Follicular helper T (TFH) cells participate in humoral responses providing selection signals to germinal center B cells. Recently, expression of CXCR5, PD-1, and the transcription factor Bcl-6 has allowed the identification of TFH cells. We found that a proportion of follicular T cells, with phenotypic characteristics of TFH cells and expressing Foxp3, are recruited during the course of a germinal center (GC) reaction. These Foxp3+ cells derive from natural regulatory T cells. To establish the in vivo physiologic importance of Foxp3+ follicular T cells, we used CXCR5-deficient Foxp3+ cells, which do not have access to the follicular region. Adoptive cell transfers of CXCR5-deficient Foxp3+ cells have shown that Foxp3+ follicular T cells are important regulators of the GC reaction following immunization with a thymus-dependent Ag. Our in vivo data show that Foxp3+ follicular T cells can limit the magnitude of the GC reaction and also the amount of secreted Ag-specific IgM, IgG1, IgG2b, and IgA. Therefore, Foxp3+ follicular regulatory T cells appear to combine characteristics of TFH and regulatory T cells for the control of humoral immune responses.

The significance of cytokine production by CD4+ regulatory T (T reg) cells after antigen exposure in vivo and its impact on their regulatory activity remains unclear. Pretreatment with donor alloantigen under the cover of anti-CD4 therapy generates alloantigen reactive T reg cells that can prevent rejection of donor-specific skin grafts that are mediated by naive CD45RBhighCD4+ T cells. To examine the kinetics and importance of cytokine gene transcription by such alloantigen-reactive T reg cells, pretreated mice were rechallenged with donor alloantigen in vivo. CD25+CD4+ T cells, but not CD25−CD4+ T cells, showed a fivefold increase in IFN-γ mRNA expression within 24 h of reencountering alloantigen in vivo. This expression kinetic was highly antigen-specific and was of functional significance. Neutralizing IFN-γ at the time of cotransfer of alloantigen reactive T reg cells, together with CD45RBhighCD4+ effector T cells into Rag −/− skin graft recipients, resulted in skin graft necrosis in all recipients; the generation and function of alloantigen-reactive T reg cells was impaired dramatically in IFN-γ–deficient mice. These data support a unique role for IFN-γ in the functional activity of alloantigen-reactive T reg cells during the development of operational tolerance to donor alloantigens in vivo.

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Regulatory CD4(+) T cells, enriched in the CD25 pool of healthy individuals, mediate natural tolerance and prevent autoimmune diseases. Despite their fundamental and potential clinical significance, regulatory T (T(R)) cells have not yet been incorporated in a coherent theory of the immune system. This article reviews experimental evidence and theoretical arguments supporting a model of T(R) cell dynamics, uncovering some of its most relevant biological implications. According to this model, the persistence and expansion of T(R) cell populations depend strictly on specific interactions they make with antigen-presenting cells (APCs) and conventional effector T (T(E)) cells. This three-partner crossregulation imposes that T(R) cells feed on the specific autoimmune activities they suppress, with implications ranging from their interactions with other cells to their repertoire selection in the periphery and in the thymus, and to the relationship between these cells and the innate immune system. These implications stem from the basic prediction that the peripheral dynamics sort the CD4(+) T-cell repertoire into two subsets: a less diverse set of small clones of autoreactive effector and regulatory cells that regulate each other's growth, and a more diverse set of barely autoreactive T(E) cell clones, whose expansion is limited only by APC availability. It is argued that such partitioning of the repertoire sets the ground for self-non-self discrimination.

The use of small organic molecules as catalysts has gained increasing importance recently. These substances, the so-called organocatalysts, present a lot of advantages, like being less toxic, less polluting, and more economically viable than the organometallic catalysts that dominate asymmetric synthesis. This work intends to briefly show some classic works and recent publications, explaining the advantages of organocatalysis and the different types of compounds used in this field, as well as their course of action.

Classical in vitro Treg conversion assays, which rely on optimal T-cell activation in the presence of exogenous TGF-b, induce Foxp3 expression at a frequency far above that which is observed in vivo in Treg-dependent models of oral or transplantation tolerance. We have found that suboptimal murine T-cell activation in vitro results in induction of Foxp3 expression, in the absence of exogenous TGF-b, at a frequency similar to that which we found in vivo upon anti-CD4-induced transplantation tolerance. We show that TCR triggering with either low-dose anti-CD3 or low-dose agonist peptide, as well as downmodulation of the TCR signal with non-depleting anti-CD4, promotes TGF-b production by T cells, an event that precedes Foxp3 expression and is Foxp3 independent. These findings support the view that sub-immunogenic regimens lead to dominant tolerance as a result of T-cell intrinsic properties.

Regulatory T cells (Treg) have been shown to play a role in the prevention of autoimmune diseases and transplant rejection. Based on an established protocol known to generate alloantigen reactive Treg in vivo, we have developed a strategy for the in vitro selection of Treg. Stimulation of unfractionated CD4+ T cells from naive CBA.Ca (H2k) mice with C57BL/10 (H2b) splenocytes in the presence of an anti-CD4 antibody, YTS 177, resulted in the selection of Treg able to inhibit proliferation of naive T cells. In vivo, the cells were able to prevent rejection of 80% C57BL/10 skin grafts when co-transferred to CBA.Rag–/– mice together with naive CD45RBhighCD4+ cells. Purification of CD62L+CD25+CD4+ cells from the cultures enriched for cells with regulatory activity; as now 100% survival of C57BL/10 skin grafts was achieved. Furthermore, differentiation of Treg could be also achieved when using purified CD25–CD4+ naive T cells as a starting population. Interestingly, further in vitro expansion resulted in a partial loss of CD4+ cells expressing both CD62L and CD25 and abrogation of their regulatory activity in vivo. This study shows that alloantigen stimulation in the presence of anti-CD4 in vitro provides a simple and effective strategy to generate alloreactive Treg.

BackgroundRheumatoid Arthritis (RA) is a chronic immune mediated disease associated with deregulation of many cell types. It has been reported that different T cell subsets have opposite effects in disease pathogenesis, in particular Th17 and Treg cells.Methodology and FindingsWe investigated whether non-depleting anti-CD4 monoclonal antibodies, which have been reported as pro-tolerogenic, can lead to protection from chronic autoimmune arthritis in SKG mice – a recently described animal model of RA – by influencing the Th17/Treg balance. We found that non-depleting anti-CD4 prevented the onset of chronic autoimmune arthritis in SKG mice. Moreover, treated mice were protected from the induction of arthritis up to 60 days following anti-CD4 treatment, while remaining able to mount CD4-dependent immune responses to unrelated antigens. The antibody treatment also prevented disease progression in arthritic mice, although without leading to remission. Protection from arthritis was associated with an increased ratio of Foxp3, and decreased IL-17 producing T cells in the synovia. In vitro assays under Th17-polarizing conditions showed CD4-blockade prevents Th17 polarization, while favoring Foxp3 induction.ConclusionsNon-depleting anti-CD4 can therefore induce long-term protection from chronic autoimmune arthritis in SKG mice through reciprocal changes in the frequency of Treg and Th17 cells in peripheral tissues, thus shifting the balance towards immune tolerance.