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Eradication of HIV-1 from the Macrophage Reservoir: An Uncertain Goal?

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  • SBA School of Science and Engineering, Lahore University of Management Sciences

Abstract and Figures

Human immunodeficiency virus type 1 (HIV-1) establishes latency in resting memory CD4+ T cells and cells of myeloid lineage. In contrast to the T cells, cells of myeloid lineage are resistant to the HIV-1 induced cytopathic effect. Cells of myeloid lineage including macrophages are present in anatomical sanctuaries making them a difficult drug target. In addition, the long life span of macrophages as compared to the CD4+ T cells make them important viral reservoirs in infected individuals especially in the late stage of viral infection where CD4+ T cells are largely depleted. In the past decade, HIV-1 persistence in resting CD4+ T cells has gained considerable attention. It is currently believed that rebound viremia following cessation of combination anti-retroviral therapy (cART) originates from this source. However, the clinical relevance of this reservoir has been questioned. It is suggested that the resting CD4+ T cells are only one source of residual viremia and other viral reservoirs such as tissue macrophages should be seriously considered. In the present review we will discuss how macrophages contribute to the development of long-lived latent reservoirs and how macrophages can be used as a therapeutic target in eradicating latent reservoir.
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Viruses 2015, 7, 1578-1598; doi:10.3390/v7041578
viruses
ISSN 1999-4915
www.mdpi.com/journal/viruses
Review
Eradication of HIV-1 from the Macrophage Reservoir:
An Uncertain Goal?
Wasim Abbas 1, Muhammad Tariq 1, Mazhar Iqbal 2, Amit Kumar 3 and Georges Herbein 3,*
1 Department of Biology, SBA School of Science and Engineering,
Lahore University of Management Sciences, Lahore 54792, Pakistan;
E-Mails: wazim_cemb@hotmail.com (W.A.); m.tariq@lums.edu.pk (M.T.)
2 Laboratory of Drug Discovery and Structural Biology, Health Biotechnology Division,
National Institute for Biotechnology and Genetic Engineering (NIBGE), Faisalabad 38000,
Pakistan; E-Mail: hamzamgondal@gmail.com
3 Department of Virology, University of Franche-Comté, CHRU Besançon,
UPRES EA4266 Pathogens and Inflammation, SFR FED 4234, 25030 Besançon, France;
E-Mail: amit.aiims2005@gmail.com
* Author to whom correspondence should be addressed; E-Mail: georges.herbein@univ-fcomte.fr;
Tel.: +33-381-21-88-77; Fax: +33-381-66-56-95.
Academic Editor: Eric O. Freed
Received: 4 February 2015 / Accepted: 24 March 2015 / Published: 31 March 2015
Abstract: Human immunodeficiency virus type 1 (HIV-1) establishes latency in resting
memory CD4+ T cells and cells of myeloid lineage. In contrast to the T cells, cells of myeloid
lineage are resistant to the HIV-1 induced cytopathic effect. Cells of myeloid lineage
including macrophages are present in anatomical sanctuaries making them a difficult drug
target. In addition, the long life span of macrophages as compared to the CD4+ T cells make
them important viral reservoirs in infected individuals especially in the late stage of
viral infection where CD4+ T cells are largely depleted. In the past decade, HIV-1
persistence in resting CD4+ T cells has gained considerable attention. It is currently believed
that rebound viremia following cessation of combination anti-retroviral therapy (cART)
originates from this source. However, the clinical relevance of this reservoir has been
questioned. It is suggested that the resting CD4+ T cells are only one source of residual
viremia and other viral reservoirs such as tissue macrophages should be seriously considered.
In the present review we will discuss how macrophages contribute to the development of
OPEN ACCESS
Viruses 2015, 7 1579
long-lived latent reservoirs and how macrophages can be used as a therapeutic target in
eradicating latent reservoir.
Keywords: HIV-1; cART; latency; reservoirs; macrophage
1. Introduction
More than 35 million people have been infected with human immunodeficiency virus type-1 (HIV-1)
worldwide [1,2]. With the introduction of combination anti-retroviral therapy (cART) in 1996 HIV-1
infection has become treatable but yet not curable [37]. Today, more than 30 different antiretroviral
drugs have been approved for HIV treatment [2,8]. These drugs drive the viral load down to undetectable
levels. However, the persistence of latent reservoirs of replication-competent non-induced proviruses
remains a major obstacle in HIV-1 eradication [3,916]. These latent reservoirs are established early
during acute viral infection [1719]. Macrophages and latently infected resting CD4+ T cells are
reservoirs of HIV-1 [2023]. These reservoirs are fully capable of producing infectious viral particles
when cART is discontinued [11,15,19,24].
Based on the integration status of HIV-1 proviral DNA into the host chromatin, latency has been
classified as pre and post integration latency [2528]. The role of unintegrated forms of HIV-1 DNA in
the formation of viral reservoir is not well established. However, tissue specific cells retain these forms
for a longer period of time [29,30]. Post-integration latency occurs when a provirus fails to adequately
express its genome and becomes reversibly silenced after integration into the host genome. This latent
state is exceptionally stable and mechanisms that maintain HIV-1 latency in vivo are not fully
understood. Several factors contribute to the silencing of integrated HIV-1 provirus such as the site and
orientation of integration into the host genome. These factors include the absence of crucial inducible
host factors, the presence of transcriptional repressors, the chromatin structure and epigenetic control of
HIV-1 promoter, sequestration of cellular positive transcription factors and the suboptimal concentration
of viral transactivators, and inhibition of HIV-1 translation by microRNAs [15,3136]. Most of these
mechanisms have been elucidated using transformed cell lines and recently developed primary cell
models of HIV-1 latency. However, the relative importance of each mechanism in maintaining viral
latency in vivo is not fully established.
Reports suggest the HIV-1 infection of circulating monocytes in vivo. The infected monocytes can
cross the blood-tissue barrier and can differentiate into macrophages [18,26,3739]. Moreover, HIV-1
infected macrophages release several immunoregulatory and inflammatory cytokines including TNF-α,
interleukin (IL)-1, and IL-7, which in turn influence viral replication and disease associated with viral
infection [40,41]. The successful blockade of HIV-1 replication by cART has shifted the medical
research from developing novel antiretroviral drugs towards the eradication of viral reservoirs. A better
understanding in the formation of HIV-1 reservoirs will be necessary to uncover the novel targets and
methods for purging or eradicating the latent reservoirs. The purpose of this review is to precisely define
the viral reservoirs for therapeutic applications.
Viruses 2015, 7 1580
2. HIV-1 Infection of Monocytes/Macrophages
Macrophages play a crucial role in the initial infection, and contribute to HIV-1 pathogenesis
throughout the course of viral infection. Since macrophages are an important part of innate immunity
and participate indirectly to the adaptive immunity to clear the infection, this makes them a central
target of HIV-1 [37,4250]. HIV-1 targets the monocyte/macrophage lineage at varying stages of
differentiation [48,49]. For instance data suggests the involvement of a particular monocyte subtype in
HIV-1 infection [51]. Phenotypical comparative studies demonstrate that CD14++CD16+ monocytes are
more permissive to productive HIV-1 infection and harbor HIV-1 in infected individuals on cART as
compare to the majority of blood monocytes (CD14++CD16). In healthy individuals, the CD14++CD16+
monocytes represent 10% of circulating monocytes [52]. The characteristics have been studied in rhesus
macaques. In acute infection, there was an increase in CD14++CD16+ and CD14+CD16++ monocytes,
while CD14++CD16 monocytes decreased two weeks after infection [53]. Similarly, there was increase
in CD14++CD16+ and CD14+CD16++ monocytes subsets in rhesus macaques with chronic infection and
high viral load [53,54]. Moreover, in HIV-1 infected patients, the preferential expansion of
CD14++CD16+ monocyte subset is associated with increased intracellular level of CCL2 [55]. CCL-2 is
an important pro-inflammatory chemokine produced during HIV-1 infection and is one of the key factors
responsible for the chronic inflammation and tissue damage in HIV-infected patients [56]. For instance,
Cinque and colleagues reported a positive correlation between the levels of CCL2 in cerebrospinal fluid
of patients with the severity of HIV-1 encephalitis [57]. In another instance, role of CCL-2 has been
shown in enhancing the replication of HIV-1 in PBMCs isolated from patients [58]. These monocyte
subsets (CD14++CD16+ and CD14+CD16++) have been also reported in HCV infection demonstrating
that CD16+ monocytes may play important role in viral diseases [59,60].
2.1. Activation Status of Macrophages and HIV-1 Infection
Monocyte derived macrophages exhibits two distinct types of polarization states depending upon the
presence or absence of specific microenvironment stimuli including cytokines. Interestingly, these
cytokines also govern HIV-1 pathogenesis. These activation states (classically activated (M1) and
alternatively activated macrophages (M2)) play an important role in mediating an effective immune
response against infectious agents including HIV-1 [6165] (Figure 1). The M1 macrophages are
activated by a high amount of Th1 cytokines (IFN-γ, IL-2, IL-12), pro-inflammatory cytokines (TNF-α,
IL-1β, IL-6, IL-18) and chemokines (CCL3, CCL4, CCL5) that enhance viral replication and block viral
entry to prevent superinfection in infected macrophages [64] (Figure 1). M1 macrophages express
classical pro-inflammatory cytokines such as TNF-α while M2 macrophages produce anti-inflammatory
cytokines such as IL-4, TGF-β and IL-10 by a high amount [62]. During early stages of infection, the
M1 macrophages are predominant which cause the tissue injury specifically in lymph nodes that is
correlated with T cell apoptosis [66]. However, at later stages of viral infection, there is a shift of M1 to
M2 due to the presence of IL-4 and IL-13. The M2 macrophages favor tissue repair and help to clear the
opportunistic infections during HIV-1 infection. The progression of HIV-1 infection is accompanied by
depletion of CD4+ T cells, resulting in frequent opportunistic infections and the imbalance of Th1 and
Th2 responses leads towards the progression of AIDS [64,67].
Viruses 2015, 7 1581
Figure 1. Modulation of macrophage activity by cytokines. Classical activation of
macrophages by IFN-γ which display pro-inflammatory characteristics while the alternative
activation is mediated by IL-4 and IL-13 and express anti-inflammatory or tissue repairing
properties. Macrophages can be deactivated by IL-10.
2.2. HIV-1 Dynamics in Monocytes/Macrophages: Viral Persistence and Reservoirs
The studies on viral dynamics in monocytes demonstrate that the viral decay in monocytes is slower
than that in activated CD4+ T cells. The mean half-life of viral DNA in monocytes/macrophages is
longer than that in activated and resting CD4+ T cells suggesting the monocytes/macrophages as an
important source of ongoing viral replication in HIV-1-infected patients on cART [68]. Findings suggest
that in naïve patients, the activated CD4+ T cells accounts for most of plasma viremia (99%) while the
other 1% of the virus may be generated primarily from tissue macrophages [69]. However, in the
presence of cART, macrophages are likely the main source of plasma viremia as active viral replication
is halted in CD4+ T cells [6971]. Furthermore, it has been reported that circulating monocytes are not
a major reservoir of HIV-1 in elite suppressors [72].
2.3. Monocytes/Macrophages versus CD4+ T Cells in HIV-1 Infection
Monocyte/macrophages facilitate the transmission and establishment of HIV-1 infection to the CD4+
T cells. Macrophage-tropic HIV-1 variants have been detected during all stages of HIV-1 infection [73].
The chemokine receptor CCR5 is the principal coreceptor for macrophage-tropic HIV-1 on CD4+ T
cells and monocytes/macrophages. Several macrophage-tropic variants such as HIV-1BAL (lung
macrophages), HIV-1JR-FL (isolated from brain tissue), and HIV-1Ada (from PBMCs) have been isolated
Viruses 2015, 7 1582
from HIV-1 infected patients [7476]. Several studies have demonstrated that monocytes contain
HIV-1 variants that are genetically distinct from those observed in CD4+ T cells. Furthermore, the HIV
isolates present in monocytes/macrophages are genetically identical or closely associated with viral
variants found in the blood of suppressive cART-treated patients for longer periods of time [77,78].
Furthermore, phenotypic studies show that HIV-1 in circulating blood monocytes represents diverse
viral phenotypes with multiple coreceptor and cell tropism usage during HIV-1 infection [79,80].
It is worth mentioning that opportunistic pathogens such as Mycobacterium avium and Pneumocystis
carinii activate the macrophages and induce HIV production from infected macrophages in lymph
nodes [81,82]. These findings suggest that macrophages can be a prominent source of viremia at later
stages of HIV when lymphoid tissues are quantitatively and qualitatively impaired and opportunistic
pathogens fuel HIV pathogenesis by activating and increasing the viral production from infected
macrophages [40,71,81,82]. In addition, T cells also induce HIV-1 replication in myeloid cells. For
example, HIV-1 replication in J22-HL-60 (promonocytic cell line) has been reported following direct
contact with MOLT-4 T cells, providing the insight into the molecular mechanisms that regulate virus
production from monocytes/macrophages which are latently infected with HIV-1 [83]. Moreover,
macrophages selectively capture and engulf virally infected CD4+ T cells, a phenomenon that may
contribute to the formation or persistence of viral reservoirs [84,85].
3. Modulation of Macrophage Biology by HIV-1
The life span of macrophages varies greatly and depends upon their tissue location. The tissue
macrophages are long lived with a half-life of six weeks to several years. The cells of monocyte-
macrophage lineage are highly resistant to viral cytopathic effects and apoptosis, and exhibit longer life
spans even when they are exposed to different oxidative stress stimuli [8688]. The macrophages of
central nervous system such as microglia and perivascular macrophages produce and release toxins that
induce apoptosis of neurons and astrocytes, contributing to the HIV-1-associated dementia [87,8891].
It is worth mentioning that HIV-1 infection differentially regulates the telomerase activity in immune
cells. Several studies reported that HIV-1 negatively regulates the telomerase activity in CD4+ T cells,
CD8+ T cells and Jurkat T cells [92,93]. Furthermore, HIV-1 elite suppressors have longer telomeres
and have higher telomerase activity [94]. Interestingly, a study has recently reported that HIV-1 infection
of macrophages increases their telomerase activity. The increase in telomerase activity was specific to
HIV-1 infection and correlated with p24 antigen production [95,96]. Moreover, increase in telomerase
activity by either HIV-1 infection or by overexpression of human telomerase results in higher resistance
of macrophages against oxidative stress and DNA damage. Collectively data suggest that HIV-1
infection of macrophages provides better protection against oxidative stress which could be an important
viral strategy to make HIV-1-infected macrophages long lived and more resistant viral reservoirs (Figure
2). Furthermore, HIV-1 infection of macrophages favors the expression of macrophage colony
stimulating factor (M-CSF) [97]. M-CSF is a prosurvival cytokine that down-regulates TNF-related
apoptosis inducing ligand (TRAIL-R1/DR4) and upregulates the anti-apoptotic genes such as Bfl-1 and
Mcl-1. Subsequently HIV-1 infected macrophages are resistant to apoptosis induced by TRAIL [97].
Viruses 2015, 7 1583
Figure 2. Macrophages fuel HIV-1 pathogenesis. HIV-1 infected macrophages secrete pro-
inflammatory cytokines and chemokines that attract T cells in their vicinity, thereby
transmitting virus to uninfected CD4+ T cells. Infected CD4+ T cells die soon (due to viral
cytopathic effects or antiviral immune response) or return into memory CD4+ T cells as
latent viral reservoirs. HIV-1 infected macrophages secrete soluble CD23 and ICAM that
results in CD4+ T cell activation favoring the viral infection to CD4+ T cells. Viral gp120
increases the expression of TNF-α and TNFR2 in macrophages and T cells, resulting in
CD8+ T cell apoptosis. Bystander CD4+ T cell apoptosis is triggered by FasL ligation to
Fas receptor. HIV-1 infection of macrophages enhances its telomerase activity. HIV-1
expands macrophage survival by upregulating antiapoptotic genes. The P-glycoprotein
transporter present on macrophages pumps out the antiretroviral drugs and limits the
distribution of antiretroviral drugs to macrophages. Furthermore, macrophages spread the
virus to CD4+ T cells through virological synapses. HIV-1 infected macrophages store virus
into the intracellular cytoplasmic compartments providing the protection against antiviral
immune response. HIV-1 infection of macrophages results in the secretion of pro-inflammatory
cytokines and chemokines that ultimately accounts for the perturbation of immune trafficking.
In addition, HIV-1 infection of macrophages has been shown to modulate apoptosis and promote
infection of resting CD4+ T cells. In macrophages, Nef activates a variety of signaling pathways that
leads to the infection of bystander CD4+ T cells and hence expands viral reservoirs. Nef-expressing
macrophages enhance resting CD4+ T cells infection through multiple cellular and soluble interactions
involving macrophages and T cells [40,98]. Nef interacts with apoptosis signal regulating kinase-1
Viruses 2015, 7 1584
(ASK-1) and inhibits Fas- and TNF receptor-mediated apoptosis in HIV-1-infected CD4+ T cells [40,99].
Reports suggest that the survival of infected CD4+ T cells requires intercellular contacts between
macrophages and CD4+ T cells, and expression of Nef [100].
On the other hand, HIV-infected macrophages have been shown to induce apoptosis in uninfected
CD4+ T and CD8+ T cells. In vitro experiments demonstrated that apoptosis inducing ligands expressed
by macrophages govern apoptosis of uninfected CD4+ T cells [101103]. The expression of TNF-α and
TNFR increases during HIV-1 infection and is associated with the depletion of T cells. Following HIV-1
infection activated macrophages release TNF-α as a soluble factor or expressed as a membrane-bound
form that binds to TNFR2. The binding of TNF-α to TNFR2 triggers apoptosis in CD8+ T
cells [40,104,105]. In contrast to CD8+ T cells, TNFR2 is not increased on CD4+ T cells, and the
apoptosis of CD4+ T cells is mediated through the interaction of Fas and FasL [40,106]. Furthermore,
HIV-1 Tat upregulates the production of TRAIL in macrophages and results in the apoptosis of bystander
CD4+ T cells [107]. Moreover, the binding of gp120 to CXCR4 upregulates the expression of membrane
bound TNF-α and TNFR2 in macrophages and CD8+ T cells respectively (Figure 2). The binding of
TNF-α and TNFR2 is associated with decreased intracellular level of Bcl-XL resulting in apoptosis of
CD8+ T cells [108].
4. Macrophages Disseminate HIV-1 to CD4+ T Cells
HIV-1 infected macrophages contribute significantly to the pathogenesis of HIV infection through
transmission of virus to CD4+ T cells [42] (Figure 2). It has been reported that HIV-1 infected
macrophages fuse and transmit virus to CD4+ T cells through virological synapses [109112].
In addition to virological synapses, HIV-1 infected macrophages also secrete viral containing exosomes
and microvesicles that facilitate and enhance HIV-1 dissemination to uninfected CD4+ T cells [44]. The
production of chemokines by HIV-1-infected monocytes/macrophages favors the recruitment and the
activation of a variety of immune cells (Figure 2). In vitro, HIV infection of macrophages leads to the
production of several chemokines such as CCL-2, CCL-3, CCL-4 and CCL-5 [113115] which in turn
favor the recruitment of immune cells including monocytes, macrophages, dendritic cells and T cells.
The HIV-1 Nef protein plays a critical role for this function. The adenovirus-mediated expression of Nef
in macrophages induces chemokine production that results in chemotaxis and activation of CD4+ T cells
for productive HIV-1 infection [116118]. In addition, HIV-1 Nef intersects the macrophage CD40L
signaling pathway and promotes the resting CD4+ T cell infection by inducing soluble CD23 and soluble
ICAM [119].
5. Macrophage Infection under cART
The activity of different antiretroviral drugs has been investigated in macrophages chronically
infected with HIV-1 [120,121]. Protease inhibitors (PIs) have been shown to be a powerful therapeutic
tool to fight HIV infection [122,123]. The combination of PIs along with reverse transcriptase inhibitors
has the ability to target the viral replication at early and late stages of HIV infection. The activity of PIs
such as saquinavir and ritonavir on HIV-1 infection in monocytes/macrophages was found to be several
folds lower than in T cells [120]. Furthermore, the intracellular concentrations of active metabolites of
nucleoside analogs were significantly lower (5 to 140 fold) in macrophages than in lymphocytes. The
Viruses 2015, 7 1585
high expression of P-glycoprotein transporter in macrophages has been reported to limit the availability
and absorption of these drugs [124126]. This remarkable feature renders the macrophages resistant to
certain antiretroviral drugs and ultimately promotes the emergence of viral escape mutants [127,128].
Furthermore, pharmacological inhibition of P-glycoprotein transport enhances absorption and
distribution of HIV-1 protease inhibitors to different organs [129,130]. The relatively lower antiviral
activity of anti-HIV drugs in macrophages allows continued HIV-1 replication, which may result in the
formation of HIV-1 reservoirs and emergence of resistant virus.
In situ hybridization studies on simian immunodeficiency virus HIV type 1 chimera (SHIV) showed
that the tissue macrophages in lymph nodes contain high plasma virus in the absence of CD4+ T cells [131].
Quantitative analysis reveals that most of virus producing cells (95%) in these tissues are macrophages
and 2% are T lymphocytes. In addition, the administration of potent HIV reverse transcriptase inhibitors
blocked the virus production during early infection in T cells but not in macrophages [131]. During
macrophage infection, the presence of an individual mutation in HIV integrase is sufficient to produce
virus resistant to raltegravir [132]. A recent study by Micci and co-workers demonstrated that the
macrophages act as a prominent source of virus in the rhesus macaques that were experimentally
depleted of CD4+ T cells followed by SIV infection [133]. Altogether, these different lines of evidence
demonstrate that macrophages provide a favorable environment for HIV persistence [133,134].
6. Cellular Restrictions Factors and HIV Replication in Macrophages
The importance of macrophages in HIV-1 pathogenesis is further underlined with the discoveries of
the presence of anti-HIV-1 cellular restriction factors. Some restriction factors were found to be
macrophage-specific and some play role in several cell types. SAMHD1 (sterile alpha motif domain-
and HD domain-containing protein 1) is a cellular restriction factor that restricts the replication of
HIV-1 and Vpx deficient HIV-2 [135,136]. Noteworthy, SAMHD1 is not specific for macrophages and
was initially reported as restriction factor in dendritic cells and apparently also plays a role in CD4+
T cells. SAMHD1 has dNTPase activity that significantly reduces the dNTPs pools, thereby limiting the
reverse transcriptase (RT) activity of HIV. VPx protein of HIV-2 has been shown to promote proteasome
dependent degradation of SAMHD1 [135]. Despite of absence of Vpx in HIV-1 genome, virus
successfully replicates in the macrophages. Recently, Kyei and colleagues reported the direct involvement
of cyclin L2 in triggering the proteasomal degradation of SAMHD1 [137]. In addition to SAMHD1, p21
(also called CDKN1A) has been shown to restrict the replication of HIV-1 in MDMs by governing the
expression of ribonucleotide reductase subunit R2 [138]. This resulted in the decreased intracellular
dNTPs pools limiting the RT activity of HIV-1 [138]. Several other HIV-1 restriction factors have been
described including APOBEC3A, APOBEC3G [139143], tetherin [144,145], TRIM5-alpha [146] and
MX2 [147] suggesting the significant importance of macrophages in HIV-1 pathogenesis.
7. Post-Integration Reactivation of HIV from Macrophages
Post integrated HIV-1 DNA has been well characterized in macrophages at least in vitro and to lesser
extent in vivo [148]. Barr et al. sequenced and analyzed 754 unique integration sites in macrophages
infected with HIV-1 in vitro. They found the preferential integration of HIV-1 in active transcriptional
units [149]. HIV-1 was found to be integrated in Toll-like receptor and CAP-binding protein complex
Viruses 2015, 7 1586
interacting homologue genes [150]. The viral replication in monocytes isolated from HIV-1 patients
under cART has been reported [151,152]. However, whether HIV-1 was in unintegrated or integrated
form was not characterized [152].
Figure 3. Therapeutic approaches could favor the clearance of HIV-1 from macrophage
reservoirs. Macrophages harbor integrated as well as unintegrated proviral DNA.
Antiretroviral therapy interferes with several steps of HIV-1 life cycle including entry,
reverse transcription, proviral DNA integration, polyprotein processing and release of viral
progeny. HIV-1 infection also results in the establishment of latency in less studied
reservoirs (macrophages). Macrophages harboring latent HIV-1 [157,158] can be activated
by variety of approaches including chemokines, cytokines and HDACi. In addition several
apoptotic reagents have been also employed which can specifically induce apoptosis in
infected macrophages in vitro [44].
Several latently infected cell lines have been routinely used to study the HIV latency, such as U1
cells. Proinflammatory chemokines like TNF alpha and HDAC inhibitors (HDACi) have been found to
be effective in reactivating HIV-1 in these model latent cell lines in vitro (Figure 3). For instance, HDACi
givinostat, belinostat and panobinostat have been shown to decrease the expression of HIV-1 coreceptor
CCR5 and to increase viral growth in U1 cells [153]. In another instance the bromodomain inhibitor JQ1
has been shown to reactivate HIV-1 in U1 cells [154]. However, the impact of biological or
pharmacological HIV-1 inducers such as HDACi could be difficult to assess in latently infected
Viruses 2015, 7 1587
macrophages. The presence of multidrug pumps in macrophage and inability to reach the tissue specific
macrophages in sufficient concentration could contribute to the ineffectiveness of HIV-1 inducers in
reactivating HIV-1 in macrophages in vivo [155,156]. The study of drugs reactivating HIV from latently
infected monocytes/macrophages such as HDACi and bromodomain inhibitors and apoptosis inducing
agents [44] need further investigation especially in vivo in order to potentially clear HIV-1 from the
cellular reservoir in HIV-infected patients (Figure 3).
8. Conclusions
There are several reasons that explain why macrophages play an important role in the pathogenesis
of HIV-1. From HIV standpoint, macrophages provide an ideal environment for the formation of viral
reservoirs since they live long, are widely distributed throughout the body and are relatively resistant to
HIV-induced apoptosis. Moreover, HIV-1 infection enhances the survival of macrophages by
upregulating antiapoptotic genes. HIV-1 infection of macrophages activates host transcription factors
such as NF-kB and prevents the macrophages from TNF-induced apoptosis. Furthermore, virally
infected macrophages secrete CC-chemokines that attract the T lymphocytes in their vicinity leading to
their productive viral infection. In addition, activated macrophages could favor the depletion of both
uninfected CD4+ T cells and CD8+ T cells leading to immune deficiency. Altogether, macrophages play
a critical role in HIV pathogenesis by expanding the viral reservoir that ultimately fuels disease
progression. HIV-infected monocytes/macrophages are less sensitive to cART as compared to infected
CD4+ T cells. Therefore the development of new therapeutic approaches to clear HIV from monocyte/
macrophage reservoirs is under way although total clearance of HIV from macrophage reservoirs is still
an uncertain goal that needs to be reached in the future to definitively cure HIV-infected patients.
Acknowledgments
This work was supported by grants from the University of Franche-Comté, the gion
Franche-Comté (RECH-FON12-000013), the Agence Nationale de Recherche sur le SIDA (ANRS,
13543 and 13544) and HIVERA 2013 (EURECA project) (to GH), by grants from Higher
Education Commission (HEC) of Pakistan (to WA, MI, MT). AK is a recipient of a postdoctoral grant
of the Agence Nationale de Recherche sur le SIDA (ANRS, 13543 and 13544) and HIVERA 2013
(EURECA project).
Author Contributions
WA and AK were responsible for writing the manuscript. WA and AK created the figures. MT and
MI were responsible in organizing the contents and also assisted in revising the manuscript. GH was
involved in critical reading of the manuscript. All the authors read and approved the final manuscript.
List of Abbreviations
HIV-1: human immunodeficiency virus type-1, TNF-α: tumor necrosis factor alpha, TGF-β: transforming
growth factor beta, IL: interleukin, IFN-γ: interferon gamma, cART: combination anti-retroviral therapy,
AZT: azidothymidine, TRAIL: TNF-related apoptosis-inducing ligand, ASK-1: apoptosis signal
Viruses 2015, 7 1588
regulating kinase-1, ICAM: intercellular adhesion molecule, SAMHD1: sterile alpha motif domain- and
HD domain-containing protein 1, HDAC: histone deacetylase, HDACi: HDAC inhibitor.
Conflicts of Interest
The authors declare no conflict of interest.
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... killed by active virus replication (1). This feature enables infected myeloid cells to serve as long-term reservoirs in vivo, particularly in the CNS, where their sustained long-term low-level viral expression may also contribute to neurocognitive complica tions that develop despite antiretroviral therapy (2)(3)(4)(5). ...
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... Macrophages infected with HIV-1 demonstrate increased resistance to apoptosis and decreased sensitivity to combination antiretroviral therapy. These features make macrophages the optimal HIV-1 reservoirs and a key focus for therapeutic intervention [15,16,[20][21][22]]. An early study [22] reported that TLR5 activation by flagellin could activate NF-κB and latent HIV-1 in CD4 + T cells in HIV-1-infected individuals. ...
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Both bacteria product flagellin and macrophages are implicated in HIV-1 infection/disease progression. However, the impact of their interaction on HIV-1 infection and the associated mechanisms remain to be determined. We thus examined the effect of the flagellins on HIV-1 infection of primary human macrophages. We observed that the pretreatment of macrophages with the flagellins from the different bacteria significantly inhibited HIV-1 infection. The mechanistic investigation showed that the flagellin treatment of macrophages downregulated the major HIV-1 entry receptors (CD4 and CCR5) and upregulated the CC chemokines (MIP-1α, MIP-1β and RANTES), the ligands of CCR5. These effects of the flagellin could be compromised by a toll-like receptor 5 (TLR5) antagonist. Given the important role of flagellin as a vaccine adjuvant in TLR5 activation-mediated immune regulation and in HIV-1 infection of macrophages, future investigations are necessary to determine the in vivo impact of flagellin–TLR5 interaction on macrophage-mediated innate immunity against HIV-1 infection and the effectiveness of flagellin adjuvant-based vaccines studies.
... Macrophages infected with HIV demonstrate increased resistance to apoptosis and decreased sensitivity to combination antiretroviral therapy. These features make macrophages the optimal HIV reservoirs and a key focus for therapeutic intervention [15,16,[20][21][22]]. An early study [22] reported that TLR5 activation by flagellin could activate NF-ĸB and latent HIV in CD4+ T cells in HIV-infected individuals. ...
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Both bacteria product flagellin and macrophages are implicated in HIV infection/disease progression. However, the impact of their interaction on HIV infection and the associated mechanisms remain to be determined. We thus examined the effect of the flagellins on HIV infection of primary human macrophages. We observed that the pretreatment of macrophages with the flagellins from the different bacteria significantly inhibited HIV infection. The mechanistic investigation showed that the flagellin treatment of macrophages downregulated the major HIV entry receptors (CD4 and CCR5) and upregulated the CC chemokines (MIP-1α, MIP-1β, and RANTES), the ligands of CCR5. These effects of the flagellin could be compromised by a toll like receptor 5 (TLR5) antagonist. Given the important role of flagellin as a vaccine adjuvant in TLR5 activation-mediated immune regulation and in HIV infection of macrophages, future investigations are necessary to determine the in vivo impact of flagellin-TLR5 interaction on macrophage-mediated innate immunity against HIV infection and the effectiveness of flagellin adjuvant-based vaccines studies.
... Upon viral infection, macrophages respond with an enhanced expression of inflammatory cytokines (-pro and -anti) which leads to an activated antiviral immune response [4]. Several studies report that viruses persist in macrophages for extended periods that may have a direct bearing on disease progression [5][6][7][8][9]. ...
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... Although the role of resting memory CD4 + T cells as a reservoir of HIV infection has been clearly established, there is evidence that macrophages also represent a durable HIV reservoir. Tissue-resident macrophages, including microglia in the central nervous system (CNS), have a lifespan of months to years and are resistant to the cytopathic effects of HIV 24,25 . Studies in both SIV-infected macaques and HIV-infected humanized mice demonstrate that tissue macrophages are productively infected and represent a source of rebound viremia upon cessation of ART 15,26,27 . ...
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... HIV-1 cell-to-cell transfer between CD4+ T cells, mainly through the formation of the so-called virological synapse [40][41][42] , or from infected macrophages or dendritic cells to CD4+ T cell targets, have been extensively described in vitro 23,[43][44][45][46][47][48][49] , In addition, recent reports demonstrate that myeloid cells can be also productively infected through virus cell-to-cell transfer for more efficient spreading in these poorly susceptible cell types. Since myeloid cells are now emerging as important target cells involved in all steps of HIV-1 pathogenesis and in viral persistence in tissues of infected individuals 10,50,51 , even under conditions of anti-retroviral treatment, the goal of this review is to discuss the different mechanisms reported in the literature regarding HIV-1 cell-to-cell spread leading to productive infection of myeloid cells. The first part of the review will be related to the mechanisms of homotypic virus cell-to-cell transfer between macrophages, while the second part will focus on the two different mechanisms reported for heterotypic virus cell-to-cell dissemination from virus-donor infected CD4+ T cells to myeloid cell targets, including macrophages, DCs and OCs. ...
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This study investigates apoptosis as a mechanism for CD4+ T-cell depletion in human immunodeficiency virus type-1 (HIV-1) infection. Although several recent studies have shown that T cells of HIV-infected individuals show enhanced susceptibility to cell death by apoptosis, the mechanisms responsible for apoptosis are largely unknown. By using a flow cytometric technique and by morphology, we have quantitated the percentage of cells undergoing apoptosis in peripheral blood mononuclear cells (PBMCs) from HIV-seronegative donors and from HIV- infected asymptomatic patients. The PBMCs were cultured without any stimulus or with staphylococcus enterotoxin B, anti-T-cell receptor (TCR) alpha beta monoclonal antibody WT-31, or phytohemagglutinin for periods up to 6 days. In addition, we sought to determine whether cross- linking of CD4 followed by various modes of TCR stimulation in vitro could induce apoptosis in normal PBMCs. Here we show that (1) patient PMBCs undergo marked spontaneous apoptosis; (2) stimulation of T cells of patients as well as normal donors results in increased apoptosis; and (3) cross-linking of CD4 molecules is sufficient to induce apoptosis in CD4+ T cells if cross-linking is performed in unfractioned PBMCs, but not if CD4 molecules are cross-linked in purified T-cell preparations. These observations strongly suggest that accelerated cell death through apoptosis plays an important role in the pathogenesis of HIV-1 infection. At the same time, our observations implicate cross- linking of CD4 in vivo as a major contributor to this mechanism of accelerated cell death in HIV infection.
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Primary cultures from a brain biopsy specimen of a human T-cell lymphotropic virus type III/lymphadenopathy-associated virus (HTLV-III/LAV) seropositive patient with progressive dementia contained small numbers of monocytoid cells and showed reverse transcriptase activity that persisted for as long as 100 days. Electron microscopy of these cells revealed the presence of HTLV-III/LAV virions. Subcultured cells removed from primary cultures by trypsinization were nonspecific esterase negative and did not express virus or show evidence of HTLV-III/LAV proviral sequences, while those remaining in the original flasks were nonspecific esterase positive and continued to produce virus. Virus from primary cultures was transmitted to peripheral blood-derived monocytemacrophages and T cells. Virus production in T-cell cultures was transient while the monocyte-macrophages, like the primary cultures, produced virus for at least 120 days. Infection of several brain-derived cells with this and another HTLV-III/LAV isolate failed to demonstrate virus replication. These results indicate that the HTLV-III/LAV-infected cells recovered from the brain of this patient are cells of the mononuclear phagocyte series. (JAMA 1986;256:2365-2371)
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Apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (APOBEC3G), a cytidine deaminase, is a recently recognized innate intracellular protein with lethal activity against human immunodeficiency virus (HIV). Packaged into progeny virions, APOBEC3G enzymatic activity leads to HIV DNA degradation. As a counterattack, HIV virion infectivity factor (Vif) targets APOBEC3G for proteasomal proteolysis to exclude it from budding virions. Based on the ability of APOBEC3G to antagonize HIV infection, considerable interest hinges on elucidating its mechanism(s) of regulation. In this study, we provide the first evidence that an innate, endogenous host defense factor has the potential to promote APOBEC3G and rebuke the virus-mediated attempt to control its cellular host. We identify interferon (IFN)-α as a potent inducer of APOBEC3G to override HIV Vif neutralization of APOBEC3 proteins that pose a threat to efficient macrophage HIV replication. Our data provide a new dimension by which IFN-α mediates its antiviral activity and suggest a means to render the host nonpermissive for viral replication.
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The restriction factor SAMHD1 limits HIV-1 replication in noncycling cells. SIV and HIV-2 overcome this restriction via the accessory protein Vpx, which targets SAMHD1 for degradation through interactions with the host ubiquitin ligase adaptor DCAF1. However, the factors used by HIV-1 to replicate in macrophages, despite the presence of the restriction factor SAMHD1, are unknown. Using a yeast two-hybrid screen, we identified cyclin L2 as a DCAF1-interacting protein required for HIV-1 replication in macrophages. Knockdown of cyclin L2 results in severe attenuation of HIV-1 replication in macrophages but not cycling cells, and this effect is lost in the absence of SAMHD1. Cyclin L2 and SAMHD1 form a molecular complex that is partially dependent on the presence of DCAF1 and results in SAMHD1 degradation in a proteasome- and DCAF1-dependent manner. Therefore, cyclin L2-mediated control of SAMHD1 levels in macrophages supports HIV-1 replication. Copyright © 2015 Elsevier Inc. All rights reserved.
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Macrophages are motile leukocytes, targeted by HIV-1, thought to play a critical role in host dissemination of the virus. However, whether infection impacts their migration capacity remains unknown. We show that 2-dimensional (2D) migration and the 3D amoeboid migration mode of HIV-1-infected human monocyte-derived macrophages were inhibited, while the 3D mesenchymal migration was enhanced. The viral protein Nef was necessary and sufficient for all HIV-1-mediated effects on migration. In Nef transgenic mice, tissue infiltration of macrophages was increased in a tumor model and in several tissues at the steady state, suggesting a dominant role for mesenchymal migration in vivo. The mesenchymal motility involves matrix proteolysis and podosomes, cell structures constitutive of monocyte-derived cells. Focusing on the mechanisms used by HIV-1 Nef to control the mesenchymal migration, we show that the stability, size and proteolytic function of podosomes are increased via the phagocyte-specific kinase Hck and WASP, two major regulators of podosomes. In conclusion, HIV-1 reprograms macrophage migration, which likely explains macrophage accumulation in several patient tissues, a key step for virus spreading and pathogenesis. Moreover, Nef points out podosomes and Hck/WASP signaling pathway as good candidates to control tissue infiltration of macrophages, a detrimental phenomenon in several diseases. Copyright © 2014 American Society of Hematology.