Introduction
HIV infection is typically characterized by a prolonged asymptomatic phase prior to the development of AIDS. New insights into the nature of this deceptively quiet phase in the natural history of HIV infection, primarily revealed by the alterations in viral and cellular dynamics that occur with the initiation of highly active antiretroviral therapy (HAART), may hold the key to designing the next generation of HIV therapies. With the advent of HAART and quantitative plasma viral RNA assays, it became clear that this 'clinically latent' stage of disease represents a dynamic steady state of active infection and death of replication active cells [1,2]. In individuals who are not receiving therapy, the years following initial infection are characterized by a viremia 'set point' that is relatively constant over time within each individual but widely variable between individuals, and by progressive loss of CD4 T cells. The set point of viremia is reflective of the magnitude of viral replication and is directly related to the rate of disease progression [3]. Although HAART has resulted in a dramatic decline in the progression of HIV disease (AIDS-related opportunistic infections and death) [4-6], administering these combination regimens to patients long-term is difficult. HAART regimens often require many pills in carefully timed doses that limit patient adherence; they are expensive, and there are a growing number of potential adverse effects and complex drug interactions to consider [7-9]. The initial optimism that continual suppression of viral replication could lead to complete eradication after several years [10] has faded with the identification of a stable form of latent HIV infection in resting CD4 T cells [11-15]. Furthermore, an increasing number of reports demonstrate that a reservoir of infectious virus persists despite prolonged effective viral load suppression [16-20].
The steady state of viral replication in untreated individuals is highly dynamic and the specific viral load level at steady state is probably determined by a combination of factors. While the frequency of activated T cells is a contributing factor to the level of viral replication [21-26], there is a rapidly growing body of evidence that the dominant control mechanism is an HIV-specific cellular immune response. A distinct goal for the long-term therapy of HIV infection has recently emerged that focuses on inducing efficient immune control over viral replication, rather than the more daunting objective of complete viral eradication. The rationale for this therapeutic approach is based on the following hypotheses. (i) An HIV-specific cellular immune response is the primary mechanism that controls viral replication - this response brings the acute infection syndrome to an end and ultimately determines the steady-state 'set point' during chronic infection. (ii) HAART administration results in a significant reduction in this antigen-driven immune response, which may allow persistence of a low level of viable virus. (iii) Although deletion of relevant HIV-specific CD4 T cells may occur during acute infection, deletion is not complete and anergy mediated by HIV products may serve to render the steady-state level of immune response inefficient during chronic disease. (iv) Therapeutic immunization of patients with adequate inhibition of viral replication with HIV antigens may allow reversal of this anergic state and the establishment of efficient immune control characteristic of long-term nonprogressor patients.
Rapidly evolving experimental assays
Assessing the impact of HIV-specific cellular immune responses on HIV disease progression has been challenging, in part because the complex cellular interactions involved lead to dilemmas about which components are pertinent to measure. In addition, the results of HIV-specific in vitro assays have an uncertain relationship to actual immune function in vivo or to clinical outcomes. The standard assay for antigen-specific T helper cell responses involves measurement of cellular proliferation in culture following stimulation with soluble protein antigens. In general, lymphoproliferative assays (LPA) for HIV-specific responses are lost very early on in the course of acute HIV infection and are extremely low or nonexistent among patients with chronic HIV infection [27]. There is re-assuring evidence that LPA to several opportunistic pathogens (cytomegalovirus, mycobacterial infections) improve following HAART, but there is limited or no improvement in LPA responses to HIV antigens among patients with well-established HIV disease or AIDS [28-30]. Although proliferation after 6-day cell culture is dependent on CD4 T-cell function, the magnitude of this response may not always be directly related to the frequency of antigen-specific CD4 T cells or to the 'help' delivered to CD8 T cells. An alternative approach utilizes direct measurement of the frequency of CD4 T cells producing specific cytokines after antigen activation (usually interferon [IFN]-γ and/or tumor necrosis factor [TNF]-α have been measured). This approach confirms that CD4+ lymphocyte responses to other pathogens (like cytomegalovirus) may be recovered following HAART [31], and additionally reveals that there may be a degree of HIV-specific CD4 cell responsiveness in some chronically infected patients not reliably detected using standard LPA [32].
Assays for HIV-specific CD8 T-cell function are generally further along in development than those targeting CD4 function, but which (if any) of these assays directly reflect the in vivo ability to block HIV replication remains unclear. One standard approach is to measure the lysis of 51Cr-labeled target cells expressing relevant major histocompatibility complex (MHC) class I/peptide epitopes in the presence of patient peripheral blood mononuclear cells. Bulk assays are typically carried out using different ratios of effector to target cells. Such direct lysis assays can be performed in several formats that have different levels of sensitivity. If active cytotoxic T lymphocytes (CTLs) are present at a relatively high frequency, a direct lysis assay using effector cells directly from a patient's blood sample can be performed [33,34]. Alternatively, cells can be stimulated in vitro for several weeks in the presence of interleukin (IL)-2 and then determination of lytic activity can be carried out [35]. Direct lysis has been demonstrated during the acute HIV infection syndrome [36,37], but this assay generally does not detect CTL activity in peripheral blood mononuclear cells from chronically infected patients. Assays based on bulk stimulation and expansion in vitro, on the contrary, are capable of detecting the level of CTL precursors in selected specimens derived from patients with established, chronic HIV infection [33,38]. The frequency of precursor cells capable of generating multiple active daughter cells can also be measured in a limiting dilution format, with the actual 51Cr release assay as the method to score positive wells. Antigen-specific activation of CD8 T cells not only activates the lytic program, but also activates cytokine gene expression, particularly IFN-γ and TNF-α [39]. Cytokine expression can be assessed in several different assay formats including detection of soluble protein by enzyme-linked immunosorbent assay, the ELISPOT assay [38,40,41], or by intracellular cytokine staining with flow cytometry analysis [42-44].
Our understanding of the in vivo dynamics of CD8 T cells has also been dramatically improved using MHC-peptide tetramers capable of recognizing HIV antigen-specific cells [45]. In different immununological situations, including the response to HIV infection [46], the frequency of tetramer-positive cells is greater than the number of precursor cells detected by limiting dilution analysis. In some dramatic circumstances, for example during acute viral infections, CTLs that respond to a specific antigen may comprise a substantial fraction of all the CD8 T cells present. This analytical technique is limited by the requirement to construct a tetramer reagent for each distinct MHC class I/peptide specificity, but many of these reagents have been produced and preliminary results suggest the ability to quantitate cellular immune responses to HIV longitudinally in some cases [47]. There is also proof-of-concept that tetramer constructs can be developed to assay for class II, CD4+ pathogen-specific, cellular immune responses [48,49].
CTL responses
There is an emerging consensus that effective immune responses to HIV antigens can be evoked, at least under certain clinical situations, and that the steady-state 'set point' of plasma viral load is predominately controlled by an active CD8 T-cell-mediated immune response. Although many investigators initially concluded that a normal host immune response was incapable of controlling HIV infection, recent data indicate the existence of HIV exposed but uninfected individuals with evidence of active HIV-specific CD8 T-cell responses [41,50-52], suggesting that some individuals can mount an immune response capable of clearing initial infection. For those individuals who do become infected, development of active CTL is temporally related to the control of viral replication and the resolution of the acute retroviral syndrome [37,53-55]. In addition, HIV-specific CTL activity is detectable during chronic infection in untreated subjects using either the chromium release or tetramer methodologies [46,56,57]. Furthermore, there is evidence from viral sequence evolution studies that viral persistence and disease progression may be associated with the selection of specific CTL escape mutants [58-62]. Thus, ideal cellular immune responses that occur very quickly may conceivably eliminate infection altogether, and chronic infection is associated with consistently detectable CTL responses and ongoing selective pressure on viral growth. Although initially considered paradoxical, there are now substantial data that potent ARV suppression also leads to a significant decline in the activity of HIV-specific CTL responses [38,46,56,63,64]. Not only do CTL responses fall when ARV therapy inhibits viral replication, but the viral rebound that occurs when therapy is stopped is followed by re-emergence of the CTL response in several reports [65-67].
Further evidence for the key role of CD8 T cells in the control of viral load comes from the strong association between particular class I MHC (but not class II) alleles and the degree of viral replication [68,69]. Since the function of MHC class I antigens is to present peptide epitopes to CD8 T cells, allelic variation in these molecules gives rise to a variation in the magnitude of CD8 T-cell immune responses. The correlation of the viral load 'set point' with the genetic elements that control CD8 T-cell immune responses further supports the central role of this immune response in steady-state infection. In addition, the added benefit of heterozygosity at MHC class I loci in lowering the viral load [70] suggests that the more diverse CD8 repertoire possible with more MHC restriction molecules is also a contributor to lower viral load at steady state.
Further support for the critical role of effective cellular immunity in controlling retrovirus infection comes from studies of the SIV infection model in rhesus macaques. First, as in human infection with HIV, strong CTL responses are associated with the control of initial viremia in SIV infection [71-75]. Second, infection with a live attenuated SIV strain is associated with the induction of a vigorous immune response [76] and protection from subsequent infection with a virulent strain [77]. Furthermore, vaccination of rhesus macaques with a gp120 subunit preparation that primarily stimulates CD4 T cells results in a lower viral load set point after infection, compared with nonimmunized controls [78]. Finally, and perhaps most persuasively, two groups have independently shown that depletion of CD8 T cells with a cytotoxic anti-CD8 monoclonal antibody in SIV-infected rhesus macaques results in rapid increases in plasma viral load [79,80]. Even in SIV-infected animals with undetectable plasma HIV RNA [81], depletion of CD8 T cells (without an appreciable change in the number of target CD4 T cells) results in a 2-4 log rise in viral load within several days. Thus, the presence of CD8 T cells, presumably continuously activated by viral antigen, is required to maintain the steady-state level of viral replication.
Multiple lines of evidence thus lead to the conclusion that an active antigen-driven immune response occurs in HIV-1 infection, but this response is not efficient enough to maintain viral replication below levels that result in long-term damage to the immune system. In mathematical formulations of the dynamics between virus growth and immune response [82,83], the nonintuitive result finding is that efficiency or responsiveness of each individual determines the steady-state viral load, rather than the magnitude of the response at a particular instant. The hypothesis that steady-state viral replication is maintained by an antigen-driven immune clearance of replication active cells in competition with growth rate of the virus (which is the source of viral antigen) predicts the relatively uniform rapid rate of decline of viremia with HAART initiation [83]. Furthermore, this concept leads to a natural explanation of the rapid generation of latent infection early in disease, but subsequent stability of this pool of cells and a potential explanation for the persistence of active infection on HAART [83-85].
HIV-Specific CD4+ cell responsiveness
There is thus substantial evidence to support the view that an antigen-driven active CTL response is the dominant host mechanism controlling the extent of viral replication. The problem in most chronically HIV-infected individuals, however, is that this response is not adequately efficient. The relative inefficiency of the CD8 T-cell response may be due to the lack of an effective HIV-specific CD4 T-cell response in chronic HIV disease. The maintenance of effective CTL responses, along with a wide spectrum of humoral and cellular immune responses, appears to depend upon the orchestration provided by CD4+ T helper cells. In experimental animal models of viral infection, the critical role of an effective CD4 T-cell response to clear chronic infection is well documented [86-90], despite the dominant effector mechanism being a CTL response. The presence of strong CD4 T-cell responses by proliferation in patients who can maintain low viral load without ARV therapy [27] suggests that the presence of effective CD4 T-cell responses are critical for efficient CD8 responses to HIV-1. The subsequent observations that strong LPA responses present in subjects who initiate HAART very early after primary infection [91] suggests that chronic infection may lead to the death of most HIV-specific CD4 T cells. Since HIV preferentially infects antigen-activated CD4 T cells and either a direct viral cytopathic effect or immune elimination quickly kills infected cells, the critical viral antigen-specific CD4 T cells may be deleted very early during infection. Another possibility is that these antigen-specific cells are rendered anergic by the persistent level of HIV antigens together with the effects of potentially immunosuppressive viral products such as viral envelope proteins [92-94] and HIV-1 tat[95-97]. This concept suggests that HIV-specific CD4 T cells could be effectively immunized and the anergic effect reversed if they were stimulated while active viral replication was substantially suppressed by HAART. The observation that HIV-responsive CD4 T cells exist since they can make some cytokines after antigenic stimulation [32] is consistent with this idea. Furthermore, the frequency of CD4 T cells that can respond is increased by co-stimulation with anti-CD28 or by CD40 crosslinking [98], suggesting an altered sensitivity and/or cytokine pattern of response rather than clonal deletion as the primary mechanism of low responses during chronic infection. It is also possible that HIV products produce materials that inhibit dendritic cell function and thereby result in aberrant activation of CD4 T cells and inefficient helper activity for CD8 T cells.
Virologic control in vivo following immune-based interventions
Further evidence that the immune system can maintain control of viral replication comes from a series of tantalizing anecdotes related to interruptions of ARV therapy, which have raised hopes that a heightened immune response to HIV antigens that efficiently controls viral replication may be achievable. One of the first indications that long-term control of viral replication might be obtained is a case report of an individual (often referred to as the 'Berlin patient') who started HAART shortly after primary infection [99]. The patient briefly discontinued HAART several times due to intercurrent illnesses, and then ultimately stopped all HAART against medical advice. Subsequently, he has remained healthy with no detectable viremia for several years. As already described, early induction of HAART shortly after primary infection has been shown to preserve substantial HIV-specific immune responses (both CD4 and CD8 T cells) [27,100]. Subsequently, several other subjects who started HAART shortly after primary infection, had several episodes of intermittent adherence, and who then discontinued therapy have been described [65,91,101,102]. Although these studies have not been conducted in a randomized fashion, several subjects have experienced delayed rebound followed by a spontaneous fall in plasma viral load to below detection without re-introduction of HAART. In some subjects who underwent several repeated therapy withdrawals, preliminary reports suggested that the rebound back to baseline was slower following each subsequent treatment interruption [102]. Follow-up reports on several of these subjects demonstrates that the fall in viral load without ARV therapy is associated with measurable increases in CTL activity in the blood [66], strongly suggesting that immune control of the infection has occurred. The association between the development of improved in vitro measures of HIV-specific T-cell responses and viral control after therapy withdrawal was also documented in a recent study in which subjects with early HAART treatment underwent therapy withdrawal [67].
A key question is whether these observations in acutely infected patients can be duplicated in the much more common clinical scenario of chronic infection. If clonal deletion by HIV infection is the predominant mechanism that accounts for low CD4 T-cell responses in chronic infection, then attempts to boost this response may be futile. If the relative inefficiency of the CD8 T-cell response is due primarily to anergy induced by HIV viral products during high-level replication, then it may be possible to reverse this anergic phenotype and obtain efficient immune control of viral replication. Either therapeutic immunization with existing vaccine products, such as Remune that can stimulate significant LPA responses in chronically infected patients [103], or the canary pox vectors that have shown some success in stimulating CTL responses in HIV seronegative subjects [104] may be able to activate efficient responses in HIV-infected subjects. While larger studies designed to address the role of therapuetic immunization are ongoing, there are several reports of studies of chronically infected patients with serial structured treatment interruption (STI) designed to induce heighten immunity and control of viral replication. In a cohort from Barcelona, four of nine subjects experienced a transient burst of viremia followed by suppression in the absence of ARV drugs after the second STI, although all rebounded to near the previous set point on the first STI [18]. Patients who have received HAART and IL-2 therapy may experience dramatic viral rebound off therapy, but then quickly return to lower steady-state levels of viral load without restarting HAART [105]. On the contrary, early results from a large Swiss-Spanish cooperative study, while demonstrating that some individuals achieve lower viral load steady states after therapy interruption indicate that the majority return to their previous set point as a result of serial treatment interruptions [106].
Since CD8 T-cell-mediated immune responses seem to be functionally active, albeit inefficient, during chronic infection, the prospect of inducing efficient immune control in these patients most likely depends on reconstitution of the CD4 T-cell response to viral antigens. Whether this is a feasible prospect depends on the existence of viral antigen-specific CD4 T cells with the implication that actively growing virus produces materials that can anergize these cells as well as produce relevant antigen. Thus, release of ARV drug control of replication may simply result in re-establishing the scenario of CD4 T-cell anergy and the previous 'set point' level of replication determined by 'helper-independent' CD8 responses.
While there are many remaining questions to be answered, it is possible that STI may not only serve as a diagnostic strategy to detect increased efficiency of the immune response, but may also be important in the elicitation of this response. While the ability to induce efficient immune control in chronic infection remains speculative, the benefit of maintaining viral replication at extremely low levels is clear. If a therapeutic immunization regimen can achieve the very low levels of viral replication typical of long-term nonprogressors or of subjects successfully treated with HAART, the rate of disease progression will probably also be similarly low [67]. If such heightened immunity can maintain low viral load without the need for continual ARV therapy, even if not indefinitely, the long-term prognosis of HIV infection would be considerably brighter. Furthermore, the execution of clinical trials to test HIV-specific immunization as a therapeutic strategy for chronically infected people offers the prospect of significant new insights into HIV pathogenesis and the correlates of effective immune control. This information and empirical characterization of specific HIV vaccine agents can be directly translated into vaccine regimens that may prevent initial HIV infection.
References
1. Wei X, Ghosh SK, Taylor ME,
et al.Viral dynamics in human immunodeficiency virus type 1 infection.Nature 1995,
373:117-122.
2. Ho DD, Neumann AU, Perelson AS, Chen W, Leonard JM, Markowitz M.
Rapid turnover of plasma virions and CD4 lymphocytes in HIV-1 infection.Nature 1995,
373:123-126.
3. Mellors J, Rinaldo C, Gupta P, White RM, Todd JA, Kingsley LA.
Prognosis in HIV infection predicted by the quantity of virus in plasma.Science 1996,
272:1167-1170.
4. Hammer SM, Squires KE, Hughes MD,
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. AIDS Clinical Trials Group 320 Study Team.N Engl J Med 1997,
337:725-733.
5. Gulick RM, 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.
6. Palella FJ Jr, Delaney KM, Moorman AC,
et al.Declining morbidity and mortality among patients with advanced human immunodeficiency virus infection. HIV Outpatient Study Investigators.N Engl J Med 1998,
338:853-860.
7. Flexner C.
HIV-protease inhibitors.N Engl J Med 1998,
338:1281-1292.
8. Carr A, Samaras K, Thorisdottir A, Kaufmann GR, Chisholm DJ, Cooper DA.
Diagnosis, prediction, and natural course of HIV-1 protease inhibitor associated lipodystrophy, hyperlipidaemia, and diabetes mellitus: a cohort study.Lancet 1999,
353:2093-2908.
9. Lucas GM, Chaisson RE, Moore RD.
Highly active antiretroviral therapy in a large urban clinic: risk factors for virologic failure and adverse drug reactions.Ann Intern Med 1999,
131:81-87.
10. Perelson AS, Essunger P, Cao Y,
et al.Decay characteristics of HIV-1-infected compartments during combination therapy.Nature 1997,
387:188-191.
11. 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.
12. Wong JK, Hezareh M, Gunthard HF,
et al.Recovery of replication-competent HIV despite prolonged suppression of plasma viremia.Science 1997,
278:1291-1295.
13. 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 USA 1997,
94:13193-13197.
14. Chun TW, Engel D, Berrey MM, Shea T, Corey L, Fauci AS.
Early establishment of a pool of latently infected, resting CD4(+
) T cells during primary HIV-1 infection.Proc Natl Acad Sci USA 1998,
95:8869-8873.
15. Ho DD.
Toward HIV eradication or remission: the tasks ahead.Science 1998,
280:1866-1867.
16. Jubault V, Burgard M, Le Corfec E, Costagliola D, Rouzioux C, Viard JP.
High rebound of plasma and cellular HIV load after discontinuation of triple combination therapy [letter].AIDS 1998,
12:2358-2359.
17. Harrigan PR, Whaley M, Montaner JS.
Rate of HIV-1 RNA rebound upon stopping antiretroviral therapy.AIDS 1999,
13:F59-F62.
18. Garcia F, Plana M, Vidal C,
et al.Dynamics of viral load rebound and immunological changes after stopping effective antiretroviral therapy.AIDS 1999,
13:F79-F86.
19. Chun TW, Engel D, Mizell SB,
et al.Effect of interleukin-2 on the pool of latently infected, resting CD4+T cells in HIV-1-infected patients receiving highly active anti-retroviral therapy.Nat Med 1999,
5:651-655.
20. Davey RT Jr, Bhat N, Yoder C,
et al.HIV-1 and T cell dynamics after interruption of highly active antiretroviral therapy (HAART) in patients with a history of sustained viral suppression.Proc Natl Acad Sci USA 1999,
96:15109-15114.
21. Pantaleo G, Graziosi C, Fauci AS.
The immunopathogenesis of human immunodeficiency virus infection.N Engl J Med 1993,
328:327-335.
22. Fauci AS.
Host factors and the pathogenesis of HIV-induced disease.Nature 1996,
384:529-534.
23. Stanley SK, Ostrowski MA, Justement JS,
et al.Effect of immunization with a common recall antigen on viral expression in patients infected with human immunodeficiency virus type 1.N Engl J Med 1996,
334:1222-1230.
24. Ostrowski MA, Krakauer DC, Li Y,
et al.Effect of immune activation on the dynamics of human immunodeficiency virus replication and on the distribution of viral quasispecies.J Virol 1998,
72:7772-7784.
25. Kovacs JA, Vogel S, Albert JM,
et al.Controlled trial of interleukin-2 infusions in patients infected with the human immunodeficiency virus.N Engl J Med 1996,
335:1350-1356.
26. Phillips AN.
Reduction of HIV concentration during acute infection: independence from a specific immune response.Science 1996,
271:497-499.
27. 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.
28. Lederman MM, Connick E, Landay A,
et al.Immunologic responses associated with 12 weeks of combination antiretroviral therapy consisting of zidovudine, lamivudine, and ritonavir: results of AIDS Clinical Trials Group Protocol 315.J Infect Dis 1998,
178:70-79.
29. Autran B, Carcelain G, Li TS,
et al.Positive effects of combined antiretroviral therapy on CD4+T cell homeostasis and function in advanced HIV disease.Science 1997,
277:112-116.
30. Rinaldo CR, Liebmann JM, Huang XL,
et al.Prolonged suppression of human immunodeficiency virus type 1 (HIV-1) viremia in persons with advanced disease results in enhancement of CD4 T cell reactivity to microbial antigens but not to HIV-1 antigens.J Infect Dis 1999,
179:329-336.
31. Komanduri KV, Viswanathan MN, Wieder ED,
et al.Restoration of cytomegalovirus-specific CD4+T-lymphocyte responses after ganciclovir and highly active antiretroviral therapy in individuals infected with HIV-1.Nat Med 1998,
4:953-956.
32. Pitcher CJ, Quittner C, Peterson DM,
et al.HIV-1-specific CD4+T cells are detectable in most individuals with active HIV-1 infection, but decline with prolonged viral suppression.Nat Med 1999,
5:518-525.
33. Plata F, Autran B, Martins LP,
et al.AIDS virus-specific cytotoxic T lymphocytes in lung disorders.Nature 1987,
328:348-351.
34. Brander C, Walker BD.
T lymphocyte responses in HIV-1 infection: implications for vaccine development.Curr Opin Immunol 1999,
11:451-459.
35. Lubaki MN, Egan MA, Siliciano RF, Weinhold KJ, Bollinger RC.
A novel method for detection andex vivoexpansion of HIV type 1-specific cytolytic T lymphocytes.AIDS Res Hum Retrovirus 1994,
10:1427-1431.
36. Kahn JL, Walker BD.
Acute human immunodeficiency virus Type I infection.N Engl J Med 1998,
339:33-39.
37. Musey L, Hughes J, Schacker T, Shea T, Corey L, McElrath MJ.
Cytotoxic-T-cell responses, viral load, and disease progression in early human immundeficiency virus 1 infection.N Engl J Med 1997,
337:1267-1274.
38. Mollet L, Li TS, Samri A,
et al.Dynamics of HIV-specific CD8+T lymphocytes with changes in viral load.J Immunol 2000,
165:1692-1704.
39. Jassoy C, Harrer T, Rosenthal T, Navia BA, Worth J, Johnson RP, Walker BD.
Human immunodeficiency virus type 1-specific cytotoxic T lymphocytes release gamma interferon, tumor necrosis factor alpha (TNF-alpha), and TNF-beta when they encounter their target antigens.J Virol 1993,
67:2844-2852.
40. Czerkinsky CC, Anderson G, Ekre H-P, Nilsson L-A, Klareskog L, Ouchterlony O.
Reverse ELISPOT assay for clonal analysis of cytokine production. I. Enumeration of gamma-interferon-producing cells.J Immunol Methods 1988,
110:29-36.
41. Herr W, Protzer U, Lohse AW, Gerken G, Meyer zum Buschenfelde KH, Wolfel T.
Quantification of CD8+T lymphocytes responsive to human immunodeficiency virus (HIV) peptide antigens in HIV-infected patients and seronegative persons at high risk for recent HIV exposure.J Infect Dis 1998,
178:260-265.
42. Harrer E, Harrer T, Buchbinder S,
et al.HIV-1-specific cytotoxic T lymphocyte response in healthy, long-term nonprogressing seropositive persons.AIDS Res Hum Retrovirus 1994,
10(suppl 2):S77-S78.
43. Maino VC, Picker LJ.
Identification of functional subsets by flow cytometry: intracellular detection of cytokine expression.Cytometry 1998,
34:207-215.
44. Gea-Banacloche JC, Migueles SA, Martino L,
et al.Maintenance of large numbers of virus-specific CD8+T cells in HIV-infected progressors and long-term nonprogressors.J Immunol 2000,
165:1082-1092.
45. Altman JD, Moss PAH, Goulder PJR,
et al.Phenotypic analysis of antigen-specific T lymphocytes.Science 1996,
274:94-96.
46. Ogg GS, Jin X, Bonhoeffer S,
et al.Quantitation of HIV-1-specific cytotoxic T lymphocytes and plasma load of viral RNA.Science 1998,
279:2103-2106.
47. Ogg GS, Kostense S, Klein MR,
et al.Longitudinal phenotypic analysis of human immunodeficiency virus type 1-specific cytotoxic T lymphocytes: correlation with disease progression.J Virol 1999,
73:9153-9160.
48. Novak EJ, Liu AW, Nepom GT, Kwok WW.
MHC class II tetramers identify peptide-specific human CD4+T cells proliferating in response to influenza A antigen.J Clin Invest 1999,
104:R63-R67.
49. Kuroda MJ, Schmitz JE, Lekutis C,
et al.Human immunodeficiency virus type 1 envelope epitope-specific CD4(+
) T lymphocytes in Simian/Human immunodeficiency virus-infected and vaccinated rhesus monkeys detected using a peptide-major histocompatibility complex class II tetramer.J Virol 2000,
74:8751-8756.
50. Rowland-Jones SL, Dong T, Fowke KR,
et al.Cytotoxic T cell responses to multiple conserved HIV epitopes in HIV-resistant prostitutes in Nairobi.J Clin Invest 1998,
102:1758-1765.
51. Bernard NF, Yannakis CM, Lee JS, Tsoukas CM.
Human immunodeficiency virus (HIV)-specific cytotoxic T lymphocyte activity in HIV-exposed seronegative persons.J Infect Dis 1999,
179:538-547.
52. Goh WC, Markee J, Akridge RE,
et al.Protection against human immunodeficiency virus type 1 infection in persons with repeated exposure: evidence for T cell immunity in the absence of inherited CCR5 coreceptor defects.J Infect Dis 1999,
179:548-557.
53. Borrow P, Lewicki H, Hahn BH, Shaw GM, Oldstone MB.
Virus-specific CD8+cytotoxic T-lymphocyte activity associated with control of viremia in primary human immunodeficiency virus type 1 infection.J Virol 1994,
68:6103-6110.
54. Koup RA, Safrit JT, Cao Y,
et al.Temporal association of cellular immune responses with the initial control of viremia in primary human immunodeficiency virus type 1 syndrome.J Virol 1994,
68:4650-4655.
55. Pantaleo G, Demarest JF, Schacker T,
et al.The qualitative nature of the primary immune response to HIV infection is a prognosticator of disease progression independent of the initial level of plasma viremia.Proc Natl Acad Sci USA 1997,
94:254-258.
56. Dalod M, Dupuis M, Deschemin JC,
et al.Broad, intense antihuman immunodeficiency virus (HIV)ex vivoCD8(+
) responses in HIV type 1-infected patients: comparison with anti-Epstein-Barr virus responses and changes during antiretroviral therapy.J Virol 1999,
73:7108-7116.
57. Kalams SA, Buchbinder SP, Rosenberg ES,
et al.Association between virus-specific cytotoxic T-lymphocyte and helper responses in human immunodeficiency virus type 1 infection.J Virol 1999,
73:6715-6720.
58. Haas G, Plikat U, Debre P,
et al.Dynamics of viral variants in HIV-1 Nef and specific cytotoxic T lymphocytesin vivo.J Immunol 1996,
157:4212-4221.
59. Klenerman P, Phillips RE, Rinaldo CR,
et al.Cytotoxic T lymphocytes and viral turnover in HIV type 1 infection.Proc Natl Acad Sci USA 1996,
93:15323-15328.
60. Borrow P, Lewicki H, Wei X,
et al.Antiviral pressure exerted by HIV-1-specific cytotoxic T lymphocytes (CTLs) during primary infection demonstrated by rapid selection of CTL escape virus.Nat Med 1997,
3:205-211.
61. Haas G, Hosmalin A, Hadida F, Duntze J, Debre P, Autran B.
Dynamics of HIV variants and specific cytotoxic T-cell recognition in nonprogressors and progressors.Immunol Lett 1997,
57:63-68.
62. McMichael A.
T cell responses and viral escape.Cell 1998,
93:673-676.
63. Kalams SA, Goulder PJ, Shea AK,
et al.Levels of human immunodeficiency virus type 1-specific cytotoxic T-lymphocyte effector and memory responses decline after suppression of viremia with highly active antiretroviral therapy.J Virol 1999,
73:6721-6728.
64. Ogg GS, Jin X, Bonhoeffer S,
et al.Decay kinetics of human immunodeficiency virus-specific effector cytotoxic T lymphocytes after combination antiretroviral therapy.J Virol 1999,
73:797-800.
65. Ortiz GM, Jin X, Demoite MA,
et al.Containment of break-through HIV plasma viremia in the absence of antiretorviral drug therapy is associated with a broad and vigorous HIV specific cytotoxic T lymphocyte (CTL) response.Sixth Conference on Retroviruses and Opportunistic Infections. Chicago, January-February 1999 [abstract 256].
66. Ortiz GM, Nixon DF, Trkola A,
et al.HIV-1-Specific immune responses in subjects who temporarily contain virus replication after discontinuation of highly active antiretroviral therapy.J Clin Invest 1999,
104:R13-R18.
67. Rosenberg ES, Altfield M, Poon SH,
et al.Immune control of HIV-1 following early treatment of acute infection.Nature 2000,
407:523-526.
68. Kaslow RA, Carrington M, Apple R,
et al.Influence of combinations of human major histocompatibility complex genes on the course of HIV-1 infection.Nat Med 1996,
2:405-411.
69. Saah AJ, Hoover DR, Weng S,
et al.Association of HLA profiles with early plasma viral load, CD4+cell count and rate of progression to AIDS following acute HIV-1 infection. Multicenter AIDS Cohort Study.AIDS 1998,
12:2107-2113.
70. Carrington M, Nelson GW, Martin MP,
et al.HLA and HIV-1: heterozygote advantage and B*35-Cw*04 disadvantage.Science 1999,
283:1748-1752.
71. Reimann KA, Snyder GB, Chalifoux LV,
et al.An activated CD8+lymphocyte appears in lymph nodes of rhesus monkeys early after infection with simian immunodeficiency virus.J Clin Invest 1991,
88:1113-1120.
72. Shen L, Chen ZW, Letvin NL.
The repertoire of cytotoxic T lymphocytes in the recognition of mutant simian immunodeficiency virus variants.J Immunol 1994,
153:5849-5854.
73. Chen ZW, Shen L, Regan JD, Kou Z, Ghim SH, Letvin NL.
The T cell receptor gene usage by simian immunodeficiency virus gag-specific cytotoxic T lymphocytes in rhesus monkeys.J Immunol 1996,
156:1469-1475.
74. Connor RI, Montefiori DC, Binley JM,
et al.Temporal analyses of virus replication, immune responses, and efficacy in rhesus macaques immunized with a live, attenuated simian immunodeficiency virus vaccine.J Virol 1998,
72:7501-7509.
75. Kuroda MJ, Schmitz JE, Barouch DH,
et al.Analysis of Gag-specific cytotoxic T lymphocytes in simian immunodeficiency virus-infected rhesus monkeys by cell staining with a tetrameric major histocompatibility complex class I-peptide complex.J Exp Med 1998,
187:1373-1381.
76. Johnson RP, Glickman RL, Yang JQ,
et al.Induction of vigorous cytotoxic T-lymphocyte responses by live attenuated simian immunodeficiency virus.J Virol 1997,
71:7711-7718.
77. Shibata R, Siemon C, Czajak SC, Desrosiers RC, Martin MA.
Live, attenuated simian immunodeficiency virus vaccines elicit potent resistance against a challenge with a human immunodeficiency virus type 1 chimeric virus.J Virol 1997,
71:8141-8148.
78. Hirsch VM, Goldstein S, Hynes NA,
et al.Prolonged clinical latency and survival of macaques given a whole inactivated simian immunodeficiency virus vaccine.J Infect Dis 1994,
170:51-59.
79. Schmitz JE, Kuroda MJ, Santra S,
et al.Control of viremia in simian immunodeficiency virus infection by CD8(+
) lymphocytes.Science 1999,
283:857-860.
80. Jin X, Bauer DE, Tuttleton SE,
et al.Dramatic rise in plasma viremia after CD8(+
) T cell depletion in simian immunodeficiency virus-infected macaques.J Exp Med 1999,
189:991-998.
81. Schmitz JE, Kuroda MJ, Santra S,
et al.Evidence for importance of CD8+T cells in controlling SIVmac infection using a CD8 cell depleting mAbin vivoin Rhesus monkeys.Sixth Conference on Retroviruses at Opportunistic Infections. Chicago, January-February 1999 [abstract 253].
82. Nowak MA, Bangham CRM.
Population dynamics of immune responses to persistent viruses.Science 1996,
272:74-79.
83. Bucy RP.
Immune clearance of HIV-1 replication active cells: a model of two patterns of steady state HIV infection.AIDS Res Hum Retrovirus 1999,
15:223-227.
84. Hockett RD, Kilby JM, Derdeyn CA,
et al.Constant mean viral copy number per infected cell in tissues regardless of high, low, or undetectable plasma HIV RNA.J Exp Med 1999,
189:1545-1554.
85. Derdeyn CA, Kilby JM, Miralles GD,
et al.Evaluation of instinct blood lymphocyte populations in human immunodeficiency virus type 1-infected subjects in the absence or presence of effective therapy.J Infect Dis 1999,
180:1851-1862.
86. Ahmed R, Butler LD, Bhatti L.
T4+T helper cell functionin vivo:differential requirement for induction of antiviral cytotoxic T-cell and antibody responses.J Virol 1988,
62:2102-2106.
87. Matloubian M, Concepcion RJ, Ahmed R.
CD4+T cells are required to sustain CD8+cytotoxic T-cell responses during chronic viral infection.J Virol 1994,
68:8056-8063.
88. Ahmed R, Gray D.
Immunological memory and protective immunity: understanding their relation.Science 1996,
272:54-60.
89. Christensen JP, Doherty PC.
Quantitative analysis of the acute and long-term CD4(+
) T-cell response to a persistent gammaherpesvirus.J Virol 1999,
73:4279-4283.
90. Doherty PC, Topham DJ, Tripp RA.
Establishment and persistence of virus-specific CD4+and CD8+T cell memory.Immunol Rev 1996,
150:23-44.
91. Rosenberg ES.
Preserving HIV-1 specific T-cell help: will it prevent progression?Sixth Conference on Retroviruses at Opportunistic Infections. Chicago, January-February 1999 [abstract S41].
92. Banda NK, Bernier J, Kurahara DK,
et al.Crosslinking CD4 by human immunodeficiency virus gp120 primes T cells for activation-induced apoptosis.J Exp Med 992,
176:1099-1106.
93. Westendorp MO, Frank R, Ochsenbauer C,
et al.Sensitization of T cells to CD95-mediated apoptosis by HIV-1 Tat and gp120.Nature 1995,
375:497-500.
94. Sunila I, Vaccarezza M, Pantaleo G, Fauci AS, Orenstein JM.
gp120 is present on the plasma membrane of apoptotic CD4 cells prepared from lymph nodes of HIV-1-infected individuals: an immunoelectron microscopic study.AIDS 1997,
11:27-32.
95. Cohen SS, Li C, Ding L, Cao Y, Pardee AB, Shevach EM, Cohen DI.
Pronounced acute immunosuppression in vivo mediated by HIV Tat challenge.Proc Natl Acad Sci USA 1999,
96:10842-10847.
96. Gallo RC.
Tat as one key to HIV-induced immune pathogenesis and Tat toxoid as an important component of a vaccine.Proc Natl Acad Sci USA 1999,
96:8324-8326.
97. Ito M, Ishida T, He L,
et al.HIV type 1 Tat protein inhibits interleukin 12 production by human peripheral blood mononuclear cells.AIDS Res Hum Retrovirus 1998,
14:845-849.
98. Dybul M, Mercier G, Belson M,
et al.CD40 ligand trimer and IL-12 enhance peripheral blood mononuclear cells and CD4+T cell proliferation and production of IFN-gamma in rsponse to p24 atigen in HIV-infected individuals: potential contribution of anergy to HIV-specific unresponsiveness.J Immunol 2000,
165:1685-1691.
99. Lisziewicz J, Jessen H, Finzi D, Siliciano RF, Lori F.
HIV-1 suppression by early treatment with hydroxyurea, didanosine, and a protease inhibitor.Lancet 1998,
352:199-200.
100. Dalod M, Harzic M, Pellegrin I,
et al.Evolution of cytotoxic T lymphocyte responses to human immunodeficiency virus type 1 in patients with symptomatic primary infection receiving anti-retroviral triple therapy.J Infect Dis 1998,
178:61-69.
101. Lisziewicz J, Rosenberg ES, Lieberman J,
et al.Immune control of HIV after suspension of therapy.Sixth Conference on Retroviruses at Opportunistic Infections. Chicago, January-February 1999 [abstract 351].
102. Lori F, Zinn D, Varga G,
et al.Intermittent drug therapy increases the time to HIV rebound in humans and induces the control of SIV after treatment interruption in monkeys.Sixth Conference on Retroviruses at Opportunistic Infections. Chicago, January-February 1999 [abstract LB5].
103. Valentine FT, De Gruttola V, Kaplan M,
et al.Effects of HAART compared to HAART plus an inactivated HIV immunogen on lymphocyte proliferative responses (LPR) to HIV antigens.12th World AIDS Conference. Geneva, June/July 1998.
104. Clements-Mann ML, Weinhold K, Matthews TJ,
et al.Immune responses to human immunodeficiency virus (HIV) type 1 induced by canarypox expressing HIV-1MN gp120, HIV-1SF2 recombinant gp120, or both vaccines in seronegative adults. NIAID AIDS Vaccine Evaluation Group.J Infect Dis 1998,
177:1230-1246.
105. Smith KA, Jacobson EL, Sohn T,
et al.Cessation of HAART plus daily low-dose interleukin-2 to promote immunity to HIV.Seventh Conference on Retroviruses at Opportunistic Infections. San Francisco, January-February 2000 [abstract 355].
106. Fagard C, Lebraz M, Tortajada C,
et al.SITT: a prospective trial of strategic treatment interruptions.Seventh Conference on Retroviruses at Opportunistic Infections. San Francisco, January-February 2000 [abstract 458].
Section Description
Based on a Satellite Symposium conducted at the 39th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) conference
This supplement is sponsored by Chiron Corporation, 4560 Horton St., Emeryville, California 94608
