The immunodeficiency that follows HIV infection is related to the virus-mediated killing of infected CD4+ T cells, the chronic activation of the immune system, and the impairment of T cell production. In this study we show that in HIV-infected individuals the loss of IL-7R (CD127) expression defines the expansion of a subset of CD8+ T cells, specific for HIV as well as other Ags, that show phenotypic (i.e., loss of CCR7 and CD62 ligand expression with enrichment in activated and/or proliferating cells) as well as functional (i.e., production of IFN-γ, but not IL-2, decreased ex vivo proliferative potential and increased susceptibility to apoptosis) features of effector T cells. Importantly, in HIV-infected individuals the levels of CD8+CD127 T cells are directly correlated with the main markers of disease progression (i.e., plasma viremia and CD4+ T cell depletion) as well as with the indices of overall T cell activation. In all, these results identify the expansion of CD8+CD127 effector-like T cells as a novel feature of the HIV-associated immune perturbation. Further studies are thus warranted to determine whether measurements of CD127 expression on CD8+ T cells may be useful in the clinical management of HIV-infected individuals.

Naive CD8+ T (TN)4 cells that encounter their cognate Ag undergo a complex process of maturation and differentiation that ultimately leads to the generation of a sizeable pool of long-lived memory CD8+ T (TM) cells that mediate immune protection from subsequent challenge with the same Ag (reviewed in Ref.1). A series of elegant studies in the murine model of lymphocytic choriomeningitis virus infection show that Ag-experienced TN cells differentiate first into short-lived, rapidly proliferating effector CD8+ T (TE) cells, which express a number of activation markers (i.e., CD25, CD44), have down-regulated the lymph node homing receptors (i.e., CD62L, CCR7), and display strong immediate effector function (i.e., IFN-γ production and CTL activity) (2, 3, 4). Within a few weeks a subset of these TE cells further differentiate into long-lived, slowly proliferating memory CD8+ T (TM) cells, which re-express CD62 ligand (CD62L) and CCR7, show limited immediate effector function, but exhibit a high proliferative potential upon antigenic restimulation (2, 3, 4). Adoptive transfer experiments indicate that Ag-specific TM cells are more effective than TE cells in mediating protection from challenge (2, 3, 4).

In humans, TM cell differentiation is likely regulated in a different and more complex manner, with specific features of TM cell differentiation that may vary following infection with different pathogens, such as HIV, EBV, CMV, and hepatitis C virus (5, 6, 7, 8, 9, 10). Essentially, two main models of TM cell differentiation in humans have been proposed. In the first model, Ag-specific CD8+ T cells differentiate from CCR7+CD62L+CD45RA+ TN cells to CCR7+CD62L+CD45RA central memory T (TCM) cells, then to CCR7CD62LCD45RA effector memory (TEM) cells and finally to CCR7CD62LCD45RA+ TEM cells (TEMRA or terminally differentiated effectors) (5, 6, 7, 8). In the second model, CD27highCD28+ TN cells differentiate into early (i.e., CD27+CD28+), intermediate (CD27+CD28), and late (CD27CD28) memory T cells (9, 10). Above and beyond the use of distinct sets of markers to define specific subsets of TM cells, these two models differ in that the first describes T cell differentiation as a linear process, while the second allows for branches in cell lineage (5, 6, 7, 8, 9, 10).

HIV infection is characterized by progressive CD4+ T cell depletion and immunodeficiency that paradoxically occur in the context of a chronic state of immune system activation. Increasing evidence suggests that the generalized immune activation that follows HIV infection plays a prominent, if not predominant, role in the pathogenesis of AIDS (11, 12, 13, 14, 15) and that the extent of this immune activation is as good, or perhaps even better, a predictor of disease progression than the level of viral replication (16, 17, 18, 19, 20, 21). These observations are consistent with the report that, in the nonpathogenic SIV infection of sooty mangabeys (a natural host species for SIV infection), minimal levels of immune activation and no evidence of immunodeficiency are observed despite chronic high levels of virus replication (22). However, the mechanisms by which chronic immune activation precipitates the development of immune system compromise and susceptibility to opportunistic infections following HIV infection are still poorly understood. This lack of understanding is partly related to the fact that the functional role of the markers of T cell activation that are associated with HIV disease progression, such as the expression of CD38 and HLA-DR on CD8+ T cells, is still largely unknown.

Recent evidence indicates that expression of IL-7R α-chain (CD127) on lymphocytic choriomeningitis virus-specific CD8+ T cells during the generation of primary antiviral immune responses identifies a subset of TE cells that differentiate successfully into fully protective TM cells (4). In analogy with this model, we hypothesized that a determinant of the chronic immune activation observed in HIV-infected individuals is the expansion of activated/TE cells that do not express of CD127 and thus fail to properly differentiate in TM cells. In addition, given the known role of IL-7 in regulating T cell homeostasis, a dysregulation of the IL-7/IL-7R system may also support a pathogenic model in which the HIV-associated CD4+ T cell depletion is associated with impaired T cell production (23). We report that HIV infection is consistently associated with the expansion of CD8+CD127 T cells (specific for HIV as well as for other Ags) that show immunophenotypic and functional features of TE cells, and that the levels of CD8+CD127 T cells in HIV-infected patients correlate with markers of disease progression (i.e., plasma viremia and CD4+ T cell depletion) as well as with the overall levels of immune system activation. We propose that in HIV-infected individuals the chronic expansion of CD8+CD127 TE-like cells may be both a determinant and a consequence of the heightened levels of immune activation and bystander apoptosis that are associated with CD4+ T cell depletion and progression to AIDS.

All subjects included in this study were enrolled at the HIV Infection Clinic of the Crawford and Long Hospital, the Hope Clinic of the Emory Vaccine Center, and the Grady Infectious Disease Program (all affiliated with Emory University, Atlanta, GA), or at the Service of Clinical Immunology, University of Ancona (Ancona, Italy). Individuals included in this study were: 1) HIV-1-infected therapy-naive adults with CD4+ T cell counts >50 per μl and levels of plasma viremia >5000 HIV-1 RNA copies/ml; 2) HIV-1-infected adults treated with highly active antiretroviral therapy (HAART; at least 2 reverse transcriptase inhibitors and 1 protease inhibitor) for at least 6 mo and with <400 HIV-1 RNA copies/ml; and 3) healthy, age-matched HIV-uninfected controls. The clinical, immunologic, and virologic characteristics of the studied patients are displayed in Table I. These studies were approved by the Institutional Review Board of Emory University and all participants gave written informed consent.

Table I.

Clinical, immunologic, and virologic characteristics of the studied patients

CodeAge (y)SexCD4+ T CellViral LoadHAART
38 389 100,000 No 
57 111 43,400 Yes 
41 286 289 Yes 
43 454 110 Yes 
46 450 291,000 No 
52 431 370 Yes 
38 427 85,300 No 
47 1250 <50 Yes 
63 92 100,000 No 
10 31 385 200 Yes 
11 46 237 <50 Yes 
12 56 260 460 Yes 
13 60 400 333 Yes 
14 36 464 46,700 No 
15 40 282 <50 Yes 
16 36 332 30,100 No 
17 57 393 <50 Yes 
18 29 325 234,000 No 
19 39 392 15,500 No 
20 26 597 1300 No 
21 36 695 8600 Yes 
22 41 603 400 Yes 
23 29 312 85,500 No 
24 40 363 82,200 No 
25 49 372 502,000 No 
26 46 182 675,000 No 
27 45 586 2,100 No 
28 41 200 148,000 No 
29 45 190 88,800 No 
30 35 144 154,000 No 
31 34 600 750 No 
32 38 304 15,000 Yes 
33 43 325 74,900 No 
34 22 314 110,000 No 
35 35 179 <50 Yes 
36 32 765 16,663 No 
37 31 350 891 Yes 
38 34 195 143,000 No 
39 45 631 91,960 No 
40 31 338 10,715 Yes 
41 33 213 102,329 No 
42 41 88 400 No 
43 36 580 400 Yes 
44 38 470 60 No 
45 39 434 31,486 No 
46 37 306 150 No 
47 64 578 1,235 Yes 
48 29 637 7,033 Yes 
49 60 479 3,656 Yes 
50 44 495 8,106 No 
51 40 140 <50 Yes 
CodeAge (y)SexCD4+ T CellViral LoadHAART
38 389 100,000 No 
57 111 43,400 Yes 
41 286 289 Yes 
43 454 110 Yes 
46 450 291,000 No 
52 431 370 Yes 
38 427 85,300 No 
47 1250 <50 Yes 
63 92 100,000 No 
10 31 385 200 Yes 
11 46 237 <50 Yes 
12 56 260 460 Yes 
13 60 400 333 Yes 
14 36 464 46,700 No 
15 40 282 <50 Yes 
16 36 332 30,100 No 
17 57 393 <50 Yes 
18 29 325 234,000 No 
19 39 392 15,500 No 
20 26 597 1300 No 
21 36 695 8600 Yes 
22 41 603 400 Yes 
23 29 312 85,500 No 
24 40 363 82,200 No 
25 49 372 502,000 No 
26 46 182 675,000 No 
27 45 586 2,100 No 
28 41 200 148,000 No 
29 45 190 88,800 No 
30 35 144 154,000 No 
31 34 600 750 No 
32 38 304 15,000 Yes 
33 43 325 74,900 No 
34 22 314 110,000 No 
35 35 179 <50 Yes 
36 32 765 16,663 No 
37 31 350 891 Yes 
38 34 195 143,000 No 
39 45 631 91,960 No 
40 31 338 10,715 Yes 
41 33 213 102,329 No 
42 41 88 400 No 
43 36 580 400 Yes 
44 38 470 60 No 
45 39 434 31,486 No 
46 37 306 150 No 
47 64 578 1,235 Yes 
48 29 637 7,033 Yes 
49 60 479 3,656 Yes 
50 44 495 8,106 No 
51 40 140 <50 Yes 

PBMCs were isolated from whole blood by density gradient centrifugation using standard procedures. In some cases purified PBMCs that had been cryopreserved in liquid nitrogen were thawed before analysis; in these cases the staining involved using Abs directed against CD3, CD8, CD127, and HLA class I tetramers only. PBMCs were analyzed by four-color fluorescent Ab staining to determine the percentage and absolute number of specific cell subpopulations (calculated from the total white blood cell count and the percentage of lymphocytes in the white blood cell differential). Intracellular and surface staining were performed using the following Abs: anti-CD4 PerCP, anti-CD8 PerCP, anti-CD62L FITC, anti-CCR5 CyChrome, anti-CXCR4 CyChrome, anti-CD45RO FITC, anti-HLA-DR allophycocyanin, anti-CD95 allophycocyanin, anti-Ki-67 FITC (all from BD Pharmingen), anti-CD45RA TriColor (Caltag Laboratories), anti-CD127-PE (Corixa). Isotype-matched controls were used in all experiments. To stain for CCR7 we used the allophycocyanin-labeled tetrameric MIP3β complex (24). Flow cytometric acquisition and analysis of samples were performed on at least 10,000 acquired events, gated on lymphocytes, on a FACSCalibur flow cytometer driven by the CellQuest software package (BD Pharmingen). Data analyses were performed using the FlowJo software (TreeStar). Cell sorting experiments were performed on a MoFlow (Cytomation).

Tetrameric HLA class I molecules were produced as previously described (25). Prokaryotic expression vectors encoding the extracellular portion of the HLA-A2*0201 and β2-microglobulin molecules (tagged to a birA recognition sequence) were separately expressed in Escherichia coli after isopropyl β-d-thiogalactoside-induction and isolated from inclusion bodies. HLA-A*0201 and β2-microglobulin chains were folded in the presence of the appropriate peptides (HIVgag-p17SLYNTVATL, CMVpp65NLVPMVATV, EBVbmfl-1GLCTLVAML). Folded trimolecular complexes were biotinylated with BirA enzyme and FPLC purified on a MonoQ column. PE-streptavidin was finally mixed with biotinylated monomers to obtain the tetrameric HLA complex.

Intracellular cytokine production by PBMCs was assessed by flow cytometry. FITC-conjugated Abs against human IL-2, IFN-γ, and TNF-α (all from BD Pharmingen) were used, and staining with a pool of appropriate isotype-matched Abs was included as a negative control. PBMCs were incubated for 4 h in medium containing phorbol 12-myristate 13-acetate (10 ng/ml), the calcium ionophore A23187 (200 ng/ml), and either monensin (10 nM) or brefeldin A (5 nM) as inhibitors of Golgi transport (all from Sigma-Aldrich). Cells were first surface stained with anti-CD4 allophycocyanin, anti-CD8 PerCP, and anti-CD127 PE mAbs, and then fixed and permeabilized using the CytoFix/CytoPerm kit (BD Pharmingen).

Studies of the expression of CD127 after ex vivo stimulation were performed on PBMCs treated either with rIL-7 (10 ng/ml; BD Pharmingen) or PHA (10 μg/ml; Sigma-Aldrich). Staining for CD127 was performed at 7, 22, and 72 h posttreatment. In vitro proliferation after treatment with PHA was determined in sorted CD8+CD127+ and CD8+CD127 cells using CFSE labeling. The fraction of proliferating cells was measured by flow cytometry at 72 and 120 h postactivation. Studies of spontaneous and activation-induced apoptosis were performed in CD127+ and CD127 T cells after a 48-h incubation either with no stimulus (spontaneous apoptosis) or with PHA (activation-induced apoptosis). Rates of apoptotic cells were determined by flow cytometry as a percentage of cells reactive to Annexin VFITC (BD Pharmingen).

IL-7 levels in the plasma of HIV-infected patients and controls were estimated using the commercial sandwich enzyme immunoassay kit (R&D Systems). Anticoagulants other than heparin, which may interfere with plasma IL-7 measurements, were used at the time of blood collection. In preliminary experiments the specificity of the IL-7 ELISA was ascertained by blocking with anti-IL-7 antisera (R&D Systems). The reproducibility of the IL-7 ELISA was assessed through the incorporation of a control plasma sample in each assay, and variations were between 0.1 and 5%.

The performed analyses include the two-tailed Student t test for the comparison between groups, whereas correlations involving different sets of data within the same group were determined using the Spearman’s rank correlation coefficient. Significance was assessed at the p = 0.05 level. All analyses were performed using SAS software.

To determine whether HIV infection is associated with aberrant regulation of CD127 expression on T cells, we used multiparametric flow cytometry to perform an immunophenotypic analysis of peripheral blood lymphocytes from both HIV-infected patients (either untreated or treated with HAART) and healthy uninfected controls. The clinical, immunologic, and virologic characteristics of the studied patients are displayed in Table I. As shown in Fig. 1,A, untreated HIV-infected patients exhibited a significant decrease in CD127 expression (measured as a percentage of CD127+ cells) on both CD4+ and CD8+ T cells, with a more pronounced decrease observed in the CD8+ T cell population, a result consistent with previous studies (26, 27, 28). These described results are also consistent with the observation that the levels of CD127 mRNA are ∼4- to 6-fold decreased in sorted CD8+ T cells isolated from HIV-infected patients as compared with CD8+ T cells of uninfected controls (data not shown). When the expression of CD127 was examined in TN (CD45RA+CD62L+) and TM (i.e., non-naive) cells, we found that the decline of CD127 expression primarily involved TM lymphocytes (Fig. 1, A and B), although a small but significant reduction in CD127 expression was observed in TN cells as well. To then determine whether the decreased percentage of CD8+CD127+ memory T cells reflected an absolute reduction of this cell subset or, alternatively, an expansion of CD8+CD127 memory T cells, we calculated the absolute counts per cubic millimeter of CD127+ and CD127 cells in the total CD8+ T cell population as well as in the TN and TM cell subsets. As shown in Fig. 1,C, the absolute numbers of CD8+ T cells expressing CD127 were similar in HIV-infected and uninfected individuals, whereas the absolute counts of CD8+CD127 T cells were increased in untreated HIV-infected patients compared with both uninfected controls and HAART-treated HIV-infected patients. Importantly, a marked absolute increase of CD8+CD127 memory T cells was consistently observed in HIV-infected patients, whereas only a small increase in the number of CD8+CD127 naive T cells was present in the same group of patients (Fig. 1 C). In all, these results indicate that HIV infection is associated with a significant expansion of TM (i.e., non-naive) cells that have lost the expression of CD127.

FIGURE 1.

Decreased CD127 expression on TM (i.e., non-naive) cells from HIV-infected patients correlates with disease progression and immune activation, but not with prevailing IL-7 plasma levels. A, Mean expression of CD127 (IL-7R α-chain) on total, memory, and naive CD4+ and CD8+ T cells. Naive cells are defined as CD45RA+CD62L+, and all non-naive cells are considered memory. The analysis was performed on 18 untreated HIV-infected patients (▪), 13 HAART-treated (▦), and 10 healthy controls (□). Statistical analyses were performed between controls and untreated HIV-infected patients, and between HIV-infected patients (untreated vs HAART-treated). B, Representative dot plots showing the expression of (Figure legend continues) CD127 on CD8+ T cells in one untreated HIV-infected patient, one HAART-treated, and one control (gate on lymphocyte region). Numbers in the upper right quadrant indicate the percentage of CD8+ T cells that are CD127+ and CD127, respectively. C, Absolute counts per cubic millimeter of total, memory, and naive CD8+ T cells expressing or not expressing CD127 are shown. The analysis was performed on the same individuals as in A. D, Expansion of CD8+CD127 T cells is correlated directly with plasma viremia (top left), inversely with CD4+ T cell counts (top right), and directly with expression of Ki-67 on CD8+ T cells (bottom left). No correlation was found between CD8+CD127 T cells and plasma levels of IL-7 (bottom right). All analyses were performed on the 31 HIV-infected patients described in A. Best fitting curves are shown. E, Sequential analysis of CD127 expression on CD8+ T cells (measured as percentage of initial expression) treated with PHA (dashed line), rIL-7 (solid line), or medium alone (dotted line) for 96 h. Results represent the mean of five independent experiments performed using PBMCs from healthy donors.

FIGURE 1.

Decreased CD127 expression on TM (i.e., non-naive) cells from HIV-infected patients correlates with disease progression and immune activation, but not with prevailing IL-7 plasma levels. A, Mean expression of CD127 (IL-7R α-chain) on total, memory, and naive CD4+ and CD8+ T cells. Naive cells are defined as CD45RA+CD62L+, and all non-naive cells are considered memory. The analysis was performed on 18 untreated HIV-infected patients (▪), 13 HAART-treated (▦), and 10 healthy controls (□). Statistical analyses were performed between controls and untreated HIV-infected patients, and between HIV-infected patients (untreated vs HAART-treated). B, Representative dot plots showing the expression of (Figure legend continues) CD127 on CD8+ T cells in one untreated HIV-infected patient, one HAART-treated, and one control (gate on lymphocyte region). Numbers in the upper right quadrant indicate the percentage of CD8+ T cells that are CD127+ and CD127, respectively. C, Absolute counts per cubic millimeter of total, memory, and naive CD8+ T cells expressing or not expressing CD127 are shown. The analysis was performed on the same individuals as in A. D, Expansion of CD8+CD127 T cells is correlated directly with plasma viremia (top left), inversely with CD4+ T cell counts (top right), and directly with expression of Ki-67 on CD8+ T cells (bottom left). No correlation was found between CD8+CD127 T cells and plasma levels of IL-7 (bottom right). All analyses were performed on the 31 HIV-infected patients described in A. Best fitting curves are shown. E, Sequential analysis of CD127 expression on CD8+ T cells (measured as percentage of initial expression) treated with PHA (dashed line), rIL-7 (solid line), or medium alone (dotted line) for 96 h. Results represent the mean of five independent experiments performed using PBMCs from healthy donors.

Close modal

We next investigated a possible correlation between the level of expansion of CD8+CD127 memory T cells and the main biological markers of disease progression during HIV infection, i.e., HIV plasma viremia and severity of CD4+ T cell depletion. As shown in Fig. 1,D, the level of expansion of CD127 TM cells was directly correlated with plasma viremia and inversely correlated with CD4+ T cell count. Consistent with the finding that the expansion of CD8+CD127 memory T cells is directly correlated with levels of HIV replication is the observation that HIV-infected patients that are treated with HAART show lower levels of CD8+CD127 memory T cells than untreated HIV-infected patients (Fig. 1, A and C). Interestingly, a significant correlation was also found between the percentage of CD8+CD127 memory T cells and the level of immune activation as measured by the expression of the proliferation marker Ki-67 in CD8+ (Fig. 1,D) and CD4+ T cells (data not shown). In addition, a direct correlation was found between the number of CD8+CD127 memory T cells and the level of T cells expressing the activation markers HLA-DR and CD95 (data not shown). In contrast, no correlation was found between plasma levels of IL-7 and either the percentage (Fig. 1,D) or the absolute count (data not shown) of CD8+CD127 memory T cells. These results suggest that the prevailing plasma levels of IL-7, which are inversely correlated with CD4+ T cell counts in HIV-infected patients (29, 30), do not determine the levels of IL-7R expression on CD8+ T cells. Because IL-7 has the potential to down-modulate CD127 expression both in vitro and in vivo (4, 31, 32, 33, 34), we sequentially measured CD127 expression on CD8+ T cells after ex vivo treatment with either rIL-7 or the mitogen PHA. As shown in Fig. 1,E, we found that a transient down-modulation of CD127 follows treatment with rIL-7, whereas a more persistent loss of CD127 was observed when CD8+ T cells were treated with PHA. Taken together with the observation that the expansion of CD8+CD127 T cells correlates with the levels of immune activation but not with plasma levels of IL-7 (Fig. 1 D), this result suggests that chronic antigenic stimulation may be more important than the plasma level of IL-7 in determining the unusual abundance of CD8+CD127 T cells seen in HIV-infected individuals. An additional alternative possibility, given the role of IL-7 in regulating T cell homeostasis, is that the peripheral (i.e., postthymic) expansion of memory CD8+CD127 T cells is related, as a compensatory homeostatic mechanism, to a state of impaired intrathymic T cell production that may follow HIV infection (23).

We next sought to determine whether the loss of CD127 expression on TM (i.e., non-naive) cells involved predominantly HIV-specific lymphocytes. To this end, we examined, in a subset of HLA-A*0201-positive HIV-infected patients, the level of CD127 expression on HIV-specific CD8+ T cells using the HIVgag-p17SLYNTVATL tetramer. As shown in Fig. 2, we found similarly reduced levels of CD127 expression in the HIV-specific CD8+ T cells as in the bulk population of TM cells of the same patients. Interestingly, CD127 expression on CD8+ T cells specific for other Ags, such as the HLA-A*0201-restricted CMVpp65NLVPMVATV and EBVbmfl-1GLCTLVAML epito-pes, was significantly reduced in HIV-infected patients as compared with EBV- and CMV-specific CD8+ T cells isolated from healthy HIV-uninfected controls (Fig. 2 B). These results indicate that the HIV-associated expansion of CD8+CD127 memory T cells does not involve exclusively HIV-specific TM cells.

FIGURE 2.

Decreased CD127 expression on Ag-specific CD8+ T cells. A, CD127 expression in HIVgag-p17SLYNTVATL- and EBVbmfl-1GLCTLVAML-specific CD8+ T cells from a representative HIV-infected patient (top four dot plots), and from an uninfected healthy control (bottom two dot plots) is shown. The CD127 vs tetramer dot plots are pregated on total CD8+ T cells. Numbers in the top quadrants indicate the percentage of CD127+ cells among the tetramer-positive and the tetramer-negative populations, respectively. B, Mean expression of CD127 on CD8+ T cells specific for HIVgag-p17SLYNTVATL, CMVpp65NLVPMVATV, and EBVbmfl-1GLCTLVAML in 19 HLA-A*0201-positive HIV-infected patients and 5 HLA-A*0201-pos healthy controls.

FIGURE 2.

Decreased CD127 expression on Ag-specific CD8+ T cells. A, CD127 expression in HIVgag-p17SLYNTVATL- and EBVbmfl-1GLCTLVAML-specific CD8+ T cells from a representative HIV-infected patient (top four dot plots), and from an uninfected healthy control (bottom two dot plots) is shown. The CD127 vs tetramer dot plots are pregated on total CD8+ T cells. Numbers in the top quadrants indicate the percentage of CD127+ cells among the tetramer-positive and the tetramer-negative populations, respectively. B, Mean expression of CD127 on CD8+ T cells specific for HIVgag-p17SLYNTVATL, CMVpp65NLVPMVATV, and EBVbmfl-1GLCTLVAML in 19 HLA-A*0201-positive HIV-infected patients and 5 HLA-A*0201-pos healthy controls.

Close modal

To better characterize the immunophenotypic features of the expanded CD8+CD127 T cells we used multiparametric flow cytometric analysis to assess the expression of memory T cell differentiation markers on both the CD8+CD127+ and the CD8+CD127 T cell subsets. Specifically, we used staining with CD45RA, CD62L, and CCR7 to determine the distribution of CD8+CD127+ and CD8+CD127 T cells within the subsets of TN (CD62L+, CD45RA+, and CCR7+), TCM cells (CD62L+, CD45RA, CCR7+), TEM cells (CD62L, CD45RA, CCR7), and CD45RA+ TEMRA CELLS (CD62L, CD45RA+, CCR7) (5, 6, 7, 8). As shown in Fig. 3,A, we found that in both HIV-infected patients and controls CD8+CD127+ T cells comprised essentially cells expressing markers of TN and TCM differentiation (i.e., CCR7+ and either CD62L+CD45RA+ or CD62L+CD45RA), whereas CD8+CD127 T cells included predominantly TEM cells (i.e., CCR7 and either CD62LCD45RA or CD62LCD45RA+). The most striking and consistent association in this set of immunophenotypical analyses was found between CD127 and CCR7 expression (Fig. 3,A). Consistent with the fact that CD8+CD127 T cells show immunophenotypic features of TEM differentiation, we observed that this cell subset is enriched for the expression of activation markers such as HLA-DR and CD95 (Fig. 3,A). Interestingly, CD8+CD127+ T cells, but not CD8+CD127 T cells, included significant numbers of cells expressing the CXCR4 receptor (Fig. 3,A), whereas in HIV-infected patients, no difference in the expression of CD45RO or CCR5 was observed between CD8+CD127+ and CD8+CD127 T cells (Fig. 3 A). In all, these findings indicate that CD8+CD127+ T cells include predominantly TN and TCM cells, whereas CD8+CD127 T cells include predominantly TEM (i.e., CCR7 and either CD62LCD45RA or CD62LCD45RA+) cells, a subset of which express markers of T cell activation.

FIGURE 3.

CD8+CD127 T cells show phenotypic and functional features of TE cells whereas CD127+CD8+ T cells show features of either TN or TM cells. A, Expression of the phenotypic markers CD45RA, CD62L, CD45RO, CCR7, CCR5, CXCR4, CD95, and HLA-DR on CD8+CD127+ (▪) and CD8+CD127 (□) T cells from 31 HIV-infected patients (left) and 10 healthy controls (right). B, Production of IL-2 and IFN-γ by CD8+CD127+ (solid line) and CD8+CD127 (dotted line) in one representative HIV-infected patient after a 4-h stimulation with PMA and A23187.C, Mean of the percentages of CD8+CD127+ (▪) and CD8+CD127 (□) T cells producing IL-2, IFN-γ, and TNF-α in 10 HIV-infected patients and 10 healthy uninfected controls.

FIGURE 3.

CD8+CD127 T cells show phenotypic and functional features of TE cells whereas CD127+CD8+ T cells show features of either TN or TM cells. A, Expression of the phenotypic markers CD45RA, CD62L, CD45RO, CCR7, CCR5, CXCR4, CD95, and HLA-DR on CD8+CD127+ (▪) and CD8+CD127 (□) T cells from 31 HIV-infected patients (left) and 10 healthy controls (right). B, Production of IL-2 and IFN-γ by CD8+CD127+ (solid line) and CD8+CD127 (dotted line) in one representative HIV-infected patient after a 4-h stimulation with PMA and A23187.C, Mean of the percentages of CD8+CD127+ (▪) and CD8+CD127 (□) T cells producing IL-2, IFN-γ, and TNF-α in 10 HIV-infected patients and 10 healthy uninfected controls.

Close modal

The immunophenotypic features of CD8+CD127+ and CD8+CD127 T cells appear to recapitulate those of TN/TCM and TEM/TEMRA cells, respectively. In addition, CD8+CD127 T cells are enriched in activated T cells. We next tried to determine whether and to what extent the functional features of CD8+CD127+ and CD8+CD127 T cells are consistent with the TCM/TEM paradigm of memory T cell differentiation described in humans (5, 6, 7, 8) and/or the TM/TE paradigm of TM cell differentiation described in mice (2, 3, 4). As shown in Fig. 3, B and C, the measurement of cytokine production by CD8+ T cells from both HIV-infected patients and controls following a brief ex vivo stimulation with PMA and A23187 showed that CD8+CD127 T cells are enriched in cells capable of producing IFN-γ and almost totally depleted in cells capable of IL-2 synthesis. Conversely, CD8+CD127+ T cells showed more limited IFN-γ production while including a larger fraction of IL-2-secreting cells (Fig. 3, B and C). No differences were observed between CD8+CD127+ and CD8+CD127 T cells with respect to TNF production (Fig. 3 C). Importantly, no significant changes were observed in the percentage of CD8+CD127+ and CD8+CD127 T cells under these experimental conditions (data not shown), indicating that brief treatment with PMA and A23187 did not induce major down-regulation of CD127.

In all, these results are consistent with the hypothesis that the expanded CD8+CD127 T cells of HIV-infected patients show characteristic identifiers of TE/TEM cells, including the pattern of cytokine production as well as immunophenotypic markers of T cell differentiation. Interestingly, the finding of an expansion of CD8+CD127 T cells that produce IFN-γ but not IL-2 is consistent with previous observations indicating that advanced HIV infection is associated with a defect in IL-2 production (35, 36, 37, 38) and normal or increased IFN-γ production (35, 39, 40) by T cells.

To further examine the relationship between loss of CD127 expression and the functional features of TM cell differentiation, we assessed the proliferative potential of the CD8+CD127 T cells both in vivo and in vitro. We first examined the prevailing levels of in vivo proliferation, as assessed by expression of the Ki-67 proliferation marker, and found, in both HIV-infected patients and controls, significantly higher levels of proliferation in CD8+CD127 as compared with CD8+CD127+ T cells (Fig. 4,A). However, when the ex vivo proliferative capacity of sorted CD8+CD127+ and CD8+CD127 T cells from uninfected donors was measured in response to mitogenic stimulation (Fig. 4 B), an inverse phenomenon was observed with levels of proliferation markedly higher in CD8+CD127+ as compared with CD8+CD127 T cells. The fact that CD8+CD127 T cells are enriched in cells that are proliferating in vivo (in both HIV-infected patients and controls) but exhibit a reduced proliferative potential in response to ex vivo restimulation is consistent with the hypothesis that CD8+CD127 T cells are comprised predominantly of cells that display functional features typical of TE and/or TEM cells.

FIGURE 4.

CD8+CD127 T cells show increased levels of in vivo proliferation, but decreased ex vivo proliferative potential and increased susceptibility to apoptosis. A, Expression of the proliferation marker Ki-67 on CD8+CD127+ (▪) and CD8+CD127 (□) in 31 HIV-infected patients (left) and 10 healthy controls (right). B, Efficiency of ex vivo proliferation after PHA treatment (4 days) in sorted CD8+CD127+ (top dot plots) and CD8+CD127 (bottom dot plots) T cells from an uninfected donor. Data are representative of seven experiments. C, Levels of baseline (i.e., before culture), spontaneous (i.e., medium alone), and PHA-induced apoptosis measured as a percentage of annexin V-positive cells in CD8+CD127+ (▪) and CD8+CD127 (□) lymphocytes from seven uninfected donors and five HIV-infected patients. D, Summary of phenotypical and functional features of CD8+CD127 (TE-like) and CD8+CD127+ (TN- and TM-like) T cells.

FIGURE 4.

CD8+CD127 T cells show increased levels of in vivo proliferation, but decreased ex vivo proliferative potential and increased susceptibility to apoptosis. A, Expression of the proliferation marker Ki-67 on CD8+CD127+ (▪) and CD8+CD127 (□) in 31 HIV-infected patients (left) and 10 healthy controls (right). B, Efficiency of ex vivo proliferation after PHA treatment (4 days) in sorted CD8+CD127+ (top dot plots) and CD8+CD127 (bottom dot plots) T cells from an uninfected donor. Data are representative of seven experiments. C, Levels of baseline (i.e., before culture), spontaneous (i.e., medium alone), and PHA-induced apoptosis measured as a percentage of annexin V-positive cells in CD8+CD127+ (▪) and CD8+CD127 (□) lymphocytes from seven uninfected donors and five HIV-infected patients. D, Summary of phenotypical and functional features of CD8+CD127 (TE-like) and CD8+CD127+ (TN- and TM-like) T cells.

Close modal

We next sought to determine the level of susceptibility to spontaneous and activation-induced apoptosis in CD8+CD127+ and CD8+CD127 T cells by measuring the level of annexin V-positivity after a 48-h incubation with either medium alone or PHA, respectively. As shown in Fig. 4 C, sorted CD8+CD127 T cells derived from both HIV-infected patients and healthy uninfected controls showed a 2- to 5-fold increase in the level of both spontaneous and activation-induced apoptosis as compared with sorted CD8+CD127+ T cells from the same individuals. Because TE cells generated during an acute viral infection manifest a short lifespan in vivo unless they successfully differentiate into TM cells (2, 3, 4), the increased susceptibility to apoptosis shown by CD8+CD127 T cells is also consistent with the hypothesis that this population is enriched in cells showing functional features typical of TE cells.

The processes underlying the differentiation of Ag-specific memory CD8+ T cells that follow viral infections have been the subject of intense investigation in both the murine (1, 2, 3, 4) and the human (5, 6, 7, 8, 9, 10) systems. Although significant differences have been described between the two species, the results of these complex immunophenotypic and functional studies have allowed for the identification of certain common features of memory CD8+ T cell differentiation in mice and humans. More specifically, it is now possible to draw a tentative analogy between the memory (i.e., Ag-experienced or non-naive) CD8+ cells that are defined as TE and TM (in the murine system) and those that are defined as TEM and TCM (in the human system), respectively. Following this analogy and in an attempt to simplify and summarize the somewhat complex nomenclature used to define the various stages of memory CD8+ T cell differentiation by different authors, we will refer, in discussing the above-presented findings, to the CD8+CD127 cells that show phenotypic and functional features of TE and/or TEM cells as “TE-like cells”, and to CD8+CD127+ cells that show features of TM and/or TCM cells as “TCM-like cells”.

During chronic HIV infection, persistent viral replication coexists with high levels of CD8+ T cell activation and turnover (41, 42, 43, 44, 45). We hypothesized that the loss of CD127 expression on CD8+ T cells serves as a novel marker of the HIV-associated generalized immune activation and accelerated T cell turnover, and that in HIV-infected patients the chronic expansion of CD8+CD127 TE-like lymphocytes may have pathogenic relevance as these cells might be impaired, due to their lack of responsiveness to IL-7 signaling, in their ability to differentiate and/or revert into resting CD8+ TM/TCM cells. We found that HIV-infected patients consistently show an expansion of CD8+CD127 T cells that correlates with markers of disease progression such as the level of viral replication and the severity of CD4+ T cell depletion. In addition, we found that in both HIV-infected patients and healthy controls, the analysis of a series of phenotypic and functional features of CD8+CD127+ and CD8+CD127 T cells is compatible with the hypothesis that CD8+CD127+ T cells include TN and TCM-like cells, while CD8+CD127 T cells are comprised of TE-like cells. Fig. 4 D summarizes the immunophenotypical and functional features of CD8+CD127+ and CD8+CD127 T cells that we have described in this report. Importantly, the marked expansion of CD8+CD127 T cells seen in HIV infection was associated with absolute counts of CD8+CD127+ TCM-like cells that were similar to those observed in uninfected healthy individuals, thus failing to support the hypothesis that a generalized deficit of CD8+ TCM cells occurs during HIV infection.

In the setting of HIV infection, a marked and persistent expansion of CD8+CD127 TE-like cells might reflect a pathogenic mechanism, even in the presence of normal numbers of CD8+CD127+ TCM-like cells, by contributing to the HIV-associated chronic immune activation, which in turn is thought to be a causative factor of the CD4+ T cell depletion of AIDS patients (11, 12, 13, 14, 15). Consistent with this hypothesis is the strong direct correlation observed between the expansion of CD8+CD127 cells and the main biological markers of the HIV-associated chronic immune activation (i.e., percentage of proliferating CD4+ and CD8+ T cells and percentage of CD8+ T cells expressing activation markers such as HLA-DR and CD95). The fact that CD127 down-regulation follows T cell activation (Fig. 1 E) (4, 31, 32, 33) also supports the hypothesis of a relationship between chronic in vivo immune activation and expansion of CD8+CD127 TE-like cells.

Interestingly, our observation of large numbers of CD8+CD127 TE-like cells is consistent with, and expands upon, the findings of several previous studies conducted in HIV-infected individuals. First, a chronic expansion of CD8+CD127 TE-like cells (that may produce high levels of proinflammatory cytokines such as IFN-γ) in the setting of declining CD4+ T cell counts is consistent with the notion that in AIDS patients a state of progressive and profound immunodeficiency coexists with signs of generalized immune activation (11, 12, 13, 14, 15), and that the extent of this immune activation is as good, or perhaps even better, a predictor of disease progression than the level of viral replication (16, 17, 18, 19, 20, 21). Second, the fact that HAART corrects, at least in part, the HIV-associated expansion of TE-like CD8+CD127 cells is consistent with the observation that HAART-mediated suppression of viremia is associated with a significant reduction of the level of immune activation (46). Third, the observation that CD8+CD127 T cells isolated from healthy controls show reduced in vitro proliferative potential and increased susceptibility to apoptosis is consistent with the possibility that, in the setting of chronic HIV infection, the population of CD8+CD127 cells is also characterized by short in vivo lifespan. In this case, these expanded CD8+CD127 TE-like cells may include those fast-replicating and fast-dying CD8+ T cells described recently in HIV-infected patients using in vivo T cell labeling with deuterium-based methods (45), as well as the fraction of CD8+ T cells that appears to be exquisitely sensitive to apoptosis during HIV infection (47, 48, 49). Finally, the expansion of CD8+CD127 TE-like cells that we observed in HIV-infected patients defined as normal progressors contrasts with what is seen in HIV-infected long-term nonprogressors (who maintain effective control of HIV replication in association with low levels of immune activation) in whom a substantial fraction of their HIV-specific CD8+ T cells display functional features of TCM cells, such as a high proliferative response upon in vitro antigenic restimulation (50), thus supporting the possibility that an expansion of CD8+CD127 TE-like cells reflects a pathogenic mechanism of immunodeficiency.

An intriguing and somewhat unexpected result of the present study is the relative increase of the fraction of CD8+CD127 T cells specific for Ags other than HIV (i.e., CMV, EBV) that was observed in HIV-infected patients as compared with healthy uninfected controls. The relative expansion of EBV- and CMV-specific CD8+CD127 TE-like cells in HIV-infected patients may be interpreted alternatively as a consequence of EBV and/or CMV reactivation in an immunodeficient host (i.e., Ag-dependent activation), or as the result of the HIV-associated immune activation on the pattern of differentiation of EBV- and CMV-specific TM cells in absence of EBV and/or CMV reactivation (i.e., bystander activation). At present we do not have data to support either possibility, although studies are in progress to determine the level of CD127 expression on CD8+ T cells that are specific for Ags that are not likely to be reactivated during HIV infection.

It should be noted that increased numbers of CD8+CD127 T cells in the presence of low CD4+ T cell counts may also reflect a “homeostatic”, IL-7-independent attempt to reconstitute the pool of T cells in the context of impaired de novo T cell production by the thymus. In this perspective, an additional possibility is that the described expansion of CD8+CD127 T cells from multiple Ag specificities (i.e., EBV, CMV) is in fact the consequence of an increased extrathymic, IL-7-independent, homeostasis-driven proliferation of existing TM cell.

In summary, the data reported in this study suggest that the expansion of CD8+CD127 TE-like cells is a previously unrecognized feature of the generalized immune dysfunction that follows HIV infection. In this perspective, the expansion of CD8+CD127 cells may represent a new marker to be evaluated, in association with other known parameters of T cell activation (i.e., levels of CD38, HLA-DR), to monitor the level of immune activation in HIV-infected patients. In contrast to markers such as HLA-DR and CD38, in which in vivo function on T cells is basically unknown, the loss of CD127 expression on CD8+ T cells is a marker of the HIV-associated immune activation whose biological function may provide clues as to a pathogenic mechanism of immune dysfunction (i.e., loss of responsiveness to IL-7 and failure to differentiate and/or reconvert into TCM-like CD8+ cells). The potential pathogenic role of the expanded CD8+CD127 TE-like cells suggests that immune-based interventions in the setting of HIV infection could be aimed not only at directly reconstituting a sizeable CD4+ T cell pool and at reducing the level of immune activation (51, 52), but ideally also at favoring an appropriate balance of the different subsets of TM cells (i.e., TCM, TEM, TEMRA). The results we report indicate that further studies are warranted to determine whether and to what extent measurements of CD8+CD127 T cells may be useful in the clinical management of HIV-infected patients as a predictor of disease progression as well as a marker of immunological response to therapy.

The authors have no financial conflict of interest.

We thank Drs. Rafi Ahmed, John Wherry, Ann Chahroudi, and Silvija I. Staprans for helpful discussion, Michael Hulsey for technical assistance with FACS sorting, and the research staff of the Emory Center for AIDS Research Clinical Research and Immunology Cores and the Hope Clinic of the Emory Vaccine Center for their facilitation of these studies.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1

This work was supported by National Institutes of Health Grants R01-AI52755 (to G.S.), R01-AI049155 (to M.B.F.), and AI35522 (to D.L.S.).

4

Abbreviations used in this paper: TN, naive CD8+ T cell; TE, effector CD8+ T cell; TM, memory CD8+ T cell; TCM, central memory CD8+ T cell; TEM; effector memory CD8+ T cell; HAART, highly active antiretroviral therapy.

1
Seder, R. A., R. Ahmed.
2003
. Similarities and differences in CD4+ and CD8+ effector and memory T cell generation.
Nat. Immunol.
4
:
835
.
2
Wherry, E. J., V. Teichgraber, T. C. Becker, D. Masopust, S. M. Kaech, R. Antia, U. H. von Andrian, R. Ahmed.
2003
. Lineage relationship and protective immunity of memory CD8 T cell subsets.
Nat. Immunol.
4
:
225
.
3
Kaech, S. M., S. Hemby, E. Kersh, R. Ahmed.
2002
. Molecular and functional profiling of memory CD8 T cell differentiation.
Cell
111
:
837
.
4
Kaech, S. M., J. T. Tan, E. J. Wherry, B. T. Konieczny, C. D. Surh, R. Ahmed.
2003
. Selective IL-7R expression identifies effector CD8 T cells that give rise to long-lived memory cells.
Nat. Immunol.
4
:
1191
.
5
Sallusto, F., D. Lenig, R. Forster, M. Lipp, A. Lanzavecchia.
1999
. Two subsets of memory T lymphocytes with distinct homing potentials and effector functions.
Nature
401
:
708
.
6
Champagne, P., G. S. Ogg, A. S. King, C. Knabenhans, K. Ellefsen, M. Nobile, V. Appay, G. P. Rizzardi, S. Fleury, M. Lipp, et al
2001
. Skewed maturation of memory HIV-specific CD8 T lymphocytes.
Nature
410
:
106
.
7
Geginat, J., F. Sallusto, A. Lanzavecchia.
2001
. Cytokine-driven proliferation and differentiation of human naive, central memory, and effector memory CD4+ T cells.
J. Exp. Med.
194
:
1711
.
8
Geginat, J., A. Lanzavecchia, F. Sallusto.
2003
. Proliferation and differentiation potential of human CD8+ memory T-cell subsets in response to antigen or homeostatic cytokines.
Blood
101
:
4260
.
9
Appay, V., P. R. Dunbar, M. Callan, P. Klenerman, G. M. Gillespie, L. Papagno, G. S. Ogg, A. King, F. Lechner. C. A. Spina, et al
2002
. Memory CD8+ T cells vary in differentiation phenotype in different persistent virus infections.
Nat. Med.
8
:
379
.
10
van Baarle, D., S. Kostense, M. H. van Oers, D. Hamann, F. Miedema.
2002
. Failing immune control as a result of impaired CD8+ T-cell maturation: CD27 might provide a clue.
Trends Immunol.
23
:
586
.
11
Kinter, A., J. Arthos, C. Cicala, A. S. Fauci.
2000
. Chemokines, cytokines and HIV: a complex network of interactions that influence HIV pathogenesis.
Immunol. Rev.
177
:
88
.
12
Hazenberg, M. D., D. Hamann, H. Schuitemaker, F. Miedema.
2000
. T cell depletion in HIV-1 infection: how CD4+ T cells go out of stock.
Nat. Immunol.
1
:
285
.
13
Grossman, Z., M. Meier-Schellersheim, A. E. Sousa, R. M. Victorino, W. E. Paul.
2002
. CD4+ T cell depletion in HIV infection: are we closer to understanding the cause?.
Nat. Med.
8
:
319
.
14
Douek, D. C., L. J. Picker, R. A. Koup.
2003
. T cell dynamics in HIV infection.
Annu. Rev. Immunol.
21
:
265
.
15
Silvestri, G., M. Feinberg.
2003
. Turnover of lymphocytes and conceptual paradigms in HIV infection.
J. Clin. Invest.
112
:
821
.
16
Giorgi, J. V., L. E. Hultin, J. A. McKeating, T. D. Johnson, B. Owens, L. P. Jacobson, R. Shih, J. Lewis, D. J. Wiley, J. P. Phair, et al
1999
. Shorter survival in advanced human immunodeficiency virus type 1 infection is more closely associated with T lymphocyte activation than with plasma virus burden or virus chemokine coreceptor usage.
J. Infect. Dis.
179
:
859
.
17
Leng, Q., G. Borkow, Z. Weisman, M. Stein, A. Kalinkovich, Z. Bentwich.
2001
. Immune activation correlates better than HIV plasma viral load with CD4 T-cell decline during HIV infection.
J. Acquired Immune Defic. Syndr.
27
:
389
.
18
Sousa, A. E., J. Carneiro, M. Meier-Schellersheim, Z. Grossman, R. M. Victorino.
2002
. CD4 T cell depletion is linked directly to immune activation in the pathogenesis of HIV-1 and HIV-2 but only indirectly to the viral load.
J. Immunol.
169
:
3400
.
19
Anthony, K. B., C. Yoder, J. A. Metcalf, R. DerSimonian, J. M. Orenstein, R. A. Stevens, J. Falloon, M. A. Polis, H. C. Lane, I. Sereti.
2003
. Incomplete CD4 T cell recovery in HIV-1 infection after 12 months of highly active antiretroviral therapy is associated with ongoing increased CD4 T cell activation and turnover.
J. Acquired Immune Defic. Syndr.
33
:
125
.
20
Hazenberg, M. D., S. A. Otto, B. H. van Benthem, M. T. Roos, R. A. Coutinho, J. M. Lange, D. Hamann, M. Prins, F. Miedema.
2003
. Persistent immune activation in HIV-1 infection is associated with progression to AIDS.
AIDS
17
:
1881
.
21
Hunt, P. W., J. N. Martin, E. Sinclair, B. Bredt, E. Hagos, H. Lampiris, S. G. Deeks.
2003
. T cell activation is associated with lower CD4+ T cell gains in human immunodeficiency virus-infected patients with sustained viral suppression during antiretroviral therapy.
J. Infect. Dis.
187
:
1534
.
22
Silvestri, G., D. L. Sodora, R. A. Koup, M. Paiardini, S. P. O’Neil, H. M. McClure, S. Staprans, M. B. Feinberg.
2003
. Non-pathogenic simian immunodeficiency virus infection of sooty mangabeys is characterized by limited bystander immunopathology despite high levels viral replication.
Immunity
18
:
341
.
23
McCune, J. M..
2001
. The dynamics of CD4+ T-cell depletion in HIV disease.
Nature
410
:
974
.
24
Ravkov, E. V., C. M. Myrick, J. D. Altman.
2003
. Immediate early effector functions of virus-specific CD8+CCR7+ memory cells in humans defined by HLA and CC chemokine ligand 19 tetramers.
J. Immunol.
170
:
246
.
25
Altman, J. D., P. A. Moss, P. J. Goulder, D. H. Barouch, M. McHeyzer-Williams, J. I. Bell, A. J. McMichael, M. M. Davis.
1996
. Phenotypic analysis of antigen-specific T lymphocytes.
Science
274
:
94
.
26
Carini, C., M. F. McLane, K. H. Mayer, M. Essex.
1994
. Dysregulation of interleukin-7 receptor may generate loss of cytotoxic T cell response in human immunodeficiency virus type 1 infection.
Eur. J. Immunol.
24
:
2927
.
27
Vingerhoets, J., E. Bisalinkumi, G. Penne, R. Colebunders, E. Bosmans, L. Kestens, G. Vanham.
1998
. Altered receptor expression and decreased sensitivity of T-cells to the stimulatory cytokines IL-2, IL-7 and IL-12 in HIV infection.
Immunol. Lett.
61
:
53
.
28
MacPherson, P. A., C. Fex, J. Sanchez-Dardon, N. Hawley-Foss, J. B. Angel.
2001
. Interleukin-7 receptor expression on CD8+ T cells is reduced in HIV infection and partially restored with effective antiretroviral therapy.
J. Acquired Immune Defic. Syndr.
28
:
454
.
29
Napolitano, L. A., R. M. Grant, S. G. Deeks, D. Schmidt, S. C. De Rosa, L. A. Herzenberg, B. G. Herndier, J. Andersson, J. M. McCune.
2001
. Increased production of IL-7 accompanies HIV-1-mediated T-cell depletion: implications for T-cell homeostasis.
Nat. Med.
7
:
73
.
30
Fry, T. J., E. Connick, J. Falloon, M. M. Lederman, D. J. Liewehr, J. Spritzler, S. M. Steinberg, L. V. Wood, R. Yarchoan, J. Zuckerman, et al
2001
. A potential role for interleukin-7 in T-cell homeostasis.
Blood
97
:
2983
.
31
Sudo, T., S. Nishikawa, N. Ohno, N. Akiyama, M. Tamakoshi, H. Yoshida, S. Nishikawa.
1993
. Expression and function of the interleukin 7 receptor in murine lymphocytes.
Proc. Natl. Acad. Sci. USA
90
:
9125
.
32
Schluns, K. S., W. C. Kieper, S. C. Jameson, L. Lefrancois.
2000
. Interleukin-7 mediates the homeostasis of naive and memory CD8 T cells in vivo.
Nat. Immunol.
1
:
426
.
33
Goldrath, A. W., P. V. Sivakumar, M. Glaccum, M. K. Kennedy, M. J. Bevan, C. Benoist, D. Mathis, E. A. Butz.
2002
. Cytokine requirements for acute and basal homeostatic proliferation of naive and memory CD8+ T cells.
J. Exp. Med.
195
:
1515
.
34
Fry, T. J., M. Moniuszko, S. Creekmore, S. J. Donohue, D. C. Douek, S. Giardina, T. T. Hecht, B. J. Hill, K. Komschlies, J. Tomaszewski, et al
2003
. IL-7 therapy dramatically alters peripheral T-cell homeostasis in normal and SIV-infected nonhuman primates.
Blood
101
:
2294
.
35
Lane, H. C., J. M. Depper, W. C. Greene, G. Whalen, T. A. Waldmann, A. S. Fauci.
1985
. Qualitative analysis of immune function in patients with the acquired immunodeficiency syndrome: evidence for a selective defect in soluble antigen recognition.
N. Engl. J. Med.
313
:
79
.
36
Clerici, M., N. I. Stocks, R. A. Zajac, R. N. Boswell, D. R. Lucey, C. S. Via, G. M. Shearer.
1989
. Detection of three distinct patterns of T helper cell dysfunction in asymptomatic, human immunodeficiency virus-seropositive patients: independence of CD4+ cell numbers and clinical staging.
J. Clin. Invest.
84
:
1892
.
37
Sieg, S. F., D. A. Bazdar, C. V. Harding, M. M. Lederman.
2001
. Differential expression of interleukin-2 and γ interferon in human immunodeficiency virus disease.
J. Virol.
75
:
9983
.
38
Paiardini, M., B. Cervasi, D. Galati, G. Cannavo, D. Guetard, M. Montroni, M. Magnani, G. Piedimonte, G. Silvestri.
2001
. Exogenous IL-2 administration corrects the cell cycle perturbation in lymphocytes from HIV-infected individuals.
J. Virol.
75
:
10843
.
39
Fuchs, D., A. Hausen, G. Reibnegger, E. R. Werner, G. Werner-Felmayer, M. P. Dierich, H. Wachter.
1989
. Interferon-γ concentrations are increased in sera from individuals infected with human immunodeficiency virus type 1.
J. Acquired Immune Defic. Syndr.
2
:
158
.
40
Graziosi, C., G. Pantaleo, K. R. Gantt, J. P. Fortin, J. F. Demarest, O. J. Cohen, R. P. Sekaly, A. S. Fauci.
1994
. Lack of evidence for the dichotomy of TH1 and TH2 predominance in HIV-infected individuals.
Science
265
:
248
.
41
Hazenberg, M. D., J. W. Stuart, S. A. Otto, J. C. Borleffs, C. A. Boucher, R. J. de Boer, F. Miedema, D. Hamann.
2000
. T-cell division in human immunodeficiency virus (HIV)-1 infection is mainly due to immune activation: a longitudinal analysis in patients before and during highly active antiretroviral therapy (HAART).
Blood
95
:
249
.
42
Mohri, H., A. S. Perelson, K. Tung, R. M. Ribeiro, B. Ramratnam, M. Markowitz, R. Kost, A. Hurley, L. Weinberger, D. Cesar, et al
2001
. Increased turnover of T lymphocytes in HIV-1 infection and its reduction by antiretroviral therapy.
J. Exp. Med.
194
:
1277
.
43
Orendi, J. M., A. C. Bloem, J. C. Borleffs, F. J. Wijnholds, N. M. de Vos, H. S. Nottet, M. R. Visser, H. Snippe, J. Verhoef, C. A. Boucher.
1998
. Activation and cell cycle antigens in CD4+ and CD8+ T cells correlate with plasma human immunodeficiency virus (HIV-1) RNA level in HIV-1 infection.
J. Infect. Dis.
178
:
1279
.
44
Sachsenberg, N., A. S. Perelson, S. Yerly, G. A. Schockmel, D. Leduc, B. Hirschel, L. Perrin.
1998
. Turnover of CD4+ and CD8+ T lymphocytes in HIV-1 infection as measured by Ki-67 antigen.
J. Exp. Med.
187
:
1295
.
45
Hellerstein, M. K., R. A. Hoh, M. B. Hanley, D. Cesar, D. Lee, R. A. Neese, J. M. McCune.
2003
. Subpopulations of long-lived and short-lived T cells in advanced HIV-1 infection.
J. Clin. Invest.
112
:
956
.
46
Pomerantz, R. J., D. L. Horn.
2003
. Twenty years of therapy for HIV-1 infection.
Nat. Med.
9
:
867
.
47
Meyaard, L., S. A. Otto, R. R. Jonker, M. J. Mijnster, R. P. Keet, F. Miedema.
1992
. Programmed death of T cells in HIV-1 infection.
Science
257
:
217
.
48
Finkel, T. H., G. Tudor-Williams, N. K. Banda, M. F. Cotton, T. Curiel, C. Monks, T. W. Baba, R. M. Ruprecht, A. Kupfer.
1995
. Apoptosis occurs predominantly in bystander cells and not in productively infected cells of HIV- and SIV-infected lymph nodes.
Nat. Med.
1
:
129
.
49
Gougeon, M. L., H. Lecoeur, A. Dulioust, M. G. Enouf, M. Crouvoiser, C. Goujard, T. Debord, L. Montagnier.
1996
. Programmed cell death in peripheral lymphocytes from HIV-infected persons: increased susceptibility to apoptosis of CD4 and CD8 T cells correlates with lymphocyte activation and with disease progression.
J. Immunol.
156
:
3509
.
50
Migueles, S. A., A. C. Laborico, W. L. Shupert, M. S. Sabbaghian, R. Rabin, C. W. Hallahan, D. Van Baarle, S. Kostense, F. Miedema, M. McLaughlin, et al
2002
. HIV-specific CD8+ T cell proliferation is coupled to perforin expression and is maintained in nonprogressors.
Nat. Immunol.
3
:
1061
.
51
Kovacs, J. A., S. Vogel, J. M. Albert, J. Falloon, R. T. Davey, Jr, R. E. Walker, M. A. Polis, K. Spooner, J. A. Metcalf, M. Baseler, et al
1996
. Controlled trial of interleukin-2 infusions in patients infected with the human immunodeficiency virus.
N. Engl. J. Med.
335
:
1350
.
52
Chapuis, A. G., G. Paolo Rizzardi, C. D’Agostino, A. Attinger, C. Knabenhans, S. Fleury, H. Acha-Orbea, G. Pantaleo.
2000
. Effects of mycophenolic acid on human immunodeficiency virus infection in vitro and in vivo.
Nat. Med.
6
:
762
.