Evaluation of T cell receptor gene rearrangement excision circles after antiretroviral therapy in children infected with human immunodeficiency virus

Clinical, Virological and Immunological Subphenotypes in a Cohort of Early Treated HIV-Infected Children

Background

Identifying subphenotypes within heterogeneous diseases may have an impact in terms of therapeutic options. In this study, we aim to assess different subphenotypes in children living with human immunodeficiency virus (HIV-1), according to the clinical, virological, and immunological characteristics.

Methods

We collected clinical and sociodemographic data, baseline viral load (VL), CD4 and CD8 count and percentage, age at initiation of ART, HIV DNA reservoir size in peripheral blood mononuclear cells (PBMCs), cell-associated RNA (CA-RNA), ultrasensitive VL, CD4 subsets (T effector CD25+, activated memory cells, Treg cells), humoral-specific HIV response (T-bet B cells), innate response (CD56dim natural killer (NK) cells, NKp46+, perforin), exhaustion markers (PD-1, PD-L1, DNAM), CD8 senescence, and biomarkers for T-lymphocyte thymic output (TREC) and endothelial activation (VCAM). The most informative variables were selected using an unsupervised lasso-type penalty selection for sparse clustering. Hierarchical clustering was performed using Pearson correlation as the distance metric and WARD.D2 as the clustering method. Internal validation was applied to select the best number of clusters. To compare the characteristics among clusters, boxplot and Kruskal Wallis test were assessed.

Results

Three subphenotypes were discovered (cluster1: n=18, 45%; cluster2: n=11, 27.5%; cluster3: n=11, 27.5%). Patients in cluster1 were treated earlier, had higher baseline %CD4, low HIV reservoir size, low western blot score, higher TREC values, and lower VCAM values than the patients in the other clusters. In contrast, cluster3 was the less favorable. Patients were treated later and presented poorer outcomes with lower %CD4, and higher reservoir size, along with a higher percentage of CD8 immunosenescent cells, lower TREC, higher VCAM cytokine, and a higher %CD4 PD-1. Cluster2 was intermediate. Patients were like those of cluster1, but had lower levels of t-bet expression and higher HIV DNA reservoir size.

Conclusions

Three HIV pediatric subphenotypes with different virological and immunological features were identified. The most favorable cluster was characterized by a higher rate of immune reconstitution and a slower disease progression, and the less favorable with more senescence and high reservoir size. In the near future therapeutic interventions for a path of a cure might be guided or supported by the different subphenotypes.

The role of CD38 in HIV infection

The widely-expressed molecule CD38 is a single-stranded type II transmembrane glycoprotein that is mainly involved in regulating the differentiation and activation state of the cell. CD38 has broad and complex functions, including enzymatic activity, intercellular signal transduction, cell activation, cytokine production, receptor function and adhesion activity, and it plays an important role in the physiological and pathological processes of many diseases. Many studies have shown that CD38 is related to the occurrence and development of HIV infection, and CD38 may regulate its progression through different mechanisms. Therefore, investigating the role of CD38 in HIV infection and the potential signaling pathways that are involved may provide a new perspective on potential treatments for HIV infection. In the present review, the current understanding of the roles CD38 plays in HIV infection are summarized. In addition, the specific role of CD38 in the process of HIV infection of human CD4⁺ T lymphocytes is also discussed.

Purifying Selection in Human Immunodeficiency Virus-1 pol Gene in Perinatally Human Immunodeficiency Virus-1-Infected Children Harboring Discordant Immunological Response and Virological Nonresponse to Long-Term Antiretroviral Therapy

Background

Biological monitoring of antiretroviral treatment (ART) in human immunodeficiency virus (HIV)-infected pediatric population remains challenging. The aim of the present study was to assess the long-term HIV-1 genetic diversity in pol gene in HIV-1-infected children in virological failure under antiretroviral regimen adapted according to the successive World Health Organization (WHO) guidelines for resource-constrained settings.

Methods

HIV-1 diversity in pol gene was assessed in HIV-1-infected children and adolescents born from HIV-infected mothers (median age at follow-up: 13.8 years) in virological failure (VF⁺) despite long-term regimen recommended by the WHO. The numbers of nonsynonymous substitutions per potential nonsynonymous site (dN) and of synonymous substitutions at potential synonymous sites (dS) in HIV-1 pol gene and the dN/dS ratios were used to estimate the selective pressure on circulating HIV-1.

Results

The immunological responses to ART basically corresponded to: 1) Full therapeutic failure with immunological (I^-) and virological nonresponses in one-quarter (24.6%) of study children ((I^-, VF⁺) subgroup); 2) Discordant immunovirological responses with paradoxical high CD4 T cell counts (I⁺) and high HIV-1 RNA load in the remaining cohort patients (75.4%) ((I⁺, VF⁺) subgroup). The mean dS was 1.8-fold higher in (I⁺, VF⁺) than (I^-, VF⁺) subgroup (25.9 ± 18.4 vs. 14.3 ± 10.8). In the (I⁺, VF⁺) subgroup, the mean dS was 1.6-fold higher than the mean dN. Finally, the mean dN/dS ratio was 2.1-fold lower in (I⁺, VF⁺) than (I^-, VF⁺) subgroup (0.6 ± 0.3 vs. 1.3 ± 0.7), indicating purifying selection in the immunovirological discordant (I⁺, VF⁺) subgroup and positive selection in the immunovirological failure (I^-, VF⁺) subgroup.

Conclusions

Children and adolescents in immunovirological therapeutic failure harbor positive selection of HIV-1 strains favoring diversifying in pol-encoded amino acids. In contrast, children with persistent discordant immunovirological responses show accumulation of mutations and purifying selection in pol gene sequences, indicating limited genetic evolution and likely suggesting genetic adaptation of viruses to host functional constraints.

Escalating and sustained immunovirological dissociation among antiretroviral drug-experienced perinatally human immunodeficiency virus-1-infected children and adolescents living in the Central African Republic: A STROBE-compliant study

Sub-Saharan Africa has the vast majority (∼90%) of new pediatric acquired immunodeficiency syndrome cases worldwide. Biologically monitoring HIV-infected pediatric populations remains challenging. The differential interest of human immunodeficiency virus (HIV)-1 RNA loads and CD4 T-cell counts is debated for the treatment of pediatric acquired immunodeficiency syndrome patients.Long-term antiretroviral treatment (ART) outcomes regarding immunological and virological surrogate markers were longitudinally evaluated between 2009 and 2014 (over 57 months) in 245 perinatally HIV-1-infected children and adolescents born from HIV-infected mothers, treated at inclusion for at least 6 months by the World Health Organization-recommended ART in Bangui, Central African Republic.Patients were monitored over time biologically for CD4 T-cell counts, HIV-1 RNA loads, and drug resistance mutation genotyping.Children lost to follow-up totaled 6%. Four categories of immunovirological responses to ART were observed. At baseline, therapeutic success with sustained immunological and virological responses was observed in 80 (32.6%) children; immunological and virologic nonresponses occurred in 32 (13.0%) children; finally, the majority (133; 54.2%) of the remaining children showed discordant immunovirological responses. Among them, 33 (13.4%) children showed rapid virological responses to ART with an undetectable viral load, whereas immunological responses remained absent after 6 months of treatment and increased progressively over time in most of the cases, suggesting slow immunorestoration. Notably, nearly half of the children (40.8% at baseline and 48.2% at follow-up) harbored discordant immunovirological responses with a paradoxically high CD4 T-cell count and HIV-1 RNA load, which are always associated with high levels of drug resistance mutations. The latter category showed a significant increase over time, with a growth rate of 1.23% per year of follow-up.Our STROBE-compliant study demonstrates the high heterogeneity of biological responses under ART in children with frequent passage from 1 category to another over time. Close biological evaluation with access to routine plasma HIV-1 RNA load monitoring is crucial for adapting the complex outcomes of ART in HIV-infected children born from infected mothers.

Using CD4 percentage and age to optimize pediatric antiretroviral therapy initiation

BACKGROUND

Quantifying pediatric immunologic recovery by highly active antiretroviral therapy (HAART) initiation at different CD4 percentage (CD4%) and age thresholds may inform decisions about timing of treatment initiation.

METHODS

HIV-1-infected, HAART-naive children in Europe and the Americas were followed from 2002 through 2009 in PENPACT-1. Data from 162 vertically infected children, with at least World Health Organization "mild" immunosuppression and CD4% <10th percentile, were analyzed for improvement to a normal CD4% (≥10th percentile) within 4 years after HAART initiation. Data from 209 vertically infected children, regardless of immune status, were analyzed for CD4% outcomes at 4 years and viral failure within 4 years.

RESULTS

Seventy-two percent of baseline immunosuppressed children recovered to normal within 4 years. Compared with "severe" immunosuppression, more children with "mild" immunosuppression (difference 36%, 95% confidence interval [CI]: 22% to 49%) or "advanced" immunosuppression (difference 20.8%, 95% CI: 5.8% to 35.9%) recovered a normal CD4%. For each 5-year increase in baseline age, the proportion of children achieving a normal CD4% declined by 19% (95% CI: 11% to 27%). Combining baseline CD4% and age effects resulted in >90% recovery when initiating HAART with "mild" immunosuppression at any age or "advanced" immunosuppression at age <3 years. Baseline CD4% effects became greater with increasing age (P = .02). At 4 years, most immunologic benefits were still significant but diminished. Viral failure was highest in infancy (56%) and adolescence (63%).

CONCLUSIONS

Initiating HAART at higher CD4% and younger ages maximizes potential for immunologic recovery. Guidelines should weigh immunologic benefits against long-term risks.

Use of V(D)J recombination excision circles to identify T- and B-cell defects and to monitor the treatment in primary and acquired immunodeficiencies

T-cell receptor excision circles (TRECs) and kappa-deleting recombination excision circles (KRECs) are circular DNA segments generated in T and B cells during their maturation in the thymus and bone marrow. These circularized DNA elements persist in the cells, are unable to replicate, and are diluted as a result of cell division, thus are considered markers of new lymphocyte output. The quantification of TRECs and KRECs, which can be reliably performed using singleplex or duplex real-time quantitative PCR, provides novel information in the management of T- and B-cell immunity-related diseases. In primary immunodeficiencies, when combined with flow cytometric analysis of T- and B-cell subpopulations, the measure of TRECs and KRECs has contributed to an improved characterization of the diseases, to the identification of patients’ subgroups, and to the monitoring of stem cell transplantation and enzyme replacement therapy. For the same diseases, the TREC and KREC assays, introduced in the newborn screening program, allow early disease identification and may lead to discovery of new genetic defects. TREC and KREC levels can also been used as a surrogate marker of lymphocyte output in acquired immunodeficiencies. The low number of TRECs, which has in fact been extensively documented in untreated HIV-infected subjects, has been shown to increase following antiretroviral therapy. Differently, KREC number, which is in the normal range in these patients, has been shown to decrease following long-lasting therapy. Whether changes of KREC levels have relevance in the biology and in the clinical aspects of primary and acquired immunodeficiencies remains to be firmly established.

Effects of combined antiretroviral therapy on B- and T-cell release from production sites in long-term treated HIV-1+ patients

BACKGROUND

The immune system reconstitution in HIV-1- infected patients undergoing combined antiretroviral therapy is routinely evaluated by T-cell phenotyping, even though the infection also impairs the B-cell mediated immunity. To find new laboratory markers of therapy effectiveness, both B- and T- immune recovery were evaluated by means of a follow-up study of long-term treated HIV-1- infected patients, with a special focus on the measure of new B- and T-lymphocyte production.

METHODS

A longitudinal analysis was performed in samples obtained from HIV-1-infected patients before therapy beginning and after 6, 12, and 72 months with a duplex real-time PCR allowing the detection of K-deleting recombination excision circles (KRECs) and T-cell receptor excision circles (TRECs), as measures of bone-marrow and thymic output, respectively. A cross sectional analysis was performed to detect B- and T-cell subsets by flow cytometry in samples obtained at the end of the follow-up, which were compared to those of untreated HIV-1-infected patients and uninfected controls.

RESULTS

The kinetics and the timings of B- and T-cell release from the bone marrow and thymus during antiretroviral therapy were substantially different, with a decreased B-cell release and an increased thymic output after the prolonged therapy. The multivariable regression analysis showed that a longer pre-therapy infection duration predicts a minor TREC increase and a major KREC reduction.

CONCLUSIONS

The quantification of KRECs and TRECs represents an improved method to monitor the effects of therapies capable of influencing the immune cell pool composition in HIV-1-infected patients.

Thymopoiesis and regulatory T cells in healthy children and adolescents

OBJECTIVES

The purpose of this study was to investigate the association between T cell receptor excision circle levels in peripheral blood mononuclear cells and regulatory T cells that co-express CD25 and Foxp3 in healthy children and adolescents of different ages.

MATERIALS AND METHODS

The quantification of signal-joint T-cell receptor excision circle levels in the genomic DNA of peripheral blood mononuclear cells was performed using real-time quantitative PCR. The analysis of CD4, CD8, CD25, and Foxp3 expression was performed using flow cytometry.

RESULTS

Ninety-five healthy controls (46 females and 49 males) ranging in age from 1 to 18 years were analyzed. The mean T-cell receptor excision circle count in all individuals was 89.095 ± 36.790 T-cell receptor excision circles per microgram of DNA. There was an inverse correlation between T-cell receptor excision circles counts and age (r = -0.846; p<0.001) as well as between the proportion of CD4(+)CD25(+)Foxp3(+) T cells and age (r = -0.467; p = 0.04). In addition, we observed a positive correlation between the amount of CD4(+)CD25(+)Foxp3(+) T cells and the amount of T-cell receptor excision circles per microgram of DNA in individuals of all ages (r = -0.529; p = 0.02).

CONCLUSIONS

In this study, we observed a decrease in the thymic function with age based on the fact that the level of T-cell receptor excision circles in the peripheral blood positively correlated with the proportion of regulatory T cells in healthy children and adolescents. These findings indicate that although T-cell receptor excision circles and regulatory T cells levels decrease with age, homeostasis of the immune system and relative regulatory T cells population levels are maintained in the peripheral blood.

Developmental regulation of P-glycoprotein activity within thymocytes results in increased anti-HIV protease inhibitor activity

The thymus harbors HIV-1 and supports its replication. Treatment with PI-containing ART restores thymic output of naïve T cells. This study demonstrates that CXCR4-using WT viruses are more sensitive to PI in fetal thymcocytes than mature T cells with average IC(50) values for two PIs, RTV and IDV, of 1.5 nM (RTV) and 4.4 nM (IDV) in thymocytes versus 309.4 nM (RTV) and 27.3 nM (IDV) in mature T cells. P-gp activity, as measured using Rh123 efflux and quantitation of P-gp mRNA, increased with thymocyte maturation into CD4 and CD8 lineage T cells. P-gp activity is developmentally regulated in the thymus. Thymocytes developed increased levels of P-gp activity as maturation from DP to SP CD4 or CD8 T cells occurred, although CD4 T cells acquired activity more rapidly. Reduced P-gp activity in thymocytes is one mechanism for effectiveness of PI therapy in suppressing viral replication in the thymus and in reconstitution of naïve T cells, particularly among children receiving PI-containing ART.

Regulatory T cells and chronic immune activation in human immunodeficiency virus 1 (HIV-1)-infected children

The function of CD4(+) T cells with regulatory activity (T(regs)) is the down-regulation of immune responses. This suppressive activity may limit the magnitude of effector responses, resulting in failure to control human immunodeficiency virus 1 (HIV-1) infection, but may also suppress chronic immune activation, a characteristic feature of HIV-1 disease. We evaluated the correlation between viral load, immune activation and T(regs) in HIV-1-infected children. Eighty-nine HIV-1-infected children (aged 6-14 years) were included in the study and analysed for HIV-1 plasmaviraemia, HIV-1 DNA load, CD4 and CD8 cell subsets. T(reg) cells [CD4(+)CD25(high)CD127(low) forkhead box P3 (FoxP3(high))] and CD8-activated T cells (CD8(+)CD38(+)) were determined by flow cytometry. Results showed that the number of activated CD8(+)CD38(+)T cells increased in relation to HIV-1 RNA plasmaviraemia (r = 0·403, P < 0·0001). The proportion of T(regs) also correlated positively with HIV-1 plasmaviraemia (r = 0·323, P = 0·002), but correlated inversely with CD4(+) cells (r = -0·312, P = 0·004), thus suggesting a selective expansion along with increased viraemia and CD4(+) depletion. Interestingly, a positive correlation was found between the levels of T(regs) and CD8(+)CD38(+)T cells (r = 0·305, P = 0·005), and the percentage of T(regs) tended to correlate with HIV-1 DNA load (r = 0·224, P = 0·062). Overall, these findings suggest that immune activation contributes to the expansion of T(reg) cells. In turn, the suppressive activity of T(regs) may impair effector responses against HIV-1, but appears to be ineffective in limiting immune activation.

Microbial translocation induces persistent macrophage activation unrelated to HIV-1 levels or T-cell activation following therapy

OBJECTIVE

HIV-1 replication and microbial translocation occur concomitant with systemic immune activation. This study delineates mechanisms of immune activation and CD4 T-cell decline in pediatric HIV-1 infection.

DESIGN

Cross-sectional and longitudinal cellular and soluble plasma markers for inflammation were evaluated in 14 healthy and 33 perinatally HIV-1-infected pediatric study volunteers prior to and over 96 weeks of protease-inhibitor-containing combination antiretroviral therapy (ART). All HIV-1-infected patients reconstituted CD4 T cells either with suppression of viremia or rebound of drug-resistant virus.

METHODS

Systemic immune activation was determined by polychromatic flow cytometry of blood lymphocytes and ELISA for plasma soluble CD27, soluble CD14, and tumor necrosis factor. Microbial translocation was evaluated by limulus amebocyte lysate assay to detect bacterial lipopolysaccharide (LPS) and ELISA for antiendotoxin core antigen immunoglobulin M (IgM) antibodies. Immune activation markers were compared with viral load, CD4 cell percentage, and LPS by regression models. Comparisons between healthy and HIV-1-infected or between different viral outcome groups were performed by nonparametric rank sum.

RESULTS

Microbial translocation was detected in healthy infants but resolved with age (P < 0.05). LPS and soluble CD14 levels were elevated in all HIV-1-infected patients (P < 0.05 and P < 0.0001, respectively) and persisted even if CD4 T cells were fully reconstituted, virus optimally suppressed, and lymphocyte activation resolved by ART. Children with CD4 T-cell reconstitution but viral rebound following ART continued to display high levels of soluble CD27.

CONCLUSION

Microbial translocation in pediatric HIV-1 infection is associated with persistent monocyte/macrophage activation independent of viral replication or T-cell activation.

Thymectomy in early childhood: a model for premature T cell immunosenescence?

The thymus is the main source of recent thymic emigrants (RTE) and naïve T cells. The aging of the immune system (immunosenescence) is characterized by loss of thymic function, decreased numbers of RTE, peripheral proliferation of mature T cells, and oligoclonal expansions of specific T cell subpopulations. As shown in several studies, thymectomized patients demonstrate signs of premature immunosenescence reminiscent of aged people, such as decreased proportions of naïve T cells and RTE, a compensatory increase of mature T cell subpopulations with increased proliferation rates, restriction of the T cell receptor repertoire, and a delayed response to new antigens and vaccinations. This review demonstrates that, despite some limitations, childhood thymectomy may serve as an useful model for premature immunosenescence, mimicking changes expected after physiological thymus involution in the elderly. Thus, it may prove an insightful tool for obtaining better understanding of human naïve T cell development, thymic function, and maintenance of the naïve T cell pool.

Continuous improvement in the immune system of HIV-infected children on prolonged antiretroviral therapy

BACKGROUND

The goal of HAART is to promote reconstitution of CD4+ T cells and other immune responses. We evaluated the extent and the kinetics of immune reconstitution in HIV-infected children over 144 weeks of successful HAART.

METHODS

Thirty-seven children receiving their first HAART regimen had plasma HIV RNA; T cells and subpopulations; T-cell rearrangement excision circles (TREC) DNA; candida, HIVCD4 and HIVCD8 enzyme-linked immunospot measured at regular intervals.

RESULTS

Plasma HIV RNA became undetectable in 81% of patients at 24 weeks and remained undetectable in 77% at 144 weeks. In contrast, CD4+% continuously increased. Distribution of T-cell subpopulations changed rapidly during the first 48 weeks of HAART and more slowly thereafter. At 144 weeks, total, naive and activated CD4+% and naive CD8+% of HIV-infected children were not significantly different from those of healthy age-matched controls, whereas total and activated CD8+% remained elevated. CD4 and CD8 TREC content increased only during the first 48 weeks of HAART. They positively correlated with each other and with total CD4+%, naive CD4+% and naive CD8+%. Candida and HIVCD4 enzyme-linked immunospot increased over time reaching peak values at 48 weeks and 144 weeks, respectively. HIVCD8 enzyme-linked immunospot decreased in magnitude over 144 weeks of HAART but retained its breadth. Baseline CD4+% positively correlated with CD4+% and with functional immune reconstitution at week 144, whereas baseline TREC correlated with TREC at week 144.

CONCLUSION

HIV-infected children acquired normal distribution of CD4 T cells and other subpopulations and recovered CD4-mediated HIV immunity after 144 weeks of HAART.

Thymectomy in early childhood: significant alterations of the CD4(+)CD45RA(+)CD62L(+) T cell compartment in later life

The study was aimed to assess indicators of immunosenescence, such as the total counts of peripheral blood CD4(+)CD45RA(+)CD62L(+) (naive) T cells, the numbers of T cell receptor excision circles (TRECs), and Ki67-expression as marker of peripheral replication in thymectomized patients (TP) (n=101) compared to age-matched healthy donors (HD) (n=81). In TP, there was an inverse correlation between naive T cells and chronological age (p<0.001) or time post thymectomy (p<0.001). TP demonstrated lower TREC numbers in naive T cells compared to HD (p<0.001). TREC numbers negatively correlated with time post thymectomy (p<0.001). Percentages of Ki67-expresssing naive T cells were higher in TP compared to HD (p<0.05). The findings of the presented long-term follow up cohort of thymectomized patients indicate that changes of the peripheral naive T cell subset in TP may resemble the findings of an aging immune system in elderly persons after thymic involution. Our data provide evidence that peripheral T cell homeostasis in TP is maintained at minimal levels mainly by extrathymic expansion of existing naive T cells in the periphery to compensate the diminished thymic output.

Antiretroviral therapy corrects HIV-1-induced expansion of CD8+ CD45RA+ CD2-) CD11a(bright) activated T cells

BACKGROUND

HIV infection decreases thymic output and induces chronic T-cell activation.

OBJECTIVE

To examine the reconstitution of naive and activated T cells.

METHODS

Extended immune phenotyping of CD4(+) and CD8(+) T-cell subsets was combined with T-cell receptor rearrangement excision circle (TREC) levels and measures of T-cell receptor repertoire perturbations in CD8(+) T-cell subpopulation to define the global effect of HIV-1 on T-cell dynamics. Evaluations before and after therapy were performed in HIV-infected children and compared with those in healthy individuals.

RESULTS

Ten HIV-infected children and adolescents with a broad range of pretherapy CD4(+) T-cell counts were compared with healthy individuals. Pretherapy late activated CD8(+) T cells (CD3(+)CD8(+)CD45RA(+)CD27(-)CD11a(bright) cells) were expanded among HIV-infected subjects. Successful antiretroviral therapy increased the proportion of naive T cells (CD3(+)CD4(+)CD45RA(+)CD27(+)CD28(+) and CD3(+)CD8(+)CD45RA(+)CD27(+)CD11a(dim) cells), with a significant decrease in late activated CD8(+) T cells. The proportion of naive CD4(+) and CD8(+) T cells significantly predicted log(10) TREC copies/10(6) PBMCs in infected children and healthy control subjects, with a negative correlation in late activated CD8(+) T cells and activated CD4(+) T cells. Treatment re-established Gaussian distributions and decreased oligoclonal expansion within the Vbeta repertoire of CD8(+)CD45RA(+) T cells, but compared with that seen in healthy children, the proportion of late activated CD8(+) T cells remained increased.

CONCLUSION

HIV infection strikingly shifts the proportion of naive and late activated CD45RA(+)CD8(+) T cells. Homeostasis within this T-cell population reflects TREC levels and the extent of T-cell receptor Vbeta perturbations.

Immune reconstitution in human immunodeficiency virus type 1-infected children with different virological responses to anti-retroviral therapy

Immune repopulation, despite virological failure, often occurs in children under highly active anti-retroviral therapy (HAART). The aim of this study was to analyse the characteristics of immune repopulation and activation in children with and without virological response to HAART. Fourteen human immunodeficiency virus type 1 (HIV-1)-infected children with suppression of HIV-1 plasma viraemia (virological responders, VR) and 16 virological non-responders (VNR) to therapy were studied at baseline and after approximately 2 years of HAART. During therapy, CD4+ T cells increased in both groups, but were higher in the VR than in the VNR group. All CD4+ T cell subsets (naive, central memory, effector/memory and CD38+) increased significantly in VR children, while there was a significant increase only in naive cells in VNR children. Naive CD8+ T cells and T cell receptor rearrangement excision circles (TREC), an indicator of thymic output, increased in both VR and VNR children. Activated CD8+ CD38+ T cells decreased in VR but remained high in VNR children. Levels of circulating lipopolysaccharide (LPS), an indicator of microbial translocation, further increased in VNR children. In conclusion, HAART induced an increase in naive cells in all children, regardless of their virological response. However, the persistence of viraemia resulted in an impaired expansion of memory CD4+ T cells susceptible to HIV-1 infection, and together with the microbial translocation sustained the persistence of a high level of immune activation.

T cell receptor excision circles (TRECs) analysis during acute intrarectal infection of cynomolgus monkeys with pathogenic chimeric simian human immunodeficiency virus

Several studies have shown the importance of evaluating Recent Thymic Emigrants (RTEs) by quantification of T cell receptor-rearrangement excision circles (TRECs), as a measure of de novo T cell generation during human immunodeficiency virus-1 (HIV-1) infection. To determine whether acute viral infection may have an impact on TRECs, cynomolgus monkeys (Macaca fascicularis) were infected intrarectally with simian human immunodeficiency virus (SHIV) 89.6P(cy11) and the number of signal-joint (sj) TRECs was determined in purified CD4+ and CD8+ populations for up to 28 weeks post-infection. Four weeks after infection, TRECs levels significantly decreased in both CD3+ CD4+ and in CD3+ CD8+ T lymphocytes of infected monkeys, whereas they remained unchanged in uninfected animals. This reduction was followed by a progressive TRECs number recovery in CD3+ CD4+ T lymphocytes that positively correlated with changes in the levels of circulating CD3+ CD4+ T cells. In the CD3+ CD8+ T cell subset, TRECs number remained significantly low and inversely correlated with the increase in the percentages of CD3+ CD8+ T cells. These data suggest that SHIV89.6P(cy11) intrarectal infection of cynomolgus monkeys differently affects TRECs content in CD3+ CD4+ and CD3+ CD8+ T cell subsets.

Association of CD4+ T-lymphocyte counts and new thymic emigrants in HIV-infected children during successful highly active antiretroviral therapy

BACKGROUND

In a cohort of children receiving highly active antiretroviral therapy (HAART) with sustained plasma HIV-1 RNA < 50 copies/mL, children who reached undetectable RNA after week 8 (slow responders, median: week 20) had higher HIV-1 intracellular DNA (HIV-1 DNA) and equal or greater CD4+ T-lymphocyte counts compared with children who reached undetectable plasma HIV-1 RNA by week 8 (rapid responders) throughout HAART.

OBJECTIVE

To determine whether levels of T-cell receptor excision circles (TRECs) could explain the apparent inconsistency between the quantity of HIV-1 DNA and CD4+ T-lymphocyte counts in HIV-1-infected children receiving HAART with sustained virologic suppression.

METHODS

T-cell receptor excision circles and HIV-1 DNA and plasma HIV-1 RNA were quantified longitudinally by PCR in 31 children (median age, 5.6 years) with sustained undetectable plasma HIV-1 RNA for >104 weeks of HAART.

RESULTS

There was a positive correlation between TREC and HIV-1 DNA during HAART, notably at weeks 48 and 80 (P < .004). During the early stage of HAART, TREC levels positively correlated with CD4+ T-lymphocyte percentages (P < .02) and naive CD4+ T-lymphocyte counts (P < .001) and percentages (P = .05). Median TREC levels were consistently equal or higher in slow responders compared with rapid responders (P < .001) despite slow responders having consistently greater quantities of HIV-1 DNA.

CONCLUSION

To maintain adequate levels of CD4+ T-lymphocytes, children with high HIV-1 DNA maintain high levels of TREC while receiving HAART. Thus, a thymic control mechanism is required to maintain new CD4+ T lymphocytes in the presence of persistent virus.

CLINICAL IMPLICATIONS

The TREC level is a useful marker of thymic function in HIV-infected children.

Thymic volume, T-cell populations, and parameters of thymopoiesis in adolescent and adult survivors of HIV infection acquired in infancy

OBJECTIVES

Antiretroviral therapy has significantly prolonged the lifespan of children who acquire HIV infection in infancy, but the impact of HIV on thymus-mediated maintenance of T lymphocytes has not been studied. To examine the long-term effects of HIV infection in childhood on thymopoiesis, thymic volume and parameters of thymic function from clinically stable adolescents and young adults with HIV infection acquired in infancy were compared with those from uninfected controls.

METHODS

Thymic volume was determined using three-dimensional reconstruction and volumetric analysis of non-contrast enhanced computed tomography images of the upper chest. The degree of fat involution was assessed using a semiquantitive scoring system. CD4 and CD8 T cell populations and T cell receptor recombination excision circles (TREC) concentrations in peripheral blood lymphocytes were measured in all subjects.

RESULTS

Twenty youths (aged 17.6 +/- 2.5 years) with HIV infection acquired perinatally (n = 18) or by neonatal transfusion (n = 2) were enrolled whose HIV plasma viral load had been undetectable for a median of 3.1 years, along with 18 seronegative healthy young adults (aged 20.6 +/- 1.3 years). HIV infected subjects and controls had indistinguishable CD4 T cell counts, thymus volumes (20.5 versus 15.8 cm), thymic index scores, and TREC values. Thymic volume correlated with the number and percentage of CD4 T lymphocytes in the control group and with the number of TREC in CD4 lymphocytes in the HIV infected group.

CONCLUSIONS

Long term survivors of pediatric HIV infection appear to have retained or recovered thymic volume and thymic activity approximating uninfected youths.

Levels of recent thymic emigrant cells decrease in children undergoing partial thymectomy during cardiac surgery

The human thymus is required for establishment of a T-cell pool in fetal life, but postnatal thymectomy is not known to cause immunodeficiency. T-cell emigration from thymus (thymic recent emigrants [TRECs]) is a continuous thymic-dependent process. We studied TREC levels pre- and post-partial thymectomy in children undergoing cardiac surgery. TRECs were quantitated by real-time PCR in peripheral blood lymphocytes of 24 children (0 to 12 years). TREC values were 47916 +/- 9271 pre-partial thymectomy and 33157 +/- 8479 post-partial thymectomy in 11 paired patients (P = 0.014). Interval between pre- and post-partial thymectomy was 8.8 days +/- 5.8 days. Another group of 8 children had 30384 +/- 9748 TRECs 16 days to 6 years post-partial thymectomy. There was a significant drop in TREC values post-partial thymectomy in the immediate postoperative period compared to prethymectomy TREC levels. While decreased thymic output may persist, the long-term implications were not evaluated in this patient population.

Post-natal ontogenesis of the T-cell receptor CD4 and CD8 Vbeta repertoire and immune function in children with DiGeorge syndrome

DiGeorge syndrome (DGS) is a congenital disorder characterized by typical facial features, hypoparatyroidism, conotruncal cardiac defects and thymic hypoplasia. Although there are some reports addressing lymphocytes counts and function in DGS children over time, few data have been reported on the T-cell receptor V beta (TCRBV) repertoire in relation to disease progression. The aim of this study was to evaluate the degree and nature of immunodeficiency and to investigate a possible correlation to clinical findings. We used third complementary region (CDR3) size spectratyping as a tool for monitoring T-cell repertoire diversity in 7 DGS's children. The rate of thymic output, the phenotype and function of peripheral T-cells and the humoral immunity were also investigated. At baseline a profound alteration of the TCR repertoire was noted, mainly in the CD8+ T-cells, in DGS patients when compared to a control group. Furthermore, analysis of thymic output showed a significant decrease in TCR rearrangement excision circles (TRECs) levels in the patient group. Immunoglobulin abnormalities were also detected. The observed TCR repertoire alterations, although not statistically significant, may suggest an increased susceptibility to infections. A parallel increase in the TCR repertoire diversity and clinical improvement occurred during the follow-up. Our results confirm that the extent of immunodeficiency is highly variable and could improve through childhood, and indicate that TCR repertoire may be a useful marker to clinically monitor thymic function in this primary immunodeficiency.

Correlates of delayed disease progression in HIV-1-infected Kenyan children

Without treatment most HIV-1-infected children in Africa die before their third birthday (>89%) and long-term nonprogressors are rare. The mechanisms underlying nonprogression in HIV-1-infected children are not well understood. In the present study, we examined potential correlates of delayed HIV disease progression in 51 HIV-1-infected African children. Children were assigned to progression subgroups based on clinical characterization. HIV-1-specific immune responses were studied using a combination of ELISPOT assays, tetramer staining, and FACS analysis to characterize the magnitude, specificity, and functional phenotype of HIV-1-specific CD8(+) and CD4(+) T cells. Host genetic factors were examined by genotyping with sequence-specific primers. HIV-1 nef gene sequences from infecting isolates from the children were examined for potential attenuating deletions. Thymic output was measured by T cell rearrangement excision circle assays. HIV-1-specific CD8(+) T cell responses were detected in all progression groups. The most striking attribute of long-term survivor nonprogressors was the detection of HIV-1-specific CD4(+) Th responses in this group at a magnitude substantially greater than previously observed in adult long-term nonprogressors. Although long-term survivor nonprogressors had a significantly higher percentage of CD45RA(+)CD4(+) T cells, nonprogression was not associated with higher thymic output. No protective genotypes for known coreceptor polymorphisms or large sequence deletions in the nef gene associated with delayed disease progression were identified. In the absence of host genotypes and attenuating mutations in HIV-1 nef, long-term surviving children generated strong CD4(+) T cell responses to HIV-1. As HIV-1-specific helper cells support anti-HIV-1 effector responses in active disease, their presence may be important in delaying disease progression.

Genotypic and phenotypic correlates of the HIV Type 1 env gene evolution in infected children with discordant response to antiretroviral therapy

The genotypic-phenotypic correlates of human immunodeficiency virus type 1 (HIV-1) env gene evolution were investigated in samples from eight infected children under antiretroviral therapy (ART) and virological failure. Evolution of the gp120 C2-V5 env sequence was demonstrated in all subjects but one by sequence analysis of the replicating plasma virus collected at baseline and 12-15 months after ART initiation. The analysis of the host's selective pressure showed that in four subjects, the ratio of nonsynonymous to synonymous amino acid substitutions was higher in the V3 sequences than in the C2-V5 region ([K (a)/K (s)](V3)/[K (a)/K (s)](C2V5) >1.0). Interestingly, this feature was observed only in subjects (four of five) showing an increase in T cell receptor rearrangement excision circle (TREC)-bearing cells and in CD4(+) T-lymphocytes despite persistent viremia (discordant therapeutic response) (p = 0.02), thus suggesting that the V3 loop is a target of the immune reconstitution under ART. Using phenotypic analysis of recombinant viruses expressing exogenous V3 sequences, a reverse shift from CXCR4-tropic to CCR5-tropic variants was demonstrated in two of the four subjects, further indicating that the host's selective pressure sharply forces the V3 evolution of replicating variants. The data indicate that a complex HIV-1-host interplay occurs in children receiving antiretroviral treatments and suggest that the recovery of thymic function places a selective constraint on the viral V3 loop.

[Changes in CD4+ and CD8+ T-cell subsets in HIV-infected children differently correlate with viral replication and thymic function]

BACKGROUND AND OBJECTIVE

We intended to study the relationship between T-cell subsets with plasmatic detectable viral load (VL) and T-receptor excision circles (TREC).

PATIENTS AND METHOD

Twenty HIV-infected children on highly active antiretroviral therapy (HAART) were recruited in a 1-year longitudinal retrospective study. We analyzed the relationship between changes in peripheral blood T-cell subsets, VL and TREC markers by lineal regression.

RESULTS

Memory and activated CD4+ T-cells increases had a negative association with log10 TRECs increases. However, naive CD4+ T-cells increases had a positive association with log10 TRECs increases. In contrast, memory, activated and effector CD8+ T-cells increases positively correlated with log10 VL increases. On the other hand, naive CD8+ T-cells increases had a negative association with log10 VL increases.

CONCLUSIONS

CD4+ and CD8+ T-cells subsets change in a different way as a response to the changes produced by HAART in HIV vertically infected children. CD4+ T-cells are more dependent on thymic function and CD8+ T-cells are more dependent on viral replication. Thus, the decline in cellular activation would allow the production of more naive T-cells by the thymus.

Evaluation of the impact of highly active antiretroviral therapy on immune recovery in antiretroviral naive patients

OBJECTIVES

To examine the extent of immune reconstitution in treatment-naive patients with CD4 T-cell counts <500 cells/microL following 48 weeks of highly active antiretroviral therapy (HAART).

METHODS

Thirteen antiretroviral naive patients were evaluated longitudinally for 48 weeks on HAART utilizing immune functional and lymphocyte phenotyping assays, including lymphocyte proliferation assay, flow cytometric evaluation of cell surface markers, and delayed type hypersensitivity skin tests. Virologic responses were monitored using commercially available viral load assays and gag/pol mRNA quantification using simultaneous immunophenotyping/UltraSensitive fluorescence in situ hybridization (ViroTect In Cell HIV-1 Detection Kit; Invirion, Frankfort, MI). Thymic function was evaluated for a subset of four patients using real-time polymerase chain reaction (PCR) for T-cell receptor excision circle (TREC) quantification and thymic scans using computerized axial tomography (CT) of the thymus.

RESULTS

HAART initiation resulted in a significant decline in plasma viremia and percentage of infected peripheral blood cells, and a rise in CD4 T cells from a baseline median of 207 cells/microL to a week-48 median of 617 cells/microL. The rise was predominately in CD4 memory cells. Naive T cells also increased in number, but at a slower rate. Activated (HLA-DR CD38) CD4 and CD8 T cells were elevated at baseline (24 and 62%, respectively) and declined by week 48 (17 and 36%, respectively) but did not reach normal levels. The number of Fas CD4 T cells increased from a baseline median of 169 to 381 cells/microL at week 48. Both soluble interleukin (IL)-2 and tumour necrosis factor (TNF) II receptors declined by week 48. HIV p24 lymphocyte proliferation assay responses were transiently detected in three patients. TREC values increased from a median 6400 copies/microg at baseline to a week-48 median value of 26 697 copies/microg.

CONCLUSION

Immune functional reconstitution was not achieved in these HAART naive patients.

Lack of good correlation of serum CC-chemokine levels with human immunodeficiency virus-1 disease stage and response to treatment

Three CC-chemokines-MIP-1alpha (CCL3), MIP-1beta (CCL4), and RANTES (CCL5)-are natural ligands for the human immunodeficiency virus-1 (HIV-1) coreceptor CCR5. To determine correlations between CC-chemokines and HIV-1 disease stage or response to treatment, we examined serum levels of MIP-1alpha, MIP-1beta, and RANTES in 60 infected patients during 18 months while they were taking highly active antiretroviral therapy (HAART). Our results demonstrate that serum levels of MIP-1alpha and RANTES were increased in HIV-1-infected individuals compared with those in healthy controls. We found no significant differences among 4 clinical stages of HIV-1 infection in the serum levels of three CC-chemokines. Longitudinal HAART analyses revealed a pronounced decline in serum MIP-1alpha levels over time. We found no difference in this decline between HAART responders and nonresponders. These findings indicate that production of MIP-1alpha and RANTES changes during HIV-1 infection and treatment; however, our results suggest that serum levels of CC-chemokines should not be used as biomarkers for HIV-1 disease stage or response to treatment.

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.

T-cell receptor excision circles: a novel prognostic parameter for the outcome of transplantation in multiple myeloma patients

This study investigated whether T-cell receptor excision circles (TRECs) are a prognostic marker for the outcome of myeloma patients undergoing a tandem autologous peripheral blood stem cell transplantation (PBSCT). Twenty-five patients were enrolled. Samples were obtained at study enrollment, after conventional therapy, between first and second transplantation and 3, 6, 12 and 24 months after the second PBSCT. TRECs were quantified using real-time polymerase chain reaction. A high variation in TREC levels was found at diagnosis (median TREC level 136/10(5) peripheral blood mononuclear cells (PBMCs); range 1-1729), suggesting individual differences in thymic output of naive T cells. Patients with more than 136 TRECs/10(5) P BMCs at diagnosis had a statistically significant better overall survival (P=0.05) and event-free survival (P=0.045), whereas low TREC levels correlated with a higher incidence of infectious complications. Median TREC values were lowest after the first PBSCT (52/10(5) PBMCs) and reached the baseline 12 months after the second transplantation. Patients with high TREC levels after the second PBSCT had a significantly higher probability of being in complete or partial remission 30 months after the second PBSCT. TREC levels were not correlated with beta2-microglobulin and C-reactive protein levels at diagnosis. These data suggest that TRECs could be a relevant prognostic factor for patients who receive high-dose chemotherapy and autologous PBSCT.

Increased thymic output during acute measles virus infection

Measles virus infects thymic epithelia, induces a transient lymphopenia, and impairs cell-mediated immunity, but thymic function during measles has not been well characterized. Thirty Zambian children hospitalized with measles were studied at entry, hospital discharge, and at 1-month follow-up and compared to 17 healthy children. During hospitalization, percentages of naïve (CD62L+, CD45RA+) CD4+ and CD8+ T lymphocytes decreased (P = 0.01 for both), and activated (HLA-DR+, CD25+, or CD69+) CD4+ and CD8+ T lymphocytes increased (P = 0.02 and 0.03, respectively). T-cell receptor rearrangement excision circles (TRECs) in measles patients were increased in CD8+ T cells at entry compared to levels at hospital discharge (P = 0.02) and follow-up (P = 0.04). In CD4+ T cells, the increase in TRECS occurred later but was more sustained. At discharge, TRECs in CD4+ T cells (P = 0.05) and circulating levels of interleukin-7 (P = 0.007) were increased compared to control values and remained elevated for 1 month, similar to observations in two measles virus-infected rhesus monkeys. These findings suggest that a decrease in thymic output is not the cause of the lymphopenia and depressed cellular immunity associated with measles.

CD4+ and CD8+ T cell receptor repertoire perturbations with normal levels of T cell receptor excision circles in HIV-infected, therapy-naive adolescents

Our objective was to determine whether treatment-naive HIV-infected adolescents manifest abnormalities in thymus function and peripheral T cell repertoire, and to assess relationships of these immunologic characteristics with each other, with plasma HIV virus load, and T cell surface markers. TCR Vbeta repertoire was determined by CDR3 length spectratyping in purified CD4(+) and CD8(+) T cells of high-risk, HIV-negative adolescents and of treatment-naive, HIV-infected adolescents. Thymus function was investigated by the simultaneous examination of T cell receptor excision circles (TRECs) in the CD4(+) and CD8(+) T cell subsets. HIV-infected adolescents exhibited significantly greater perturbations in their TCR Vbeta repertoire in comparison with HIV-negative subjects. Perturbations in the CD8(+) T cell compartment were more profound in comparison with CD4(+) T cells. The CD4(+) TCR Vbeta perturbations were negatively correlated with the total and phenotypically naive CD4(+) T cells, and with CD4(+) TRECs. CD8(+) TRECs, although not correlated with CD8(+) TCR Vbeta perturbations, showed negative correlation with memory and activated CD8(+) T cells. Interestingly, TRECs in CD4(+) and CD8(+) T cells were not significantly different between HIV-infected and uninfected adolescents. The TCR Vbeta repertoire in adolescents is profoundly perturbed even in early stages of HIV infection, when total CD4(+) cell counts in most subjects are within normal limits. The correlative analyses demonstrating negative association of CD4(+) cell TRECs with CD4(+) TCR Vbeta perturbations and of CD8(+) TRECs with CD8(+) cell activation markers provide evidence of the intense activation of the central and peripheral immune compartments in this study population.

[Complete immune and clinical recovery after highly active antiretroviral therapy in advanced vertically HIV-1 infected children]

BACKGROUND AND OBJECTIVE

The aim of this study was to evaluate the ability of long-term highly active antiretroviral therapy (HAART) to fully reconstitute the immune system in children with severe AIDS.

PATIENT AND METHOD

Lymphoproliferative responses (LPR) were evaluated by incorporation of [3H]-thymidine. Cytokine production in culture (IFN- gamma, IL-5) was quantified using commercially available specific ELISA assays. T-cells subsets were determined by 3-color flow cytometry and thymic production of T-cells was assessed by quantification of TCR rearrangement excision circles (TRECs).

RESULTS

We present a vertically HIV-1-infected child at clinical category C, with long-standing CD4+ T-cells below 50/l, who was monitored during 3-years after starting HAART by quantifying the viral load (VL), naïve, memory, and activated T-cell subsets, and thymical function as well as clinical events. VL was suppressed to undetectable levels since the beginning of HAART with d4T, 3TC, nelfinavir, and efavirenz resulting in a dramatic immune reconstitution, achieving normal CD4+ T-cells counts after 6 months (25%, 597 CD4+ T-cells/l) and perdurable undetectable VL levels. Naïve CD4 and CD8 T-cells increased in parallel to TCR rearrangement excision circles (TRECs) levels, with a concomitant decrease in T-cell activation markers. Interestingly, the patient showed an increase in the lymphoproliferative responses to PHA; the IFN-gamma production by PBMCs increased with HAART while the production of IL-5 diminished, thus indicating a switch of type 2 to type 1 response. He recovered clinically and immunologically (up to normal levels) and remains asymptomatic at present.

CONCLUSION

This report demonstrates that at least in these children, an immune and clinical recovery from an advanced stage of HIV-disease can be possible throught HAART.

Assessment of thymic activity in human immunodeficiency virus-negative and -positive adolescents by real-time PCR quantitation of T-cell receptor rearrangement excision circles

Circular DNA molecules known as T-cell receptor rearrangement excision circles (TREC) arise during T-cell development and are present in cells that have recently emigrated from the thymus. In cross-sectional studies, the number of peripheral blood lymphocytes bearing TREC decreases with age, consistent with an anatomically demonstrated loss of thymic epithelial tissue. TREC numbers increase following hematopoietic stem cell transplantation and during therapy for human immunodeficiency virus (HIV) infection. Quantitation of TREC has therefore been proposed as a parameter of thymic activity. In this study, we used real-time PCR to quantify TREC in peripheral blood samples obtained longitudinally from HIV-seronegative adolescents. TREC values in peripheral blood T cells were very stable throughout adolescence, once thought to be a time of rapid involution of the thymus. In addition, in a cross-sectional analysis, we examined TREC values in a cohort of HIV-positive adolescents and found evidence of ongoing thymopoiesis in perinatally infected individuals, despite lifelong infection. These data demonstrate the utility of TREC assessment in adolescents and that HIV infection does not uniformly result in accelerated thymic involution in childhood.

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.

Immune recovery in HIV disease: role of the thymus and T cell expansion in immune reconstitution strategies

While the progressive depletion of CD4(+) T cells is the hallmark of the impact of HIV on the immune system, considerable data also point to the loss of T cell function. The question is: Can the immune system recover from this insult and what are the therapeutic strategies available to us to mediate this immune recovery? This review will focus on our current knowledge of immune recovery following treatment with highly active antiretroviral therapy (HAART). Enhancement of thymic function in generating de novo T cell synthesis post-HAART has also emerged as a viable immune recovery strategy. Advances in molecular (T cell receptor excision circle assay) and conventional (computed tomography scans of the thymus) approaches to evaluate the role of the thymus in immune recovery as well as potential agents that might enhance thymic output (interleukin-7, IL-7) will contribute greatly to the assessment of the success of these approaches as immune recovery strategies. In this review, we will integrate this new information in the context of the current strategies for HIV therapy leading to long-term immune reconstitution.

Immune reconstitution

Immune reconstitution in HIV-infected patients remains a potential mechanism to explain delayed disease progression and increased survival following suppressive therapy. Many discrepancies remain to be studied. Is an immune response to HIV protective? Why are anti-HIV CD4 responses lost so quickly in progressors and how can they be restored? What is the damage to the immune system that occurs early in disease and why can it not be overcome by simply controlling viral replication? Will management of immune reconstitution be used in future adjunct treatment strategies (vaccine or STI)? Because HAART is not the answer to long-term management of HIV throughout the world, the recovery of immune function and it's potential to control viral replication remains a key goal in the long-term management of HIV-infected persons.

Increased peripheral expansion of naive CD4+ T cells in vivo after IL-2 treatment of patients with HIV infection

Intermittent interleukin-2 (IL-2) therapy has been shown to increase the number of CD4+ T cells, preferentially cells with a naive phenotype, in patients with HIV infection. For this report we investigated the mechanism underlying this expansion by studying the relative roles of peripheral expansion and thymic output. In a cohort of six patients receiving IL-2 over a period of 1 year, the mean number of naive CD4+ T cells increased from 139 to 387 cells per microl while levels of T cell receptor rearrangement excision circles (TRECs) declined from 47,946 to 26,510 copies per 10(6) naive T cells, thus making it unlikely that the CD4+ T cell count increases were secondary to increase in thymic output. To examine directly the impact of IL-2 on peripheral expansion, peripheral blood mature, naive CD4+ T cells were labeled ex vivo with 5-bromodeoxyuridine as well as stained directly for Ki67. These studies revealed a 7-fold increase in the percentage of 5-bromodeoxyuridine-positive cells and a 20-40-fold increase in Ki67 staining in the naive CD4+ T cell pool in the setting of IL-2 administration. This degree of increase in mature CD4+ T cell turnover induced by IL-2 does not compromise the future replicative potential of these cells, because longitudinal measurements of telomere length went from 6,981 to 7,153 bp after 1 year of IL-2 therapy. These data strongly suggest that much of the increase in CD4+ cells associated with IL-2 treatment is caused by peripheral expansion of existing naive CD4+ T cells rather than increased thymic output and that these increases occur without compromising the potential of these cells for further cell division.

Reevaluation of T cell receptor excision circles as a measure of human recent thymic emigrants

The human thymus exports newly generated T cells to the periphery. As no markers have been identified for these recent thymic emigrants (RTE), it is presently impossible to measure human thymic output. T cell receptor excision circles (TREC) have been recently used to assess thymic output during both health and disease. Using a mathematical model, we quantify age-dependent changes both in the number of RTE generated per day and in TREC concentration during an 80-year lifespan. Through analyses, we demonstrate that RTE and peripheral T cell division have the same potential to affect TREC concentration at any age in healthy people. T cell death also influences TREC concentration, but to a lesser extent. During aging, our results indicate that thymic involution primarily induces an age-dependent decline in TREC concentrations within both CD4(+) and CD8(+) T cell populations. We further apply this model for studying TREC concentration during HIV-1 infection. Our analyses reveal that a decrease in thymic output is the major contributor to the decline in TREC concentration within CD4(+) T cells, whereas both increased peripheral T cell division and decreased thymic output induce the decline in TREC concentration within CD8(+) T cells. Therefore, we suggest that T cell turnover should be examined together with TREC concentration as a measure of RTE. If peripheral T cell division remains relatively unchanged, then TREC concentration indeed reflects thymic output.

Immune reconstitution in HIV-1-infected children on antiretroviral therapy: role of thymic output and viral fitness

OBJECTIVE

To investigate the role of thymic output and viral fitness in immune reconstitution in HIV-1-infected children on antiretroviral therapy.

METHODS

Thymic output was studied by measuring levels of T-cell receptor rearrangement excision circles (TREC) in peripheral blood lymphocytes, using a real-time quantitative PCR assay. Recombinant viruses containing pre-therapy or post-therapy HIV-1 protease domains were evaluated for viral infectivity in a quantitative single-cycle assay.

RESULTS

Eighteen HIV-1-infected children who showed a significant increase in CD4 T-cell count after therapy were studied; HIV-1 plasma viraemia was substantially suppressed in 12 children (virological responders), but not in the other six (virological non-responders). TREC were quantified at baseline, and sequentially during the first 12 months of therapy. Both virological responders and non-responders showed an increase in TREC levels that was inversely correlated with baseline TREC and CD4 T cell counts. Changes in TREC positively correlated with CD4 T-cell count increases in virological responders, but not in non-responders; moreover, the ratios between TREC and CD4 T-cell count increases were higher in non-responders than in responders, suggesting a persistence of peripheral CD4 T-cell loss in the former. Drug-resistant viruses with reduced replicative capacity were documented in three out of six non-responders.

CONCLUSIONS

These findings indicate that recovery of thymic function is a pivotal event in immune reconstitution, and suggest that CD4 T-cell increase despite persistent viraemia is sustained by a continuous thymic output that compensates peripheral CD4 T-cell depletion which might be slowed down by emerging viruses with reduced fitness.