HIV infection and the dynamic interplay between the thymus and the peripheral T cell pool

Hormonal control of T-cell development in health and disease

The physiology of the thymus, the primary lymphoid organ in which T cells are generated, is controlled by hormones. Data from animal models indicate that several peptide and nonpeptide hormones act pleiotropically within the thymus to modulate the proliferation, differentiation, migration and death by apoptosis of developing thymocytes. For example, growth hormone and prolactin can enhance thymocyte proliferation and migration, whereas glucocorticoids lead to the apoptosis of these developing cells. The thymus undergoes progressive age-dependent atrophy with a loss of cells being generated and exported, therefore, hormone-based therapies are being developed as an alternative strategy to rejuvenate the organ, as well as to augment thymocyte proliferation and the export of mature T cells to peripheral lymphoid organs. Some hormones (such as growth hormone and progonadoliberin-1) are also being used as therapeutic agents to treat immunodeficiency disorders associated with thymic atrophy, such as HIV infection. In this Review, we discuss the accumulating data that shows the thymus gland is under complex and multifaceted hormonal control that affects the process of T-cell development in health and disease.

Cytokines, leptin, and stress-induced thymic atrophy

Thymopoiesis is essential for development and maintenance of a robust and healthy immune system. Acute thymic atrophy is a complication of many infections, environmental stressors, clinical preparative regimens, and cancer treatments used today. This undesirable sequela can decrease host ability to reconstitute the peripheral T cell repertoire and respond to new antigens. Currently, there are no treatments available to protect against acute thymic atrophy or accelerate recovery, thus leaving the immune system compromised during acute stress events. Several useful murine models are available for mechanistic studies of acute thymic atrophy, including a sepsis model of endotoxin-induced thymic involution. We have identified the IL-6 cytokine gene family members (i.e., leukemia inhibitory factor, IL-6, and oncostatin M) as thymosuppressive agents by the observation that they can acutely involute the thymus when injected into a young, healthy mouse. We have gone on to explore the role of thymosuppressive cytokines and specifically defined a corticosteroid-dependent mechanism of action for the leukemia inhibitory factor in acute thymic atrophy. We also have identified leptin as a novel, thymostimulatory agent that can protect against endotoxin-induced acute thymic atrophy. This review will highlight mechanisms of stress-induced thymic involution and focus on thymosuppressive agents involved in atrophy induction and thymostimulatory agents that may be exploited for therapeutic use.

Human T cell reconstitution in DiGeorge syndrome and HIV-1 infection

The thymus is essential for proper development and maintenance of a broad T cell repertoire capable of recognizing a wide-range of foreign antigens. Recent advances in multicolor flow cytometry, non-invasive imaging techniques, and molecular assessments of thymic function have enabled a more comprehensive characterization of human thymic output in clinical settings than in the past. These techniques have been particularly valuable in monitoring human T cells after therapeutic thymic grafting for complete DiGeorge syndrome and during HIV-1 infection and AIDS. By defining the degree and mechanisms of T cell reconstitution in these settings, clinical investigators and primary caregivers have been able to better diagnose, treat and care for individuals with congenital or acquired immune deficiencies associated with loss of thymic function.

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.

Thymic lesions in cats infected with a pathogenic molecular clone or an ORF-A/2-deficient molecular clone of feline immunodeficiency virus

Previous studies using feline immunodeficiency virus (FIV) molecular clones lacking the putative transactivator gene (ORF-A/2) failed to address the issue of thymus pathogenesis or investigate the levels of viral replication in separate lymphoid compartments (Y. Inoshima, et al., J. Virol. 70:8518-8526, 1996; E. E. Sparger, et al., Virology 205:546-553, 1994). Using a highly pathogenic molecular clone of FIV, JSY3, and an ORF-A/2-deficient mutant, JSY3DeltaORF-A/2, we compared viral replication and the extent of thymic dysfunction as measured by the formation of lymphoid follicles and alteration of the thymocyte subsets. Viral replication was reduced in JSY3DeltaORF-A/2-infected cats as measured by lymphocyte coculture, immunohistochemistry, and quantitative PCR. Cell-associated viral load measured by lymphocyte coculture varied in a tissue-dependent manner with replication highest in lymphocytes isolated from the thymus, lower in those from the peripheral blood, and lowest in those from lymph node. Thymic proviral load and the number of viral p24 Gag-positive cells within the thymus detected by immunohistochemistry were also reduced. In addition, the onset of a reduced peripheral blood CD4/CD8 ratio was delayed in JSY3DeltaORF-A/2-infected cats. The formation and extent of thymic lymphoid follicular hyperplasia were similar in JSY3 and JSY3DeltaORF-A/2-infected cats as measured by anticytokeratin immunohistochemistry and flow cytometry for percent pan T-negative, immunoglobulin G-positive cells within the thymus. In contrast, comparison of thymocyte subpopulations demonstrated a reduced expansion of single-positive CD4(-) CD8(+) thymocytes in JSY3DeltaORF-A/2-infected cats. Level of viral replication, therefore, may not correlate with the formation of thymic lymphoid follicles but may correlate with the expansion of the single-positive CD4(-) CD8(+) thymocyte subpopulation.