Citation: Apetrei C, Pandrea I, Mellors JW (2012) Nonhuman Primate Models for HIV Cure Research. PLoS Pathog 8(8): e1002892. doi:10.1371/journal.ppat.1002892
Editor: Richard A. Koup, National Institutes of Health, National Institute of Allergy and Infectious Diseases, United States of America
Published: August 30, 2012
Copyright: © Apetrei et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Drs Apetrei and Pandrea are supported by grant R01 RR025781 from the NIH/NCRR. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Shytaj et al.  report that complete suppression of SIVmac replication can be achieved in rhesus macaques by a combination of five antiretroviral (ARV) drugs, which the authors term “mega-HAART”. This combination consists of the three-drug nucleoside reverse transcriptase inhibitor (tenofovir/emtricitabine) and integrase inhibitor (raltegravir) regimen often used in treatment studies of simian immunodeficiency virus (SIV)-infected rhesus macaques, intensified with the protease inhibitor darunavir (pharmacokinetically enhanced by ritonavir) and the CCR5 antagonist maraviroc. Achieving complete suppression of SIVmac in rhesus macaques is an important step in developing an animal model for HIV-1 cure research because it parallels the effects of antiretroviral therapy in HIV-infected humans. Without complete suppression, testing of therapeutic strategies to reduce viral reservoirs is confounded by ongoing cycles of viral replication that can replete such reservoirs.
Over the last two decades, rhesus macaque models of AIDS have revealed key aspects of HIV-1 pathogenesis, such as virus transmission and early events post-infection, the sites of viral replication and CD4+ T cell depletion, and virus and cell turnover –. These models have also been instrumental for vaccine research, allowing the evaluation of increasingly potent DNA and vector immunogens and combinations of these vectors in various prime-boost combinations –. Macaque models of preexposure prophylaxis (PrEP) have also helped elucidate the ARV exposures and the timing of exposure required to maximize protection from virus challenge .
By contrast, the rhesus macaque/SIV model has contributed less to the development and optimization of ARV therapy . The main reasons for this include the natural resistance of SIVs to nonnucleoside reverse transcriptase inhibitors (NNRTIs) , , major differences in ARV pharmacokinetics between humans and macaques , , and divergent interactions of SIVmac and HIV-1 with host restriction factors . Importantly, because the pandemic HIV-1 subtypes do not replicate in monkey species , simian counterparts derived from naturally infected sooty mangabeys, such as SIVmac/smm, must be used . Although SIVsmm is not completely distinct from its HIV relatives, being the cause of the HIV-2 epidemic , the differences in ARV susceptibility and pharmacokinetics have restricted the use of the RM/SIVmac models for antiretroviral therapy.
Nevertheless, many ARVs are active in vitro and in vivo against SIVmac , , and studies of antiretroviral therapy (ART) in macaques have been reported –. In most of these studies, however, complete control of viral replication has not been achieved . In addition to suboptimal pharmacokinetics, failure to achieve complete control of viral replication is likely related to the biology of SIVmac infection in rhesus macaques. SIVmac is more virulent than HIV-1 . The set point of viremia in SIV-infected macaques is 10- to 100-fold higher than in HIV-1 infection, and progression to AIDS occurs in 1–2 years and more quickly (<1 year) in up to 40% of macaques , . Alternatives to SIVmac have been reported, most notably the use of chimeric HIV-SIV viruses (called simian-human immunodeficiency viruses or SHIVs) in which SIVmac reverse transcriptase (RT) is replaced by HIV-1 RT (RT-SHIVs) –. RT-SHIVs have the advantage of being susceptible to both nucleoside and non-nucleoside RT inhibitors similar to HIV-1. These chimeric viruses also have limitations, most notably that, similar to the parental virus, RT-SHIVmac is difficult to suppress with the same three-drug combination (tenofovir/emtricitabine/efavirenz) that is most commonly used in humans . As an alternative approach, RT-SHIVmne was constructed using the SIVmne isolate from pigtailed macaques and is used to infect this species. RT-SHIVmne is less virulent than SIVmac and not infrequently can be controlled by the host without intervention. Even so, ART with tenofovir/emtricitabline/efavirenz failed to completely control RT-SHIVmne replication in chronically infected pigtailed macaques, as evidenced by both persistent viral replication and sequence evolution under treatment , .
The recent report that one patient (“the Berlin patient”) was cured of HIV infection  has renewed enthusiasm for a “cure research” aimed at understanding the mechanisms of HIV-1 persistence and developing therapeutic strategies to reduce and ultimately eliminate viral reservoirs. Limitations of human clinical studies, especially invasive sampling of multiple reservoir sites, make it imperative to develop analogous and tractable animal models for cure research. Several groups are trying to achieve this goal by (i) ARV intensification in SIVmac-infected rhesus macaques to completely suppress viral replication, similar to Shytaj et al.  (J. Lifson, unpublished data; P. Luciw, unpublished data), (ii) use of Chinese macaques infected with SIVmac (B. Ling, unpublished data), in which viral replication at set point viral replication is lower than in Indian rhesus macaques , which have dominated AIDS research, and (iii) development of an animal model of functional cure in the absence of ART . Additionally, humanized mouse models of HIV-1 infection have been developed for cure research .
How these varied animal models should be used for cure research is not well defined. Several different therapies could have contributed to the cure of the Berlin patient, including myeloablative chemotherapy, total body irradiation, transplantation of allogeneic Δ32/Δ32stem cells, immunosuppressive agents, and graft-versus-host disease, for which animal models are impractical . Irrespective of the mechanisms of the cure, careful characterization of viral reservoirs is the keystone of cure research. Such studies should involve invasive sampling and detailed description of the sites of virus persistence on completely suppressive ART, as well as identifying the source of viral rebound after cessation of ART. New therapeutic strategies can then be tested, including selective activators of viral expression, reversal of immune exhaustion, and enhancement of viral specific immune responses, with careful quantification of effects on viral reservoirs in different anatomic sites, including the brain. For obvious ethical reasons, such studies cannot be performed in humans. In addition, because the time of infection and the virus inocula can be controlled in animal models, studies can be performed to assess the optimal timing of cure interventions as has been reported for PrEP . In addition, animal models for cure research can establish “proof of concept” for the many new therapeutic strategies emerging in the field before testing in humans. It is essential, however, that the therapeutic benefits observed in animal models are validated in human studies (and vice versa), and that the cause of discrepancies be elucidated. Because none of the current animal models perfectly reproduce HIV-1 infection and ART, it is likely that several different models will be needed to understand virus persistence, latency, reactivation, and eradication.
As such, establishing a nonhuman primate model of complete viral suppression as reported by Shytaj et al.  is a step forward for cure research. One may argue that achieving complete control of viral replication is just a baby step towards the overall goal of virus eradication and that much more needs to be accomplished with regard to developing more sensitive virological assays , small molecules to activate latent virus –, and biologics to clear viral reservoirs , . Nevertheless, approaches like that of Shytaj et al.  indicate that control of viral replication is possible in macaques if drug potent, multidrug combinations are employed and that nonhuman primate models for cure research are here to stay.
We would like to thank Jeffrey Lifson, Louis Picker, Guido Silvestri and Zandrea Ambrose for helpful discussion.
- 1. Shytaj IL, Norelli S, Chirullo B, Della Corte A, Collins M, et al. (2012) A highly intensified ART regimen induces long-term viral suppression and restriction of the viral reservoir in a simian AIDS model. PLoS Pathog 8: e1002774 doi:10.1371/journal.ppat.1002774.
- 2. Haase AT (2010) Targeting early infection to prevent HIV-1 mucosal transmission. Nature 464: 217–223.
- 3. Lackner AA, Veazey RS (2007) Current concepts in AIDS pathogenesis: insights from the SIV/macaque model. Annu Rev Med 58: 461–476.
- 4. Veazey RS, DeMaria M, Chalifoux LV, Shvetz DE, Pauley DR, et al. (1998) Gastrointestinal tract as a major site of CD4+ T cell depletion and viral replication in SIV infection. Science 280: 427–431.
- 5. Smit-McBride Z, Mattapallil JJ, McChesney M, Ferrick D, Dandekar S (1998) Gastrointestinal T lymphocytes retain high potential for cytokine responses but have severe CD4(+) T-cell depletion at all stages of simian immunodeficiency virus infection compared to peripheral lymphocytes. J Virol 72: 6646–6656.
- 6. Li Q, Duan L, Estes JD, Ma ZM, Rourke T, et al. (2005) Peak SIV replication in resting memory CD4+ T cells depletes gut lamina propria CD4+ T cells. Nature 434: 1148–1152.
- 7. Chen HY, Di Mascio M, Perelson AS, Ho DD, Zhang L (2007) Determination of virus burst size in vivo using a single-cycle SIV in rhesus macaques. Proc Natl Acad Sci U S A 104: 19079–19084.
- 8. Picker LJ, Hagen SI, Lum R, Reed-Inderbitzin EF, Daly LM, et al. (2004) Insufficient production and tissue delivery of CD4+ memory T cells in rapidly progressive simian immunodeficiency virus infection. J Exp Med 200: 1299–1314.
- 9. Picker LJ, Hansen SG, Lifson JD (2012) New paradigms for HIV/AIDS vaccine development. Annu Rev Med 63: 95–111.
- 10. Shedlock DJ, Silvestri G, Weiner DB (2009) Monkeying around with HIV vaccines: using rhesus macaques to define ‘gatekeepers’ for clinical trials. Nat Rev Immunol 9: 717–728.
- 11. Watkins DI, Burton DR, Kallas EG, Moore JP, Koff WC (2008) Nonhuman primate models and the failure of the Merck HIV-1 vaccine in humans. Nat Med 14: 617–621.
- 12. Heneine W, Kashuba A (2012) HIV Prevention by Oral Preexposure Prophylaxis. Cold Spring Harb Perspect Med 2: a007419.
- 13. Ambrose Z, KewalRamani VN, Bieniasz PD, Hatziioannou T (2007) HIV/AIDS: in search of an animal model. Trends Biotechnol 25: 333–337.
- 14. Deere JD, Schinazi RF, North TW (2011) Simian immunodeficiency virus macaque models of HIV latency. Curr Opin HIV AIDS 6: 57–61.
- 15. Clements JE, Gama L, Graham DR, Mankowski JL, Zink MC (2011) A simian immunodeficiency virus macaque model of highly active antiretroviral treatment: viral latency in the periphery and the central nervous system. Curr Opin HIV AIDS 6: 37–42.
- 16. Kinman L, Brodie SJ, Tsai CC, Bui T, Larsen K, et al. (2003) Lipid-drug association enhanced HIV-1 protease inhibitor indinavir localization in lymphoid tissues and viral load reduction: a proof of concept study in HIV-2287-infected macaques. J Acquir Immune Defic Syndr 34: 387–397.
- 17. Kirchhoff F (2010) Immune evasion and counteraction of restriction factors by HIV-1 and other primate lentiviruses. Cell Host Microbe 8: 55–67.
- 18. Stremlau M, Owens CM, Perron MJ, Kiessling M, Autissier P, et al. (2004) The cytoplasmic body component TRIM5alpha restricts HIV-1 infection in Old World monkeys. Nature 427: 848–853.
- 19. Desrosiers RC (2004) Prospects for an AIDS vaccine. Nat Med 10: 221–223.
- 20. VandeWoude S, Apetrei C (2006) Going wild: lessons from naturally occurring T-lymphotropic lentiviruses. Clin Microbiol Rev 19: 728–762.
- 21. Van Rompay KK (2012) The use of nonhuman primate models of HIV infection for the evaluation of antiviral strategies. AIDS Res Hum Retroviruses 28: 16–35.
- 22. Van Rompay KK, Cherrington JM, Marthas ML, Berardi CJ, Mulato AS, et al. (1996) 9-[2-(Phosphonomethoxy)propyl]adenine therapy of established simian immunodeficiency virus infection in infant rhesus macaques. Antimicrob Agents Chemother 40: 2586–2591.
- 23. Lewis MG, Norelli S, Collins M, Barreca ML, Iraci N, et al. (2010) Response of a simian immunodeficiency virus (SIVmac251) to raltegravir: a basis for a new treatment for simian AIDS and an animal model for studying lentiviral persistence during antiretroviral therapy. Retrovirology 7: 21.
- 24. Lifson JD, Rossio JL, Arnaout R, Li L, Parks TL, et al. (2000) Containment of simian immunodeficiency virus infection: cellular immune responses and protection from rechallenge following transient postinoculation antiretroviral treatment. J Virol 74: 2584–2593.
- 25. North TW, Van Rompay KK, Higgins J, Matthews TB, Wadford DA, et al. (2005) Suppression of virus load by highly active antiretroviral therapy in rhesus macaques infected with a recombinant simian immunodeficiency virus containing reverse transcriptase from human immunodeficiency virus type 1. J Virol 79: 7349–7354.
- 26. Kearney M, Spindler J, Shao W, Maldarelli F, Palmer S, et al. (2011) Genetic diversity of simian immunodeficiency virus encoding HIV-1 reverse transcriptase persists in macaques despite antiretroviral therapy. J Virol 85: 1067–1076.
- 27. Ambrose Z, Palmer S, Boltz VF, Kearney M, Larsen K, et al. (2007) Suppression of viremia and evolution of human immunodeficiency virus type 1 drug resistance in a macaque model for antiretroviral therapy. J Virol 81: 12145–12155.
- 28. Allers K, Hutter G, Hofmann J, Loddenkemper C, Rieger K, et al. (2011) Evidence for the cure of HIV infection by CCR5Delta32/Delta32 stem cell transplantation. Blood 117: 2791–2799.
- 29. Ling B, Veazey RS, Luckay A, Penedo C, Xu K, et al. (2002) SIV(mac) pathogenesis in rhesus macaques of Chinese and Indian origin compared with primary HIV infections in humans. AIDS 16: 1489–1496.
- 30. Pandrea I, Gaufin T, Gautam R, Kristoff J, Mandell D, et al. (2011) Functional cure of SIVagm infection in rhesus macaques results in complete recovery of CD4+ T cells and is reverted by CD8+ cell depletion. PLoS Pathog 7: e1002170 doi:10.1371/journal.ppat.1002170.
- 31. Denton PW, Garcia JV (2009) Novel humanized murine models for HIV research. Curr HIV/AIDS Rep 6: 13–19.
- 32. Hilldorfer BB, Cillo AR, Besson GJ, Bedison MA, Mellors JW (2012) New tools for quantifying HIV-1 reservoirs: plasma RNA single copy assays and beyond. Curr HIV/AIDS Rep 9: 91–100.
- 33. Planelles V, Wolschendorf F, Kutsch O (2011) Facts and fiction: cellular models for high throughput screening for HIV-1 reactivating drugs. Curr HIV Res 9: 568–578.
- 34. Bosque A, Famiglietti M, Weyrich AS, Goulston C, Planelles V (2011) Homeostatic proliferation fails to efficiently reactivate HIV-1 latently infected central memory CD4+ T cells. PLoS Pathog 7: e1002288 doi:10.1371/journal.ppat.1002288.
- 35. Bosque A, Planelles V (2009) Induction of HIV-1 latency and reactivation in primary memory CD4+ T cells. Blood 113: 58–65.
- 36. Trono D, Van Lint C, Rouzioux C, Verdin E, Barre-Sinoussi F, et al. (2010) HIV persistence and the prospect of long-term drug-free remissions for HIV-infected individuals. Science 329: 174–180.
- 37. Dyavar Shetty RD, Velu V, Titanji K, Bosinger SE, Freeman GJ, et al. (2012) PD-1 blockade during chronic SIV infection reduces hyperimmune activation and microbial translocation in rhesus macaques. J Clin Invest 122: 1712–1716.
- 38. Velu V, Titanji K, Zhu B, Husain S, Pladevega A, et al. (2009) Enhancing SIV-specific immunity in vivo by PD-1 blockade. Nature 458: 206–210.