Interactions between HIV-infected monocytes and the extracellular matrix: HIV-infected monocytes secrete neutral metalloproteases that degrade basement membrane protein matrices

Blood-brain barrier disruption in dementia: Nano-solutions as new treatment options

Candidate drugs targeting the central nervous system (CNS) demonstrate extremely low clinical success rates, with more than 98% of potential treatments being discontinued due to poor blood-brain barrier (BBB) permeability. Neurological conditions were shown to be the second leading cause of death globally in 2016, with the number of people currently affected by neurological disorders increasing rapidly. This increasing trend, along with an inability to develop BBB permeating drugs, is presenting a major hurdle in the treatment of CNS-related disorders, like dementia. To overcome this, it is necessary to understand the structure and function of the BBB, including the transport of molecules across its interface in both healthy and pathological conditions. The use of CNS drug carriers is rapidly gaining popularity in CNS research due to their ability to target BBB transport systems. Further research and development of drug delivery vehicles could provide essential information that can be used to develop novel treatments for neurological conditions. This review discusses the BBB and its transport systems and evaluates the potential of using nanoparticle-based delivery systems as drug carriers for CNS disease with a focus on dementia.

Human Immunodeficiency Virus (HIV) Infection and Use of Illicit Substances Promote Secretion of Semen Exosomes that Enhance Monocyte Adhesion and Induce Actin Reorganization and Chemotactic Migration

Semen exosomes (SE) from HIV-uninfected (HIV−) individuals potently inhibit HIV infection in vitro. However, morphological changes in target cells in response to SE have not been characterized or have the effect of HIV infection or the use of illicit substances, specifically psychostimulants, on the function of SE been elucidated. The objective of this study was to evaluate the effect of HIV infection, psychostimulant use, and both together on SE-mediated regulation of monocyte function. SE were isolated from semen of HIV− and HIV-infected (HIV+) antiretroviral therapy (ART)-naive participants who reported either using or not using psychostimulants. The SE samples were thus designated as HIV−Drug−, HIV−Drug+, HIV+Drug−, and HIV+Drug+. U937 monocytes were treated with different SEs and analyzed for changes in transcriptome, morphometrics, actin reorganization, adhesion, and chemotaxis. HIV infection and/or use of psychostimulants had minimal effects on the physical characteristics of SE. However, different SEs had diverse effects on the messenger RNA signature of monocytes and rapidly induced monocyte adhesion and spreading. SE from HIV infected or psychostimulants users but not HIV−Drug− SE, stimulated actin reorganization, leading to the formation of filopodia-like structures and membrane ruffles containing F-actin and vinculin that in some cases were colocalized. All SE stimulated monocyte chemotaxis to HIV secretome and activated the secretion of matrix metalloproteinases, a phenotype exacerbated by HIV infection and psychostimulant use. SE-directed regulation of cellular morphometrics and chemotaxis depended on the donor clinical status because HIV infection and psychostimulant use altered SE function. Although our inclusion criteria specified the use of cocaine, humans are poly-drug and alcohol users and our study participants used psychostimulants, marijuana, opiates, and alcohol. Thus, it is possible that the effects observed in this study may be due to one of these other substances or due to an interaction between different substances.

In vitro effect of antiretroviral drugs on cultured primary astrocytes: analysis of neurotoxicity and matrix metalloproteinase inhibition

There is little information available on the possible toxic effects that antiretroviral (ARV) drugs used for the treatment of human immunodeficiency virus (HIV)-infected subjects, may have on the central nervous system (CNS) resident cells. Moreover, it remains unclear whether the efficacy of the ARV drugs may also be due to their ability to exert extravirological effects on factors responsible for the development of HIV brain injury, e.g., matrix metalloproteinases (MMPs). This study investigates the toxicity of three different ARV drugs and on their ability to modulate levels and expression of gelatinases A (MMP-2) and B (MMP-9) in astrocytes. Primary cultures of rat astrocytes were activated by exposure to lipopolysaccaride (LPS) and simultaneously treated with darunavir, maraviroc, or raltegravir, used alone or in combination. Among the tested drugs, maraviroc was the less toxic for astrocytes. At toxic concentration (TC₅₀ ), the studied drugs induced the production of reactive oxygen species (ROS), suggesting that the oxidative stress may represent a mechanism of ARV toxicity. As assessed by gelatin zymography and RT-PCR, the single antiretroviral drugs reduced levels and expression of both MMP-2 and MMP-9 through the inhibition of the signaling transduction pathway of extracellular signal-regulated kinase1/2, which is involved in the regulation of MMP-9 gene. A synergistic inhibition of MMP-2 and MMP-9 was observed with combinations of the studied ARV drugs. The present results indicate that maraviroc, darunavir, and raltegravir, through their ability to inhibit MMP-2 and MMP-9 at doses non-toxic for astrocytes, might have a great potential for the management of HIV-associated neurological complications.

HIV-1 and the immune response to TB

TB causes 1.4 million deaths annually. HIV-1 infection is the strongest risk factor for TB. The characteristic immunological effect of HIV is on CD4 cell count. However, the risk of TB is elevated in HIV-1 infected individuals even in the first few years after HIV acquisition and also after CD4 cell counts are restored with antiretroviral therapy. In this review, we examine features of the immune response to TB and how this is affected by HIV-1 infection and vice versa. We discuss how the immunology of HIV-TB coinfection impacts on the clinical presentation and diagnosis of TB, and how antiretroviral therapy affects the immune response to TB, including the development of TB immune reconstitution inflammatory syndrome. We highlight important areas of uncertainty and future research needs.

Methamphetamine and HIV-1 gp120 effects on lipopolysaccharide stimulated matrix metalloproteinase-9 production by human monocyte-derived macrophages

Monocytes/macrophages are a primary source of human immunodeficiency virus (HIV-1) in the central nervous system (CNS). Macrophages infected with HIV-1 produce a plethora of factors, including matrix metalloproteinase-9 (MMP-9) that may contribute to the development of HIV-1-associated neurocognitive disorders (HAND). MMP-9 plays a pivotal role in the turnover of the extracellular matrix (ECM) and functions to remodel cellular architecture. We have investigated the role of methamphetamine and HIV-1 gp120 in the regulation of lipopolysaccaride (LPS) induced-MMP-9 production in monocyte-derived macrophages (MDM). Here, we show that LPS-induced MMP-9 gene expression and protein secretion are potentiated by incubation with methamphetamine alone and gp120 alone. Further, concomitant incubation with gp120 and methamphetamine potentiated LPS-induced MMP-9 expression and biological activity in MDM. Collectively methamphetamine and gp120 effects on MMPs may modulate remodeling of the extracellular environment enhancing migration of monocytes/macrophages to the CNS.

Cell adhesion through alphaV-containing integrins is required for efficient HIV-1 infection in macrophages

Monocytes and macrophages are an important reservoir of human immunodeficiency virus (HIV) and may represent the largest reservoir of this virus in tissues. Differentiation of monocytes into macrophages leads to cell attachment and susceptibility to infection and replication of HIV. Among other cell-surface molecules, integrins are overexpressed during monocyte-macrophage differentiation and may play a role in the replication cycle of envelope viruses including HIV. Here, we show that inhibition of alphaV integrin in monocyte-derived macrophages, by RNA interference or their inhibition by a selective small heterocyclic RGD-mimetic nonpeptide compound, inhibited the replication of HIV in the absence of cytotoxicity. Interference or inhibition of alphaV integrins triggered a signal transduction pathway, leading to down-regulation of nuclear factor-kappaB-dependent HIV-1 transcription. Such inhibition was mediated by a MAP-kinase signaling cascade, probably involving ERK1/2, p38-mitogen-activated protein kinases, and HSP27. In conclusion, our results reveal a significant role of integrin alphaV-mediated adhesion in HIV-1 infection of macrophages.

Matrix metalloproteinase dysregulation in HIV infection: implications for therapeutic strategies

The emerging role of immune activation and inflammation in the pathogenesis of human immunodeficiency virus (HIV) disease has stimulated the search for new approaches for managing HIV infection. Recent evidence suggests that an imbalance between matrix metalloproteinases (MMPs) and endogenous tissue inhibitors of MMPs (TIMPs) might contribute to HIV-associated pathology by inducing remodelling of the extracellular matrix. Here, we discuss the evidence and the potential mechanisms for altered MMP or TIMP function in HIV infection and disease. Furthermore, we outline the possible medical implications for the use of compounds that target MMP activity, and we propose that antiretroviral drugs, particularly HIV protease inhibitors (PIs), and compounds with anti-inflammatory properties, such as statins, natural omega-3 fatty acids and tetracyclines, which inhibit MMP function, might represent useful therapeutic approaches to mitigate potential MMP-related damage during HIV infection.

Molecular and cellular mechanisms of neuronal cell death in HIV dementia

The deaths of neurons, astrocytes and endothelial cells have been described in patients with HIV (human immunodeficiency virus) dementia. HIV-1 does not infect neurons; instead, neurotoxic substances shed by infected glia and macrophages can induce a form of programmed cell death called apoptosis in neurons. These neurotoxins include the HIV-1 proteins Tat and gp120, as well as pro-inflammatory cytokines, chemokines, excitotoxins and proteases. In this article we review the evidence for apoptosis of various cell types within the brain of HIV-infected patients, and describe in vitro and in vivo experimental studies that have elucidated the mechanisms by which HIV causes apoptosis of brain cells.

Impaired oxygen on-kinetics in persons with human immunodeficiency virus are not due to highly active antiretroviral therapy

OBJECTIVE

To determine the effects of human immunodeficiency virus (HIV) and highly active antiretroviral therapy (HAART) on oxygen on-kinetics in HIV-positive persons.

DESIGN

Quasi-experimental cross-sectional.

SETTING

Infectious disease clinic and exercise laboratory.

PARTICIPANTS

Referred participants (N=39) included 13 HIV-positive participants taking HAART, 13 HIV-positive participants not taking HAART, and 13 noninfected controls.

INTERVENTIONS

Participants performed 1 submaximal exercise treadmill test below the ventilatory threshold, 1 above the ventilatory threshold, and 1 maximal treadmill exercise test to exhaustion.

MAIN OUTCOME MEASURES

Change in oxygen consumption (Delta.VO2) and oxidative response index (Delta.VO2/mean response time).

RESULTS

Delta.VO2 was significantly lower in both HIV-positive participants taking (946.5+/-68.1mL) and not taking (871.6+/-119.6mL) HAART than in controls (1265.3+/-99.8mL) during submaximal exercise above the ventilatory threshold. The oxidative response index was also significantly lower (P<.05) in HIV-positive participants both taking (15.0+/-1.3mL/s) and not taking (15.1+/-1.7mL/s) HAART than in controls (20.8+/-2.1mL/s) during exercise above the ventilatory threshold.

CONCLUSION

Oxygen on-kinetics during submaximal exercise above the ventilatory threshold was impaired in HIV-positive participants compared with a control group, and it appeared that the attenuated oxygen on-kinetic response was primarily caused by HIV infection rather than HAART.

Decreased peak arteriovenous oxygen difference during treadmill exercise testing in individuals infected with the human immunodeficiency virus

OBJECTIVE

To determine if arteriovenous oxygen difference was lower in asymptomatic individuals with human immunodeficiency virus (HIV) infection than in sedentary but otherwise healthy controls.

DESIGN

Quasi-experimental cross-sectional.

SETTING

Clinical exercise laboratory.

PARTICIPANTS

Fifteen subjects (10 men, 5 women) with HIV and 15 healthy gender- and activity level-matched controls (total N=30).

INTERVENTION

Participants performed an incremental maximal exercise treadmill test to exhaustion. Electrocardiogram, metabolic, and noninvasive cardiac output measurements were evaluated at rest and throughout the tests. Data were analyzed by using analysis of covariance.

MAIN OUTCOME MEASURES

Peak oxygen consumption (Vo(2)), cardiac output, stroke volume, and arteriovenous oxygen difference. The arteriovenous oxygen difference was determined indirectly using the Fick equation.

RESULTS

Peak VO(2) was significantly lower (P<.0005) in participants with HIV (24.6+/-1.2mL.kg(-1).min(-1)) compared with controls (32.0+/-1.2mL.kg(-1).min(-1)). There were no significant intergroup differences in cardiac output or stroke volume at peak exercise. Peak arteriovenous oxygen difference was significantly lower (P<.04) in those infected with HIV (10.8+/-0.5 volume %) than in controls (12.4+/-0.5 volume %).

CONCLUSION

The observed deficit in aerobic capacity in the participants with HIV appeared to be the result of a peripheral tissue oxygen extraction or utilization limitation. In addition to deconditioning, potential mechanisms for this significant attenuation may include HIV infection and inflammation, highly active antiretroviral therapy medication regimens, or a combination of these factors.

Microglia in HIV-associated neurological diseases

Human immunodeficiency virus type-1 (HIV-1) is a neurotropic virus linked to a variety of progressive neurologic disorders. This review describes our current understanding of how HIV-1 enters the nervous system and interacts with neuronal and non-neuronal cells to initiate and sustain neurologic dysfunction. The overwhelming majority of cells infected with HIV-1 in the nervous system are microglia/macrophages. Microglial/macrophage infection leads to immune dysregulation as well as production and release of cytotoxic molecules. Interaction of these infected cells with astrocytes may accelerate neurotoxic mechanisms. A hypothetical scenario for how HIV-1 infection leads to neurologic disease is presented.

HIV-1-associated central nervous system dysfunction

Despite more than 15 years of extensive investigative efforts, a complete understanding of the neurological consequences of HIV-1 CNS infection remains elusive. Although the resources of numerous investigators have been focused on studies of HIV-1-associated CNS disease, the complex nature of the disease processes that underlie the clinical, pathological, and cellular manifestations of HIV-1 CNS infection have required a larger volume of studies than was initially envisioned. Several major areas remain as the focus of current research efforts. One of the more pressing issues facing researchers and clinicians alike is the search for correlates to the development of HIV-1-associated CNS neuropathology and the onset of HIVD. Although numerous parameters have been studied, none have been shown to be absolute predictors or markers of HIV-1-related CNS dysfunction. The identification of solid correlates of HIVD is an important goal that would permit clinical identification of individuals at risk for developing potentially crippling, life-threatening CNS abnormalities and would facilitate early treatment of nascent neurological problems. A more complete comprehension of the cellular foundations of CNS dysfunction and HIVD is also a fundamental part of strategies designed to treat or prevent HIV-1-associated CNS disease. Future investigations will strive to expand the body of knowledge concerning the complex interactions between infected and uninfected neuroglial cells and the roles of numerous cytokines, chemokines, and other soluble agents that are deregulated during HIV-1 CNS infection. In particular, a thorough understanding of the mechanisms of neurotoxicity may facilitate the development of new therapies that alleviate or eliminate the clinical consequences of CNS infection. Finally, investigators will continue to study HIVD within the context of single and combination drug therapies used in the treatment of HIV-1 infection and AIDS. As newer and more effective systemic treatments for HIV-1 infection and AIDS are introduced, the effects of these treatments on the onset, incidence, and severity of HIVD will also require intensive study. The impact of drug therapies on the ability of the CNS to act as an HIV-1 reservoir will also need to be addressed. Introduction of each new drug or drug combination will necessitate studies of drug penetration into the CNS and efficacy against the development of CNS abnormalities. Furthermore, as more effective treatments prolong the lifespan of individuals infected with HIV-1, the impact of extended survival on the occurrence and severity of HIVD will also require further investigations. The quest for answers to these and other questions will be complicated by the diversity of experimental systems used to study different aspects of HIV-1 CNS infection and HIVD. Each system has its own unique strengths and weaknesses. Clinical observations provide a continuous spectrum of symptomatic findings but reveal little about the underlying mechanisms of disease. In vivo imaging techniques, such as CT and MRI, also provide a continuum of observations, but the images are limited in their resolution. Neuropathological examinations of postmortem HIV-1-infected brains offer gross, cellular, and molecular views (including phenotypic and genotypic analyses of CNS viral isolates) of the diseased brain, but only provide a snapshot of the end-stage neurologic dysfunction. Studies that rely on animal surrogates for HIV-1, including SIV, simian-HIV (SHIV), feline immunodeficiency virus (FIV), visna virus, and HIV-1 SCID-hu models, permit experimental protocols that cannot be carried out in humans, but are limited by the fidelity with which each virus and animal model emulates the conditions and events observed in the human host. Finally, in vitro techniques, which include the use of primary cells and cell lines, adult or fetal human cell cultures, and BBB barrier model systems, are also convenient means by which aspe

HIV-1 Tat increases endothelial solute permeability through tyrosine kinase and mitogen-activated protein kinase-dependent pathways

OBJECTIVE

HIV-1 infection is associated with alterations of several vascular endothelial functions including adhesion molecule expression, growth, and vascular permeability. The bases of these errors are not known, but might involve secretion of the HIV-1 derived transcription factor 'Tat-1'. This study investigated Tat-1 mediated endothelial barrier changes and second message regulation of this phenomenon.

METHODS

We exposed human umbilical vein endothelial cell monolayers to Tat-1 (0-150 ng/ml) for up to 48 h and measured resulting changes in monolayer permeability. We also investigated the role of tyrosine and mitogen activated protein (MAP) kinases, and protein kinase G using the pharmacological blockers genistein, PD98059 and KT5823 respectively.

RESULTS

Tat-1 significantly reduced monolayer barrier and increased albumin permeability within 24 h. Tat-1 also stimulated tyrosine phosphorylation of multiple endothelial proteins, disorganized junctional phosphotyrosine staining and increased the number of these immunostaining structures. The increased permeability produced by Tat-1 was blocked by genistein and PD98059, but not by KT5823. Genistein and PD98059 pretreatment also prevented the changes in phosphotyrosine immunostaining produced by Tat-1 and blocked phosphorylation of several proteins including MAP kinase.

CONCLUSION

These results suggest that HIV may dysregulate endothelial barrier through the effects of Tat-1. These blocker experiments suggest that the effects of Tat are transcription/translation-dependent. These data demonstrate that Tat increases endothelial albumin permeability in vitro through tyrosine kinase and MAP kinase, but not protein kinase G pathways.

72-kDa and 92-kDa gelatinases in saliva of patients with human immunodeficiency virus infection

Human immunodeficiency virus (HIV) infection has been associated with periodontal diseases in HIV-seropositive patients. In periodontal diseases, matrix metalloproteinases (MMPs) may play key roles in the extracellular matrix, basement membrane, serpin degradation, and modification of cytokine action. We characterized the 72 kDa type IV collagenase (gelatinase A, MMP-2) and 92 kDa type IV collagenase (gelatinase B, MMP-9) in the saliva of HIV-seropositive patients and seronegative healthy controls by activity measurements and quantitative immunoblotting. Immunoblot analysis with specific antibodies against MMP-2 and MMP-9 and their tissue inhibitors (TIMP-1, TIMP-2) disclosed that, independent of the phase of the patients' HIV infection, their salivary samples contained higher amounts of MMP-2 and MMP-9 immunoreactivities in pro- and active forms and the TIMP-1 and TIMP-2 inhibitors than did the control samples. Healthy control saliva contained only slight immunoreactivities for gelatinases and TIMPs. However, as judged by the studied clinical and microbiologic indicators, HIV-seropositive patients showed only a slight tendency to develop periodontitis. Overall, an increased amount of gelatinases in saliva may reflect increased host response and defense activities in HIV infection.

The expression of Fas Ligand by macrophages and its upregulation by human immunodeficiency virus infection

Fas/Fas Ligand (FasL) interactions play a significant role in peripheral T lymphocyte homeostasis and in certain pathological states characterized by T cell depletion. In this study, we demonstrate that antigen-presenting cells such as monocyte-derived human macrophages (MDM) but not monocyte-derived dendritic cells express basal levels of FasL. HIV infection of MDM increases FasL protein expression independent of posttranslational mechanisms, thus highlighting the virus-induced transcriptional upregulation of FasL. The in vitro relevance of these observations is confirmed in human lymphoid tissue. FasL protein expression is constitutive and restricted to tissue macrophages and not dendritic cells. Moreover, a significant increase in macrophage-associated FasL is observed in lymphoid tissue from HIV (+) individuals (P < 0.001), which is further supported by increased levels of FasL mRNA using in situ hybridization. The degree of FasL protein expression in vivo correlates with the degree of tissue apoptosis (r = 0.761, P < 0. 001), which is significantly increased in tissue from HIV-infected patients (P < 0.001). These results identify human tissue macrophages as a relevant source for FasL expression in vitro and in vivo and highlight the potential role of FasL expression in the immunopathogenesis of HIV infection.

HIV-I induced destruction of neocortical extracellular matrix components in AIDS victims

Neurological dysfunction is not uncommon in patients suffering from acquired immunodeficiency syndrome (AIDS) and, when manifested, intimates involvement of the central nervous system. Here, the human immunodeficiency virus (HIV) infects preferentially microglial cells, which thereby release substances known to interfere with neuronal function. One class of agents set free in this manner are proteases; these degrade certain components within, and thereby undermine the integrity of, the extracellular matrix (ECM) compartment, which plays a vital role in cell-to-cell communication. We wished to ascertain whether the ECM compartment is indeed disrupted in the brains of AIDS victims. We examined the neocortical areas of 27 AIDS autopsy cases, including 9 with diagnosed HIV-encephalopathy (HIVE); 8 HIV-seronegative cases with various types of brain lesion, including viral infections, were also included in this study. HIV-antigens and DNA were identified by use of immunohistochemistry and in situ hybridization, and ECM components by lectin staining and immunohistochemistry. Of the 27 AIDS cases examined, each of the 9 with HIVE was completely devoid of labeled ECM components; 8 of the 18 without HIVE had incurred substantial losses, and only 2 manifested a normal complement of constituents within this compartment. With respect to stratal and topographic variations, layers II and III were less affected than layers V to VII, as was the frontal cortex relative to other areas. These findings confirmed our expectations of the brain's ECM undergoing degradation following HIV infection, and these changes may well underlie the neurological disturbances manifested in AIDS patients.

Matrix metalloproteinases-1, -3 and -8 and myeloperoxidase in saliva of patients with human immunodeficiency virus infection

OBJECTIVE

Human immunodeficiency virus (HIV)-seropositive patients have frequently severe gingival inflammation and/or attachment loss. In addition many infectious diseases affect their periodontium with varying clinical manifestations. Matrix metalloproteinases seem to play a key role in physiological periodontal remodelling and pathological tissue destruction. The aim of the present study was to characterize the presence, molecular forms, cellular sources, activities, and relative amounts of fibroblast-type (matrix metalloproteinase [MMP]-1) and neutrophil (MMP-8) collagenases, as well as their potential activator stromelysin-I (MMP-3) and myeloperoxidase in saliva of HIV-seropositive patients at different phases of HIV-infection. HIV-seronegative, healthy, age-matched patients served as controls.

PATIENTS AND METHODS

Saliva samples were characterized by Western blotting using antibodies specific for MMP-1, MMP-3 and MMP-8. Interstitial collagenase activities were measured using quantitative sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis/laser densitometry assay. Myeloperoxidase was analysed using quantitative dot blotting.

RESULTS

Clinical and microbiological evaluation of HIV-seropositive patients' periodontium showed the presence of putative periodontopathogens ie Actinobacillus actinomycetemcomitans (Aa), Porphyromonas gingivalis (Pg), Prevotella intermedia (Pi), Peptostreptococcus micros (Psm) and Campylobacter rectus (Cr) in their periodontal pockets. The amount of Candida increased with the severity of HIV-infection. Clinical and microbiological findings of HIV-seropositive patients suggested that they have a tendency to develop periodontal disease. Interstitial collagenase activities were found to be increased in saliva of different phases of HIV-infected patients compared to the controls. Independent of the phase of HIV-infection saliva samples contained pro- and active forms of MMP-1, -3 and -8 using Western blotting. Saliva samples from healthy controls were found to contain hardly any immunoreactivities for MMP-1 or MMP-8, but considerable amounts of MMP-3 were detected. Quantitative dot blotting demonstrated increased amounts of myeloperoxidase in HIV-patients' saliva relative to controls.

CONCLUSION

The present results showed increased amounts of MMP-1, -3, -8 and myeloperoxidase in HIV-patients' saliva. MMP-1 and -8 may have been activated by MMP-3 and/or oxidants generated by myeloperoxidase. The increased amounts of MMPs and myeloperoxidase may reflect and directly participate in HIV-infection associated periodontitis.

Impaired antigen presentation to CD4+ T-cells by HIV-infected monocytes is related to down-modulation of CD4 expression on helper T-cells: possible involvement of HIV-induced cellular factors

Defective antigen presentation by HIV-infected monocytes is related to severe immune dysfunction in patients with AIDS, although the mechanism by which this process occurs is not well defined. Here we report that reduced capacity by HIV-infected monocytes to stimulate or present antigen to CD4+ T-cells was mediated by cellular factors associated with the plasma membranes of HIV-infected monocytes. In contrast, soluble factors secreted by HIV-infected monocytes had little or no effect on T-cell stimulation. Reduced T-cell stimulation by HIV-infected monocytes was related to down-modulation of CD4 expression on helper T-cells and was not affected by the inclusion of anti-HIV-gpl20 Ab, indicating the involvement of soluble or cell-associated viral envelope protein to be less likely. Exposure of CD4+ T-cells, that had been in co-culture with HIV-infected monocytes, to uninfected monocytes partially restored impaired T-cell stimulation. Thus, for the first time we report that altered capacity of HIV-infected monocytes to stimulate and present antigen to CD4+ T-cells is related to down-modulation of CD4 expression on T-cells, and appears to occur via membrane-associated cellular factors on HIV-infected monocytes.