Blockade of costimulation through B7/CD28 inhibits experimental autoimmune uveoretinitis, but does not induce long-term tolerance

It has been reported that costimulation blockade can result in T cell anergy. We investigated the effects of blocking costimulatory molecules in vivo on the development of experimental autoimmune uveoretinitis (EAU), a model for autoimmune uveitis in humans that is induced in mice by immunization with the retinal Ag interphotoreceptor retinoid binding protein. B10.A mice immunized with a uveitogenic regimen of interphotoreceptor retinoid-binding protein were treated with Abs to B7.1 and B7.2 for 2 wk. Evaluation of EAU and immunological responses 1 wk later showed that disease had been abrogated, and cellular responses were suppressed. To determine whether the costimulation blockade resulted in tolerance, adult-thymectomized mice immunized for uveitis and treated with anti-B7 or anti-CD28 were rechallenged for uveitis induction 5 wk after the initial immunization. Although confirmed to be disease free after the initial immunization, both anti-B7- and anti-CD28-treated mice developed severe EAU and elevated cellular responses after the rechallenge, equivalent to those of control mice. We conclude that in this model costimulatory blockade in vivo prevents the development of autoimmune disease, but does not result in long-term tolerance. The data are compatible with the interpretation that B7/CD28 blockade prevents generation of effector, but not of memory, T cells.     

Oral tolerance: immune mechanisms and the generation of Th3-type TGF-beta-secreting regulatory cells

Oral tolerance is a long recognized method to induce peripheral immune tolerance. Oral tolerance has been used successfully to treat animal models of autoimmune diseases and is being tested in human diseases. Low doses of oral antigen induce active suppression, whereas high doses induce clonal anergy and deletion. Oral antigen preferentially generates a Th2(IL-4/IL-10)- or a Th3(TGF-beta)-type response. Th3-type cells are a unique T-cell subset which primarily secrete TGF-beta, provide help for IgA and have suppressive properties for Th1 and other immune cells. Th3-type cells appear distinct from the Th2 cells as CD4(+) TGF-beta-secreting cells with suppressive properties in the gut have been generated from IL-4-deficient animals. In vitro differentiation of Th3-type cells from Th0 precursors from TCR transgenic mice is enhanced by culture with TGF-beta, IL-4, IL-10 and anti-IL-12. Because regulatory T cells generated by oral antigen are triggered in an antigen-specific fashion but suppress in an antigen-nonspecific fashion, they mediate bystander suppression when they encounter the fed autoantigen at the target organ. Thus, mucosal tolerance can be used to treat inflammatory processes that are not autoimmune in nature. Mucosal antigen has also been used to treat animal models of stroke and of Alzheimer's disease. Induction of low-dose oral tolerance is enhanced by oral administration of IL-4 and IL-10. Coupling antigen to CTB or administration of Flt-3 ligand enhances oral tolerance. Anti-B7.2 but not anti-B7.1 blocks low-dose, but not high-dose oral tolerance. High-dose oral tolerance is blocked by anti-CTLA-4. CD25(+) CD4(+) regulatory T-cell function also appears to be related to TFG-beta.     

Break of neonatal Th1 tolerance and exacerbation of experimental allergic encephalomyelitis by interference with B7 costimulation

Ig-PLP1 is an Ig chimera expressing proteolipid protein-1 (PLP1) peptide corresponding to aa residues 139-151 of PLP. Newborn mice given Ig-PLP1 in saline on the day of birth and challenged 7 wk later with PLP1 peptide in CFA develop an organ-specific neonatal immunity that confers resistance against experimental allergic encephalomyelitis. The T cell responses in these animals comprise Th2 cells in the lymph node and anergic Th1 lymphocytes in the spleen. Intriguingly, the anergic splenic T cells, although nonproliferative and unable to produce IFN-gamma or IL-4, secrete significant amounts of IL-2. In this work, studies were performed to determine whether costimulation through B7 molecules plays any role in the unusual form of splenic Th1 anergy. The results show that engagement of either B7.1 or B7.2 with anti-B7 Abs during induction of EAE in adult mice that were neonatally tolerized with Ig-PLP1 restores and exacerbates disease severity. At the cellular level, the anergic splenic T cells regain the ability to proliferate and produce IFN-gamma when stimulated with Ag in the presence of either anti-B7.1 or anti-B7.2 Ab. However, such restoration was abolished when both B7.1 and B7.2 molecules were engaged simultaneously, indicating that costimulation is necessary for reactivation. Surprisingly, both anti-B7.1 and anti-B7.2 Abs triggered splenic dendritic cells to produce IL-12, a key cytokine required for restoration of the anergic T cells. Thus, recovery from neonatally induced T cell anergy requires B7 molecules to serve double functions, namely, costimulation and induction of cytokine production by APCs.     

IL-3 induces B7.2 (CD86) expression and costimulatory activity in human eosinophils.

Eosinophils in tissues are often present in intimate contact with T cells in allergic and parasitic diseases. Resting eosinophils do not express MHC class II proteins or costimulatory B7 molecules and fail to induce proliferation of T cells to Ags. IL-5 and GM-CSF induce MHC class II and B7 expression on eosinophils and have been reported in some studies to induce eosinophils to present Ag to T cells. The cytokine IL-3, like IL-5 and GM-CSF, is a survival and activating factor for eosinophils and the IL-3 receptor shares with the IL-5 and GM-CSF receptors a common signal transducing beta-chain. IL-3-treated eosinophils expressed HLA-DR and B7.2, but not B7.1 on their surface and supported T cell proliferation in response to the superantigen toxic shock syndrome toxin 1, as well as the proliferation of HLA-DR-restricted tetanus toxoid (TT) and influenza hemagglutinin-specific T cell clones to antigenic peptides. This was inhibited by anti-B7.2 mAb. In contrast, IL-3-treated eosinophils were unable to present native TT Ag to either resting or TT-specific cloned T cells. In parallel experiments, eosinophils treated with IL-5 or GM-CSF were also found to present superantigen and antigenic peptides, but not native Ag, to T cells. These results suggest that eosinophils are deficient in Ag processing and that this deficiency is not overcome by cytokines that signal via the beta-chain. Nevertheless, our findings suggest that eosinophils activated by IL-3 may contribute to T cell activation in allergic and parasitic diseases by presenting superantigens and peptides to T cells.     

Oral administration of myelin basic protein is superior to myelin in suppressing established relapsing experimental autoimmune encephalomyelitis

Oral administration of a myelin component, myelin basic protein (MBP), induces immunological unresponsiveness to CNS Ags and ameliorates murine relapsing experimental autoimmune encephalomyelitis (REAE). However, a recent clinical trial in which multiple sclerosis patients were treated with repeated doses of oral myelin was unsuccessful in reducing disease exacerbations. Therefore, we directly compared the tolerizing capacity of myelin vs MBP during REAE in B10.PL mice. Oral administration of high doses of myelin, either before disease induction or during REAE, did not provide protection from disease or decrease in vitro T cell responses. In contrast, repeated oral administration of high doses of MBP suppressed established disease and MBP-specific T cell proliferation and cytokine responses. The frequency of IL-2-, IFN-gamma-, and IL-5-secreting MBP-specific T cells declined with MBP feeding, implicating anergy and/or deletion as the mechanism(s) of oral tolerance after high Ag doses. We have previously shown that the dosage and timing of Ag administration are critical parameters in oral tolerance induction. Studies presented here demonstrate that Ag homogeneity is also important, i.e., homogeneous Ag (MBP) is more effective at inducing oral tolerance than heterogeneous Ag (myelin).     

B cell-deficient (mu MT) mice have alterations in the cytokine microenvironment of the gut-associated lymphoid tissue (GALT) and a defect in the low dose mechanism of oral tolerance

Peripheral immune tolerance following i.v. administration of Ag has been shown to occur in the absence of B cells. Because different mechanisms have been identified for i.v. vs low dose oral tolerance and B cells are a predominant component of the gut-associated lymphoid tissue (GALT) they may play a role in tolerance induction following oral Ag. To examine the role of B cells in oral tolerance we fed low doses of OVA or myelin oligodendrocyte glycoprotein to B cell-deficient ( microMT) and wild-type C57BL/6 mice. Results showed that the GALT of naive wild-type and microMT mice was characterized by major differences in the cytokine microenvironment. Feeding low doses of 0.5 mg OVA or 250 microg myelin oligodendrocyte glycoprotein resulted in up-regulation of IL-4, IL-10, and TGF-beta in the GALT of wild-type but not microMT mice. Upon stimulation of popliteal node cells, in vitro induction of regulatory cytokines TGF-beta and IL-10 was observed in wild-type but not microMT mice. Greater protection against experimental autoimmune encephalomyelitis was found in wild-type mice. Oral tolerance in microMT and wild-type mice was found to proceed by different mechanisms. Anergy was observed from 0.5 mg to 250 ng in microMT mice but not in wild-type mice. Increased Ag was detected in the lymph of microMT mice. No cytokine-mediated suppression was found following lower doses from 100 ng to 500 pg in either group. These results demonstrate the importance of the B cell for the induction of cytokine-mediated suppression associated with low doses of Ag.     

B7-H1 (programmed death-1 ligand) on dendritic cells is involved in the induction and maintenance of T cell anergy

In an effort to identify immunoregulatory molecules on dendritic cells (DC), we generated and screened for mAbs capable of modulating the T cell stimulatory function of DC. A particularly interesting mAb was mAb DF272. It recognizes monocyte-derived DC, but not blood monocytes or lymphocytes, and has profound immunomodulatory effects on DC. Treatment of DC with intact IgG or Fab of mAb DF272 enhanced their T cell stimulatory capacity. This effect on DC was accompanied by neither an up-regulation of costimulatory molecules such as B7.1 (CD80), B7.2 (CD86), and MHC class II molecules nor by an induction of cytokine production, including IL-1, TNF-alpha, IL-10, and IL-12. Moreover, the well-established inhibitory function of IL-10-treated DC could be reverted with mAb DF272. Even T cells, anergized because of stimulation with IL-10-treated DC, could be reactivated and induced to proliferate upon stimulation with mAb DF272-treated DC. Furthermore, mAb DF272-treated DC favored the induction of a type-1 cytokine response in T cells and inhibited IL-10 production. By using a retrovirus-based cDNA expression library generated from DC, we cloned and sequenced the mAb DF272-defined cell surface receptor and could demonstrate that it is identical with B7-H1 (programmed death-1 ligand), a recently identified new member of the B7 family of costimulatory molecules. Our results thus demonstrate that the mAb DF272-defined surface molecule B7-H1 represents a unique receptor structure on DC that might play a role in the induction and maintenance of T cell anergy.     

B7 costimulation in the development of lupus: autoimmunity arises either in the absence of B7.1/B7.2 or in the presence of anti-b7.1/B7.2 blocking antibodies.

Costimulatory molecules, termed B7.1 and B7.2, are present on the surfaces of APC and are important for the activation of T lymphocytes specific for both foreign Ags and autoantigens. We have examined the role of B7 costimulation in the MRL-lpr/lpr murine model of human systemic lupus erythematosus. MRL-lpr/lpr mice receiving both anti-B7.1 and anti-B7.2 Abs expressed significantly lower anti-small nuclear ribonucleoprotein particles (snRNP) and anti-dsDNA autoantibodies than did untreated mice. Anti-B7.2 Ab treatment alone inhibited anti-dsDNA autoantibody expression while having no effect on anti-snRNP autoantibody expression. Anti-B7.1 Ab treatment alone did not change the expression of either anti-snRNP or anti-dsDNA autoantibodies. Parallel studies performed in MRL-lpr/lpr mice genetically deficient in either B7.1 or B7.2 expressed autoantibody profiles comparable to those found in wild-type MRL-lpr/lpr mice. However, B7.1-deficient MRL-lpr/lpr mice exhibited distinct and more severe glomerulonephritis while B7.2-deficient MRL-lpr/lpr mice had significantly milder or absent kidney pathology as compared with age-matched wild-type mice. These studies indicate that each B7 costimulatory signal may control unique pathological events in murine systemic lupus erythematosus that may not always be apparent in autoantibody titers alone.     

Induction of bystander suppression by feeding antigen occurs despite normal clonal expansion of the bystander T cell population

The induction of bystander suppression, whereby the response against one Ag is suppressed when it is presented in the context of an Ag to which tolerance is already established, would be an important property of oral tolerance, because it would allow treatment of autoimmune and hypersensitivity responses where the initiating Ag is not known. Although bystander suppression has been described in oral tolerance, it is not known how its effects are mediated at the level of the bystander T cells. In addition, previous studies have not compared regimes in which Ag is fed in a tolerogenic or immunogenic manner, meaning that the possible effects of Ag competition have not been excluded. In this study we have used two populations of Ag-specific TCR transgenic CD4(+) T cells to examine the cellular basis of bystander suppression associated with oral tolerance in mice in vitro and in vivo. Our results show that bystander responses can be inhibited by feeding Ag and that these effects are more pronounced in mice fed protein in tolerogenic form than after feeding Ag with mucosal adjuvant. However, the expansion of the bystander-specific CD4(+) T cells is not influenced by the presence of oral tolerance. Thus, bystander suppression does not reflect clonal deletion or reduced clonal expansion of the bystander T cells, but may act by altering the functional differentiation of bystander T cells.     

B7 interactions with CD28 and CTLA-4 control tolerance or induction of mucosal inflammation in chronic experimental colitis

CD28-B7 interaction plays a critical costimulatory role in inducing T cell activation, while CTLA-4-B7 interaction provides a negative signal that is essential in immune homeostasis. Transfer of CD45RB(high)CD4(+) T cells from syngeneic mice induces transmural colon inflammation in SCID recipients. This adoptive transfer model was used to investigate the contribution of B7-CD28/CTLA-4 interactions to the control of intestinal inflammation. CD45RB(high)CD4(+) cells from CD28(-/-) mice failed to induce mucosal inflammation in SCID recipients. Administration of anti-B7.1 (but not anti-B7.2) after transfer of wild-type CD45RB(high)CD4(+) cells also prevented wasting disease with colitis, abrogated leukocyte infiltration, and reduced production of proinflammatory cytokines IL-2 and IFN-gamma by lamina propria CD4(+) cells. In contrast, anti-CTLA-4 treatment led to deterioration of disease, to more severe inflammation, and to enhanced production of proinflammatory cytokines. Of note, CD25(+)CD4(+) cells from CD28(-/-) mice similar to those from the wild-type mice were efficient to prevent intestinal mucosal inflammation induced by the wild-type CD45RB(high) cells. The inhibitory functions of these regulatory T cells were effectively blocked by anti-CTLA-4. These data show that the B7-CD28 costimulatory pathway is required for induction of effector T cells and for intestinal mucosal inflammation, while the regulatory T cells function in a CD28-independent way. CTLA-4 signaling plays a key role in maintaining mucosal lymphocyte tolerance, most likely by activating the regulatory T cells.     

CTLA-4 is not required for induction of CD8(+) T cell anergy in vivo.

Recent studies of T cell anergy induction have produced conflicting conclusions as to the role of the negative regulatory receptor, CTLA-4. Several in vivo models of tolerance have implicated the interaction of CTLA-4 and its ligands, B7.1 and B7.2, as an essential step in induction of anergy, while results from a number of other systems have indicated that signals from the TCR/CD3 complex alone are sufficient to induce T cell unresponsiveness. One explanation for this disparity is that the requirements for anergy induction depend closely on the details of the system: in vivo vs in vitro, route of stimulus administration, naive vs memory cells, CD4(+) vs CD8(+) cells, etc. To test this possibility, we established an in vivo anergy model using mice transgenic for the 2C TCR on a recombination-activating gene-2-deficient background, that either express or lack the CTLA-4 molecule. This system provides us with a very homogeneous pool of naive Ag-specific CD8(+) T cells, allowing us to control some of the conditions mentioned above. We found that T cells from CTLA-4-deficient mice were anergized by injections of soluble antigenic peptide as efficiently as were CTLA-4-expressing cells. These results indicate that CTLA-4 is not universally required for in vivo T cell anergy induction and may point to distinctions between regulation of peripheral tolerance in CD4(+) and CD8(+) T cells.     

CCR5 regulates high dose oral tolerance by modulating CC chemokine ligand 2 levels in the GALT

Oral tolerance is an immunomodulatory mechanism used by gut tissues to induce systemic tolerance to ingested proteins. In models of disease, such as experimental autoimmune encephalomyelitis, oral tolerance has been used to protect against paralysis induced by immunization with myelin proteins. Previous work in our laboratory has shown a role for the chemokine, CCL2, and its receptor in the induction of high dose oral tolerance. In the present study, we report that two CCR5 ligands, CCL4 and CCL5, are expressed in gut tissues after Ag feeding. CCR5(-/-) mice were unable to be tolerized by feeding a high dose of Ag and were not protected from developing experimental autoimmune encephalomyelitis. Moreover, CCR5(-/-) mice did not display cytokine deviation as normally seen after high dose oral Ag. Using a selective CCR5 antagonist, methionine-RANTES, CCL2 expression was inhibited, resulting in enhanced IL-12 production and the inability for mice treated with methionine-RANTES to become orally tolerized. This current study suggests that CCR5 ligands may function to modulate CCL2 levels in the gut after Ag feeding, promoting a cellular environment that favors tolerance rather than immunity.     

CD28 signaling augments Elk-1-dependent transcription at the c-fos gene during antigen stimulation.

Untransformed CD4(+) Th1 cells stimulated with Ag and APC demonstrated a dependence on B7- and CD28-mediated costimulatory signals for the expression and function of AP-1 proteins. The induction of transactivation by the c-fos gene regulator Elk-1 mirrored this requirement for TCR and CD28 signal integration. c-Jun N-terminal kinase (JNK) (but not extracellular signal-regulated kinase or p38) protein kinase activity was similarly inhibited by neutralizing anti-B7 mAbs. Blockade of JNK protein kinase activity with SB 202190 prevented both Elk-1 transactivation and c-Fos induction. These results identify a unique role for B7 costimulatory molecules and CD28 in the activation of JNK during Ag stimulation in Th1 cells, and suggest that JNK regulates Elk-1 transactivation at the c-fos gene to promote the formation of AP-1 complexes important to IL-2 gene expression.     

Kinetic analysis of oral tolerance: memory lymphocytes are refractory to oral tolerance.

Oral administration of soluble Ag before immunization induces peripheral tolerance and is effective in suppressing animal models of autoimmune diseases. Although tolerance induction in primed animals is more clinically relevant, it is not well studied. Therefore, this study was designed to examine the feeding effects on different phases of the immune response. We observed that feeding a single high dose (250 mg) of OVA to OVA-primed BALB/c mice could induce OVA-specific suppression in the Ab production and T cell proliferation only at the naive and the activation phases of the immune response, whereas multiple high doses (100 mg/feed for 10 days) were effective at the effector phase. OVA-specific IL-4 production in culture supernatant was also suppressed in the tolerized groups. However, when the mice had resting memory lymphocytes, even multiple feeding regimens were not effective in tolerance induction, although multiple low doses (1 mg/feed for 10 days) partially suppressed Ab production. This phenomenon was confirmed by adoptive transfer study. Nevertheless, the reactivated memory response was suppressed partially by multiple high doses. Our findings have an important implication for understanding the mechanism of oral tolerance and for the therapeutic applications of oral tolerance to autoimmune diseases.     

CD4+ T cells from lupus-prone mice avoid antigen-specific tolerance induction in vivo

Activated T cells in spontaneous lupus presumably bypass normal tolerance mechanisms in the periphery, since thymic tolerance appears intact. To determine whether such T cells indeed avoid in vivo peripheral tolerance mechanisms, we assessed their activation and recall responses after in vivo Ag stimulation in the absence of exogenously supplied costimulatory signals. Naive CD4(+) AND (transgenic mice bearing rearranged TCR specific for pigeon cytochrome c, peptides 88-104) TCR-transgenic T cells, specific for pigeon cytochrome c, from lupus-prone Fas-intact MRL/Mp+(Fas-lpr) and from H-2(k)-matched control CBA/CaJ and B10.BR mice (MRL.AND, CBA.AND, and B10.AND, respectively) were adoptively transferred into (MRL x CBA)F(1) or (MRL x B10)F(1) recipients transgenically expressing membrane-bound pigeon cytochrome c as a self-Ag. MRL.AND and control CBA.AND and B10.AND-transgenic T cells were activated and divided after transfer, indicating encounter with their cognate Ag; however, T cells from CBA.AND and B10.AND mice were impaired in their ability to proliferate and produce IL-2 after challenge with pigeon cytochrome c in ex vivo recall assays, a typical phenotype of anergized cells. By contrast, MRL.AND T cells proliferated more, and a significantly higher percentage of such cells produced IL-2, compared with control T cells. This observation that MRL T cells avoided anergy induction in vivo was confirmed in an in vitro system where the cells were stimulated with an anti-CD3 in the absence of a costimulatory signal. These experiments provide direct evidence that CD4(+) T cells from Fas-intact lupus-prone MRL mice are more resistant than nonautoimmune control cells to anergy induction. Anergy avoidance in the periphery might contribute to the characteristic finding in lupus of inappropriate T cell activation in response to ubiquitous self-Ags.     

Intercellular adhesion molecule 1 is critical for activation of CD28-deficient T cells

Presentation of Ag to T lymphocytes in the absence of the requisite costimulatory signals leads to an Ag-specific unresponsiveness termed anergy, whereas Ag presentation in conjunction with costimulation leads to clonal expansion. B7/CD28 signaling has been shown to provide this critical costimulatory signal and blockade of this pathway may inhibit in vitro and in vivo immune responses. Although T cells from CD28-deficient mice are lacking in a variety of responses, they nonetheless are capable of various primary and secondary responses without the induction of anergy expected in the absence of costimulation. This suggests that there may be alternative costimulatory pathways that can replace CD28 signaling under certain circumstances. In this paper, we show that ICAM-1becomes a dominant costimulatory molecule for CD28-deficient T cells. ICAM-1 costimulates anti-CD3-mediated T cell proliferation and IL-2 secretion in CD28-deficient murine T cells. Furthermore, splenocytes from ICAM-1-deficient mice could not activate CD28-deficient T cells and splenocytes lacking both ICAM and CD28 fail to proliferate in response to anti-CD3-induced T cell signals. This confirms that not only can ICAM-1 act as a CD28-independent costimulator, but it is the dominant, requisite costimulatory molecule for the activation of T cells in the absence of B7/CD28 costimulation.     

The in vivo elimination of CD4+ T cells prevents the induction but not the expression of carrier-induced epitopic suppression.

Injection of mice with an immunogenic dose of carrier (keyhole limpet hemocyanin (KLH)) followed by immunization with hapten-carrier conjugate (TNP-KLH) selectively suppresses anti-hapten antibody response. In this study, the cellular basis of this epitopic suppression and also of the suppression induced by a high dose of carrier were analyzed by in vivo depletion of CD4+ or CD8+ T cell subsets by using mAb. The mAb treatments were performed either at the time of carrier priming or at the time of hapten-carrier immunization. The elimination of CD8+ T cells has not modified the anti-carrier antibody response, whether this treatment was performed at the time of KLH-priming or during TNP-KLH immunization. Moreover, the in vivo treatment with the anti-CD8 mAb did not modify the carrier-induced epitopic suppression induced either by a low immunogenic dose of KLH or by a high dose of this Ag. The elimination of CD4+ T cells at the time of KLH immunization has prevented the induction of a memory response to KLH, clearly establishing that CD4+ T cells are essential in memory B cell development to T-dependent Ag. Moreover, this treatment has totally abrogated the epitopic suppression induced either by low or high dosages of KLH. In contrast, the in vivo elimination of CD4+ T cells after carrier immunization did not abolish the secondary anti-carrier antibody response and did not prevent the expression of epitopic suppression. These data indicate that primed CD4+ T cells are required neither for memory B cell expression nor for the expression of suppression. Finally, once induced, the suppression can be evidenced after in vivo depletion of both primed CD4+ and CD8+ T cells. These data support the view that epitopic suppression is induced through the expansion of carrier-specific B cells and resulted from intramolecular antigenic competition between hapten and carrier epitopes.     

Anergy in memory CD4+ T cells is induced by B cells

Induction of tolerance in memory T cells has profound implications in the treatment of autoimmune diseases and transplant rejection. Previously, we reported that the presentation of low densities of agonist peptide/MHC class II complexes induced anergy in memory CD4(+) T cells. In the present study, we address the specific interaction of different types of APCs with memory CD4(+) T cells. A novel ex vivo anergy assay first suggested that B cells induce anergy in memory T cells, and an in vivo cell transfer assay further confirmed those observations. We demonstrated that B cells pulsed with defined doses of Ag anergize memory CD4 cells in vivo. We established that CD11c(+) dendritic cells do not contribute to anergy induction to CD4 memory T cells, because diphtheria toxin receptor-transgenic mice that were conditionally depleted of dendritic cells optimally induced anergy in memory CD4(+) T cells. Moreover, B cell-deficient muMT mice did not induce anergy in memory T cells. We showed that B2 follicular B cells are the specific subpopulation of B cells that render memory T cells anergic. Furthermore, we present data showing that anergy in this system is mediated by CTLA-4 up-regulation on T cells. This is the first study to demonstrate formally that B cells are the APCs that induce anergy in memory CD4(+) T cells.     

Distinct requirements for deletion versus anergy during CD8 T cell peripheral tolerance in vivo

Activation of naive T cells by quiescent APCs results in tolerance through deletion and anergy. The underlying basis for these distinct fates is unclear. Using clone 4 TCR transgenic animals as a source of naive CD8 T cells, we examined the requirements for peripheral deletion in vivo. Our results demonstrate that independent of the amount of Ag used for stimulation, a single dose was insufficient to achieve complete clonal deletion. Instead, further antigenic exposure was required to completely eliminate all of the activated T cells. Additionally, consecutive stimulations with low doses of Ag were highly effective in promoting deletion. In contrast, although stimulation with high doses of Ag initially led to the apoptosis of many of the activated T cells, it induced hyporesponsiveness in a portion of the responding cells, thereby sparing them from further activation and deletion. These data explain why some conditions promote tolerance through clonal deletion whereas others promote anergy. Furthermore, these data provide a framework to devise protocols for effective deletion of potentially autoreactive T cells.