September-December 2003 Basic Science Review
T cell Turnover in HIV Infection
As reported previously in TAG's Basic Science Review, the once-popular "tap and drain" theory of CD4 T cell depletion in HIV infection has been replaced by a more nuanced understanding of the effects of the virus on the immune system. A paper by Mark Hellerstein and colleagues, along with a commentary by Mark Feinberg and Guido Silvestri, further refines this newly emerging picture of HIV pathogenesis. As Feinberg and Silvestri note, Hellerstein's data confirm the central role of HIV-induced immune activation in driving the turnover (proliferation and death) of both CD4 and CD8 T cells in the setting of untreated HIV infection. This contrasts starkly with the tap and drain theory, which proposed that HIV directly kills CD4 T cells (the drain), leading other CD4 T cells to proliferate in an attempt to replace the lost cells (the tap).
| Free full text access online: http://www.jci.org/cgi/content/full/112/6/821 J Clin Invest. 2003 Sep; 112(6): 821-4.. |
| Comment on: J Clin Invest. 2003 Sep;112(6):956-66. Turnover of lymphocytes and conceptual paradigms in HIV infection Silvestri G, Feinberg MB. Emory Vaccine Center, 954 Gatewood Road, Atlanta, Georgia 30329, USA. |
| Prevailing views concerning the pathogenic mechanisms of AIDS have shifted from models that focus primarily on direct HIV-mediated killing of CD4+ T cells to models that emphasize the pathogenic role of generalized immune system activation. The observation that increases in T cell turnover seen in HIV-infected individuals primarily reflect increased proliferation of effector-memory T cells supports the concept that chronic immune activation plays a prominent, if not predominant, role in the pathogenesis of AIDS. |
| Free full text access online: http://www.jci.org/cgi/content/full/112/6/956 J Clin Invest. 2003 Sep; 112(6): 956-66. |
| Comment on: J Clin Invest. 2003 Sep;112(6):821-4. Subpopulations of long-lived and short-lived T cells in advanced HIV-1 infection. Hellerstein MK, Hoh RA, Hanley MB, Cesar D, Lee D, Neese RA, McCune JM. University of California, Berkeley, 119 Morgan Hall, Berkeley, California 94720-3104, USA. march@nature.berkeley.edu |
| Antigenic stimulation of T cells gives rise to short-lived effector cells and long-lived memory cells. We used two stable isotope-labeling techniques to identify kinetically distinct subpopulations of T cells and to determine the effect of advanced infection with HIV-1. Long-term deuterated water (2H2O) incorporation into DNA demonstrated biphasic accrual of total and of memory/effector (m/e)-phenotype but not naive-phenotype T cells, consistent with the presence of short-lived and longer-lived subpopulations within the m/e-phenotype T cell pool. These results were mirrored by biphasic die-away kinetics in m/e- but not naive-phenotype T cells after short-term 2H-glucose labeling. Persistent label retention was observed in a subset of m/e-phenotype T cells (presumably memory T cells), confirming the presence of T cells with very different life spans in humans. In advanced HIV-1 infection, much higher proportions of T cells were short-lived, compared to healthy controls. Effective long-term anti-retroviral therapy restored values to normal. These results provide the first quantitative evidence that long-lived and quiescent T cells do indeed predominate in the T cell pool in humans and determine T cell pool size, as in rodents. The greatest impact of advanced HIV-1 infection is to reduce the generation of long-lived, potential progenitor T cells. |
CD4 T cell Responses to HIV: Correlate of Immune Control?
The question of whether T cells targeting HIV (HIV-specific T cells) play a role in controlling viral replication is highly controversial. Some studies have found that the number of HIV-specific T cells correlates inversely with viral load (e.g. the higher the HIV-specific T cell response, the lower the viral load) whereas others have reported opposite findings (the higher the viral load, the greater the numbers of HIV-specific T cells). Two new papers suggest that differences in the techniques used to quantify HIV-specific T cells may explain these seemingly irreconcilable findings. The papers focus on HIV-specific CD4 T cell responses, which previous studies have generally assessed using either a lymphoproliferation assay (a cumbersome six-day test that roughly gauges the ability of CD4 T cells to proliferate in response to stimulation with HIV antigens) or an interferon-gamma ELISpot assay (a rapid test that quantifies HIV-specific T cells based on their ability to produce the cytokine interferon-gamma after stimulation with HIV antigens). Generally, lymphoproliferation assays have revealed an inverse correlation between HIV-specific CD4 T cell responses and viral load, whereas interferon-gamma ELISpots have shown a positive correlation.
The two new papers—one from Guiseppe Pantaleo's research group in Switzerland and the other from Rafik Sejaly's group in Montreal—evaluated HIV-specific CD4 T cell responses using an ELISpot assay that captured cells based on their ability to produce of the cytokine interleukin-2 (IL-2). The results showed that this technique captured cells capable of proliferation in response to HIV antigens, and higher numbers of IL-2-producing HIV-specific CD4 T cells were associated with lower viral loads. In contrast, higher viral loads were associated with increased numbers of interferon-gamma-producing HIV-specific CD4 T cells but reduced or absent IL-2-producing HIV-specific CD4 T cells.
Sekaly and colleagues suggest that persistently high levels of HIV replication prevent the establishment of long-lived HIV-specific memory CD4 T cells capable of producing IL-2. Pantaleo's study adds the potentially encouraging finding that the number of these cells increases after initiation of HAART, although on average they made up 0.04% of total CD4 T cells in this setting, compared to 0.1% of T cells in long-term non-progressors (who have low viral loads in the absence of any therapeutic intervention). It will be important for future studies to delineate whether IL-2-producing HIV-specific CD4 T cells truly play a causative role in reducing viral replication, or whether low levels of replication simply allows the generation of these cells.
| http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14676292&dopt=Abstract |
| J Exp Med. 2003 Dec 15; 198(12): 1773-7. Progress in defining CD4 helper cell responses in chronic viral infections. Day CL, Walker BD. Peter Medawar Building for Pathogen Research, Nuffield Department of Medicine, University of Oxford, Oxford, UK. |
| J Exp Med. 2003 Dec 15; 198(12): 1909-22. HIV-1 viremia prevents the establishment of interleukin 2-producing HIV-specific memory CD4+ T cells endowed with proliferative capacity. Younes SA, Yassine-Diab B, Dumont AR, Boulassel MR, Grossman Z, Routy JP, Sekaly RP. Departement de Microbiologie et Immunologie, Universite de Montreal, 2900 Edouard-Montpetit Boulevard, Montreal H3T 1J4, Canada. |
| CD4+ T cell responses are associated with disease control in chronic viral infections. We analyzed human immunodeficiency virus (HIV)-specific responses in ten aviremic and eight viremic patients treated during primary HIV-1 infection and for up to 6 yr thereafter. Using a highly sensitive 5-(and-6)-carboxyfluorescein diacetate-succinimidyl ester-based proliferation assay, we observed that proliferative Gag and Nef peptide-specific CD4+ T cell responses were 30-fold higher in the aviremic patients. Two subsets of HIV-specific memory CD4+ T cells were identified in aviremic patients, CD45RA- CCR7+ central memory cells (Tcm) producing exclusively interleukin (IL)-2, and CD45RA- CCR7- effector memory cells (Tem) that produced both IL-2 and interferon (IFN)-gamma. In contrast, in viremic, therapy-failing patients, we found significant frequencies of Tem that unexpectedly produced exclusively IFN-gamma. Longitudinal analysis of HIV epitope-specific CD4+ T cells revealed that only cells that had the capacity to produce IL-2 persisted as long-term memory cells. In viremic patients the presence of IFN-gamma-producing cells was restricted to periods of elevated viremia. These findings suggest that long-term CD4+ T cell memory depends on IL-2-producing CD4+ T cells and that IFN-gamma only-producing cells are short lived. Our data favor a model whereby competent HIV-specific Tcm continuously arise in small numbers but under persistent antigenemia are rapidly induced to differentiate into IFN-gamma only-producing cells that lack self-renewal capacity. |
| http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12958069&dopt=Abstract |
| Blood. 2003 Sep 4 [Epub ahead of print] Skewed representation of functionally distinct populations of virus-specific CD4 T cells in HIV-1-infected subjects with progressive disease: changes after antiretroviral therapy. Harari A, Petitpierre S, Vallelian F, Pantaleo G. Laboratory of AIDS Immunopathogenesis, Divisions of Immunology and Allergy and Infectious Diseases, Department of Medicine, Centre Hospitalier Universitaire Vaudois, University of Lausanne, Lausanne, Switzerland. |
| HIV-1- and cytomegalovirus (CMV)-specific CD4 T-cell mediated antiviral immunity was evaluated by assessing the frequency of IL-2 and IFN-gamma secreting cells following antigen-specific stimulation in blood and lymph node. HIV-1 infected subjects with progressive disease at early stage of infection with no previous history of antiretroviral therapy (ART), subjects with non-progressive disease and HIV-negative subjects were studied. On the basis of the ability to secrete IL-2 and IFN-gamma three functionally distinct populations of CD4 T cells were identified: a) IL-2 secreting cells; b) IL-2-IFN-gamma secreting cells; and c) IFN-gamma secreting cells. CMV-specific CD4 T cells were almost equally distributed within the three functionally distinct cell populations in the three study groups as well as HIV-1-specific CD4 T cells in subjects with non-progressive disease. However, a skewing towards IFN-gamma secreting cells (70% of HIV-1-specific CD4 T cells) was observed in subjects with progressive disease and IL-2 and IL-2-IFN-gamma secreting cells were almost absent. The frequencies of IL-2 and of IL-2-IFN-gamma secreting HIV-1-specific CD4 T cells were negatively correlated with the levels of viremia. Interestingly, prolonged ART was able to correct the skewed representation of different populations of HIV-1-specific CD4 T cells but was associated with only a partial recovery of IL-2 secreting cells. These results indicate that the composition of the pool of functionally distinct virus-specific CD4 T cells is important for virus control. |
| http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14512540&dopt=Abstract |
| J Virol. 2003 Oct; 77(20): 10900-9. Diminished proliferation of human immunodeficiency virus-specific CD4+ T cells is associated with diminished interleukin-2 (IL-2) production and is recovered by exogenous IL-2. Iyasere C, Tilton JC, Johnson AJ, Younes S, Yassine-Diab B, Sekaly RP, Kwok WW, Migueles SA, Laborico AC, Shupert WL, Hallahan CW, Davey RT Jr, Dybul M, Vogel S, Metcalf J, Connors M. Laboratory of Immunoregulation, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892-1876, USA. |
| Virus-specific CD4(+) T-cell function is thought to play a central role in induction and maintenance of effective CD8(+) T-cell responses in experimental animals or humans. However, the reasons that diminished proliferation of human immunodeficiency virus (HIV)-specific CD4(+) T cells is observed in the majority of infected patients and the role of these diminished responses in the loss of control of replication during the chronic phase of HIV infection remain incompletely understood. In a cohort of 15 patients that were selected for particularly strong HIV-specific CD4(+) T-cell responses, the effects of viremia on these responses were explored. Restriction of HIV replication was not observed during one to eight interruptions of antiretroviral therapy in the majority of patients (12 of 15). In each case, proliferative responses to HIV antigens were rapidly inhibited during viremia. The frequencies of cells that produce IFN-gamma in response to Gag, Pol, and Nef peptide pools were maintained during an interruption of therapy. In a subset of patients with elevated frequencies of interleukin-2 (IL-2)-producing cells, IL-2 production in response to HIV antigens was diminished during viremia. Addition of exogenous IL-2 was sufficient to rescue in vitro proliferation of DR0101 class II Gag or Pol tetramer(+) or total-Gag-specific CD4(+) T cells. These observations suggest that, during viremia, diminished in vitro proliferation of HIV-specific CD4(+) T cells is likely related to diminished IL-2 production. These results also suggest that relatively high frequencies of HIV-specific CD4(+) T cells persist in the peripheral blood during viremia, are not replicatively senescent, and proliferate when IL-2 is provided exogenously. |
HIV Under the Atom Microscope
A new study, published in the Journal of Virology by Y. G. Kuznetsov and colleagues from the University of California-Irvine, offers a series of stunning images of HIV virions obtained using a technique called atomic force microscopy (AFM). AFM produces an image after scanning biological materials using a method similar to the way a record play stylus scans the groove of a vinyl record. The images of HIV reveal particles averaging 120 nanometers in diameter, with "tufts" of gp120 protein on the outer surface. The researchers note that the images call into question the widely-held idea that gp120 proteins form triple structures called trimers on the outside of virions. Rather, the conclude that "it seems probable that the individual gp120 monomers at the ends of gp41 stems, like heads of flowers in a bouquet, are otherwise separated from one another and make no systematic protein-protein contacts. Associations are arbitrary with neighbors, and the groupings therefore exhibit no consistent symmetry or shape."
| http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14581526&dopt=Abstract |
| J Virol. 2003 Nov; 77(22): 11896-909. Atomic force microscopy investigation of human immunodeficiency virus (HIV) and HIV-infected lymphocytes. Kuznetsov YG, Victoria JG, Robinson WE Jr, McPherson A. Department of Molecular Biology and Biochemistry, University of California-Irvine, Irvine, California 92697-3900, USA. |
| Isolated human immunodeficiency virus (HIV) and HIV-infected human lymphocytes in culture have been imaged for the first time by atomic force microscopy (AFM). Purified virus particles spread on glass substrates are roughly spherical, reasonably uniform, though pleomorphic in appearance, and have diameters of about 120 nm. Similar particles are also seen on infected cell surfaces, but morphologies and sizes are considerably more varied, possibly a reflection of the budding process. The surfaces of HIV particles exhibit "tufts" of protein, presumably gp120, which do not physically resemble spikes. The protein tufts, which number about 100 per particle, have average diameters of about 200 A, but with a large variance. They likely consist of arbitrary associations of small numbers of gp120 monomers on the surface. In examining several hundred virus particles, we found no evidence that the gp120 monomers form threefold symmetric trimers. Although >95% of HIV-infected H9 lymphocytic cells were producing HIV antigens by immunofluorescent assay, most lymphocytes displayed few or no virus on their surfaces, while others were almost covered by a hundred or more viruses, suggesting a dependence on cell cycle or physiology. HIV-infected cells treated with a viral protease inhibitor and their progeny viruses were also imaged by AFM and were indistinguishable from untreated virions. Isolated HIV virions were disrupted by exposure to mild neutral detergents (Tween 20 and CHAPS) at concentrations from 0.25 to 2.0%. Among the products observed were intact virions, the remnants of completely degraded virions, and partially disrupted particles that lacked sectors of surface proteins as well as virions that were split or broken open to reveal their empty interiors. Capsids containing nucleic acid were not seen, suggesting that the capsids were even more fragile than the envelope and were totally degraded and lost. From these images, a good estimate of the thickness of the envelope protein-membrane-matrix protein outer shell of the virion was obtained. Treatment with even low concentrations (<0.1%) of sodium dodecyl sulfate completely destroyed all virions but produced many interesting products, including aggregates of viral proteins with strands of nucleic acid. |
Fair Use Notice: The abstracts included in this review are reproduced as fair use of copyrighted material for nonprofit educational purposes, as provided for in Section 107 of the US Copyright Law (Title 17 U.S.C. Sect. 107).
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