Improved thymic index, density and output in HIV-infected patients following low-dose growth hormone therapy: a placebo controlled study

The potential role of the thymus in immunotherapies for acute myeloid leukemia

Understanding the factors which shape T-lymphocyte immunity is critical for the development and application of future immunotherapeutic strategies in treating hematological malignancies. The thymus, a specialized central lymphoid organ, plays important roles in generating a diverse T lymphocyte repertoire during the infantile and juvenile stages of humans. However, age-associated thymic involution and diseases or treatment associated injury result in a decline in its continuous role in the maintenance of T cell-mediated anti-tumor/virus immunity. Acute myeloid leukemia (AML) is an aggressive hematologic malignancy that mainly affects older adults, and the disease's progression is known to consist of an impaired immune surveillance including a reduction in naïve T cell output, a restriction in T cell receptor repertoire, and an increase in frequencies of regulatory T cells. As one of the most successful immunotherapies thus far developed for malignancy, T-cell-based adoptive cell therapies could be essential for the development of a durable effective treatment to eliminate residue leukemic cells (blasts) and prevent AML relapse. Thus, a detailed cellular and molecular landscape of how the adult thymus functions within the context of the AML microenvironment will provide new insights into both the immune-related pathogenesis and the regeneration of a functional immune system against leukemia in AML patients. Herein, we review the available evidence supporting the potential correlation between thymic dysfunction and T-lymphocyte impairment with the ontogeny of AML (II-VI). We then discuss how the thymus could impact current and future therapeutic approaches in AML (VII). Finally, we review various strategies to rejuvenate thymic function to improve the precision and efficacy of cancer immunotherapy (VIII).

End-Stage Renal Disease-Related Accelerated Immune Senescence: Is Rejuvenation of the Immune System a Therapeutic Goal?

End-stage renal disease (ESRD) patients exhibit clinical features of premature ageing, including frailty, cardiovascular disease, and muscle wasting. Accelerated ageing also concerns the immune system. Patients with ESRD have both immune senescence and chronic inflammation that are resumed in the so-called inflammaging syndrome. Immune senescence is particularly characterised by premature loss of thymic function that is associated with hyporesponsiveness to vaccines, susceptibility to infections, and death. ESRD-related chronic inflammation has multiple causes and participates to accelerated cardiovascular disease. Although, both characterisation of immune senescence and its consequences are relatively well-known, mechanisms are more uncertain. However, prevention of immune senescence/inflammation or/and rejuvenation of the immune system are major goal to ameliorate clinical outcomes of ESRD patients.

Hormone replacement therapy in children with growth hormone deficiency: impact on immune profile

Growth hormone (GH) may influence the immune system. The aim of this study was to assess the immune profile after treatment with GH in pre-pubertal children with a deficiency of this hormone. The study was carried out in two phases. Two groups were included in the first phase: group A) children treated with GH; group B) untreated children, prior to starting treatment. In the second phase, group B children were assessed 6 months after starting treatment. In the first phase, groups A and B were compared (case-control study). In the second phase, group B was compared in terms of baseline and final times (before and after study). We analysed: humoral immunity (immunoglobulin (Ig)A, IgG, IgM, C1 inhibitor, C3, and C4) and cell-mediated immunity (CD3+, CD4+, CD8+, CD4+/CD8+, CD19+ lymphocytes, and natural killer (NK) cells). Insulin-like growth factor 1 (IGF-1) and insulin-like growth factor-binding protein 3 (IGFBP-3) were also determined. In the first phase, CD3+, CD4+, CD19+, and NK cells and CD4+/CD8+ were greater in the treated group. CD8+ was lower in this group. No variations were seen in immunoglobulins and the complement between both groups. There were no changes to the complement in either of the two phases. In the second phase, untreated patients were assessed after 6 months of treatment. When comparing the baseline and final immune profiles, a statistically significant decrease in IgG and IgM was observed, and an increase of IGF-1 levels and monocytes. In conclusion, our study shows changes in the cellular and humoral immune profiles in children with GH deficiency who were treated.

Growth Hormone Releasing Hormone Reduces Circulating Markers of Immune Activation in Parallel with Effects on Hepatic Immune Pathways in Individuals with HIV-Infection and Nonalcoholic Fatty Liver Disease

BACKGROUND

The growth hormone (GH)/insulin-like growth factor-1 (IGF-1) axis modulates critical metabolic pathways; however, little is known regarding effects of augmenting pulsatile GH secretion on immune function in humans. This study used proteomics and gene set enrichment analysis to assess effects of a GH releasing hormone (GHRH) analog, tesamorelin, on circulating immune markers and liver tissue in people with HIV (PWH) and NAFLD.

METHODS

92 biomarkers associated with immunity, chemotaxis, and metabolism were measured in plasma samples from 61 PWH with NAFLD who participated in a double-blind, randomized trial of tesamorelin versus placebo for 12 months. Gene set enrichment analysis was performed on serial liver biopsies targeted to immune pathways.

RESULTS

Tesamorelin, compared to placebo, decreased interconnected proteins related to cytotoxic T-cell and monocyte activation. Circulating concentrations of 13 proteins were significantly decreased, and no proteins increased, by tesamorelin. These included four chemokines (CCL3, CCL4, CCL13 [MCP4], IL8 [CXCL8]), two cytokines (IL-10 and CSF-1), and four T-cell associated molecules (CD8A, CRTAM, GZMA, ADGRG1), as well as ARG1, Gal-9, and HGF. Network analysis indicated close interaction among the gene pathways responsible for these proteins, with imputational analyses suggesting down regulation of a closely related cluster of immune pathways. Targeted transcriptomics using liver tissue confirmed a significant end-organ signal of down-regulated immune activation pathways.

CONCLUSIONS

Long-term treatment with a GHRH analog reduced markers of T-cell and monocyte/macrophage activity, suggesting that augmentation of the GH axis may ameliorate immune activation in an HIV population with metabolic dysregulation, systemic and end organ inflammation.

Evaluating Thymic Function After Human Hematopoietic Stem Cell Transplantation in the Personalized Medicine Era

Hematopoietic stem cell transplantation (HSCT) is an effective treatment option for several malignant and non-malignant hematological diseases. The clinical outcome of this procedure relies to a large extent on optimal recovery of adaptive immunity. In this regard, the thymus plays a central role as the primary site for de novo generation of functional, diverse, and immunocompetent T-lymphocytes. The thymus is exquisitely sensitive to several insults during HSCT, including conditioning drugs, corticosteroids, infections, and graft-vs.-host disease. Impaired thymic recovery has been clearly associated with increased risk of opportunistic infections and poor clinical outcomes in HSCT recipients. Therefore, better understanding of thymic function can provide valuable information for improving HSCT outcomes. Recent data have shown that, besides gender and age, a specific single-nucleotide polymorphism affects thymopoiesis and may also influence thymic output post-HSCT, suggesting that the time of precision medicine of thymic function has arrived. Here, we review the current knowledge about thymic role in HSCT and the recent work of genetic control of human thymopoiesis. We also discuss different transplant-related factors that have been associated with impaired thymic recovery and the use of T-cell receptor excision circles (TREC) to assess thymic output, including its clinical significance. Finally, we present therapeutic strategies that could boost thymic recovery post-HSCT.

Thymic Atrophy: Experimental Studies and Therapeutic Interventions

The thymus is essential for T cell development and maturation. It is extremely sensitive to atrophy, wherein loss in cellularity of the thymus and/or disruption of the thymic architecture occur. This may lead to lower naïve T cell output and limited TCR diversity. Thymic atrophy is often associated with ageing. What is less appreciated is that proper functioning of the thymus is critical for reduction in morbidity and mortality associated with various clinical conditions including infections and transplantation. Therefore, therapeutic interventions which possess thymopoietic potential and lower thymic atrophy are required. These treatments enhance thymic output, which is a vital factor in generating favourable outcomes in clinical conditions. In this review, experimental studies on thymic atrophy in rodents and clinical cases where the thymus atrophies are discussed. In addition, mechanisms leading to thymic atrophy during ageing as well as during various stress conditions are reviewed. Therapies such as zinc supplementation, IL7 administration, leptin treatment, keratinocyte growth factor administration and sex steroid ablation during thymic atrophy involving experiments in animals and various clinical scenarios are reviewed. Interventions that have been used across different scenarios to reduce the extent of thymic atrophy and enhance its output are discussed. This review aims to speculate on the roles of combination therapies, which by acting additively or synergistically may further alleviate thymic atrophy and boost its function, thereby strengthening cellular T cell responses.

Low-dose growth hormone therapy reduces inflammation in HIV-infected patients: a randomized placebo-controlled study

BACKGROUND

Combination antiretroviral therapy (cART) has drastically increased the life expectancy of HIV-infected patients. However, HIV-infected patients exhibit increased inflammation and 33-58% exhibit a characteristic fat re-distribution termed HIV-associated lipodystrophy syndrome (HALS). Recombinant human growth hormone (rhGH) has been tested as treatment of HALS. Low-dose rhGH therapy improves thymopoiesis and fat distribution in HIV-infected patients and appears to be well tolerated. However, since high-dose rhGH is associated with adverse events related to inflammation, we wanted to investigate the impact of low-dose rhGH therapy on inflammation in HIV-infected patients.

METHODS

Forty-six cART-treated HIV-infected men were included in the HIV-GH low-dose (HIGH/Low) study: a randomized, placebo-controlled, double-blinded trial. Subjects were randomized 3:2 to 0.7 mg/day rhGH, or placebo for 40 weeks. rhGH was self-administered between 1 pm and 3 pm. The primary outcome of this substudy was changes in inflammation measured by plasma C-reactive protein (CRP) and soluble urokinase plasminogen activator receptor (suPAR).

RESULTS

Both CRP (-66%, p = 0.002) and suPAR (-9.7%, p = 0.06) decreased in the rhGH group compared to placebo; however, only CRP decreased significantly. The effect of rhGH on inflammation was not mediated through rhGH-induced changes in insulin-like growth factor 1, body composition, or immune parameters.

CONCLUSION

Daily 0.7 mg rhGH treatment for 40 weeks, administered at nadir endogenous GH secretion, significantly reduced CRP. The effect does not appear to be mediated by other factors. Our findings suggest that low-dose rhGH treatment may minimize long-term risks associated with high-dose rhGH therapy.

T-lymphocyte subset dynamics in well-treated HIV-infected men during a bout of exhausting exercise

In healthy individuals the substantial lymphocytosis during a bout of exhausting exercise constitutes primarily mature T cells from the peripheral lymphoid organs but naïve T cells are also recruited. This study investigated whether the defective CD4 + T-lymphocyte count in peripheral blood during rest in human immunodeficiency virus (HIV)-infected patients would also be observed following a maximal output ergometer bicycle test. At rest, in 45 well-treated HIV-infected patients, mature and naïve CD4 + T-lymphocyte counts were decreased whereas the less immune active CD8 + T lymphocytes were increased compared with 10 healthy control subjects. In response to exercise mature and naïve CD4 + T lymphocytes increased less and mature and naïve CD8 + T lymphocytes increased most in HIV-infected patients. In conclusion, defective resting mature and naïve CD4 + T lymphocytes in well-treated HIV-infected patients are also reflected in defective acutely mobilized active immune cells following exhausting exercise. The CD4 + T-lymphocyte count is highly sensitive to physical activity.

Multifarious immunotherapeutic approaches to cure HIV-1 infection

Immunotherapy in the context of treated HIV-1 infection aims to improve immune responses to achieve better control of the virus. To date, multifaceted immunotherapeutic approaches have been shown to reduce immune activation and increase CD4 T-lymphocyte counts, further to the effects of antiretroviral therapy alone, in addition to improving HIV-1-specific T-cell responses. While sterilizing cure of HIV-1 would involve elimination of all replication-competent virus, a functional cure in which the host has long-lasting control of viral replication may be more feasible. In this commentary, we discuss novel strategies aimed at targeting the latent viral reservoir with cure of HIV-1 infection being the ultimate goal, an achievement that would have considerable impact on worldwide HIV-1 infection.

Therapeutic immunisation plus cytokine and hormone therapy improves CD4 T-cell counts, restores anti-HIV-1 responses and reduces immune activation in treated chronic HIV-1 infection

BACKGROUND

This randomised, open label, phase I, immunotherapeutic study investigated the effects of interleukin (IL)-2, granulocyte-macrophage colony-stimulating factor (GM-CSF), recombinant human growth hormone (rhGH), and therapeutic immunisation (a Clade B DNA vaccine) on combination antiretroviral therapy (cART)-treated HIV-1-infected individuals, with the objective to reverse residual T-cell dysfunction.

METHODS

Twelve HIV-1(+) patients on suppressive cART with baseline CD4 T-cell counts >400 cells/mm(3) blood were randomised into one of three groups: (1) vaccine, IL-2, GM-CSF and rhGH (n=3); (2) vaccine alone (n=4); or (3) IL-2, GM-CSF and rhGH (n=5). Samples were collected at weeks 0, 1, 2, 4, 6, 8, 12, 16, 24 and 48. Interferon (IFN)-γ, IL-2, IL-4 and perforin ELISpot assays performed at each time point quantified functional responses to Gag p17/p24, Nef, Rev, and Tat peptides; and detailed T-cell immunophenotyping was undertaken by flow cytometry. Proviral DNA was also measured.

RESULTS

Median baseline CD4 T-cell count was 757 cells/mm(3) (interquartile range [IQR] 567-886 cells/mm(3)), median age 48 years (IQR 42-51 years), and plasma HIV-1-RNA <50 copies/ml for all subjects. Patients who received vaccine plus IL-2, GM-CSF and rhGH (group 1) showed the most marked changes. Assessing mean changes from baseline to week 48 revealed significantly elevated numbers of CD4 T cells (p=0.0083) and improved CD4/CD8 T-cell ratios (p=0.0033). This was accompanied by a significant reduction in expression of CD38 on CD4 T cells (p=0.0194), significantly increased IFN-γ and IL-2 production in response to Gag (p=0.0122) and elevated IFN-γ production in response to Tat (p=0.041) at week 48 compared to baseline. Subjects in all treatment groups showed significantly reduced PD-1 expression at week 48 compared to baseline, with some reductions in proviral DNA.

CONCLUSIONS

Multifarious immunotherapeutic approaches in the context of fully suppressive cART further reduce immune activation, and improve both CD4 T-lymphocyte counts and HIV-1-specific T-cell responses (NCT01130376).

The immune system in children with malnutrition--a systematic review

BACKGROUND

Malnourished children have increased risk of dying, with most deaths caused by infectious diseases. One mechanism behind this may be impaired immune function. However, this immune deficiency of malnutrition has not previously been systematically reviewed.

OBJECTIVES

To review the scientific literature about immune function in children with malnutrition.

METHODS

A systematic literature search was done in PubMed, and additional articles identified in reference lists and by correspondence with experts in the field. The inclusion criteria were studies investigating immune parameters in children aged 1-60 months, in relation to malnutrition, defined as wasting, underweight, stunting, or oedematous malnutrition.

RESULTS

The literature search yielded 3402 articles, of which 245 met the inclusion criteria. Most were published between 1970 and 1990, and only 33 after 2003. Malnutrition is associated with impaired gut-barrier function, reduced exocrine secretion of protective substances, and low levels of plasma complement. Lymphatic tissue, particularly the thymus, undergoes atrophy, and delayed-type hypersensitivity responses are reduced. Levels of antibodies produced after vaccination are reduced in severely malnourished children, but intact in moderate malnutrition. Cytokine patterns are skewed towards a Th2-response. Other immune parameters seem intact or elevated: leukocyte and lymphocyte counts are unaffected, and levels of immunoglobulins, particularly immunoglobulin A, are high. The acute phase response appears intact, and sometimes present in the absence of clinical infection. Limitations to the studies include their observational and often cross-sectional design and frequent confounding by infections in the children studied.

CONCLUSION

The immunological alterations associated with malnutrition in children may contribute to increased mortality. However, the underlying mechanisms are still inadequately understood, as well as why different types of malnutrition are associated with different immunological alterations. Better designed prospective studies are needed, based on current understanding of immunology and with state-of-the-art methods.

A call to include severe combined immunodeficiency in newborn screening program

Quantification of the T cell receptor excision circles (TRECs) has recently emerged as a useful non-invasive clinical and research tool to investigate thymic activity. It allows the identification of T cell production by the thymus. Quantification of TREC copies has recently been implemented as the preferred test to screen neonates with severe combined immunodeficiency (SCID) or significant lymphopenia. Neonatal genetic screening for SCID is highly important in countries with high rates of consanguinous marriages, such as Israel, and can be used for early diagnosis, enabling prompt therapeutic intervention that will save lives and improve the outcome of these patients. TREC measurement is also applicable in clinical settings where T cell immunity is involved, including any T cell immunodeficiencies, HIV infection, the aging process, autoimmune diseases, and immune reconstitution after bone marrow transplantation.

TAKE-HOME MESSAGES

Severe combined immunodeficiency, a life-threatening condition, can be detected by neonatal screening.The earlier the detection and the quicker the implementation of appropriate treatment, the greater the likelihood for improved outcome, even cure, for the affected children.TRECs and KRECs quantification are useful screening tests for severe T and B cell immunodeficiency and can be used also to evaluate every medical condition involving T and B cell immunity.

Low-dose growth hormone for 40 weeks induces HIV-1-specific T cell responses in patients on effective combination anti-retroviral therapy

Recombinant human growth hormone (rhGH) administered to combination anti-retroviral therapy (cART)-treated human immunodeficiency virus-1 (HIV-1)-infected individuals has been found to reverse thymic involution, increase total and naive CD4 T cell counts and reduce the expression of activation and apoptosis markers. To date, such studies have used high, pharmacological doses of rhGH. In this substudy, samples from treated HIV-1(+) subjects, randomized to receive either a physiological dose (0·7 mg) of rhGH (n = 21) or placebo (n = 15) daily for 40 weeks, were assessed. Peptide-based enzyme-linked immunospot (ELISPOT) assays were used to enumerate HIV-1-specific interferon (IFN)-γ-producing T cells at baseline and week 40. Individuals who received rhGH demonstrated increased responses to HIV-1 Gag overlapping 20mer and Gag 9mer peptide pools at week 40 compared to baseline, whereas subjects who received placebo showed no functional changes. Subjects with the most robust responses in the ELISPOT assays had improved thymic function following rhGH administration, as determined using CD4(+) T cell receptor rearrangement excision circle (TREC ) and thymic density data from the original study. T cells from these robust responders were characterized further phenotypically, and showed decreased expression of activation and apoptosis markers at week 40 compared to baseline. Furthermore, CD4 and CD8 T cell populations were found to be shifted towards an effector and central memory phenotype, respectively. Here we report that administration of low-dose rhGH over 40 weeks with effective cART resulted in greater improvement of T lymphocyte function than observed with cART alone, and provide further evidence that such an approach could also reduce levels of immune activation.

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.

Growth hormone modulates migration of thymocytes and peripheral T cells

In the context of immunoneuroendocrine cross talk, growth hormone (GH) exerts pleiotropic effects in the immune system. For example, GH-transgenic mice, as well as animals and humans treated with GH, exhibit enhanced cellularity in the thymus. GH also stimulates the thymic microenvironment, augmenting chemokine and extracellular matrix (ECM) production, with consequent increase in ECM- and chemokine-driven thymocyte migratory responses. Peripheral T cell migration triggered by laminin or fibronectin was enhanced in cells from GH-transgenic versus wild-type control adult mice, as seen for CD4(+) and CD8(+) T cells from mesenteric lymph nodes. Migration of these T lymphocytes, triggered by the chemokine CXCL12, in conjunction with laminin or fibronectin, was also enhanced compared with control counterparts. Considering that GH can be used as an adjuvant therapy in immunodeficiencies, including AIDS, the concepts defined herein, that GH enhances developing and peripheral T cell migration, provide new clues for future GH-related immune interventions.

Improving immunity in the elderly: current and future lessons from nonhuman primate models

The immune system must overcome daily challenges from pathogens to protect the body from infection. The success of the immune response to infection relies on the ability to sense and evaluate microbial threats and organize their elimination, while limiting damage to host tissues. This delicate balance is achieved through coordinated action of the innate and adaptive arms of the immune system. Aging results in several structural and functional changes in the immune system, often described under the umbrella term "immune senescence". Age-related changes affect both the innate and adaptive arms of the immune system and are believed to result in increased susceptibility and severity of infectious diseases, which is further exacerbated by reduced vaccine efficacy in the elderly. Therefore, multiple strategies to improve immune function in the aged are being investigated. Traditionally, studies on immune senescence are conducted using inbred specific pathogen free (SPF) rodents. This animal model has provided invaluable insight into the mechanisms of aging. However, the limited genetic heterogeneity and the SPF status of this model restrict the successful transfer of immunological discoveries between murine models and the clinical setting. More recently, nonhuman primates (NHPs) have emerged as a leading translational model to investigate immune senescence and to test interventions aimed at delaying/reversing age-related changes in immune function. In this article, we review and summarize advances in immuno-restorative approaches investigated in the NHP model system and discuss where the NHP model can support the development of novel therapeutics.

Effect of intermittent interleukin-2 therapy on CD4+ T-cell counts following antiretroviral cessation in patients with HIV

BACKGROUND

Interleukin (IL)-2 therapy impacts T-cell homeostasis. Whether IL-2 expanded CD4(+) T cells may persist following viral rebound has not been fully investigated.

METHODS

Patients with CD4(+) T cells 500/μl or more and HIV RNA less than 50 copies/ml were randomized to continue antiretroviral therapy (ART) either alone (n = 67) or combined with three IL-2 cycles (n = 81; 6 million units) twice daily for 5 days at weeks 0, 8, and 16 before stopping ART (week 24). Patients were followed up to 168 weeks.

RESULTS

At week 24, median CD4(+) T-cell counts were 1198 and 703 cells/μl in the IL-2 and control groups, respectively (P < 0.001). At week 72, 27% (IL-2 group) and 45% (control group; P = 0.03) of patients were in failure (defined as no interruption of ART at week 24, CD4 drop below 350 cells/μl or ART resumption). After week 24, a biphasic decline (before and after week 32) of CD4 was noted -106 and -7 cells/μl per month in controls and -234 and -17 in IL-2 group (all P ≤ 0.0001). At week 96, IL-2-expanded CD4(+)CD25(+) T cells remained higher than in the control group (26 vs. 16%, P = 0.006).

CONCLUSION

In IL-2-treated patients, CD4(+)CD25(+) T cells persisting despite viral replication allow a longer period of ART interruption.

Incomplete immune recovery in HIV infection: mechanisms, relevance for clinical care, and possible solutions

Treatment of HIV-infected patients with highly active antiretroviral therapy (HAART) usually results in diminished viral replication, increasing CD4+ cell counts, a reversal of most immunological disturbances, and a reduction in risk of morbidity and mortality. However, approximately 20% of all HIV-infected patients do not achieve optimal immune reconstitution despite suppression of viral replication. These patients are referred to as immunological nonresponders (INRs). INRs present with severely altered immunological functions, including malfunction and diminished production of cells within lymphopoetic tissue, perturbed frequencies of immune regulators such as regulatory T cells and Th17 cells, and increased immune activation, immunosenescence, and apoptosis. Importantly, INRs have an increased risk of morbidity and mortality compared to HIV-infected patients with an optimal immune reconstitution. Additional treatment to HAART that may improve immune reconstitution has been investigated, but results thus far have proved disappointing. The reason for immunological nonresponse is incompletely understood. This paper summarizes the known and unknown factors regarding the incomplete immune reconstitution in HIV infection, including mechanisms, relevance for clinical care, and possible solutions.

The role of naïve T-cells in HIV-1 pathogenesis: an emerging key player

Functional naïve T-cells are critical for an effective immune response to multiple pathogens. HIV leads to a significant reduction in CD4+ naïve T-cell number and impaired function and there is incomplete recovery following combination antiretroviral therapy (cART). Here we review the basic homeostatic mechanisms that maintain naïve CD4+ T-cells and discuss recent developments in understanding the impact of HIV infection on naïve CD4+ T-cells. Finally we review therapeutic interventions in HIV-infected individuals aimed at specifically enhancing recovery of naïve CD4+ T-cells.

The reconstitution of the thymus in immunosuppressed individuals restores CD4-specific cellular and humoral immune responses

Infection with HIV-1 frequently results in the loss of specific cellular immune responses and an associated lack of antibodies. Recombinant growth hormone (rGH) administration reconstitutes thymic tissue and boosts the levels of peripheral T cells, so rGH therapy may be an effective adjuvant through promoting the recovery of lost cellular and T-cell-dependent humoral immune responses in immunosuppressed individuals. To test this concept, we administered rGH to a clinically defined group of HIV-1-infected subjects with defective cellular and serological immune responses to at least one of three commonly employed vaccines (hepatitis A, hepatitis B or tetanus toxoid). Of the original 278 HIV-1-infected patients entering the trial, only 20 conformed to these immunological criteria and were randomized into three groups: Group A (n = 8) receiving rGH and challenged with the same vaccine to which they were unresponsive and Groups B (n = 5) and C (n = 7) who received either rGH or vaccination alone, respectively. Of the eight subjects in Group A, five recovered CD4 cellular responses to vaccine antigen and four of these produced the corresponding antibodies. In the controls, three of the five in group B recovered cellular responses with two producing antibodies, whereas three of the seven in Group C recovered CD4 responses, with only two producing antibodies. Significantly, whereas seven of ten patients receiving rGH treatment in Group A (six patients) and B (one patient) recovered T-cell responses to HIVp24, only two of six in Group C responded similarly. In conclusion, reconstitution of the thymus in immunosuppressed adults through rGH hormone treatment restored both specific antibody and CD4 T-cell responses.

Thymopoietic and bone marrow response to murine Pneumocystis pneumonia

CD4(+) T cells play a key role in host defense against Pneumocystis infection. To define the role of naïve CD4(+) T cell production through the thymopoietic response in host defense against Pneumocystis infection, Pneumocystis murina infection in the lung was induced in adult male C57BL/6 mice with and without prior thymectomy. Pneumocystis infection caused a significant increase in the number of CCR9(+) multipotent progenitor (MPP) cells in the bone marrow and peripheral circulation, an increase in populations of earliest thymic progenitors (ETPs) and double negative (DN) thymocytes in the thymus, and recruitment of naïve and total CD4(+) T cells into the alveolar space. The level of murine signal joint T cell receptor excision circles (msjTRECs) in spleen CD4(+) cells was increased at 5 weeks post-Pneumocystis infection. In thymectomized mice, the numbers of naïve, central memory, and total CD4(+) T cells in all tissues examined were markedly reduced following Pneumocystis infection. This deficiency of naïve and central memory CD4(+) T cells was associated with delayed pulmonary clearance of Pneumocystis. Extracts of Pneumocystis resulted in an increase in the number of CCR9(+) MPPs in the cultured bone marrow cells. Stimulation of cultured bone marrow cells with ligands to Toll-like receptor 2 ([TLR-2] zymosan) and TLR-9 (ODN M362) each caused a similar increase in CCR9(+) MPP cells via activation of the Jun N-terminal protein kinase (JNK) pathway. These results demonstrate that enhanced production of naïve CD4(+) T lymphocytes through the thymopoietic response and enhanced delivery of lymphopoietic precursors from the bone marrow play an important role in host defense against Pneumocystis infection.