Lymphopenia-induced proliferation is a potent activator for CD4+ T cell-mediated autoimmune disease in the retina

Follicular helper- and peripheral helper-like T cells drive autoimmune disease in human immune system mice

Human immune system (HIS) mice constructed in various ways are widely used for investigations of human immune responses to pathogens, transplants and immunotherapies. In HIS mice that generate T cells de novo from hematopoietic progenitors, T cell-dependent multisystem autoimmune disease occurs, most rapidly when the human T cells develop in the native NOD.Cg- Prkdc scid Il2rg tm1Wjl (NSG) mouse thymus, where negative selection is abnormal. Disease develops very late when human T cells develop in human fetal thymus grafts, where robust negative selection is observed. We demonstrate here that PD-1 + CD4 + peripheral (Tph) helper-like and follicular (Tfh) helper-like T cells developing in HIS mice can induce autoimmune disease. Tfh- like cells were more prominent in HIS mice with a mouse thymus, in which the highest levels of IgG were detected in plasma, compared to those with a human thymus. While circulating IgG and IgM antibodies were autoreactive to multiple mouse antigens, in vivo depletion of B cells and antibodies did not delay the development of autoimmune disease. Conversely, adoptive transfer of enriched Tfh- or Tph-like cells induced disease and autoimmunity-associated B cell phenotypes in recipient mice containing autologous human APCs without T cells. Tfh/Tph cells from mice with a human thymus expanded and induced disease more rapidly than those originating in a murine thymus, implicating HLA-restricted T cell-APC interactions in this process. Since Tfh, Tph, autoantibodies and lymphopenia-induced proliferation (LIP) have all been implicated in various forms of human autoimmune disease, the observations here provide a platform for the further dissection of human autoimmune disease mechanisms and therapies.

Parabiosis reveals the correlation between the recruitment of circulating antigen presenting cells to the retina and the induction of spontaneous autoimmune uveoretinitis

BACKGROUND

Characterizing immune cells and conditions that govern their recruitment and function in autoimmune diseases of the nervous system or in neurodegenerative processes is an area of active investigation. We sought to analyze the origin of antigen presenting cells associated with the induction of retinal autoimmunity using a system that relies on spontaneous autoimmunity, thus avoiding uncertainties associated with immunization with adjuvants at remotes sites or adoptive transfer of in vitro activated T cells.

METHODS

R161H mice (B10.RIII background), which spontaneously and rapidly develop severe spontaneous autoimmune uveoretinitis (SAU), were crossed to CD11cDTR/GFP mice (B6/J) allowing us to track the recruitment to and/or expansion within the retina of activated, antigen presenting cells (GFPhi cells) in R161H+/- × CD11cDTR/GFP F1 mice relative to the course of SAU. Parabiosis between R161H+/- × CD11cDTR/GFP F1 mice and B10.RIII × B6/J F1 (wild-type recipient) mice was done to explore the origin and phenotype of antigen presenting cells crucial for the induction of autoimmunity. Analysis was done by retinal imaging, flow cytometry, and histology.

RESULTS

Onset of SAU in R161H+/- × CD11cDTR/GFP F1 mice was delayed relative to B10.RIII-R161H+/- mice revealing a disease prophase prior to frank autoimmunity that was characterized by expansion of GFPhi cells within the retina prior to any clinical or histological evidence of autoimmunity. Parabiosis between mice carrying the R161H and CD11cDTR/GFP transgenes and transgene negative recipients showed that recruitment of circulating GFPhi cells into retinas was highly correlative with the occurrence of SAU.

CONCLUSIONS

Our results here contrast with our previous findings showing that retinal antigen presenting cells expanding in response to either sterile mechanical injury or neurodegeneration were derived from myeloid cells within the retina or optic nerve, thus highlighting a unique facet of retinal autoimmunity.

Lymphopenia, Lymphopenia-Induced Proliferation, and Autoimmunity

Immune homeostasis is a tightly regulated system that is critical for defense against invasion by foreign pathogens and protection from self-reactivity for the survival of an individual. How the defects in this system might result in autoimmunity is discussed in this review. Reduced lymphocyte number, termed lymphopenia, can mediate lymphopenia-induced proliferation (LIP) to maintain peripheral lymphocyte numbers. LIP not only occurs in normal physiological conditions but also correlates with autoimmunity. Of note, lymphopenia is also a typical marker of immune aging, consistent with the fact that not only the autoimmunity increases in the elderly, but also autoimmune diseases (ADs) show characteristics of immune aging. Here, we discuss the types and rates of LIP in normal and autoimmune conditions, as well as the coronavirus disease 2019 in the context of LIP. Importantly, although the causative role of LIP has been demonstrated in the development of type 1 diabetes and rheumatoid arthritis, a two-hit model has suggested that the factors other than lymphopenia are required to mediate the loss of control over homeostasis to result in ADs. Interestingly, these factors may be, if not totally, related to the function/number of regulatory T cells which are key modulators to protect from self-reactivity. In this review, we summarize the important roles of lymphopenia/LIP and the Treg cells in various autoimmune conditions, thereby highlighting them as key therapeutic targets for autoimmunity treatments.

Treatment With FoxP3+ Antigen-Experienced T Regulatory Cells Arrests Progressive Retinal Damage in a Spontaneous Model of Uveitis

We specify the clinical features of a spontaneous experimental autoimmune uveitis (EAU) model, in which foreign hen-egg lysozyme (HEL) is expressed in the retina, controlled by the promoter for interphotoreceptor retinol binding protein (IRBP). We previously reported 100% P21 (post-partum day) IRBP:HEL single transgenic (sTg) mice, when crossed to transgenic T cell receptor mice (3A9) generating the double transgenic (dTg) genotype, develop EAU despite profound lymphopenia (thymic HEL-specific T cell deletion). In this work, we characterized the immune component of this model and found conventional dTg CD4+ T cells were less anergic than those from 3A9 controls. Furthermore, prior in vitro HEL-activation of 3A9 anergic T cells (T_an) rendered them uveitogenic upon adoptive transfer (Tx) to sTg mice, while antigen-experienced (AgX, dTg), but not naïve (3A9) T cells halted disease in P21 dTg mice. Flow cytometric analysis of the AgX cells elucidated the underlying pathology: FoxP3+CD25^hiCD4+ T regulatory cells (T_reg) comprised ∼18%, while FR4+CD73+FoxP3-CD25^lo/–CD4+ T_an comprised ∼1.2% of total cells. Further T_reg-enrichment (∼80%) of the AgX population indicated FoxP3+CD25^hiCD4+ T_reg played a key role in EAU-suppression while FoxP3-CD25^lo/–CD4+ T cells did not. Here we present the novel concept of dual immunological tolerance where spontaneous EAU is due to escape from anergy with consequent failure of T_reg induction and subsequent imbalance in the [T_reg:T_effector] cell ratio. The reduced numbers of T_an, normally sustaining T_reg to prevent autoimmunity, are the trigger for disease, while immune homeostasis can be restored by supplementation with AgX, but not naïve, antigen-specific T_reg.

Prior to Peripheral Tolerance, Newly Generated CD4 T Cells Maintain Dangerous Autoimmune Potential: Fas- and Perforin-Independent Autoimmunity Controlled by Programmed Death-1

Lymphopenia can result from various factors, including viral infections, clinical interventions, or as a normal property of the fetal/neonatal period. T cells in a lymphopenic environment undergo lymphopenia-induced proliferation (LIP) to fill the available “niche” as defined by peptide–MHC and homeostatic cytokine resources. We recently reported systemic autoimmunity following reconstitution of the lymphoid compartment of Rag1^−/− mice with PD-1^−/− hematopoietic stem cells or by transfer of thymocytes, but not splenocytes, suggesting that programmed death-1 (PD-1) plays a crucial role in controlling recent thymic emigrants (RTE) and preventing autoimmunity upon their LIP. However, it is unclear whether RTE residing within the periphery of a lymphoreplete host maintain enhanced autoimmune generating potential or if this property only manifests if RTE experience a lymphopenic periphery immediately after export from the thymus. Furthermore, it is unclear which of a variety of T cell effector mechanisms generate pathology when control of RTE by PD-1 is lacking. Herein, we determined that PD-1 is upregulated on CD4 T cells undergoing the natural LIP characteristic of the neonatal period. Newly generated T cells lacking PD-1 maintained an enhanced autoimmune potential even after residence in a lymphoreplete periphery, emphasizing the importance of PD-1 in the establishment of peripheral tolerance. Neither Fas nor perforin-dependent killing mechanisms were required for autoimmunity, while host MHC-II expression was critical, suggesting that LIP-driven autoimmunity in the absence of PD-1 may primarily result from a CD4 T cell-mediated systemic cytokinemia, a feature potentially shared by other autoimmune or inflammatory syndromes associated with immune reconstitution and LIP.

Ocular antigen does not cause disease unless presented in the context of inflammation

Ocular antigens are sequestered behind the blood-retina barrier and the ocular environment protects ocular tissues from autoimmune attack. The signals required to activate autoreactive T cells and allow them to cause disease in the eye remain in part unclear. In particular, the consequences of peripheral presentation of ocular antigens are not fully understood. We examined peripheral expression and presentation of ocular neo-self-antigen in transgenic mice expressing hen egg lysozyme (HEL) under a retina-specific promoter. High levels of HEL were expressed in the eye compared to low expression throughout the lymphoid system. Adoptively transferred naïve HEL-specific CD4⁺ T cells proliferated in the eye draining lymph nodes, but did not induce uveitis. By contrast, systemic infection with a murine cytomegalovirus (MCMV) engineered to express HEL induced extensive proliferation of transferred naïve CD4⁺ T cells, and significant uveoretinitis. In this model, wild-type MCMV, lacking HEL, did not induce overt uveitis, suggesting that disease is mediated by antigen-specific peripherally activated CD4⁺ T cells that infiltrate the retina. Our results demonstrate that retinal antigen is presented to T cells in the periphery under physiological conditions. However, when the same antigen is presented during viral infection, antigen-specific T cells access the retina and autoimmune uveitis ensues.

PD-1 Controls Tonic Signaling and Lymphopenia-Induced Proliferation of T Lymphocytes

Recovery of the T lymphocyte compartment within a lymphopenic host by lymphopenia-induced proliferation (LIP) is regulated by inter- and intraclonal competition for limited resources, including homeostatic cytokines and peptide:MHC (pMHC) complexes with which the TCR can interact at least weakly to yield a tonic signal. Importantly, the process of LIP can synergize with other factors that promote T cell activation to drive inflammatory disease. While reconstitution of the lymphoid compartment of immune deficient Rag^-/- mice by transfer of wild-type hematopoietic stem cells (HSC) does not generally result in an overt disease phenotype, transfer of HSC deficient in expression of the co-inhibitory molecule PD-1 results in severe systemic autoimmunity driven by newly generated T cells that emerge from the thymus into the periphery and undergo LIP. Importantly, autoimmunity does not appear to depend on a response to exogenous (i.e., gut flora-derived) antigens. PD-1 is well known to be upregulated during T cell activation in response to cognate antigens, but it is unclear whether PD-1 has a role in controlling LIP of T cells in the absence of cognate antigen, i.e., in response to tonic pMHC. We examined whether PD-1 controls LIP of newly generated T cells by controlling the response to tonic pMHC or the homeostatic cytokine IL-7. We found that PD-1-deficient T cells have a proliferative advantage over WT T cells during LIP and this effect is MHC-II dependent and independent of IL-7Rα signaling. Furthermore, our data suggest that signals through IL-7Rα can be dispensable for LIP and may instead be of increased importance for T cell survival in conditions of high competition for limited pMHC (e.g., post-LIP, in a lymphoreplete host). We hypothesize that autoimmunity post-PD-1^-/- HSC transplant is the result of an overzealous T cell response to normally tonic self-pMHC precipitated by the synergy of LIP and PD-1 deficiency. Furthermore, potentiation of TCR signals in response to normally tonic self-pMHC may contribute to the success of PD-1 blockade in cancer immunotherapy.

Retinal antigen-specific regulatory T cells protect against spontaneous and induced autoimmunity and require local dendritic cells

BACKGROUND

We previously reported that the peripheral regulatory T cells (pTregs) generated 'on-demand' in the retina were crucial to retinal immune privilege, and in vitro analysis of retinal dendritic cells (DC) showed they possessed antigen presenting cell (APC) activity that promoted development of the Tregs and effector T cells (Teffs). Here, we expanded these findings by examining whether locally generated, locally acting pTregs were protective against spontaneous autoimmunity and autoimmunity mediated by interphotoreceptor retinoid-binding protein (IRBP). We also examined the APC capacity of retinal DC in vivo.

METHODS

Transgenic (Tg) mice expressing diphtheria toxin receptor (DTR) and/or green fluorescent protein (GFP) under control of the endogenous FoxP3 promoter (GFP only in FG mice, GFP and DTR in FDG mice) or the CD11c promoter (GFP and DTR in CDG mice) were used in conjunction with Tg mice expressing beta-galactosidase (βgal) as retinal neo-self antigen and βgal-specific TCR Tg mice (BG2). Retinal T cell responses were assayed by flow cytometry and retinal autoimmune disease assessed by histological examination.

RESULTS

Local depletion of the Tregs enhanced actively induced experimental autoimmune uveoretinitis to the highly expressed retinal self-antigen IRBP in FDG mice and spontaneous autoimmunity in βgal-FDG-BG2 mice, but not in mice lacking autoreactive T cells or their target antigen in the retina. The presence of retinal βgal downregulated the generation of antigen-specific Teffs and pTregs within the retina in response to local βgal challenge. Retinal DC depletion prevented generation of Tregs and Teffs within retina after βgal injection. Microglia remaining after DC depletion did not make up for loss of DC-dependent antigen presentation.

CONCLUSIONS

Our results suggest that local retinal Tregs protect against spontaneous organ-specific autoimmunity and that T cell responses within the retina require the presence of local DC.

Lymphopenia-induced proliferation in aire-deficient mice helps to explain their autoimmunity and differences from human patients

Studies on autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED) and its mouse model – both caused by mutant AIRE – have greatly advanced the understanding of thymic processes that generate a self-tolerant T-cell repertoire. Much is now known about the molecular mechanisms by which AIRE induces tissue-specific antigen expression in thymic epithelium, and how this leads to negative selection of auto-reactive thymocytes. However, we still do not understand the processes that lead to the activation of any infrequent naïve auto-reactive T-cells exported by AIRE-deficient thymi. Also, the striking phenotypic differences between APECED and its mouse models have puzzled researchers for years. The aim of this review is to suggest explanations for some of these unanswered questions, based on a fresh view of published experiments. We review evidence that auto-reactive T-cells can be activated by the prolonged neonatal lymphopenia that naturally develops in young Aire-deficient mice due to delayed export of mature thymocytes. Lymphopenia-induced proliferation (LIP) helps to fill the empty space; by favoring auto-reactive T-cells, it also leads to lymphocyte infiltration in the same tissues as in day 3 thymectomized animals. The LIP becomes uncontrolled when loss of Aire is combined with defects in genes responsible for anergy induction and Treg responsiveness, or in signaling from the T-cell receptor and homeostatic cytokines. In APECED patients, LIP is much less likely to be involved in activation of naïve auto-reactive T-cells, as humans are born with a more mature immune system than in neonatal mice. We suggest that human AIRE-deficiency presents with different phenotypes because of additional precipitating factors that compound the defective negative selection of potentially autoaggressive tissue-specific thymocytes.

Breakdown of immune privilege and spontaneous autoimmunity in mice expressing a transgenic T cell receptor specific for a retinal autoantigen

Despite presence of circulating retina-specific T cells in healthy individuals, ocular immune privilege usually averts development of autoimmune uveitis. To study the breakdown of immune privilege and development of disease, we generated transgenic (Tg) mice that express a T cell receptor (TCR) specific for interphotoreceptor retinoid-binding protein (IRBP), which serves as an autoimmune target in uveitis induced by immunization. Three lines of TCR Tg mice, with different levels of expression of the transgenic R161 TCR and different proportions of IRBP-specific CD4⁺ T cells in their peripheral repertoire, were successfully established. Importantly, two of the lines rapidly developed spontaneous uveitis, reaching 100% incidence by 2 and 3 months of age, respectively, whereas the third appeared "poised" and only developed appreciable disease upon immune perturbation. Susceptibility roughly paralleled expression of the R161 TCR. In all three lines, peripheral CD4⁺ T cells displayed a naïve phenotype, but proliferated in vitro in response to IRBP and elicited uveitis upon adoptive transfer. In contrast, CD4⁺ T cells infiltrating uveitic eyes mostly showed an effector/memory phenotype, and included Th1, Th17 as well as T regulatory cells that appeared to have been peripherally converted from conventional CD4⁺ T cells rather than thymically derived. Thus, R161 mice provide a new and valuable model of spontaneous autoimmune disease that circumvents the limitations of active immunization and adjuvants, and allows to study basic mechanisms involved in maintenance and breakdown of immune homeostasis affecting immunologically privileged sites such as the eye.

Local "on-demand" generation and function of antigen-specific Foxp3+ regulatory T cells

Extrathymically derived regulatory T cells (iTregs) protect against autoimmunity to tissue-specific Ags. However, whether Ag-specific iTreg generation and function is limited to secondary lymphoid tissue or whether it can occur within the tissue-specific local environment of the cognate Ag remains unresolved. Mice expressing β-galactosidase (βgal) on a retina-specific promoter (βgal mice) in conjunction with mice expressing GFP and diphtheria toxin (DTx) receptor (DTR) under control of the Foxp3 promoter, and βgal-specific TCR transgenic (BG2) mice were used to examine this question. Local depletion (ocular DTx), but not systemic depletion (i.p. DTx), of βgal-specific iTregs enhanced experimental autoimmune uveoretinitis induced by activated βgal-specific effector T cells. Injections of small amounts of βgal into the anterior chamber of the eye produced similar numbers of βgal-specific iTregs in the retina whether the mouse was depleted of pre-existing, circulating Tregs. Taken together, these results suggest that protection from tissue-specific autoimmunity depends on the function of local Ag-specific iTregs and that the retina is capable of local, "on-demand" iTreg generation that is independent of circulating Tregs.

Regulation of CD8(+) T Cell Responses to Retinal Antigen by Local FoxP3(+) Regulatory T Cells

While pathogenic CD4 T cells are well known mediators of autoimmune uveoretinitis, CD8 T cells can also be uveitogenic. Since preliminary studies indicated that C57BL/6 mice were minimally susceptible to autoimmune uveoretinitis induction by CD8 T cells, the basis of the retinal disease resistance was sought. Mice that express β-galactosidase (βgal) on a retina-specific promoter (arrβgal mice) were backcrossed to mice expressing green fluorescent protein (GFP) and diphtheria toxin (DTx) receptor (DTR) under control of the Foxp3 promoter (Foxp3-DTR/GFP mice), and to T cell receptor transgenic mice that produce βgal-specific CD8 T cells (BG1 mice). These mice were used to explore the role of regulatory T cells in the resistance to retinal autoimmune disease. Experiments with T cells from double transgenic BG1 × Foxp3-DTR/GFP mice transferred into Foxp3-DTR/GFP × arrβgal mice confirmed that the retina was well protected from attempts to induce disease by adoptive transfer of activated BG1 T cells. The successful induction of retinal disease following unilateral intraocular administration of DTx to deplete regulatory T cells showed that the protective activity was dependent on local, toxin-sensitive regulatory T cells; the opposite, untreated eye remained disease-free. Although there were very few Foxp3(+) regulatory T cells in the parenchyma of quiescent retina, and they did not accumulate in retina, their depletion by local toxin administration led to disease susceptibility. We propose that these regulatory T cells modulate the pathogenic activity of βgal-specific CD8 T cells in the retinas of arrβgal mice on a local basis, allowing immuno regulation to be responsive to local conditions.

Local activation of dendritic cells alters the pathogenesis of autoimmune disease in the retina

Interest in the identities, properties, functions, and origins of local APC in CNS tissues is growing. We recently reported that dendritic cells (DC) distinct from microglia were present in quiescent retina and rapidly responded to injured neurons. In this study, the disease-promoting and regulatory contributions of these APC in experimental autoimmune uveoretinitis (EAU) were examined. Local delivery of purified, exogenous DC or monocytes from bone marrow substantially increased the incidence and severity of EAU induced by adoptive transfer of activated, autoreactive CD4 or CD8 T cells that was limited to the manipulated eye. In vitro assays of APC activity of DC from quiescent retina showed that they promoted generation of Foxp3(+) T cells and inhibited activation of naive T cells by splenic DC and Ag. Conversely, in vitro assays of DC purified from injured retina demonstrated an enhanced ability to activate T cells and reduced induction of Foxp3(+) T cells. These findings were supported by the observation that in situ activation of DC before adoptive transfer of β-galactosidase-specific T cells dramatically increased severity and incidence of EAU. Recruitment of T cells into retina by local delivery of Ag in vivo showed that quiescent retina promoted development of parenchymal Foxp3(+) T cells, but assays of preinjured retina did not. Together, these results demonstrated that local conditions in the retina determined APC function and affected the pathogenesis of EAU by both CD4 and CD8 T cells.

Generation of Regulatory T Cells to Antigen Expressed in the Retina

Regulatory T cells (Tregs) are generated to antigens (Ag) found in the retina. Some Tregs are the result of ectopic expression of the retinal Ags in the thymus, where developing T cells are committed to enter the regulatory lineage. However, the generation of retinal Ag-specific Tregs independent of the thymus was uncertain. Our studies show that Tregs can be generated from mature, peripheral T cells based on exposure to retinal Ags. These peripherally induced Tregs limited immune responses and experimental autoimmune disease induced by retinal Ags and thus constitute a crucial component of retinal immune privilege.

Th1-driven immune reconstitution disease in Mycobacterium avium-infected mice

Following antiretroviral therapy, a significant proportion of HIV(+) patients with mycobacterial coinfections develop a paradoxical, poorly understood inflammatory disease termed immune reconstitution inflammatory syndrome (IRIS). Here, we show that Mycobacterium avium-infected T cell-deficient mice injected with CD4 T cells also develop an immune reconstitution disease (IRD) manifesting as weight loss, impaired lung function, and rapid mortality. This form of IRD requires Ag recognition and interferonγ production by the donor CD4 T cells and correlates with marked alterations in blood and tissue CD11b(+) myeloid cells. Interestingly, disease is associated with impaired, rather than augmented, T-cell expansion and function and is not strictly dependent on lymphopenia-induced T-cell proliferation. Instead, our findings suggest that mycobacterial-associated IRIS results from a heightened sensitivity of infected lymphopenic hosts to the detrimental effects of Ag-driven CD4 T-cell responses.

Microbiota innate stimulation is a prerequisite for T cell spontaneous proliferation and induction of experimental colitis

Little is known about how the microbiota regulates T cell proliferation and whether spontaneous T cell proliferation is involved in the pathogenesis of inflammatory bowel disease. In this study, we show that stimulation of innate pathways by microbiota-derived ligands and antigen-specific T cell stimulation are both required for intestinal inflammation. Microbiota-derived ligands promoted spontaneous T cell proliferation by activating dendritic cells (DCs) to produce IL-6 via Myd88, as shown by the spontaneous proliferation of T cells adoptively transferred into specific pathogen-free (SPF) RAG-/- mice, but not in germfree RAG-/- mice. Reconstitution of germfree RAG-/- mice with cecal bacterial lysate-pulsed DCs, but not with IL-6-/- or Myd88-/- DCs, restored spontaneous T cell proliferation. CBir1 TCR transgenic (CBir1 Tg) T cells, which are specific for an immunodominant microbiota antigen, induced colitis in SPF RAG-/- mice. Blocking the spontaneous proliferation of CBir1 Tg T cells by co-transferring bulk OT II CD4+ T cells abrogated colitis development. Although transferred OT II T cells underwent spontaneous proliferation in RAG-/- mice, the recipients failed to develop colitis because of the lack of cognate antigen in the intestinal lumen. Collectively, our data demonstrate that induction of colitis requires both spontaneous proliferation of T cells driven by microbiota-derived innate signals and antigen-specific T cell proliferation.

Irradiation and IL-15 promote loss of CD8 T-cell tolerance in response to lymphopenia

Functional inactivation of self-reactive T lymphocytes contributes to the maintenance of immunologic self-tolerance. At the same time, tolerance induction limits immune responses against tumors expressing tolerizing self-antigens. Some cancer therapies include the adoptive transfer of tumor-reactive T lymphocytes into lymphopenic patients. Lymphopenia provides an activation signal to T lymphocytes, which undergo lymphopenia-induced proliferation (LIP), acquire effector functions, and reject tumors. However, it is so far unknown to which extent LIP may result in reversal of established antigen-specific CD8 T-cell tolerance. Here, we report that neonatally induced dominant CD8 T-cell tolerance remained stable under lymphopenic conditions also in the presence of systemic inflammation induced by Toll-like receptor ligands. However, when lymphopenic recipients were irradiated, the tolerant status was lost, because CD8 T cells acquired effector functions in an interleukin-15-dependent fashion and efficiently rejected tumors. In conclusion, we show that lymphopenia is not sufficient to break CD8 T-cell tolerance. Furthermore, we demonstrate that pretreatment regimens are crucial to circumvent this problem and to optimize adoptive T-cell therapy.

Peripheral induction of tolerance by retinal antigen expression

The contribution of peripheral expression of tissue-specific CNS Ags to the generation of tolerance is uncertain. To study this question, we examined mice transgenic (Tg) for expression of beta-galactosidase (beta gal) on the retinal photoreceptor cell arrestin promoter, in conjunction with TCR Tg mice producing CD4(+) T cells specific for beta gal (beta galTCR). Several strategies were used to test the hypothesis that betagal expressed in the retina supported thymus-independent tolerance and regulatory T cell development. Retinal expression generated an immunoregulatory response that depressed development of immune responses to beta gal following systemic immunization with beta gal. This regulation was transferable to naive mice by CD3(+)4(+)25(+) T cells from naive retinal beta gal(+) donors. Experiments that removed the beta gal(+) retina by enucleation showed that subsequent development of a regulatory response was lost. Adoptive transfer of CD25(-) beta galTCR T cells into retinal beta gal Tg mice on the Rag(-/-) background led to regulatory activity that limited lymphopenia-induced proliferation of beta galTCR T cells in mice with retinal expression of beta gal and inhibited the ear-swelling assay for delayed type hypersensitivity. These results show that retinal expression of very small amounts of a tissue-specific Ag can generate tolerance that includes regulatory T cells.