variant in mice. Second, we demonstrate that human umbilical vein endothelial cells and human lung microvascular endothelial cells that were inhibited by CXL10 did not express CXCR3 by FACS analysis. Third, two different neutralizing CXCR3 antibodies did not inhibit the anti-proliferative effect of CXCL10. Finally, fourth, utilizing a panel of CXCL10 mutants, we show that the ability to inhibit endothelial cell proliferation correlates with CXCL10s glycosaminoglycan binding affinity and not with its CXCR3 binding and signaling. Thus, using a very defined system, we show that CXCL10 can inhibit endothelial cell proliferation through a CXCR3-independent mechanism.