High prevalence of thymic tissue in adults with human immunodeficiency virus-1 infection

Correlation of T-lymphocyte subpopulations with immunological markers in HIV-1-infected Indian patients

Progressive HIV disease is characterized by CD4 T cell decline and activation of the immune system. We aimed to study the quantitative alterations in the naive (CD45RA+CD62L+), memory/effector (CD45RO+) and activated (HLA-DR+CD38+) T-lymphocyte subpopulations in antiretroviral treatment naive, HIV-1 infected Indian patients by three-color multi-parametric flow cytometry. The association of different CD4+ and CD8+ T cell subsets with the immunological markers- CD4+ and CD8+ T cell percentages was examined by calculating the partial correlation coefficients. We also observed significant differences in the expression of different CD4+ and CD8+ T-cell subsets among the two groups of patients formed using the median CD4+ T cell percentage value (15%) of the study population. The correlations of different CD4+ and CD8+ T cell subsets reflected the quantitative alterations in the T-lymphocyte subpopulations and activation of the immune system during HIV-infection. The study outcome also emphasizes the significance of the CD38+CD8+ T-lymphocyte subset as a prognostic marker for HIV management and ART monitoring in resource-limited settings of developing countries like India.

Naïve T Cells Are Maintained in the Periphery During the First3 Months of Acute HIV-1 Infection: Implications for Analysis of Thymus Function

A key determinant of T cell dynamics in HIV-1 infection is the status of thymic function. To date, most studies of the impact of HIV-1 on the thymus during early HIV-1 infection have been done in samples collected in the interval of 3-12 months after infection. In this study, we have probed the status of thymic function and peripheral naive T cells in patients with acute HIV-1 infection diagnosed 18-72 days after the onset of symptoms. We found that peripheral CD4 and CD8 T cell proliferation was initially elevated, then waned over time. The fall in T cell proliferation correlated with a reduction in HIV-1 viral RNA levels and a rise in peripheral blood CD4+ CD25+ T cells. In spite of elevated T cell proliferation early on in primary HIV-1 infection, levels of naive phenotype CD4 and CD8 T cells and T cell receptor excision circle positive cells (sjTREC(+)) remained constant. Taken together with the observation that T cell proliferation normally dilutes peripheral T cell episomal sjTREC levels, these data suggested that thymopoiesis contributes to maintenance of the naive T cell pool during the earliest stages of HIV-1 infection (18-72 days).

Glucose transporter 1 expression identifies a population of cycling CD4+ CD8+ human thymocytes with high CXCR4-induced chemotaxis

GLUT1, the major glucose transporter in peripheral T lymphocytes, is induced upon T cell receptor activation. However, the role of GLUT1 during human thymocyte differentiation remains to be evaluated. Our identification of GLUT1 as the human T lymphotrophic virus (HTLV) receptor has enabled us to use tagged HTLV-receptor-binding domain fusion proteins to specifically monitor surface GLUT1 expression. Here, we identify a unique subset of CD4+ CD8+ double-positive (DP) thymocytes, based on their GLUT1 surface expression. Whereas these cells express variable levels of CD8, they express uniformly high levels of CD4. Glucose uptake was 7-fold higher in CD4(hi) DP thymocytes than in CD4(lo) DP thymocytes (P = 0.0002). Further analyses indicated that these GLUT1+ thymocytes are early post-beta-selection, as demonstrated by low levels of T cell receptor (TCR)alphabeta and CD3. This population of immature GLUT1+ DP cells is rapidly cycling and can be further distinguished by specific expression of the transferrin receptor. Importantly, the CXCR4 chemokine receptor is expressed at 15-fold higher levels on GLUT1+ DP thymocytes, as compared with the DP GLUT1- subset, and the former cells show enhanced chemotaxis to the CXCR4 ligand CXCL12. Thus, during human thymopoiesis, GLUT1 is up-regulated after beta-selection, and these immature DP cells constitute a population with distinct metabolic and chemotactic properties.

Age-related thymic activity in adults following chemotherapy-induced lymphopenia

BACKGROUND

The potential role of the adult thymus in T-cell homeostasis subsequent to lymphopenia remains the subject of debate. We examined whether thymic activity contributes to reconstitution of the peripheral T-cell pool, a critical process for patients recovering from antineoplastic therapy.

METHODS

In selected patients with various neoplastic diseases we assessed peripheral blood lymphocyte subsets by flow-cytometry, including thymus-derived, CD4+ T cells expressing the CD45RA molecule, and thymic size rebound by CT scan before, and 3, 6 and 12 months after completion of cytotoxic therapy.

RESULTS

Adult patients (n = 21, mean age of 30 years, range 18-49) had higher baseline numbers of B and lower numbers of NK cells than elderly patients (n = 15, mean age of 79 years, range 70-91), while total T-cell numbers did not differ. Despite the reduction of lymphocyte counts being comparable in the adult (mean of 45%) and elderly (mean of 49%) groups, occurring at, or near, completion of treatment, an enlargement of the previously atrophic thymus was evident in 63% of the adult, but in none of the elderly, subjects. In 22 patients who remained active disease-free during the following year, B cells and NK cells recovered to pretreatment levels as soon as at 3 months, whereas overall T-cell recovery occurred at 6 months post-treatment. Thymic rebound, observed in 11 of 22 patients who were of younger age, correlated significantly with a faster and more complete recovery of CD45RA+ CD4+ (mainly helper-naïve) T cells.

CONCLUSION

The adult thymus appears capable of regeneration, at least up to middle age, contributing significantly to the reconstitution of the peripheral T-cell pool following chemotherapy-induced lymphopenia. In advanced age, however, although peripheral homeostatic pathways appear intact, regeneration of the naïve repertoire is incomplete.

Multiparameter evaluation of human thymic function: interpretations and caveats

After the provision of highly active antiretroviral therapy (HAART), the level of circulating CD4+ T cells increases in many adults infected with the human immunodeficiency virus, type 1 (HIV). To study factors involved in immune reconstitution, we have measured thymic abundance by CT scans, circulating naive-phenotype CD4+ T cells by flow cytometry, and T cell receptor (TCR) rearrangement excision circles (TRECs) by quantitative PCR in 40 virologically suppressed, HIV-infected adults and 33 age-matched, HIV-uninfected controls. In HIV-uninfected subjects, naive T cell numbers, thymic abundance, and the frequency of circulating naive CD4+ T cells bearing TRECs decreased with age, as expected. When corrected for this relationship with age, naive T cell numbers correlated significantly with naive T cell TREC frequencies. Virologically suppressed HIV-infected subjects had higher TREC frequencies, and subjects over the age of 39 were more likely to have abundant thymus compared to age-matched, HIV-uninfected adults. Nevertheless, all HIV-infected subjects had reduced absolute numbers of naive T cells, irrespective of thymic size, age, or TREC frequencies. These data illustrate the complex relationship between these measures of thymic size and function and underscore the need to develop more definitive measures of thymic function in the future.

Disease progression in macaques with low SIV replication levels: on the relevance of TREC counts

BACKGROUND

An attenuated immunodeficiency virus has been long considered innocuous. Nevertheless, converging data suggest that low levels of viral replication can still provoke AIDS. Pathogenesis of these attenuated infections is not understood.

OBJECTIVES

To determine the pathogenicity of a long-term attenuated infection and to delineate T-cell dynamics during such an infection.

METHODS

This is a cross-sectional study of 12 rhesus macaques infected with SIV Delta nef for 8 years. We evaluated apoptosis (annexin V), activation (HLA-DR, Ki67), and newly generated T cells (TCR excision circle: TREC).

RESULTS

Infection with SIV Delta nef induced pathological CD4 T-cell depletion after 8 years of infection. Virus replication and CD8 T-cell activation positively correlated with the rate of disease progression. The frequency of TREC within CD8+CD45RA+ cells increased in SIV Delta nef-infected animals compared to age-matched non-infected controls. Moreover, in the cohort of infected animals, TREC+CD45RA+CD4+ T-cell counts correlated strongly with non-progression to AIDS. The animal with the lowest rate of disease progression exhibited a 115-fold increase in TREC+CD45RA+CD4+ T-cell counts compared to age-matched non-infected controls. In contrast, the animal showing the fastest rate of progression to AIDS displayed 600-fold lower TREC+CD45RA+CD4+ T-cell counts compared to age-matched non-infected controls.

CONCLUSIONS

Our results suggest that the thymus plays a major role in the pathogenesis of an attenuated SIV infection and that a sustained thymic output could maintain CD4 T-cell homeostasis in the context of low viral loads.

Age-dependent incidence, time course, and consequences of thymic renewal in adults

Homeostatic regulation of T cells involves an ongoing balance of new T cell generation, peripheral expansion, and turnover. The recovery of T cells when this balance is disrupted provides insight into the mechanisms that govern homeostasis. In a long-term, single cohort study, we assessed the role of thymic function after autologous transplant in adults, correlating serial computed tomography imaging of thymic size with concurrent measurements of peripheral CD4(+) T cell populations. We established the age-dependent incidence, time course, and duration of thymic enlargement in adults and demonstrated that these changes were correlated with peripheral recovery of naive CD45RA(+)CD62L(+) and signal-joint TCR rearrangement excision circle-bearing CD4(+) populations with broad TCR diversity. Furthermore, we demonstrated that renewed thymopoiesis was critical for the restoration of peripheral CD4(+) T cell populations. This recovery encompassed the recovery of normal CD4(+) T cell numbers, a low ratio of effector to central memory cells, and a broad repertoire of TCR Vbeta diversity among these memory cells. These data define the timeline and consequences of renewal of adult thymopoietic activity at levels able to quantitatively restore peripheral T cell populations. They further suggest that structural thymic regrowth serves as a basis for the regeneration of peripheral T cell populations.

Recovery from and consequences of severe iatrogenic lymphopenia (induced to treat autoimmune diseases)

To ascertain the consequences of severe leukopenia and the tempo of recovery, we studied the immunity of 56 adult patients treated for multiple sclerosis or systemic sclerosis with autologous CD34 cell transplantation using extremely lymphoablative conditioning. NK cell, monocyte, and neutrophil counts recovered to normal by 1 month; dendritic cell and B cell counts by 6 months; and T cell counts by 2 years posttransplant, although CD4 T cell counts remained borderline low. Initial peripheral expansion was robust for CD8 T cells but only moderate for CD4 T cells. Subsequent thymopoiesis was slow, especially in older patients. Importantly, levels of antibodies, including autoantibodies, did not drop substantially. Infections were frequent during the first 6 months, when all immune cells were deficient, and surprisingly rare (0.21 per patient year) at 7-24 months posttransplant, when only T cells (particularly CD4 T cells) were deficient. In conclusion, peripheral expansion of CD8 but not CD4 T cells is highly efficient. Prolonged CD4 lymphopenia is associated with relatively few infections, possibly due to antibodies produced by persisting pretransplant plasma cells.

Expression of Werner and Bloom syndrome genes is differentially regulated by in vitro HIV-1 infection of peripheral blood mononuclear cells

In HIV infection, continuous immune activation leads to accelerated ageing of the adaptive immune system, similar to that observed in elderly people. We investigated the expression of WRN and BLM (genes involved in disorders characterized by premature ageing, genomic instability and cancer predisposition) in peripheral blood mononuclear cells (PBMC) activated in vitro with phytohaemagglutinin (PHA) and infected with different HIV-1 strains. The steady state levels of mRNA were analysed by reverse transcription-polymerase chain reaction (RT-PCR), and protein expression was assayed using immunocytochemistry and Western blot techniques. In uninfected PBMC, PHA stimulation induced an increase in BLM mRNA and protein expression, while WRN expression remained virtually unchanged. When PBMC were infected in vitro with a lymphotropic HIV-1 strain, the level of BLM mRNA showed a peak at 24 h of infection, followed by a decline to uninfected culture levels. A similar result failed to be seen using an R5-tropic HIV-1 strain. In accordance with mRNA expression, in HIV-infected cultures PBMC were stained more frequently and more intensely by a BLM-specific antibody as compared to uninfected cultures, staining peaking at 24. Conversely, WRN expression was not modulated by HIV-1. The proportion of cells showing BLM up-regulation, established by immunocytochemical staining, was much greater than the proportion of productively infected PBMC, as established by proviral DNA measurement. This result indicates that BLM up-regulation is probably a result of an indirect bystander cell effect. Activation of the BLM gene in infected PBMC suggests that premature ageing could be a further immunopathogenetic mechanism involved in HIV-induced immunodeficiency, and points to a possible new candidate target for innovative therapeutic intervention.

HIV Infection Rapidly Induces and Maintains a Substantial Suppression of Thymocyte Proliferation

The supply of naive T cells by the thymus normally requires precursor T cell proliferation within the thymus and would be particularly important in the setting of HIV infection when both naive and memory T cells are progressively depleted. As a robust, quantitative index of intrathymic proliferation, the ratio of different T cell receptor excision circles (TRECs), molecular markers of distinct T cell receptor rearrangements occurring at different stages of thymocyte development, was measured in peripheral blood-mononuclear cells (PBMCs). This ratio has the virtue that it is a "signature" of thymic emigrants throughout their entire life and, thus, can be measured in peripheral cell populations that are easy to obtain. Using the new assay, we evaluated the effect of HIV infection on intrathymic precursor T cell proliferation by longitudinal analysis of PBMCs from recently infected individuals. Our findings reveal a substantial reduction in intrathymic proliferation. The analysis also indicates the existence of a compensatory mechanism acting to sustain the numbers of recent thymic emigrants (RTEs) in the periphery.

Characterization of a thymus-tropic HIV-1 isolate from a rapid progressor: role of the envelope

Loss of T cell homeostasis usually precedes the onset of AIDS. We hypothesized that rapid progressors may be transmitted with HIV-1 that is particularly able to perturb T cell homeostasis. To this end, we have tested two transmitted, syncytium-inducing (SI) viral isolates from a rapid progressor in two thymus models. One of the isolates (R3A) exhibited markedly rapid kinetics of replication and thymocyte depletion. These phenotypes mapped to the envelope, as a recombinant NL4-3 virus encoding the R3A envelope had similar phenotypes, even in the absence of nef. Notably, the viruses with high pathogenic activity in the thymus (R3A and NL4-R3A) did not show enhanced replication or cytopathicity in PHA-stimulated PBMCs. Furthermore, NL4-R3A did not enhance replication of the coinfected NL4-3 virus in the thymus, suggesting an intrinsic advantage of the R3A envelope. The R3A envelope showed higher entry activity in infecting human T cells and in depleting CD4+ thymocytes when expressed in trans. These data suggest that SI viruses with unique envelope functions which can overcome barriers to transmission may hasten disease progression by perturbing T cell homeostasis.

Is thymus redundant after adulthood?

Thymus is considered to involute with age with a decline in thymic function. However, this generality is not universally and incontrovertibly true. Many studies performed in animals and men have proved to the contrary that thymic activity and function appear to be well maintained in the old age and may be indispensable for T cell reconstitution in different immunological settings. During some clinical situations where T cell pool needs to be regenerated, renewal of thymic activity and mass has been observed in an otherwise dormant thymic remnant. New studies have revealed a dynamic interplay between postnatal thymus output and peripheral T cell pool. Moreover, age-related loss of thymic function appears to be only quantitative and not qualitative. This review, thus, focuses on the different conditions that lead to thymic involution and attempts to bring about the emerging notion and the clinical relevance of continuous thymic activity well beyond the adulthood to optimise the function of the immune system in the context of cancer and infectious diseases.

Apoptosis of naive CD4+ T-cells from HIV-infected patients with poor immune response to HAART is enhanced in vitro by steroid

OBJECTIVE

Because the absence of immune restoration in HIV-infected patients efficiently treated by highly active antiretroviral therapy (HAART) may be due to excessive immune activation, we prospectively studied the effect of hydrocortisone on T-cell apoptosis in a cohort of patients with satisfactory virologic response.

METHODS

Apoptosis of T-cell subsets including naïve CD45RA(+)CD4+ T-cells was determined at baseline and at months 1 and 3 after initiation of HAART. A satisfactory immune response was defined as an increase >100/microL CD4+ T-cells at month 3 compared to baseline.

RESULTS

Twenty out of 63 patients showed undetectable viral load at month 3, among whom eight exhibited a satisfactory immune response. Down-regulation spontaneous CD4+T-cell apoptosis was significant in the group of patients with a satisfactory immune response compared to the other patients. However, hydrocortisone up-regulated apoptosis of naïve CD4+ CD45RA+ T-cells, specifically in group of patients with poor immune response, whatever the time point considered: percentage of apoptotic CD4 T-cells was 16+/-16% without hydrocortisone and 22+/-22% with hydrocortisone at month 1, and respectively, 10+/-9 and 17+/-15% at month 3 (P < 0.05) Hydrocortisone had no impact on CD8+ T-cell apoptosis, whatever the considered group.

CONCLUSION

Our results suggest to not use steroid therapy as adjuvant immunotherapy in patients with less than optimal immunologic response to HAART.

Increased interleukin-7 plasma levels are associated with recovery of CD4+ T cells in HIV-infected children

To asses the role of interleukin 7 (IL-7) in the thymic reconstitution of CD4 T cells observed in children after successful antiretroviral therapy, a longitudinal study in five vertically HIV-1-infected children was carried out. Thymic function, IL-7 plasma levels, viral load, and T-lymphocytes subsets were determined every 2 or 3 months for about 90 months. In all the children, the drop in CD4+ T cells below 5-10% was associated with a marked increase in IL-7 plasma levels. The drastic decrease in viral load after treatment, led in all the cases to a recover of CD4 to levels higher than 30%, which was associated to an increase in thymic production of T cells and followed by a decrease in IL-7 to the normal levels. We conclude that the drop in CD4 in HIV children would induce an increase of IL-7 as part of a homeostatic mechanism. IL-7 would induce the thymus to produce new T cells to recover the normal levels of CD4 when the viral load was low and so the thymic function was not inhibited. The increase in the thymic production of new T cells recovers the CD4 population, and leads to a normalization of IL-7 levels.

Stem cells and immune reconstitution in AIDS

The hematopoietic system generally has reserve sufficient to tolerate significant insult and regenerative capacity to overcome most damage due to infectious agents. However, HIV infection results in a progressive decline in hematopoietic function and even in the context of potent, anti-retroviral therapy is able to only incompletely reconstitute immune function. The ability of the immune system to respond to HIV itself remains compromised, a defect that leaves infected individuals with a lifelong dependence on medications. The capability of stem cells and the thymus to restore function and their limitations in the context of HIV infection are discussed in this review.

Decreases in percentages of naïve CD4 and CD8 T cells and increases in percentages of memory CD8 T-cell subsets in the peripheral blood lymphocyte populations of A-bomb survivors

Our previous studies have revealed a clear dose-dependent decrease in the percentage of naïve CD4 T cells that are phenotypically CD45RA+ in PBL among A-bomb survivors. However, whether there is a similar radiation effect on CD8 T cells has remained undetermined because of the unreliability of CD45 isoforms as markers of naïve and memory subsets among the CD8 T-cell population. In the present study, we used double labeling with CD45RO and CD62L for reliable identification of naïve and memory cell subsets in both CD4 and CD8 T-cell populations among 533 Hiroshima A-bomb survivors. Statistically significant dose-dependent decreases in the percentages of CD45RO-/CD62L+ naïve cells were found in the CD8 T-cell population as well as in the CD4 T-cell population. Furthermore, the percentages of CD45RO+/CD62L+ and CD45RO+/CD62L- memory T cells were found to increase significantly with increasing radiation dose in the CD8 T-cell population but not in the CD4 T-cell population. These results suggest that the prior A-bomb exposure has induced long-lasting deficits in both naïve CD4 and CD8 T- cell populations along with increased proportions of these particular subsets of the memory CD8 T-cell population.

Apparent induction of partial thymic regeneration in a normal human subject: a case report

Magnetic resonance imaging was used to detect signs of regeneration of the thymus after approximately one month of human growth hormone administration. A 46-year-old human volunteer was placed on a regimen of recombinant human growth hormone and pharmaceutical grade dehydroepiandrosterone for one month. Mediastinal magnetic resonance images were collected at baseline and after the study period. Thymic cross sections were analyzed for total area and for the total gray area, which was taken to represent functional mass. Baseline and post-treatment blood samples were taken to follow changes in IGF-1 levels and related metabolites. The setting was an informal, non-institutional trial supervised by a physician will full informed consent of the volunteer. Visual inspection and image analysis demonstrated limited but distinct enlargement of the thymus after treatment, and an increase in the percent of thymic cross section represented by gray-appearing (functional) mass. Estimated total thymic functional volume was within the normal range at baseline, but after treatment was more than three standard deviations above the expected mean for a subject of this age, thus meeting a proposed definition of thymic hyperplasia for individuals. IGF-1 levels were confined to the upper range of normal for young adults. The present observations apparently provide the first demonstration of growth hormone induced partial reversal of established thymic involution in a normal human subject, and are consistent with previous measurements of restored immune function after the administration of human growth hormone to elderly individuals.

Evidence of thymic reconstitution after highly active antiretroviral therapy in HIV-1 infection

OBJECTIVES

We aimed to provide evidence of thymic reconstitution after highly active antiretroviral therapy (HAART) in HIV-1 infected patients and to correlate this with the restoration of peripheral naïve T cells.

METHODS

Positron emission tomography (PET) enables definitive evidence of thymic activity, indicating functional potential. In this case study, a single patient who initiated HAART demonstrated reconstitution of the naïve T-cell pool and underwent thymic PET scans at baseline and 2 and 6 months following initiation of therapy. Two patients who failed to demonstrate such reconstitution acted as controls. These patients (mean age 27 years) had chronic HIV infection with low CD4 T-cell counts (mean 82, range 9-160 cells/microL blood). Increased function of the thymus visualized by PET was correlated with phenotypic changes in CD4 and CD8 T cells in the periphery measured by flow cytometry, and with numbers of recent thymic emigrants measured by quantification of the numbers of T-cell receptor excision circles (TRECs) in peripheral cells.

RESULTS

In one patient, clear correlations could be drawn between visible activity within the thymus, as measured by increased [F18]fluorodeoxyglucose (FDG) uptake, and regeneration of naïve CD4 (CD45RA/CD62L) T cells, increased numbers of CD4 T cells, controlled viraemia and increased numbers of recent thymic emigrants. A second patient displayed no increase in peripheral CD4 count and no increase in thymic activity. The third patient elected to stop therapy following the 2-month time point.

CONCLUSIONS

The use of PET suggests that thymic activity may increase after HAART, indicating that the thymus has the potential to be functional even in HIV-1 infected persons with low CD4 T-cell counts.

The impact of HIV on mortality rates in the complete UK haemophilia population

OBJECTIVE

To estimate the effect of HIV-1 infection on subsequent mortality in a complete population.

DESIGN

Prospective cohort study.

SUBJECTS

A total of 7250 haemophilic males were registered in the UK Haemophilia Centre Doctors' Organisation database, 1977-1998. Most were infected with hepatitis C virus. In the early 1980s, 1246 were infected with HIV-1 from contaminated clotting factor concentrate. The main outcome measure was the date of death.

RESULTS

During 1977-1984 annual mortality in severely haemophilic males was 0.9%. For those with HIV, annual mortality increased progressively from 1985 reaching over 10% during 1993-1996 before falling to 5% in 1997-1999, whereas without HIV it remained approximately 0.9% throughout 1985-1999. For moderately/mildly haemophilic males the annual mortality was 0.4% during 1977-1984. Without HIV it remained approximately 0.4% throughout 1985-1999, but with HIV it was similar to that in severe haemophilia with HIV. Survival was strongly related to age at HIV infection. The large temporal changes in mortality with HIV were largely accounted for by HIV-related conditions. Without HIV annual liver disease mortality remained below 0.2% throughout 1985-1999, but with HIV it was 0.2% during 1985-1990, 0.8% during 1991-1996, and 0.8% during 1997-1999.

CONCLUSION

These data provide a direct estimate of the effect of HIV-1 infection on subsequent mortality in a population with a high prevalence of hepatitis C. From approximately 3 years after HIV infection, large, progressive increases in mortality were seen. From 1997, after the introduction of effective treatment, substantial reductions occurred, although mortality from liver disease remained high.

Impaired TCR-mediated induction of Ki67 by naive CD4+ T cells is only occasionally corrected by exogenous IL-2 in HIV-1 infection

Perturbations in naive T cell homeostasis and function may play a major role in the immunodeficiency that accompanies HIV infection. By examining naive CD4(+) T cell function on a single cell basis, we provide evidence that these cells have significant qualitative defects in HIV disease. Ki67, a molecule expressed during cell cycle progression, is induced less efficiently among naive CD4(+) T cells from HIV-infected individuals following activation with anti-TCR Ab. The impairment in Ki67 expression is evident even when a separate function, CD62L down-modulation, is within normal ranges. Moreover, the defects in Ki67 induction are only sometimes corrected by the addition of rIL-2 to cell cultures. An initial assessment of IL-2 unresponsiveness in cells from selected HIV-infected individuals suggests that the defect is not a consequence of impaired IL-2R expression or IL-2R signaling capability. Qualitative defects in naive T cells that cannot be routinely corrected by IL-2 have significant implications for disease pathogenesis and for strategies using IL-2 as a vaccine adjuvant in HIV disease.

Graves' disease during immune reconstitution after highly active antiretroviral therapy for HIV infection: evidence of thymic dysfunction

A patient with HIV infection who experienced immune reconstitution after highly active antiretroviral therapy (HAART) [increase in CD4 T cell count from <1/microl to >600/microl] presented with severe Graves' disease 32 months after commencing HAART. A comprehensive clinical and laboratory study demonstrated pronounced regional lymphadenopathy and thymic enlargement at presentation, and that the onset of thyrotropin receptor antibody production was associated with increased production of soluble CD30 (a marker of type 2 immune responses). Blood naive CD8 T cell counts and TREC levels in both CD4 and CD8 T cells were increased at multiple time points compared with carefully selected controls. We conclude that the Graves' disease in this patient was associated with abnormally high blood counts of thymus-derived T cells, and propose that Graves' disease after HAART in this and other HIV patients may result from failure to delete autoreactive T cell clones in the regenerating thymus.

Histologic and molecular assessment of human thymus

Most work describing the histopathology of normal human thymus has focused on pediatric thymus because of tissue availability and high thymopoietic activity. However, pathologic examination of the thymus can provide information about immune status that is relevant to the clinical care of patients of all ages. Understanding age-related changes in the relative abundance and composition of anatomic compartments within the thymus is critical for evaluation of the thymus in normal adults and patients with diseases that affect the thymus. The purpose of this review is to acquaint diagnostic pathologists with some of the newer histologic, flow cytometric, and molecular techniques for assessment of non-neoplastic thymus. Diagnostic criteria are presented for assessment of thymic function and for determining the mechanisms underlying thymic hyperplasia. Accurate assessment of thymic function is critical for the diagnosis and treatment of patients with complete DiGeorge syndrome and can complement the clinical care of patients with a variety of disorders that affect the immune system.

Changes in T cell receptor excision DNA circle (TREC) levels in HIV type 1-infected subjects pre- and post-highly active antiretroviral therapy

The T cell receptor excision DNA circle (TREC) level is an independent predictor of HIV-1 disease prognosis. We studied the temporal changes in TREC levels prior to and after highly active antiretroviral therapy (HAART) in a cohort of 131 Greek men with hemophilia who were followed up for up to 20 years since seroconversion (SC). TREC levels were determined in all available cryopreserved samples of peripheral blood mononuclear cells (PBMCs) using a multiplex real-time polymerase chain reaction (PCR) assay. Trends in log(10) TREC values were described using random effects models. Prior to HAART initiation TREC levels tended to decrease over time (mean rate of drop 19% per year; 95% CI: 16-22%). Initial TREC values were higher with younger age at SC, but the subsequent rate of drop did not differ significantly by age at SC. There was a monotonic relationship between baseline HIV-RNA levels and TREC slopes with steeper slopes at higher levels of HIV-RNA. The TREC slopes differed significantly by clinical outcome being steeper in subjects who progressed to AIDS sooner. After HAART initiation, TREC values tended to increase on average by 35% per year (95% CI: -7-94%) but the increase was evident only in subjects with a pre-HAART CD4 count below 80 cells/microl. TREC values, which likely represent a simple indicator of naive T-lymphocyte reserve, may be a clinically useful marker for long-term prognosis of HIV-1 infection and for immune reconstitution after successful HAART.

Increases in CD4+ T lymphocytes occur without increases in thymic size in HIV-infected subjects receiving interleukin-2 therapy

OBJECTIVE

To examine the potential contribution of the thymus to CD4+ T-lymphocyte increases in HIV-infected patients receiving intermittent interleukin-2 (IL-2) therapy.

DESIGN

Fifteen HIV-infected patients treated with antiretroviral regimens who were enrolled in a study of intermittent IL-2 therapy and were willing to undergo serial thymic computed tomography (CT) were prospectively studied.

METHODS

Thymic CT was performed before and approximately 6 and 12-17 months after intermittent IL-2 therapy was started. Scans were graded in a blinded manner. Changes in lymphocyte subpopulations were determined by flow cytometry.

RESULTS

Statistically significant increases in CD4+ T lymphocytes occurred with IL-2 administration, with a preferential increase in naive relative to memory CD4+ T cells. Despite this increase in naive CD4+ T cells, overall there was a modest decrease in thymic volume observed during the study period. No correlation was found between changes in thymic volume indices and total, naive, or memory CD4+ T-lymphocyte counts.

CONCLUSIONS

These findings demonstrate that the profound CD4+ T-lymphocyte increases seen with intermittent IL-2 administration are not associated with increases in thymic volume and more likely are due to peripheral expansion rather than increased thymic output.

Immune Mechanisms in HIV Infection

Recent advances in immunobiology have led to a greater understanding of the healthy immune system and the complex pathogenesis of human immunodeficiency virus (HIV) infection. Knowledge of the mechanisms underlying the decrease in CD4+ T cells is rapidly evolving as a result of new assays, genetic advances, and recombinant DNA technologies. Studying the immune responses of long-term non-progressors is also providing insight into the immunopathology of HIV. Trials using highly active antiretroviral therapy and immune modulators have shown that it may be possible to reverse damage to the immune system and increase CD4+ T-cell numbers. Current and future findings might provide the knowledge necessary to identify effective HIV drugs and vaccines with acceptable toxicity profiles and to determine whether it will be possible to fully restore immune system function in patients with HIV disease.

IL-7 Stimulates T Cell Renewal Without Increasing Viral Replication in Simian Immunodeficiency Virus-Infected Macaques

The main failure of antiretroviral therapy is the lack of restoration of HIV-specific CD4(+) T cells. IL-7, which has been shown to be a crucial cytokine for thymopoiesis, has been envisaged as an additive therapeutic strategy. However, in vitro studies suggest that IL-7 might sustain HIV replication in thymocytes and T lymphocytes. Therefore, in the present study, we evaluated the effect of IL-7 on both T cell renewal and viral load in SIVmac-infected young macaques in the absence of antiretroviral therapy. This evaluation was conducted during the asymptomatic phase in view of a potential treatment of HIV patients. We show that IL-7 induces both a central renewal and a peripheral expansion of T lymphocytes associated with cell activation. No alarming modulation of the other hemopoietic cells was observed. No increase in the viral load was shown in blood or lymph nodes. These data strengthen the rationale for the use of IL-7 as an efficient immunotherapy in AIDS.

Hematopoietic stem cells in HIV disease

The hematopoietic stem cell has long been hypothesized to be a target of human immunodeficiency virus type-1 (HIV) infection that limits the potential for compensatory immune cell production. Data have recently emerged documenting stem cell dysfunction in HIV disease and indicating that immune recovery from potent antiretroviral therapy is partly driven by new T-cell generation. Effects of HIV on stem cell physiology, however, appear to be indirect, as stem cells are highly resistant to HIV infection. Despite the presence of surface receptors for HIV, the hematopoietic stem cell is not infectible with HIV. However, stem transduction can be achieved with HIV constructs in which the envelope glycoproteins have been replaced by vesicular stomatitis virus G protein. Therefore, hematopoietic stem cells are likely participants in HIV-related cytopenias, but they are spared direct infection and can serve as a resource for cellular therapies for AIDS.

HIV-1 replication and pathogenesis in the human thymus

How HIV replicates and causes destruction of the thymus, and how to restore thymic function, are among the most important questions of HIV-1 pathogenesis and therapy in adult as well as pediatric patients. The thymus appears to function, albeit at reduced levels, throughout the life of adults, to respond to T cell depletion induced by HIV and to be suppressed by HIV. In this review, we summarize recent findings concerning HIV replication and pathogenesis in the human thymus, focusing on mechanistic insights gleaned from studies in the SCID-hu Thy/Liv mouse and human fetal-thymus organ culture (HF-TOC) models. First, we discuss HIV viral determinants and host factors involved in the replication of HIV in the thymus. Second, we consider evidence that both viral factors and host factors contribute to HIV-induced thymocyte depletion. We thus propose that multiple mechanisms, including depletion and suppression of progenitor cells, paracrine and direct lytic depletion of thymocytes, and altered thymocyte selection are involved in HIV-induced pathology in the thymus. With the SCID-hu Thy/Liv mouse and HF-TOC models, it will be important in the coming years to further clarify the virological, cell biological, and immunological mechanisms of HIV replication and pathogenesis in human thymus, and to correlate their significance in HIV disease progression.

Endogenous IL-7 is associated with increased thymic volume in adult HIV-infected patients under highly active antiretroviral therapy

OBJECTIVE

Immune reconstitution after highly active antiretroviral therapy (HAART) in HIV-infected patients has led to an increase in the number of new CD4 T lymphocytes. Neolymphopoiesis in the thymus has been proposed as a mechanism in T-cell regeneration. Nevertheless, factors involved in the regeneration of T cells by thymic-dependent pathways in HIV-infected patients under HAART are still unknown and might be of relevance in HIV infection. The aim of this work was to study the role of IL-7 in the thymic rebound of HIV-infected adults under HAART.

DESIGN

To study the association between IL-7 and thymic function-related markers, these variables were measured in 49 antiretroviral-naive HIV-infected patients at baseline and at weeks 12, 24, 36 and 48 of treatment.

METHODS

Thymic function-related markers: thymic volume, naive phenotype, and T-cell receptor excision circles (TREC) bearing-cells, were evaluated by computed tomography, flow cytometry, and quantitative polymerase chain reaction, respectively. IL-7 levels were evaluated using a high sensitivity colorimetric enzyme-linked immunosorbent assay.

RESULTS

At baseline, we found an inverse correlation between IL-7 levels and thymic function-associated parameters: thymic volume, naive T cells and TREC-bearing cells. After 48 weeks of therapy increased levels of thymic function-related markers along with a significant decrease in IL-7 levels were found. IL-7 levels at baseline were the only independently associated variable with respect to changes in thymic volume at weeks 12, 24 and 48 of follow-up.

CONCLUSION

These data suggest that IL-7 plays an important role in thymic rebound in adult HIV-infected patients under HAART.

Comparison of Thymic Function-Related Markers to Predict Early Cd4 T-Cell Repopulation in Adult HIV-Infected Patients on Haart

The aim of this work was to compare thymic function-related markers for predicting early CD4 T-cell repopulation in adult HIV-infected patients under HAART. Forty-three consecutive antiretroviral-naive patients were prospectively analysed for clinical, biochemical, immunological and virological parameters at starting HAART, and followed for 4 weeks and every 12 weeks thereafter. At baseline, all patients underwent a thoracic computer tomography scan, in order to measure thymic volume, as well, T-cell phenotype (naive CD4 and CD8 T cells) and the number of TREC-bearing cells were obtained. CD4 cell repopulation was considered as an increase ≥200 cells/mm3 above baseline count. Twenty-seven patients (62.8%) increased ≥200 cells/mm3 above baseline levels during the follow-up. The median time to event was 182 days (84–537 days). On the univariate analysis, to be younger than 36 years, showing a CD4 cell count ≥272 cells/mm3, a total naive T-cell count ≥128 cells/mm3, a TREC-bearing cell count ≥0.74 cells/mm3, and a thymic volume ≥3.07 cc at baseline were statistically associated to the event studied. However, when the multivariate analysis was performed, only thymic volume at baseline was independently associated ( P=0.002) to CD4 cell recovery. This co-variable was identified as a positive predictor [hazard ratio, 1.22 (95% confidence interval: 1.16–1.28)]. In summary, data presented herewith show that thymic volume is the best thymic function-related marker for predicting early CD4 T-cell recovery in adult HIV-infected patients under HAART.

Impact of simian immunodeficiency virus (SIV) infection on lymphocyte numbers and T-cell turnover in different organs of rhesus monkeys

HIV infection leads to reduced numbers and increased turnover of CD4(+) T cells in blood. However, blood represents only 2% of the total lymphocyte pool, and information about other organs is lacking, leading to controversy about the effects of HIV infection on T-cell homeostasis. Therefore, we have determined phenotype and turnover of lymphocyte subsets in various tissues of macaques. Infection with simian immunodeficiency virus (SIV) resulted in increased proliferation rates of T cells in all organs. Despite reduced CD4 counts in blood, absolute numbers of CD4(+) T cells were increased in spleen and lymph nodes and remained stable in nonlymphoid organs such as liver, lung, bone marrow, and brain during the asymptomatic phase, indicative for an altered tissue distribution. In animals killed with first signs of AIDS, total body CD4 counts and proliferation rates had returned to control levels, whereas thymocytes were almost completely absent. Our data show that a drastically increased turnover in the early stages of HIV infection, driven by a generalized immune activation rather than a homeostatic response to CD4(+) T-cell destruction, is followed by exhaustion of the regenerative capacity of the immune system.

Thymic output during initial highly active antiretroviral therapy (HAART) and during HAART supplementation with interleukin 2 and/or with HIV type 1 immunogen (Remune)

The thymic output of patients receiving highly active antiretroviral therapy (HAART) was assessed by sjTREC (signal joint T cell receptor rearrangement excision circle) analysis to determine the thymic contribution to CD4(+) T cell reconstitution during initial therapy and during interleukin 2 (IL-2) and/or Remune supplementation of HAART. Levels of sjTRECs were observed to decline dramatically after the first 4 weeks of HAART and then increased without significant associated changes in CD4(+) T cell counts. HAART supplementation with IL-2 was observed to lead to rapid increases in CD4(+) T cells that were accompanied by sjTREC decreases. No notable changes in CD4(+) T cell counts and sjTRECs were seen in patients receiving HAART supplemented with Remune alone. The results indicate CD4(+) T cell maintenance during initial treatment of HIV-1 with HAART and early CD4(+) T cell reconstitution of patients receiving IL-2 with HAART is largely due to thymus-independent mechanisms, with the thymus making a limited contribution.

Changes in thymus volume in adult HIV-infected patients under HAART: correlation with the T-cell repopulation

An important thymus role has been suggested in T-cell repopulation after HAART in adult HIV-1 infected patients. Thymus volume increase after treatment has been described in HIV-1 infected children but not in adult patients. The objective of this work was to evaluate the effect of HAART on the thymic volume of adult HIV-1 infected patients and its relation with the T-cell repopulation. Twenty-one adult patients following 24 weeks under HAART were included in the study. All patients underwent a thoracic computed tomography (CT) evaluation for the measurement of thymic volumes at weeks 0, 12 and 24. Baseline thymus volume showed a significant correlation with the patient's age. Thymic volume significantly increased after 24 weeks of HAART. Besides, a significant correlation between changes in the thymus volume and changes in both total and naïve CD4+ cell counts was found. Only patients with increases > or =100 CD4+ cell counts after treatment significantly increased the thymic volume. These data show the first evidence of an early change in thymic volume of adult HIV-1 infected patients under HAART. This increase was related to the rise of both total and naïve CD4+ cell counts suggesting a functional role of thymic volume increase.

Entrapment of recent thymic emigrants in lymphoid tissues from HIV-infected patients: association with HIV cellular viral load

OBJECTIVE(S)

Depletion of thymus derived naive T-cells is a feature of HIV infection. Here the impact of HIV infection on the compartmentalization of recent thymic emigrants of (RTE) and naive T-cells was examined.

METHODS

Peripheral blood mononuclear cells (PBMC) and lymphoid tissue (LT) from 43 HIV-infected patients and 12 controls were examined for RTE distribution by measuring coding joint T-cell receptor excisional circles (cjTREC) by PCR and naive and memory T-cell subsets and adhesion molecules (L-selection, LFA-1) by flow cytometry.

RESULTS

In HIV-infected patients, the RTE as quantified by cjTRECs in CD4 LT cells were significantly higher than in PBMC. Their values, however, were less than in control subjects, in both the LT and PBMC compartments. This was associated with an increase in L-selectin and LFA-1 expression on LT derived T cells. In PBMC, a significant positive relationship between TREC and naive CD4 cells and an inverse relationship between TREC and cellular viral load (CVL) was observed. Whereas in LT, there was a positive relationship between cjTREC and both naive CD4 cell percentage and CVL.

CONCLUSIONS

Collectively, the data suggests that LT is a significant reservoir for RTE. The RTE appeared to be entrapped in LT from HIV-infected subjects. Such entrapment is probably a response to the high viral load in these tissues. These observations may partially explain the decline in RTE observed in the peripheral blood of HIV-infected patients, and the delay in recovery of naive cells in blood after initiation of HAART.

Dynamics of T cells and TCR excision circles differ after treatment of acute and chronic HIV infection

We quantified T cell proliferation and thymic function in primary HIV infection (PHI; n = 19) and chronic HIV infection (CHI; n = 14) by measuring Ki67 staining and TCR excision circle (TREC) number. After antiretroviral therapy of PHI there is a profound decrease in the number and percentage of Ki67(+) T cells (<6% Ki67(+)) with no significant increase in TREC per million cells and a transient increase in TREC per milliliter. In contrast, after antiretroviral therapy of CHI there is a reduction in the percentage but little change in the total number of Ki67(+)CD4(+) T cells associated with increases in both TREC per million cells and TREC per milliliter. Using a mathematical model that accounts for proliferation, death, and redistribution of T cells, we find that redistribution is consistent with the TREC changes observed during treatment of PHI and that an increase in thymic output is needed to explain the increase in TREC during treatment of CHI. Consideration of TREC per milliliter shows that changes in proliferation alone cannot explain the changes in TREC. In addition, although increased proliferation of memory cells in HIV infection has been established, we find no difference in TREC per million CD45RA(-) "memory" T cells between healthy and infected individuals (p = 0.154 for CD4(+); p = 0.383 for CD8(+)). Finally, although the number of TREC per million cells is always much lower in memory T cells than in naive T cells, in the setting of HIV infection, given that memory cells make up a larger proportion of total T cells, we find that 50% of TREC per milliliter in CD4(+) T cells is harbored in the CD45RA(-) "memory" subset of our infected subjects.

Development of alphabeta T cells in the human thymus

The thymus is the main producer of alphabeta T cells and is, therefore, crucial for a normal immune system. The intrathymic developmental pathway of human alphabeta T cells has now been delineated. The production of new T cells by the thymus decreases with age, and the thymus was thought to be redundant in adults once the peripheral T-cell pool has been formed early in life. However, recent work has shown that the thymus can function even at an advanced age. Research into the production of T cells in clinical settings that are associated with loss of T cells in the periphery has sparked renewed interest in the function of the human thymus.

Toward cellular-based therapies for HIV infection

Infection with HIV-1 progressively erodes immune function, leading ultimately to multiple hematopoietic cytopenias. In advanced HIV disease, anemia, neutropenia, and thrombocytopenia occur in a large fraction of patients and are reflective of a striking inability of the bone marrow to accomplish a compensatory increase in production. No failure of compensatory production is greater than that of the T lymphoid system, where despite apparently effective control of HIV replication, restoration of anti-HIV immunity does not occur. Recent technical developments have provided considerable insight into hematopoietic dysfunction in HIV disease and, in particular, in vivo T cell generation. This article will review some of the mechanisms participating in immune regeneration, the stakes involved in effectively accomplishing full reconstitution, and potential approaches to enhance the limited endogenous regenerative process.

Antivirals against HIV-1

Morbidity and mortality associated with HIV infection increased rapidly following the recognition of this syndrome in 1981, and by 1994, AIDS was the leading cause of death in the United States among men and women ages 15-45. The antiretroviral therapy era began in 1987 following the Food and Drug Administration approval of zidovudine, a nucleoside analog reverse transcriptase inhibitor; however, it was not until 1996, when HIV viral load assays were developed that could quantify the copy number of HIV RNA present in plasma, that investigators and clinicians could appropriately evaluate the antiviral efficacy of therapy. This important technical breakthrough and the availability of HIV protease inhibitors led to a dramatic decline in the morbidity and mortality associated with the disease.

Baseline predictors of CD4 T-lymphocyte recovery with combination antiretroviral therapy

CD4 T-cell recovery with potent antiretroviral therapy varies considerably among HIV-1-infected patients. Data from two studies that enrolled subjects at different stages of disease progression were retrospectively combined. This analysis assessed the association between patient-specific factors and three measures of CD4 T-cell recovery after the initiation of triple-drug therapy: overall changes in CD4 cell counts; changes in CD4 cell counts during the first 8 weeks (phase I); and changes in CD4 cell counts during weeks 8-24 (phase II). Higher initial HIV-1 RNA values corresponded to greater increases in overall and phase I changes in mean CD4 cell counts, particularly among subjects with less advanced disease. In the overall and phase II cases, those subjects with suppressed HIV RNA levels had consistently higher increases in mean CD4 cell counts across both baseline HIV-1 RNA levels and CD4 cell counts than did the respective unsuppressed group. Based on a multivariate model, increases in mean phase I CD4 cell counts corresponded to higher log baseline HIV-1 RNA levels (p =.0001) and log changes in HIV-1 RNA levels at week 4 (p =.03). Patients with earlier stages of disease (p =.0001) and females (p =.01) had higher increases in phase I changes. Phase II CD4 cell counts did not depend solely on baseline HIV-1 RNA levels and CD4 cell counts but on their interaction (p =.0001) as well as on achieving virologic suppression (p =.0009).

Features of 'CD4-exploders', HIV-positive patients with an optimal immune reconstitution after potent antiretroviral therapy

OBJECTIVE

To identify crucial immunological characteristics of a group of patients, defined 'CD4-exploders', who were able to fully reconstitute their immune system after receiving highly active antiretroviral therapy (HAART).

PATIENTS

Among a population of 540 HIV-positive patients treated with HAART, six individuals were identified who experienced a nadir of less than 85 x 106 CD4+ cells/l, had major opportunistic infections (four out of six), started HAART in 1996 or 1997, and showed a rapid and relevant CD4+ lymphocyte increase (mainly due to virgin cells), in some cases regardless of virological control.

METHODS

Enzyme-linked immunosorbent assay for the determination of interleukin (IL)-7 plasma levels; flow cytometry to analyse surface antigens and CD127 (IL-7 receptor alpha-chain) expression; quantitative-competitive (QC) PCR for detecting cells containing T-cell receptor rearrangement excision circles (TREC); chest-computed tomography (CT) to analyse thymus volume and content.

RESULTS

In 'CD4-exploders', high levels of TREC+ lymphocytes were found among CD4+ T cells, which also contained a high percentage of naive cells. However, CT revealed a dramatic depletion of intrathymic lymphoid tissue. Plasma levels of IL-7 were significantly high. Most CD4+ and CD8+ T lymphocytes expressed CD127, whose level was similar to that of healthy donors. CD127 expression on CD8+ lymphocytes was markedly higher than in HIV-positive individuals treated with the same therapy or in patients with AIDS.

CONCLUSION

In 'CD4-exploders', HAART-induced reconstitution of the T-cell compartment is independent from thymus volume, and is favoured by the upregulation of the IL-7/IL-7 receptor system.

Hematologic Aspects of HIV/AIDS

This review addresses various aspects of HIV infection pertinent to hematology, including the consequences of HIV infection on specific aspects of hematopoiesis and an update on the current biologic, epidemiologic and therapeutic aspects of AIDS-related lymphoma and Hodgkin's disease. The results of the expanding use of progenitor cell transplantation in HIV infected patients are also reviewed. In Section I, Dr. Scadden reviews the basis for HIV dysregulation of blood cell production, focusing on the role of the stem cell in HIV disease. T cell production and thymic function are discussed, with emphasis placed upon the mechanisms of immune restoration in HIV infected individuals. Results of clinical and correlative laboratory studies are presented. In Section II, Dr. Levine reviews the recent epidemiologic trends in the incidence of lymphoma, since the widespread availability of highly active anti-retroviral therapy (HAART). The biologic aspects of AIDS-lymphoma and Hodgkin's disease are discussed in terms of pathogenesis of disease. Various treatment options for these disorders and the role of concomitant anti-retroviral and chemotherapeutic intervention are addressed. Drs. Zaia and Krishnan will review the area of stem cell transplantation in patients with AIDS related lymphoma, presenting updated information on clinical results of this procedure. Additionally, they report on the use of gene therapy, with peripheral blood CD34+ cells genetically modified using a murine retrovirus, as a means to treat underlying HIV infection. Results of gene transfer experiments and subsequent gene marking in HIV infected patients are reviewed.

Increased thymic mass and circulating naive CD4 T cells in HIV-1-infected adults treated with growth hormone

OBJECTIVE

To determine whether treatment with growth hormone (GH) enhances thymopoiesis in individuals infected with HIV-1.

METHODS

Five HIV-1-infected adults were treated with GH for 6-12 months in a prospective open-label study. Immunological analyses were performed before GH treatment and repeated at 3 month intervals after GH initiation. Thymic mass was analysed using computed tomography with quantitative density and volume analysis. Analysis of circulating lymphocytes, including naive and memory T cell subsets, was performed using multiparameter flow cytometry.

RESULTS

GH treatment was associated with a marked increase in thymic mass in all GH recipients. Circulating naive CD4 T cells also increased significantly in all patients during GH therapy, suggesting an enhancement of thymopoiesis.

CONCLUSION

GH has significant effects on the human immune system, including the reversal of thymic atrophy in HIV-1-infected adults. De-novo T cell production may thus be inducible in immunodeficient adults.

T-cell repopulation and thymic volume in HIV-1-infected adult patients after highly active antiretroviral therapy

The origin of T cells after highly active antiretroviral therapy (HAART) in patients infected with human immunodeficiency virus 1 (HIV-1) is now under discussion. The possibility of renewed lymphopoiesis in aged thymuses is still controversial. In this work we combine the analysis of naïve T cells, T-cell receptor excision circles (TRECs), and computed tomography scanning of thymic tissue to further assess whether the thymus is involved in immune reconstitution. Fifteen antiretroviral-naïve HIV-1-infected patients were evaluated during 48 weeks of HAART. At baseline, significant correlation was present among age and both thymic volume and TRECs, and between naïve T cells and TRECs. After starting HAART, there was a significant increase at week 12 in naïve CD4(+) and CD8(+) T cells, TRECs, and thymic volume. The initial net increases in naïve T cells and TREC counts were significantly correlated. Changes in thymic volume and TRECs were also indirectly related; splitting the population into 2 groups of high and low baseline TREC levels, only the group with low TREC levels had significant increases in both TRECs and thymic volume. Thus, the increase in thymic volume might be functional, in response to depleted TREC levels. Taken together, our data strongly suggest a thymic role in immune reconstitution, at least in patients with depleted baseline TREC levels. (Blood. 2002;99:3702-3706)

Dynamics of naive and memory CD4+ T lymphocytes in HIV-1 disease progression

Understanding the dynamics of naive and memory CD4+ T cells in the immune response to HIV-1 infection can help elucidate typical disease progression patterns observed in HIV-1 patients. Although infection markers such as CD4+ T-cell count and viral load are monitored in patient blood, the lymphatic tissues (LT) have been shown to be an important viral reservoir. Here, we introduce the first comprehensive theoretical model of disease progression based on T-cell subsets and virus circulating between the two compartments of LT and blood. We use this model to predict several trademarks observed in adult HIV-1 disease progression such as the establishment of a setpoint in the asymptomatic stage. Our model predicts that both host and viral elements play a role in determining different disease progression patterns. Viral factors include viral infectivity and production rates, whereas host factors include elements of specific immunity. We also predict the effect of highly active antiretroviral therapy and treatment cessation on cellular and viral dynamics in both blood and LT.