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THE INTRODUCTION FOUR years ago of highlyactive anti-retroviral therapy1,2 (HAART)

for the treatment of human immunodefi-ciency virus (HIV)-1 infection has resultedin a substantial decrease in death ratesfrom AIDS in the US (ref. 3). Most HAARTregimens consist of combinations of threedrugs, typically two inhibitors of HIV-1 re-verse transcriptase and one inhibitor ofHIV-1 protease. In many patients respond-ing to HAART, the amount of free virus inthe plasma decreases to undetectable levels(fewer than 50 copies of genomic viralRNA/ml). Drugs targeting the other essen-tial viral enzyme, integrase, and drugs tar-geting the initial step in the virus life cycle,virus entry, are being studied and mayeventually become part of HAART regi-mens. In contrast to the rapid progressmade in the development of anti-retroviraldrugs, less progress has been made in thedevelopment of drugs that target the cellsin which the virus replicates. A paper byChapuis et al. in the current issue of NatureMedicine is a step in the direction of target-ing the cells in which HIV replicates4.

The principal target cell for HIV-1 in vivois the activated CD4+ T lymphocyte5. Thevirus does not replicate in CD4+ T cells thatare in a resting state, although it can estab-lish a state of latent infection in these cells6.Targeting activated CD4+ T cells has theobvious problem that it may cause addi-tional immunosuppression. However, withHAART, virus-induced immunosuppres-sion is halted and partially reversed, and itis conceivable that therapeutic strategiesdirected at activated CD4+ T cells mightbenefit patients by helping to contain thelow level of viral replication that continuesin patients on HAART (ref. 7). With thisidea in mind, Chapuis et al. studied theeffects of the immunosuppressive agentmycophenolic acid (MPA) on HIV-1 repli-cation in vitro and in vivo. MPA is aninhibitor of inosine monophosphate dehy-drogenase, an enzyme involved in theguanosine nucleotide synthesis pathway.Lymphocytes depend on de novo synthesisof purines, whereas many other proliferat-ing cells can generate purines throughboth the de novo and salvage pathways.Therefore, MPA selectively inhibits lym-phocyte proliferation. Mycophenolatemofetil (MMF, or CellCept®), the 2-mor-

pholinoethyl ester of MPA, is used in con-junction with other immunosuppressiveagents to prevent allograft rejection in kid-ney transplant recipients.

In principle, MPA might inhibit HIV-1replication through several mechanisms(Fig. 1). By depleting pools of guanosinenucleotides, it might inhibit reverse tran-scription, as the synthesis of proviral DNAby reverse transcriptase is dependent onprecursor nucleotides, as is the synthesis ofcellular DNA. Moreover, depletion of nu-cleotide pools might potentiate the actionof abacavir, a potent HIV-1 reverse tran-scriptase inhibitor that is a guanosine ana-log. Substantial synergy between abacavirand MPA in the inhibition of HIV-1 replica-tion has been described in an in vitro study8.Finally, because MPA inhibits cellular DNA

synthesis in lymphocytes, it actually causesthe death of activated CD4+ T cells, therebyreducing the number of susceptible targetcells for HIV-1 replication. Chapuis et al.confirm that the drug causes the death ofactivated CD4+ T cells in vitro4. In addition,they show in a small clinical study that theadministration of MMF results in a sub-stantial reduction in the number of prolif-erating CD4+ and CD8+ T cells in patientson HAART. Total numbers of CD4 andCD8 cells were stable over the course of thestudy. Thus, it seems that the drug mightbe capable of reducing the number of sus-ceptible targets cells without affecting theoverall size of the lymphocyte compart-ment and hopefully without producingsubstantial immunosuppression.

At present, it remains unclear whetheraddition of immunosuppressive agents likeMMF to HAART regimens will have clinicalbenefit. The patients studied by Chapuis etal. all had excellent suppression of viral

Taking aim at HIV replicationImpressive progress has been made in the development of drugs that inhibit human immunodeficiency virus replication.A new study indicates that mycophenolic acid, a selective inhibitor of lymphocyte proliferation, might also be useful in

controlling viral replication through several mechanisms (pages 762–768).

DIANA FINZI & ROBERT F. SILICIANO

MPART1, MPA P1

MPA? MPA?

NaiveCD4+

T cell

ActivatedCD4+

T cell

Free virus

MemoryCD4+

T cell

MemoryCD4+

T cell

+Ag

_Ag _Ag Postintegrationlatency

Fig. 1 When resting CD4+ T cells encounter the appropriate foreign antigen (Ag), they enlarge,enter the cell cycle and proliferate. These activated cells fulfill their functions, and then some revertback to a resting state as memory cells capable of responding to future encounters with the sameantigen. A gradual process of proliferative renewal may maintain the memory cell pool. While in theactivated state, CD4+ T cells are very susceptible to productive infection by HIV-1. The infection pro-gresses quickly through the successive steps of virus entry, reverse transcription, integration of the re-verse transcribed HIV-1 DNA (red line) into the host genome, viral gene expression and virusproduction. Many of these activated infected cells die from cytopathic effects of the virus or host im-munological mechanisms. However, some of the productively infected CD4+ T cells escape both theviral cytopathic effects and immunological effector mechanisms and revert to a resting state carryingintegrated provirus. In the resting state, these memory CD4+ T cells are likely to have little or no viralgene expression and are therefore also ‘invisible’ to the immune system. Because the viral DNA is inan integrated state in these cells, it is very stable. Thus, cells in this state of post-integration latencypotentially represent a long-term, stable reservoir for the virus. This reservoir may also be maintainedby proliferative renewal. Reverse transcriptase inhibitors (RTI) prevent the formation of a double-stranded DNA copy of the RNA genome, and protease inhibitors (PI) prevent the maturation of thevirus particle. Mycophenolic acid (MPA) acts by decreasing the proliferation of activated CD4+ T cells,by potentiating the activity of certain reverse transcriptase inhibitors and, possibly, by interferingwith the proliferative renewal of latently infected cells.

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HIGHLY ACTIVE RETROVIRAL therapy (HAART)can control replication of human im-

munodeficiency virus (HIV)-1 in vivo soeffectively that the virus becomes unde-tectable in the plasma. Based on experi-mental observations in conjunction withtheoretical projections, it was suggestedthat virions and productively infected cellpopulations could be completely elimi-nated in 2–3 years on HAART, assumingthe absence of additional viral reservoirs orresidual replication1. However, as this asser-

tion was made, reports emerged describingthe presence of an unforeseen latent reser-voir comprising infectious provirus inte-grated quiescently within resting memoryCD4 T cells2, as well as its persistence onprolonged treatment3. Moreover, manylaboratories subsequently found evidenceof residual viral replication, albeit at lowlevels, despite complete suppression of

plasma viremia on HAART (refs. 4,5). It istherefore not unexpected that HIV-1 re-bounds promptly after cessation of drugtherapy6. In this issue of Nature Medicine,Chun et al.7 describe results of a compari-son between the genotype of the rebound-ing virus and that in the latent reservoir. Ofnine patients studied, six had a detectablelatent reservoir before discontinuing ther-apy. The rebounding virus was geneticallyclosely related to the latent reservoir virusin two cases, but non-identity of viruses

HIV-1 rebound after anti-retroviral therapyIn patients with human immunodeficiency virus 1, a comparison of the genotype of rebounding virus to that in the

latent reservoir after highly active anti-retroviral therapy is discontinued indicates that the latent reservoir is not thecause of viral rebound in most cases. However, the precise anatomic location of the source of viral rebound is still

undefined (pages 757–761).

DAVID D. HO & LINQI ZHANG

736 NATURE MEDICINE • VOLUME 6 • NUMBER 7 • JULY 2000

NEWS & VIEWS

replication on an abacavir-containingHAART regimen for at least 36 weeks beforethe addition of MMF to the regimen.Therefore, it is difficult to determine usingstandard assays of plasma HIV-1 RNAwhether the addition of MMF allowed bet-ter control of viral replication. However,the authors do report that the frequency ofperipheral blood T cells containing replica-tion-competent virus was substantially re-duced (one log or more) in three of sixpatients on HAART plus MMF. Similar re-ductions were not found in control pa-tients on HAART alone. The mechanism ofthis antiviral effect is unclear. The analysisdone by Chapuis et al. detected a decreasein the amount of virus that could be cul-tured from combined pools of resting andactivated CD4+ T cells. It is apparent that inpatients on HAART, HIV-1 can persist forlong periods of time in a latent form in rest-ing CD4+ T cells9–12. Because the restingCD4+ T cells that contain latent provirusare not likely to be affected by MPA, the ob-served reductions may reflect a decrease inthe number of activated CD4+ T cells thatsustain the very low level of active replica-tion that continues in patients on HAART(ref. 7). Alternatively, the drug may inter-fere with the proliferative renewal of thememory T-cell compartment that containslatent virus. In most patients on HAART,the decay rate of the pool of latentlyinfected cells is so slow that lifetimepersistence of the virus is guaranteed12.Therefore, it will be essential to determinewhether the addition of MMF actually ac-celerates the decay of the latent reservoir.This will require additional longitudinalstudies using special assays that detect la-

tently infected cells.The idea of using drugs that target acti-

vated CD4+ T cells to treat HIV-1 infectionis not new. Previous studies have focusedon hydroxyurea, a cytostatic drug used totreat some myeloproliferative disorders.Hydroxyurea inhibits ribonucleosidediphosphate reductase, a cellular enzymethat converts ribonucleotides to deoxyri-bonucleotides. It therefore inhibits synthe-sis of proviral DNA by reverse transcriptaseas well as the synthesis of cellular DNA inproliferating cells13. Decreased intracellularlevels of dNTPs lead to cell cycle arrest andalso increase the probability that chain-ter-minating nucleoside analog inhibitors ofreverse transcriptase will be incorporatedduring proviral DNA synthesis. Thus,hydroxyurea potentiates the activity ofnucleoside analog inhibitors of reversetranscriptase, particularly didanosine.Hydroxyurea has been used in the treat-ment of primary HIV-1 infection inconjunction with didanosine and pro-tease inhibitors with impressive results14.Determining the precise therapeutic func-tion of drugs like MMF and hydroxyureawill require mechanistic studies of the ef-fects of these drugs on residual viral replica-tion and the decay of the latent reservoir,an evaluation of long-term toxicities ofthese agents in the setting of HAART and,ultimately, a demonstration of clinicalbenefit.

1. Gulick, R.M. et al. Treatment with indinavir, zidovu-dine, and lamivudine in adults with human immunod-eficiency virus infection and prior antiretroviraltherapy. N. Engl. J. Med. 337, 734–739 (1997).

2. Hammer, S.M. et al. A controlled trial of two nucleo-side analogues plus indinavir in persons with humanimmunodeficiency virus infection and CD4 cell counts

of 200 per cubic millimeter or less. AIDS Clinical TrialsGroup 320 Study Team. N. Engl. J. Med. 337, 725–733(1997).

3. Palella, F.J. et al. Declining morbidity and mortalityamong patients with advanced human immunodefi-ciency virus infection. N. Engl. J. Med. 338, 853-860(1998).

4. Chapuis, A.G. et al. Effects of mycophenolic acid onhuman immunodeficiency virus infection in vitro andin vivo. Nature Med. 6, 762–768 (2000).

5. Haase, A. T. Population biology of HIV-1 infection:Viral and CD4+ T cell demographics and dynamics inlymphatic tissues. Annu. Rev. Immunol. 17, 625-656(1999).

6. Chun, T.-W. et al. Fate of HIV-1-infected T cells in vivo:Rates of transition to stable latency. Nature Med. 1,1284–1290 (1995).

7. Dornadula, G. et al. Residual HIV-1 RNA in bloodplasma of patients taking suppressive highly activeantiretroviral therapy. J. Am. Med. Assoc. 282,1627–1632 (1999).

8. Margolis, D. et al. Abacavir and mycophenolic acid, aninhibitor of inosine monophosphate dehydrogenase,have profound and synergistic anti-HIV activity. J.Acquir. Immune Defic. Syndr. 21, 362–370 (1999).

9. Finzi, D. et al. Identification of a reservoir for HIV-1 inpatients on highly active antiretroviral therapy. Science278, 1295–1300 (1997).

10. Wong, J. K. et al. Recovery of replication-competentHIV despite prolonged suppression of plasma viremia.Science 278, 1291–1295 (1997).

11. Chun, T.W. et al. Presence of an inducible HIV-1 latentreservoir during highly active antiretroviral therapy.Proc. Natl. Acad. Sci. USA 94, 13193–13197 (1997).

12. Finzi, D. et al. Latent infection of CD4+ T cells providesa mechanism for lifelong persistence of HIV-1, even inpatients on effective combination therapy. NatureMed. 5, 512–517 (1999).

13. Lori, F. et al. Hydroxyurea as an inhibitor of human im-munodeficiency virus-type 1 replication. Science 266,801–805 (1994).

14. Lori, F. et al. Treatment of human immunodeficiencyvirus infection with hydroxyurea, didanosine, and aprotease inhibitor before seroconversion is associatedwith normalized immune parameters and limited viralreservoir. J. Infect. Dis. 180, 1827–1832 (1999).

Department of MedicineJohns Hopkins University School of MedicineRoom 1049 Ross Building720 Rutland AvenueBaltimore, Maryland 21205, USAEmail: rsilicia@welch.jhu.edu

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