HIV/AIDS in 1998 -- Gaining the Upper Hand?
Published in The Journal of the American Medical Association
July 1, 1998
The global community that is dedicated to understanding, treating, and preventing human immunodeficiency virus (HIV) infection is gathering in Geneva, Switzerland, from June 28 through July 3, 1998, for the 12th International Conference on AIDS (acquired immunodeficiency syndrome). Whereas steady incremental advances characterized the field of HIV research throughout the first 10 international conferences on AIDS, the 11th conference in Vancouver, British Columbia, focused on quantum leaps within the areas of therapeutics (i.e., introduction of protease inhibitors for use in combination antiretroviral regimens) and pathogenesis (i.e., identification of chemokine receptors as HIV coreceptors). Early optimism regarding the possibility of eradication of HIV has yielded to the more realistic concept of long-term control of virus replication. In addition, the many obstacles to effective therapy for persons with HIV infection highlight the theme of 2 years ago, "much accomplished, much to do."
Considerable optimism has been generated as a result of advances in HIV therapeutics. The use of combinations of potent antiretroviral agents capable of long-term, profound suppression of plasma viremia has been shown to prolong survival for individuals infected with HIV.[2,3,4] Consensus guidelines have been created for the optimal use of the increasingly complex armamentarium of antiretroviral drugs; 11 antiretroviral agents are now licensed for use in HIV infection, and this number is likely to increase in the near future.[5,6] Appropriate application of these guidelines has dramatically altered the prognosis for patients with late-stage HIV disease, and it is likely that treatment of individuals during the acute stage of HIV infection will dramatically lower the viral load set point and thereby significantly alter the natural history of the disease process.
These dramatic improvements in prognosis achieved with potent antiretroviral therapy are tempered by the many challenges that remain. Although the mortality rate due to HIV infection has decreased significantly in the era of potent antiretroviral therapy, the incidence of new infections has not seen such a dramatic downturn in the United States, and indeed the epidemic rages virtually out of control in parts of the world such as Southeast Asia and the Indian Subcontinent. Access to potent antiretroviral therapy is extremely limited in the developing world and in many segments of the developed world. These potent regimens are expensive and complex, with high pill burdens, numerous adverse effects, and myriad drug interactions. All of these factors raise quality-of-life issues with attendant challenges for adherence to the drug regimen. Emergence of a drug-resistant virus is a serious consequence of imperfect drug regimens and/or suboptimal compliance. In this regard, the success rate for potent antiretroviral regimens in reducing plasma viral load to undetectable levels is considerably lower in an urban clinic setting compared with a clinical trial setting, and is lower when drugs are added sequentially rather than simultaneously.
The existence of latently infected CD4+ T cells poses perhaps the greatest challenge to the long-term control of HIV infection in persons receiving potent antiretroviral therapy. Latently infected cells cannot be eliminated by the host immune response because of the absence of expression of viral antigens on the cell surface; furthermore, integrated proviral DNA cannot be targeted by currently available antiretroviral agents. Therefore, the natural half-life of latently infected resting CD4+ T cells appears to be the rate-limiting step in eradicating HIV infection. In fact, recent studies have established that replication-competent HIV can be recovered from cells of patients receiving potent antiretroviral therapy with undetectable levels of plasma virus for prolonged periods.[12,13,14] Taken together, these studies suggest that the half-life of latently infected resting CD4+ T cells may be very long, and that many years of complete suppression of virus replication may be required before elimination of HIV infection is feasible. Efforts are currently under way to combine potent antiretroviral therapy with interventions aimed at activating latently infected cells. In this situation, latently infected cells might be driven to a state of productive infection and die, while infectious virus released from these cells would be unable to infect new cells under the cover of potent therapy.
The feasibility of immune reconstitution following successful suppression of virus replication by potent antiretroviral therapy remains a topic of considerable interest and debate. It has recently become clear that in the first weeks following initiation of potent antiretroviral therapy, most of the sharp rise in the CD4+ cell count occurs as a result of redistribution of memory cells.[16,17] However, the mechanisms involved in the subsequent slow increase that is observed in the number of naive CD4+ cells remain enigmatic. The depletion of cell populations with certain immunologic specificities during the course of HIV infection creates "holes" in the immunologic repertoire that result in increased susceptibility to opportunistic infections. Therefore, the appearance of naive cells with specificities that had previously been lost would constitute true immune reconstitution. The interpretation of some data would suggest that such reconstitution can indeed occur during potent therapy; however, many questions concerning immune reconstitution remain unanswered. In this regard, the contribution, if any, of the thymus in immune reconstitution in HIV disease remains unclear. Immune-based therapies directed toward boosting production of naive CD4+ cells will be an important adjunct to potent antiretroviral therapy.
New avenues in potential therapies for HIV infection have been opened by the discovery that certain chemokine receptors can serve as coreceptors for HIV entry into CD4+ cells. Molecular epidemiologic data linking mutations in genes for HIV coreceptors with reduced susceptibility to HIV infection or with attenuated rates of disease progression in infected individuals highlight the importance of these receptors in the pathogenesis of HIV infection. In addition, polymorphisms in the genes for the ligands to the HIV coreceptors, modulation of expression of these genes, or both may hinder access of HIV to potential target cells, resulting in delayed progression of HIV disease.[23,24,25] Strategies aimed at blocking access of HIV to its coreceptors attempt to recapitulate the scenarios resulting from polymorphisms or down-regulation of these receptors. Several agents have already been identified that are capable of binding to CCR5 or CXCR4, the major coreceptors for macrophage- and T-cell-tropic strains of HIV, respectively. As is the case with the immunoregulatory network in general, caution is clearly warranted in manipulating the chemokine-chemokine receptor axis. Of note, unanticipated results of coreceptor blockade have been observed in vitro, including enhancement of replication of T-cell-tropic strains of HIV following blockade of CCR5 (the coreceptor for macrophage-tropic strains) (A. Kinter and A. S. Fauci, MD, unpublished data, 1997). Such observations highlight the importance of gaining a fuller understanding of the intracellular signaling events initiated by ligation of chemokine receptors.
Pathogenic events surrounding primary HIV infection are being actively studied by several groups throughout the world. An intervention that prevents establishment of a reservoir of latently infected cells following primary HIV infection would be invaluable. Such a goal will be a great challenge, given the rapidity with which a latent reservoir is established in primary HIV infection. Exhaustion of clones of HIV-specific CD8+ cytolytic T cells following primary HIV infection is another key event in the early pathogenesis of HIV disease that may represent a target for intervention. Persistently high levels of viral antigen, as well as the early loss of HIV-specific CD4+ cell proliferative responses, likely contribute to this CD8+ cell clonal exhaustion. It may be possible to prevent the loss of these CD4+ cell responses with early treatment of primary HIV infection by using potent antiretroviral therapy. The long-term impact of maintaining these cellular responses remains to be seen, but will likely be favorable.
Development of a safe and effective vaccine for HIV infection remains the "holy grail" of AIDS research. The grim global statistics of the HIV epidemic are potent reminders of the pressing need for such a vaccine. In 1997, nearly 6 million people became infected with HIV, and more than 90% of these new infections occurred in developing countries. In these countries, where annual per capita health care expenditures may total only a few dollars, access to potent antiretroviral therapy is essentially nonexistent. The development of a safe and effective vaccine continues to encounter a host of sobering challenges, including geographic variability of HIV subtypes, and the lack of correlates of protective immunity in HIV infection. Clinical trials of several vaccine candidates are currently in progress. Studies of experimental vaccines that are protective in the simian immunodeficiency virus model, and of immune responses in long-term nonprogressors with HIV infection continue to provide clues regarding immune responses that may be particularly beneficial. New concepts in vaccine design, such as vaccination with DNA and the use of cytokines to elicit a "custom-made" immune response, are also being applied to the search for an HIV vaccine.
Finally, are we, as the scientific community, now poised to gain the upper hand against HIV infection? In settings where access to potent antiretroviral therapy and to experienced care providers is possible, where conditions are amenable to adherence to a complicated medical regimen, and where drug adverse effects and quality-of-life issues are manageable, the answer is that long-term control of viral replication is quite likely. However, of the more than 30 million people worldwide who are living with HIV infection, only a tiny proportion has access to the conditions mentioned above. Gaining the upper hand against HIV infection will require a deeper understanding of the pathogenic events that cause HIV-induced disease, wider access to medical care and potent antiretroviral regimens, education about prevention (such as strategies to reduce high-risk behavior), access to interventions aimed at prevention, and the development and worldwide dissemination of a safe and effective HIV vaccine.
Oren J. Cohen, MD
From the National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Md.
Reprints: Anthony S. Fauci, MD, 31 Center Dr, MSC 2520, Bldg 31, Room 7A03, Bethesda, MD 20892-2520.
Editorials represent the opinions of the authors and THE JOURNAL and not those of the American Medical Association.
1. Fauci AS. AIDS in 1996: much accomplished, much to do. JAMA. 1996;276:155-156.
2. Gulick R, Mellors J, Havlir D, et al. Treatment with indinavir, zidovudine, and lamivudine in adults with human immunodeficiency virus infection and prior antiretroviral therapy. N Engl J Med. 1997;337:734-739.
3. Hammer S, Squires K, Hughes M, et al. A controlled trial of two nucleoside analogues plus indinavir in persons with human immunodeficiency virus infection and CD4 cell counts of 200 per cubic millimeter or less. N Engl J Med. 1997;337:725-733.
4. Palella FJ Jr, Delaney KM, Moorman AC, et al, and the HIV Outpatient Study Investigators. Declining morbidity and mortality among patients with advanced human immunodeficiency virus infection. N Engl J Med. 1998;338:853-860.
5. Carpenter C, Fischl M, Hammer S, et al. Antiretroviral therapy for HIV infection in 1997. JAMA. 1997;277:1962-1969.
6. Department of Health and Human Services and Henry J. Kaiser Family Foundation. Guidelines for the use of antiretroviral agents in HIV-infected adults and adolescents. MMWR Morb Mortal Wkly Rep. 1998;(RR-5) 47:43-82.
7. Mellors JW, Kingsley LA, Rinaldo CR, et al. Quantitation of HIV-1 RNA in plasma predicts outcome after seroconversion. Ann Intern Med. 1995;122:573-579.
8. Centers for Disease Control and Prevention. Diagnosis and reporting of HIV and AIDS in states with integrated HIV and AIDS surveillanceUnited States, January 1994-June 1997. MMWR Morb Mortal Wkly Rep. 1998;47:309-314.
9. World Health Organization. HIV/AIDS. Wkly Epidemiol Rec. 1997;72:17-24.
10. Deeks S, Loftus R, Cohen P, et al. Incidence and predictors of virologic failure to indinavir (IDV) or/and ritonavir (RTV) in an urban health clinic. In: Programs and abstracts of the 37th Interscience Congress on Antimicrobial Agents and Chemotherapy; Toronto, Ontario; September 28-October 1, 1997. Abstract LB-2.
11. Gulick R, Mellors J, Havlir D, et al. Simultaneous vs sequential initiation of therapy with indinavir, zidovudine, and lamivudine: 100-week follow-up. JAMA. 1998;280:35-41.
12. Finzi D, Hermankova M, Pierson T, et al. Identification of a reservoir for HIV-1 in patients on highly active antiretroviral therapy. Science. 1997;278:1295-1300.
13. Wong JK, Hezareh M, Gunthard HF, et al. Recovery of replication-competent HIV despite prolonged suppression of plasma viremia. Science. 1997;278:1291-1295.
14. Chun TW, Stuyver L, Mizell SB, et al. Presence of an inducible HIV-1 latent reservoir during highly active antiretroviral therapy. Proc Natl Acad Sci U S A. 1997;94:13193-13197.
15. Chun T-W, Engel D, Ehler L, Mizell S, Fauci A. Induction of replication-competent HIV in latently infected resting CD4+ T cells using cytokines. J Exp Med. In press.
16. Autran B, Carcelain G, Li T, et al. Positive effects of combined antiretroviral therapy on CD4+ T cell homeostasis and function in advanced HIV disease. Science. 1997;277:112-116.
17. Pakker NG, Notermans DW, de Boer RJ, et al. Biphasic kinetics of peripheral blood T cells after triple combination therapy in HIV-1 infection: a composite of redistribution and proliferation. Nat Med. 1998;4:208-214.
18. Connors M, Kovacs JA, Krevat S, et al. HIV infection induces changes in CD4+ T-cell phenotype and depletions within the CD4+ T-cell repertoire that are not immediately restored by antiviral or immune-based therapies. Nat Med. 1997;3:533-540.
19. Gorochov G, Neumann AU, Kereveur A, et al. Perturbation of CD4+ and CD8+ T-cell repertoires during progression to AIDS and regulation of the CD4+ repertoire during antiviral therapy. Nat Med. 1998;4:215-221.
20. Withers-Ward ES, Amado RG, Koka PS, et al. Transient renewal of thymopoiesis in HIV-infected human thymic implants following antiviral therapy. Nat Med. 1997;3:1102-1109.
21. Moore JP, Trkola A, Dragic T. Co-receptors for HIV-1 entry. Curr Opin Immunol. 1997;9:551-562.
22. Cohen O, Kinter A, Fauci A. Host factors in the pathogenesis of HIV infection. Immunol Rev. 1997;159:31-48.
23. Winkler C, Modi W, Smith MW, et al. Genetic restriction of AIDS pathogenesis by an SDF-1 chemokine gene variant. Science. 1998;279:389-393.
24. Zagury D, Lachgar A, Chams V, et al. C-C chemokines, pivotal in protection against HIV type 1 infection. Proc Natl Acad Sci U S A. 1998;95:3857-3861.
25. Ullum H, Lepri A, Victor J, et al. Production of beta-chemokines in human immunodeficiency virus (HIV) infection. J Infect Dis. 1998;177:331-336.
26. Baggiolini M, Moser B. Blocking chemokine receptors. J Exp Med. 1997;186:1189-1191.
27. Dolei A, Biolchini A, Serra C, Curreli S, Gomes E, Dianzani F. Increased replication of T-cell-tropic HIV strains and CXC-chemokine receptor-4 induction in T cells treated with macrophage inflammatory protein (MIP)-1alpha, MIP-1beta and RANTES beta-chemokines. AIDS. 1998;12:183-190.
28. Chun T-W, Engel D, Berrey M, Corey L, Fauci A. Early establishment of a pool of latently infected resting CD4+ T cells during primary HIV-1 infection. Proc Natl Acad Sci U S A. In press.
29. Pantaleo G, Soudeyns H, Demarest J, et al. Evidence for rapid disappearance of initially expanded HIV-specific CD8+ T-cell clones during primary HIV infection. Proc Natl Acad Sci U S A. 1997;94:9848-9853.
30. Rosenberg ES, Billingsley JM, Caliendo AM, et al. Vigorous HIV-1-specific CD4+ T-cell responses associated with control of viremia. Science. 1997;278:1447-1450.
31. Tighe H, Corr M, Roman M, Raz E. Gene vaccination: plasmid DNA is more than just a blueprint. Immunol Today. 1998;19:89-97.
This article was provided by U.S. National Institute of Allergy and Infectious Diseases.