CD4+ T cells in the absence of the CD8+ cytotoxic T cells are critical and sufficient for NKT cell-dependent tumor rejection

NKT cells perform crucial roles in tumor surveillance, functioning as regulators of early host response. In this study, we have assessed the effects of NKT activation at the time of tumor Ag immunization, and have evaluated the contributions of CD4+ and CD8+ T cells in tumor rejection during adaptive immune response against live tumor cells. Our data indicate that CD4+ T cells play critical roles, not only in assisting CTL, but also in the orchestration of host response against the tumor. The CD4+ T cells were found to reject the transplanted tumor cells very efficiently under conditions in which the CTLs were removed either genetically, or via the action of anti-CD8 Ab in mice that had been immunized with tumor extracts and alpha-galactosylceramide. Immunization resulted in an NKT cell-dependent antitumor adaptive immune response, which was associated with both CD4+ T cells and cytokine IFN-gamma.     

A plant-based allergy vaccine suppresses experimental asthma via an IFN-gamma and CD4+CD45RBlow T cell-dependent mechanism

Allergic asthma is currently considered a chronic airway inflammatory disorder associated with the presence of activated CD4(+) Th2-type lymphocytes, eosinophils, and mast cells. Interestingly, therapeutic strategies based on immune deviation and suppression have been shown to successfully attenuate the development of the asthma phenotype. In this investigation, we have for the first time used a genetically modified (GM) plant, narrow leaf lupin (Lupinus angustifolius L.), expressing a gene for a potential allergen (sunflower seed albumin) (SSA-lupin) to examine whether a GM plant/food-based vaccine strategy can be used to suppress the development of experimental asthma. We show that oral consumption of SSA-lupin promoted the induction of an Ag-specific IgG2a Ab response. Furthermore, we demonstrate that the plant-based vaccine attenuated the induction of delayed-type hypersensitivity responses and pathological features of experimental asthma (mucus hypersecretion, eosinophilic inflammation, and enhanced bronchial reactivity (airways hyperreactivity). The suppression of experimental asthma by SSA-lupin was associated with the production of CD4(+) T cell-derived IFN-gamma and IL-10. Furthermore, we show that the specific inhibition of experimental asthma was mediated via CD4(+)CD45RB(low) regulatory T cells and IFN-gamma. Thus, our data demonstrate that a GM plant-based vaccine can promote a protective immune response and attenuate experimental asthma, suggesting that plant-based vaccines may be potentially therapeutic for the protection against allergic diseases.     

IFN-gamma-dependent inhibition of tumor angiogenesis by tumor-infiltrating CD4+ T cells requires tumor responsiveness to IFN-gamma

The importance of CD4(+) T cells in the induction of an optimal antitumor immune response has largely been attributed to their ability to provide costimulatory signals for the priming of MHC class I-restricted CD8(+) CTL. However, many reports have demonstrated a requirement for CD4(+) T cells in the effector phase of tumor rejection indicating a greater responsibility for CD4(+) T cells in controlling tumor outgrowth. We demonstrate here a critical role for CD4(+) T cells in restraining initial tumor development through the inhibition of tumor angiogenesis. Using a tumor variant that is unresponsive to IFN-gamma, we show that tumor responsiveness to IFN-gamma is necessary for IFN-gamma-dependent inhibition of tumor angiogenesis by CD4(+) T cells. These studies reveal a pivotal role for CD4(+) T cells in controlling early tumor development through inhibition of tumor angiogenesis.     

CD8+ T cell homing to the pancreas in the nonobese diabetic mouse is CD4+ T cell-dependent.

The adoptive transfer of type I diabetes in nonobese diabetic mice requires the contribution of both CD4+ and CD8+ T cells. To further elucidate the cellular pathway(s) of beta-cell destruction and the responsibility of each subset, high doses of committed T cells from diabetic mice purified to single subsets, were injected into syngeneic nonobese diabetic neonates. The recipients of single or mixed subsets were followed for clinical manifestations of diabetes and examined at 30 days of age for in situ lesions. None of the animals injected with either CD4+ or CD8+ T cells became overtly diabetic during the 30 days of observation whereas 8 of 23 mice inoculated with a mixture of the two subsets developed glycosuria and hyperglycemia. However, insulitis was found in 6 of the 13 mice injected with CD4+ T cells whereas only 1 of the 9 mice injected with CD8+ T cells showed marginal infiltration of the pancreas. The lesions initiated by CD4+ T cells alone were considerably less severe than those induced by the mixture of both subsets, corroborating the fact that overt disease did not occur in the former group. Together, these results suggest a distinct function for each diabetogenic T cell subset. CD4+ T cells, which have the capacity to home to the pancreas, promote in turn the influx of CD8+ effector T cells that do not by themselves accumulate in this organ. These results illustrate a novel form of T-T cell interactions leading to organ specific autoimmune lesions.     

CD4+CD25+ T cell-dependent inhibition of autoimmunity in transgenic mice overexpressing human Bcl-2 in T lymphocytes

Regulation of lymphocyte survival is essential for the maintenance of lymphoid homeostasis preventing the development of autoimmune diseases. Recently, we described a systemic lupus erythematosus associated with an IgA nephropathy in autoimmune-prone (NZW x C57BL/6)F(1) overexpressing human Bcl-2 (hBcl-2) in B cells (transgenic (Tg) 1). In the present study, we analyze in detail a second line of hBcl-2 Tg mice overexpressing the transgene in all B cells and in a fraction of CD4(+) and CD8(+) T cells (Tg2). We demonstrate here that the overexpression of hBcl-2 in T cells observed in Tg2 mice is associated with a resistance to the development of lupus disease and collagen type II-induced arthritis in both (NZW x C57BL/6)F(1) and (DBA/1 x C57BL/6)F(1) Tg2 mice, respectively. The disease-protective effect observed in autoimmune-prone Tg2 mice is accompanied by an increase of peripheral CD4(+)CD25(+) hBcl-2(+) regulatory T cells (T(regs)), expressing glucocorticoid-induced TNFR, CTLA-4, and FoxP3. Furthermore, the in vivo depletion of CD4(+)CD25(+) T(regs) in (DBA/1 x C57BL/6)F(1) Tg2 mice promotes the development of a severe collagen type II-induced arthritis. Taken together, our results indicate that the overexpression of hBcl-2 in CD4(+) T cells alters the homeostatic mechanisms controlling the number of CD4(+)CD25(+) T(regs) resulting in the inhibition of autoimmune diseases.     

Role of the L3T4+ T cell in allograft rejection

Pancreatic islet and fetal pancreas allotransplantation has been used to examine the role of the L3T4+ T cell in allograft rejection. Tissues were grafted into recipient animals depleted of peripheral L3T4+ T cells by in vivo administration of GK1.5 (anti-L3T4) monoclonal antibody to ask the question: is there a requirement for the L3T4+ T cell in graft rejection? Data show that the requirement for the L3T4+ T cell depends on either the type of tissue transplanted or type of the antigenic disparity between donor and recipient. Data also indicate that islet allograft acceptance achieved after GK1.5 treatment of the recipient is not due to tolerance induction. We therefore conclude that the cellular requirements for allograft rejection are determined by the type of tissue transplanted and the genetic disparity between donor and recipient.     

CD8+ T cells circumvent immune privilege in the eye and mediate intraocular tumor rejection by a TNF-alpha-dependent mechanism

Although intraocular tumors reside in an immune-privileged environment, T cells can circumvent immune privilege and mediate tumor rejection without inducing damage to normal ocular tissue. In this study, we used a well-characterized tumor, Ad5E1 (adenovirus type 5 early region 1), to analyze the role of CD8+ T cells in the pristine rejection of intraocular tumors. It has been previously documented that Ad5E1 tumor rejection can occur in the absence of CD8+ T cells. However, here we find that CD8+ T cells infiltrated intraocular Ad5E1 tumors in C57BL/6 mice. Surprisingly, CD8+ T cells from tumor-rejector mice could mediate intraocular tumor rejection following adoptive transfer to SCID mice. In determining the mechanisms behind CD8+ T cell-mediated tumor rejection, we discovered that antitumor CTL activity was neither observed nor necessary for rejection of the intraocular tumors. CD8+ T cells from rejector mice did not produce IFN-gamma in response to Ad5E1 tumor Ags or use FasL to mediate intraocular tumor rejection. Also, CD8+ T cells did not use perforin or TRAIL, as CD8+ T cells from perforin knockout (KO) and TRAIL KO mice conferred protection to SCID recipient mice following adoptive transfer. We discovered that CD8+ T cells used TNF-alpha to mediate tumor rejection, because Ad5E1 tumor cells were highly sensitive to TNF-alpha-induced apoptosis and CD8+ T cells from TNF-alpha KO mice did not protect SCID mice from progressive Ad5E1 tumor growth. The results indicate that CD8+ T cells circumvent immune privilege and mediate intraocular tumor rejection by a TNF-alpha-dependent manner while leaving the eye intact and vision preserved.     

TRAIL suppresses tumor growth in mice by inducing tumor-infiltrating CD4+CD25+ Treg apoptosis

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), a promising and novel anticancer cytokine, specifically kills numerous tumor cells by apoptosis. However, some malignancies are resistant to TRAIL treatment in clinical trials, thus limiting its therapeutic potential. In the present study, the TRAIL-resistant murine hepatocellular carcinoma cell line Hepa1-6 was used to elucidate the physiological significance of TRAIL resistance, especially with respect to the immune regulatory function of TRAIL. Hepa1-6 cells were resistant to TRAIL-induced apoptosis in vitro; however, intratumoral injection of recombinant soluble TRAIL inhibited tumor growth and prolonged survival time in tumor-bearing mice. Local TRAIL treatment decreased the number of intratumoral CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs) but did not affect CD4⁺CD25⁺Foxp3⁺ Tregs in the draining lymph nodes and spleen. Further investigation showed that TRAIL induced apoptosis of tumor-activated CD4⁺CD25⁺Foxp3⁺ Tregs, but not of CD4⁺CD25^? T cells. Moreover, mouse TRAIL receptor DR5 expression was detected on the surface of the tumor-infiltrating CD4⁺CD25⁺Foxp3⁺ Tregs, but not on naïve CD4⁺CD25⁺Foxp3⁺ Tregs. Interestingly, intratumoral injection of TRAIL not only decreased the number of CD4⁺CD25⁺Foxp3⁺ Tregs but also increased the number of tumor-specific CD8⁺ CTL and augmented their cytotoxicity to the tumor cells. These data provide the novel evidence for an immune regulatory function of TRAIL and may shed light on the clinical application of TRAIL.     

Conservation of CD4+ T cell-dependent developmental mechanisms in the blood fluke pathogens of humans

Schistosoma blood flukes are trematode parasites with a cosmopolitan distribution that infect over 200 million people globally. We previously showed that Schistosoma mansoni growth and development in the mammalian host is dependent on signals from host CD4+ T cells. To gain insight into the mechanisms that underlie this dependence, we sought to determine the evolutionary origins and limits of this aspect of the host-pathogen relationship. By infecting RAG-1-/- mice with a range of different schistosome species and strains, we tested several hypotheses concerning the time during Schistosoma evolution at which this dependence arose, and whether this dependence is specific to Schistosoma or is also found in other blood flukes. Our data indicate that the developmental dependence on CD4+ T cells previously described for S. mansoni is conserved in the evolutionarily basal species Schistosoma japonicum, suggesting this developmental adaptation arose early in Schistosoma evolution. We also demonstrate that the development of the more evolutionarily derived species Schistosoma haematobium and Schistosoma intercalatum are dependent on adaptive immune signals. Together, these data suggest that the blood fluke parasites of humans utilise common mechanisms to infect their hosts and to co-opt immune signals in the coordination of parasite development. Thus, exploitation of host-schistosome interactions to impair or prevent parasite development may represent a novel approach to combating all of the schistosome pathogens of humans.     

Injection of soluble antigen into the anterior chamber of the eye induces expansion and functional unresponsiveness of antigen-specific CD8+ T cells

The injection of soluble Ag into the anterior chamber (a.c.) of the eye induces systemic tolerance, termed a.c.-associated immune deviation (ACAID), characterized by Ag-specific inhibition of delayed-type hypersensitivity responses and a reduction in complement-fixing Abs. Recently, we have shown that CD8(+) CTL responses are also inhibited in ACAID. In this study, we have used an adoptive transfer approach to follow the fate of Ag-specific CD8(+) TCR transgenic (OT-I) T cells in vivo during the induction and expression of ACAID. C57BL/6 (B6) recipients of OT-I splenocytes that were injected with chicken OVA in the a.c. displayed reduced OVA-specific delayed-type hypersensitivity and CTL responses, compared with those of mice given OVA in the subconjunctiva or an irrelevant Ag human IgG in the a.c. OT-I T cells increased 9-fold in the submandibular lymph nodes and 3-fold in the spleen following an a.c. injection with OVA, indicating that expansion rather than deletion of Ag-specific CD8(+) T cells was induced by this treatment. OT-I T cells expanded equivalently upon administration of OVA in CFA to mice previously given OVA in the a.c. or subconjunctiva. However, the lytic activity attributed to OT-I T cells was reduced on a per-cell basis in mice previously given OVA in the a.c. We conclude that tolerance of CTL responses in mice given Ag via the a.c. results from unresponsiveness of Ag-specific CD8(+) T cells.     

IFN-gamma and CD8+ T cells restore host defenses against Pneumocystis carinii in mice depleted of CD4+ T cells

Host defenses against infection are profoundly compromised in HIV-infected hosts due to progressive depletion of CD4+ T lymphocytes and defective cell-mediated immunity. Although recent advances in antiretroviral therapy can dramatically lower HIV viral load, blood CD4+ T lymphocytes are not restored to normal levels. Therefore, we investigated mechanisms of host defense other than those involving CD4+ T lymphocytes against a common HIV-related opportunistic infection, Pneumocystis carinii (PC) pneumonia. Using CD4-depleted mice, which are permissive for chronic PC infection, we show that up-regulation of murine IFN-gamma by gene transfer into the lung tissue results in clearance of PC from the lungs in the absence of CD4+ lymphocytes. This resolution of infection was associated with a >4-fold increase in recruited CD8+ T lymphocytes and NK cells into the lungs. The role of CD8+ T cells as effector cells in this model was further confirmed by a lack of an effect of IFN-gamma gene transfer in scid mice or mice depleted of both CD4+ and CD8+ T cells. Cytokine mRNA analysis revealed that recruited, lung-derived CD8+ T cells had greater expression of IFN-gamma message in animals treated with the IFN-gamma gene. These results indicate that CD8+ T cells are capable of clearing PC pneumonia in the absence of CD4+ T cells and that this host defense function of CD8+ T cells, as well as their cytokine repertoire, can be up-regulated through cytokine gene transfer.     

Accumulation of immunosuppressive CD11b+ myeloid cells correlates with the failure to prevent tumor growth in the anterior chamber of the eye

The purpose of these studies is to determine why an immunogenic tumor grows unchecked in the anterior chamber (a.c.) of the eye. The OVA-expressing EL4 tumor, E.G7-OVA, was injected into the a.c. or skin of immunocompetent and immunodeficient mice. Tumor growth and tumor-specific immune responses were monitored. Ocular tumor-infiltrating leukocytes were characterized phenotypically and functionally. Growth of E.G7-OVA was inhibited when limiting numbers of cells were injected in the skin but not in the a.c. of C57BL/6 mice, although both routes primed OVA-specific immune responses, which prevented the growth of a subsequent injection with E.G7-OVA in the skin or opposite eye. Tumor regression was OVA-specific because growth of the parental EL-4 tumor was not inhibited in primed mice. E.G7-OVA growth in the skin was not inhibited in immunodeficient Rag(-/-) or CD8 T cell-deficient mice, suggesting that CD8(+) CTLs mediate tumor elimination. CD8(+) T cell numbers were significantly increased in eyes of mice primed with E.G7-OVA, but few were detected in primary ocular tumors. Nevertheless, growth of E.G7-OVA was retarded in the a.c. of TCR-transgenic OT-I mice, and CD8(+) T cell numbers were increased within eyes, suggesting that tumor-specific CD8(+) CTLs migrated into and controlled primary ocular tumor growth. E.G7-OVA did not lose antigenicity or become immunosuppressive after 13 days of growth in the eye. However, CD11b(+) cells accumulated in primary ocular tumors and contained potent immunosuppressive activity when assayed in vitro. Thus, CD11b(+) cells that accumulate within the eye as tumors develop in the a.c. may contribute to immune evasion by primary ocular tumors by inhibiting CTLs within the eye.     

An essential contribution by IFN-gamma to CD8+ T cell-mediated rejection of pancreatic islet allografts

CD8+ T cells have long been considered to be the prototypical cytotoxic lymphocyte subpopulation. However, whether alloreactive CD8+ T cells require traditional cytolytic pathways such as perforin and Fas ligand (FasL) to mediate graft rejection has been a controversial issue. In the present studies, we examined the role of varied effector pathways in CD8+ T cell-mediated rejection of pancreatic islet allografts. Our goal was to systematically determine the relative requirements, if any, of perforin and FasL as well as the proinflammatory cytokine IFN-gamma in triggering graft destruction. To study CD8+ T cell effector pathways independently of other lymphocyte populations, purified alloreactive CD8+ T cells were adoptively transferred into severe combined immune-deficient (SCID) recipients bearing established islet allografts. Results indicate that to reject established islet allografts, primed CD8+ T cells do not require the individual action of the conventional cytotoxic effectors perforin and Fas ligand. In contrast, the ability to produce IFN-gamma is critical for efficient CD8+ T cell-mediated rejection of established islet allografts. Furthermore, alloreactive CD8+ TCR transgenic T cells (2C) also show IFN-gamma dependence for mediating islet allograft rejection in vivo. We speculate from these results that the production of IFN-gamma by alloreactive CD8+ T cells is a rate-limiting step in the process of islet allograft rejection.     

TRAIL deficiency does not rescue impaired CD8+ T cell memory generated in the absence of CD4+ T cell help

Ag-specific CD8(+) T cells immunized in the absence of CD4(+) T cell help, so-called "unhelped" CD8(+) T cells, are defective in function and survival. We investigated the role of the proapoptotic molecule TRAIL in this defect. We first demonstrate that TRAIL does not contribute to the CD8(+) T cell response to Listeria monocytogenes strain expressing OVA (LmOVA) in the presence of CD4(+) T cells. Secondly, we generated mice doubly deficient in CD4(+) T cells and TRAIL and analyzed their CD8(+) T cell response to LmOVA. Memory CD8(+) T cells in double-deficient mice waned over time and were not protective against rechallenge, similar to their TRAIL-sufficient unhelped counterparts. To avoid the effects of CD4(+) T cell deficiency during memory maintenance, and to address whether TRAIL plays a role in the early programming of the CD8(+) T cell response, we performed experiments using heterologous prime and early boost immunizations. We did not observe activation-induced cell death of unhelped CD8(+) T cells when mice were infected with followed vaccinia virus expressing OVA 9 days later by LmOVA infection. Furthermore, primary immunization of CD4(+) T cell-deficient mice with cell-associated Ag followed by LmOVA infection did not reveal a role for TRAIL-mediated activation-induced cell death. Overall, our results suggest that CD4(+) T cell help for the CD8(+) T cell response is not contingent on the silencing of TRAIL expression and prevention of TRAIL-mediated apoptosis.     

IFN-gamma controls the generation/activation of CD4+ CD25+ regulatory T cells in antitumor immune response

Immunization with serological identification of Ags by recombinant expression cloning (SEREX)-defined self-Ags leads to generation/activation of CD4+ CD25+ regulatory T cells with suppressive activities and enhanced expression of Foxp3. This is associated with increased susceptibility to pulmonary metastasis following challenge with syngeneic tumor cells and enhanced development of 3-methylcholanthrene-induced primary tumors. In contrast, coimmunization with the same SEREX-defined self-Ags mixed with a CTL epitope results in augmented CTL activity and heightened resistance to pulmonary metastasis, both of which depend on CD4+ Th cells. These active regulatory T cells and Th cells were derived from two distinct CD4+ T cell subsets, CD4+ CD25+ T cells and CD4+ CD25- T cells, respectively. In the present study, IFN-gamma was found to abrogate the generation/activation of CD4+ CD25+ regulatory T cells by immunization with SEREX-defined self-Ag. CD4+ CD25+ T cells from these IFN-gamma-treated mice failed to exhibit immunosuppressive activity as measured by 1) increased number of pulmonary metastasis, 2) enhanced development of 3-methylcholanthrene-induced primary tumors, 3) suppression of peptide-specific T cell proliferation, and 4) enhanced expression of Foxp3. The important role of IFN-gamma produced by CD8+ T cells was shown in experiments demonstrating that CD4+ CD25+ T cells cotransferred with CD8+ T cells from IFN-gamma(-/-) mice, but not from wild-type BALB/c mice, became immunosuppressive and enhanced pulmonary metastasis when recipient animals were subsequently immunized with a SEREX-defined self-Ag and a CTL epitope. These findings support the idea that IFN-gamma regulates the generation/activation of CD4+ CD25+ regulatory T cells.     

Human CD4+ T cells mediate rejection of porcine xenografts

It has previously been demonstrated that xenograft rejection in rodents is dependent on CD4+ T cells. However, because of the lack of an appropriate in vivo model, little is known about the cellular basis of human T cell-mediated rejection of xenografts. In this study, we have evaluated the ability of human T cells to mediate rejection of porcine skin grafts in a novel in vivo experimental system using immunodeficient mice as recipients. Recombinase-activating gene-1-deficient mice (R-) lacking mature B and T cells were grafted with porcine skin and received human lymphocytes stimulated in vitro with irradiated porcine PBMC. Skin grafts on mice given either unseparated, activated human lymphocytes, or NK cell-depleted lymphocyte populations were rejected within 18 days after adoptive cell transfer. In contrast, skin grafts on mice given T cell-depleted human lymphocytes or saline showed no gross or histologic evidence of rejection up to 100 days after adoptive transfer. Purified CD4+ T cells were also able to mediate rejection of porcine skin grafts. These data suggest that human CD4+ T cells are sufficient to induce rejection of porcine xenografts. Thus, strategies directed toward CD4+ T cells may effectively prevent cellular rejection of porcine xenografts in humans.     

CD4 T cell-dependent conditioning of dendritic cells to produce IL-12 results in CD8-mediated graft rejection and avoidance of tolerance

Rejection of ectopic heart transplants expressing OVA requires OVA-specific CD4 and CD8 T cells. In the absence of CD4 T cells, OVA-specific CD8 T cells proliferate and migrate to the graft, but fail to develop cytolytic functions. With CD4 T cells present, clonal expansion of the CD8 T cells is only marginally increased but the cells now develop effector functions and mediate rapid graft rejection. In the presence of CD4 T cells, Ag and B7 levels do not increase on dendritic cells but IL-12 production is up-regulated, and this requires CD154 expression on the CD4 T cells. OVA-specific CD8 T cells lacking the IL-12 receptor fail to differentiate or mediate graft rejection even when CD4 T cells are present. Thus, CD4 T cells condition dendritic cells by inducing the production of IL-12, which is needed as the "third signal" for CD8 T cell differentiation and avoidance of tolerance.     

Donor IFN-gamma receptors are critical for acute CD4(+) T cell-mediated cardiac allograft rejection.

Recent studies using mouse models demonstrate that CD4(+) T cells are sufficient to mediate acute cardiac allograft rejection in the absence of CD8(+) T cells and B cells. However, the mechanistic basis of CD4-mediated rejection is unclear. One potential mechanism of CD4-mediated rejection is via elaboration of proinflammatory cytokines such as IFN-gamma. To determine whether IFN-gamma is a critical cytokine in CD4-mediated acute cardiac allograft rejection, we studied whether the expression of IFN-gamma receptors on the donor heart was required for CD4-mediated rejection. To investigate this possibility, purified CD4(+) T cells were transferred into immune-deficient mice bearing heterotopic cardiac allografts from IFN-gamma receptor-deficient (GRKO) donors. While CD4(+) T cells triggered acute rejection of wild-type heart allografts, they failed to trigger rejection of GRKO heart allografts. The impairment in CD4-mediated rejection of GRKO hearts appeared to primarily involve the efferent phase of the immune response. This conclusion was based on the findings that GRKO stimulator cells provoked normal CD4 proliferation in vitro and that intentional in vivo challenge of CD4 cells with wild-type donor APC or the adoptive transfer of in vitro primed CD4 T cells failed to provoke acute rejection of GRKO allografts. In contrast, unseparated lymph node cells acutely rejected both GRKO and wild-type hearts with similar time courses, illustrating the existence of both IFN-gamma-dependent and IFN-gamma-independent mechanisms of acute allograft rejection.     

Prevention of acute and chronic allograft rejection with CD4+CD25+Foxp3+ regulatory T lymphocytes

A major challenge in transplantation medicine is controlling the very strong immune responses to foreign antigens that are responsible for graft rejection. Although immunosuppressive drugs efficiently inhibit acute graft rejection, a substantial proportion of patients suffer chronic rejection that ultimately leads to functional loss of the graft. Induction of immunological tolerance to transplants would avoid rejection and the need for lifelong treatment with immunosuppressive drugs. Tolerance to self-antigens is ensured naturally by several mechanisms; one major mechanism depends on the activity of regulatory T lymphocytes. Here we show that in mice treated with clinically acceptable levels of irradiation, regulatory CD4+CD25+Foxp3+ T cells stimulated in vitro with alloantigens induced long-term tolerance to bone marrow and subsequent skin and cardiac allografts. Regulatory T cells specific for directly presented donor antigens prevented only acute rejection, despite hematopoietic chimerism. By contrast, regulatory T cells specific for both directly and indirectly presented alloantigens prevented both acute and chronic rejection. Our findings demonstrate the potential of appropriately stimulated regulatory T cells for future cell-based therapeutic approaches to induce lifelong immunological tolerance to allogeneic transplants.