CD4+CD25+ regulatory T cells in HIV infection

The immune system faces the difficult task of discerning between foreign, potentially pathogen-derived antigens and self-antigens. Several mechanisms, including deletion of self-reactive T cells in the thymus, have been shown to contribute to the acceptance of self-antigens and the reciprocal reactivity to foreign antigens. Over the last decade it has become increasingly clear that CD4(+)CD25(+) T(Reg) cells are crucial for maintenance of T cell tolerance to self-antigens in the periphery, and to avoid development of autoimmune disorders. Recently, evidence has also emerged that demonstrates that CD4(+)CD25(+) T(Reg) cells can also suppress T cell responses to foreign pathogens, including viruses such as HIV. In this article we review the current knowledge and potential role of CD4(+)CD25(+) T(Reg) cells in HIV infection.     

Antigen-specific T cell repertoire modification of CD4+CD25+ regulatory T cells

T cell immune responses are regulated by the interplay between effector and suppressor T cells. Immunization with Ag leads to the selective expansion and survival of effector CD4(+) T cells with high affinity TCR against the Ag and MHC. However, it is not known if CD4(+)CD25(+) regulatory T cells (T(reg)) recognize the same Ag as effector T cells or whether Ag-specific TCR repertoire modification occurs in T(reg). In this study, we demonstrate that after a primary Ag challenge, T(reg) proliferate and TCR repertoire modification is observed although both of these responses were lower than those in conventional T cells. The repertoire modification of Ag-specific T(reg) after primary Ag challenge augmented the total suppressive function of T(reg) against TCR repertoire modification but not against the proliferation of memory CD4(+) T cells. These results reveal that T cell repertoire modification against a non-self Ag occurs in T(reg), which would be crucial for limiting excess primary and memory CD4(+) T cell responses. In addition, these studies provide evidence that manipulation of Ag-specific T(reg) is an ideal strategy for the clinical use of T(reg).     

CD4+CD25bright T cells in human intestinal lamina propria as regulatory cells

It is well known that immune responses in the intestine remain in a state of controlled inflammation, suggesting that not only active suppression by regulatory T cells plays an important role in the normal intestinal homeostasis, but also its dysregulation leads to the development of inflammatory bowel disease. In this study, we demonstrate that the CD4(+)CD25(bright) T cells reside in the human intestinal lamina propria (LP) and functionally retain regulatory activities. All human LP CD4(+) T cells regardless of CD25 expression constitutively expressed CTLA-4, glucocorticoid-induced TNFR family-related protein, and Foxp3 and proliferate poorly. Although LP CD4(+)CD25(-) T cells showed an activated and anergic/memory phenotype, they did not retain regulatory activity. In LP CD4(+)CD25(+) T cells, however, cells expressing CD25 at high levels (CD4(+)CD25(bright)) suppressed the proliferation and various cytokine productions of CD4(+)CD25(-) T cells. LP CD4(+)CD25(bright) T cells by themselves produced fewer amounts of IL-2, IFN-gamma, and IL-10. Interestingly, LP CD4(+)CD25(bright) T cells with regulatory T activity were significantly increased in patients with active inflammatory bowel disease. These results suggest that CD4(+)CD25(bright) T cells found in the normal and inflamed intestinal mucosa selectively inhibit the host immune response and therefore may contribute to the intestinal immune homeostasis.     

Influence of CD4+CD25+ regulatory T cells on low/high-avidity CD4+ T cells following peptide vaccination

We have recently reported that NY-ESO-1-specific naive CD4+ T cell precursors exist in most individuals but are suppressed by CD4+CD25+ regulatory T cells (Tregs), while memory CD4+ T cell effectors against NY-ESO-1 are found only in cancer patients with spontaneous Ab responses to NY-ESO-1. In this study, we have analyzed mechanisms of CD4+ T cell induction following peptide vaccination in relation to susceptibility to Tregs. Specific HLA-DP4-restricted CD4+ T cell responses were elicited after vaccination with NY-ESO-1(157-170) peptide (emulsified in IFA) in patients with NY-ESO-1-expressing epithelial ovarian cancer. These vaccine-induced CD4+ T cells were detectable from effector/memory populations without requirement for in vitro CD4+CD25+ T cell depletion. However, they were only able to recognize NY-ESO-1(157-170) peptide but not naturally processed NY-ESO-1 protein and had much lower avidity compared with NY-ESO-1-specific pre-existing naive CD4+CD25- T cell precursors or spontaneously induced CD4+ T cell effectors of cancer patients with NY-ESO-1 Ab. We propose that vaccination with NY-ESO-1(157-170) peptide recruits low-avidity T cells with low sensitivity to Tregs and fails to modulate the suppressive effect of Tregs on high-avidity NY-ESO-1-specific T cell precursors.     

CD4+CD25+ regulatory T cells inhibit the antigen-dependent expansion of self-reactive T cells in vivo

A deficiency of CD4+CD25+ regulatory T cells (CD25+ Tregs) in lymphopenic mice can result in the onset of autoimmune gastritis. The gastric H/K ATPase alpha (H/Kalpha) and beta (H/Kbeta) subunits are the immunodominant autoantigens recognized by effector CD4+ T cells in autoimmune gastritis. The mechanism by which CD25+ Tregs suppress autoimmune gastritis in lymphopenic mice is poorly understood. To investigate the antigenic requirements for the genesis and survival of gastritis-protecting CD25+ Tregs, we analyzed mice deficient in H/Kbeta and H/Kalpha, as well as a transgenic mouse line (H/Kbeta-tsA58 Tg line 224) that lacks differentiated gastric epithelial cells. By adoptive transfer of purified T cell populations to athymic mice, we show that the CD25+ Treg population from mice deficient in either one or both of H/Kalpha and H/Kbeta, or from the H/Kbeta-tsA58 Tg line 224 mice, is equally effective in suppressing the ability of polyclonal populations of effector CD4+ T cells to induce autoimmune gastritis. Furthermore, CD25+ Tregs, from either wild-type or H/Kalpha-deficient mice, dramatically reduced the expansion of pathogenic H/Kalpha-specific TCR transgenic T cells and the induction of autoimmune gastritis in athymic recipient mice. Proliferation of H/Kalpha-specific T cells in lymphopenic hosts occurs predominantly in the paragastric lymph node and was dependent on the presence of the cognate H/Kalpha Ag. Collectively, these studies demonstrate that the gastritis-protecting CD25+ Tregs do not depend on the major gastric Ags for their thymic development or their survival in the periphery, and that CD25+ Tregs inhibit the Ag-specific expansion of pathogenic T cells in vivo.     

CD4+CD25+ T regulatory cells suppress NK cell-mediated immunotherapy of cancer

CD4+CD25+ regulatory T cells (Treg) that suppress T cell-mediated immune responses may also regulate other arms of an effective immune response. In particular, in this study we show that Treg directly inhibit NKG2D-mediated NK cell cytotoxicity in vitro and in vivo, effectively suppressing NK cell-mediated tumor rejection. In vitro, Treg were shown to inhibit NKG2D-mediated cytolysis largely by a TGF-beta-dependent mechanism and independently of IL-10. Adoptively transferred Treg suppressed NK cell antimetastatic function in RAG-1-deficient mice. Depletion of Treg before NK cell activation via NKG2D and the activating IL-12 cytokine, dramatically enhanced NK cell-mediated suppression of tumor growth and metastases. Our data illustrate at least one mechanism by which Treg can suppress NK cell antitumor activity and highlight the effectiveness of combining Treg inhibition with subsequent NK cell activation to promote strong innate antitumor immunity.     

Gamma irradiation alters the phenotype and function of CD4CD25 regulatory T cells

To examine the effects of gamma irradiation on Tregs, changes in phenotype and suppression function in Tregs treated with or without gamma ray were analyzed. Purified CD4⁺CD25⁺ regulatory T cells were irradiated at different dosages with a ¹³⁷Cs source gamma ray at 4.8 Gy/min. After culture, the phenotype and function changes were determined by flow cytometry and [³H]-thymidine incorporation, respectively. A dose-dependent reduction of Tregs proliferation in response to gamma irradiation was noted, which paralleled the apoptosis induction of Tregs. Gamma irradiation downregulated the Tregs expression of CD45RO, CD62L, FOXP3, membrane TGF-β, but upregulated Bax and GITR. High dose gamma irradiation (30 Gy) significantly abolished the suppression of Tregs on CD4⁺CD25⁻ T cells proliferation. Thus Tregs not only influences the phenotype but also alters their suppressive capacities. Our findings suggest that radiotherapy may be an important strategy to alter the immunologic balance of Tregs and effector cells in cancer therapy.     

Therapeutic potential of CD4+ CD25+ regulatory T cells in allogeneic transplantation

The subpopulation of CD4+ CD25+ immunoregulatory T cells constitutes less than of the entire CD4+ T-cell pool in mice and 2% in humans. These cells play a crucial role in the control of autoimmune processes. More recently, in vitro and in vivo data also indicate that CD4+ CD25+ immunoregulatory T cells can regulate alloreactivity. This renders them good candidates for innovative strategies in the field of transplantation. Inducing a state of immune tolerance with immunoregulatory T cells would alleviate the need for immunosuppression, and the occurrence of late allograft failure represents a major goal of transplantation immunology. Here we discuss how these naturally occurring CD4+ CD25+ immunoregulatory T cells can be used to modulate alloreactivity in hematopoietic stem cell and solid organ transplantation.     

CD4+CD25+ regulatory T cells control innate immune reactivity after injury

Major injury initiates a systemic inflammatory response that can be detrimental to the host. We have recently reported that burn injury primes innate immune cells for a progressive increase in TLR4 and TLR2 agonist-induced proinflammatory cytokine production and that this inflammatory phenotype is exaggerated in adaptive immune system-deficient (Rag1(-/-)) mice. The present study uses a series of adoptive transfer experiments to determine which adaptive immune cell type(s) has the capacity to control innate inflammatory responses after injury. We first compared the relative changes in TLR4- and TLR2-induced TNF-alpha, IL-1beta, and IL-6 production by spleen cell populations prepared from wild-type (WT), Rag1(-/-), CD4(-/-), or CD8(-/-) mice 7 days after sham or burn injury. Our findings indicated that splenocytes prepared from burn-injured CD8(-/-) mice displayed TLR-induced cytokine production levels similar to those in WT mice. In contrast, spleen cells from burn-injured CD4(-/-) mice produced cytokines at significantly higher levels, equivalent to those in Rag1(-/-) mice. Moreover, reconstitution of Rag1(-/-) or CD4(-/-) mice with WT CD4(+) T cells reduced postinjury cytokine production to WT levels. Additional separation of CD4(+) T cells into CD4(+)CD25(+) and CD4(+)CD25(-) subpopulations before their adoptive transfer into Rag1(-/-) mice showed that CD4(+)CD25(+) T cells were capable of reducing TLR-stimulated cytokine production levels to WT levels, whereas CD4(+)CD25(-) T cells had no regulatory effect. These findings suggest a previously unsuspected role for CD4(+)CD25(+) T regulatory cells in controlling host inflammatory responses after injury.     

Activation of CD25(+)CD4(+) regulatory T cells by oral antigen administration

CD25(+)CD4(+) T cells are naturally occurring regulatory T cells that are anergic and have suppressive properties. Although they can be isolated from the spleens of normal mice, there are limited studies on how they can be activated or expanded in vivo. We found that oral administration of OVA to OVA TCR transgenic mice resulted in a modification of the ratio of CD25(+)CD4(+) to CD25(-)CD4(+) cells with an increase of CD25(+)CD4(+) T cells accompanied by a decrease of CD25(-)CD4(+) T cells. The relative increase in CD25(+)CD4(+) T cells persisted for as long as 4 wk post feeding. We also found that CTLA-4 was dominantly expressed in CD25(+)CD4(+) T cells and there was an increase in the percentage of CD25(+)CD4(+) T cells expressing CTLA-4 in OVA-fed mice. In contrast to CD25(-)CD4(+) cells, CD25(+)CD4(+) cells from fed mice proliferated only minimally to OVA or anti-CD3 and secreted IL-10 and elevated levels of TGF-beta(1) following anti-CD3 stimulation. CD25(+)CD4(+) cells from fed mice suppressed the proliferation of CD25(-)CD4(+) T cells in vitro more potently than CD25(+)CD4(+) T cells isolated from unfed mice, and this suppression was partially reversible by IL-10 soluble receptor or TGF-beta soluble receptor and high concentration of anti-CTLA-4. With anti-CD3 stimulation, CD25(+)CD4(+) cells from unfed mice secreted IFN-gamma, whereas CD25(+)CD4(+) cells from fed mice did not. Adoptive transfer of CD25(+)CD4(+) T cells from fed mice suppressed in vivo delayed-type hypersensitivity responses in BALB/c mice. These results demonstrate an Ag-specific in vivo method to activate CD25(+)CD4(+) regulatory T cells and suggest that they may be involved in oral tolerance.     

CD25- T cells generate CD25+Foxp3+ regulatory T cells by peripheral expansion

Naturally occurring CD4(+) regulatory T cells are generally identified through their expression of CD25. However, in several experimental systems considerable T(reg) activity has been observed in the CD4(+)CD25(-) fraction. Upon adoptive transfer, the expression of CD25 in donor-derived cells is not stable, with CD4(+)CD25(+) cells appearing in CD4(+)CD25(-) T cell-injected animals and vice versa. We show in this study that CD25(+) cells arising from donor CD25(-) cells upon homeostatic proliferation in recipient mice express markers of freshly isolated T(reg) cells, display an anergic state, and suppress the proliferation of other cells in vitro. The maintenance of CD25 expression by CD4(+)CD25(+) cells depends on IL-2 secreted by cotransferred CD4(+)CD25(-) or by Ag-stimulated T cells in peripheral lymphoid organs.     

Human CD4+CD25+ regulatory T cells share equally complex and comparable repertoires with CD4+CD25- counterparts

CD4(+)CD25(+) T cells are critical mediators of peripheral immune tolerance. However, many developmental and functional characteristics of these cells are unknown, and knowledge of human regulatory T cells is particularly limited. To better understand how human CD4(+)CD25(+) T cells develop and function, we examined the diversity of CD4(+)CD25(+) and CD4(+)CD25(-) T cell repertoires in both thymus and peripheral blood. Levels of T receptor excision circles (TREC) were comparable in purified CD4(+)CD25(+) and CD4(+)CD25(-) thymic populations, but were significantly higher than those in samples derived from peripheral blood, consistent with murine studies demonstrating thymic development of CD4(+)CD25(+) regulatory T cells. Surprisingly, CD4(+)CD25(-) T cells isolated from peripheral blood had greater TREC quantities than their CD4(+)CD25(+) counterparts, supporting the possibility of extrathymic expansion as well. CD4(+)CD25(+) and CD4(+)CD25(-) T cells from a given individual showed overlapping profiles with respect to diversity by Vbeta staining and spectratyping. Interestingly, CD4(+)CD25(+) T cells have lower quantities of CD3 than CD4(+)CD25(-) T cells. Collectively, these data suggest that human CD4(+)CD25(+) T cells recognize a similar array of Ags as CD4(+)CD25(-) T cells. However, reduced levels of TCR on regulatory T cells suggest different requirements for activation and may contribute to how the immune system regulates whether a particular response is suppressed or augmented.     

Platelet factor 4 differentially modulates CD4+CD25+ (regulatory) versus CD4+CD25- (nonregulatory) T cells

Active suppression mediated by CD4(+)CD25(+) T regulatory (Tr) cells plays an important role in the down-regulation of T cell responses to both foreign and self-Ags. Platelet factor 4 (PF4), a platelet-derived CXC chemokine, has been shown to strongly inhibit T cell proliferation as well as IFN-gamma and IL-2 release by isolated T cells. In this report we show that human PF4 stimulates proliferation of the naturally anergic human CD4(+)CD25(+) Tr cells while inhibiting proliferation of CD4(+)CD25(-) T cells. In coculture experiments we found that CD4(+)CD25(+) Tr cells exposed to PF4 lose the ability to inhibit the proliferative response of CD4(+)CD25(-) T cells. Our findings suggest that human PF4, by inducing Tr cell proliferation while impairing Tr cell function, may play a previously unrecognized role in the regulation of human immune responses. Because platelets are the sole source of PF4 in the circulation, these findings may be relevant to the pathogenesis of certain immune-mediated disorders associated with platelet activation, such as heparin-induced thrombocytopenia and autoimmune thrombocytopenic purpura.     

Immunomodulation after allogeneic bone marrow transplantation by CD4+CD25+ regulatory T cells

Graft-versus-host disease is a major complication after allogeneic bone marrow transplantation (BMT) caused by donor T cells. Immunosuppression mediated by CD4(+)CD25(+) regulatory T cells has been shown to ameliorate such pathogenic immune responses in animal models. Here, we summarize recent findings from experimental and clinical studies and propose a model for peripheral tolerance induction after BMT.     

Distinct IL-2 receptor signaling pattern in CD4+CD25+ regulatory T cells

Despite expression of the high-affinity IL-2R, CD4(+)CD25(+) regulatory T cells (Tregs) are hypoproliferative upon IL-2R stimulation in vitro. However the mechanisms by which CD4(+)CD25(+) T cells respond to IL-2 signals are undefined. In this report, we examine the cellular and molecular responses of CD4(+)CD25(+) Tregs to IL-2. IL-2R stimulation results in a G(1) cell cycle arrest, cellular enlargement and increased cellular survival of CD4(+)CD25(+) T cells. We find a distinct pattern of IL-2R signaling in which the Janus kinase/STAT pathway remains intact, whereas IL-2 does not activate downstream targets of phosphatidylinositol 3-kinase. Negative regulation of phosphatidylinositol 3-kinase signaling and IL-2-mediated proliferation of CD4(+)CD25(+) T cells is inversely associated with expression of the phosphatase and tensin homologue deleted on chromosome 10, PTEN.     

CD4+CD25+ regulatory T cells control the severity of viral immunoinflammatory lesions

CD4(+)CD25(+) regulatory T cells (T(reg)) can inhibit a variety of autoimmune and inflammatory diseases, but their involvement in regulating virus-induced immunopathology is not known. We have evaluated the role of T(reg) in viral immunopathological lesion stromal keratitis. This frequent cause of human blindness results from a T cell-mediated immunoinflammatory response to HSV in the corneal stroma. The results show that lesions were significantly more severe if mice were depleted of T(reg) before infection. The T(reg) was also shown to modulate lesion expression induced by adoptive transfer of pathogenic CD4(+) T cells in infected SCID recipients. The mechanism of T(reg) control of stromal keratitis involved suppressed antiviral immunity and impaired expression of the molecule required for T cell migration to lesion sites. Interestingly, T(reg) isolated from ocular lesions in nondepleted mice showed in vitro inhibitory effects involving IL-10, but were not very effective in established lesions. Our results decipher the in vivo role of T(reg) in a virus-induced immunopathology and imply that manipulation of regulatory cell function represents a useful approach to control viral-induced immunoinflammatory disease.