B7+ iris pigment epithelium induce CD8+ T regulatory cells; both suppress CTLA-4+ T cells

Ocular pigment epithelia contribute to immune privilege by suppressing T cell activation and converting T cells into regulatory T regulatory cells (Tregs) that inhibit bystander T cell activation. Iris pigment epithelium (IPE) does so through direct cell-cell contact with naive T cells, and this suppressive contact is via interactions between B7 expressed constitutively on IPE cells and CTLA-4 expressed on a subpopulation of CD8+ T cells. We have now examined whether TGFbeta is required in this process. We report that IPE produces both soluble and membrane-bound active TGFbeta, but that only the latter is actually delivered to CD8+ T cells. In turn, these T cells become IPE Tregs by up-regulating their own expression of B7-1/B7-2 and soluble and membrane-bound TGFbeta. IPE Tregs through their expression of B7 are able to engage CTLA-4+ bystander T cells, and thus precisely, target delivery of membrane-bound TGFbeta. We propose that this mechanism of suppression via TGFbeta ensures that soluble active TGFbeta is not released into the ocular microenvironment where it can have unregulated and deleterious effects, including elevation of intraocular pressure and development of glaucoma.     

B7-deficient autoreactive T cells are highly susceptible to suppression by CD4(+)CD25(+) regulatory T cells

CD4(+)CD25(+) regulatory T cells (Tregs) suppress immunity to infections and tumors as well as autoimmunity and graft-vs-host disease. Since Tregs constitutively express CTLA-4 and activated T cells express B7-1 and B7-2, it has been suggested that the interaction between CTLA-4 on Tregs and B7-1/2 on the effector T cells may be required for immune suppression. In this study, we report that autopathogenic T cells from B7-deficient mice cause multiorgan inflammation when adoptively transferred into syngeneic RAG-1-deficient hosts. More importantly, this inflammation is suppressed by adoptive transfer of purified wild-type (WT) CD4(+)CD25(+) T cells. WT Tregs also inhibited lymphoproliferation and acquisition of activation markers by the B7-deficient T cells. An in vitro suppressor assay revealed that WT and B7-deficient T cells are equally susceptible to WT Treg regulation. These results demonstrate that B7-deficient T cells are highly susceptible to immune suppression by WT Tregs and refute the hypothesis that B7-CTLA-4 interaction between effector T cells and Tregs plays an essential role in Treg function.     

Ligation of B7-1/B7-2 by human CD4+ T cells triggers indoleamine 2,3-dioxygenase activity in dendritic cells

Human monocyte-derived dendritic cells (DCs) are capable of expressing the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO), which allows them to suppress Ag-driven proliferation of T cells in vitro. In DCs that express IDO, the activity of the enzyme is tightly regulated, with the protein being constitutively expressed, but functional activity requiring an additional set of triggering signals supplied during Ag presentation. We now show that triggering of functional IDO obligately requires ligation of B7-1/B7-2 molecules on the DCs by CTLA4/CD28 expressed on T cells. When this interaction was disrupted, IDO remained in the inactive state, and the DCs were unable to inhibit T cell proliferation. Inhibition could be fully restored by direct Ab-mediated cross-linking of B7-1/B7-2. Although both CD4(+) and CD8(+) T cells were susceptible to inhibition once IDO was induced, the ability to trigger functionally active IDO was strictly confined to the CD4(+) subset. Thus, the ability of CD4(+) T cells to induce IDO activity in DCs allowed the CD4(+) population to dominantly inhibit proliferation of the CD8(+) population via the bridge of a conditioned DC. We hypothesize that IDO activation via engagement of B7-1/B7-2 molecules on DCs, specifically, engagement by CTLA4 expressed on regulatory CD4(+) T cells, may function as a physiologic regulator of T cell responses in vivo.     

B7-independent inhibition of T cells by CTLA-4

CTLA-4 is an inhibitory molecule that regulates T cell expansion and differentiation. CTLA-4 binding to B7-1/B7-2 is believed to be crucial for its inhibitory signal both by competing for CD28 binding to the same ligands and aggregating CTLA-4 to deliver negative signals. In this study, we demonstrate that B7 binding is not essential for CTLA-4 activity. CTLA-4 knockout T cells are hyperresponsive compared with wild-type T cells in B7-free settings. Expression of a B7-nonbinding CTLA-4 mutant inhibited T cell proliferation, cytokine production, and TCR-mediated ERK activation in otherwise CTLA-4-deficient T cells. Finally, transgenic expression of the ligand-nonbinding CTLA-4 mutant delayed the lethal lymphoproliferation observed in CTLA-4-deficient mice. These results suggest that ligand binding is not essential for the CTLA-4 function and supports an essential role for CTLA-4 signaling during T cell activation.     

Targeted CTLA-4 engagement induces CD4+CD25+CTLA-4high T regulatory cells with target (allo)antigen specificity

CTLA-4 (CD152) is actively involved in down-regulating T cell activation and maintaining lymphocyte homeostasis. Our earlier studies showed that targeted engagement of CTLA-4 can down-modulate T cell response and suppress allo- and autoimmune responses. In this study, we report that targeted CTLA-4 engagement can induce immune tolerance to a specific target through selective induction of an Ag-specific CD4(+)CD25(+)CTLA-4(high) regulatory T cell (Treg cell) population. Allogeneic cells coated with anti-CTLA-4 Ab induced immune hyporesponsiveness through suppression of proinflammatory cytokines IFN-gamma and IL-2, and up-regulation of the regulatory cytokines IL-10, TGF-beta1, and IL-4, presumably through the engagement of CTLA-4 on activated T cells. Although rechallenge with alloantigen failed to break the unresponsiveness, a transient recovery from tolerance was observed in the presence of high concentrations of exogenous IL-2, saturating concentrations of neutralizing anti-TGF-beta1 and anti-IL-10 Abs, and blocking anti-CTLA-4 Ab, and upon depletion of CD4(+)CD25(+) Treg cells. The CD4(+)CD25(+)CTLA-4(high) Treg cells from tolerant mice suppressed the effector function of CD25(-) T cells from Ag-primed mice. Adoptive transfer of these Treg cells into Ag-primed mice resulted in a significantly reduced alloantigen-specific response. Further characterization demonstrated that the Treg cells with memory phenotype (CD62L(-)) were more potent in suppressing the alloantigen-specific T cell response. These results strongly support that the targeted engagement of CTLA-4 has therapeutic potential for the prevention of transplant rejection.     

CD80 (B7-1) and CD86 (B7-2) are functionally equivalent in the initiation and maintenance of CD4+ T-cell proliferation after activation with suboptimal doses of PHA

Effective activation of T cells requires engagement of two separate T-cell receptors. The antigen-specific T-cell receptor (TCR) binds foreign peptide antigen-MHC complexes, and the CD28 receptor binds to the B7 (CD80/CD86) costimulatory molecules expressed on the surface of antigen-presenting cells (APC). The simultaneous triggering of these T-cell surface receptors with their specific ligands results in an activation of this cell. In contrast, CTLA-4 (CD152) is a distinct T-cell receptor that, upon binding to B7 molecules, sends an inhibitory signal to T cell activation. Many in vitro and in vivo studies demonstrated that both CD80 and CD86 ligands have an identical role in the activation of T cells. Recently, functions of B7 costimulatory molecules in vivo have been investigated in B7-1 and/or B7-2 knockout mice, and the authors concluded that CD86 could be more important for initiating T-cell responses, while CD80 could be more significant for maintaining these immune responses. In this study, we directly compared the role of CD80 and CD86 in initiating and maintaining proliferation of resting CD4(+) T cells in an in vitro mode system that allowed to provide the first signal-to-effector cells through the use of suboptimal doses of PHA and the second costimulatory signal through cells expressing CD80 or CD86, but not any other costimulatory molecules. Using this experimental system we demonstrate that the CD80 and CD86 molecules can substitute for each other in the initial activation of resting CD4(+) T cells and in the maintenance of their proliferative response.     

Enhanced engagement of CTLA-4 induces antigen-specific CD4+CD25+Foxp3+ and CD4+CD25- TGF-beta 1+ adaptive regulatory T cells

CTLA-4 is a critical negative regulator of T cell response and is instrumental in maintaining immunological tolerance. In this article, we report that enhanced selective engagement of CTLA-4 on T cells by Ag-presenting dendritic cells resulted in the induction of Ag-specific CD4(+)CD25(+)Foxp3(+) and CD4(+)CD25(-)TGF-beta1(+) adaptive Tregs. These cells were CD62L(low) and hyporesponsive to stimulation with cognate Ag but demonstrated a superior ability to suppress Ag-specific effector T cell response compared with their CD62L(high) counterparts. Importantly, treatment of mice with autoimmune thyroiditis using mouse thyroglobulin (mTg)-pulsed anti-CTLA-4 agonistic Ab-coated DCs, which results in a dominant engagement of CTLA-4 upon self-Ag presentation, not only suppressed thyroiditis but also prevented reemergence of the disease upon rechallenge with mTg. Further, the disease suppression was associated with significantly reduced mTg-specific T cell and Ab responses. Collectively, our results showed an important role for selective CTLA-4 signaling in the induction of adaptive Tregs and suggested that approaches that allow dominant CTLA-4 engagement concomitant with Ag-specific TCR ligation can be used for targeted therapy.     

Intestinal lamina propria retaining CD4+CD25+ regulatory T cells is a suppressive site of intestinal inflammation

It is well known that immune responses in the intestine remain in a state of controlled inflammation, suggesting that not only does active suppression by regulatory T (T(REG)) cells play an important role in the normal intestinal homeostasis, but also that its dysregulation of immune response leads to the development of inflammatory bowel disease. In this study, we demonstrate that murine CD4(+)CD25(+) T cells residing in the intestinal lamina propria (LP) constitutively express CTLA-4, glucocorticoid-induced TNFR, and Foxp3 and suppress proliferation of responder CD4(+) T cells in vitro. Furthermore, cotransfer of intestinal LP CD4(+)CD25(+) T cells prevents the development of chronic colitis induced by adoptive transfer of CD4(+)CD45RB(high) T cells into SCID mice. When lymphotoxin (LT)alpha-deficient intercrossed Rag2 double knockout mice (LTalpha(-/-) x Rag2(-/-)), which lack mesenteric lymph nodes and Peyer's patches, are transferred with CD4(+)CD45RB(high) T cells, they develop severe wasting disease and chronic colitis despite the delayed kinetics as compared with the control LTalpha(+/+) x Rag2(-/-) mice transferred with CD4(+)CD45RB(high) T cells. Of note, when a mixture of splenic CD4(+)CD25(+) T(REG) cells and CD4(+)CD45RB(high) T cells are transferred into LTalpha(-/-) x Rag2(-/-) recipients, CD4(+)CD25(+) T(REG) cells migrate into the colon and prevent the development of colitis in LTalpha(-/-) x Rag2(-/-) recipients as well as in the control LTalpha(+/+) x Rag2(-/-) recipients. These results suggest that the intestinal LP harboring CD4(+)CD25(+) T(REG) cells contributes to the intestinal immune suppression.     

Loss of IL-7 receptor alpha on CD4+ T cells defines terminally differentiated B cell-helping effector T cells in a B cell-rich lymphoid tissue

IL-7 plays important roles in development and homeostatic proliferation of lymphocytes. IL-7 uses a receptor composed of IL-7Ralpha (CD127) and the common gamma-chain (CD132) to transmit its signal. It has been unknown how CD127 is regulated during Th cell differentiation to the B cell-helping T cell lineage. In this study, we report that loss of CD127 defines terminally differentiated B cell-helping effector T cells in human tonsils. Although naive CD4(+) T cells uniformly express CD127, the memory/effector (non-FOXP3(+)) CD4(+) T cells are divided into CD127(+) and CD127(-) cells. The CD127(-) T cells are exclusively localized within the germinal centers where B cells become plasma and memory B cells, whereas CD127(+) T cells are found in T cell areas and the area surrounding B cell follicles. Consistently, the CD127(-) T cells highly express the B cell zone homing receptor CXCR5 with concomitant loss of CCR7. Compared with CD127(+) memory T cells, CD127(-) T cells have considerably shorter telomeres, do not proliferate in response to IL-7, and are prone to cell death. The CD127(-) T cells produce a large amount of the B cell follicle-forming chemokine CXCL13 upon stimulation with B cells and Ags. Most importantly, they are highly efficient in helping B cells produce Igs of all isotypes in a manner dependent on CD40L and ICOS and inducing activation-induced cytidine deaminase and Ig class switch recombination. The selective loss of CD127 on the B cell-helping effector T cells would have implications in regulation and termination of Ig responses.     

Spontaneous colitis occurrence in transgenic mice with altered B7-mediated costimulation

The B7 costimulatory molecules govern many aspects of T cell immune responses by interacting with CD28 for costimulation, but also with CTLA-4 for immune suppression. Although blockade of CTLA-4 with Ab in humans undergoing cancer immune therapy has led to some cases of inflammatory bowel disease, spontaneous animal models of colitis that depend upon modulation of B7 interactions have not been previously described. In this study, we demonstrate that mice expressing a soluble B7-2 Ig Fc chimeric protein spontaneously develop colitis that is dependent on CD28-mediated costimulation of CD4(+) T cells. We show that the chimeric protein has mixed agonistic/antagonist properties, and that it acts in part by blocking the cell intrinsic effects on T cell activation of engagement of CTLA-4. Disease occurred in transgenic mice that lack expression of the endogenous B7 molecules (B7 double knock-out mice), because of the relatively weak costimulatory delivered by the chimeric protein. Surprisingly, colitis was more severe in this context, which was associated with the decreased number of Foxp3(+) regulatory T cells in transgenic B7 double knock-out mice. This model provides an important tool for examining how B7 molecules and their effects on CTLA-4 modulate T cell function and the development of inflammatory diseases.     

Human CD25+ regulatory T cells maintain immune tolerance to nickel in healthy, nonallergic individuals

We investigated the capacity of CD25(+) T regulatory cells (Treg) to modulate T cell responses to nickel, a common cause of allergic contact dermatitis. CD4(+) T cells isolated from the peripheral blood of six healthy, nonallergic individuals showed a limited capacity to proliferate in response to nickel in vitro, but responsiveness was strongly augmented (mean increment +/- SD, 240 +/- 60%) when cells were depleted of CD25(+) Treg. Although CD25(+) Treg were anergic to nickel, a small percentage up-regulated membrane CTLA-4 upon nickel exposure. CD25(+) Treg strongly and dose-dependently inhibited nickel-specific activation of CD25(-) T lymphocytes in coculture experiments in a cytokine-independent, but cell-to-cell contact-dependent, manner. Approximately 30% of circulating CD25(+) Treg expressed the cutaneous lymphocyte-associated Ag (CLA), and CLA(+)CD25(+) Treg were more efficient than CLA(-)CD25(+) cells in suppressing nickel responsiveness of CD25(-) T cells. The site of a negative patch test in response to nickel showed an infiltrate of CD4(+)CLA(+) cells and CD25(+) cells, which accounted for approximately 20% of the total T cells isolated from the tissue. Skin-derived T cells suppressed nickel-specific responses of peripheral blood CD25(-) T cells. In addition, 60 +/- 14% of peripheral blood CD25(+) Treg expressed the chemokine receptor CCR7 and strongly inhibited naive T cell activation in response to nickel. Finally, CD25(+) T cells isolated from peripheral blood of nickel-allergic patients showed a limited or absent capacity to suppress metal-specific CD4(+) and CD8(+) T cell responses. The results indicates that in healthy individuals CD25(+) Treg can control the activation of both naive and effector nickel-specific T cells.     

Human peripheral blood T regulatory cells (Tregs), functionally primed CCR4+ Tregs and unprimed CCR4- Tregs, regulate effector T cells using FasL

Regulatory CD25(+)CD4(+) T cells (Tregs) play an important role in the control of peripheral tolerance. In this study we demonstrate that human peripheral blood Tregs can be divided into two distinct populations based on the expression of CCR4. The majority ( approximately 75%) of freshly isolated Tregs express CCR4 and presumably represent memory-type Tregs. Interestingly, CCR4(-) Tregs require anti-CD3 Ab-mediated activation to acquire a regulatory activity, while CCR4(+) Tregs appear to be already primed to suppress the proliferation of CD8(+) T cells. CCR4 is also expressed on CD25(low)CD4(+) T cells (CCR4(+) non-Tregs) that mostly suppress Th1-type polarization without affecting T cell proliferation, presumably via the production of immunomodulatory cytokines like IL-10. In contrast, CCR4(+) Tregs express FasL to primarily regulate T cell proliferation via a contact-mediated process involving FasL/Fas signaling, a major regulatory pathway of T cell homeostasis. Finally, we also demonstrate that the depletion of CCR4(+) T cells leads to Th1-type polarization of CD4(+) T cells and augmentation of CD8(+) T cell responses to tumor Ags.     

Distinct effects of STAT5 activation on CD4+ and CD8+ T cell homeostasis: development of CD4+CD25+ regulatory T cells versus CD8+ memory T cells

Using transgenic mice that express a constitutively active version of STAT5b, we demonstrate that STAT5 plays a key role in governing B cell development and T cell homeostasis. STAT5 activation leads to a 10-fold increase in pro-B, but not pro-T, cells. Conversely, STAT5 signaling promotes the expansion of mature alphabeta T cells (6-fold increase) and gammadelta and NK T cells (3- to 4-fold increase), but not of mature B cells. In addition, STAT5 activation has dramatically divergent effects on CD8(+) vs CD4(+) T cells, leading to the selective expansion of CD8(+) memory-like T cells and CD4(+)CD25(+) regulatory T cells. These results establish that activation of STAT5 is the primary mechanism underlying both IL-7/IL-15-dependent homeostatic proliferation of naive and memory CD8(+) T cells and IL-2-dependent development of CD4(+)CD25(+) regulatory T cells.     

CD4+CD25+ and CD4+CD25- T cells act respectively as inducer and effector T suppressor cells in superantigen-induced tolerance

The repeated injection of low doses of bacterial superantigens (SAg) is known to induce specific T cell unresponsiveness. We show in this study that the spleen of BALB/c mice receiving chronically, staphylococcal enterotoxin B (SEB) contains SEB-specific CD4(+) TCRBV8(+) T cells exerting an immune regulatory function on SEB-specific primary T cell responses. Suppression affects IL-2 and IFN-gamma secretion as well as proliferation of T cells. However, the suppressor cells differ from the natural CD4(+) T regulatory cells, described recently in human and mouse, because they do not express cell surface CD25. They are CD152 (CTLA-4)-negative and their regulatory activity is not associated with expression of the NF Foxp3. By contrast, after repeated SEB injection, CD4(+)CD25(+) splenocytes were heterogenous and contained both effector as well as regulatory cells. In vivo, CD4(+)CD25(-) T regulatory cells prevented SEB-induced death independently of CD4(+)CD25(+) T cells. Nevertheless, SEB-induced tolerance could not be achieved in thymectomized CD25(+) cell-depleted mice because repeated injection of SEB did not avert lethal toxic shock in these animals. Collectively, these data demonstrate that, whereas CD4(+)CD25(+) T regulatory cells are required for the induction of SAg-induced tolerance, CD4(+)CD25(-) T cells exert their regulatory activity at the maintenance stage of SAg-specific unresponsiveness.     

Role of thrombospondin-1 in T cell response to ocular pigment epithelial cells

Ocular pigment epithelium (PE) cells promote the generation of T regulators (PE-induced Treg cells). Moreover, T cells exposed to PE acquire the capacity to suppress the activation of bystander T cells via TGFbeta. Membrane-bound TGFbeta on iris PE cells interacts with TGFbeta receptors on T cells, leading to the conversion of T cells to CD8(+) Treg cells via a cell contact-dependent mechanism. Conversely, soluble forms of TGFbeta produced by retinal PE cells can convert CD4(+) T cells into Treg cells in a manner that is independent of cell contact. In this study, we looked at the expression of immunoregulatory factors (TGFbeta, thrombospondins, CD59, IL-1 receptor antagonist, etc.) in PE cells as identified via an oligonucleotide microarray. Several thrombospondin-binding molecules were detected, and thus we focused subsequent analyses on thrombospondins. Via the conversion of latent TGFbeta to an active form that appears to be mediated by thrombospondin 1 (TSP-1), cultured iris PE and retinal PE cells induce a PE-induced Treg cell fate. After conversion, both ocular PE and PE-induced Treg cells express TSP-1. Regulatory T cell generation was amplified when the T cells also expressed TSP-1. In addition, PE-induced Treg cells significantly suppressed activation of bystander T cells via TSP-1. These results strongly suggest that the ability of ocular PE and PE-induced Treg cells to suppress bystander T cells depends on their capacity to produce TSP-1. Thus, intraocular TSP-1 produced by both ocular parenchymal cells and regulatory T cells is essential for immune regulation in the eye.     

CD28-independent costimulation of T cells in alloimmune responses.

T cell costimulation by B7 molecules plays an important role in the regulation of alloimmune responses. Although both B7-1 and B7-2 bind CD28 and CTLA-4 on T cells, the role of B7-1 and B7-2 signaling through CTLA-4 in regulating alloimmune responses is incompletely understood. To address this question, we transplanted CD28-deficient mice with fully allogeneic vascularized cardiac allografts and studied the effect of selective blockade of B7-1 or B7-2. These mice reject their grafts by a mechanism that involves both CD4(+) and CD8(+) T cells. Blockade of CTLA-4 or B7-1 significantly accelerated graft rejection. In contrast, B7-2 blockade significantly prolonged allograft survival and, unexpectedly, reversed the acceleration of graft rejection caused by CTLA-4 blockade. Furthermore, B7-2 blockade prolonged graft survival in recipients that were both CD28 and CTLA-4 deficient. Our data indicate that B7-1 is the dominant ligand for CTLA-4-mediated down-regulation of alloimmune responses in vivo and suggest that B7-2 has an additional receptor other than CD28 and CTLA-4 to provide a positive costimulatory signal for T cells.     

Selective survival of naturally occurring human CD4+CD25+Foxp3+ regulatory T cells cultured with rapamycin

Naturally occurring CD4(+)CD25(+) regulatory T (nTreg) cells are essential for maintaining T cell tolerance to self Ags. We show that discrimination of human Treg from effector CD4(+)CD25(+) non-nTreg cells and their selective survival and proliferation can now be achieved using rapamycin (sirolimus). Human purified CD4(+)CD25(high) T cell subsets stimulated via TCR and CD28 or by IL-2 survived and expanded up to 40-fold in the presence of 1 nM rapamycin, while CD4(+)CD25(low) or CD4(+)CD25(-) T cells did not. The expanding pure populations of CD4(+)CD25(high) T cells were resistant to rapamycin-accelerated apoptosis. In contrast, proliferation of CD4(+)CD25(-) T cells was blocked by rapamycin, which induced their apoptosis. The rapamycin-expanded CD4(+)CD25(high) T cell populations retained a broad TCR repertoire and, like CD4(+) CD25(+) T cells freshly obtained from the peripheral circulation, constitutively expressed CD25, Foxp3, CD62L, glucocorticoid-induced TNFR family related protein, CTLA-4, and CCR-7. The rapamycin-expanded T cells suppressed proliferation and effector functions of allogeneic or autologous CD4(+) and CD8(+) T cells in vitro. They equally suppressed Ag-specific and nonspecific responses. Our studies have defined ex vivo conditions for robust expansion of pure populations of human nTreg cells with potent suppressive activity. It is expected that the availability of this otherwise rare T cell subset for further studies will help define the molecular basis of Treg-mediated suppression in humans.     

CD4+CD25high regulatory cells in human peripheral blood

Thymectomy in mice on neonatal day 3 leads to the development of multiorgan autoimmune disease due to loss of a CD(+)CD25(+) T cell regulatory population in their peripheral lymphoid tissues. Here, we report the identification of a CD4(+) population of regulatory T cells in the circulation of humans expressing high levels of CD25 that exhibit in vitro characteristics identical with those of the CD4(+)CD25(+) regulatory cells isolated in mice. With TCR cross-linking, CD4(+)CD25(high) cells did not proliferate but instead totally inhibited proliferation and cytokine secretion by activated CD4(+)CD25(-) responder T cells in a contact-dependent manner. The CD4(+)CD25(high) regulatory T cells expressed high levels of CD45RO but not CD45RA, akin to the expression of CD45RB(low) on murine CD4(+)CD25(+) regulatory cells. Increasing the strength of signal by providing either costimulation with CD28 cross-linking or the addition of IL-2 to a maximal anti-CD3 stimulus resulted in a modest induction of proliferation and the loss of observable suppression in cocultures of CD4(+)CD25(high) regulatory cells and CD4(+)CD25(-) responder cells. Whereas higher ratios of CD4(+)CD25(high) T cells are required to suppress proliferation if the PD-L1 receptor is blocked, regulatory cell function is shown to persist in the absence of the PD-1/PD-L1 or CTLA-4/B7 pathway. Thus, regulatory CD4 T cells expressing high levels of the IL-2 receptor are present in humans, providing the opportunity to determine whether alterations of these populations of T cells are involved in the induction of human autoimmune disorders.     

Cutting edge: a crucial role for B7-CD28 in transmitting T help from APC to CTL

Although APC activation via CD40-CD40L signaling plays a critical role in enabling CD4(+) T cells to provide the "help" necessary for cross-priming of naive CTL, it is unclear how this makes the APC competent for priming. We have investigated the roles of B7-1/B7-2 and their receptors [corrected] CD28/CTLA-4 in cross-priming of CD4-dependent CTL in vivo. We find that both CD28 and B7-1/B7-2 are required for CD40-activated APC to cross-prime CTL, and that priming by CD40-activated APC was prevented by blockade of CD28. Conversely, augmenting CD28 signals with an agonistic Ab bypassed the requirement for CD4(+) T help or CD40 activation. Interestingly, blockade of the negative regulatory B7 receptor CTLA-4 failed to prime CTL in the absence of T help. These results support a model in which activation-induced up-regulation of B7 molecules on APC leads to increased CD28 signaling and a commitment to cross-priming of CD4-dependent CTL.