Targeting HIV-1 innate immune responses therapeutically

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CE: Madhur; COH/200544; Total nos of Pages: 9;

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Targeting HIV-1 innate immune
responses therapeuticallyRada Ellegard, Esaki M. Shankar and Marie Larsson
Division of Molecular Virology, Department of Clinicaland Experimental Medicine, Linkoping University,Linkoping, Sweden

Correspondence to Marie Larsson, PhD, Division ofMolecular Virology, Linkoping University, Lab 1, Plan13 HU, 581 85 Linkoping, SwedenTel: +46 10 1031055; e-mail: [email protected]

Current Opinion in HIV and AIDS 2011,6:000–000

Purpose of review

The early stage of HIV-1 infection is when the virus is most vulnerable, and should

therefore offer the best opportunity for therapeutic interventions. This review addresses

the recent progress in the understanding of innate immune responses against HIV-1

with focus on the potential targets for prevention of viral acquisition, replication and

dissemination.

Recent findings

Research indicates that the host-derived factor trappin-2/elafin is protective against

HIV, whereas semen-derived enhancer of viral infection and defensins 5 and 6 enhance

viral transmission. Further, studies suggest that stimulation of TLR4 and inhibition of

TLR7–9 pathways may be HIV suppressive. The regulation and function of viral

restriction factors tetherin and APOBEC3G have been investigated and a molecule

mimicking the premature uncoating achieved by TRIM5a, PF74, has been identified.

Chloroquine has been shown to inhibit plasmacytoid dendritic cell activation and

suppress negative modulators of T-cell responses. Blockade of HMBG1 has been

found to restore natural-killer-cell-mediated killing of infected dendritic cells, normally

suppressed by HIV-1. Interestingly, when used as adjuvants, EAT-2 and heat shock

protein gp96 reportedly enhance innate immune responses.

Summary

Several targets for innate immunity-mediated therapeutics have been identified.

Nonetheless, more research is required to unveil their underlying mechanisms and

interactions before testing these molecules in clinical trials.

Keywords

HIV, innate, microbicides, therapeutics

Curr Opin HIV AIDS 6:000–000� 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins1746-630X

Introduction

The innate immune system is the first line of defense

against HIV-1 and is crucial for the initial antiviral activity

which reduces viral replication and protects surrounding

cells from infection, as well as for the development of

appropriate adaptive immune responses [1]. To date, the

vast majority of HIV-1 research has focused on the adap-

tive immunity in order to formulate an effective vaccine.

Due to the limited success of the recent vaccine candidates

[2,3], research on innate-mediated antiviral immunity is on

the increase.

Following mucosal sexual transmission of HIV-1, viral

RNA initially remains undetectable in the peripheral

blood. Studies in the SIV-macaque model have demon-

strated that infection may be initiated in dendritic cells

and Langerhans cells [4], followed by establishment in

certain CD4þ T-cell populations (40–50 cells) approxi-

mately 3–4 days after exposure [5,6]. When viral RNA,

that is, HIV-1, appears in the circulation (approximately

opyright © Lippincott Williams & Wilkins. Unauth

1746-630X � 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins

7 days later), the CD4þT cells in the gut are substantially

depleted [7] and a persistent infection has already been

established in the lymphatic tissue [8]. This implies that

the early stage of infection is when HIV-1 is most

vulnerable, and should offer the best opportunity for

rapid therapeutic intervention. In addition, HIV-1 elite

controllers frequently lack the standard pathognomonic

features of primary infection [9] and can control viral

replication before the onset of adaptive immune

responses [10]. This control is not because of the intrinsic

resistance of CD4þ T cells to infection [11��] and

suggests the potential of innate immunity as the key

determinant of HIV disease progression.

Recent findings with respect to innate-mediated immune

responses against HIV-1 could lead to the development

of potent microbicides/antiviral agents that can success-

fully prevent viral acquisition, replication and dissemina-

tion as well as valuable insights into possible adjuvants

that can shape innate immune responses to enhance the

effect of future vaccines.

orized reproduction of this article is prohibited.

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2 Innate immunity

Key points

� Exploiting or inhibiting endogenous microbicides

could generate effective HIV protection, although

the development of therapeutics is complicated by

the complexity of mucosal environment.

� Studies on factors associated with protection,

especially in frequently exposed uninfected com-

mercial sex workers, may give insight into other

potential targets for HIV therapy and prevention.

� Research on TLR pathways and HIV-1 infection

has so far mainly been performed on single-cell

types in vitro, but may lead to valuable tools for

both prevention and treatment of HIV.

� Manipulating naturally occurring antiviral factors,

such as APOBEC3G and tetherin, or finding new

molecules that mimic antiviral factor activity, such

as PF74, may be useful to control HIV patho-

genesis, but their mechanisms have not been

investigated thoroughly enough to ensure their

HIV protective or suppressive properties in vivo.

� Agents in clinical use for other purposes, glycyr-

rhizin and chloroquine, may help to eliminate

infected dendritic cells during early stages of

HIV infection and interfere with chronic immune

activation, respectively.

Microbicides in genital and rectal mucosaIn addition to posing a mechanical barrier to viral entry,

the host genital and rectal mucosa offers a plethora of

microbicides that can prevent infection and viral replica-

tion, as well as decrease inflammation (Fig. 1)

[12��,13�,14�,15–17,18�]. As existing inflammation in

the mucosa, due to HIV/SIV itself or a preexisting coin-

fection such as HSV [19,20], aids in establishment and

expansion of HIV/SIV, the anti-inflammatory properties

of microbicides at mucosal sites may prevent viral dis-

semination. Although none of the synthetic microbicides

that have completed phase III clinical trials appear to

offer good protection correlates [21,22], research on novel

antimicrobial molecules should generate better results.

A serine protease inhibitor produced by epithelial cells

in the human female reproductive tract, trappin-2/ela-

fin, has been identified to possess both anti-inflamma-

tory and HIV-inhibitory activities [12��], and has been

associated with protection against HIV-1 acquisition

[23].

Not all secreted antimicrobials are protective, mucosal

defensins 5 and 6 increase HIV infectivity in vitroand have been proposed to have partially caused the

failure of some microbicide candidates in trials [13�].

Semen-derived enhancer of viral infection (SEVI), a type

opyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.

Figure 1 Factors influencing HIV infectivity at the site of entry

(1) Trappin-2/elafin produced by epithelial cells directly interacts with HIV, reducing infectivity [12��]. (2) Defensins 5 and 6 are constitutively expressedin the human mucosa and significantly upregulated in response to sexually transmitted infections [13�]. They directly enhance the infection by aidingviral entry and inhibit HIV-suppressive polyanions. (3) Semen-derived enhancer of viral infection aggregates found in seminal fluid aid in viral attachment[14�], their positive charge may help the virus overcome the electrostatic repulsion between the negatively charged viral and target cell membranes.Complement opsonization increases infectivity and cell–cell transmission of HIV [15,16]. (4) Inflammation and coinfections aid in the establishmentand dissemination of HIV by increasing mucosal permeability and attracting target cells [17,18�].

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Targeting HIV-1 innate immune responses Ellegard et al. 3

of amyloid aggregates found in semen, enhance viral

attachment to target cells and their levels have been

correlated to HIV infectivity [14�]. In addition, HIV has

developed mechanisms to exploit the human comp-

lement system by incorporating regulators of comp-

lement activation into its envelope while budding from

infected host cells [15], leading to enhanced infection

and transmission from dendritic cells to T cells [16]. The

inactivated complement fragments present on the viral

surface might be involved in the induction of tolerance

and immune suppression [24,25]. Of note, frequently

exposed uninfected commercial sex workers expressed

less complement component C3 in their genital mucosa

compared with HIV-infected sex workers and unin-

fected low-risk groups [26]. In the light of these findings,

the effects of complement opsonization on infectivity

and induction of innate immune responses are important

to investigate when developing therapeutics and

vaccines.

In order to successfully identify or develop agents with

HIV-protective properties, the great complexity of

innate-immunity regulation in human mucosa must be

taken into consideration. Dissecting the roles and mech-

anisms of action of individual innate factors combined


Figure 2 Targeting Toll-like receptor pathways therapeutically

nucleu

CpG

Bac

terr

ia

LPS

dsRNA

T cell

Prothymosin-α

Poly I:C

IFN

TLR3

TLR

TLR2

TLR4

APOBEC3G

Tetherin

1.

(1) Signaling through Toll-like receptor 3 (TLR3), induced by virus-derivedcommensal bacteria or prothymosin-a, lead to HIV suppression via the pStimulation of TLR2, 7 and 9 has been associated with the inflammation anphosphorothioate 20 deoxyribose oligomers significantly inhibits the infection oand DC-SIGN signaling in order to achieve productive infection. Viral ssRNinduces Raf1 through DC-SIGN. Raf1 phosphorylates NFkB, leading to viral rinhibitor GW5074 blocks Raf1 and inhibits HIV replication [32��] and may

with a greater understanding of how they interact with

and are influenced by hormonal fluctuations [27], coin-

fections [17,18�], as well as the effect of semen on the

mucosal milieu and HIV susceptibility will be challen-

ging, will be valuable tools in the development of

prevention modalities.

Induction of innate immune responsesthrough Toll-like receptor pathwaysActivation of pattern recognition receptors (PRRs) of the

innate immune system is crucial for early antiviral

defense, which includes production of interferons (IFNs)

and interferon-inducible factors (Fig. 2) [28�,29,30�,

31��,32��,33�]. The most studied class of PRRs, Toll-like

receptors (TLRs) have been examined to evaluate

whether their signaling can be exploited to generate

effective vaccines and therapeutic interventions.

Although all TLRs (1–10) can be found in the human

mucosa, TLR3, 4, 7, 8 and 9 preferentially induce IFN

production, and thereby exhibit antiviral activity.

Research showing that HIV has developed mechanisms

to evade triggering TLR-induced responses [34��,35�]

supports their potential as targets for innate-mediated

interventions.


s

ssRNADN

A

ssR

NA

gp120

GW5074NFkB

Raf1

Productive infection

Phosphorothioate2' deoxyribose oligomers

9 TLR7

TLR8

Inflammation

DC-SIGN

2.

3.

dsRNA or synthetic poly I:C, and signaling through TLR4, induced byroduction of interferon (IFN) and IFN-induced factors [28�–30�]. (2)d enhanced HIV replication [29,31��]. Blocking TLR7 and TLR9 usingf peripheral blood mononuclear cells by HIV [31��]. (3) HIV exploits TLR8A induces NFkB through TLR8, whereas viral envelope protein gp120eplication and cell–cell transmission of HIV-1 [32��]. The small moleculehave therapeutic potential if developed further.

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4 Innate immunity

The major concern with TLRs is their association with

inflammation and other immunoregulatory factors that

may favor HIV replication and dissemination. Inducing

high levels of IFN to provide potent innate antiviral

immunity, but bypassing the induction of inflammation

and recruitment of leukocytes to sites of infection is a

challenge. Insights into which TLRs are suitable tar-

gets for HIV prevention have been generated by stu-

dies on mucosal commensal bacteria and their effect on

HIV susceptibility. One study showed that in macro-

phages, whereas bacteria with HIV-suppressive

effects preferentially stimulated TLR4, the ones that

enhanced HIV replication stimulated TLR2 [28�].

These results are consistent with effects seen in den-

dritic cells following TLR4 and TLR2 stimulation [29].

Furthermore, HIV inhibition through targeting TLR4

utilizing prothymosin-a, a naturally occurring TLR4

ligand produced by CD8þ T cells, potently suppressed

HIV after entry into macrophages via production of

type-I IFN [30�].

Stimulation of TLR3, which has been shown to act

independently of inflammation [36], in macrophages

through dsRNA (poly I:C) significantly inhibited HIV

infection and replication by inducing type-I IFN indu-

cible factors such as apolipoprotein B mRNA-editing

enzyme 3G (APOBEC3G) and tetherin [37�]. However,

the suitability of TLR3 ligands for HIV suppression

remains dubious as they also have been shown to

increase Langerhans cell maturation and HIV trans-

mission [38].

Although some results indicate that signaling through

TLR9 may be HIV suppressive [39�], most studies claim

that stimulation of TLR7 and TLR9 induces inflam-

mation and augments HIV replication and spread

[31��,40]. Recent results showed that phosphorothioate

20 deoxyribose oligomers, which specifically block these

TLRs, significantly inhibited the infection of peripheral

blood mononuclear cells (PBMCs) by HIV [31��]. In

addition, the oligomers did not appear to be toxic in vitro[31��], making them viable microbicide candidates that

merit further investigation.

A possible means of avoiding inflammation resulting from

TLR activation is through the use of miRNAs, which may

reduce the expression of TLRs or inhibit their down-

stream signaling. Let-7e and miR-105 have been shown

to decrease the expression of TLR4 [41] and TLR2 [42],

respectively, whereas miR-155 has been reported to

inhibit key proteins in downstream TLR signaling

[43]. However, despite the potential advances in mol-

ecular medicine this approach may not be optimal for

HIV therapy as blocking TLR signaling may impair

responses to other infections and/or cell functions such

as antigen presentation [44��].

opyright © Lippincott Williams & Wilkins. Unautho

In myeloid dendritic cells (mDCs), corresponding to tissue

dendritic cells, HIV achieved productive infection by

exploiting TLR8 signaling in combination with the C-

type lectin DC-SIGN, as blocking either pathway abro-

gated replication and prevented HIV transmission [32��].

This mechanism could potentially be exploited to prevent

infection and viral dissemination.

A study in mouse dendritic cells showed that expression

of A20, an inducible feedback inhibitor of retinoid-indu-

cible gene-1 (RIG-1), and TLR signaling pathways,

inhibited robust mucosal and systemic HIV-specific cel-

lular and humoral responses [45�]. When A20 expression

was silenced it enhanced dendritic cell’s ability to home

to gut-associated lymphoid tissues following systemic

administration, as well as inhibiting the expression of

gut-homing receptors on T and B cells [45�], which would

be beneficial in a HIV vaccine.

Viral restriction factorsViral restriction factors are constitutive or IFN-induced

components of the innate immune response interfering

with different stages of viral replication (Fig. 3) [46,47��–

49��,50,51,52�,53�,54]. Three classes of restriction factors

affecting lentiviruses have been identified: cytidine dea-

minases, such as APOBEC3G, which induce lethal

hypermutations in the viral genome; tripartite motif-

containing factor (TRIM) proteins, which may affect

stages of the replication cycle such as uncoating and

budding; and tetherin, which prevents mature HIV

particles from being released from infected cells [55,56].

APOBEC3G inhibits reverse transcription and integration

eventsduringthecourseofHIV’s lifecycle.However,upon

infecting a cell, the viral protein Vif targets APOBEC3G for

proteasomal degradation, abrogating or severely diminish-

ing its antiviral functions [50]. In addition, APOBEC3G is

thought to be able to exert antiviral activity only if it is

incorporated into the viral particles [57]. Nevertheless,

upregulation of APOBEC3G expression in PBMCs by

immunizing macaques with SIV antigens and CCR5 pep-

tides linked to a 70-kDa heat-shock protein (HSP) offered

protection from subsequent SIV challenge [51]. There is

some concern that instead of decreasing HIV infectivity in

patients, APOBEC3G upregulation may raise mutagenic

activity to a level that is insufficient to render the virions

noninfectious and instead aid in the evolution of drug-

resistantphenotypes[52�,53�,58�].Althoughrecent studies

have shed some light on the regulatory mechanisms of

ABOBEC3G [59�,60�], it is likely that development of

novel probes and research reagents will be necessary in

order to dissect the mechanisms that determine what

precise role APOBEC3G plays in infected cells.

Although TRIM5a is responsible for the complete block

of HIV replication in Old World monkeys, its human

rized reproduction of this article is prohibited.

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Figure 3 Viral restriction factors

CypA

Debio-025

TRIM5α/PF74

Vif

Vpu

TRIM22

Tetherin

APOBEC3G

1.

2.

5.

3.

4.

(1) Cyclophilin A (CypA) interacts with HIV-1 protein p24 and helps viralattachment as well as aiding in later stages of the viral life cycle [46].Debio-025 inhibits CypA and suppresses HIV replication [47��]. (2) Themolecule PF74 mimics the activity of simian TRIM5a and prematurelyuncoats HIV virions [48��]. (3) TRIM22 suppresses HIV at later stages ofits replication cycle, such as transcription, although its exact mechanismof action is still unclear [49��]. (4) APOBEC3G is incorporated intobudding virions and causes lethal hypermutations, but is marked forproteasomal degradation by Vif [50]. Although upregulation of APO-BEC3G has been achieved in a SIV-macaque model [51], it is still underdebate whether this strategy would be beneficial to HIV-infectedpatients [52�,53�]. (5) The recently discovered viral restriction factortetherin is induced by interferon and holds mature virions from buddingfrom infected cells, but is downregulated by viral protein Vpu [54].Elucidation of its mechanisms may lead to new antiviral strategies.

homolog has no HIV-suppressive properties. However,

the molecule PF74 has been found to prematurely uncoat

HIV in target cells, by mimicking the activity of simian

TRIM5a [48��]. Another IFN-induced TRIM protein

which inhibits viral assembly, TRIM22, has been associ-

ated with significantly lower risk of HIV acquisition and

lower viral loads or higher CD4þ T-cell counts during

primary infection [49��], although its exact mechanisms

of action and regulation have not yet been fully eluci-

dated.

Ever since the discovery of its viral restriction properties

in 2008 [54,61], the IFN-a-induced protein tetherin has

been intensively studied. Tetherin is constitutively

expressed by the plasmacytoid dendritic cells (pDCs)

and restricts productive HIV cell–cell transmission [62�],

as well as participating in an important negative feedback

loop limiting IFN responses to viral infections [63]. The

HIV-1 protein Vpu interferes with tetherin [54]. The

fact that HIV-1 has developed highly specific tools

to antagonize tetherin clearly suggests that this host


restriction factor could be a potent inhibitor of viral

replication in vivo.

In contrast to the factors discussed above, the peptidyl-

propyl isomerase cyclophilin A (CypA) protein expressed

by host cells helps in HIV replication, by binding the HIV

capsid protein p24 [46]. It has recently been discovered

that a CypA inhibitor, Debio-025, blocks this interaction

and reduces viral replication [47��], and may therefore

have therapeutic potential.

Dendritic cells and natural killer cellspDCs produce more type-I IFN in response to HIV than

any other cell type in the body and subsequently activate

mDCs and natural killer (NK) cells as well as inducing

adaptive immune responses [64�,65,66��,67��,68�,69�,70]

(Fig. 4). IFN-a is both beneficial, as it inhibits virus

replication early during infection, and harmful, as it

contributes to chronic immune activation that drives

progression to AIDS [71]. Blood pDCs and mDCs are

hyperresponsive in HIV-infected individuals, indicating

that they play an important role in chronic immune

activation [72�]. It is hypothesized that some of the

detrimental effects of chronically activated dendritic cells

and extended IFN production are mediated by indolea-

mine 2,3-dioxygenase (IDO) which leads to disruption of

Th17/Treg T-cell balance [64�,65]. In-vitro studies show

that when IFN-a is blocked using chloroquine, an anti-

malaria drug, pDC activation is inhibited and negative

modulators of T-cell responses, such as IDO and pro-

grammed death ligand-1 (PDL-1) are suppressed [66��].

The use of chloroquine along with antiretroviral therapy

may interfere with chronic immune activation in

infected individuals.

Although the role of pDCs in later stages of HIV infec-

tion is not clearly understood, studies show that while

HIV-infected subjects with progressive disease have a

significant decline in pDCs over time, elite suppressors

maintain high levels of these cells [73], indicating that

they are important factors in the innate immune

response. A novel association between polymorphisms

in interferon-regulatory factor-7 (IRF7), a master regu-

lator of IFN-a, and the ability of pDCs to produce IFN-a

in response to HIV-1 has recently been described [74�].

Further insight into the early events in primary infection

and the underlying mechanisms behind elite controller’s

ability to maintain pDC levels and functions will assume

paramount importance to achieve elite control of

HIV replication.

NK cells interact with T cells and dendritic cells to shape

the magnitude and quality of adaptive immune

responses [70]. This ‘editing process’ is impaired when

the dendritic cells are infected with HIV-1, and novel


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6 Innate immunity

Figure 4 Targeting dendritic cell and natural killer cell function

TLR

MyD88

IRF4/7

Chloroquine

IFN

PD1-L

IDO

TH17

T reg

DC

NK cell

c-FLIP

c-IAP2

TRAIL

DR4

HMGB1

Glycyrrhizin

Glycyrrhizin

(a) (b) (c)

NTB-A

HMGB1

Vpu

DC

NK cell

infectedcell

CD4T cell

NKp44L

(a) Chronic plasmacytoid dendritic cell and myeloid dendritic cell activation leads to the disruption of Th17/Treg T-cell balance via induction of PD1-Land IDO [64�,65]. Chloroquine inhibits Toll-like receptor signaling and downregulates negative modulators of T-cell responses and interferonproduction, and may be useful as a therapeutic agent in infected patients [66��]. (b) Natural killer (NK)-mediated killing of infected dendritic cells isdisrupted by the HIV-mediated upregulation of HMGB1 secretion, which induces expression of the antiapoptotic molecules cellular FLICE-inhibitoryprotein and cellular inhibitor of apoptosis 2 and inhibition of TRAIL-mediated killing [67��]. Blocking HMBG1 using the anti-inflammatory agentGlycyrrhizin, which is currently in clinical use for other purposes, restored NK-mediated killing of infected dendritic cells [67��] and may be useful as atherapeutic during early HIV infection. (c). HIV disease progression is associated with the upregulation of NKp44L on CD4 T cells, causing autologousNK lysis [68�]. In addition, viral protein Vpu downregulates the surface expression of NTB-A and thereby suppresses the lysis of other infected cells[69�]. Investigating these mechanisms may further lead to new strategies for HIV therapy.

findings show that this occurs because of the upregula-

tion of two antiapoptotic molecules, cellular FLICE-

inhibitory protein (c-FLIP) and cellular inhibitor of

apoptosis 2 (c-IAP2) [67��]. This upregulation is report-

edly controlled by high-mobility group box1 (HMGB1),

a key mediator of natural killer cell–dendritic cell cross-

talk. Blockade of c-FLIP and c-IAP2 or HMGB1 restored

NK cell’s capability to destroy HIV-infected dendritic

cells [67��]. These mechanisms could possibly be tar-

geted to eliminate infected dendritic cells early in

HIV infection.

NK cells play an important role in the control of HIV

infection by lysing infected cells [70]. The exact mech-

anisms that mediate the recognition of HIV-infected cells

are not known, detection could either occur directly,

through receptor-mediated interactions that have not

yet been identified, or indirectly by antibody-mediated

crosslinking of CD16, an Fc receptor for IgG [75]. HIV

Vpu has recently been shown to inhibit surface expres-

sion of NTB-A (NK-cell, T-cell and B-cell antigen) in

infected cells, thus protecting the cells from lysis [69�].

Also, the effects that HIV infection exerts on NK-cell

functions remain to be elucidated as alteration in cell

phenotype but cell levels has not been reported [76].

Identifying the receptors responsible for NK-cell recog-

nition of HIV and the effects HIV viremia exerts on NK-

cell functions remains a key research area. One study

found that HIV progressors overexpressed NKp44L on

their CD4þT cells as compared with HIV controllers and


healthy donors, causing autologous NK lysis, and that this

lysis was abrogated after treatment with anti-NKp44L

mAb [68�]. Such research on the specific functional

activity of NK cells may have important implications

for the design of new anti-HIV therapeutical strategies.

Manipulating dendritic cell, macrophage and NK-cell

responses may be useful strategies in vaccine develop-

ment. In a mouse model, coexpression of HIV antigens

and EWS-FLI1-activated transcript 2 (EAT-2), an

adapter protein that leads to activation of the signaling

lymphocytic activation molecule (SLAM) receptors on

dendritic cells and macrophages, enhanced the induction

of antigen-specific cellular immune responses and facili-

tated bystander activation of NK, NKT, B and T cells

[77�]. Seeing that the EAT-2 adapter is conserved in both

mice and humans, human vaccination strategies that

specifically facilitate SLAM signaling may improve

vaccine potency. The use of a genetically engineered,

secreted form of heat shock protein gp96 (HSP gp96), an

endoplastic-reticulum-resident chaperone, as an adjuvant

in a SIV-macaque model showed some success in the

induction of high levels of antigen-specific CD8þ T-cell

and memory responses in rectal and vaginal mucosa [78�].

ConclusionMounting evidence suggests that innate immune

responses determine susceptibility as well as disease

progression in HIV infection. The goal for therapeutics


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targeting innate factors is a high level of protection or

optimally a block of infection. Given that a recent trial has

shown that mucosally applied inhibitors can confer pro-

tection from HIV-1 infection [79��], the application

of potent drugs targeting innate factors present in the

genital mucosa is a viable prospect for inhibiting HIV

infection.

AcknowledgementsThis work has been supported by the grant AI52731 from the SwedishResearch Council, The Swedish Physicians against AIDS ResearchFoundation, The Swedish International Development CooperationAgency, SIDA SARC, VINNMER for Vinnova, Linkoping UniversityHospital Research Fund, CALF and The Swedish Society of Medicine.

References and recommended readingPapers of particular interest, published within the annual period of review, havebeen highlighted as:� of special interest�� of outstanding interest

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��Rabi S, O’Connell K, Nikolaeva D, et al. Unstimulated primary CD4þ T cellsfrom HIV-1-positive elite suppressors are fully susceptible to HIV-1 entry andproductive infection. J Virol 2011; 85:979–986.

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12

��Ghosh M, Shen Z, Fahey J, et al. Trappin-2/Elafin: a novel innate antihumanimmunodeficiency virus-1 molecule of the human female reproductive tract.Immunology 2010; 129:207–219.

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�Ding J, Rapista A, Teleshova N, et al. Mucosal human defensins 5 and 6antagonize the anti-HIV activity of candidate polyanion microbicides. J InnateImmunol 2011; 3:208–212.

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14

�Kim K, Yolamanova M, Zirafi O, et al. Semen-mediated enhancement of HIVinfection is donor-dependent and correlates with the levels of SEVI. Retro-virology. 2010; 7:55.

This study describes how one of the many factors present in semen has a highimpact on HIV infectivity.

15 Stoiber H, Pruenster M, Ammann C, Dierich M. Complement-opsonized HIV:the free rider on its way to infection. Mol Immunol 2005; 42:153–160.

16 Bouhlal H, Chomont N, Requena M, et al. Opsonization of HIV with comple-ment enhances infection of dendritic cells and viral transfer to CD4 T cells in aCR3 and DC-SIGN-dependent manner. J Immunol 2007; 178:1086–1095.

17 Thurman AR, Doncel GF. Innate immunity and inflammatory response toTrichomonas vaginalis and bacterial vaginosis: relationship to HIV acquisition.Am J Reprod Immunol 2011; 65:89–98.

18

�De Jong M, de Witte L, Taylor M, Geijtenbeek T. Herpes simplex virus type 2enhances HIV-1 susceptibility by affecting Langerhans cell function. J Im-munol 2010; 185:1633–1641.

This study identifies Langerhans cell maturation induced by HSV-2 via TLR3signaling as the one potential mechanism leading to increased HIV susceptibility inHSV-2 infected individuals.

19 Li Q, Estes J, Schlievert P, et al. Glycerol monolaurate prevents mucosal SIVtransmission. Nature 2009; 458:1034–1038.

20 Freeman E, Weiss H, Glynn J, et al. Herpes simplex virus 2 infection increasesHIV acquisition in men and women: systematic review and meta-analysis oflongitudinal studies. AIDS 2006; 20:73–83.

21 Feldblum P, Adeiga A, Bakare R, et al. SAVVY vaginal gel (C31G) forprevention of HIV infection: a randomized controlled trial in Nigeria. PLoSOne 2008; 3:e1474.

22 Halpern V, Ogunsola F, Obunge O, et al. Effectiveness of cellulose sulfatevaginal gel for the prevention of HIV infection: results of a phase III trial inNigeria. PLoS One 2008; 3:e3784.

23 Iqbal S, Ball T, Levinson P, et al. Elevated elafin/trappin-2 in the female genitaltract is associated with protection against HIV acquisition. AIDS 2009;23:1669–1677.

24 Sohn J, Bora P, Suk H, et al. Tolerance is dependent on complement C3fragment iC3b binding to antigen-presenting cells. Nat Med 2003; 9:206–212.

25 Verbovetski I, Bychkov H, Trahtemberg U, et al. Opsonization of apoptoticcells by autologous iC3b facilitates clearance by immature dendritic cells,down-regulates DR and CD86, and up-regulates CC chemokine receptor 7. JExp Med 2002; 196:1553–1561.

26 Burgener A, Boutilier J, Wachihi C, et al. Identification of differentiallyexpressed proteins in the cervical mucosa of HIV-1-resistant sex workers.J Proteasome Res 2008; 7:4446–4454.

27 Wira C, Fahey J, Ghosh M, et al. Hormone regulation of innate immunity in thefemale reproductive tract: the role of epithelial cells in balancing reproductivepotential with protection against sexually transmitted pathogens. Am J ReprodImmunol 2010; 63:544–565.

28

�Ahmed N, Hayashi T, Hasegawa A, et al. Suppression of human immunode-ficiency virus type 1 replication in macrophages by commensal bacteriapreferentially stimulating Toll-like receptor 4. J Gen Virol 2010; 91:2804–2813.

This study examines how TLR stimulation by commensal bacteria affects HIVsusceptibility, although performed on a single cell type.

29 Thibault S, Fromentin R, Tardif M, Tremblay M. TLR2 and TLR4 triggering exertscontrasting effects with regard to HIV-1 infection of human dendritic cells andsubsequent virus transfer to CD4þ T cells. Retrovirology 2009; 6:42.

30

�Mosoian A, Teixeira A, Burns C, et al. Prothymosin-alpha inhibits HIV-1 via Toll-like receptor 4-mediated type I interferon induction. Proc Natl Acad Sci USA2010; 107:10178–10183.

This study describes a molecule naturally produced by CD8þ T cells that acts as aTLR4 ligand and stimulates type-I IFN production to potently suppress HIV-1 afterentry into cells.

31

��Fraietta J, Mueller Y, Do D, et al. Phosphorothioate 20 deoxyribose oligomersas microbicides that inhibit human immunodeficiency virus type 1 (HIV-1)infection and block Toll-like receptor 7 (TLR7) and TLR9 triggering by HIV-1.Antimicrob Agents Chemother 2010; 54:4064–4073.

This study shows that HIV can be suppressed by inhibiting TLR7 and TLR9signaling. The inhibitor used did not appear to be toxic in vitro, and may thereforehave therapeutic potential if studied further.

32

��Gringhuis S, van der Vlist M, van den Berg L, et al. HIV-1 exploits innatesignaling by TLR8 and DC-SIGN for productive infection of dendritic cells. NatImmunol 2010; 11:419–426.

This study sheds light on a mechanism exploited by HIV to achieve productiveinfection in dendritic cells, which could possibly be targeted therapeutically, as theinhibitor used, GW5074, does not appear to be toxic.


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8 Innate immunity

33

�Lu J, Pan Q, Rong L, et al. The IFITM proteins inhibit HIV-1 infection. J Virol2011; 85:2126–2137.

This study suggests that induction of IFITM proteins through type-I IFN is animportant innate mechanism limiting HIV infection by interfering with viral entry.

34

��Manel N, Hogstad B, Wang Y, et al. A cryptic sensor for HIV-1 activatesantiviral innate immunity in dendritic cells. Nature 2010; 467:214–217.

This study shows that HIV has developed evasion strategies to avoid the triggeringof TLR-mediated responses in dendritic cells, which suggests that targeting TLRsmay be a viable approach for HIV intervention.

35

�Yan N, Regalado-Magdos A, Stiggelbout B, et al. The cytosolic exonucleaseTREX1 inhibits the innate immune response to human immunodeficiency virustype 1. Nat Immunol 2010; 11:1005–1013.

This study describes the mechanism whereby HIV avoids triggering of TLRresponses in dendritic cells. Further elucidation of this mechanism may lead toHIV-intervention strategies.

36 Ashkar A, Yao X, Gill N, et al. Toll-like receptor (TLR)-3, but not TLR4, agonistprotects against genital herpes infection in the absence of inflammation seenwith CpG DNA. J Infect Dis 2004; 190:1841–1849.

37

�Zhou Y, Wang X, Liu M, et al. A critical function of toll-like receptor-3 in theinduction of antihuman immunodeficiency virus activities in macrophages.Immunology 2010; 131:40–49.

This shows that TLR3 stimulation can confer HIV protection in macrophagesthrough the upregulation of APOBEC3G and tetherin as well as HIV-inhibitorymicroRNAs.

38 Ogawa Y, Kawamura T, Kimura T, et al. Gram-positive bacteria enhance HIV-1susceptibility in Langerhans cells, but not in dendritic cells, via Toll-likereceptor activation. Blood 2009; 113:5157–5166.

39

�Brichacek B, Vanpouille C, Kiselyeva Y, et al. Contrasting roles for TLR ligandsin HIV-1 pathogenesis. PLoS One 2010; 5:e12831.

This study examines the effect of stimulating different TLRs on HIV susceptibility.

40 Trifonova RT, Doncel GF, Fichorova RN. Polyanionic microbicides modify Toll-like receptor-mediated cervicovaginal immune responses. Antimicrob AgentsChemother 2009; 53:1490–1500.

41 Androulidaki A, Iliopoulos D, Arranz A, et al. The kinase Akt1 controlsmacrophage response to lipopolysaccharide by regulating microRNAs. Im-munity 2009; 31:220–231.

42 Benakanakere MR, Li Q, Eskan MA, et al. Modulation of TLR2 proteinexpression by miR-105 in human oral keratinocytes. J Biol Chem 2009;284:23107–23115.

43 Ceppi M, Pereira PM, Dunand-Sauthier I, et al. MicroRNA-155 modulates theinterleukin-1 signaling pathway in activated human monocyte-derived den-dritic cells. Proc Natl Acad Sci U S A 2009; 106:2735–2740.

44

��Liu X, Zhan Z, Xu L, et al. MicroRNA-148/152 impair innate response andantigen presentation of TLR-triggered dendritic cells by targeting CaMKIIal-pha. J Immunol 2010; 185:7244–7251.

This study suggests that inhibiting TLR signaling may have unwanted effects suchas impairing innate responses or Ag-presenting capacity of dendritic cells.

45

�Hong B, Song X, Rollins L, et al. Mucosal and systemic anti-HIV immunitycontrolled by A20 in mouse dendritic cells. J Clin Invest 2011; 121:739–751.

This study describes a promising adjuvant that modulates innate immune re-sponses.

46 Saphire A, Bobardt M, Gallay P. Human immunodeficiency virus type 1 hijackshost cyclophilin A for its attachment to target cells. Immunol Res 2000; 21(2–3):211–217.

47

��Daelemans D, Dumont J, Rosenwirth B, et al. Debio-025 inhibits HIV-1 byinterfering with an early event in the replication cycle. Antiviral Res 2010;85:418–421.

This study shows that HIV can be inhibited using a CypA inhibitor, which may bedeveloped further and used therapeutically.

48

��Shi J, Zhou J, Shah V, et al. Small-molecule inhibition of human immunode-ficiency virus type 1 infection by virus capsid destabilization. J Virol 2011;85:542–549.

This study introduces a molecule that mimics the premature uncoating of HIVachieved by simian TRIM5a, which may have therapeutic potential.

49

��Singh R, Gaiha G, Werner L, et al. Association of TRIM22 with the type 1interferon response and viral control during primary HIV-1 infection. J Virol2011; 85:208–216.

This study demonstrates the HIV-protective properties of TRIM22, although itsmechanisms of action are not clear.

50 Sheehy AM, Gaddis NC, Choi JD, Malim MH. Isolation of a human gene thatinhibits HIV-1 infection and is suppressed by the viral Vif protein. Nature2002; 418:646–650.

51 Wang Y, Bergmeier L, Stebbings R, et al. Mucosal immunization in macaquesupregulates the innate APOBEC 3G antiviral factor in CD4(þ) memory Tcells. Vaccine 2009; 27:870–881.


52

�Kim E, Bhattacharya T, Kunstman K, et al. Human APOBEC3G-mediatedediting can promote HIV-1 sequence diversification and accelerate adapta-tion to selective pressure. J Virol 2010; 84:10402–10405.

This study suggests that APOBEC3G can help generate viral sequence diversi-fication and the evolution of beneficial viral variants.

53

�Sadler H, Stenglein M, Harris R, Mansky L. APOBEC3G contributes to HIV-1variation through sublethal mutagenesis. J Virol 2010; 84:7396–7404.

This study raises the concern that APOBEC3G upregulation may raise HIVmutagenicity that leads to the evolution of immune escape and drug-resistantphenotypes.

54 Neil S, Zang T, Bieniasz P. Tetherin inhibits retrovirus release and is antag-onized by HIV-1 Vpu. Nature 2008; 451:425–430.

55 Malim MH, Emerman M. HIV-1 accessory proteins – ensuring viral survival in ahostile environment. Cell Host Microbe 2008; 3:388–398.

56 Neil S, Bieniasz P. Human immunodeficiency virus, restriction factors, andinterferon. J Interferon Cytokine Res 2009; 29:569–580.

57 Khan M, Kao S, Miyagi E, et al. Viral RNA is required for the association ofAPOBEC3G with human immunodeficiency virus type 1 nucleoprotein com-plexes. J Virol 2005; 79:5870–5874.

58

�Albin J, Hache G, Hultquist J, et al. Long-term restriction by APOBEC3Fselects human immunodeficiency virus type 1 variants with restored Viffunction. J Virol 2010; 84:10209–10219.

This study suggests that HIV can adapt to replicate efficiently in an environmentwhere APOBEC3G is upregulated.

59

�Ma J, Li X, Xu J, et al. The cellular source for APOBEC3G’s incorporation intoHIV-1. Retrovirology 2011; 8:2.

This study investigates the intracellular regulation of APOBEC3G activity throughactive and inactive forms of the protein.

60

�Farrow M, Kim E, Wolinsky S, Sheehy A. NFAT and IRF proteins regulatetranscription of the anti-HIV gene, APOBEC3G. J Biol Chem 2011;286:2567–2577.

This study describes the potential mechanisms that regulate APOBEC3G tran-scription.

61 Van Damme N, Goff D, Katsura C, et al. The interferon-induced protein BST-2restricts HIV-1 release and is downregulated from the cell surface by the viralVpu protein. T Cell Host Microbe 2008; 3:245–252.

62

�Casartelli N, Sourisseau M, Feldmann J, et al. Tetherin restricts productiveHIV-1 cell-to-cell transmission. PLoS Pathog 2010; 6:e1000955.

This study verifies tetherin’s antiviral activity, and expands its known functions toinclude inhibition of cell–cell transmission.

63 Cao W, Bover L, Cho M, Wen X, et al. Regulation of TLR7/9 responses inplasmacytoid dendritic cells by BST2 and ILT7 receptor interaction. J ExpMed 2009; 206:1603–1614.

64

�Favre D, Mold J, Hunt P, et al. Tryptophan catabolism by indoleamine 2,3-dioxygenase 1 alters the balance of TH17 to regulatory T cells in HIV disease.Sci Transl Med 2010; 2:32–36.

This study shows that IDO mediates the reduction of Th17 T cells leading to thedisruption of the mucosal barrier.

65 Manches O, Munn D, Fallahi A, et al. HIV-activated human plasmacytoid DCsinduce Tregs through an indoleamine 2,3-dioxygenase-dependent mechan-ism. J Clin Invest 2008; 2008:10:3431–3439.

66

��Martinson J, Montoya C, Usuga X, et al. Chloroquine modulates HIV-1-induced plasmacytoid dendritic cell alpha interferon: implication for T-cellactivation. Antimicrob Agents Chemother 2010; 54:871–881.

This study shows that chloroquine may be useful as a therapeutic agent in HIV-infected patients with chronic immune activation as it downregulates IFN andnegative T-cell responses.

67

��Melki M, Saıdi H, Dufour A, et al. Escape of HIV-1-infected dendritic cells fromTRAIL-mediated NK cell cytotoxicity during NK-DC cross-talk: a pivotal role ofHMGB1. PLoS Pathog 2010; 6:e1000862.

This study describes an immune evasion strategy that may be targeted for HIVinhibition once its mechanisms have been fully elucidated. The HMBG1 inhibitorused, Glycyrrhizin is in clinical use for other purposes and is anti-inflammatory,making it a viable therapeutic candidate.

68

�Vieillard V, Fausther-Bovendo H, Samri A, et al. Specific phenotypic andfunctional features of natural killer cells from HIV-infected long-term nonpro-gressors and HIV controllers. J Acquir Defic Syndr 2010; 53:564–573.

This study investigates what differences in NK-cell phenotype that aid in the controlof HIV infection.

69

�Shah AH, Sowrirajan B, Davis ZB, et al. Degranulation of natural killer cellsfollowing interaction with HIV-1-infected cells is hindered by downmodulationof NTB-A by Vpu. Cell Host Microbe 2010; 8:397–409.

This study deals with a HIV strategy to avoid NK-mediated lysis of infected cells.

70 Altfeld M, Fadda L, Frleta D, Bhardwaj N. DCs and NK cells: critical effectorsin the immune response to HIV-1. Nat Rev Immunol 2011; 11:176–186.


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71 Herbeuvala J-P, Shearerb GM. HIV-1 immunopathogenesis: how good inter-feron turns bad. Clin Immunol 2007; 123:121–128.

72

�Sabado R, O’Brien M, Subedi A, et al. Evidence of dysregulation of dendriticcells in primary HIV infection. Blood 2010; 116:3839–3852.

This study describes hyperresponsiveness in mDCs and pDCs and indicates thatthey play a key role in chronic immune activation.

73 Blankson J. Control of HIV-1 replication in elite suppressors. Discov Med2010; 9:261–266.

74

�Chang J, Lindsay R, Kulkarni S, et al. Polymorphisms in interferon regulatoryfactor 7 reduces interferon-a responses of plasmacytoid dendritic cells toHIV-1. AIDS 2011; 25:715–717.

This study elucidates the control of pDC IFN production by IRF-7. Furtherelucidation of the regulation of IFN production will be valuable for the controlof chronic immune activation.

75 Stratov I, Chung A, Kent SJ. Robust NK cell-mediated human immunodefi-ciency virus (HIV)-specific antibody-dependent responses in HIV-infectedsubjects. J Virol 2008; 82:5450–5459.


76 Alter G, Teigen N, Davis B, et al. Sequential deregulation of NK cell subsetdistribution and function starting in acute HIV-1 infection. Blood 2005;106:3366–3369.

77

�Aldhamen Y, Appledorn D, Seregin S, et al. Expression of the SLAM family ofreceptors adapter EAT-2 as a novel strategy for enhancing beneficial immuneresponses to vaccine antigens. J Immunol 2011; 186:722–732.

This study describes an adjuvant that acts by activating dendritic cells andmacrophages, which may be useful in a HIV vaccine.

78

�Strbo N, Vaccari M, Pahwa S, et al. Gp96(SIV)Ig immunization induces potentpolyepitope specific, multifunctional memory responses in rectal and vaginalmucosa. Vaccine 2011; 29:2619–2625.

This study suggests the use of a heat shock protein as an adjuvant to enhancemucosal vaccine responses.

79

��Abdool Karim Q, Abdool Karim S, Frohlich J, et al. Effectiveness and safety oftenofovir gel, an antiretroviral microbicide, for the prevention of HIV infection inwomen. Science 2010; 329:1168–1174.

This study indicates that mucosal administration of antiviral agents can conferprotection against HIV.


Targeting HIV-1 innate immune responses therapeutically

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