Analysis of fractalkine receptor CX(3)CR1 function by targeted deletion and green fluorescent protein reporter gene insertion

The seven-transmembrane receptor CX(3)CR1 is a specific receptor for the novel CX(3)C chemokine fractalkine (FKN) (neurotactin). In vitro data suggest that membrane anchoring of FKN, and the existence of a shed, soluble FKN isoform allow for both adhesive and chemoattractive properties. Expression on activated endothelium and neurons defines FKN as a potential target for therapeutic intervention in inflammatory conditions, particularly central nervous system diseases. To investigate the physiological function of CX(3)CR1-FKN interactions, we generated a mouse strain in which the CX(3)CR1 gene was replaced by a green fluorescent protein (GFP) reporter gene. In addition to the creation of a mutant CX(3)CR1 locus, this approach enabled us to assign murine CX(3)CR1 expression to monocytes, subsets of NK and dendritic cells, and the brain microglia. Analysis of CX(3)CR1-deficient mice indicates that CX(3)CR1 is the only murine FKN receptor. Yet, defying anticipated FKN functions, absence of CX(3)CR1 interferes neither with monocyte extravasation in a peritonitis model nor with DC migration and differentiation in response to microbial antigens or contact sensitizers. Furthermore, a prominent response of CX(3)CR1-deficient microglia to peripheral nerve injury indicates unimpaired neuronal-glial cross talk in the absence of CX(3)CR1.

T-bet is rapidly induced by interferon-gamma in lymphoid and myeloid cells

Differentiation of naive CD4(+) T cells into IFN-gamma-producing T helper 1 (T(H)1) cells is pivotal for protective immune responses against intracellular pathogens. T-bet, a recently discovered member of the T-box transcription factor family, has been reported to play a critical role in this process, promoting IFN-gamma production. Although terminal T(H)1 differentiation occurs over days, we now show that challenge of mice with a prototypical T(H)1-inducing stimulus, Toxoplasma gondii soluble extract, rapidly induced IFN-gamma and T-bet; T-bet induction was substantially lower in IFN-gamma-deficient mice. Naive T cells expressed little T-bet, but this transcription factor was induced markedly by the combination of IFN-gamma and cognate antigen. Human myeloid antigen-presenting cells showed T-bet induction after IFN-gamma stimulation alone, and this induction was antagonized by IL-4 and granulocyte/macrophage colony-stimulating factor. Although T-bet was induced rapidly and directly by IFN-gamma, it was not induced by IFN-alpha, lipopolysaccharide, or IL-1, indicating that this action of IFN-gamma was specific. Moreover, T-bet induction was dependent on Stat1 but not Stat4. These data argue for a model in which IFN-gamma gene regulation involves an autocrine loop, whereby the cytokine regulates a transcription factor that promotes its own production. These findings substantially alter the current view of T-bet in IFN-gamma regulation and promotion of cell-mediated immune responses.

CD40 triggering of heterodimeric IL-12 p70 production by dendritic cells in vivo requires a microbial priming signal

CD40 ligation triggers IL-12 production by dendritic cells (DC) in vitro. Here, we demonstrate that CD40 cross-linking alone is not sufficient to induce IL-12 production by DC in vivo. Indeed, resting DC make neither the IL-12 p35 nor IL-12 p40 subunits and express only low levels of CD40. Nevertheless, after DC activation by microbial stimuli that primarily upregulate IL-12 p40 and augment CD40 expression, CD40 ligation induces a significant increase in IL-12 p35 and IL-12 p70 heterodimer production. Similarly, IL-12 p70 is produced during T cell activation in the presence but not in the absence of microbial stimuli. Thus, production of bioactive IL-12 by DC can be amplified by T cell-derived signals but must be initiated by innate signals.

CCR5 provides a signal for microbial induced production of IL-12 by CD8 alpha+ dendritic cells

The activation of dendritic cells (DC) to produce interleukin 12 (IL-12) is thought to be a key step in the initiation of cell-mediated immunity to intracellular pathogens. Here we show that ligation of the C-C chemokine receptor (CCR) 5 can provide a major signal for the induction of IL-12 synthesis by the CD8 alpha+ subset of DC and that this pathway is important in establishing interferon gamma-dependent resistance to the protozoan parasite Toxoplasma gondii. These findings support the concept that the early induction of chemokines by invading pathogens is a critical step not only for the recruitment of DC but also for the determination of their subsequent immunologic function.

Lipoxin-mediated inhibition of IL-12 production by DCs: a mechanism for regulation of microbial immunity

Lipoxins are eicosanoid mediators that show potent inhibitory effects on the acute inflammatory process. We show here that the induction of lipoxin A(4) (LXA(4)) accompanied the in vivo suppression of interleukin 12 (IL-12) responsiveness of murine splenic dendritic cells (DCs) after microbial stimulation with an extract of Toxoplasma gondii. This paralysis of DC function could not be triggered in mice that were deficient in a key lipoxygenase involved in LXA(4) biosynthesis. In addition, DCs pre-treated with LXA(4) became refractory to microbial stimulation for IL-12 production in vitro and mice injected with a stable LXA(4) analog showed reduced splenic DC mobilization and IL-12 responses in vivo. Together, these findings indicate that the induction of lipoxins in response to microbial stimulation can provide a potent mechanism for regulating DC function during the innate response to pathogens.

Paralysis of dendritic cell IL-12 production by microbial products prevents infection-induced immunopathology

Interleukin-12 plays a major role in immunity to intracellular pathogens by governing the development of IFNgamma-dependent host resistance. Nevertheless, unregulated IL-12 synthesis can lead to immunopathology, an outcome prevented by the concurrent expression of interleukin-10. Dendritic cells (DC) are an important source of the initial IL-12 stimulated by microbial agents. Here, we show that, following systemic triggering, DC can no longer be restimulated to produce IL-12 in vivo while continuing to respond in vitro. When infected with Toxoplasma gondii during this refractory state, mice mount impaired acute IFNgamma responses and, in the case of IL-10-deficient animals, are protected from cytokine-induced mortality. These findings demonstrate a previously unrecognized form of immunologic paralysis involving DC that can protect from infection-induced immunopathology.

Native and aspirin-triggered lipoxins control innate immunity by inducing proteasomal degradation of TRAF6

Innate immune signaling is critical for the development of protective immunity. Such signaling is, perforce, tightly controlled. Lipoxins (LXs) are eicosanoid mediators that play key counterregulatory roles during infection. The molecular mechanisms underlying LX-mediated control of innate immune signaling are of interest. In this study, we show that LX and aspirin (ASA)-triggered LX (ATL) inhibit innate immune signaling by inducing suppressor of cytokine signaling (SOCS) 2-dependent ubiquitinylation and proteasome-mediated degradation of TNF receptor-associated factor (TRAF) 2 and TRAF6, which are adaptor molecules that couple TNF and interleukin-1 receptor/Toll-like receptor family members to intracellular signaling events. LX-mediated degradation of TRAF6 inhibits proinflammatory cytokine production by dendritic cells. This restraint of innate immune signaling can be ablated by inhibition of proteasome function. In vivo, this leads to dysregulated immune responses, accompanied by increased mortality during infection. Proteasomal degradation of TRAF6 is a central mechanism underlying LX-driven immune counterregulation, and a hitherto unappreciated mechanism of action of ASA. These findings suggest a new molecular target for drug development for diseases marked by dysregulated inflammatory responses.

Positive and negative regulation of pathogen induced dendritic cell function by G-protein coupled receptors

The induction of IL-12 from dendritic cells (DC) is a major initiating step in host resistance to intracellular pathogens. We have studied the regulation of this response using an in vivo model in which IL-12 production by splenic CD8alpha+ DC is followed after injection of a soluble extract (STAg) of the protozoan parasite Toxoplasma gondii. Our findings indicate that the potent IL-12 response observed is highly dependent on both the chemokine receptor CCR5 and G(i)-protein coupled signaling. In addition, we have examined the basis of the unresponsiveness of DC to secondary STAg injection which occurs following primary exposure to this parasite stimulus. Our results demonstrate that this refractory state correlates with the down-regulation of CCR5 expression on DC which, in turn, appears to depend on the induction of endogenous lipoxin A(4) (LXA(4)), a product of arachidonic acid metabolism. Since LXA(4) is known to also signal through a G-protein coupled receptor pathway, these findings taken together support a major role for G-protein signaling in the regulation of microbial-induced DC function.

Platelet disturbances correlate with endothelial cell activation in uncomplicated Plasmodium vivax malaria

Platelets drive endothelial cell activation in many diseases. However, if this occurs in Plasmodium vivax malaria is unclear. As platelets have been reported to be activated and to play a role in inflammatory response during malaria, we hypothesized that this would correlate with endothelial alterations during acute illness. We performed platelet flow cytometry of PAC-1 and P-selectin. We measured platelet markers (CXCL4, CD40L, P-selectin, Thrombopoietin, IL-11) and endothelial activation markers (ICAM-1, von Willebrand Factor and E-selectin) in plasma with a multiplex-based assay. The values of each mediator were used to generate heatmaps, K-means clustering and Principal Component analysis. In addition, we determined pair-wise Pearson’s correlation coefficients to generate correlation networks. Platelet counts were reduced, and mean platelet volume increased in malaria patients. The activation of circulating platelets in flow cytometry did not differ between patients and controls. CD40L levels (Median [IQ]: 517 [406–651] vs. 1029 [732–1267] pg/mL, P = 0.0001) were significantly higher in patients, while P-selectin and CXCL4 showed a nonsignificant trend towards higher levels in patients. The network correlation approach demonstrated the correlation between markers of platelet and endothelial activation, and the heatmaps revealed a distinct pattern of activation in two subsets of P. vivax patients when compared to controls. Although absolute platelet activation was not strong in uncomplicated vivax malaria, markers of platelet activity and production were correlated with higher endothelial cell activation, especially in a specific subset of patients.

Endothelial cell activation is a key process in the pathogenesis of Plasmodium vivax malaria. Platelets are classically involved in endothelial cell activation in several diseases, but their role in the context of vivax malaria remains unclear. Thrombocytopenia is the most common hematological disturbance in P. vivax-infected patients, and platelets have been implicated in parasitemia control. In this work, we studied the activation of platelets in association with endothelial cell activation in vivax malaria. Platelets retrieved from infected peripheral blood were non-activated when analyzed by flow cytometry; however, they displayed higher mean volume and significantly reduced counts. We also found higher levels of circulating factors associated with platelet activation (especially soluble CD40L), thrombopoiesis and endothelial cell activation in infected patients. Further, through pair-wise correlation and clustering analysis, we found a subgroup of patients showing significant associations between markers of platelet and endothelial activation in a pattern different from that of endemic controls. Collectively, our findings indicate a role of platelets in endothelial cell activation in vivax malaria and indicate a heterogeneous host response in the setting of uncomplicated disease, a finding to be further explored in future studies.

Slower rescue of ER homeostasis by the unfolded protein response pathway associated with common variable immunodeficiency

Common variable immunodeficiency (CVID) is a primary immunodeficiency characterized by hypogammaglobulinemia and recurrent infections. Herein we addressed the role of unfolded protein response (UPR) in the pathogenesis of the disease. Augmented unspliced X-box binding protein 1 (XBP-1) mRNA concurrent with co-localization of IgM and BiP/GRP78 were found in one CVID patient. At confocal microscopy analysis this patient's cells were enlarged and failed to present the typical surface distribution of IgM, which accumulated within an abnormally expanded endoplasmic reticulum. Sequencing did not reveal any mutation on XBP-1, neither on IRE-1alpha that could potentially prevent the splicing to occur. Analysis of spliced XBP-1, IRE-1alpha and BiP messages after LPS or Brefeldin A treatment showed that, unlike healthy controls that respond to these endoplasmic reticulum (ER) stressors by presenting waves of transcription of these three genes, this patient's cells presented lower rates of transcription, not reaching the same level of response of healthy subjects even after 48 h of ER stress. Treatment with DMSO rescued IgM and IgG secretion as well as the expression of spliced XBP-1. Our findings associate diminished splicing of XBP-1 mRNA with accumulation of IgM within the ER and lower rates of chaperone transcription, therefore providing a mechanism to explain the observed hypogammaglobulinemia.

Pathophysiological Mechanisms in Gaseous Therapies for Severe Malaria

Over 200 million people worldwide suffer from malaria every year, a disease that causes 584,000 deaths annually. In recent years, significant improvements have been achieved on the treatment of severe malaria, with intravenous artesunate proving superior to quinine. However, mortality remains high, at 8% in children and 15% in adults in clinical trials, and even worse in the case of cerebral malaria (18% and 30%, respectively). Moreover, some individuals who do not succumb to severe malaria present long-term cognitive deficits. These observations indicate that strategies focused only on parasite killing fail to prevent neurological complications and deaths associated with severe malaria, possibly because clinical complications are associated in part with a cerebrovascular dysfunction. Consequently, different adjunctive therapies aimed at modulating malaria pathophysiological processes are currently being tested. However, none of these therapies has shown unequivocal evidence in improving patient clinical status. Recently, key studies have shown that gaseous therapies based mainly on nitric oxide (NO), carbon monoxide (CO), and hyperbaric (pressurized) oxygen (HBO) alter vascular endothelium dysfunction and modulate the host immune response to infection. Considering gaseous administration as a promising adjunctive treatment against severe malaria cases, we review here the pathophysiological mechanisms and the immunological aspects of such therapies.