Regulatory T cells (TReg) control immune responses to self and nonself Ags. The relationship between Ag-driven IL-10-secreting TReg (IL-10-TReg) and naturally occurring CD4+CD25+ TReg is as yet unclear. We show that mouse IL-10-TReg obtained using either in vitro or in vivo regimens of antigenic stimulation did not express the CD4+CD25+ TReg-associated transcription factor Foxp3. However, despite the absence of Foxp3 expression, homogeneous populations of IL-10-TReg inhibited the in vitro proliferation of CD4+CD25− T cells with a similar efficiency to that of CD4+CD25+ TReg. This inhibition of T cell proliferation by IL-10-TReg was achieved through an IL-10-independent mechanism as seen for CD4+CD25+ TReg and was overcome by exogenous IL-2. Both IL-10-TReg and CD4+CD25+ TReg were similar in that they produced little to no IL-2. These data show that Foxp3 expression is not a prerequisite for IL-10-TReg activity in vitro or in vivo, and suggest that IL-10-TReg and naturally occurring CD4+CD25+ TReg may have distinct origins.

The mechanisms of transcriptional activation in heterochromatin were investigated by using FISH to directly visualize changes in chromatin organization during activation of a heterochromatic lambda5 transgene. A DNase I hypersensitive site was shown to relocate the transgene to the outside of the pericentromeric heterochromatin complex in the absence of transcription. Activation of transcription, which is dependent on the transcription factor EBF, occurs in a stochastic manner that resembles telomeric silencing in yeast, with the transcribed gene remaining closely associated with the heterochromatin complex. Reducing the dosage of EBF results in a reduced frequency of localization of the transgene to the outside of the heterochromatin complex and lower levels of transcription. These data provide evidence that transcription factors can initiate changes in higher order chromatin structure during the earliest stages of gene activation.

Repetitive antigen stimulation induces peripheral T cell tolerance in vivo. It is not known, however, whether multiple stimulations merely suppress T cell activation or, alternatively, change the transcriptional program to a distinct, tolerant state. In this study, we have discovered that STAT3 and STAT5 were activated in response to antigen stimulation in vivo, in marked contrast to the suppression of AP-1, NF-jB and NFAT. In addition, a number of transcription factors were induced in tolerant T cells following antigen challenge in vivo, including T-bet, Irf-1 and Egr-2. The altered transcription program in tolerant cells associates closely with the suppression of cell cycle progression and IL-2 production, as well as with the induction of IL-10. Studies of T-bet and Egr-2 show that the function of T-bet in peptide treatment-induced regulatory T cells is not associated with Th1 differentiation, but correlates with the suppression of IL-2, whereas expression of Egr-2 led to an up-regulation of the cell cycle inhibitors p21 cip1 and p27 kip . Our results demonstrate a balanced transcription program regulated by different transcription factors for T cell activation and/or tolerance during antigen-induced T cell responses. Persistent antigen stimulation can induce T cell tolerance by changing the balance of transcription factors.See accompanying commentary http://www.dx

It is generally acknowledged that cytotoxic T-lymphocyte-associ-

Intranasal administration of peptide Ac1–9[4Y], based on the N-terminal epitope of myelin basic protein, can induce CD4+ T cell tolerance, and suppress experimental autoimmune encephalomyelitis induction. The peptide-induced regulatory T (PI-TReg) cells failed to produce IL-2, but expressed IL-10 in response to Ag and could suppress naive T cell responses in vitro. Analysis of Jak-STAT signaling pathways revealed that the activation of Jak1, STAT3, and STAT5 were induced in tolerant T cells after Ag stimulation in vivo. In addition, the expression of suppressor of cytokine signaling 3 was induced in tolerant T cells, suggesting that cytokines regulate the tolerant state of the PI-TReg cells. Stimulation of PI-TReg cells in vitro with IL-10 induced Jak1 and STAT3 activation, but not STAT5, suggesting that IL-10 is important, but not the only cytokine involved in the development of T cell tolerance. Although IL-2 expression was deficient, stimulation with IL-2 in vitro induced Jak1 and STAT5 activation in PI-TReg cells, restored their proliferative response to antigenic stimulation, and abrogated PI-TReg-mediated suppression in vitro. However, the addition of IL-2 could not suppress IL-10 expression, and the IL-2 gene remained inactive. After withdrawal of IL-2, the PI-TReg cells regained their nonproliferative state and suppressive ability. These results underline the ability of the immune system to maintain tolerance to autoantigens, but at the same time having the ability to overcome the suppressive phenotype of tolerant T cells by cytokines, such as IL-2, during the protective immune response to infection.

Inhibition of cytotoxicity by two different human A-LAK cell populations through F(abЈ) 2 of affinity-purified anti-laminin Ab. Sorted Leu-19 ϩ /CD3 Ϫ and Leu-19 ϩ /CD3 ϩ A-LAK cell populations were mixed with Cr-labeled target cells in the continued presence of 150 g/ml Ab for 4 h, and the resulting lytic activity was calculated as specified in Materials and Methods.

Recent studies have emphasized the importance of T cells with regulatory/suppressor properties in controlling autoimmune diseases. A number of different types of regulatory T cells have been described with the best characterized being the CD25+ population. In addition, it has been shown that regulatory T cells can be induced by specific Ag administration. In this study, we investigate the relationship between peptide-induced, CD4+ regulatory T cells and naturally occurring CD4+CD25+ cells derived from the Tg4 TCR-transgenic mouse. Peptide-induced cells were FoxP3− and responded to Ag by secreting IL-10, whereas CD25+ cells failed to secrete this cytokine. Both cell types were able to suppress the proliferation of naive lymphocytes in vitro although with distinct activation sensitivities. Depletion of CD25+ cells did not affect the suppressive properties of peptide-induced regulators. Furthermore, peptide-induced regulatory/suppressor T cells could be generated in RAG−/−, TCR-transgenic mice that do not spontaneously generate CD25+ regulatory cells. These results demonstrate that these natural and induced regulatory cells fall into distinct subsets.

Summary An elderly patient with no abnormal bleeding presented with prolongation of the activated partial thromboplastin time (aPTT). Preincubation of plasma with aPTT reagent caused shortening of the abnormal clotting time. Plasma prekallikrein (PK) activity and antigen were <5 u/dL. Molecular analysis showed a homozygous Arg94Stop substitution in the PK gene, predicted to prevent expression of the mutant allele. The five heterozygous offspring of the proband each showed a normal aPTT but reduced PK activity and antigen. This is the first description of a kindred in which absence of expression of one or both PK alleles has been confirmed by genotype.