The low number of CD4+ CD25+ regulatory T cells (Tregs), their anergic phenotype, and diverse antigen specificity present major challenges to harnessing this potent tolerogenic population to treat autoimmunity and transplant rejection. In this study, we describe a robust method to expand antigen-specific Tregs from autoimmune-prone nonobese diabetic mice. Purified CD4+ CD25+ Tregs were expanded up to 200-fold in less than 2 wk in vitro using a combination of anti-CD3, anti-CD28, and interleukin 2. The expanded Tregs express a classical cell surface phenotype and function both in vitro and in vivo to suppress effector T cell functions. Most significantly, small numbers of antigen-specific Tregs can reverse diabetes after disease onset, suggesting a novel approach to cellular immunotherapy for autoimmunity.

Human immunodeficiency virus (HIV) disease is typified by declining CD4+ T lymphocyte counts in the peripheral circulation, a loss which may be secondary to accelerated destruction, to suppressed differentiation, and/or to sequestration of circulating cells into tissue spaces. As it is hard to distinguish between these possibilities in human subjects, the pathogenic mechanisms associated with HIV infection are unclear. In particular, little is known about the events that occur within infected lymphoid organs in which most CD4 T lymphocytes mature and function. To obtain a better description of HIV pathogenesis in vivo, we have implanted human haematolymphoid organs into the immunodeficient SCID mouse to create the SCID-hu mouse. We have previously shown that these organ systems promote long-term multilineage human haematopoiesis and are permissive for infection with HIV. Here we report that human thymopoiesis is suppressed by HIV infection, thereby precluding regeneration of the peripheral T-cell compartment.

Direct and indirect cytopathic mechanisms have been proposed to account for the loss of CD4+ T cells after infection with human immunodeficiency virus type 1 (HIV-1). We report here that HIV-1 infection of the human thymus in vivo results in thymocyte depletion by at least two different mechanisms. Thymocytes within multiple stages of differentiation are induced to die of apoptosis; most of these cells are uninfected. Additionally, thymopoiesis is interrupted by direct infection and destruction of intrathymic CD3-CD4+CD8- progenitor cells. These mechanisms are differentially induced by distinct isolates of HIV-1.

SummaryInfection with the human immunodeficiency virus (HIV) results in immunosuppression and depletion of circulating CD4 + T cells. Since the thymus is the primary organ in which T cells mature it is of interest to examine the effects of HIV infection in this tissue. HIV infection has been demonstrated in the thymuses of infected individuals and thymocytes have been previously demonstrated to be susceptible to HIV infection both in vivo, using the SCID-hu mouse, and in vitro. The present study sought to determine which subsets of thymocytes were infected in the SCID-hu mouse modal and to evaluate HIV-related alterations in the thymic microenvironment. Using two different primary HIV isolates, infection was found in CD4 +/CD8 + double positive thymocytes as well as in both the CD4 + and CD8 + single positive subsets of thymocytes. The kinetics of infection and resulting viral burden differed among the three thymocyte subsets and depended on which HIV isolate was used for infection. Thymic epithelial (TE) cells were also shown to endocytose virus and to often contain copious amounts of viral RNA in the cytoplasm by in situ hybridization, although productive infection of these cells could not be definitively shown. Furthermore, degenerating TE cells were observed even without detection of HIV in the degenerating cells. Two striking morphologic patterns of infection were seen, involving either predominantly thymocyte infection and depletion, or TE cell involvement with detectable cytoplasmic viral RNA and/or TE cell toxicity. Thus, a variety of cells in the human thymus is susceptible to HIV infection, and infection with HIV results in a marked disruption of the thymic microenvironment leading to depletion of thymocytes and degeneration of TE ceils.