Impaired complement-mediated phagocytosis by HIV type-1-infected human monocyte-derived macrophages involves a cAMP-dependent mechanism

Role of Type I Interferons during Mycobacterium tuberculosis and HIV Infections

Tuberculosis and AIDS remain two of the most relevant human infectious diseases. The pathogens that cause them, Mycobacterium tuberculosis (Mtb) and HIV, individually elicit an immune response that treads the line between beneficial and detrimental to the host. Co-infection further complexifies this response since the different cytokines acting on one infection might facilitate the dissemination of the other. In these responses, the role of type I interferons is often associated with antiviral mechanisms, while for bacteria such as Mtb, their importance and clinical relevance as a suitable target for manipulation are more controversial. In this article, we review the recent knowledge on how these interferons play distinct roles and sometimes have opposite consequences depending on the stage of the pathogenesis. We highlight the dichotomy between the acute and chronic infections displayed by both infections and how type I interferons contribute to an initial control of each infection individually, while their chronic induction, particularly during HIV infection, might facilitate Mtb primo-infection and progression to disease. We expect that further findings and their systematization will allow the definition of windows of opportunity for interferon manipulation according to the stage of infection, contributing to pathogen clearance and control of immunopathology.

The cytokine/chemokine response in Leishmania/HIV infection and co-infection

HIV infection progressively weakens the immune system by infecting and destroying cells involved in host defense. Viral infection symptoms are generated and aggravated as immunosuppression progresses, triggered by the presence of opportunistic infections: among these is leishmaniasis, a disease caused by the intracellular parasite Leishmania. The incidence of this co-infection is growing progressively due to the geographic distribution overlap. Both pathogens infect monocytes/macrophages and dendritic cells, although they can also modulate the activity of other cells without co-infecting, such as T and B lymphocytes. Leishmania/HIV co-infection could be described as a system comprising modulations of cell surface molecule expression, production of soluble factors, and intracellular death activities, leading ultimately to the potentiation of infectivity, replication, and spread of both pathogens. This review describes the cytokine/chemokine response in Leishmania/HIV infection and co-infection, discussing how these molecules modulate the course of the disease and analyzing the therapeutic potential of targeting this network.

Exchange Protein Directly Activated by cAMP 2 Enhances Respiratory Syncytial Virus-Induced Pulmonary Disease in Mice

Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract infection in young children. It is also a significant contributor to upper respiratory tract infections, therefore, a major cause for visits to the pediatrician. High morbidity and mortality are associated with high-risk populations including premature infants, the elderly, and the immunocompromised. However, no effective and specific treatment is available. Recently, we discovered that an exchange protein directly activated by cyclic AMP 2 (EPAC2) can serve as a potential therapeutic target for RSV. In both lower and upper epithelial cells, EPAC2 promotes RSV replication and pro-inflammatory cytokine/chemokine induction. However, the overall role of EPAC2 in the pulmonary responses to RSV has not been investigated. Herein, we found that EPAC2-deficient mice (KO) or mice treated with an EPAC2-specific inhibitor showed a significant decrease in body weight loss, airway hyperresponsiveness, and pulmonary inflammation, compared with wild-type (WT) or vehicle-treated mice. Overall, this study demonstrates the critical contribution of the EPAC2-mediated pathway to airway diseases in experimental RSV infection, suggesting the possibility to target EPAC2 as a promising treatment modality for RSV.

The HIV Reservoir in Monocytes and Macrophages

In people living with HIV (PLWH) who are failing or unable to access combination antiretroviral therapy (cART), monocytes and macrophages are important drivers of pathogenesis and progression to AIDS. The relevance of the monocyte/macrophage reservoir in PLWH receiving cART is debatable as in vivo evidence for infected cells is limited and suggests the reservoir is small. Macrophages were assumed to have a moderate life span and lack self-renewing potential, but recent discoveries challenge this dogma and suggest a potentially important role of these cells as long-lived HIV reservoirs. This, combined with new HIV infection animal models, has led to a resurgence of interest in monocyte/macrophage reservoirs. Infection of non-human primates with myeloid-tropic SIV implicates monocyte/macrophage activation and infection in the brain with neurocognitive disorders, and infection of myeloid-only humanized mouse models are consistent with the potential of the monocyte/macrophage reservoir to sustain infection and be a source of rebound viremia following cART cessation. An increased resistance to HIV-induced cytopathic effects and a reduced susceptibility to some antiretroviral drugs implies macrophages may be relevant to residual replication under cART and to rebound viremia. With a reappraisal of monocyte circulation dynamics, and the development of techniques to differentiate between self-renewing tissue-resident, and monocyte-derived macrophages in different tissues, a new framework exists to contextualize and evaluate the significance and relevance of the monocyte/macrophage HIV reservoir. In this review, we discuss recent developments in monocyte and macrophage biology and appraise current and emerging techniques to quantify the reservoir. We discuss how this knowledge influences our evaluation of the myeloid HIV reservoir, the implications for HIV pathogenesis in both viremic and virologically-suppressed PLWH and the need to address the myeloid reservoir in future treatment and cure strategies.

Adenosine signaling and adenosine deaminase regulation of immune responses: impact on the immunopathogenesis of HIV infection

Infection by human immunodeficiency virus (HIV) causes the acquired immune deficiency syndrome (AIDS), which has devastating effects on the host immune system. HIV entry into host cells and subsequent viral replication induce a proinflammatory response, hyperactivating immune cells and leading them to death, disfunction, and exhaustion. Adenosine is an immunomodulatory molecule that suppresses immune cell function to protect tissue integrity. The anti-inflammatory properties of adenosine modulate the chronic inflammation and immune activation caused by HIV. Lack of adenosine contributes to pathogenic events in HIV infection. However, immunosuppression by adenosine has its shortcomings, such as impairing the immune response, hindering the elimination of the virus and control of viral replication. By attempting to control inflammation, adenosine feeds a pathogenic cycle affecting immune cells. Deamination of adenosine by ADA (adenosine deaminase) counteracts the negative effects of adenosine in immune cells, boosting the immune response. This review comprises the connection between adenosinergic system and HIV immunopathogenesis, exploring defects in immune cell function and the role of ADA in protecting these cells against damage.

Epigenetic alterations are associated with monocyte immune dysfunctions in HIV-1 infection

Monocytes are key cells in the immune dysregulation observed during human immunodeficiency virus (HIV) infection. The events that take place specifically in monocytes may contribute to the systemic immune dysfunction characterized by excessive immune activation in infected individuals, which directly correlates with pathogenesis and progression of the disease. Here, we investigated the immune dysfunction in monocytes from untreated and treated HIV + patients and associated these findings with epigenetic changes. Monocytes from HIV patients showed dysfunctional ability of phagocytosis and killing, and exhibited dysregulated cytokines and reactive oxygen species production after M. tuberculosis challenge in vitro. In addition, we showed that the expression of enzymes responsible for epigenetic changes was altered during HIV infection and was more prominent in patients that had high levels of soluble CD163 (sCD163), a newly identified plasmatic HIV progression biomarker. Among the enzymes, histone acetyltransferase 1 (HAT1) was the best epigenetic biomarker correlated with HIV - sCD163 high patients. In conclusion, we confirmed that HIV impairs effector functions of monocytes and these alterations are associated with epigenetic changes that once identified could be used as targets in therapies aiming the reduction of the systemic activation state found in HIV patients.

HIV and the Macrophage: From Cell Reservoirs to Drug Delivery to Viral Eradication

Macrophages serve as host cells, inflammatory disease drivers and drug runners for human immunodeficiency virus infection and treatments. Low-level viral persistence continues in these cells in the absence of macrophage death. However, the cellular microenvironment changes as a consequence of viral infection with aberrant production of pro-inflammatory factors and promotion of oxidative stress. These herald viral spread from macrophages to neighboring CD4⁺ T cells and end organ damage. Virus replicates in tissue reservoir sites that include the nervous, pulmonary, cardiovascular, gut, and renal organs. However, each of these events are held in check by antiretroviral therapy. A hidden and often overlooked resource of the macrophage rests in its high cytoplasmic nuclear ratios that allow the cell to sense its environment and rid it of the cellular waste products and microbial pathogens it encounters. These phagocytic and intracellular killing sensing mechanisms can also be used in service as macrophages serve as cellular carriage depots for antiretroviral nanoparticles and are able to deliver medicines to infectious disease sites with improved therapeutic outcomes. These undiscovered cellular functions can lead to reductions in persistent infection and may potentially facilitate the eradication of residual virus to eliminate disease.

Defective Phagocytic Properties of HIV-Infected Macrophages: How Might They Be Implicated in the Development of Invasive Salmonella Typhimurium?

Human immunodeficiency virus type 1 (HIV-1) infects and kills T cells, profoundly damaging the host-specific immune response. The virus also integrates into memory T cells and long-lived macrophages, establishing chronic infections. HIV-1 infection impairs the functions of macrophages both in vivo and in vitro, which contributes to the development of opportunistic diseases. Non-typhoidal Salmonella enterica serovar Typhimurium has been identified as the most common cause of bacterial bloodstream infections in HIV-infected adults. In this review, we report how the functions of macrophages are impaired post HIV infection; introduce what makes invasive Salmonella Typhimurium specific for its pathogenesis; and finally, we discuss why these bacteria may be particularly adapted to the HIV-infected host.

The Immune Interaction between HIV-1 Infection and Mycobacterium tuberculosis

The modulation of tuberculosis (TB)-induced immunopathology caused by human immunodeficiency virus (HIV)-1 coinfection remains incompletely understood but underlies the change seen in the natural history, presentation, and prognosis of TB in such patients. The deleterious combination of these two pathogens has been dubbed a "deadly syndemic," with each favoring the replication of the other and thereby contributing to accelerated disease morbidity and mortality. HIV-1 is the best-recognized risk factor for the development of active TB and accounts for 13% of cases globally. The advent of combination antiretroviral therapy (ART) has considerably mitigated this risk. Rapid roll-out of ART globally and the recent recommendation by the World Health Organization (WHO) to initiate ART for everyone living with HIV at any CD4 cell count should lead to further reductions in HIV-1-associated TB incidence because susceptibility to TB is inversely proportional to CD4 count. However, it is important to note that even after successful ART, patients with HIV-1 are still at increased risk for TB. Indeed, in settings of high TB incidence, the occurrence of TB often remains the first presentation of, and thereby the entry into, HIV care. As advantageous as ART-induced immune recovery is, it may also give rise to immunopathology, especially in the lower-CD4-count strata in the form of the immune reconstitution inflammatory syndrome. TB-immune reconstitution inflammatory syndrome will continue to impact the HIV-TB syndemic.

HIV-1 Tat inhibits phagocytosis by preventing the recruitment of Cdc42 to the phagocytic cup

Most macrophages remain uninfected in HIV-1-infected patients. Nevertheless, the phagocytic capacity of phagocytes from these patients is impaired, favouring the multiplication of opportunistic pathogens. The basis for this phagocytic defect is not known. HIV-1 Tat protein is efficiently secreted by infected cells. Secreted Tat can enter uninfected cells and reach their cytosol. Here we found that extracellular Tat, at the subnanomolar concentration present in the sera of HIV-1-infected patients, inhibits the phagocytosis of Mycobacterium avium or opsonized Toxoplasma gondii by human primary macrophages. This inhibition results from a defect in mannose- and Fcγ-receptor-mediated phagocytosis, respectively. Inhibition relies on the interaction of Tat with phosphatidylinositol (4,5)bisphosphate that interferes with the recruitment of Cdc42 to the phagocytic cup, thereby preventing Cdc42 activation and pseudopod elongation. Tat also inhibits FcγR-mediated phagocytosis in neutrophils and monocytes. This study provides a molecular basis for the phagocytic defects observed in uninfected phagocytes following HIV-1 infection.

The immunology of Leishmania/HIV co-infection

Leishmaniases are emerging as an important disease in human immunodeficiency virus (HIV)-infected persons living in several sub-tropical and tropical regions around the world, including the Mediterranean. The HIV/AIDS pandemic is spreading at an alarming rate in Africa and the Indian subcontinent, areas with very high prevalence of leishmaniases. The spread of HIV into rural areas and the concomitant spread of leishmaniases to suburban/urban areas have helped maintain the occurrence of Leishmania/HIV co-infection in many parts of the world. The number of cases of Leishmania/HIV co-infection is expected to rise owing to the overlapping geographical distribution of the two infections. In Southwestern Europe, there is also an increasing incidence of Leishmania/HIV co-infection (particularly visceral leishmaniasis) in such countries as France, Italy, Spain and Portugal. Studies suggest that in humans, very complex mechanisms involving dysregulation of host immune responses contribute to Leishmania-mediated immune activation and pathogenesis of HIV. In addition, both HIV-1 and Leishmania infect and multiply within cells of myeloid or lymphoid origin, thereby presenting a perfect recipe for reciprocal modulation of Leishmania and HIV-1-related disease pathogenesis. Importantly, because recovery from leishmaniases is associated with long-term persistence of parasites at the primary infection sites and their draining lymph nodes, there is very real possibility that HIV-mediated immunosuppression (due to CD4(+) T cell depletion) could lead to reactivation of latent infections (reactivation leishmaniasis) in immunocompromised patients. Here, we present an overview of the immunopathogenesis of Leishmania/HIV co-infection and the implications of this interaction on Leishmania and HIV disease outcome.

Activation, exhaustion, and persistent decline of the antimicrobial MR1-restricted MAIT-cell population in chronic HIV-1 infection

Mucosal-associated invariant T (MAIT) cells are an evolutionarily conserved antimicrobial MR1-restricted T-cell subset. MAIT cells are CD161(+), express a V7.2 TCR, are primarily CD8(+) and numerous in blood and mucosal tissues. However, their role in HIV-1 infection is unknown. In this study, we found levels of MAIT cells to be severely reduced in circulation in patients with chronic HIV-1 infection. Residual MAIT cells were highly activated and functionally exhausted. Their decline was associated with time since diagnosis, activation levels, and the concomitant expansion of a subset of functionally impaired CD161(+) V7.2(+) T cells. Such cells were generated in vitro by exposure of MAIT cells to Escherichia coli. Notably, whereas the function of residual MAIT cells was at least partly restored by effective antiretroviral therapy, levels of MAIT cells in peripheral blood were not restored. Interestingly, MAIT cells in rectal mucosa were relatively preserved, although some of the changes seen in blood were recapitulated in the mucosa. These findings are consistent with a model in which the MAIT-cell compartment, possibly as a result of persistent exposure to microbial material, is engaged, activated, exhausted, and progressively and persistently depleted during chronic HIV-1 infection.

HIV-1 promotes intake of Leishmania parasites by enhancing phosphatidylserine-mediated, CD91/LRP-1-dependent phagocytosis in human macrophages

Over the past decade, the number of reported human immunodeficiency virus type-1 (HIV-1)/Leishmania co-infections has risen dramatically, particularly in regions where both diseases are endemic. Although it is known that HIV-1 infection leads to an increase in susceptibility to Leishmania infection and leishmaniasis relapse, little remains known on how HIV-1 contributes to Leishmania parasitaemia. Both pathogens infect human macrophages, and the intracellular growth of Leishmania is increased by HIV-1 in co-infected cultures. We now report that uninfected bystander cells, not macrophages productively infected with HIV-1, account for enhanced phagocytosis and higher multiplication of Leishmania parasites. This effect can be driven by HIV-1 Tat protein and transforming growth factor-beta (TGF-β). Furthermore, we show for the first time that HIV-1 infection increases surface expression of phosphatidylserine receptor CD91/LRP-1 on human macrophages, thereby leading to a Leishmania uptake by uninfected bystander cells in HIV-1-infected macrophage populations. The more important internalization of parasites is due to interactions between the scavenger receptor CD91/LRP-1 and phosphatidylserine residues exposed at the surface of Leishmania. We determined also that enhanced CD91/LRP-1 surface expression occurs rapidly following HIV-1 infection, and is triggered by the activation of extracellular TGF-β. Thus, these results establish an intricate link between HIV-1 infection, Tat, surface CD91/LRP-1, TGF-β, and enhanced Leishmania phosphatidylserine-mediated phagocytosis.

HIV-1 inhibits phagocytosis and inflammatory cytokine responses of human monocyte-derived macrophages to P. falciparum infected erythrocytes

HIV-1 infection increases the risk and severity of malaria by poorly defined mechanisms. We investigated the effect of HIV-1_Ba-L infection of monocyte-derived macrophages (MDM) on phagocytosis of opsonised P. falciparum infected erythrocytes (IE) and subsequent proinflammatory cytokine secretion. Compared to mock-infected MDM, HIV-1 infection significantly inhibited phagocytosis of IE (median (IQR) (10 (0–28) versus (34 (27–108); IE internalised/100 MDM; p = 0.001) and decreased secretion of IL-6 (1,116 (352–3,387) versus 1,552 (889–6,331); pg/mL; p = 0.0078) and IL-1β (16 (7–21) versus 33 (27–65); pg/mL; p = 0.0078). Thus inadequate phagocytosis and cytokine production may contribute to impaired control of malaria in HIV-1 infected individuals.

Inhibition of phagocytosis in HIV-1–infected macrophages relies on Nef-dependent alteration of focal delivery of recycling compartments

Phagocytosis in macrophages is receptor mediated and relies on actin polymerization coordinated with the focal delivery of intracellular membranes that is necessary for optimal phagocytosis of large particles. Here we show that phagocytosis by various receptors was inhibited in primary human macrophages infected with wild-type HIV-1 but not with a nef-deleted virus. We observed no major perturbation of F-actin accumulation, but adaptor protein 1 (AP1)–positive endosome recruitment was inhibited in HIV-1–infected cells. Expression of negative factor (Nef) was sufficient to inhibit phagocytosis, and myristoylation as well as the LL and DD motifs involved in association of Nef with AP complexes were important for this inhibition. We observed that Nef interferes with AP1 in association with membranes and/or with a cleaved regulatory form of AP1. Finally, an alteration of the recruitment of vesicle-associated membrane protein (VAMP3)– and tumor necrosis factor-α (TNFα)–positive recycling endosomes regulated by AP1, but not of VAMP7-positive late endosomes, was observed in phagocytic cups of HIV-1–infected macrophages. We conclude that HIV-1 impairs optimal phagosome formation through Nef-dependent perturbation of the endosomal remodeling relying on AP1. We therefore identified a mechanism of macrophage function down-regulation in infected cells.

Complement, Fc receptors and antibodies: a Trojan horse in HIV infection?

PURPOSE OF REVIEW

Because complement is present in all fluids of the body, including serum, saliva and seminal fluid, and is found at mucosal surfaces and in the brain, all pathogens have to deal with complement proteins. Thus, immediately upon entering the host, independent on the route of infection, HIV activates the complement system. Although a first line of immune defense, complement cannot eliminate retroviral infections completely.

RECENT FINDINGS

Recent data indicate that complement, in concert with non-neutralizing antibodies, contributes to the control of HIV replication at early stages of infection. In parallel or at later stages, complement and non-neutralizing antibodies may counteract the immune response by enhancing HIV infection via complement and Fc-receptor-positive cells in 'cis' and 'trans'.

SUMMARY

This review highlights current knowledge in this field and emphasizes the contribution of complement and non-neutralizing antibodies in controlling versus and enhancing infection.

Effects of HIV-1 infection in vitro on transendothelial migration by monocytes and monocyte-derived macrophages

Monocytes constitutively migrate from the bloodstream across the vascular endothelium for systemic immune surveillance and maintenance of macrophage populations. They also perform reverse transendothelial migration (TEM) across the endothelium, which is required for entry of tissue monocytes/macrophages into the lymphatics or back into the bloodstream. We have modeled these processes previously using HUVEC monolayers grown on three-dimensional collagen matrices. The aim of the present study was to determine whether HIV-1 infection of monocytes/macrophages in vitro affects TEM. Purified primary human monocytes and monocyte-derived macrophages (MDM) expressed important TEM proteins such as CD62L, CD18, PECAM-1, CCR2, and CCR8. Purified monocytes underwent efficient forward and reverse TEM across HUVEC, and this function was maintained by MDM after up to 15 days of culture. Monocytes exposed to HIV-1 for 2 days had unaltered forward or reverse TEM. However, HIV-1 infection of MDM for 7 days decreased reverse TEM by an average of 66.5% compared with mock-infected MDM (n=9 independent donors; P=0.004), without affecting forward TEM. Decreased reverse TEM by HIV-infected MDM required viral RT and was not a result of alterations in surface expression of CCR8 or p-glycoprotein or a general impairment in mobility, as assessed by migration toward fMLP. This study indicates that HIV-1 infection of macrophages reduces their capacity to emigrate from the subendothelial extracellular matrix in vitro, which could result in defective cell-mediated immune responses to infections and promote establishment of viral reservoirs of HIV in tissue macrophages in vivo.

Antibody-mediated opsonization of red blood cells in parvovirus B19 infection

Red blood cells (RBCs) express abundantly parvovirus B19 receptor, and their role in the dissemination or clearance of B19 infection is unknown. In this study, we report that in early, acute or persistent infection, B19 viremia is mostly associated with RBCs. The capacity of different patients' plasma or IgG to opsonize RBCs collected from patients with early B19 infection, was investigated. The highest opsonization activity was observed with plasma or IgG fractions from patients with past B19 infection. In contrast, IgG samples from patients with acute or persistent infection showed no or little opsonization activity. The depletion of antibodies specific to B19 VP1, but not VP2, from IgG samples, resulted in a significant suppression of opsonization. Furthermore, IgG samples preincubated with heated B19 particles exposing VP1-unique (VP1u) region were unable to opsonize RBCs. These observations clearly suggest a role for anti-VP1u IgG in the opsonization of RBC-bound B19 particles.

Genome-wide innate immune responses in HIV-1-infected macrophages are preserved despite attenuation of the NF-kappa B activation pathway

Macrophages contribute to HIV-1 infection at many levels. They provide permissive cells at the site of inoculation, augment virus transfer to T cells, generate long-lived viral reservoirs, and cause bystander cell apoptosis. A body of evidence suggests that the role of macrophages in cellular host defense is also compromised by HIV-1 infection. In this respect, macrophages are potent cells of the innate immune system that initiate and regulate wide-ranging immunological responses. This study focuses on the effect of HIV-1 infection on innate immune responses by macrophages at the level of signal transduction, whole genome transcriptional profiling, and cytokine secretion. We show that in an ex vivo model, M-CSF-differentiated monocyte-derived macrophages uniformly infected with replicating CCR5-tropic HIV-1, without cytopathic effect, exhibit selective attenuation of the NF-kappaB activation pathway in response to TLR4 and TLR2 stimulation. However, functional annotation clustering analysis of genome-wide transcriptional responses to LPS stimulation suggests substantial preservation of gene expression changes at the systems level, with modest attenuation of a subset of up-regulated LPS-responsive genes, and no effect on a selection of inflammatory cytokine responses at the protein level. These results extend existing reports of inhibitory interactions between HIV-1 accessory proteins and NF-kappaB signaling pathways, and whole genome expression profiling provides comprehensive assessment of the consequent effects on immune response gene expression. Unexpectedly, our data suggest innate immune responses are broadly preserved with limited exceptions, and pave the way for further study of the complex relationship between HIV-1 and immunological pathways within macrophages.

Cyclic AMP plays a critical role in C3a-receptor-mediated regulation of dendritic cells in antigen uptake and T-cell stimulation

The biochemical basis for complement acting directly on antigen-presenting cells to enhance their function in T-cell stimulation has been unclear. Here we present evidence that engagement of C3a receptor (C3aR) on the surface of dendritic cells (DCs) leads to alterations in the level of intracellular cyclic adenosine monophosphate (cAMP), a potent negative regulator of inflammatory cytokines. C3aR activation-induced depression of cAMP was associated with enhanced capacity of DCs for antigen uptake and T-cell stimulation. Conversely, C3aR-deficient DCs showed elevation of cAMP and impaired properties for antigen uptake and immune stimulation. Similarities in the phenotype of C3-deficient and C3aR-deficient DCs suggest that local production of C3 with extracellular metabolism to C3a is an important driver of DC alterations in cAMP. The finding of a link between complement and adaptive immune stimulation through cAMP offers new insight into how innate and adaptive immunity combine to generate efficient effector and memory responses.

Cyclic AMP: master regulator of innate immune cell function

Cyclic adenosine monophosphate (cAMP) was the original "second messenger" to be discovered. Its formation is promoted by adenylyl cyclase activation after ligation of G protein-coupled receptors by ligands including hormones, autocoids, prostaglandins, and pharmacologic agents. Increases in intracellular cAMP generally suppress innate immune functions, including inflammatory mediator generation and the phagocytosis and killing of microbes. The importance of the host cAMP axis in regulating antimicrobial defense is underscored by the fact that microbes have evolved virulence-enhancing strategies that exploit it. Many clinical situations that predispose to infection are associated with increases in cAMP, and therapeutic strategies to interrupt cAMP generation or actions have immunostimulatory potential. This article reviews the anatomy of the cAMP axis, the mechanisms by which it controls phagocyte immune function, microbial strategies to dysregulate it, and its clinical relevance.

Antibodies for HIV treatment and prevention: window of opportunity?

Monoclonal antibodies are routinely used as therapeutics in a number of disease settings and have thus also been explored as potential treatment for human immunodeficiency virus (HIV)-1 infection. Antibodies targeting viral antigens, and those directed to the cellular receptors, have been considered for use in prevention and therapy. For virus-targeted antibodies, attention has focused primarily on their neutralizing activity, but such antibodies also have the potential to exert antiviral effects via effector functions, such as antibody-dependent cellular cytotoxicity (ADCC), opsonization, or complement activation. Anti-cell antibodies act through occlusion or down-modulation of the viral receptors with notable impact in vivo, as recent trials have shown. This review summarizes the diverse specificities and modes of action of therapeutic antibodies against HIV-1 infection. Successes, challenges, and future opportunities of harnessing antibodies for therapy of HIV-1 infection are discussed.

Humoral immunity to HIV-1: neutralization and beyond

Humoral immunity is considered a key component of effective vaccines against HIV-1. Hence, an enormous effort has been put into investigating the neutralizing antibody response to HIV-1 over the past 20 years which generated key information on epitope specificity, potency, breadth and in vivo activity of the neutralizing antibodies. Less clear is still the role of antibody-mediated effector functions (antibody-dependent cellular cytotoxicity, phagocytosis, complement system) and uncertainty prevails whether Fc-mediated mechanisms are largely beneficial or detrimental for the host. The current knowledge on the manifold functions of the humoral immune response in HIV infection, their underlying mechanisms and potential in vaccine-induced immunity will be discussed in this review.

Prostaglandin E2 suppresses bacterial killing in alveolar macrophages by inhibiting NADPH oxidase

Prostaglandin E2 (PGE2) is a potent lipid mediator that effects changes in cell functions through ligation of four distinct G protein-coupled E prostanoid (EP) receptors (EP1-EP4). PGE2 inhibits bacterial killing and reactive oxygen intermediate (ROI) production by alveolar macrophages (AMs), although little is known about the operative molecular mechanisms. The aims of this study were to evaluate the molecular mechanisms and the specific EP receptors through which PGE2 inhibits killing of Klebsiella pneumoniae by AMs. The treatment of AMs with PGE2 suppressed the killing of K. pneumoniae, and this effect was blocked by an adenylyl cyclase inhibitor and mimicked by agonists for the stimulatory G protein (G(s))-coupled EP2 and EP4 receptors. Conversely, microbicidal activity was augmented by pretreatment with the cyclooxygenase inhibitor, indomethacin, and antagonists of EP2 and EP4. Similar results were found when ROI production was examined. PGE2 inhibition of killing and ROI generation was associated with its activation of the cAMP effectors, protein kinase A and exchange protein directly activated by cAMP-1, as well as attenuation of the phosphorylation and translocation of the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase component, p47phox, to the phagosomal membrane. We conclude that PGE2 suppresses the microbicidal activity of AMs through the G(s)-coupled EP2/EP4 receptors, with increased cAMP inhibiting the assembly and activation of p47phox.

The mechanism underlying defective Fcgamma receptor-mediated phagocytosis by HIV-1-infected human monocyte-derived macrophages

Clearance of IgG-opsonized erythrocytes is impaired in HIV-1-infected patients, suggesting defective FcgammaR-mediated phagocytosis in vivo. We have previously shown defective FcgammaR-mediated phagocytosis in HIV-1-infected human monocyte-derived macrophages (MDM), establishing an in vitro model for defective tissue macrophages. Inhibition was associated with decreased protein expression of FcR gamma-chain, which transduces immune receptor signals via ITAM motifs. FcgammaRI and FcgammaRIIIa signal via gamma-chain, whereas FcgammaRIIa does not. In this study, we showed that HIV-1 infection inhibited FcgammaRI-, but not FcgammaRIIa-dependent Syk activation in MDM, showing that inhibition was specific for gamma-chain-dependent signaling. HIV-1 infection did not impair gamma-chain mRNA levels measured by real-time PCR, suggesting a posttranscriptional mechanism of gamma-chain depletion. HIV-1 infection did not affect gamma-chain degradation (n = 7, p = 0.94) measured in metabolic labeling/chase experiments, whereas gamma-chain biosynthesis was inhibited (n = 12, p = 0.0068). Using an enhanced GFP-expressing HIV-1 strain, we showed that FcgammaR-mediated phagocytosis inhibition is predominantly due to a bystander effect. Experiments in which MDM were infected in the presence of the antiretroviral drug 3TC suggest that active viral replication is required for inhibition of phagocytosis in MDM. These data suggest that HIV-1 infection may affect only gamma-chain-dependent FcgammaR functions, but that this is not restricted to HIV-1-infected cells.