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

The challenge of viral reservoirs in HIV-1 infection

A viral reservoir is a cell type or anatomical site in association with which a replication-competent form of the virus accumulates and persists with more stable kinetic properties than the main pool of actively replicating virus. This article reviews several cell types and anatomical sites proposed as potential reservoirs for HIV-1. It is now clear that HIV-1 persists in a small reservoir of latently infected resting memory CD4(+) T cells, which shows minimal decay even in patients on highly active antiretroviral therapy (HAART). The persistence of virus in this reservoir is consistent with the biology of these cells and the long-term persistence of immunologic memory. The viral replication that continues in patients on suppressive HAART may also contribute to the stability of this reservoir. There may be other reservoirs, but the latent reservoir in resting CD4(+) T cells appears to be sufficient to guarantee lifetime persistence of HIV-1 in the majority of patients on current HAART regimens, and unless new approaches are developed, eradication will not be possible. The clinical implications of this and other HIV-1 reservoirs are discussed.

Role of antiretroviral regimes in HIV-1 patients in reducing immune activation

We assessed whether antiretroviral regimes are able to diminish apoptosis and markers of lymphocyte activation and restore lymphocyte proliferation. T-cell subset, spontaneous and induced apoptosis, CD95 and soluble Fas antigen and cell proliferation were analysed in 41 human immunodeficiency virus type 1-positive patients. Twenty-five were in asymptomatic stage A and 16 were in stage B/C. Thirty-five received antiretroviral treatment: 18 received two inhibitors of reverse transcriptase and one protease inhibitor and 17 received three inhibitors of reverse transcriptase. Six patients did not receive treatment, for different reasons, but continued to participate in the study. Studies were performed at baseline, 3, 6 and 12 months. Levels of CD4 increased slightly until 6 months of antiretroviral treatment, as a whole, in all the patients treated. Naïve CD4 lymphocytes, as well as memory CD4 lymphocytes, remained constant. Spontaneous apoptosis of lymphocytes, after 72 hr of culture, decreased in all patients treated, but to a much smaller extent than phytohaemagglutinin-induced apoptosis. In both groups treated, levels of soluble Fas decreased until 6 months of treatment and then increased again. Lymphocyte proliferation reached normal levels after 1 year of treatment. In patients without treatment CD4 cells decreased slowly and no modification in activation markers was found. Antiretroviral regimes decrease immune activation as well as viral load and this deactivation restores lymphocyte proliferation.

Immune reconstitution in patients with HIV infection

The peripheral T cell pool is damaged by HIV-1 infection and can be regenerated by production of new T lymphocytes either from the thymus or from proliferation of post-thymic T cells. A critical question for AIDS patients is whether treatment with antiretroviral drugs can restore the capability to produce new T lymphocytes. The development of a new assay of thymus function in adults (the measurement of T cell receptor excision circles, TRECs), and studies of thymus biopsies in untreated and treated HIV-1-infected patients, have suggested that in select patients the thymus can regenerate on antiretroviral therapy. New strategies to overcome the thymic atrophy of aging are needed to improve thymic function in the majority of AIDS patients.

Effects of exogenous interleukin-7 on human thymus function

Immune reconstitution is a critical component of recovery after treatment of human immunodeficiency virus (HIV) infection, cancer chemotherapy, and hematopoietic stem cell transplantation. The ability to enhance T-cell production would benefit such treatment. We examined the effects of exogenous interleukin-7 (IL-7) on apoptosis, proliferation, and the generation of T-cell receptor rearrangement excision circles (TRECs) in human thymus. Quantitative polymerase chain reaction demonstrated that the highest level of TRECs (14 692 copies/10 000 cells) was present in the CD1a(+)CD3(-)CD4(+)CD8(+) stage in native thymus, suggesting that TREC generation occurred following the cellular division in this subpopulation. In a thymic organ culture system, exogenous IL-7 increased the TREC frequency in fetal as well as infant thymus, indicating increased T-cell receptor (TCR) rearrangement. Although this increase could be due to the effect of IL-7 to increase thymocyte proliferation and decrease apoptosis of immature CD3(-) cells, the in vivo experiments using NOD/LtSz-scid mice given transplants of human fetal thymus and liver suggested that IL-7 can also directly enhance TREC generation. Our results provide compelling evidence that IL-7 has a direct effect on increasing TCR-alphabeta rearrangement and indicate the potential use of IL-7 for enhancing de novo naïve T-cell generation in immunocompromised patients.

Baseline Thymic Volume is a Predictor for CD4 T Cell Repopulation in Adult HIV-Infected Patients under Highly Active Antiretroviral Therapy

The objective of the present study was to assess the impact of baseline thymic volume on the CD4 cell repopulation induced by highly active antiretroviral therapy (HAART) in HIV-infected adults. Therefore, 37 previously untreated HIV-1-infected adults were prospectively followed from August 1998 to September 2001. All patients underwent a thorax CT scan at starting HAART to measure thymic volume. The median follow-up time was 189 [87.5–498.5] days. CD4 cell repopulation was considered as an increase ≥200 cells/mm3 above baseline count. Twenty-one (57%) patients achieved this CD4 repopulation. Baseline thymic volume was the main independent factor associated with CD4 repopulation ( P=0.016); this factor was a positive predictor (adjusted hazard ratio: 1.25 [95% confidence interval 1.1–1.4]). Although, CD4 cell count and non-AIDS diagnosis at baseline were associated with CD4 cell repopulation on the univariate analysis ( P=0.03 and P=0.02, respectively), no statistical signification was found on the multivariate test. In summary, baseline thymic volume is a predictor of early CD4 cell repopulation in adult HIV-1-infected patients taking HAART.

Plasticity of lymphoid compartments during HIV infection and treatments: hopes and limits

Immune reconstitution during antiretroviral therapy has recently been shown to depend upon multiple factors at work in T-cell homeostasis, amongst which the reduction of thymus dysfunction and of immune hyperactivation is instrumental. The restoration of host defenses against opportunistic pathogens is, however, balanced by the poor immunity restored against HIV thus giving a satisfying link between antigen stimulation and the reconstitution of immune responses to pathogens.

A broad T-cell repertoire diversity and an efficient thymic function indicate a favorable long-term immune reconstitution after cord blood stem cell transplantation

Cord blood (CB) is used increasingly as a source of hematopoietic stem cells because of a lower risk of acute and chronic graft-versus-host disease (GVHD). However, there is some concern regarding the ability to adequately reconstitute host immune response due to the immaturity and naivety of CB T cells. This study was designed to evaluate T-cell reconstitution using combined approaches of phenotyping, analysis of alphabeta T-cell receptor (TCR) diversity, and assessment of ex vivo thymic function by measuring TCR rearrangement excision circles (TRECs). Ten patients who underwent CB transplantation for high-risk hematologic disorders were compared to a reference group of 19 age- and GVHD-matched patients who underwent transplantation with non-T cell-depleted bone marrow from an HLA-identical sibling donor. TREC values correlated with the relative number of naive T cells and with TCR repertoire polyclonality. During the first year after transplantation, TCR repertoires were highly abnormal and TREC values low in both groups. Notably, 2 years after transplantation onward TREC values as well as TCR diversity were higher in CB recipients than in recipients of bone marrow transplants. These data indicate an efficient thymic regeneration pathway from CB lymphoid progenitors despite the low number of cells infused compared to bone marrow, arguing for a complete clinical immune recovery after CB transplantation.

Evidence of immune reconstitution in antiretroviral drug-experienced patients with advanced HIV disease

Highly active antiretroviral therapy (HAART) of HIV disease is associated with effective virologic control, immune reconstitution, and clinical improvements. This study addresses the potential for improvements in lymphocyte phenotype and virologic responses of HIV-infected persons with extensive experience with dual nucleoside reverse transcriptase (NRTI) treatment and advanced HIV disease after a change to a potent antiretroviral therapy (NRTI + protease inhibitor). The majority of participants achieved virologic success. There was a median rise in CD4+ lymphocytes of 99 cells/mm(3) by 48 weeks, because of an increase in memory CD4+ cells at 4 and 16 weeks, followed by a later increase in naive CD4+ cells between weeks 16 and 48. The proportion of activated, DR+ CD38+ CD8+ lymphocytes decreased during the 48 weeks of follow-up. The immunologic findings (increased memory and naive T cells and reduced activation levels) were significantly improved in participants with persistent suppression of viral replication over the 48 weeks of the study. Baseline HIV RNA copy number was lower (median, 14,784 copies/ml) in persons who responded virologically than in those not suppressing viral replication (median, 49,454 copies/ml). CD4+ cell counts above the median (125/mm(3)) at time 0 for the participants, was the only baseline immunologic marker significantly associated with viral suppression at week 48. Participants older than 40 years of age demonstrated less immunologic recovery. The results of the study show that patients with extensive experience with NRTIs respond both virologically and immunologically during the first 48 weeks of therapy with a potent antiretroviral regimen.

Evidence for increased T cell turnover and decreased thymic output in HIV infection

The effects of HIV infection upon the thymus and peripheral T cell turnover have been implicated in the pathogenesis of AIDS. In this study, we investigated whether decreased thymic output, increased T cell proliferation, or both can occur in HIV infection. We measured peripheral blood levels of TCR rearrangement excision circles (TREC) and parameters of cell proliferation, including Ki67 expression and ex vivo bromodeoxyuridine incorporation in 22 individuals with early untreated HIV disease and in 15 HIV-infected individuals undergoing temporary interruption of therapy. We found an inverse association between increased T cell proliferation with rapid viral recrudescence and a decrease in TREC levels. However, during early HIV infection, we found that CD45RO-CD27high (naive) CD4+ T cell proliferation did not increase, despite a loss of TREC within naive CD4+ T cells. A possible explanation for this is that decreased thymic output occurs in HIV-infected humans. This suggests that the loss of TREC during HIV infection can arise from a combination of increased T cell proliferation and decreased thymic output, and that both mechanisms can contribute to the perturbations in T cell homeostasis that underlie the pathogenesis of AIDS.

Thymopoiesis in HIV-infected adults after highly active antiretroviral therapy

The thymus of HIV-seropositive patients can enlarge as CD4+ T cell counts increase on highly active anti-retroviral therapy (HAART). This may indicate development of new T cells or represent mature peripheral T cells recirculating to the thymus. To define the etiology of the enlargement, the thymuses of two HIV-infected individuals on HAART were biopsied. For more than 3 years before initiation of HAART, both patients (38 and 41 years of age) had documented CD4+ T lymphopenia. Peripheral blood samples were obtained to assess circulating CD4+ CD45RA+ CD62L+ T cells, which were thought to have recently developed in the thymus. Peripheral blood T cells from both patients and thymocytes from the second patient were also tested for levels of DNA episomes formed during T cell receptor gene rearrangement (T cell receptor rearrangement excision circles, TRECs). With HAART, peripheral blood CD4+ T cell counts increased from approximately 60/mm(3) to 552/mm(3) and 750/mm(3) for patients 1 and 2, respectively. Thymic biopsies from both patients showed normal thymus histology with active thymopoiesis. Percentages of peripheral blood CD4+ CD45RA+ CD62L+ T cells and quantitation of T cell TRECs also reflected active thymopoiesis in both patients. Thus, in these two HIV-seropositive adults examined after initiation of HAART, thymic enlargement represented active thymopoiesis. Thymopoiesis in adult AIDS patients may contribute to immune reconstitution even after prolonged CD4+ T lymphopenia.

Interleukin-7 in plasma correlates with CD4 T-cell depletion and may be associated with emergence of syncytium-inducing variants in human immunodeficiency virus type 1-positive individuals

Human immunodeficiency virus type 1 (HIV-1) primary infection is characterized by the use of CCR5 as a coreceptor for viral entry, which is associated with the non-syncytium-inducing (NSI) phenotype in lymphoid cells. Syncytium-inducing (SI) variants of HIV-1 appear in advanced stages of HIV-1 infection and are characterized by the use of CXCR4 as a coreceptor. The emergence of SI variants is accompanied by a rapid decrease in the number of T cells. However, it is unclear why SI variants emerge and what factors trigger the evolution of HIV from R5 to X4 variants. Interleukin-7 (IL-7), a cytokine produced by stromal cells of the thymus and bone marrow and by keratin, is known to play a key role in T-cell development. We evaluated IL-7 levels in plasma of healthy donors and HIV-positive patients and found significantly higher levels in HIV-positive patients. There was a negative correlation between circulating IL-7 levels and CD4(+) T-cell count in HIV-positive patients (r = -0.621; P < 0.001), suggesting that IL-7 may be involved in HIV-induced T-cell depletion and disease progression. IL-7 levels were higher in individuals who harbored SI variants and who had progressed to having CD4 cell counts of lower than 200 cells/microl than in individuals with NSI variants at a similar stage of disease. IL-7 induced T-cell proliferation and up-regulated CXCR4 expression in peripheral blood mononuclear cells in vitro. Taken together, our results suggest a role for IL-7 in the maintenance of T-cell regeneration and depletion by HIV in infected individuals and a possible relationship between IL-7 levels and the emergence of SI variants.

Interleukin-7 and infection itself by human immunodeficiency virus 1 favor virus persistence in mature CD4(+)CD8(-)CD3(+) thymocytes through sustained induction of Bcl-2

The sequence of events and the mechanisms leading to the destruction of the thymus during human immunodeficiency virus (HIV) infection are still poorly characterized. Investigated here are the survival capacity on HIV-1 infection of the mature single-positive CD4(+)CD8(-)CD3(+) (SP CD4(+)) and the intermediate CD4(+) CD8(-)CD3(-) thymocytes previously shown to be able to replicate the virus in the thymic microenvironment. It is demonstrated that the mature SP CD4(+) thymocytes exhibit a high survival capacity despite the production of a high yield of viruses. Interleukin-7, reported to be a crucial cofactor of tumor necrosis factor (TNF) to promote HIV replication, is shown here to counteract the apoptotic activity of TNF. Resistance to apoptosis of SP CD4(+) cells is conferred by a high expression of the IL-7 receptor (IL-7R) associated with the capacity of IL-7 to permanently up-regulate Bcl-2. In addition, this high Bcl-2 level is further enhanced by infection itself. In contrast, intermediate thymocytes, which replicate the virus at a lower level, are more sensitive to apoptosis, and their differentiation into double-positive CD4(+)CD8(+)CD3(-) (DP CD3(-)) cells strongly increases their death rate on infection. This sensitivity is related to a lower expression of IL-7R and Bcl-2 in intermediate thymocytes, which further decreases at the DP CD3(-) stage. In addition, a decreased level of Bcl-2 is observed in this subset during infection. Altogether these data suggest that in vivo, HIV infection might create a persistent virus reservoir within the SP CD4(+) thymocytes, whereas the later infection of intermediate cells might lead to thymopoiesis failure.

Poor CD4 T cell restoration after suppression of HIV-1 replication may reflect lower thymic function

OBJECTIVE

To characterize immune phenotype and thymic function in HIV-1-infected adults with excellent virologic and poor immunologic responses to highly active antiretroviral therapy (HAART).

METHODS

Cross-sectional study of patients with CD4 T cell rises of > or = 200 x 10(6) cells/l (CD4 responders; n = 10) or < 100 x 10(6) cells/l (poor responders; n = 12) in the first year of therapy.

RESULTS

Poor responders were older than CD4 responders (46 versus 38 years; P < 0.01) and, before HAART, had higher CD4 cell counts (170 versus 35 x 106 cells/l; P = 0.11) and CD8 cell counts (780 versus 536 x 10(6) cells/l; P = 0.02). After a median of 160 weeks of therapy, CD4 responders had more circulating naive phenotype (CD45+CD62L+) CD4 cells (227 versus 44 x 10(6) cells/l; P = 0.001) and naive phenotype CD8 cells (487 versus 174 x 10(6) cells/l; P = 0.004) than did poor responders (after 130 weeks). Computed tomographic scans showed minimal thymic tissue in 11/12 poor responders and abundant tissue in 7/10 responders (P = 0.006). Poor responders had fewer CD4 cells containing T cell receptor excision circles (TREC) compared with CD4 responders (2.12 versus 27.5 x 10(6) cells/l; P = 0.004) and had shorter telomeres in CD4 cells (3.8 versus 5.3 kb; P = 0.05). Metabolic labeling studies with deuterated glucose indicated that the lower frequency of TREC-containing lymphocytes in poor responders was not caused by accelerated proliferation kinetics.

CONCLUSION

Poor CD4 T cell increases observed in some patients with good virologic response to HAART may be caused by failure of thymic T cell production.

T cell receptor excision circle (TREC) content following maximum HIV suppression is equivalent in HIV-infected and HIV-uninfected individuals

BACKGROUND

The adult human thymus contributes to de novo T cell synthesis; such synthesis can be assessed by analyzing T cell receptor excision circles (TREC).

METHODS

TREC levels were measured in total peripheral blood mononuclear cells (PBMC) and CD4- and CD8-enriched cells of 29 HIV-positive patients with maximal viral suppression. The expression of CD45RA+CD45RO-, CD45RA+CD62L+, CD45RO-CD27+CD95low and HLA-DR+CD38+ was assessed using three-color flow cytometric analysis of whole blood. Thymic index score was based on computed tomographic scans of the thymus. The relationship of TREC with thymic index and the expression of the naive phenotypes was evaluated.

RESULTS

TREC expression was not statistically different in these HIV-positive patients from that in age-matched HIV-negative controls. Among HIV-positive patients with CD4 cell count of > 500 x 10(6) cells/l after antiretroviral therapy (n = 15), PBMC TREC levels correlated with the expression of CD45RA+CD45RO- and CD45RA+CD62L+ naive phenotypes, and inversely correlated with the expression of HLA-DR+CD38+. The change between pre- and post-therapy CD4 cell counts for these 15 patients significantly correlated with both thymic index and expression of the CD45RA+CD45RO- phenotype.

CONCLUSIONS

The finding that TREC expression was equivalent between HIV-positive patients after therapy and HIV-negative donors suggests that there is no reduction in thymic output among HIV-positive individuals after therapy. Given that TREC is inversely correlated with HLA-DR/CD38 expression, its analysis in studies of thymopoiesis should be evaluated in the context of maximum viral suppression to reduce HIV-mediated immune activation and/or by normalizing for cell turnover.

Human immunodeficiency virus pathogenesis: insights from studies of lymphoid cells and tissues

Although plasma virus load is invaluable for monitoring human immunodeficiency virus (HIV) infection, key pathogenesis events and most viral replication take place in lymphoid tissues. Decreases in virus load associated with therapy occur in plasma and tissues, but persistent latent infection and ongoing viral replication are evident. Many unanswered questions remain regarding mechanisms of HIV-associated lymphocyte depletion, but partial CD4(+) cell reconstitution after therapy likely reflects retrafficking from inflamed tissues, increased thymic or peripheral production, and decreased destruction. Rapid establishment of latent infection and the follicular dendritic cell-associated viral pool within lymphoid tissues suggest that only early intervention could substantially alter the natural history of HIV. If therapy is started prior to seroconversion, some individuals retain potent HIV-specific cellular immune responsiveness that is suggestive of delayed progression. Although complete virus eradication appears out of reach at present, more attention is being directed toward the prospect of boosting HIV-specific immune responses to effect another type of "clinical cure": immune-mediated virus suppression in the absence of therapy.

The human thymus during aging

The human thymus is required for establishment of a normal T cell repertoire in fetal development, as children born without a thymus (DiGeorge Syndrome) lack thymus-derived (T) and T cell immunity. While the function of the thymus in children for production of new T cells is clear, it has not been obvious that the adult thymus can produce significant numbers of new T cells. Until recently, no assays were available to directly evaluate postnatal thymic function. This paper reviews work on human thymic aging at Duke University School of Medicine and discusses the relevance of this work to devising new strategies for T cell immune reconstitution in man.

Immunology and the REACH study: HIV immunology and preliminary findings. Reaching for Excellence in Adolescent Care and Health

This review paper presents the immunology findings in human immunodeficiency virus (HIV) infected and uninfected youth in the Reaching for Excellence in Adolescent Care and Health (REACH) Project within the context of basic and HIV immunology concepts. Methods employed in the study for specimen collection, management, and laboratory analysis are presented. This paper reviews published analyses of cross-sectional data; longitudinal analyses are underway. These preliminary data extend the work of others in demonstrating the potential for substantial thymic reserve in youth. This finding in HIV infected adolescents has implications for a fuller response to antiretroviral or immune-based therapies compared to that seen in adults. Dysregulation in mucosal immunity may appear before systemic HIV effects are seen and requires attention particularly to screening and treatment of genital co-infections. REACH has demonstrated gender differences in immunologic measures irrespective of HIV infection status.

Changes in thymic function in HIV-positive patients treated with highly active antiretroviral therapy and interleukin-2

Despite its potent antiviral activity, highly active antiretroviral therapy (HAART) only exerts a marginal effect on CD4+ T-cell regeneration in HIV-infected subjects. Combination therapies aimed at boosting T-cell activity and maturation may provide an important contribution to the restoration of immune function. Here, we report the results obtained by a two-year follow-up of a cohort of HIV-infected patients treated with a combination of HAART and interleukin-2 (IL-2). In these patients, in addition to a series of quantitative virological and immunological parameters, we investigated T-cell regeneration by an immunophenotypic assay monitoring CD4+ naïve T cells, and by analysis of thymic function, through the quantification of the excision DNA products of T-cell receptor rearrangement (TRECs) in lymphocytes. Compared with HAART alone, we found that the IL-2 combination therapy was equally effective in reducing the levels of viremia and marginally more effective in decreasing proviral DNA load. Strikingly, the IL-2 combination produced a marked increase in the number of CD4+ T cells bearing a naïve phenotype (CD45RA+, CD62L+), which was apparent for over 96 weeks after therapy. To assess whether these cells were the product of improved T-cell generation, we exploited a competitive quantitative molecular assay to quantify TRECs in peripheral blood lymphocytes. Surprisingly, we found that the levels of these molecules were unchanged in these patients. These findings indicate that improved thymic function does not account for the early rise of CD4 naïve cells in HIV-positive patients treated with IL-2, and suggest that alternative mechanisms of T-cell maturation and differentiation are responsible for this event.

Immunity to infections following hematopoietic cell transplantation

Hematopoietic cell transplantation has progressed from the use of unpurified bone marrow cells or mobilized peripheral blood cells to the use of purified stem cells and progenitor cells. These kinds of transplants can be designed to provide not only hematopoietic rescue but also augmented innate and acquired immunity.

Reconstitution of CD4+ T lymphocytes in HIV-infected individuals following antiretroviral therapy

Immune reconstitution during antiretroviral therapy has recently been shown to depend upon multiple factors at work in T cell homeostasis, amongst which the reduction of thymus dysfunction and of immune hyperactivation are instrumental. The optimism that has been raised by the restoration of hosts' defenses against opportunistic pathogens is, however, balanced by the poor immunity restored against HIV; thus, innovative immune interventions are required.

Immune Reconstitution Strategies in HIV

Although potent antiretroviral therapy can dramatically decrease HIV replication and improve some aspects of host immunity, incomplete immune reconstitution persists even after several years of fully suppressive therapy. In addition, long-term toxicities of antiretroviral medications and the probability of developing multidrug-resistant virus with long-term use indicate that alternate means of controlling viral replication are needed for more durable suppression of HIV. Immune-based therapies may help potentiate the host's own defenses against HIV and other pathogens, and may ultimately result in more durable viral suppression and lower incidence of antiretroviral therapy-related side effects and toxicities.

Impaired replication of protease inhibitor-resistant HIV-1 in human thymus

Many HIV-1-infected patients treated with protease inhibitors (PI) develop PI-resistant HIV-1 variants and rebounds in viremia, but their CD4+ T-cell counts often do not fall. We hypothesized that in these patients, T-cell counts remain elevated because PI-resistant virus spares intrathymic T-cell production. To test this, we studied recombinant HIV-1 clones containing wild-type or PI-resistant protease domains, as well as uncloned isolates from patients, in activated peripheral blood mononuclear cells, human thymic organ cultures and human thymus implants in SCID-hu Thy/Liv mice. In most cases, wild-type and PI-resistant HIV-1 isolates replicated to similar degrees in peripheral blood mononuclear cells. However, the replication of PI-resistant but not wild-type HIV-1 isolates was highly impaired in thymocytes. In addition, patients who had PI-resistant HIV-1 had abundant thymus tissue as assessed by computed tomography. We propose that the inability of PI-resistant HIV-1 to replicate efficiently in thymus contributes to the preservation of CD4+ T-cell counts in patients showing virologic rebound on PI therapy.

Immunopathogenesis of human immunodeficiency virus: implications for immune-based therapies

Human immunodeficiency virus (HIV) infection leads to a state of CD4 lymphopenia and generalized immune activation with subsequent development of opportunistic infections and neoplasms. The use of highly active antiretroviral treatment has dramatically improved the clinical outcome for HIV-infected patients, but the associated cost and toxicity and the eventual development of drug resistance have underscored the need for additional therapeutic strategies. Immune-based therapies, such as treatment with cytokines or immunosuppressants, adoptive immunotherapy, and therapeutic immunizations, are being intensely investigated as potential supplements to antiretroviral therapy. Although much data have been generated as a result of these efforts, to date there has been little evidence of the clinical efficacy of these strategies. Randomized clinical studies remain critical in evaluating the clinical significance and the role of immune-based therapies in the therapeutic armamentarium against HIV.

HIV chemotherapy

The use of chemotherapy to suppress replication of the human immunodeficiency virus (HIV) has transformed the face of AIDS in the developed world. Pronounced reductions in illness and death have been achieved and healthcare utilization has diminished. HIV therapy has also provided many new insights into the pathogenesis and the viral and cellular dynamics of HIV infection. But challenges remain. Treatment does not suppress HIV replication in all patients, and the emergence of drug-resistant virus hinders subsequent treatment. Chronic therapy can also result in toxicity. These challenges prompt the search for new drugs and new therapeutic strategies to control chronic viral replication.

The dynamics of CD4+ T-cell depletion in HIV disease

The size and composition of the CD4+ T-cell population is regulated by balanced proliferation of progenitor cells and death of mature progeny. After infection with the human immunodeficiency virus, this homeostasis is often disturbed and CD4+ T cells are instead depleted. Such depletion cannot result simply from accelerated destruction of mature CD4+ T cells - sources of T-cell production must also fail. Ironically, this failure may be precipitated by physiological mechanisms designed to maintain homeostasis in the face of accelerated T-cell loss.

Down-regulation of CD8+ T-cell expansions in patients with human immunodeficiency virus infection receiving highly active combination therapy

Analysis of T-cell receptor (TCR) repertoire usage made by peripheral T lymphocytes during the chronic phase of HIV-1 infection has revealed the presence of clonal expansions of CD8 T cells that are also shown to be largely HIV-specific. Yet, it remains unclear whether the global repertoire perturbation observed during the chronic phase of the infection is also HIV-related and reversible in the long term with the application of highly active antiretroviral therapy. Furthermore, the diversity and the stability of repertoire usage after a relapse of viral replication were never examined. Eight patients were observed longitudinally up to 31 months under triple-association therapy. When viral replication was steadily suppressed, CD8 repertoires were significantly stabilized. Conversely, in situations of incomplete or only transient viral suppression, persistence or rebound in repertoire perturbation was observed. Finally, a T-cell response remarkably different from baseline, as reflected by a repertoire switch, was generated after the discontinuation of highly active therapy. In conclusion, a sustained control of HIV replication correlated with profound modifications of the CD8 repertoire usage. These data also suggested that autovaccination by the withdrawal of antiviral drugs would result in the selection and expansion of T-cell clones that were not necessarily dominant before the onset of treatment.

Circulating levels of interleukin-7 in antiretroviral-naïve and highly active antiretroviral therapy-treated HIV-infected patients

Interleukin (IL)-7 is a critical cytokine regulating T-lymphocyte development, regeneration, and function.

PURPOSE

This study analyzes the endogenous IL-7 production in HIV-infected patients receiving highly active antiretroviral treatment (HAART).

METHOD

Plasma levels of IL-7 were measured by enzyme-linked immunosorbent assay (ELISA) in 11 patients with untreated advanced HIV disease, in 8 patients who successfully responded to HAART, and in 9 individuals with virological and immunological treatment failure.

RESULTS

We found that in the patients with advanced HIV disease and no treatment IL-7 concentrations were elevated and were inversely related to both CD4 + and CD8 + T-cell counts. When IL-7 was assessed in treated patients, this cytokine was below the detection limit of the assay in all participants who responded to HAART. On the contrary, patients with evidence of HAART failure had increased concentrations of IL-7 that were comparable to those found in the untreated group with progressive disease.

CONCLUSION

These data suggest that IL-7 may play a role in the immune reconstitution of T-cells during HIV infection, especially in the context of potent antiretroviral treatments.

The DiGeorge anomaly

The DiGeorge anomaly, originally considered a clinical paradigm for isolated thymus deficiency, has now been redefined as a member of a group of disorders that share in common a chromosome deletion resulting in monosomy 22q11 (CATCH-22 or DGA/VCFS). In addition to the thymus defect, conotruncal heart anomalies, dysmorphism, hypoparathyroidism, and cleft palate are prominent features. Despite the emphasis on thymus involvement in DGA, a clinically significant thymus defect is found only in a small percentage of these patients probably occurring in less than 5% of the cases. Maldescent of the thymus, however, is extremely common, leading to an absence of mediastinal thymic tissue in nearly all cases. The basic embryological fault in these disorders is an inadequate development of the facial neural-crest tissues, resulting in defective organogenesis of pharyngeal pouch derivatives that receive cephalic neural-crest contribution to the mesenchmyme. The causes for this maldevelopment are both genetic and extragenetic in origin and the genetic lesions act in concert with random environmental events to produce the ultimate clinical picture. The modern research approaches now available have cleared away most of the confusion clouding the clinical and theoretical aspects of DGA and related disorders, providing the clinician with useful landmarks to assess and treat these intriguing clinical challenges.

Relative significance of different pathways of immune reconstitution in HIV type 1 infection as estimated by mathematical modeling

A major goal of antiretroviral HIV-1 therapy is the reversal of HIV-1-associated immunological dysfunction. However, the pathogenetic mechanisms involved and their significance are largely unknown. On the basis of the life cycle of naive, activated, and memory CD4(+) T cell subsets, a mathematical model of immune reconstitution was developed and applied to data for T cell subsets in individuals with acute or chronic HIV-1 infection receiving antiretroviral therapy. The final model that most accurately fitted the data, and resulted in realistic estimates for CD4(+) T cell turnover, considered three pathways of immune reconstitution for naive cells, including thymic production, peripheral expansion, and redistribution of naive cells from lymphoid tissue. The reconstitution of the memory compartment was fitted through differentiation and expansion of naive cells and peripheral expansion of memory cells as well as redistribution of memory cells trapped in the lymphoid tissue. Estimated median half-lives for naive and memory CD4(+) T cells were 114 and 21 days, while total production rates were 9.1 x 10(7) and 2.4 x 10(9) cells/day, respectively. Peripheral expansion and thymic production contributed equally to the regeneration of naive cells, but peripheral expansion of memory cells was larger than production of these cells by differentiation of naive cells. A comparison of immune reconstitution in acute and chronic HIV-1 infection revealed that, after adjustment for age, the main difference was the more rapid release of a larger number of naive cells in treated acute HIV-1 infection. Thymic function and peripheral expansion rates, however, were similar in both cohorts.

CD4+,CD28- T cells in rheumatoid arthritis patients combine features of the innate and adaptive immune systems

OBJECTIVE

To determine whether CD4+,CD28- T cells, which are expanded in patients with rheumatoid arthritis (RA), express receptors that typically regulate the function of natural killer (NK) cells.

METHODS

Expression of the NK cell surface molecules CD158, p70, CD94, CD161, and CD8alpha on T cell subsets was determined by multicolor flow cytometric analysis of peripheral blood mononuclear cells from 36 RA patients. Expression of CD161 on tissue-infiltrating CD4 T cells was determined by 2-color immunohistochemistry analysis of synovial tissue samples.

RESULTS

Killer cell-inhibitory receptors (KIR) and killer cell-activating receptors (KAR) were exclusively expressed on CD4+,CD28- T cells, with the CD158b molecule being the most frequently detected isoform. A coordinated mechanism inducing KIR/KAR expression was suggested by similarities in the expression of CD158b on CD4 and CD8 T cells. CD4+,CD28- T cells were also positive for CD8-alphaalpha homodimers, another characteristic shared with NK cells. Of the C-type lectin NK cell receptors (NK receptors), CD94 was consistently absent, but CD161 was found on a CD4 T cell population that is significantly expanded in RA patients (P = 0.01). Involvement in disease of NK receptor-expressing CD4 T cells was suggested by the presence of CD4+,CD161+ T cells in follicular microstructures typical of rheumatoid synovitis.

CONCLUSION

Patients with RA have an expanded and unusual subset of CD4 T cells that infiltrates the tissue lesions and is characterized by a deficiency of CD28, the expression of CD8-alphaalpha homodimers, and the expression of several types of HLA class I-recognizing NK receptors. CD4 T cells bearing NK receptors can bridge functions of the innate and adaptive immune systems, such as responsiveness to specific antigen, rapid release of interferon-gamma, cytotoxicity, independence from classic costimulatory pathways, and integration of multiple activating and inhibitory signals to control effector functions.

Increased production of IL-7 accompanies HIV-1-mediated T-cell depletion: implications for T-cell homeostasis

We hypothesized that HIV-1-mediated T-cell loss might induce the production of factors that are capable of stimulating lymphocyte development and expansion. Here we perform cross-sectional (n = 168) and longitudinal (n = 11) analyses showing that increased circulating levels of interleukin (IL)-7 are strongly associated with CD4+ T lymphopenia in HIV-1 disease. Using immunohistochemistry with quantitative image analysis, we demonstrate that IL-7 is produced by dendritic-like cells within peripheral lymphoid tissues and that IL-7 production by these cells is greatly increased in lymphocyte-depleted tissues. We propose that IL-7 production increases as part of a homeostatic response to T-cell depletion.

Cellular restoration in HIV infected persons treated with abacavir and a protease inhibitor: age inversely predicts naive CD4 cell count increase

OBJECTIVE

To characterize early and later indices of cellular restoration among HIV-1 infected persons treated with abacavir and one protease inhibitor and to identify predictors of CD4 cell increases.

METHODS

Flow-cytometric analyses of lymphocyte phenotypes among 71 antiretroviral treatment naive adults in a 48 week treatment trial.

RESULTS

During the first 4 weeks of therapy, increases in naive and memory CD4 cells and in B cells were seen; naive CD8 cells increased while CD8 cells remained stable as memory CD8 cells decreased. During the second phase total CD4 and naive CD4 and CD8 cells increased while total CD8 and memory CD8 cells decreased. The numbers of CD4 cells that expressed CD28 increased from a median of 308 x 10(6)/l at baseline to 477 x 10(6)/l at week 48. Higher baseline plasma HIV-1 RNA levels predicted the magnitude of early CD4 (r = 0.35; P = 0.01), memory CD4 (r = 0.38; P = 0.001) and CD28 CD4 cell (r = 0.29; P = 0.01) restoration but was not related to second phase changes. Younger age predicted a greater second phase (but not first phase) increase in naive CD4 cells (r = -0.31; P = 0.03).

CONCLUSIONS

Higher baseline levels of HIV-1 replication determine the magnitude of first phase CD4 cell increases after suppression of HIV-1 replication. Second phase (primarily naive) CD4 cell increases are not related to HIV-1 replication but are inversely relate to age suggesting that thymic potential is a major determinant of long term cellular restoration in HIV-1 infected persons receiving antiretroviral therapy.

Clinical progression, survival, and immune recovery during antiretroviral therapy in patients with HIV-1 and hepatitis C virus coinfection: the Swiss HIV Cohort Study

BACKGROUND

Hepatitis C virus (HCV) infection is highly prevalent among HIV-1-infected individuals, but its contribution to the morbidity and mortality of coinfected patients who receive potent antiretroviral therapy is controversial. We used data from the ongoing Swiss HIV Cohort Study to analyse clinical progression of HIV-1, and the virological and immunological response to potent antiretroviral therapy in HIV-1-infected patients with or without concurrent HCV infection.

METHODS

We analysed prospective data on survival, clinical disease progression, suppression of HIV-1 replication, CD4-cell recovery, and frequency of changes in antiretroviral therapy according to HCV status in 3111 patients starting potent antiretroviral therapy.

RESULTS

1157 patients (37.2%) were coinfected with HCV, 1015 of whom (87.7%) had a history of intravenous drug use. In multivariate Cox's regression, the probability of progression to a new AIDS-defining clinical event or to death was independently associated with HCV seropositivity (hazard ratio 1.7 [95% CI 1.26-2.30]), and with active intravenous drug use (1.38 [1.02-1.88]). Virological response to antiretroviral therapy and the probability of treatment change were not associated with HCV serostatus. In contrast, HCV seropositivity was associated with a smaller CD4-cell recovery (hazard ratio for a CD4-cell count increase of at least 50 cells/microL=0.79 [0.72-0.87]).

INTERPRETATION

HCV and active intravenous drug use could be important factors in the morbidity and mortality among HIV-1-infected patients, possibly through impaired CD4-cell recovery in HCV seropositive patients receiving potent antiretroviral therapy. These findings are relevant for decisions about optimum timing for HCV treatment in the setting of HIV infection.

T cell depletion in HIV-1 infection: how CD4+ T cells go out of stock

HIV-1 infection is characterized by a gradual loss of CD4+ T cells and progressive immune deficiency that leads to opportunistic infections, otherwise rare malignancies and ultimately death. Extensive research over the past two decades has increased our insight into the pathogenic mechanisms underlying these features of HIV-1 infection. Here, we will give a brief overview of the most recent findings and present a model that fits most of the relevant aspects of HIV-1 infection as known. We hypothesize that HIV-1 infection depletes T cell supplies (which are not replaced because of low and static thymic function) by direct infection and killing of cells and through hyperactivation of the immune system.

Coinfection of SCID-hu Thy/Liv mice with human herpesvirus 6 and human immunodeficiency virus type 1

Human herpesvirus 6 (HHV-6) has been proposed as a potential cofactor in the progression of human immunodeficiency virus type 1 (HIV-1) disease. We used the SCID-hu Thy/Liv mouse model to evaluate the in vivo interactions between HHV-6 and HIV-1. Our results demonstrate that HHV-6 and HIV-1 can simultaneously replicate in the human thymus in vivo. In this model, however, the presence of one virus appears not to modify the replication or cytopathicity of the other.

Model of HIV-1 disease progression based on virus-induced lymph node homing and homing-induced apoptosis of CD4+ lymphocytes

Several proposed theories have described the progression of HIV infection. Even so, no concrete evidence supports any as comprehensive, including, for example, why the CD4+ T-cell counts fall from 1000/mm3 of blood to roughly 100/mm3 over an average 10-year period, whereas concomitant viral loads are relatively constant, increasing by several orders of magnitude in late-stage disease. Here, we develop and validate a theoretical model that altered lymphocyte circulation patterns between the lymph system and blood due to HIV-induced enhanced lymph-node homing and subsequent apoptosis of resting CD4+ T cells can explain many aspects of HIV-1 disease progression. These results lead to a recalculation of the CD4+ lymphocyte dynamics during highly active antiretroviral therapy, and also suggest new targets for therapy.

The role of the thymus in immune reconstitution in aging, bone marrow transplantation, and HIV-1 infection

The human thymus is a complex chimeric organ comprised of central (thymic epithelial space) and peripheral (perivascular space) components that functions well into adult life to produce naive T lymphocytes. Recent advances in identifying thymic emigrants and development of safe methods to study thymic function in vivo in adults have provided new opportunities to understand the role that the human thymus plays in immune reconstitution in aging, in bone marrow transplantation, and in HIV-1 infection. The emerging concept is that there are age-dependent contributions of thymic emigrants and proliferation of postthymic T cells to maintain the peripheral T cell pool and to contribute to T cell regeneration, with the thymus contributing more at younger ages and peripheral T cell expansion contributing more in older subjects. New studies have revealed a dynamic interplay between postnatal thymus output and peripheral T cell pool proliferation, which play important roles in determining the nature of immune reconstitution in congenital immunodeficiency diseases, in bone marrow transplantation, and in HIV-1 infection. In this paper, we review recent data on human postnatal thymus function that, taken together, support the notion that the human thymus is functional well into the sixth decade and plays a role throughout life to optimize human immune system function.

T-cell progenitor function during progressive human immunodeficiency virus-1 infection and after antiretroviral therapy

Impairment of T-cell renewal has been proposed as contributing to CD4+ T-cell depletion in persons infected with human immunodeficiency virus-1. We analyzed the T-cell development capacity of progenitors using fetal thymus organ culture. Those who progressed to AIDS had a dramatic loss in T-cell development capacity shortly after seroconversion. In contrast, long-term nonprogressors retained progenitor capacity 8 years after seroconversion. Approximately 70% of patients experienced an improvement in T-cell development capacity after receiving 6 months of potent antiretroviral therapy. Improvement in T-cell development in fetal thymus organ culture correlated with an increase in the number of naive CD4+ T cells in peripheral blood. Numbers of progenitors in blood and bone marrow after seroconversion or during therapy did not correlate with the change observed in T-cell development capacity. These data provide evidence that HIV-1 infection can interfere with T-cell renewal at the level of the progenitor cell. Interference with T-cell renewal may contribute to CD4+ T-cell depletion.

T-cell progenitor function during progressive human immunodeficiency virus-1 infection and after antiretroviral therapy

Impairment of T-cell renewal has been proposed as contributing to CD4+ T-cell depletion in persons infected with human immunodeficiency virus-1. We analyzed the T-cell development capacity of progenitors using fetal thymus organ culture. Those who progressed to AIDS had a dramatic loss in T-cell development capacity shortly after seroconversion. In contrast, long-term nonprogressors retained progenitor capacity 8 years after seroconversion. Approximately 70% of patients experienced an improvement in T-cell development capacity after receiving 6 months of potent antiretroviral therapy. Improvement in T-cell development in fetal thymus organ culture correlated with an increase in the number of naive CD4+ T cells in peripheral blood. Numbers of progenitors in blood and bone marrow after seroconversion or during therapy did not correlate with the change observed in T-cell development capacity. These data provide evidence that HIV-1 infection can interfere with T-cell renewal at the level of the progenitor cell. Interference with T-cell renewal may contribute to CD4+ T-cell depletion.

Regulation of extrathymic T cell development and turnover by oncostatin M

Chronic exposure to oncostatin M (OM) has been shown to stimulate extrathymic T cell development. The present work shows that in OM transgenic mice, 1) massive extrathymic T cell development takes place exclusively the lymph nodes (LNs) and not in the bone marrow, liver, intestines, or spleen; and 2) LNs are the sole site where the size of the mature CD4+ and CD8+ T cell pool is increased (6- to 7-fold). Moreover, when injected into OM transgenic mice, both transgenic and nontransgenic CD4+ and CD8+ T cells preferentially migrated to the LNs rather than the spleen. Studies of athymic recipients of fetal liver grafts showed that lymphopoietic pathway modulated by OM was truly thymus independent, and that nontransgenic progenitors could generate extrathymic CD4+CD8+ cells as well as mature T cells under the paracrine influence of OM. The progeny of the thymic-independent differentiation pathway regulated by OM was polyclonal in terms of Vbeta usage, exhibited a phenotype associated with previous TCR ligation, and displayed a rapid turnover rate (5-bromo-2'-deoxyuridine pulse-chase assays). This work suggests that chronic exposure to OM 1) discloses a unique ability of LNs to sustain extrathymic T cell development, and 2) increases the number and/or function of LN niches able to support seeding of recirculating mature T cells. Regulation of the lymphopoietic pathway discovered in OM transgenic mice could be of therapeutic interest for individuals with thymic hypoplasia or deficient peripheral T cell niches.