David M. Sansom

Trans-Endocytosis of CD80 and CD86: A Molecular Basis for the Cell Extrinsic Function of CTLA-4

An inhibitory T cell receptor acts by stripping activating ligands off dendritic cells. Cytotoxic T lymphocyte antigen 4 (CTLA-4) is an essential negative regulator of T cell immune responses whose mechanism of action is the subject of debate. CTLA-4 shares two ligands (CD80 and CD86) with a stimulatory receptor, CD28. Here, we show that CTLA-4 can capture its ligands from opposing cells by a process of trans-endocytosis. After removal, these costimulatory ligands are degraded inside CTLA-4–expressing cells, resulting in impaired costimulation via CD28. Acquisition of CD86 from antigen-presenting cells is stimulated by T cell receptor engagement and observed in vitro and in vivo. These data reveal a mechanism of immune regulation in which CTLA-4 acts as an effector molecule to inhibit CD28 costimulation by the cell-extrinsic depletion of ligands, accounting for many of the known features of the CD28–CTLA-4 system.

Normal human pregnancy is associated with an elevation in the immune suppressive CD25+ CD4+ regulatory T-cell subset

CD4+ CD25+ T regulatory cells (TReg), suppress antigen‐specific immune responses and are important for allograft tolerance. During pregnancy the mother tolerates an allograft expressing paternal antigens (the fetus) requiring substantial changes in immune regulation over a programmed period of time. We analysed whether immune‐suppressive TReg cells were altered during pregnancy and therefore might play a part in this tolerant state. The presence of TReg cells was assessed in the blood of 25 non‐pregnant, 63 pregnant and seven postnatal healthy women by flow cytometry. We observed an increase in circulating TReg cells during early pregnancy, peaking during the second trimester and then a decline postpartum. Isolated CD25+ CD4+ cells expressed FoxP3 messenger RNA, a marker of TReg cells, and suppressed proliferative responses of autologous CD4+ CD25– T cells to allogeneic dendritic cells. These data support the concept that normal pregnancy is associated with an elevation in the number of TReg cells which may be important in maintaining materno‐fetal tolerance.

1,25-Dihydroxyvitamin D3 and IL-2 Combine to Inhibit T Cell Production of Inflammatory Cytokines and Promote Development of Regulatory T Cells Expressing CTLA-4 and FoxP3

The active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), has potent immunomodulatory properties that have promoted its potential use in the prevention and treatment of infectious disease and autoimmune conditions. A variety of immune cells, including macrophages, dendritic cells, and activated T cells express the intracellular vitamin D receptor and are responsive to 1,25(OH)2D3. Despite this, how 1,25(OH)2D3 regulates adaptive immunity remains unclear and may involve both direct and indirect effects on the proliferation and function of T cells. To further clarify this issue, we have assessed the effects of 1,25(OH)2D3 on human CD4+CD25− T cells. We observed that stimulation of CD4+CD25− T cells in the presence of 1,25(OH)2D3 inhibited production of proinflammatory cytokines including IFN- γ, IL-17, and IL-21 but did not substantially affect T cell division. In contrast to its inhibitory effects on inflammatory cytokines, 1,25(OH)2D3 stimulated expression of high levels of CTLA-4 as well as FoxP3, the latter requiring the presence of IL-2. T cells treated with 1,25(OH)2D3 could suppress proliferation of normally responsive T cells, indicating that they possessed characteristics of adaptive regulatory T cells. Our results suggest that 1,25(OH)2D3 and IL-2 have direct synergistic effects on activated T cells, acting as potent anti-inflammatory agents and physiologic inducers of adaptive regulatory T cells.

Extra-renal 25-hydroxyvitamin D3-1α-hydroxylase in human health and disease

Although ectopic expression of 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1alpha-OHase) has been recognized for many years, the precise function of this enzyme outside the kidney remains open to debate. Three specific aspects of extra-renal 1alpha-OHase have attracted most attention: (i) expression and regulation in non-classical tissues during normal physiology; (ii) effects on the immune system and inflammatory disease; (iii) expression and function in tumors. The most well-recognized manifestation of extra-renal 1alpha-OHase activity remains that found in some patients with granulomatous diseases where locally synthesized 1alpha,25(OH)(2)D(3) has the potential to spill-over into the general circulation. However, immunohistochemistry and mRNA analyses suggest that 1alpha-OHase is also expressed by a variety of normal human tissues including the gastrointestinal tract, skin, vasculature and placenta. This has promoted the idea that autocrine/paracrine synthesis of 1,25(OH)(2)D(3) contributes to normal physiology, particularly in mediating the potent effects of vitamin D on innate (macrophage) and acquired (dendritic cell) immunity. We have assessed the capacity for synthesis of 1,25(OH)(2)D(3) in these cells and the functional significance of autocrine responses to 1alpha-hydroxylase. Data suggest that local synthesis of 1,25(OH)(2)D(3) may be a preferred mode of response to antigenic challenge in many tissues.

The emerging role of CTLA4 as a cell-extrinsic regulator of T cell responses

The T cell protein cytotoxic T lymphocyte antigen 4 (CTLA4) was identified as a crucial negative regulator of the immune system over 15 years ago, but its mechanisms of action are still under debate. It has long been suggested that CTLA4 transmits an inhibitory signal to the cells that express it. However, not all the available data fit with a cell-intrinsic function for CTLA4, and other studies have suggested that CTLA4 functions in a T cell-extrinsic manner. Here, we discuss the data for and against the T cell-intrinsic and -extrinsic functions of CTLA4.

Autosomal dominant immune dysregulation syndrome in humans with CTLA4 mutations

The protein cytotoxic T lymphocyte antigen-4 (CTLA-4) is an essential negative regulator of immune responses, and its loss causes fatal autoimmunity in mice. We studied a large family in which five individuals presented with a complex, autosomal dominant immune dysregulation syndrome characterized by hypogammaglobulinemia, recurrent infections and multiple autoimmune clinical features. We identified a heterozygous nonsense mutation in exon 1 of CTLA4. Screening of 71 unrelated patients with comparable clinical phenotypes identified five additional families (nine individuals) with previously undescribed splice site and missense mutations in CTLA4. Clinical penetrance was incomplete (eight adults of a total of 19 genetically proven CTLA4 mutation carriers were considered unaffected). However, CTLA-4 protein expression was decreased in regulatory T cells (Treg cells) in both patients and carriers with CTLA4 mutations. Whereas Treg cells were generally present at elevated numbers in these individuals, their suppressive function, CTLA-4 ligand binding and transendocytosis of CD80 were impaired. Mutations in CTLA4 were also associated with decreased circulating B cell numbers. Taken together, mutations in CTLA4 resulting in CTLA-4 haploinsufficiency or impaired ligand binding result in disrupted T and B cell homeostasis and a complex immune dysregulation syndrome.

The role of CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA-4) in regulatory T-cell biology

Summary:  The profound influence of CD28 and cytotoxic T‐lymphocyte antigen‐4 (CTLA‐4) on T‐cell immunity has been known for over a decade, yet the precise roles played by these molecules still continue to emerge. Initially viewed as molecules that provide cell‐intrinsic costimulatory and inhibitory signals, recent evidence suggests that both CD28 and CTLA‐4 are also important in the homeostasis and function of a population of suppressive cells, termed regulatory T cells (Tregs). Here we review the main features of the CD28 and CTLA‐4 system and examine how these impact upon Treg biology.

CTLA4 gene polymorphism and autoimmunity

Summary:  CD28 and cytotoxic T‐lymphocyte antigen‐4 (CTLA4) are two receptors that have critical but opposing functions in T‐cell stimulation. CD28 promotes a number of T‐cell activities, whereas in contrast CTLA4 is an essential inhibitor of T‐cell responses. Because of its inhibitory role, CTLA4 is a strong candidate susceptibility gene in autoimmunity and several studies suggest disease‐associated polymorphisms. In this review, we discuss recent progress in relating CTLA4 polymorphisms to disease susceptibility and consider the putative mechanisms by which CTLA4 may act to inhibit autoimmunity.

CD86 and CD80 differentially modulate the suppressive function of human regulatory T cells.

Regulatory T cells (Treg) are important in maintaining tolerance to self tissues. As both CD28 and CTLA-4 molecules are implicated in the function of Treg, we investigated the ability of their two natural ligands, CD80 and CD86, to influence the Treg-suppressive capacity. During T cell responses to alloantigens expressed on dendritic cells, we observed that Abs against CD86 potently enhanced suppression by CD4+CD25+ Treg. In contrast, blocking CD80 enhanced proliferative responses by impairing Treg suppression. Intriguingly, the relative expression levels of CD80 and CD86 on dendritic cells are modulated during progression from an immature to a mature state, and this correlates with the ability of Treg to suppress responses. Our data show that CD80 and CD86 have opposing functions through CD28 and CTLA-4 on Treg, an observation that has significant implications for manipulation of immune responses and tolerance in vivo.

What's the difference between CD80 and CD86?

CD28 and CD152 have crucial yet opposing functions in T-cell stimulation, in which CD28 promotes but CD152 inhibits T-cell responses. Intriguingly, they share two ligands, CD80 and CD86, but at present there is no clear model for understanding whether a ligand will promote or inhibit responses. Current perceptions are based around the concept that CD86 is the initial co-stimulatory ligand based on its more abundant and earlier expression pattern; CD80 has a role following antigen-presenting-cell activation. We describe an alternative view in which CD80 is the initial ligand, responsible for maintaining aspects of immune tolerance through interactions with CD152. These inhibitory functions can then be over-ridden by the upregulation of CD86 on dendritic cells as a result of inflammatory stimuli, leading to immune activation.