Peripheral Immune Compartment Research Articles

Maintenance of host leukocytes in peripheral immune compartments following lethal irradiation and bone marrow reconstitution: Implications for graft versus host disease

Bone marrow reconstitution is utilized as a tool for disease treatment and as a research technique to elucidate the function of bone marrow derived cells. Clinically successful engraftment is indicated by the development of a functioning immune repertoire. In research, reconstitution is considered successful if >85% of splenic leukocytes are of donor origins. Previous work suggests that splenic reconstitution may not be indicative of reconstitution in the mucosa. We sought to evaluate mucosal reconstitution in animals following a standard bone marrow eradication and reconstitution technique. Bone marrow was harvested from adult B6.SJL donor mice (CD45.1) and injected via either the retro-orbital or intraperitoneal route into lethally irradiated B6 (CD45.2) adult or neonatal recipients respectively. The expression of CD45 by flow cytometry was used to calculate reconstitution with respect to immune compartment and cell type. In reconstituted adult animals 93.2±1.5% of splenic leukocytes expressed the donor CD45.1 antigen thus meeting the standard definition of reconstitution, however only 58.6±13.6% of intestinal lamina propria lymphocytes and 52.4±16.0% of intestinal intraepithelial lymphocytes were of donor origin, confirming splenic reconstitution fails to represent peripheral immune reconstitution. T-cells in the gastrointestinal tract are the most poorly reconstituted, while B-cells appear to be almost universally replaced by donor cells. The inadequate mucosal reconstitution was not corrected by evaluating later time points or by performing the bone marrow transfer during the neonatal period. This demonstration that substantial host T-cells remain in the intestinal mucosa after a “successful” bone marrow transplantation should cause a re-evaluation of data from research bone marrow chimera experiments, as well as the mechanisms for complications after clinical bone marrow transplantation.

Brain dendritic cells: biology and pathology.

Dendritic cells (DC) are the professional antigen-presenting cells of the immune system. In their quiescent and mature form, the presentation of self-antigens by DC leads to tolerance; whereas, antigen presentation by mature DC, after stimulation by pathogen-associated molecular patterns, leads to the onset of antigen-specific immunity. DC have been found in many of the major organs in mammals (e.g. skin, heart, lungs, intestines and spleen); while the brain has long been considered devoid of DC in the absence of neuroinflammation. Consequently, microglia, the resident immune cell of the brain, have been charged with many functional attributes commonly ascribed to DC. Recent evidence has challenged the notion that DC are either absent or minimal players in brain immune surveillance. This review will discuss the recent literature examining DC involvement within both the young and aged steady-state brain. We will also examine DC contributions during various forms of neuroinflammation resulting from neurodegenerative autoimmune disease, injury, and CNS infections. This review also touches upon DC trafficking between the central nervous system and peripheral immune compartments during viral infections, the new molecular technologies that could be employed to enhance our current understanding of brain DC ontogeny, and some potential therapeutic uses of DC within the CNS.

Peripheral Immune Cell Functionality in Amyotrophic Lateral Sclerosis (P03.184)

Objective: To investigate the functionality of circulating immune cells in ALS patients. Background Amyotrophic lateral sclerosis (ALS) axonal pathology precedes motoneuron degeneration during disease progression in patients and the mSOD1 mouse model. We and others have shown that a functional immune system is neuroprotective in mSOD1 mice, enhancing survival and promoting facial motoneuron (FMN) survival levels after axotomy. Increased levels of FMN loss after axotomy were seen in immunodeficient mice, but wildtype levels of FMN survival could be restored by adoptively transferring wildtype splenocytes or CD4+ T cells prior to axotomy. Interestingly, the adoptive transfer of mSOD1 whole splenocytes did not rescue axotomy-induced FMN death in immunodeficient mice, but isolated CD4 + T cells (from the splenocyte population) effectively mediated neuroprotection to similar levels as wildtype CD4 + T cells, suggesting that the defect in immune neuroprotection was NOT explained by a functional defect in CD4 + T cells. Previous studies examining the peripheral immune compartment in ALS patients have yielded varied results regarding levels/function of CD4 + T cells. Based on the results above, we hypothesize that the peripheral immune microenvironment in ALS activates CD4 + T cells early on, but the neuroprotective effects of certain T cell subsets are subsequently inhibited by non-Tcell components of the immune system. Design/Methods: In vitro T cell proliferation assays were utilized to assess the function of ALS T cell subsets activated with anti-CD3 stimulation and compared to controls. Results: ALS CD4 + T cell subsets maintain their functional and proliferative capabilities when removed from the ALS microenvironment and activated polyclonally. No differences in T regulatory cell function or FoxP3 expression were detected in ALS samples compared to controls. Conclusions: ALS CD4 + T cells function normally in response to polyclonal stimulation in vitro , suggesting that other components of the ALS immune system (B cells, dendritic cells, or macrophages) underlie the defect in neuroprotective CD4 + T cell functionality. Supported by: The ALS Association (JR). Disclosure: Dr. Mesnard has nothing to disclose. Dr. Muthusamy has nothing to disclose. Dr. Xin has nothing to disclose. Dr. Sheng has nothing to disclose. Dr. Jones has received research support from Veterans Affairs, the National Institutes of Health, and the Les Turneer ALS Foundation. Dr. Prabhakar has received personal compensation for activities with Gliknik Inc. as a consultant.Dr. Prabhakar has received research support from NIH. Dr. Rowin has nothing to disclose. Dr. Meriggioli has received personal compensation for activities with Athena Diagnostics, Talecris Biotherapeutics, Gliknik, and Celegene Corporation. Dr. Meriggioli receives patent payments for a patent on bispecific antibody coated dendritic cells. Dr. Meriggioli has received research support from the NIH/NINDS and the Muscular Dystrophy Association.

Cellular Immune Responses to Nine Mycobacterium tuberculosis Vaccine Candidates following Intranasal Vaccination

BackgroundThe identification of Mycobacterium tuberculosis vaccines that elicit a protective immune response in the lungs is important for the development of an effective vaccine against tuberculosis.Methods and Principal FindingsIn this study, a comparison of intranasal (i.n.) and subcutaneous (s.c.) vaccination with the BCG vaccine demonstrated that a single moderate dose delivered intranasally induced a stronger and sustained M. tuberculosis-specific T-cell response in lung parenchyma and cervical lymph nodes of BALB/c mice than vaccine delivered subcutaneously. Both BCG and a multicomponent subunit vaccine composed of nine M. tuberculosis recombinant proteins induced strong antigen-specific T-cell responses in various local and peripheral immune compartments. Among the nine recombinant proteins evaluated, the alanine proline rich antigen (Apa, Rv1860) was highly antigenic following i.n. BCG and immunogenic after vaccination with a combination of the nine recombinant antigens. The Apa-induced responses included induction of both type 1 and type 2 cytokines in the lungs as evaluated by ELISPOT and a multiplexed microsphere-based cytokine immunoassay. Of importance, i.n. subunit vaccination with Apa imparted significant protection in the lungs and spleen of mice against M. tuberculosis challenge. Despite observed differences in the frequencies and location of specific cytokine secreting T cells both BCG vaccination routes afforded comparable levels of protection in our study.Conclusion and SignificanceOverall, our findings support consideration and further evaluation of an intranasally targeted Apa-based vaccine to prevent tuberculosis.

IgA-Producing Plasma Cells Originate From Germinal Centers That Are Induced by B-Cell Receptor Engagement in Humans

IgA contributes to homeostatic balance between host and intestinal microbiota. Mechanisms that initiate the IgA response are unclear and likely to differ between humans and animal models. We used multiple experimental approaches to investigate the origin of human intestinal plasma cells that produce IgA in the gastrointestinal tract. Complexity of IgA-producing plasma cell populations in human gastrointestinal mucosa and bone marrow and the specific response to oral cholera vaccine were compared by analysis of immunoglobulin genes. Flow cytometry, gene expression analysis, and immunohistochemistry were used to analyze signaling pathways induced by B-cell receptor engagement in human gut-associated lymphoid tissue (GALT) and involvement of innate immunity in B-cell activation in GALT compared with nonintestinal sites. Human intestinal IgA-producing plasma cells appeared to be of germinal center origin; there was no evidence for the population complexity that accompanies multiple pathways of derivation observed in bone marrow. In germinal center B cells of human GALT, Btk and Erk are phosphorylated, CD22 is down-regulated, Lyn is translocated to the cell membrane, and Fos and Jun are up-regulated; these features indicate B-cell receptor ligation during germinal center evolution. No differences in innate activation of B cells were observed in GALT, compared with peripheral immune compartments. IgA-producing plasma cells appear to be derived from GALT germinal centers in humans. B-cell receptor engagement promotes formation of germinal centers of GALT, with no more evidence for innate immune receptor activation in the mucosa than nonintestinal immune compartments. Germinal centers in GALT should be targets of mucosal vaccinations because they are the source of human intestinal IgA response.

MicroRNAs and Multiple Sclerosis

MicroRNAs (miRNAs) have recently emerged as a new class of modulators of gene expression. miRNAs control protein synthesis by targeting mRNAs for translational repression or degradation at the posttranscriptional level. These noncoding RNAs are endogenous, single-stranded molecules approximately 22 nucleotides in length and have roles in multiple facets of immunity, from regulation of development of key cellular players to activation and function in immune responses. Recent studies have shown that dysregulation of miRNAs involved in immune responses leads to autoimmunity. Multiple sclerosis (MS) serves as an example of a chronic and organ-specific autoimmune disease in which miRNAs modulate immune responses in the peripheral immune compartment and the neuroinflammatory process in the brain. For MS, miRNAs have the potential to serve as modifying drugs. In this review, we summarize current knowledge of miRNA biogenesis and mode of action and the diverse roles of miRNAs in modulating the immune and inflammatory responses. We also review the role of miRNAs in autoimmunity, focusing on emerging data regarding miRNA expression patterns in MS. Finally, we discuss the potential of miRNAs as a disease marker and a novel therapeutic target in MS. Better understanding of the role of miRNAs in MS will improve our knowledge of the pathogenesis of this disease.

Biomarkers for Early and Late Stage Chronic Allograft Nephropathy by Proteogenomic Profiling of Peripheral Blood

BackgroundDespite significant improvements in life expectancy of kidney transplant patients due to advances in surgery and immunosuppression, Chronic Allograft Nephropathy (CAN) remains a daunting problem. A complex network of cellular mechanisms in both graft and peripheral immune compartments complicates the non-invasive diagnosis of CAN, which still requires biopsy histology. This is compounded by non-immunological factors contributing to graft injury. There is a pressing need to identify and validate minimally invasive biomarkers for CAN to serve as early predictors of graft loss and as metrics for managing long-term immunosuppression.MethodsWe used DNA microarrays, tandem mass spectroscopy proteomics and bioinformatics to identify genomic and proteomic markers of mild and moderate/severe CAN in peripheral blood of two distinct cohorts (n = 77 total) of kidney transplant patients with biopsy-documented histology.FindingsGene expression profiles reveal over 2400 genes for mild CAN, and over 700 for moderate/severe CAN. A consensus analysis reveals 393 (mild) and 63 (moderate/severe) final candidates as CAN markers with predictive accuracy of 80% (mild) and 92% (moderate/severe). Proteomic profiles show over 500 candidates each, for both stages of CAN including 302 proteins unique to mild and 509 unique to moderate/severe CAN.ConclusionsThis study identifies several unique signatures of transcript and protein biomarkers with high predictive accuracies for mild and moderate/severe CAN, the most common cause of late allograft failure. These biomarkers are the necessary first step to a proteogenomic classification of CAN based on peripheral blood profiling and will be the targets of a prospective clinical validation study.

MyD88 Expression by CNS-Resident Cells is Pivotal for Eliciting Protective Immunity in Brain Abscesses

MyD88 KO (knockout) mice are exquisitely sensitive to CNS (central nervous system) infection with Staphylococcus aureus, a common aetiological agent of brain abscess, exhibiting global defects in innate immunity and exacerbated tissue damage. However, since brain abscesses are typified by the involvement of both activated CNS-resident and infiltrating immune cells, in our previous studies it has been impossible to determine the relative contribution of MyD88-dependent signalling in the CNS compared with the peripheral immune cell compartments. In the present study we addressed this by examining the course of S. aureus infection in MyD88 bone marrow chimaera mice. Interestingly, chimaeras where MyD88 was present in the CNS, but not bone marrow-derived cells, mounted pro-inflammatory mediator expression profiles and neutrophil recruitment equivalent to or exceeding that detected in WT (wild-type) mice. These results implicate CNS MyD88 as essential in eliciting the initial wave of inflammation during the acute response to parenchymal infection. Microarray analysis of infected MyD88 KO compared with WT mice revealed a preponderance of differentially regulated genes involved in apoptotic pathways, suggesting that the extensive tissue damage characteristic of brain abscesses from MyD88 KO mice could result from dysregulated apoptosis. Collectively, the findings of the present study highlight a novel mechanism for CNS-resident cells in initiating a protective innate immune response in the infected brain and, in the absence of MyD88 in this compartment, immunity is compromised.

IL-17 and Th17 Cells

CD4+ T cells, upon activation and expansion, develop into different T helper cell subsets with different cytokine profiles and distinct effector functions. Until recently, T cells were divided into Th1 or Th2 cells, depending on the cytokines they produce. A third subset of IL-17-producing effector T helper cells, called Th17 cells, has now been discovered and characterized. Here, we summarize the current information on the differentiation and effector functions of the Th17 lineage. Th17 cells produce IL-17, IL-17F, and IL-22, thereby inducing a massive tissue reaction owing to the broad distribution of the IL-17 and IL-22 receptors. Th17 cells also secrete IL-21 to communicate with the cells of the immune system. The differentiation factors (TGF-beta plus IL-6 or IL-21), the growth and stabilization factor (IL-23), and the transcription factors (STAT3, RORgammat, and RORalpha) involved in the development of Th17 cells have just been identified. The participation of TGF-beta in the differentiation of Th17 cells places the Th17 lineage in close relationship with CD4+CD25+Foxp3+ regulatory T cells (Tregs), as TGF-beta also induces differentiation of naive T cells into Foxp3+ Tregs in the peripheral immune compartment. The investigation of the differentiation, effector function, and regulation of Th17 cells has opened up a new framework for understanding T cell differentiation. Furthermore, we now appreciate the importance of Th17 cells in clearing pathogens during host defense reactions and in inducing tissue inflammation in autoimmune disease.

Spleen versus CNS immunoglobulin G in multiple sclerosis: an isoelectric focusing study

Immunoglobulin G (IgG) band patterns were investigated in a peripheral immune compartment, the spleen, and the central nervous system (CNS) compartment in 6 multiple sclerosis (MS) subjects and 3 controls in a search for systemic humoral immune response that may be related to the localized immune response seen in MS. Using the isoelectric focusing (IEF) technique with immunoperoxidase staining of proteins transferred to nitrocellulose membranes, we were not able to demonstrate any qualitative abnormalities of spleen or serum IgG in the MS cases, whilst all matched CNS tissues and CSF specimens were clearly positive for oligoclonal IgG bands. Our results suggest that there is no systemic oligoclonal IgG secretion in MS. This further supports the presence of a compartmentalized IgG intrathecal immune response in MS and emphasizes the organ specificity of MS.

Analysis of lymphocyte subpopulations in cerebrospinal fluid and peripheral blood in patients with multiple sclerosis and inflammatory diseases of the nervous system.

We analysed different subsets of lymphocytes from peripheral blood (PB) and cerebrospinal fluid (CSF) by flow cytometry in order to determinate alterations in patients with multiple sclerosis (MS) in acute relapse and viral inflammatory neurological disease (IND). We found increased levels of adhesion molecules (LFA-1 and beta1 integrin) in the CSF of patients with MS and IND compared to NIND. CD4+/CD8+ ratio was significantly higher in CSF of MS as compared with all groups analysed and compared with PB. We detected a significantly higher expression of the interleukin-2 receptor in PB of MS patients when compared with other groups. In patients with IND a significant higher expression of the interleukin-2 receptor was found in the CSF compared with MS and NIND. Our findings indicate that the activation of T lymphocytes primarily occurs in the peripheral immune compartment in MS and the increase of adhesion molecules in CSF is related to inflammatory disorders and not only to MS.

Immune activation in the peripheral blood of patients with acute ischemic stroke

Lymphocytes, neutrophils and macrophages are found in the brain in areas of acute ischaemic stroke. There is also evidence of modulation of systemic immune function after stroke, with post-stroke immunosuppression being observed. Because lymphocytes are activated in the peripheral immune compartment, before entry to the target organ, we reasoned that activated lymphocytes would be present in the circulation, prior to entering the brain, in patients after stroke. Because immune responses are controlled by regulatory mechanisms, we also reasoned that the post-stroke immunosuppression would involve T regulatory cells. The aim of the study was to look for evidence of immune activation and alterations in regulatory T cells in the peripheral blood of patients after acute ischaemic stroke, in comparison to age-matched healthy controls and patients with other neurological diseases (OND), and to determine the phenotype of the activated cells. The percentages of total and activated T cells, B cells, monocyte/ macrophages, and NK/NK-T cells were determined by labelling peripheral blood leukocytes with specific cell surface markers and analysis with 4-colour flow cytometry. The percentages of activated T cells and regulatory T cells were significantly increased in patients with ischemic stroke compared to healthy subjects and patients with OND. There was also an increase in the percentage of CCR7+ T cells. There were no significant differences in the activation of other cell types. In conclusion, there is evidence of immune activation and Treg cells in acute ischaemic stroke.

Pathophysiology of multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system (CNS). Both genetic and environmental causes for MS have been suggested. Recent genome-wide association studies revealed new susceptibility alleles for MS besides the HLA complex that are all related with immune functions. Whereas there is little evidence to support a purely environmental trigger for the disease in the sense of an infectious agent, the autoimmune hypothesis of MS is well established. Myelin antigen-specific CD4+ T cells become activated in the peripheral immune compartment, cross the blood-brain barrier and trigger the disease. Here, current concepts of the commitment of T cells to pro-inflammatory effector T helper cell lineages including Th17 cells that appear to be important inducers of organ-specific autoimmunity will be discussed.

PPAR gamma ablation in CD4+ T-cells augments T-cell responses in vivo and facilitates disease induction in experimental autoimmune encephalomyelitis

The peroxisome proliferator-activated receptor gamma (PPARg) belongs to a group of ligand-activated transcription factors involved in the regulation of metabolism and inflammation. PPARg is expressed in cells of the peripheral immune compartment like lymphocytes and dendritic cells, but also within the CNS, mainly in microglial cells and in neurons. Interestingly, oral administration of PPARg agonists ameliorates the clinical course and histopathological features in experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. However, it is still unclear which cell type is primarily involved in PPARg-mediated suppression of (auto)-immunity. Therefore, we generated mice with a T cell specific knock-out of PPARg employing a Cre-recombinase mediated ablation of PPARg which is limited to CD4-expressing T cells. In these mice (CD4-PPARg-KO) and their Cre negative wild-type littermates, we investigated both disease course and T cell responses during actively induced MOG-EAE. CD4-PPARg-KO mice exhibited both an earlier disease onset and signficantly increased disease severity during the induction phase of EAE when compared to their wild-type littermates. Accordingly, both peripheral T cell responses and T cell infiltration of the CNS were strongly enhanced in CD4-PPARg-KO mice. CNS infiltration in CD4-PPARg-KO mice was characterized by a profound increase in the frequency of IFN-g and IL-17 producing antigen-specific T cells. On the molecular level, PPARg ablation in T cells resulted in a profound increase in NF-kB baseline activity and an enhanced induction of NF-kB DNA-binding activity upon T cell receptor stimulation, thus indicating that PPARg operates as a brake of NF-kB-mediated T cell activity. These data demonstrate that ablation of PPARg enhances CD4+ T cell autoreactivity resulting in pronounced T cell responses and increased disease induction in a model of CD4+ T cell mediated CNS autoimmunity. Selective activation of PPARg in T cells therefore represents a promising future target for control of multiple sclerosis.

Retinoic Acid Increases Foxp3+ Regulatory T Cells and Inhibits Development of Th17 Cells by Enhancing TGF-β-Driven Smad3 Signaling and Inhibiting IL-6 and IL-23 Receptor Expression

The de novo generation of Foxp3+ regulatory T (Treg) cells in the peripheral immune compartment and the differentiation of Th17 cells both require TGF-beta, and IL-6 and IL-21 are switch factors that drive the development of Th17 cells at the expense of Treg cell generation. The major vitamin A metabolite all-trans retinoic acid (RA) not only enforces the generation of Treg cells but also inhibits the differentiation of Th17 cells. Herein we show that RA enhances TGF-beta signaling by increasing the expression and phosphorylation of Smad3, and this results in increased Foxp3 expression even in the presence of IL-6 or IL-21. RA also inhibits the expression of IL-6Ralpha, IRF-4, and IL-23R and thus inhibits Th17 development. In vitro, RA significantly promotes Treg cell conversion, but in vivo during the development of experimental autoimmune encephalomyelitis it does not increase the frequency of Treg cells in the face of an ongoing inflammation. However, RA suppresses the disease very efficiently by inhibiting proinflammatory T cell responses, especially pathogenic Th17 responses. These data not only identify the signaling mechanisms by which RA can affect both Treg cell and Th17 differentiation, but they also highlight that in vivo during an autoimmune reaction, RA suppresses autoimmunity mainly by inhibiting the generation of effector Th17 cells.

Myelin specific regulatory T cells expand from naturally occurring regulatory T cells and accumulate in the CNS during EAE (128.8)

Abstract FoxP3 is a lineage specific marker for regulatory T cells (T-reg). We have generated FoxP3 knock-in mice by introducing a bicistronic GFP reporter into the endogenous FoxP3 locus, allowing us to faithfully track T-reg in vivo. We have also generated a MOG 35-55/IAb-tetramer. The combination of these two technologies enables us to study the in vivo behavior of myelin specific T-reg and effector T cells (T-eff) during EAE. Upon immunization with MOG 35-55, we identified a population of MOG-tetramer-reactive T-reg in the peripheral lymphoid compartment. T-reg trafficked to the CNS where they were readily detected as early as day 10 following immunization. At this stage and till the peak of the disease, an overwhelming number of T-eff also targeted the CNS. Thus, the ratio of T-reg and T-eff at the initiation of disease was 1:15. However at the onset of recovery, this ratio changed in favor of T-reg (T-reg/T-eff close to 1:1. During recovery, 50% of the CNS-derived T-reg were IL-10 producers compared to only 10% in the peripheral immune compartment. FOXP3+ T cells isolated from the CNS are effective in suppressing naive MOG-specific T cells, but fail to control CNS-derived T-eff that secrete IL-6 and TNF. Our data suggest that in order for CD4+FOXP3+ T-reg to effectively control autoimmune reactions in the target organ, it may also be necessary to control tissue inflammation

Rapamycin, not cyclosporine, permits thymic generation and peripheral preservation of CD4+CD25+FoxP3+ T cells

Graft-versus-host-disease (GVHD) is the most common cause of poor outcome after allogeneic stem cell transplantation (SCT). Of late, exploitation of FOXP3(+) regulatory T-cell (T(REG)) function is emerging as a promising strategy in suppression of GVHD, while preserving graft-versus-leukemia (GVL). Cyclosporine and rapamycin reduce the expansion of effector T cells by blocking interleukin (IL)-2, but signaling by IL-2 is pivotal for T(REG) homeostasis. The resolution of GVHD is critically dependent on thymus-dependent reconstitution of the immunoregulatory system. Thus, there has been concern about the impact of blocking IL-2 signaling by immunosuppressive agents on T(REG) homeostasis. Here we demonstrate in a mouse model that in contrast to rapamycin, cyclosporine compromises not only the thymic generation of CD4(+)CD25(+)FoxP3(+) T cells but also their homeostatic behavior in peripheral immune compartments. Treatment with cyclosporine resulted in a sharp reduction of peripheral CD25(+)FoxP3(+) T cells in all immune compartments studied. Prolonged rapamycin treatment allowed for thymic generation of CD4(+)FoxP3(+) T cells, whereas treatment with cyclosporine led to a reduced generation of these cells. In conclusion, cyclosporine and rapamycin differentially affect homeostasis of CD4(+)FoxP3(+) T(REG) in vivo. As peripheral tolerance induction is a prerequisite for successful treatment outcome after allogeneic SCT, these findings are of potential clinical relevance.

T‐cell activation in the intestinal mucosa

The vast majority of peripheral T cells exist as resting lymphocytes until a signal for activation has been received. In response to antigen, this activation involves ligation of the T-cell receptor (TCR) and signal transmission through the CD3 complex, which then initiates a cascade of intracellular events that lead to the expression of genes used in T-cell activation. T-cell activation also requires soluble mediators in the form of cytokines and chemokines that regulate the process in both positive and negative ways, and costimulatory signals received in conjunction with TCR/CD3 signaling are important in the activation of T cells. Unlike T cells in other peripheral immune compartments, small and large intestinal intraepithelial lymphocytes (IELs) bear some but not all properties of activated T cells, suggesting that they constitute a large population of 'partially activated' effector cells. Thus, regulation of the IEL activation process must be held in tight check, yet it must be ready to respond to foreign antigen rapidly and effectively. We discuss how costimulatory molecules may hold the key to controlling IEL activation through a multiphase process beginning with cells that have already entered into the early stage of activation.

Th3 Cells in Peripheral Tolerance. II. TGF-β-Transgenic Th3 Cells Rescue IL-2-Deficient Mice from Autoimmunity

We developed a transgenic (Tg) mouse that expresses TGF-beta under control of the IL-2 promoter to investigate Th3 cell differentiation both in vitro and in vivo. We previously found that repetitive in vitro Ag stimulation results in constant expression of Foxp3 in TGF-beta-Tg Th3 cells that acquire regulatory function independent of surface expression of CD25. To examine the differentiation and function of Th3 cells in vivo and to compare them with thymic-derived CD4(+)CD25(+) regulatory T cells (Treg), we introduced the TGF-beta transgene into T cells of IL-2-deficient (IL-2(-/-)) mice. We found that the induction, differentiation, and function of TGF-beta-derived Foxp3(+) Th3 cells were independent of IL-2, which differs from thymic Tregs. In an environment that lacks functional CD25(+) thymic-derived Tregs, expression of the TGF-beta transgene in IL-2(-/-) mice led to the induction of distinct CD25(-) regulatory cells in the periphery. These cells expressed Foxp3 and efficiently controlled hyperproliferation of T cells and rescued the IL-2(-/-) mouse from lethal autoimmunity. Unlike IL-2(-/-) animals, TGF-beta/IL-2(-/-) mice had normal numbers of T cells, B cells, macrophages, and dendritic cells and did not have splenomegaly, lymphadenopathy, or inflammation in multiple organs. Accumulation of Foxp3(+) cells over time, however, was dependent on IL-2. Our results suggest that TGF-beta-derived Foxp3(+)CD25(+/-) Th3 regulatory cells represent a different cell lineage from thymic-derived CD25(+) Tregs in the periphery but may play an important role in maintaining thymic Tregs in the peripheral immune compartment by secretion of TGF-beta.

Th3 Cells in Peripheral Tolerance. I. Induction of Foxp3-Positive Regulatory T Cells by Th3 Cells Derived from TGF-β T Cell-Transgenic Mice

TGF-beta has been shown to be critical in the generation of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs). Because Th3 cells produce large amounts of TGF-beta, we asked whether induction of Th3 cells in the periphery was a mechanism by which CD4(+)CD25(+) Tregs were induced in the peripheral immune compartment. To address this issue, we generated a TGF-beta1-transgenic (Tg) mouse in which TGF-beta is linked to the IL-2 promoter and T cells transiently overexpress TGF-beta upon TCR stimulation but produce little or no IL-2, IL-4, IL-10, IL-13, or IFN-gamma. Naive TGF-beta-Tg mice are phenotypically normal with comparable numbers of lymphocytes and thymic-derived Tregs. We found that repeated antigenic stimulation of pathogenic myelin oligodendrocyte glycoprotein (MOG)-specific CD4(+)CD25(-) T cells from TGF-beta Tg mice crossed to MOG TCR-Tg mice induced Foxp3 expression in both CD25(+) and CD25(-) populations. Both CD25 subsets were anergic and had potent suppressive properties in vitro and in vivo. Furthermore, adoptive transfer of these induced regulatory CD25(+/-) T cells suppressed experimental autoimmune encephalomyelitis when administrated before disease induction or during ongoing experimental autoimmune encephalomyelitis. The suppressive effect of TGF-beta on T cell responses was due to the induction of Tregs and not to the direct inhibition of cell proliferation. The differentiation of Th3 cells in vitro was TGF-beta dependent as anti-TGF-beta abrogated their development. Thus, Ag-specific TGF-beta-producing Th3 cells play a crucial role in inducing and maintaining peripheral tolerance by driving the differentiation of Ag-specific Foxp3(+) regulatory cells in the periphery.