Combination of rapamycin and IL-2 increases de novo induction of human CD4(+)CD25(+)FOXP3(+) T cells

Intact extracellular matrix and the maintenance of immune tolerance: high molecular weight hyaluronan promotes persistence of induced CD4+CD25+ regulatory T cells

The composition of the ECM provides contextual cues to leukocytes in inflamed and healing tissues. One example of this is HA, where LMW-HA, generated during active inflammation, is a TLR ligand and an endogenous "danger signal," and HMW-HA, predominant in healing or intact tissues, functions in an inverse manner. Our data suggest that HMW-HA actively promotes immune tolerance by augmenting CD4+CD25+ T(Reg) function, and LMW-HA does not. Using a human iT(Reg) model, we demonstrate that HMW-HA but not LMW-HA provides a costimulatory signal through cross-linking CD44 which promotes Foxp3 expression, a critical signaling molecule associated with T(Reg). This effect, in part, may be mediated by a role for intact HMW-HA in IL-2 production, as T(Reg) are highly IL-2-dependent for their survival and function. We propose that HMW-HA contributes to the maintenance of immune homeostasis in uninjured tissue and effectively communicates an "all-clear" signal to down-regulate the adaptive immune system through T(Reg) after tissue matrix integrity has been restored.

FOXP3 and the regulation of Treg/Th17 differentiation

CD4 T cell lineages are marked by the signature transcription factor each lineage expresses. For example, regulatory T cells (Tregs) are characterized by expression of FOXP3, which is either induced during thymic development for natural Tregs (nTregs), or in the periphery in the presence of TGF-beta and retinoic acid for induced Tregs (iTreg). Interestingly, recent work has shown that the signature transcription factor for Th17 cells, RORgammat, is also induced by TGF-beta, thus linking the differentiation of the Treg and Th17 lineages. In the absence of a second signal from a proinflammatory cytokine, FOXP3 can inhibit RORgammat function and drive Treg differentiation. However, when the cell also receives a signal from a proinflammation cytokine (e.g., IL-6), FOXP3 function is inhibited and the Th17 differentiation pathway is induced. Therefore, it is the balance between FOXP3 and RORgammat function that determines CD4 T cell fate and the type of immune response that will be generated.

Functional islet-specific Treg can be generated from CD4+CD25- T cells of healthy and type 1 diabetic subjects

CD4(+)CD25(+)FOXP3(+) Treg cells require TCR engagement for suppressive function, thus ensuring that suppression occurs only in the presence of specific antigens; however, to date no studies have addressed the function of self-antigen-specific Treg in humans. These studies were designed to determine whether peripheral generation and function of islet antigen-specific adaptive Treg are defective in human subjects with type 1 diabetes (T1D). Islet antigen-specific adaptive Treg were induced in vitro by activation of CD4(+)FOXP3(-) T cells with glutamic acid decarboxylase and islet-specific glucose-6-phosphate catalytic subunit-related protein peptides in the context of T1D-associated HLA-DRbeta alleles. Antigen-specific Treg were characterized using flow cytometry for FOXP3 and class II tetramer and assessed for the ability to inhibit proliferation. These adaptive Treg were then compared with influenza-specific Treg from the same study population. The function of tetramer(+) cells that expressed FOXP3 was similar for both influenza and islet antigens generated from control and T1D subjects. In fact, the potency of suppression correlated with FOXP3 expression, not antigen specificity. Thus, these data suggest that development of functional adaptive Treg can occur in response to islet antigens and activation of islet-specific Treg may potentially be used as a targeted immunotherapy in T1D.

Functional analysis of FOXP3

Tolerance to self antigens is established in two ways: first in the thymus through the deletion of thymocytes expressing self-reactive T cell receptors; and second, in the periphery through multiple mechanisms involving deletion, anergy, and suppression. Dominant tolerance to self antigens in the periphery is primarily the function of the CD4(+)CD25(+)FOXP3(+) subset of T cells, which have the capability of suppressing autoreactive T cells that have escaped deletion during thymic selection. The essential role of the transcription factor FOXP3 in the development and function of these cells has been well documented. However, the underlying mechanisms by which FOXP3 controls this process are less well understood. This review will focus on the role of FOXP3 in regulating CD4 T cell function in both humans and mice, with an emphasis on recent work in human systems.

The effector T cells of diabetic subjects are resistant to regulation via CD4+ FOXP3+ regulatory T cells

Defects in immune regulation have been implicated in the pathogenesis of diabetes in mouse and in man. In vitro assays using autologous regulatory (Treg) and responder effector (Teff) T cells have shown that suppression is impaired in diabetic subjects. In this study, we addressed whether the source of this defect is intrinsic to the Treg or Teff compartment of diabetic subjects. We first established that in type 1 diabetes (T1D) individuals, similar levels of impaired suppression were seen, irrespective of whether natural (nTreg) or adaptive Treg (aTreg) were present. Then using aTreg, we examined the ability of T1D aTreg to suppress Teff of healthy controls, as compared with the ability of control aTreg to suppress Teff of diabetic subjects. Taking this approach, we found that the aTregs from T1D subjects function normally in the presence of control Teff, and that the T1D Teff were resistant to suppression in the presence of control aTreg. This escape from regulation was seen with nTreg as well and was not transferred to control Teff coincubated with T1D Teff. Thus, the "defective regulation" in T1D is predominantly due to the resistance of responding T cells to Treg and is a characteristic intrinsic to the T1D Teff. This has implications with respect to pathogenic mechanisms, which underlie the development of disease and the target of therapies for T1D.