Thymus-derived CD4+CD25+ regulatory T (Treg) cells are essential for the maintenance of immunologic self-tolerance. Despite their critical role in the active suppression of experimental autoimmune disorders, little is known about their involvement in human autoimmune diseases. Myasthenia gravis (MG) is a CD4+ T cell–dependent autoimmune disease and the thymus is assumed to be the initiation site. To identify possible defects in the Treg cells in MG, we analyzed CD4+CD25+ cells in thymi from patients with MG compared to those from healthy subjects. We found a normal CD4+CD25+ number but a severe functional defect in their regulatory activity together with a decreased expression of the transcription factor, Foxp3, which is essential for T-cell regulatory function. The phenotypic analysis of CD4+CD25+ thymocytes revealed an increased number of activated effector cells with strong Fas expression in patients with MG. However, whatever their level of Fas, CD4+CD25+ thymocytes from patients with MG remained unable to suppress the proliferation of responding cells, indicating that the impaired Treg cell function is not due to contamination by activated effector T cells. These data are the first to demonstrate a severe functional impairment of thymic Treg cells in MG, which could contribute to the onset of this autoimmune disease.

The immune system contains natural regulatory T cells that control the magnitude of the immune response during physiologic and pathologic conditions. Although this suppressive function was historically attributed to CD8 T cells, most recent reports have focused on natural regulatory CD4 T cells. In the present study, we describe a new subset of natural CD8 regulatory T cells in normal healthy animals. This subset expresses low levels of CD45RC at its surface (CD45RClow); produces mainly interleukin-4 (IL-4), IL-10, and IL-13 cytokines upon in vitro stimulation; expresses Foxp3 and cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4); and is not cytotoxic against allogeneic targets. This subset suppresses the proliferation and differentiation of autologous CD4 T cells into type-1 cytokines producing T cells after stimulation with allogeneic accessory cells. We also provide evidence that this regulatory subset mediates its suppression by cell-to-cell contact and not through secretion of suppressive cytokines. Finally, the regulatory activity of CD8 CD45RClow cells is also demonstrated in vivo in a rat model of CD4-dependent graft-versus-host disease. Collectively, these data demonstrate for the first time that freshly isolated rat CD8 CD45RClow T cells contain T cells with regulatory properties, a result that enlarges the general picture of T-cell-mediated regulation. (Blood. 2004;104:3294-3301)

Treatment with CD40Ig results in indefinite allograft survival in a complete MHC-mismatched heart allograft model in the rat. Here we show that serial second, third, and fourth adoptive transfers of total splenocytes from CD40Ig-treated recipients into secondary recipients led to indefinite donor-specific allograft acceptance. Purification of splenocyte subpopulations from CD40Ig-treated recipients demonstrated that only the adoptively transferred CD8⁺CD45RC^low subset resulted in donor-specific long-term survival, whereas CD8⁺CD45RC^low T cells from naive animals did not. Accepted grafts displayed increased indoleamine 2,3-dioxygenase (IDO) expression restricted in the graft to ECs. Coculture of donor ECs with CD8⁺CD45RC^low T cells purified from CD40Ig-treated animals resulted in donor-specific IDO expression dependent on IFN-γ. Neutralization of IFN-γ or IDO triggered acute allograft rejection in both CD40Ig-treated and adoptively transferred recipients. This study demonstrates for what we believe to be the first time that interference in CD40–CD40 ligand (CD40-CD40L) interactions induces allospecific CD8⁺ Tregs that maintain allograft survival. CD8⁺CD45RC^low T cells act through IFN-γ production, which in turn induces IDO expression by graft ECs. Thus, donor alloantigen-specific CD8⁺ Tregs may promote local graft immune privilege through IDO expression.

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) with an increasing incidence in developed countries. Recent reports suggest that modulation of the gut microbiota might be one promising therapy for MS. Here, we investigated whether the probiotic Escherichia coli strain Nissle 1917 (ECN) could modulate the outcome of experimental autoimmune encephalomyelitis (EAE), a murine model of MS. We evidenced that daily oral treatment with ECN, but not with the archetypal K12 E. coli strain MG1655, reduced the severity of EAE induced by immunization with the MOG35-55 peptide. This beneficial effect was associated with a decreased secretion of IFN-g and an increased production of the anti-inflammatory cytokine IL-10 by autoreactive CD4 T cells, both in peripheral lymph nodes and CNS. Interestingly, ECN-treated mice exhibited increased numbers of MOG-specific CD4+ T cells in the periphery contrasting with severely reduced numbers in the CNS, suggesting that ECN might affect T cells migration from the periphery to the CNS through a modulation of their activation and/or differentiation. In addition, we demonstrated that EAE is associated with a profound defect in the intestinal barrier function and that treatment with ECN, but not with MG1655, repaired intestinal permeability dysfunction. Collectively, our data reveal that EAE induces a disruption of the intestinal homeostasis and that ECN protects from disease and restores the intestinal barrier function.