Regulation of T-cell functions by MHC class II self-presentation

Induction of Immune Tolerance in Islet Transplantation Using Apoptotic Donor Leukocytes

Allogeneic islet transplantation has become an effective treatment option for severe Type 1 diabetes with intractable impaired awareness due to hypoglycemic events. Although current immunosuppressive protocols effectively prevent the acute rejection associated with initial T cell activation in recipients, chronic rejection has remained an obstacle for achieving long-term allogeneic islet engraftment. The development of donor-specific immune tolerance to the allograft is the ultimate goal given its potential ability to overcome chronic rejection and disregard the need for maintenance immunosuppression, which may be toxic to islet grafts. Recently, a breakthrough in tolerance induction during allogeneic islet transplantation using apoptotic donor lymphocytes (ADLs) in a non-human primate model had been reported. Several studies have suggested that the clonal depletion, anergy, and expansion of the antigen-specific regulatory immune network are the mechanisms for donor-specific tolerance with ADLs, which act synergistically to induce robust transplant tolerance. This achievement represents a huge step forward toward the clinical application of immune tolerance induction. We herein summarize the reported operational induction therapies in islet transplantation using the ADLs. Moreover, a few obstacles for the engraftment of transplanted islets, such as islet immunogenicity and instant blood-mediated response, which need to be resolved in the future, are also discussed.

Long-term tolerance of islet allografts in nonhuman primates induced by apoptotic donor leukocytes

Immune tolerance to allografts has been pursued for decades as an important goal in transplantation. Administration of apoptotic donor splenocytes effectively induces antigen-specific tolerance to allografts in murine studies. Here we show that two peritransplant infusions of apoptotic donor leukocytes under short-term immunotherapy with antagonistic anti-CD40 antibody 2C10R4, rapamycin, soluble tumor necrosis factor receptor and anti-interleukin 6 receptor antibody induce long-term (≥1 year) tolerance to islet allografts in 5 of 5 nonsensitized, MHC class I-disparate, and one MHC class II DRB allele-matched rhesus macaques. Tolerance in our preclinical model is associated with a regulatory network, involving antigen-specific Tr1 cells exhibiting a distinct transcriptome and indirect specificity for matched MHC class II and mismatched class I peptides. Apoptotic donor leukocyte infusions warrant continued investigation as a cellular, nonchimeric and translatable method for inducing antigen-specific tolerance in transplantation.

Injection of donor apoptotic cells induces graft tolerance in mice. Here the authors combine this approach with short immunosuppressive therapy to achieve long-term tolerance to allogeneic islets and restoration of normoglycemia in diabetic nonhuman primates, and delineate cellular and molecular correlates of tolerance induction.

Use of a 3D Floating Sphere Culture System to Maintain the Neural Crest-Related Properties of Human Dental Pulp Stem Cells

Human dental pulp is considered an interesting source of adult stem cells, due to the low-invasive isolation procedures, high content of stem cells and its peculiar embryological origin from neural crest. Based on our previous findings, a dental pulp stem cells sub-population, enriched for the expression of STRO-1, c-Kit, and CD34, showed a higher neural commitment. However, their biological properties were compromised when cells were cultured in adherent standard conditions. The aim of this study was to evaluate the ability of three dimensional floating spheres to preserve embryological and biological properties of this sub-population. In addition, the expression of the inwardly rectifying potassium channel Kir4.1, Fas and FasL was investigated in 3D-sphere derived hDPSCs. Our data showed that 3D sphere-derived hDPSCs maintained their fibroblast-like morphology, preserved stemness markers expression and proliferative capability. The expression of neural crest markers and Kir4.1 was observed in undifferentiated hDPSCs, furthermore this culture system also preserved hDPSCs differentiation potential. The expression of Fas and FasL was observed in undifferentiated hDPSCs derived from sphere culture and, noteworthy, FasL was maintained even after the neurogenic commitment was reached, with a significantly higher expression compared to osteogenic and myogenic commitments. These data demonstrate that 3D sphere culture provides a favorable micro-environment for neural crest-derived hDPSCs to preserve their biological properties.

Allorecognition by T Lymphocytes and Allograft Rejection

Recognition of donor antigens by recipient T cells in secondary lymphoid organs initiates the adaptive inflammatory immune response leading to the rejection of allogeneic transplants. Allospecific T cells become activated through interaction of their T cell receptors with intact allogeneic major histocompatibility complex (MHC) molecules on donor cells (direct pathway) and/or donor peptides presented by self-MHC molecules on recipient antigen-presenting cells (APCs) (indirect pathway). In addition, recent studies show that alloreactive T cells can also be stimulated through recognition of allogeneic MHC molecules displayed on recipient APCs (MHC cross-dressing) after their transfer via cell-cell contact or through extracellular vesicles (semi-direct pathway). The specific allorecognition pathway used by T cells is dictated by intrinsic and extrinsic factors to the allograft and can influence the nature and magnitude of the alloresponse and rejection process. Consequently, various organs and tissues such as skin, cornea, and solid organ transplants are recognized differently by pro-inflammatory T cells through these distinct pathways, which may explain why these grafts are rejected in a different fashion. On the other hand, the mechanisms by which anti-inflammatory regulatory T cells (Tregs) recognize alloantigen and promote transplantation tolerance are still unclear. It is likely that thymic Tregs are activated through indirect allorecognition, while peripheral Tregs recognize alloantigens in a direct fashion. As we gain insights into the mechanisms underlying allorecognition by pro-inflammatory and Treg cells, novel strategies are being designed to prevent allograft rejection in the absence of ongoing immunosuppressive drug treatment in patients.

Donor Lymphocyte Infusion-Mediated Graft-versus-Host Responses in a Preclinical Swine Model of Haploidentical Hematopoietic Cell Transplantation

We previously described successful hematopoietic stem cell engraftment across MHC barriers in miniature swine without graft-versus-host disease (GVHD) using novel reduced-intensity conditioning regimens consisting of partial transient recipient T cell-depletion, thymic or low-dose total body irradiation, and a short course of cyclosporine A. Here we report that stable chimeric animals generated with these protocols are strongly resistant to donor leukocyte infusion (DLI)-mediated GVH effects. Of 33 total DLIs in tolerant chimeras at clinical doses, 21 failed to induce conversion to full donor hematopoietic chimerism or cause GVHD. We attempted to overcome this resistance to conversion through several mechanisms, including using sensitized donor lymphocytes, increasing the DLI dose, removing chimeric host peripheral blood cells through extensive recipient leukapheresis before DLI, and using fully mismatched lymphocytes. Despite our attempts, the resistance to conversion in our model was robust, and when conversion was achieved, it was associated with GVHD in most animals. Our studies suggest that delivery of unmodified hematopoietic stem cell doses under reduced-intensity conditioning can induce a potent, GVHD-free, immune tolerant state that is strongly resistant to DLI.

Immunosenescence and immune response in organ transplantation

The immune system undergoes a complex and continuous remodeling with aging. Immunosenescence results into both quantitative and qualitative changes of specific cellular subpopulations that have major impact on allorecognition and alloresponse, and consequently on graft rejection and tolerance. Here, we are going to review the immunological changes associated with the aging process relevant for transplantation. Interventions to selectively target changes associated with the senescence process seem promising therapeutic strategies to improve transplantation outcome.

Characterization of antigenic variants of hepatitis C virus in immune evasion

BACKGROUND

Antigenic variation is an effective way by which viruses evade host immune defense leading to viral persistence. Little is known about the inhibitory mechanisms of viral variants on CD4 T cell functions.

RESULTS

Using sythetic peptides of a HLA-DRB1*15-restricted CD4 epitope derived from the non-structural (NS) 3 protein of hepatitis C virus (HCV) and its antigenic variants and the peripheral blood mononuclear cells (PBMC) from six HLA-DRB1*15-positive patients chronically infected with HCV and 3 healthy subjects, the in vitro immune responses and the phenotypes of CD4+CD25+ cells of chronic HCV infection were investigated. The variants resulting from single or double amino acid substitutions at the center of the core region of the Th1 peptide not only induce failed T cell activation but also simultaneously up-regulate inhibitory IL-10, CD25-TGF-β+ Th3 and CD4+IL-10+ Tr1 cells. In contrast, other variants promote differentiation of CD25+TGF-β+ Th3 suppressors that attenuate T cell proliferation.

CONCLUSIONS

Naturally occuring HCV antigenic mutants of a CD4 epitope can shift a protective peripheral Th1 immune response into an inhibitory Th3 and/or Tr1 response. The modulation of antigenic variants on CD4 response is efficient and extensive, and is likely critical in viral persistence in HCV infection.

H-2 alleles contribute to antigen 85-specific interferon-gamma responses during Mycobacterium tuberculosis infection

The in vitro immune responses to mycobacterial antigens have been linked to the H-2 loci in mice. We evaluated in vitro and in vivo immune responses during early Mycobacterium tuberculosis (M.tb) pulmonary infection of C57BL/6 (H-2(b)), C57BL/6 (H-2(k)), CBA/J (H-2(k)), and C3H/HeJ (H-2(k)) mice to determine H-2(k)-dependent and -independent effects. H-2(k)-dependent effects included delayed and diminished Ag85-specific Th1 cell priming, a reduced frequency of Ag85-specific IFN-γ producing cells, reduced IFN-γ protein in vivo, and increased M.tb lung burden as demonstrated by C57BL/6 H-2(k) mice vs. C57BL/6 mice. H-2(k)-independent factors controlled the amount of Ag85-specific IFN-γ produced by each cell, T cell numbers, granuloma size, and lymphocytic infiltrates in the lungs. Overall, these results suggest that an H-2(k)-dependent suboptimal generation of Ag85-specific cells impairs control of early M.tb growth in the lungs. H-2(k)-independent factors influence the potency of IFN-γ producing cells and immune cell trafficking during pulmonary M.tb infection.

Regulatory T cells for tolerance therapy: revisiting the concept

The discovery of regulatory T cells (Tregs) as a crucial component of peripheral down-regulation of immunity to self and allogeneic antigens has raised legitimate hope for the development of Treg-based clinical protocols for tolerance to allografts. The present review addresses the question of whether therapeutic Tregs are ready to enter the clinical transplantation arena. In light of recent experimental observations, we will revisit some fundamentals of T cell and Treg biology that stress the need for further studies prior to applications and provide conceptual cues for novel therapeutic approaches.

Phenotype, distribution and alloreactive properties of memory T cells from cynomolgus monkeys

The high frequency of memory T cells present in primates is thought to represent a major barrier to tolerance induction in transplantation. Therefore, it is crucial to characterize these memory T cells and determine their functional properties. High numbers of memory T cells were detected in peripheral blood and all lymphoid tissues except lymph nodes, which were essentially the site of naïve T cells. The majority of CD8(+) memory T cells were effector memory cells located in the blood and bone marrow while most CD4(+) memory T cells were central memory cells present in the spleen. Next, memory T cells from over 100 monkeys were tested for their response to alloantigens by ELISPOT. Memory alloreactivity mediated via direct but not indirect allorecognition was detected in all animals. The frequency of allospecific memory T cells varied dramatically depending upon the nature of the responder/stimulator monkey combination tested. MHC gene matching was generally associated with a low-memory alloreactivity. Nevertheless, low anamnestic alloresponses were also found in a significant number of fully MHC-mismatched monkey combinations. These results show that selected donor/recipient combinations displaying a low memory alloresponsiveness can be found. These combinations may be more favorable for transplant tolerance induction.

The locus control region of the MHC class II promoter acts as a repressor element, the activity of which is inhibited by CIITA

The closest region of the promoter of MHC II genes and particularly three conserved boxes (X, Y and S) are fundamental for the transcriptional regulation. A second set of conserved sequences is present approximately 1200-1500 bp upstream in opposite orientation. In transient transfection experiments in IFN-gamma-treated macrophages and in B lymphocytes, we determined the expression of a fragment of 2035 bp of the I-Abeta gene, which contains the upstream boxes. Mutation of the distal boxes increased induction, thereby suggesting a repressive effect on transcription. In vitro, the proximal and distal ends of I-Abeta promoter were ligated in the presence of nuclear extracts from untreated macrophages but not when the extracts were obtained from IFN-gamma-stimulated cells. The mutation of distal or proximal boxes resulted in a decrease in the ligation assay. The addition of recombinant CIITA to untreated nuclear extracts decreased the capacity of the promoter to be ligated. Finally, we observed increased capacity to ligate the promoter in extracts from B cells lacking CIITA, but not from B cells lacking RFXANK. These results allow us to postulate a model where the proteins in the proximal and distal conserved sequences interact. When CIITA is induced, these proteins make an enhanceosome, allowing chromatin to open and initiate transcription.

Rapid deletional peripheral CD8 T cell tolerance induced by allogeneic bone marrow: role of donor class II MHC and B cells

Mixed chimerism and donor-specific tolerance are achieved in mice receiving 3 Gy of total body irradiation and anti-CD154 mAb followed by allogeneic bone marrow (BM) transplantation. In this model, recipient CD4 cells are critically important for CD8 tolerance. To evaluate the role of CD4 cells recognizing donor MHC class II directly, we used class II-deficient donor marrow and were not able to achieve chimerism unless recipient CD8 cells were depleted, indicating that directly alloreactive CD4 cells were necessary for CD8 tolerance. To identify the MHC class II(+) donor cells promoting this tolerance, we used donor BM lacking certain cell populations or used positively selected cell populations. Neither donor CD11c(+) dendritic cells, B cells, T cells, nor donor-derived IL-10 were critical for chimerism induction. Purified donor B cells induced early chimerism and donor-specific cell-mediated lympholysis tolerance in both strain combinations tested. In contrast, positively selected CD11b(+) monocytes/myeloid cells did not induce early chimerism in either strain combination. Donor cell preparations containing B cells were able to induce early deletion of donor-reactive TCR-transgenic 2C CD8 T cells, whereas those devoid of B cells had reduced activity. Thus, induction of stable mixed chimerism depends on the expression of MHC class II on the donor marrow, but no requisite donor cell lineage was identified. Donor BM-derived B cells induced early chimerism, donor-specific cell-mediated lympholysis tolerance, and deletion of donor-reactive CD8 T cells, whereas CD11b(+) cells did not. Thus, BM-derived B cells are potent tolerogenic APCs for alloreactive CD8 cells.

Immature and neurally differentiated mouse embryonic stem cells do not express a functional Fas/Fas ligand system

The potential of pluripotent embryonic stem (ES) cells to develop into functional cells or tissue provides an opportunity in the development of new therapies for many diseases including neurodegenerative disorders. The survival of implanted cells usually requires systemic immunosuppression, however, which severely compromises the host immune system, leading to complications in clinical transplantation. An optimal therapy would therefore be the induction of specific tolerance to the donor cells, while otherwise preserving functional immune responses. Fas ligand (FasL) is expressed in activated lymphocytes as well as cells in "immune-privileged" sites including the central nervous system. Its receptor, Fas, is expressed on various immune-reactive cell types, such as activated natural killer and T cells, monocytes, and polymorphic mononucleocytes, which can undergo apoptosis upon interaction with FasL. To render transplanted cells tolerant to host cellular immune responses, we genetically engineered mouse ES cells to express rat FasL (rFasL). The rFasL-expressing ES cells were analyzed for survival during in vitro neurodifferentiation and after transplantation to the rat brain without further immunosuppression. Although control transfected HEK-293T cells expressed functional rFasL, immature and differentiated mouse ES cells did not express the recombinant rFasL surface protein. Furthermore, there was no evidence for functional endogenous Fas and FasL expression on either ES cells or on neural cells after in vitro differentiation. Moreover, implanted rFasL-engineered ES cells did not survive in the rat brains in the absence of the immunosuppressive agent cyclosporine A. Our results indicate that immature and differentiated mouse ES cells do not express a functional Fas/FasL system. Disclosure of potential conflicts of interest is found at the end of this article.

Lipopolysaccharide up-regulates MHC class II expression on dendritic cells through an AP-1 enhancer without affecting the levels of CIITA

The expression of MHC class II genes is strictly tissue specific. In a limited number of cells, the expression of these genes is inducible by cytokines and only in dendritic and B cells is expression constitutive. LPS blocks the cytokine-dependent induction of these genes, but enhances their expression in dendritic and the B cell line A20. We have observed that LPS increased surface expression by raising I-A protein and mRNA levels. LPS does not enhance the expression of the transactivator CIITA. In transient transfection experiments, LPS induced the expression of the I-Abeta promoter, which contains an AP-1 box located between 1722 and 1729 bp upstream of the transcriptional start site. Mutation of this box abrogated the effect of LPS. The AP-1 box still responded to LPS when we moved it to -611 bp or even when it was in the opposite direction. LPS induced a complex that bound to the AP-1 box. However, in dendritic cells, the complex comprised c-jun and c-fos while in A20 cells only c-jun. This was confirmed by chromatin immune precipitation assays and the distinct induction of c-jun and c-fos mRNAs. Therefore, our results indicate that LPS exerts a novel regulatory mechanism in the control of MHC class II gene expression.

Analysis of Immune Markers in Human Cardiac Allograft Recipients and Association With Coronary Artery Vasculopathy

BACKGROUND

Because coronary artery vasculopathy (CAV) is a common cause of late cardiac allograft loss in humans, there is a need to develop and test non-invasive surrogate markers capable of detecting and predicting this disease entity.

METHODS

We performed a cross-sectional analysis of immune-based surrogate markers in 65 primary cardiac allograft recipients with or without angiographically documented CAV. Anti-donor cellular immunity was determined by interferon gamma (IFN)-gamma enzyme-linked immunosorbent spot (ELISPOT) assays using donor HLA-derived peptides (indirect pathway), and anti-donor alloantibodies were detected by flow cytometry using HLA-coated beads.

RESULTS

Anti-donor cellular and humoral immunity were detected more frequently in patients with CAV (17 of 32, 53.1%) than in controls (4 of 33, 12.1%) (p < 0.001). Anti-donor cellular and humoral immunity were detected in different sub-groups of CAV patients; peripheral blood lymphocytes (PBLs) from only 1 of 32 CAV patients reacted to donor peptides with simultaneous detection of peripheral anti-donor alloantibodies.

CONCLUSIONS

Immune reactivity in cardiac transplant recipients with CAV differs significantly from those without CAV and the detected responses are heterogeneous in nature. Serial assessments of anti-donor immunity using different methods will be required to detect and possibly predict outcome in cardiac transplant recipients.

Systemic immunomodulation of autoimmune disease using MHC-derived recombinant TCR ligands

Human autoimmune disease involves local activation of antigen-specific CD4(+) T cells that produce inflammatory Th1 cytokines leading to the further recruitment and activation of lymphocytes and monocytes, resulting ultimately in the destruction of target tissue. Antigen presenting cells (APCs) initiate activation of CD4(+) T cells in a multistep process that minimally involves co-ligation of the TCR and CD4 by the MHC class II/peptide complex and costimulation through additional T cell surface molecules such as CD28. Disruption of this highly orchestrated series of events can result in the direct modulation of CD4(+) T cell behavior. The interaction between MHC and TCR holds unique promise as a focal point for therapeutic intervention in the pathology of CD4(+) T cell-mediated diseases, and MHC class II-derived Recombinant TCR Ligands ("RTLs") have emerged as a new class of therapeutics with potent clinical efficacy in a diverse set of animal models for multiple sclerosis. Here I review the systemic effect that RTL therapy has on the intact immune system and present an overview of a molecular mechanism by which RTL therapy could induce these systemic changes.

CD4+ CD25+ T cells regulate vaccine-generated primary and memory CD8+ T-cell responses against herpes simplex virus type 1

It has become evident that naturally occurring CD25(+) regulatory T cells (T(reg) cells) not only influence self-antigen specific immune response but also dampen foreign antigen specific immunity. This report extends our previous findings by demonstrating that immunity to certain herpes simplex virus (HSV) vaccines is significantly elevated and more effective if T(reg) cell response is curtailed during either primary or recall immunization. The data presented here show that removal of CD25(+) T(reg) cells prior to SSIEFARL-CpG or gB-DNA immunization significantly enhanced the resultant CD8(+) T-cell response to the immunodominant SSIEFARL peptide. The enhanced CD8(+) T-cell reactivity in T(reg) cell-depleted animals was between two- and threefold and evident in both acute and memory stages. Interestingly, removal of CD25(+) T(reg) cells during the memory recall response to plasmid immunization resulted in a twofold increase in CD8(+) T-cell memory pool. Moreover, in the challenge experiments, memory CD8(+) T cells generated with plasmid DNA in the absence of T(reg) cells cleared the virus more effectively compared with control groups. We conclude that CD25(+) T(reg) cells quantitatively as well as qualitatively affect the memory CD8(+) T-cell response generated by gB-DNA vaccination against HSV. However, it remains to be seen if all types of vaccines against HSV are similarly affected by CD25(+) T(reg) cells and if it is possible to devise means of limiting T(reg) cell activity to enhance vaccine efficacy.