Background: The role of the thymus in the depletion or restoration of T-cell pool in HIV infection is still debatable. Studies are hampered by the lack of valuable tools to investigate thymic activity.
Methods: We have evaluated thymic activity using 18F-fluorodeoxyglucose-positron emission tomography/computed tomography and molecular and phenotypic analyses of thymic precursors. Longitudinal analyses were performed in HIV-infected patients either treatment naive with indication to initiate combination antiretroviral therapy (c-ART) (n = 11) or stable under c-ART (n = 9).
Results: Thymic standardized uptake value was significantly lower in c-ART–treated patients as compared with historical age-matched HIV-negative controls. In c-ART–naive patients, baseline thymic standardized uptake value correlated with T-cell repector excision circle levels and naive CD4+ T cells. These patients exhibited a high metabolic lymph node activity positively correlated to the percentage of activated HLA-DR+CD38+ T cells. Basal metabolic thymic activity predicts the gain in CD4+ T cells after c-ART initiation. A decrease of thymic activity, which paralleled circulating plasma IL-7 levels, was noted after c-ART initiation.
Discussion: A metabolic thymic activity is detectable in c-ART naive and correlates with indirect phenotypic and molecular markers of thymic output. This activity may participate to the pool of peripheral naive CD4+ T cells and predicts the magnitude of T-cell reconstitution under treatment.
*INSERM, Unite U955, Creteil, France
†Faculte de Medecine, Universite Paris-Est, UMR-S 955 Creteil, France
‡AP-HP, Groupe Henri-Mondor Albert-Chenevier, service d’immunologie clinique, Creteil, France
§AP-HP, Groupe Henri-Mondor Albert-Chenevier, Service de Médecine Nucléaire, UMR CNRS 7054, Créteil, France
‖Institut Pasteur, Département de Virologie, Paris, France
¶INSERM, Unité U897, Bordeaux, France
#Université Bordeaux Segalen, ISPED, Bordeaux, France.
Correspondence to: Prof. Jean-Daniel Lelièvre, MD, PhD, Service d’Immunologie Clinique, CHU Henri Mondor, 51 avenue du Marechal De Lattre de Tassigny, 94010 Créteil, France (e-mail: email@example.com).
Supported by a grant from the ANRS (Agence Nationale de Recherche sur le SIDA et les hépatites virales—Study ANRS EP 41).
Jean-Daniel Lelièvre and Giovanna Melica contributed equally to this work.
The authors have no conflicts of interest to disclose.
Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions this article on the journal's Web site (www.jaids.com).
Received December 22, 2011
Accepted May 24, 2012
Restoration of T-cell homeostasis after T-cell depletion involves both peripheral proliferation and central regeneration from the thymus. Peripheral expansion is increased when thymic function is impaired.1 Thymus plays an important role in T-cell regeneration during childhood. Several studies indicated that the thymus is still active during adulthood and contribute to the replenishment of the T-cell pool during lymphopenic conditions such as bone marrow transplantation or after chemotherapy.2 Although the nature of all the homeostatic mechanisms driving thymic rebound after lymphopenia remains to be clearly defined, interleukin-7 is thought to be a major actor of this effect.3 An inverse relationship between circulating interleukin-7 concentrations and peripheral CD4 T-cell number has been demonstrated in adults or children with T-cell depletion of various origins.3
HIV/simian immunodeficiency virus (SIV) infection of the thymus has been described in human and in nonhuman primate models.4,5 This infection is thought to lead to a dramatic decrease of thymic activity related to gland destruction, similar to age-induced thymic involution.6 However it is not clear if this thymic impairment is a constant feature of HIV infection or if it only appears during late stages of HIV infection. Moreover several studies have established the contribution of the thymus to the recovery of peripheral naive T-cell numbers during antiretroviral treatment.7–10
The evaluation of thymic function is difficult because there is no unequivocal marker of a functional thymic activity.11 Previous studies appreciating thymic activity during HIV infection were based on computed tomography (CT) scan imaging and sj T-cell repector excision circles (TREC) quantification which has been reported as a good marker of thymic functionality in healthy individuals.12 There are several form of TREC; sjTRECs, which are produced during the rearrangement of the alpha chain of T-cell receptor, have been the more widely used to estimate thymic function in various clinical settings including in HIV infection.13,14 However, there are some caveats in the interpretation of TREC data because TREC levels are dependent not only of thymic output but are also affected by cell division and death which are common features of peripheral T cells during HIV infection.12,13 Moreover, long-term detection of TREC has been evidenced after thymectomy, suggesting peripheral compensatory mechanisms leading to increased recent thymic emigrants survival.14,15 This problem could be circumvented in some cases by the use of sj/βTREC ratio as a marker of intrathymic precursor T-cell proliferation6 or mathematical modeling using TREC and Ki67 measurements.16 Quantification of naive T cells is an indirect and the simplest way to estimate thymic function. The quantification of the number of circulating cell with a naive T-cell phenotype is not sufficient per se to appreciate the functionality of the thymus, as these cells may proliferate in periphery. Identification of a subpopulation of recent thymic emigrants within naive T cells could help to overcome the limitation of phenotypic studies. However, the precise phenotype of this rare population remains to be determined.17,18
CT scan evaluation of the thymus is also currently used to explore the thymus gland. However CT scan appearance of the thymus cannot be related to a metabolic activity of this gland. 18F-fluorodeoxyglucose-positron emission tomography/computed tomography (FDG-PET/CT) imaging is a noninvasive method that identifies tissues with increased glucose metabolism. Advantages of FDG-PET/CT over many cross-sectional imaging rely on the high lesion-to-background contrast and whole-body data acquisition that make this exam especially useful during the management of non Hodgkin lymphoma.19 It allows the determination of several following parameters: an anatomical description of the tissue; the uptake of radiolabeled glucose analog; and the determination of Houndsfield unit (HU) that allows estimating the histological density of the tissue, fat having a low score unit (−100) in contrast to solid tissue (>0).
In the present study, we examined the interest of FDG-PET/CT imaging to evaluate thymic function during HIV infection. For this purpose, we compared the results of thymic FDG-PET/CT imaging measurements to the results of T-cell phenotype of naive and activated population and TREC quantification in patients before and after combination antiretroviral therapy (c-ART) initiation or stable on c-ART for several months. Finally, we compared thymic standardized uptake value (SUV) measurements in HIV-infected patients and age-matched HIV-uninfected subjects.
PATIENTS AND METHODS
This is a prospective study conducted in a single clinical site in France. Enrolled patients were HIV-1 positive and had never received antineoplastic or immunomodulating agents (cytokine or immunosuppressive drugs). Naive patients (11 patients—group 1 c-ART−) were included just before receiving c-ART. Chronically HIV-infected patients (9 patients—group 2 c-ART+) have been receiving c-ART for at least 1 year (mean duration 99 ± 38 months) and exhibited CD4+ T-cell counts above 350 cells per cubic millimeter and plasma HIV viral load below 50 copies per milliliter. Clinical, biological, and radiological evaluation were performed at baseline (both groups) and after 1 (group 1), 3, and 6 months (both groups). HIV-uninfected control group for FDG-PET/CT thymic evaluation included patients referred to the department of nuclear medicine of Henri Mondor hospital for the evaluation of abnormal radiological images or pretherapeutic staging of neoplastic disorder that did not involve thymus. All patients provided written informed consent. The study was approved by the ethical committee of Henri Mondor hospital, Créteil, France.
Biological Evaluation of Thymic Output and T-Cell Activation
Surface expression of specific markers on peripheral blood mononuclear cells was performed on EDTA anticoagulated blood specimens. Samples were stained for 4-color flow cytometry using a whole blood labeling method, and samples were analyzed on a FacsCalibur cytometer using CellQuest software (BD, Biosciences, San José, CA). Naive T cells (CCR7+CD45RA+) and activated T cells (CD4+CD38+HLA-DR+ or CD8+CD38+HLA-DR+) were characterized using a combination of directly conjugated monoclonal antibodies as follows: anti-CD3 APC, anti-CD4 PerCP, anti-CD8 PerCP, anti-CCR7 FITC, anti-CD45RA APC, anti-CD38 FITC and anti-HLA-DR PE, respectively. We used directly conjugated mouse immunoglobulin-G isotype controls to ascertain background staining. All antibodies were purchased from BD Biosciences. Results were expressed either as frequency of positive cells among CD8+ and CD4+ T cells or as absolute cells number/milliliter by multiplying the frequency of the subset within CD4+ and CD8+ T cells absolute numbers.
Quantification of each TREC together with the CD3 chain (used as a housekeeping gene) was performed for each sample using the Light Cycler technology (Roche Diagnostics, Mannheim, Germany).20 peripheral blood mononuclear cells were lysed for 30 minutes at 56°C and then 15 minutes at 98°C. Multiplex polymerase chain reaction (PCR) was performed for sjTREC together with the CD3 chain using outer 3'/5' primer pairs. PCR conditions in the LightCycler experiments, performed on 1/100th of the initial PCR, were as follows: 1 minute initial denaturation at 95°C, 1 second at 95°C, 10 seconds at 60°C, 15 seconds at 72°C for 40 cycles. Fluorescence measurements were performed at the end of the elongation steps. sjTRECs and CD3γ Light Cycler quantifications were performed in independent experiments, but on the same first-round serially diluted standard curve. This highly sensitive nested quantitative PCR assay allows the detection of 1 copy of sjTREC in 105 cells. The sjTRECs were quantified in triplicate.
Serum IL7 Quantification
Serum IL7 quantification was performed using commercial kit (Diaclone; Besançon, France).
Patients underwent serial PET/CT scanning. Each acquisition was performed on a fasting patient, after having controlled blood glucose level, which was targeted ≤7 mM. Patients were injected intravenously with 5 MBq/kg FDG and rested for 1 hour to avoid muscular uptake. They were scanned on a Gemini GXL PET/CT hybrid camera (Philips Medical Systems, DA Best, The Netherlands). The acquisition first featured a low-dose transmission CT (100 kV, 60 mAs, beam collimation 2 mm) from mid-thigh to the top of the skull, then followed by a 3-dimensional emission scan in 9–11 overlapping bed shifts (18-cm field-of-view) of 2-minutes duration each. PET images were reconstructed using a row action maximum-likelihood algorithm, including scatter, random, and attenuation corrections, in 144 × 144 matrices of 4 × 4 × 4 mm3 voxels. On the maximum intensity projections, all foci of abnormal uptake were located, including head and neck lymphoid organs, node areas, thymus, and spleen. In each area, a region of interest was drawn manually on the corresponding PET/CT slice, and the maximum SUV was recorded. In addition, the largest diameter of each enlarged node or organ was measured. Finally, a volumetric region of interest was drawn on 5 contiguous CT slices in the thymic area, encompassing the gland if any or the thymic fat if no gland was present, and the mean HU were measured.
Results are provided as median and interquartile ranges. Wilcoxon tests (for paired data when needed) and spearman correlation were performed using SAS version 9.2 statistical software (SAS Institute, Cary, NC).
FDG-PET Scanning Showed Nodal and Thymic Metabolic Activity in HIV-Infected Patients
Total body FDG-PET/CT scans have been performed in 2 groups of HIV-1–infected patients (Fig. 1). At the entry in the study, median CD4+ T-cell counts were 282 cells per cubic millimeter for group 1 (c-ART–naive patients) and 588 cells per cubic millimeter for group 2 (c-ART–treated patients) (P = 0.004), whereas median viral load were 170.550 copies per milliliter and under detectable value for group 2, respectively (see Table, Supplemental Digital Content 1, http://links.lww.com/QAI/A337). Patients were younger in group 1 than in group 2 (median age 37 vs. 47 years; P = 0.05). At baseline, a metabolic activity could be detected in lymph node, spleen, tonsil (Figs. 1A, C), and thymus (Figs. 1B, C) in both groups. SUV measurements showed significant or a trend to higher values in c-ART–naive patients depending on the site as follows: tonsillar (median 5.1 vs. 4; P = 0.1), nodal (2.2 vs. 0; P = 0.02), and splenic (2.1 vs. 1.4; P = 0.05) (Fig. 1C).
Regarding thymic structure, HU measurement and CT scans showed a thymic mass clearly distinct from adjacent tissues, in 5/11 of group 1 and 1/9 of group 2 patients, (Fig. 1D; P < 0.05). An empty thymic space was noted in 1/11 of group 1 and 2/9 of group 2 patients. In the remaining patients, a fat densification was found (Fig. 1D). In both groups, a metabolic activity, as assessed by maximum SUV measurements, was detectable without significant difference between groups (median 1.4 vs 1.2; P = 0.12) (Fig. 1C). We then compared SUV thymic values observed in our patients with those of age-matched HIV-negative controls (see Patients and Methods). We confirmed that SUV activity correlated negatively with age independently of HIV status (Fig. 2A). We found that c-ART–naive patients, but not c-ART–treated patients, exhibited a thymic metabolic activity closed to HIV-negative age-matched controls (Fig. 2B). Altogether, these data indicated that a metabolic activity is detectable in HIV-infected patients in different lymphoid tissues including the thymus. In untreated HIV-infected patients, thymic activity remained close to a group of age-matched HIV-negative controls.
Lymph Node But Not Thymic SUV Correlates With T-Cell Activation
We investigated whether metabolic activity in lymphoid organs detected by FDG-PET/CT correlated with the expression of HLA-DR and CD38 among T cells, 2-well validated markers of T-cell activation in HIV infection.21 We found a positive correlation between lymph node SUV measures and percentages of HLA-DR+CD38+ expressing cells both among CD4+ T (R = 0.77; P < 0.001) or CD8+ T populations (R = 0.77; P < 0.001) (Figs. 3A, B). A weaker correlation was found between CD4+HLADR+CD38+ or CD8+HLADR+CD38+ T cells and splenic SUV (Figs. 3A, B). In contrast, thymic SUV measures did not correlate with percentages of activated T cell (Figs. 3A, B). As expected, follow-up of patients from group 1 over 6 months after c-ART initiation showed a decrease in the percentages of CD4+HLADR+CD38+ and CD8+HLADR+CD38+ T cells, whereas these percentages remained stable during follow-up in chronically treated patients (Figs. 3C, D). Interestingly, in both groups lymph node SUV measures behave similarly as T-cell activation markers (Figs. 1A, 3E). Altogether these results indicated a correlation between lymph node, but not thymic, metabolic activity and peripheral T-cell activation.
Thymic Activity Evaluated by FDG-PET Scan is Correlated With Biological Parameters of Thymic Function in Untreated HIV-Infected Patients
A complete analysis of thymic SUV was performed longitudinally in 7 HIV-infected patients before and after c-ART initiation. At entry, a positive correlation between thymic SUV measurement and with both sjTREC concentration in blood and the number of naive T cells (R = 0.6, P = 0.04 and R = 0.66, P = 0.03, respectively, Figs. 4A, B) was found. Moreover, baseline thymic SUV also correlated with the gain of naive CD4 T cells at month 6 of treatment (R = 0.68, P = 0.02 Fig. 4C). In c-ART+ patients, no correlation could be observed between FDG-PET and biological markers of thymic activity (data not shown).
Similar Evolution Patterns Between Thymic Activity and IL-7 Concentration in the Serum of HIV-Infected Patients
Interleukin 7 is the main cytokine involved in T-cell homeostasis, it regulates notably the initial steps of T-cell development.22 Therefore, we looked at changes in IL-7 plasma levels and thymic SUV in patients who started c-ART. Although thymic SUV remained stable overtime in chronically c-ART+–treated patients (Fig. 5A; M0 = 1.2; M3 = 1.25; M6 = 1.1; P = 0.7 and 0.8 for comparison between M0 vs M3 and M0 vs M6, respectively), we observed a decrease in thymic SUV measurement in patients after c-ART initiation (Fig. 5B. M0 = 1.45; M1 = 1.3; M3 = 1.15; M6 = 1.25; P = 0.03 and 0.008 for comparison between M0 vs. M1 and M0 vs. M3, respectively). In parallel, we found a decrease in IL-7 plasma levels after c-ART initiation in patients (Fig. 5C; median M0 = 10.6 pg/mL; M1 = 3.73 pg/mL; M3 = 0.4 pg/mL; M6 = 0 pg/mL; P = 0.08 for comparison between M0 vs. M1 and P = 0.01 for comparison between M0 and both M3 or M6). IL-7 values remained under detectable levels in chronically c-ART+ patients throughout the follow-up (data not shown).
Thymus is the organ of T-cell ontogeny. It is essential in early life for the establishment of a complete T-cell repertoire and could participate in T-cell renewal even in adults.2,23 Investigation of thymus functionality remains challenging. CT scan imaging has been used to explore the thymus. A typical image of the thymus is the presence of a triangular mass in the anterior mediastinum. Although evaluation of thymic volume has been used as a marker of thymic activity,7,24 discrepancies between radiological aspects and thymic function have been clearly demonstrated in patients in whom radiological and histological confrontations were performed.25
In our study, we have evaluated the potential interest of thymic SUV measurement by 18F-FDG PET as a marker of thymic activity. In the last years, 18F-FDG PET has grown in importance in the primary diagnosis and course control of malignancies. Several reports have also suggested that 18F-FDG PET could be used to explore thymic function during childhood, but also in adults.26–29 In the setting of HIV/SIV infection, several studies have investigated thymic activity using 18F-FDG PET/CT scans.8,30–39 However, in contrast to results reported here, the majority of these studies involved a limited number of patients. Interpretation of thymic uptake by 18F-FDG PET, both in HIV-infected and HIV-uninfected individuals, raises several concerns such as the definition of a threshold of a positive value for SUV measurement and the specificity of 18F-FDG uptake. Finally, evaluation of a thymic metabolic activity does not infer only to thymopoiesis. In contrast to naive T cell, activated memory may re-enter the thymus40 and HIV-1 expressing cells have been found in both thymic compartments.25
All these limitations led us to combine 18F-FDG PET, phenotypic and molecular markers, to investigate thymic activity in HIV-infected patients. Moreover, we took the opportunity, in patients who started c-ART, to evaluate longitudinally these markers. Interestingly, we were able to detect and quantify a metabolic thymic activity in all HIV-infected patients in contrast with previous reports exploring healthy adult patients.26 In most of our cases, thymic 18F-FDG uptake was also associated with the detection of a thymic tissue, that is, a true thymic gland. Thymic 18F-FDG uptake is associated with higher serum IL-7 levels which are usually inversely correlated with peripheral CD4 T-cell counts.41 IL-7 may induce upregulation of the glucose transporter Glut-1 and increased glucose uptake which could explain the thymic 18F-FDG uptake in our patients. Thymic 18F-FDG uptake was correlated with sjTREC concentration in blood, baseline naive CD4+ T-cell count, and gain in naive CD4+ T cells after c-ART initiation, which argue that metabolic activity detected by SUV measurement is associated with a functional activity and the production of naive T cells.
We also showed that 18F-FDG uptake could also reflect the presence of an ongoing infectious or inflammatory process in peripheral tissues.31 This has been used by several authors to explore the site of presumptive HIV replication in the body. Scharko et al35 reported a diffuse 18-FDG uptake in SIV-infected macaques. Sites of 18-FDG uptake varied according to the stage of infection with a peripheral (cervical and axillary) lymph node accumulation in early infection and a predominant 18-FDG uptake in intra abdominal lymph nodes in late stages.35 In situ hybridization showed that PET images correlated with SIV replication.35 Similar patterns of predominant 18-FDG uptake in the upper body after primary infection or in the abdomen in late stages were observed in the simian-human immunodeficiency virus model38 and in HIV-infected patients.36 Our results, in accordance with previous data,42–45 showed a correlation between 18-FDG uptake in lymph node and peripheral markers of T-cell activation. Elevated SUV in lymph node in untreated HIV-infected patients correlated with the frequency of CD4+ and CD8+ T cells expressing HLA-DR and CD38 activation markers. Interestingly, we observed a parallel decline of T-cell activation markers and lymph node uptake after c-ART initiation in naive patients that reinforced the association between these 2 parameters. Furthermore, we were able to show that despite a significant decline of lymph node 18F-FDG uptake after c-ART initiation, SUV values remained elevated in treated patients receiving c-ART for up to 8 years. Likely, this observation reflects either persistent immune activation and/or viral replication.46 Finally, we found that in contrast to lymph node SUV measurement, thymic 18F-FDG uptake was not correlated with the expression of activation markers by peripheral T cells.
The role of thymus function in HIV infection is still a matter of debate.47 Alterations of T-cell production by the thymus have been described during HIV-1 infection. Histological examination of thymic tissues of patients who died from HIV infection revealed profound modifications with usually disappearance of sign of active thymopoiesis.25,48 More recent histological study performed in 7 HIV-infected patients showed evidence for thymic activity despite many years of ongoing viral replication.49 Our results are consistent with this observation. The role of the thymus in T-cell reconstitution during c-ART therapy has been also largely analyzed.10,14,47 Recently it has been shown that reduced thymic output could explain the immune reconstitution failure in immunological nonresponders.50 We found using 18F-FDG/Pet that chronically treated patients have a less functional gland. These results are in accordance with recent published data51; whereas in our study, the fact seems not to be limited to nonresponder subjects. Furthermore, we found a decline of thymic activity after c-ART initiation in naive patients. Globally, our data suggest that thymic activity could be driven by the high rate of CD4+ T-cell destruction in the periphery in patients with ongoing active viral replication. These patients exhibit also high levels of circulating IL-7, a major cytokine driving T-cell production by the thymus. On the other hand, control of viral load is associated with an increase of T-cell survival and a predominant peripheral expansion of T cells in line with our observation of lower plasma IL-7 levels and thymic activity in chronically treated patients.
Our study indicates that FDG-PET/CT in HIV may serve as a new marker for the evaluation of thymic function in HIV-infected patients. It also shows that thymic function is increased in c-ART–naive HIV and relatively impaired in older chronically treated patients arguing for a place in the treatment of HIV infection of strategies aimed to restore thymic function.52
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