CD4+CD25highFoxp3+ regulatory T cells (Tregs) can suppress other immune cells and, thus, are critical mediators of peripheral self-tolerance. On the one hand, Tregs avert autoimmune disease and allergies. On the other hand, Tregs can prevent immune reactions against tumors and pathogens. Despite the importance of Tregs, the molecular mechanisms of suppression remain incompletely understood and controversial. Proliferation and cytokine production of CD4+CD25− conventional T cells (Tcons) can be inhibited directly by Tregs. In addition, Tregs can indirectly suppress Tcon activation via inhibition of the stimulatory capacity of antigen presenting cells. Direct suppression of Tcons by Tregs can involve immunosuppressive soluble factors or cell contact. Different mechanisms of suppression have been described, so far with no consensus on one universal mechanism. Controversies might be explained by the fact that different mechanisms may operate depending on the site of the immune reaction, on the type and activation state of the suppressed target cell as well as on the Treg activation status. Further, inhibition of T cell effector function can occur independently of suppression of proliferation. In this review, we summarize the described molecular mechanisms of suppression with a particular focus on suppression of Tcons and rapid suppression of T cell receptor-induced calcium (Ca2+), NFAT, and NF-κB signaling in Tcons by Tregs.

CD4+CD25+FoxP3+ regulatory T cells (Treg) suppress T cell function and protect rodents from autoimmune disease. Regulation of Treg during an immune response is of major importance. Enhanced survival of Treg is beneficial in autoimmune disease, whereas increased depletion by apoptosis is advantageous in cancer. We show here that freshly isolated FACS-sorted Treg are highly sensitive toward CD95-mediated apoptosis, whereas other T cell populations are resistant to CD95-induced apoptosis shortly after isolation. In contrast, TCR restimulation of Treg in vitro revealed a reduced sensitivity toward activation-induced cell death compared with CD4+CD25− T cells. Thus, the apoptosis phenotype of Treg is unique in comparison to other T cells, and this might be further explored for novel therapeutic modulations of Treg.

IntroductionThere is now clear evidence that CD4 ϩ T cells contain a population of naturally immunosuppressive T cells characterized by constitutive expression of CD25, CTLA-4, and FOXP3. 1 Because not all potentially autoreactive T cells are deleted in the thymus, 2,3 peripheral control of T-cell responses by naturally occurring CD4 ϩ CD25 ϩ FOXP3 ϩ immunoregulatory T cells (T regs ) is crucial to prevent autoimmunity. 1 Depletion of T regs contributes to the induction of severe autoimmune diseases in animal models, and several studies have reported a defect of T regs in various human autoimmune diseases. 1,4,5 Despite extensive research to unravel the immunosuppressive function of T reg , the exact molecular mechanism of immunosuppression is still elusive. Promising targets for pharmacologic mimicking of T regs function remain undefined. Consequently, direct use of T regs for therapy is currently under examination. Therapeutic expansion or depletion of T regs with defined antigen specificity offers new treatment options for human diseases. 6 Because accumulation of T regs has been shown to be detrimental in cancer, 7 new insights into mechanisms of T reg homeostastis are required.To gain new insight into the modulation of T reg numbers as well as their antigen specificity, the development of natural T regs during ontogeny and in the adult needs to be explored. The peripheral T reg compartment consists mainly of thymus-derived T regs . 1 However, under specific circumstances T regs can also be generated out of conventional T cells (T convs ) (eg, if the antigen is targeted to immature dendritic cells [DCs]). [8][9][10] It has yet to be established how much "converted" T regs contribute to the total number of T regs in the periphery of adult mice and humans.At a given time, the overall number of T regs is defined by a balance of generation and demise of T regs . At the end of an immune reaction, T cells are depleted by apoptosis. Activation-induced cell death (AICD) through CD95/CD95L has been described as such an apoptosis-inducing mechanism. 11 While naive T cells are CD95 Ϫ and resistant to apoptosis induction, activated T cells (CD45RO hi ) up-regulate CD95 and become sensitive to apoptosis. Upon T-cell receptor (TCR) restimulation, activated effector T cells (T effs ) up-regulate CD95L and induce AICD through crosslinking of CD95 by CD95L. AICD eliminates T effs after an immune response and contributes to T-cell homeostasis. We have previously shown that most T regs constitutively express CD95 and can easily be killed via crosslinking of this death receptor by CD95L. 12 Together with the fact that most T regs express high levels of CD45RO, we contributed expertise in microarray analysis; J.P. contributed clinical samples; P.K. analyzed data and assisted in writing the paper; O.L. contributed clinical samples and assisted in writing the paper; and E.S.-P. designed research and wrote the paper.The online version of this article contains a data supplement. For personal use only. on April 5, 2019. by guest www....

Objective. To define the phenotype and function of CD4؉,CD25؉ regulatory T cells (Treg) in patients with cutaneous lupus erythematosus (CLE), a heterogeneous autoimmune disease characterized primarily by inflammatory skin lesions.Methods. The number of Treg in skin specimens obtained from patients with various subtypes of CLE was investigated by immunohistochemical analysis, using anti-Foxp3 and anti-CD4 monoclonal antibodies. Furthermore, characterization of peripheral blood CD4؉,CD25؉ Treg from normal healthy donors and patients with CLE was carried out by flow cytometry, analyzing the expression of Foxp3 and Treg subpopulations. We also purified CD4؉,CD25 high Treg obtained from patients with CLE and tested the sensitivity of these cells to CD95L-mediated apoptosis.Results. Quantitative analysis of CD4؉ T cells in skin lesions from patients with CLE revealed that the number was similar to that in lesions from patients with other chronic inflammatory diseases, but the number of Foxp3؉ Treg in CLE was significantly reduced. There was no correlation between disease subtype and the frequency of Foxp3؉ Treg in the skin of patients with CLE. In peripheral blood, no significant differences were observed in the number and phenotype of CD4؉,CD25؉ Treg or in the sensitivity to apoptosis of CD4؉,CD25 high Treg derived from patients with CLE and those derived from normal healthy donors.Conclusion. These data suggest that an organspecific abnormality of Treg in the skin underscores the importance of analyzing Treg in the affected tissue. Such a local process might give insight into the pathogenic mechanisms of CLE and differs from a global peripheral dysfunction as reported for patients with a systemic manifestation of the disease.