Fas Ligand Deficiency Impairs Tumor Immunity by Promoting an Accumulation of Monocytic Myeloid-Derived Suppressor Cells.

The Fas receptor ligand FasL regulates immune cell levels by inducing apoptosis of Fas receptor-positive cells. Here, we studied the impact of host FasL on tumor development in mice. Genetically targeting FasL in naïve mice increased myeloid cell populations, but, in marked contrast, it reduced the levels of myeloid-derived suppressor cells (MDSC) in mice bearing Lewis lung carcinoma tumors. Analysis of the MDSC subset distribution revealed that FasL deficiency skewed cell populations toward the M-MDSC subset, which displays a highly immunosuppressive activity. Furthermore, tumor-bearing mice that were FasL-deficient displayed an enhanced proportion of tumor-associated macrophages and regulatory T cells. Overall, the immunosuppressive environment produced by FasL targeting correlated with reduced survival of tumor-bearing mice. These results disclose a new role for FasL in modulating immunosuppressive cells.

Characterization of the kynurenine pathway and quinolinic Acid production in macaque macrophages.

The kynurenine pathway (KP) and one of its end-products, the excitotoxin quinolinic acid (QUIN), are involved in the pathogenesis of several major neuroinflammatory brain diseases. A relevant animal model to study KP metabolism is now needed to assess whether intervention in this pathway may improve the outcome of such diseases. Humans and macaques share a very similar genetic makeup. In this study, we characterized the KP metabolism in macaque primary macrophages of three different species in comparison to human cells. We found that the KP profiles in simian macrophages were very similar to those in humans when challenged with inflammatory cytokines. Further, we found that macaque macrophages are capable of producing a pathophysiological concentration of QUIN. Our data validate the simian model as a relevant model to study the human cellular KP metabolism in the context of inflammation.

EAAT expression by macrophages and microglia: still more questions than answers.

Glutamate is the main excitatory amino acid, but its presence in the extracellular milieu has deleterious consequences. It may induce excitotoxicity and also compete with cystine for the use of the cystine-glutamate exchanger, blocking glutathione neosynthesis and inducing an oxidative stress-induced cell death. Both mechanisms are critical in the brain where up to 20% of total body oxygen consumption occurs. In normal conditions, the astrocytes ensure that extracellular concentration of glutamate is kept in the micromolar range, thanks to their coexpression of high-affinity glutamate transporters (EAATs) and glutamine synthetase (GS). Their protective function is nevertheless sensitive to situations such as oxidative stress or inflammatory processes. On the other hand, macrophages and microglia do not express EAATs and GS in physiological conditions and are the principal effector cells of brain inflammation. Since the late 1990s, a number of studies have now shown that both microglia and macrophages display inducible EAAT and GS expression, but the precise significance of this still remains poorly understood. Brain macrophages and microglia are sister cells but yet display differences. Both are highly sensitive to their microenvironment and can perform a variety of functions that may oppose each other. However, in the very particular environment of the healthy brain, they are maintained in a repressed state. The aim of this review is to present the current state of knowledge on brain macrophages and microglial cells activation, in order to help clarify their role in the regulation of glutamate under pathological conditions as well as its outcome.

Nerve growth factor stimulation promotes CXCL-12 attraction of monocytes but decreases human immunodeficiency virus replication in attracted population.

The neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) are key molecules in the central nervous system development, which also exert specific effects on cells of the immune system. With regard to the latter, in vitro as well as in vivo data suggested that neurotrophins may play a role in human immunodeficiency virus (HIV) infection, especially in perivascular spaces where infiltrated macrophages express NGF. In the present study, we examined the expression of neurotrophins and their receptors in human monocyte-derived macrophages (MDMs) during infection by the R5 prototype HIV1/Ba-L strain. We then assessed to what extent neurotrophins themselves modulate infected macrophage survival and the level of virus production. The data show that neurotrophins and neurotrophin receptors are not modulated during HIV replication. Likewise, exogenous neurotrophins, or alternatively the blocking of neurotrophin receptors, neither modulated MDM sensitivity to HIV infection and replication nor altered their viability. In contrast, NGF clearly increased CD184 expression in macrophages, but this did not sensitize them to the X4 isolate HIV-1/Lai infection. Nevertheless, NGF enhanced monocyte chemotactic response to low CXCL-12 concentration regardless of infection. Surprisingly, CXCL-12-attracted monocytes from NGF-stimulated, HIV-infected cultures produced decreased amounts of virus progeny than their non-NGF-stimulated counterparts. This suggests a preferential effect on uninfected monocytes. Together these findings suggest a role for NGF in the continuous attraction of activated monocytes to the perivascular spaces, contributing to the chronic inflammatory state rather than neuroinvasion by HIV.

Comparative effects of two type I interferons, human IFN-alpha and ovine IFN-tau on indoleamine-2,3-dioxygenase in primary cultures of human macrophages.

Type I interferons (IFNs) are widely used to treat viral diseases. Depressive symptoms and suicide attempts are common neuropsychiatric side-effects during treatment with type I IFNs. Activation of indoleamine-2,3-dioxygenase (IDO), the first and rate-limiting enzyme of the kynurenine pathway by IFNs, leads to an increase in tryptophan (Trp) catabolism. Low levels of Trp lead to decrease of serotonin synthesis, which is likely to be related to the depressive symptoms. Ovine type I interferon-tau (IFN-tau) has a more potent antiretroviral effect and is less toxic than human type I IFN-alpha. Effects of IFN-tau and IFN-alpha on IDO expression and activity in primary cultures of human macrophages were compared in parallel to those of IFN-gamma, considered as one of the most potent IDO inducer. We found that both IFN-alpha and IFN-tau were poor inducers of IDO compared with IFN-gamma. However, IDO activation was slightly and significantly lower with ovine IFN-tau than human IFN-alpha, suggesting that ovine IFN-tau might have a lower impact on serotoninergic pathway compared with human IFN-alpha.

The glutamate-glutamine cycle as an inducible, protective face of macrophage activation.

Neuronal damage in HIV infection results mainly from chronic activation of brain tissue and involves inflammation, oxidative stress, and glutamate-related neurotoxicity. Glutamate toxicity acts via two distinct pathways: an excitotoxic one, in which glutamate receptors are hyperactivated, and an oxidative one, in which cystine uptake is inhibited, resulting in glutathione depletion, oxidative stress, and cell degeneration. A number of studies have shown that astrocytes normally take up glutamate, keeping extracellular glutamate concentration low in the brain and preventing excitotoxicity. They, in turn, provide the trophic amino acid glutamine via their expression of glutamine synthetase. These protective and trophic actions are inhibited in HIV infection, probably as a result of the effects of inflammatory mediators and viral proteins. In vitro and in vivo studies have demonstrated that activated microglia and brain macrophages (AMM) express the transporters and enzymes of the glutamate cycle. This suggests that in addition to their recognized neurotoxic properties in HIV infection, these cells exhibit some neuroprotective properties, which may partly compensate for the inhibited astrocytic function. This hypothesis might explain the discrepancy between microglial activation, which occurs early in the disease, and neuronal apoptosis and neuronal loss, which are late events. In this review, we discuss the possible neuroprotective and neurotrophic roles of AMM and their relationships with inflammation and oxidative stress.

Characterization of human monocyte-derived microglia-like cells.

Microglial cells are central to brain immunity and intervene in many human neurological diseases. The aim of this study was to develop a convenient cellular model for human microglial cells, suitable for HIV studies. Microglia derive from the hematogenous myelomonocytic lineage, possibly as a distinct subpopulation but in any case able to invade the CNS, proliferate, and differentiate into ameboid and then ramified microglia in the adult life. We thus attempted to derive microglia-like cells from human monocytes. When cultured with astrocyte-conditioned medium (ACM), monocytes acquired a ramified morphology, typical of microglia. They overexpressed substance P and the calcium binding protein Iba-1 and dimly expressed class II MHC, three characteristics of microglial cells. Moreover, they also expressed a potassium inward rectifier current, another microglia-specific feature. These monocyte-derived microglia-like cells (MDMi) were CD4(+)/CD14(+), evocative of an activated microglia phenotype. When treated with lipopolysaccharide (LPS), MDMi lost their overexpression of substance P, which returned to untreated monocyte-derived macrophage (MDM) level. Compared with MDM, MDMi expressed higher CD4 but lower CCR5 levels; they could be infected by HIV-1(BaL), but produced less virus progeny than MDM did. This model of human microglia may be an interesting alternative to primary microglia for large scale in vitro HIV studies and may help to better understand HIV-associated microgliosis and chronic inflammation in the brain.

Glutamate metabolism in HIV-infected macrophages: implications for the CNS.

Central nervous system disorders are still a common complication of human immunodeficiency virus (HIV) infection and can lead to dementia and death. They are mostly the consequences of an inflammatory macrophagic activation and relate to glutamate-mediated excitotoxicity. However, recent studies also suggest neuroprotective aspects of macrophage activation through the expression of glutamate transporters and glutamine synthetase. We thus aimed to study whether HIV infection or activation of macrophages could modulate glutamate metabolism in these cells. We assessed the effect of HIV infection on glutamate transporter expression as well as on glutamate uptake by macrophages and showed that glutamate transport was partially decreased in the course of virus replication, whereas excitatory amino acid transporter-2 (EAAT-2) gene expression was dramatically increased. The consequences of HIV infection on glutamine synthetase were also measured and for the first time we show the functional expression of this key enzyme in macrophages. This expression was repressed during virus production. We then quantified EAAT-1 and EAAT-2 gene expression as well as glutamate uptake in differentially activated macrophages and show that the effects of HIV are not directly related to pro- or anti-inflammatory mediators. Finally, this study shows that glutamate transport by macrophages is less affected than what has been described in astrocytes. Macrophages may thus play a role in neuroprotection against glutamate in the infected brain, through their expression of both EAATs and glutamine synthetase. Because glutamate metabolism by activated macrophages is sensitive to both HIV infection and inflammation, it may thus be of potential interest as a therapeutic target in HIV encephalitis.

Macrophage activation and human immunodeficiency virus infection: HIV replication directs macrophages towards a pro-inflammatory phenotype while previous activation modulates macrophage susceptibility to infection and viral production.

Macrophages are pivotal for the regulation of immune and inflammatory responses, but whether their role in HIV infection is protective or deleterious remains unclear. In this study, we investigated the effect of pro- and anti-inflammatory stimuli on macrophage sensitivity to two different aspects of HIV infection: their susceptibility to infection stricto sensu, which we measured by endpoint titration method, and their ability to support virus spread, which we measured by using an RT activity assay in infection kinetics. We show a partially protective role for pro-inflammatory agents as well as for IL-4. We also illustrate that various different stimuli display differential effects on macrophage susceptibility to HIV and on virus replication that occurs thereafter. On the other hand, HIV replication strongly repressed CD206 and CD163 expression, thus clearly orientating macrophages towards a pro-inflammatory phenotype, but independently of TNF. Taken together, our results emphasize that HIV infection of macrophages sets up inflammation at the cell level but through unexpected mechanisms. This may limit target susceptibility and participate in virus clearance but may also result in tissue damage.