Jeffrey Pido-Lopez

Both age and gender affect thymic output: more recent thymic migrants in females than males as they age

The thymus undergoes age‐associated involution, with studies showing thymic size decreasing from birth at a rate of approximately 3% per year until middle age, and at a rate of 1% per year thereafter. The aim of this study was to determine the effect of thymic atrophy on T‐lymphocyte production by the thymus, and to clarify the ongoing uncertainty regarding gender differences in thymic function. We quantified recent thymic emigrants (RTEs) in blood through the measurement of signal joint T‐cell receptor rearrangement excision circles (sjTRECs), and showed that the decline in the number of RTEs in the blood with increasing age is gender‐linked. Peripheral blood from females contained significantly higher levels of sjTRECs per CD3+ T cell than blood from males (P = 0·002), despite there being no significant gender difference in the absolute number of CD3+ T cells in the populations analysed (P > 0·10). Our findings suggest better thymic function in females compared with males, providing females with a higher number of recent thymic emigrants for longer periods of life. Such a finding provides a plausible explanation for the immunological gender differences observed in previous studies and possibly, for the general longer life expectancy in females compared with males.

Molecular quantitation of thymic output in mice and the effect of IL-7

Aging of the murine thymus is accompanied by a measurable loss of weight and cellularity and a marked reduction in its output of T lymphocytes. This study employs a molecular approach to determine changes in the output of the murine thymus using a novel assay system based on the detection and quantitation of the excised TCRδ DNA locus from within the TCRα chain genes. In α β+ T cells such δ excision circles (δEC) are present at higher levels in naive T cells as compared with memory T cell populations, are non‐replicating, and diluted within the total peripheral α β+ T cell pool with advancing age. This assay permits the assessment of thymic output in older animals where previous analysis was hampered by the transient nature ofthe naive T cell surface phenotype, and so allows the assessment of the efficacy of IL‐7 as an agent to reverse thymic atrophy. Treatment of old mice with IL‐7 although producing no overall change in the total number of α β+ T cells in the peripheral T cell pool altered the component subsets. Mice treated with IL‐7 showed increases in the number of α β+TCR cells possessing δEC commensurate with improved thymic output, and the splenic T cells from IL‐7‐treated mice performed significantly better in in vitro functional assays compared to those from age‐matched saline‐treated controls.

Reversal of thymic atrophy

Age-associated thymic atrophy is a key event preceding the inefficient functioning of the immune system, resulting in a diminished capacity to generate new T-cells. This thymic involution has been proposed to be due to changes in the thymic microenvironment resulting in its failure to support thymopoiesis. A key cytokine in the early stages of thymocyte development is IL-7 and expression levels are greatly reduced with age. The ability of IL-7 to restore the immune system by enhancing thymic output remains controversial. In this review, we highlight the advances in molecular approaches used to evaluate recent thymic emigrants and assess the success of these strategies in determining whether IL-7 can lead to immune reconstitution.

Immune responses and reconstitution in HIV-1 infected individuals: impact of anti-retroviral therapy, cytokines and therapeutic vaccination

Most patients with chronic HIV-1 infection lack functional CD4(+) and CD8(+) HIV-1-specific T cells with proliferative and cytolytic capacity, respectively. This is despite being able to produce intracellular cytokines in response to viral antigens. Protease inhibitor (PI)-based highly active anti-retroviral therapy (HAART) is unable to completely eradicate virus and fails to enable total restoration of immunity including induction of anti-HIV-1 responses. We have taken novel approaches towards the treatment of chronic HIV-1 disease with the aim of instigating long-term non-progressor status and depletion of virus reservoirs. HIV-1-specific CD4(+) and CD8(+) T cell responses were measured following the administration of cytokines, during therapeutic vaccination, and following treatment interruption (TI) or drug therapy change. Administration of cytokines, with or without therapeutic vaccination, in HAART treated patients, improved both CD4(+) and CD8(+) HIV-1-specific T cell responses even in late-stage disease. Virus-specific T cell responses were also seen during TI or when transient viraemia was apparent, and following therapy change from a PI- to a non-nucleoside-based HAART regimen. Reconstitution of HIV-1-specific immune responses was found to be transient and reversal to the previous anergic state was rapid. Viral reservoirs in the latently infected resting CD4(+) T cells, on follicular dendritic cells of germinal centers or even in infected thymic epithelium may be involved in clonal suppression and anergy. These may present major obstacles to the maintenance of HIV-1-specific responses and the eventual eradication of HIV-1.

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

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

Interleukin‐7: An Interleukin for Rejuvenating the Immune System

Abstract: Infection of an individual (aged 20‐30 years) by a virus will cause a response from the T (thymus derived) lymphocytes of which there are approximately 3 × 1011. If the individual has not met the virus before, the response will come from the naive T cell subset (50 ± 10% of the total T cell pool at this age) containing recent thymic emigrants produced from the thymus at approximately 108 per day. Their antigen‐specific receptor has a defined specificity governed by the conformation of its two chains (α and β), and the repertoire of specificities is somewhere in the region of 2 × 107 to 108. A successful response leads to clonal expansion and the generation of memory T cells to the infecting agent.

A pilot study of the safety and efficacy of thymosin α1 in augmenting immune reconstitution in HIV-infected patients with low CD4 counts taking highly active antiretroviral therapy

To study the safety and efficacy of thymosin α1 in stimulating immune reconstitution in combination with highly active antiretroviral therarpy (HAART), a phase II randomized, controlled open‐label trial of subcutaneous thymosin α1 was undertaken for 12 weeks. Twenty clinically stable patients with viral loads <400 copies/ml and CD4 counts less than 200 cells/µl were randomized to receive 3·2 mg thymosin α1 subcutaneous injections twice weekly or no injections for 12 weeks. CD4 and CD8 counts, CD45 RO+ and RA+ subsets and signal joint T cell receptor excision circles (sjTREC) in peripheral blood mononuclear cells (PBMCs) were measured every 2 weeks. Thirteen patients received thymosin α1 and seven were controls. Thymosin α1 was well tolerated and there were no serious adverse events. There was no significant difference between the thymosin α1 and control groups in CD4, CD8 and CD45 lymphocyte subset changes at week 12; however, PBMC sjTREC levels increased significantly in the thymosin α1‐treated patients compared to controls at week 12. In conclusion, the increase in PBMC sjTREC levels in patients taking thymosin α1 may represent enhanced immune reconstitution; however, the clinical benefits and long‐term consequences remain to be determined.

Transient nature of long-term nonprogression and broad virus-specific proliferative T-cell responses with sustained thymic output in HIV-1 controllers.

Background HIV-1+ individuals who, without therapy, conserve cellular anti-HIV-1 responses, present with high, stable CD4+ T-cell numbers, and control viral replication, facilitate analysis of atypical viro-immunopathology. In the absence of universal definition, immune function in such HIV controllers remains an indication of non-progression. Methodology/Principal Findings CD4 T-cell responses to a number of HIV-1 proteins and peptide pools were assessed by IFN-γ ELISpot and lymphoproliferative assays in HIV controllers and chronic progressors. Thymic output was assessed by sjTRECs levels. Follow-up of 41 HIV-1+ individuals originally identified as “Long-term non-progressors” in 1996 according to clinical criteria, and longitudinal analysis of two HIV controllers over 22 years, was also performed. HIV controllers exhibited substantial IFN-γ producing and proliferative HIV-1-specific CD4 T-cell responses to both recombinant proteins and peptide pools of Tat, Rev, Nef, Gag and Env, demonstrating functional processing and presentation. Conversely, HIV-specific T-cell responses were limited to IFN-γ production in chronic progressors. Additionally, thymic output was approximately 19 fold higher in HIV controllers than in age-matched chronic progressors. Follow-up of 41 HIV-1+ patients identified as LTNP in 1996 revealed the transitory characteristics of this status. IFN-γ production and proliferative T-cell function also declines in 2 HIV controllers over 22 years. Conclusions Although increased thymic output and anti-HIV-1 T-cell responses are observed in HIV controllers compared to chronic progressors, the nature of nonprogressor/controller status appears to be transitory.

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.

My T's gone cold, I'm wondering why.

Due to an ever-increasing elderly population, there is a mounting drive to understand immunological defects associated with aging. However, the history of aging research and the very nature of the research itself has hampered progress.