CD4+CD25+ regulatory T cells utilize FasL as a mechanism to restrict DC priming functions in cutaneous immune responses

Recent studies have suggested Fas‐mediated elimination of antigen‐presenting cells as an important mechanism down‐regulating the induction of autoimmune responses. It remains unknown whether this mechanism restricts the magnitude of immune responses to non‐self antigens. We used a mouse model of a cutaneous CD8⁺ T‐cell‐mediated immune response (contact hypersensitivity, CHS) to test if CD4⁺CD25⁺ T cells expressing FasL regulate hapten‐specific effector CD8⁺ T cell expansion through the elimination of Fas‐expressing hapten‐presenting DC. In WT mice, attenuation of CD4⁺CD25⁺ T regulatory cell activity by anti‐CD25 mAb increased hapten‐presenting DC numbers in skin‐draining LN, which led to increased effector CD8⁺ T‐cell priming for CHS responses. In contrast, CD4⁺CD25⁺ T cells did not regulate hapten‐specific CD8⁺ T‐cell priming and CHS responses initiated by Fas‐defective (lpr) DC. Thus, restricting DC priming functions through Fas–FasL interactions is a potent mechanism employed by CD4⁺CD25⁺ regulatory cells to restrict CD8⁺ T‐cell‐mediated allergic immune responses in the skin.     

Alloreactive CD8 T cells rescued from apoptosis during co‐stimulation blockade by Toll‐like receptor stimulation remain susceptible to Fas‐induced cell death

Blockade of co‐stimulatory signals to T cells is extremely effective for the induction of transplantation tolerance in immunologically naive rodents. However, infections and inflammation compromise the efficacy of co‐stimulation blockade regimens for the induction of tolerance, thereby stimulating the rejection of allografts. Previous studies have shown that stimulation of innate immunity abrogates tolerance induction by preventing the deletion of alloreactive CD8⁺ T cells that normally occurs during co‐stimulation blockade. Although inflammation prevents the deletion of alloreactive T cells during co‐stimulation blockade, it is not known if this resistance to cell death is the result of a mechanism intrinsic to the T cell. Here, we used syngeneic bone marrow chimeric mice that contain a trace population of T‐cell receptor transgenic alloreactive CD8⁺ T cells to investigate the early apoptotic signature and activation status of alloreactive T cells following exposure to inflammatory signals during co‐stimulation blockade with an antibody specific for CD154. Our findings revealed that the presence of bacterial lipopolysaccharide during co‐stimulation blockade enhanced the early activation of alloreactive CD8⁺ T cells, as indicated by the up‐regulation of CD25 and CD69, suppressed Fas ligand expression, and prevented apoptotic cell death. However, alloreactive CD8⁺ T cells from lipopolysaccharide‐treated mice remained sensitive to Fas‐mediated apoptosis in vitro. These findings suggest that alloreactive T cells rescued from deletion during co‐stimulation blockade by inflammation are still sensitive to pro‐apoptotic signals and that stimulating these apoptotic pathways during co‐stimulation blockade may augment the induction of tolerance.     

CD40L expression by CD4+ but not CD8+ T cells regulates antiviral immune responses in acute LCMV infection in mice

CD40‐CD40 ligand (CD40L) signaling plays multiple indispensable roles in cellular and humoral immunity. Impaired memory T‐cell responses in the absence of CD40L have been well documented, but the requirement of this interaction for efficient priming of CD8⁺ T cells especially under inflammatory conditions has been under debate. In contrast to previous publications, we report here that virus‐specific CD8⁺ T‐cell responses as well as viral clearance are affected not only in the memory but also in the effector phase in CD40L^?/? mice infected with lymphocytic choriomeningitis virus (LCMV) Armstrong strain. Interestingly, a considerable part of the LCMV‐specific effector and memory T cells consists of CD40L⁺ CD8⁺ T cells. However, deficiency of CD40L in CD8⁺ T cells did influence neither the quantity nor the quality of primary T‐cell responses in LCMV infection. Virus‐specific CD8⁺ T cells in conditional knockout mice, with a selective deletion of the CD40L in CD8⁺ T cells, were fully functional regarding cytokine production and efficient pathogen clearance. Thus, our results unambiguously demonstrate that while CD40L is critical to generate effective primary CD8⁺ T‐cell responses also under inflammatory conditions, CD40L expression by CD8⁺ T cells themselves is dispensable in acute LCMV infection.     

T‐cell help dependence of memory CD8+ T‐cell expansion upon vaccinia virus challenge relies on CD40 signaling

Due to their capacity to differentiate into long‐lived memory cells, CD8⁺ T cells are able to resolve subsequent infections faster than during the primary response. Among other factors, CD4⁺ T cells play a crucial role during primary and secondary CD8⁺ T‐cell responses. However, the timing and mechanisms by which they influence CD8⁺ T cells may differ in primary and secondary responses. Here, we demonstrate that during both primary and secondary vaccinia virus infection, CD4⁺ T cells are necessary to promote CD8⁺ T‐cell responses. While CD4⁺ T cells contributed to memory CD8⁺ T‐cell development, they were even more important during memory recall responses during challenge, as absence of CD4⁺ T cells during challenge resulted in markedly decreased proliferation and increased apoptosis. T‐cell help during primary and secondary responses was mediated via CD40 signaling, with DCs being an integral part of that pathway. As opposed to primary CD8⁺ T‐cell responses where only a combination of agonistic CD40 signaling and provision of IL‐2 could substitute for T‐cell help, agonistic CD40 triggering alone was sufficient to rescue memory CD8⁺ T‐cell responses in absence of T‐cell help in the context of vaccinia virus infection.     

CD26‐mediated co‐stimulation in human CD8+ T cells provokes effector function via pro‐inflammatory cytokine production

CD26 is an activation marker of human CD4⁺ T cells, and is associated with T‐cell signal transduction processes as a co‐stimulatory molecule. We have previously demonstrated that high CD26 cell surface expression on CD4⁺ T cells is correlated with the production of T helper type 1 cytokines, whereas CD26⁺ T helper cells stimulate antibody synthesis in B cells. Although the cellular and molecular mechanisms involved in CD26‐mediated CD4⁺ T‐cell activation have been extensively evaluated by our group and others, the role of CD26 in CD8⁺ T cells has not been clearly elucidated. In the present study, we examine the effector function of CD8⁺ T cells via CD26‐mediated co‐stimulation in comparison with CD28‐mediated co‐stimulation. We found that CD26^high CD8⁺ T cells belong to the early effector memory T‐cell subset, and that CD26‐mediated co‐stimulation of CD8⁺ T cells exerts a cytotoxic effect preferentially via granzyme B, tumour necrosis factor‐α, interferon‐γ and Fas ligand. The effector function associated with CD26‐mediated co‐stimulation is enhanced compared with that obtained through CD28‐mediated co‐stimulation, suggesting that the CD26 co‐stimulation pathway in CD8⁺ T cells is distinct from the CD28 co‐stimulation pathway. Targeting CD26 in CD8⁺ T cells therefore has the potential to be useful in studies of immune responses to new vaccine candidates as well as innovative therapy for immune‐mediated diseases.     

Homeostatic regulation of CD8+ T cells after antigen challenge in the absence of Fas (CD95)

The role of Fas in the homeostatic regulation of CD8⁺ T cells after antigen challenge was analyzed in the murine model of lymphocytic choriomeningitis virus (LCMV) infection. Mice homozygous for the lpr mutation and carrying T cell receptor (TCR) αβ transgenes specific for the LCMV glycoprotein peptide aa 33–41 in the context of H-2D^b were used. Five main results emerged: first, development of lymphadenopathy and of CD4⁻CD8⁻ double-negative B220⁺ T cells in lpr mice was not inhibited by the αβ TCR transgenes; second, tolerance induction and peripheral deletion of CD8⁺ T cells induced by LCMV glycoprotein peptide injection was independent of Fas expression; third, clonal down-regulation of Fas-deficient TCR-transgenic CD8⁺ T cells after acute LCM virus infection was identical to the decline of transgenic T cells expressing Fas; fourth, in vivo activated CD8⁺ effector T cells from TCR transgenic and TCR-lpr/lpr mice were equally susceptible to activation-induced cell death in vitro; and fifth, transgenic effector T cells from lpr/lpr mice were cleared in the declining phase of the immune response in vivo without giving rise to CD4⁻CD8⁻ double-negative T cells. Taken together, these data suggest that the homeostatic regulation of CD8⁺ T cells after antigen challenge in vivo is regulated by mechanisms that do not require Fas.     

Steady‐state antigen‐expressing dendritic cells terminate CD4+ memory T‐cell responses

CD4⁺ T cells are important effectors of inflammation and tissue destruction in many diseases of immune dysregulation. As memory T cells develop early during the preclinical stages of autoimmune and inflammatory diseases, immunotherapeutic approaches to treatment of these diseases, once established, must include the means to terminate memory T‐cell responses. Traditionally, it has been considered that, due to their terminally differentiated nature, memory T cells are resistant to tolerance induction, although emerging evidence indicates that some immunotherapeutic approaches can terminate memory T‐cell responses. Here, we demonstrate that CD4⁺ memory T‐cell responses can be terminated when cognate antigen is transgenically expressed in steady‐state DC. Transfer of in‐vitro‐generated CD4⁺ memory T cells establishes, in nontransgenic recipients, a stable and readily recalled memory response to cognate antigen. In contrast, upon transfer to mice expressing cognate antigen targeted to DC, memory CD4⁺ T cells undergo a phase of limited proliferation followed by substantial deletion, and recall responses are effectively silenced. This finding is important in understanding how to effectively apply immunotherapy to ongoing T‐cell‐mediated autoimmune and inflammatory diseases.     

Human primary adenotonsillar naïve phenotype CD45RA CD4 T lymphocytes undergo apoptosis upon stimulation with a high concentration of CD3 antibody

Young children need to develop immune tolerance to harmless foreign antigens such as digested nutrients and various inhaled airborne antigens. Because of its anatomical location, pharyngeal adenotonsillar tissue is a potential site for the establishment of this immune tolerance. To characterize possible mechanisms of peripheral immune tolerance, we studied human primary adenotonsillar naïve phenotype CD45RA⁺ CD4⁺ T cells, which represent cells that have not previously encountered foreign antigens. It was found that these CD45RA⁺ CD4⁺ T cells expressed higher levels of the activation marker CD69 as compared with peripheral blood CD45RA⁺ CD4⁺ T cells. Upon stimulation with a high concentration of CD3 antibody, which mimics the encounter of a high antigen dose, adenotonsillar CD45RA⁺ CD4⁺ T lymphocytes, but not peripheral blood CD45RA⁺ CD4⁺ T cells, underwent apoptosis. After 6 h stimulation with a high concentration of CD3 antibody, over 25% of the cells were apoptotic. Interfering with the Fas–FasL interaction with recombinant Fas or an antibody against Fas-ligand partially inhibited apoptosis. Our study results suggest that high concentrations of antigens, such as various nutrients and airborne antigens, may induce peripheral immune tolerance by selectively deleting naïve phenotype CD45RA⁺ CD4⁺ T cells via T-cell receptor-triggered apoptosis in human adenotonsillar tissue.     

Induction and mechanism of action of transforming growth factor-β-secreting Th3 regulatory cells

Summary: Th3 CD4⁺ regulatory cells were identified during the course of investigating mechanisms associated with oral tolerance. Different mechanisms of tolerance are induced following oral antigen administration, including active suppression, clonal anergy and deletion. Low doses favor active suppression whereas high doses favor anergy/deletion. Th3 regulatory cells form a unique T‐cell subset which primarily secretes transforming growth factor (TGF)‐β, provides help for IgA and has suppressive properties for both Th1 and Th2 cells. Th3 type cells are distinct from the Th2 cells, as CD4⁺ TGF‐β‐secreting cells with suppressive properties have been generated from interleukin (IL)‐4‐deficient animals. In vitro differentiation of Th3 cells from Th precursors from T‐cell antigen receptor (TCR) transgenic mice is enhanced by culture with TGF‐β, IL‐4, IL‐10, and anti‐IL‐12. Th3 CD4⁺ myelin basic protein regulatory clones are structurally identical to Th1 encephalitogenic clones in TCR usage, MHC restriction and epitope recognition, but produce TGF‐β with various amounts of IL‐4 and IL‐10. Because Th3 regulatory cells are triggered in an antigen‐specific fashion but suppress in an antigen‐non‐specific fashion, they mediate “bystander suppression” when they encounter the fed autoantigen at the target organ. In vivo induction of Th3 cells and low dose oral tolerance is enhanced by oral administration of IL‐4. Anti‐CD86 but not anti‐CD80 blocks the induction of Th3 cells associated with low dose oral tolerance. Th3 regulatory cells have been described in other systems (e.g. recovery from experimental allergic encephalomyelitis) but may be preferentially generated following oral antigen administration due to the gut immunologic milieu that is rich in TGF‐β and has a unique class of dendritic cells. CD4⁺CD25⁺ regulatory T‐cell function also appears related to TGF‐β.     

Induction of peripheral CD4+ T‐cell tolerance and CD8+ T‐cell cross‐tolerance by dendritic cells

DC can present and cross‐present self‐antigens to autoreactive CD4⁺ and CD8⁺ T cells, respectively, and incapacitate them by inducing anergy, deletion or converting them into Treg. In this review, we summarize the recent progress in immune tolerance research, which has been achieved by employing antigen‐ and TCR‐transgenic mice. We cover the numerous discoveries that have furthered our knowledge of the DC subsets and maturation pathways involved in tolerance; the signals, such as CD70, TGF‐β, B7‐H1/PD‐L1, which dictate the decision between immunity and tolerance; and the in vivo role of DC in the maintenance of CD4⁺ T‐cell tolerance and CD8⁺ T‐cell cross‐tolerance.     

Selective antigen‐specific CD4+ T‐cell,but not CD8+ T‐ or B‐cell,tolerance corrupts cancer immunotherapy

Self‐tolerance, presumably through lineage‐unbiased elimination of self‐antigen‐specific lymphocytes (CD4⁺ T, CD8⁺ T, and B cells), creates a formidable barrier to cancer immunotherapy. In contrast to this prevailing paradigm, we demonstrate that for some antigens, self‐tolerance reflects selective elimination of antigen‐specific CD4⁺ T cells, but preservation of CD8⁺ T‐ and B‐cell populations. In mice, antigen‐specific CD4⁺ T‐cell tolerance restricted CD8⁺ T‐ and B‐cell responses targeting the endogenous self‐antigen guanylyl cyclase c (GUCY2C) in colorectal cancer. Although selective CD4⁺ T‐cell tolerance blocked GUCY2C‐specific antitumor immunity and memory responses, it offered a unique solution to the inefficacy of GUCY2C vaccines through recruitment of self‐antigen‐independent CD4⁺ T‐cell help. Incorporating CD4⁺ T‐cell epitopes from foreign antigens into vaccines against GUCY2C reconstituted CD4⁺ T‐cell help, revealing the latent functional capacity of GUCY2C‐specific CD8⁺ T‐ and B‐cell pools, producing durable antitumor immunity without autoimmunity. Incorporating CD4⁺ T‐cell epitopes from foreign antigens into vaccines targeting self‐antigens in melanoma (Trp2) and breast cancer (Her2) produced similar results, suggesting selective CD4⁺ T‐cell tolerance underlies ineffective vaccination against many cancer antigens. Thus, identification of self‐antigens characterized by selective CD4⁺ T‐cell tolerance and abrogation of such tolerance through self‐antigen‐independent T‐cell help is essential for future immunotherapeutics.     

Self-encounters of the third kind: lymph node stroma promotes tolerance to peripheral tissue antigens

Multiple mechanisms have evolved to maintain tolerance among CD8⁺ T cells to innocuous antigens that arise in cutaneous and mucosal tissues. In the thymus, medullary thymic epithelial cells directly present peripheral tissue antigens (PTAs) and incite the deletion of self-reactive thymocytes. Cross-presentation of PTAs by functionally immature, CD8α⁺ dendritic cells can lead to the deletion of self-reactive CD8⁺ T cells in secondary lymphoid organs. A third mechanism of deletional tolerance has recently been uncovered in which lymph node-resident stromal cells of non-hematopoietic origin present endogenously expressed PTAs to circulating CD8⁺ T cells. Emerging data suggest that lymph node stroma is a unique niche for controlling self-reactive T cells.     

IL‐27 enhances the survival of tumor antigen‐specific CD8+ T cells and programs them into IL‐10‐producing,memory precursor‐like effector cells

IL‐27 is a member of the IL‐12 family of cytokines that is comprised of an IL‐12 p40‐related protein subunit, EBV‐induced gene 3, and a p35‐related subunit, p28. IL‐27 functions through IL‐27R and has been shown to have potent antitumor activity via activation of a variety of cellular components, including antitumor CD8⁺ T‐cell responses. However, the exact mechanisms of how IL‐27 enhances antitumor CD8⁺ T‐cell responses remain unclear. Here we show that IL‐27 significantly enhances the survival of activated tumor antigen‐specific CD8⁺ T cells in vitro and in vivo, and programs tumor antigen‐specific CD8⁺ T cells into memory precursor‐like effector cells, characterized by upregulation of Bcl‐6, SOCS3, Sca‐1, and IL‐10. While STAT3 activation and the CTL survival‐enhancing effects can be independent of CTL IL‐10 production, we show here that IL‐27‐induced CTL IL‐10 production contributes to memory precursor cell phenotype induction, CTL memory, and tumor rejection. Thus, IL‐27 enhances antitumor CTL responses via programming tumor antigen‐specific CD8⁺ T cells into a unique memory precursor type of effector cells characterized by a greater survival advantage. Our results have important implications for designing immunotherapy against human cancer.     

CD40 engagement of CD4+ CD40+ T cells in a neo-self antigen disease model ablates CTLA-4 expression and indirectly impacts tolerance

Biomarkers defining pathogenic effector T (Teff) cells slowly have been forthcoming and towards this we identified CD4⁺ T cells that express CD40 (CD4⁺CD40⁺) as pathogenic in the NOD type 1 diabetes (T1D) model. CD4⁺CD40⁺ T cells rapidly and efficiently transfer T1D to NOD.scid recipients. To study the origin of CD4⁺CD40⁺ T cells and disease pathogenesis, we employed a dual transgenic model expressing OVA_323–339 peptide as a neo‐self antigen on islet β cells and medullary thymic epithelial cells (mTECs) and a transgenic TCR recognizing the OVA_323–339 peptide. CD4⁺CD40⁺ T cells and Treg cells each recognizing the cognate neo‐antigen, rather than being deleted through central tolerance, drastically expanded in the thymus. In pancreatic lymph nodes of DO11.RIPmOVA mice, CD4⁺CD40⁺ T cells and Treg cells are expanded in number compared with DO11 mice and importantly, Treg cells remain functional throughout the disease process. When exposed to neo‐self antigen, CD4⁺CD40⁺ T cells do not express the auto‐regulatory CTLA‐4 molecule while naïve CD4⁺CD40⁺ T cells do. DO11.RIPmOVA mice develop autoimmune‐type diabetes. CD40 engagement has been shown to prevent CTLA‐4 expression and injecting anti‐CD40 in DO11.RIPmOVA mice significantly exacerbates disease. These data suggest a unique means by which CD4⁺CD40⁺ T cells thwart tolerance.     

CD4+ T‐cell development in a mouse expressing a transgenic TCR derived from a Treg

CD4⁺Foxp3⁺ Treg maintain peripheral tolerance and influence immune responses to foreign antigens. The thymus is an important source of Treg, but controversy exists as to whether T cells are selected into the Treg lineage based on signals received through TCR specific for self‐peptides. To examine the specificity of TCR expressed by Treg and its effect on CD4⁺ T‐cell development, we generated Treg‐TCR transgenic mice. Deletion of >90% of CD4⁺ T cells in RAG‐sufficient mice, and nearly 100% deletion in RAG^?/? mice expressing this TCR indicate that the TCR is specific for an unknown, naturally expressed peptide in the thymus. Deletion occurs late in development, suggesting this peptide is presented by APC in the thymic medulla. These studies are the first to describe the effects of expressing a Treg‐TCR on CD4⁺ T‐cell development. The implications of our data for models of Treg selection are discussed.     

Human CD4+ invariant NKT cells are involved in antibacterial immunity against Brucella suis through CD1d‐dependent but CD4‐independent mechanisms

Human invariant NKT (iNKT) cells are a unique subset of T cells, which recognize glycolipids presented by the CD1d. Among the iNKT cells, several functionally distinct subsets have been characterized according to CD4 and/or CD8 co‐receptor expression. The current study is focussed on the CD4⁺ iNKT cell subset and its role in an anti‐infectious response. We have examined the role of CD4⁺ iNKT cells on the intracellular Brucella suis growth. Our results indicate that CD4⁺ iNKT cells impair the intramacrophagic growth of Brucella. This inhibition is due to a combination of soluble and contact‐dependent mechanisms: IFN‐γ is weakly involved while cytotoxic activities such as the induction of the Fas pathway and the release of lytic granules are major mechanisms. The impairment of Brucella growth by CD4⁺ iNKT cells requires an interaction with CD1d on macrophage surface. Also, we have shown that although CD4 regulates several biological responses of CD4⁺ iNKT cells, it is not involved in their antibacterial activity. Here, we have shown for the first time that the CD4⁺ iNKT cell population has antibacterial activity and thus, participates directly in the elimination of bacteria and/or in the control of bacterial growth by killing infected cells.     

Non-exclusive Fas control and age dependence of viral superantigen-induced clonal deletion in lupus-prone mice

To investigate the role of Fas in the induction of tolerance by viral superantigen (SAG), we infected MRL-+/+ and MRL-lpr (Fas mutant) mice with mouse mammary tumor virus (MMTV) (SW), a virus encoding an SAG with the same specificity as endogenous Mtv-7-SAG. In normal mice, this infection has two distinct consequences on specific Vβ6⁺CD4⁺ T cells, consisting of activation followed by clonal deletion. MMTV (SW)-SAG-induced activation in vivo was identical in MRL-+/+ and MRL-lpr mice. In contrast, clonal deletion showed age-dependent impairment. Early infection (5 weeks) led to identical clonal deletion of specific T cells in blood lymphocytes from MRL-+/+ and MRL-lpr mice, although clonal deletion was slightly impaired in the MRL-lpr lymph nodes. Late infection (10 weeks) of MRL-lpr mice led to markedly delayed and reduced clonal deletion. Vβ6⁺CD4⁺ T cells which escaped clonal deletion in aging MRL-lpr mice were not anergized by interaction with SAG. These results show that peripheral clonal deletion induced by viral SAG in adult mice is controlled by Fas, but not exclusively so.     

Sublingual tolerance induction with antigen conjugated to cholera toxin B subunit induces Foxp3+CD25+CD4+ regulatory T cells and suppresses delayed-type hypersensitivity reactions

Although sublingual (s.l.) immunotherapy with selected allergens is safe and often effective for treating patients with allergies, knowledge of the immunological mechanisms involved remains limited. Can s.l. administration of antigen (Ag) induce peripheral immunological tolerance and also suppress delayed‐type hypersensitivity (DTH) responses? To what extent can s.l.‐induced tolerance be explained by the generation of Foxp3⁺CD25⁺CD4⁺ regulatory T cells (T_reg)? This study addressed these questions in mice and compared the relative efficacy of administering ovalbumin (OVA) conjugated to cholera toxin B (CTB) subunit with administration of the same Ag alone. We found that s.l. administration of a single or even more efficiently three repeated 40‐μg doses of OVA/CTB conjugate suppressed T‐cell proliferative responses to OVA by cervical lymph node (CLN), mesenteric lymph node (MLN) and spleen cells and concurrently strongly increased the frequency of Ag‐specific T_reg in CLN, MLN and spleen and also transforming growth factor‐β (TGF‐β) levels in serum. The CLN and splenic cells from OVA/CTB‐treated BALB/c mice efficiently suppressed OVA‐specific T‐cell receptor (TCR) transgenic (DO11.10) CD25^?CD4⁺ effector T‐cell proliferation in vitro. Further, s.l. treatment with OVA/CTB completely suppressed OVA‐specific DTH responses in vivo and T‐cell proliferative responses in mice immunized subcutaneously with OVA in Freund's complete adjuvant. The intracellular expression of Foxp3 was strongly increased in OVA‐specific (KJ1‐26⁺) CD4⁺ T cells from OVA/CTB‐treated mice. Thus, s.l. administration of CTB‐conjugated Ag can efficiently induce peripheral T‐cell tolerance associated with strong increases in serum TGF‐β levels and in Ag‐specific Foxp3⁺CD25⁺CD4⁺ T_reg cells.     

Involvement of the intrinsic and extrinsic cell‐death pathways in the induction of apoptosis of mature lymphocytes by the Escherichia coli heat‐labile enterotoxin

Escherichia coli heat‐labile enterotoxin (LT) exhibits a broad range of immunomodulatory activities, including the induction of lymphocyte‐programmed cell death. In previous studies, we have demonstrated that in vivo LT promotes apoptosis of immature T and B cells through the stimulation of endogenous glucocorticoids. In the present study, we show that the extrinsic cell‐death pathway as well as the apoptosis‐inducing factor do not participate in the LT‐induced elimination of thymocytes. In contrast to developing lymphocytes, LT promotes the death of mature lymphocytes by both glucocorticoid‐ and Fas death receptor/Fas ligand‐dependent mechanisms. However, the dependency of these mechanisms in the LT‐induced cell‐death activity seems to be different among CD4⁺ and CD8⁺ T cells. Altogether, our study shows that the same bacterial toxin can induce apoptosis of lymphoid cells through several mechanisms depending on the status of differentiation of these cells.     

Antigen‐specific Treg impair CD8+ T‐cell priming by blocking early T‐cell expansion

Foxp3⁺ Treg are crucial for the maintenance of self‐tolerance and have been shown to control CD8⁺ T‐cell effector functions. In addition, Treg are thought to control the priming of CD8⁺ T cells, which recognize the same antigens as Treg. Taking advantage of our model of peripheral tolerance induction to influenza hemagglutinin (HA) after HA gene transfer, we found that HA‐specific Treg suppress antigen‐linked CTL responses through early blockade of CD8⁺ T‐cell expansion. Confronted with their cognate antigen, Treg expand more rapidly than CD8⁺ T cells and are highly suppressive only during the initial stages of immune priming. They nullify HA‐specific CD8⁺ T‐cell responses, local inflammatory responses and rejection of HA transduced cells. When HA gene transfer is performed with extensive tissue inflammation, HA‐specific Treg are less effective but still reduce the frequency of newly primed HA‐specific CD8⁺ T cells and the ensuing frequency of memory CD8⁺ T cells. Our results demonstrate that Treg control CTL priming in an antigen‐specific manner at the level of T‐cell expansion, highlighting how self‐reactive Treg could prevent the induction of autoimmune responses through selective blockade of autoreactive T‐cell proliferation.