Tolerogenic dendritic cells (a. k. a. tol-DCs, tDCs, or DCregs) are heterogenous pool of dendritic cells with immuno-suppressive properties, priming immune system into tolerogenic state against various antigens. These tolerogenic effects are mostly mediated through regulation of T cells such as inducing T cell anergy, T cell apoptosis and induction of Tregs. [1] Tol-DCs also affect local micro-environment toward tolerogenic state by producing anti-inflammatory cytokines.
Tol-DCs are not lineage specific and their immune-suppressive functions is due to their state of activation and/or differentiation. Generally, properties of all types of dendritic cells can be highly affected by local micro-environment such as presence of pro or anti-inflammatory cytokines, therefore tolerogenic properties of tol-DCs are often context dependant and can be even eventually overridden into pro-inflammatory phenotype. [2] [3] [4]
Tolerogenic DCs present a potential strategy for treatment of autoimmune diseases, allergic diseases and transplant rejections. Moreover, Ag-specific tolerance in humans can be induced in vivo via vaccination with Ag-pulsed ex vivo generated tolerogenic DCs. [5] For that reason, tolerogenic DCs are an important promising therapeutic tool. [6]
Dendritic cells (DCs) were first discovered and described in 1973 by Ralph M. Steinman. They represent a bridge between innate and adaptive immunity and play a key role in the regulation of initiation of immune responses. DCs populate almost all body surfaces and they do not kill the pathogens directly, they utilize and subsequently degrade antigens to peptides by their proteolytic activity. After that, they present these peptides in complexes together with their MHC molecules on their cell surface. DCs are also the only cell type which can activate naïve T cells and induce antigen-specific immune responses. [6] [7]
Therefore, their role is crucially important in balance between tolerance and immune response.
Tolerogenic DCs are essential in maintenance of central and peripheral tolerance through induction of T cell clonal deletion, T cell anergy and generation and activation of regulatory T (Treg) cells. For that reason, tolerogenic DCs are possible candidates for specific cellular therapy for treatment of allergic diseases, autoimmune diseases (e.g. type 1 diabetes, multiple sclerosis, rheumatoid arthritis) or transplant rejections. [8] [9] [6]
Tolerogenic DCs often display an immature or semi-mature phenotype with characteristically low expression of costimulatory (e.g. CD80, CD86) and MHC molecules on their surface. Tolerogenic DCs also produce different cytokines as mature DCs (e.g. anti-inflammatory cytokines interleukin (IL)-10, transforming growth factor-β (TGF-β)). Moreover, tolerogenic DCs may also express various inhibitory surface molecules (e.g. programmed cell death ligand (PDL)-1, PDL-2) or can modulate metabolic parameters and change T cell response. For example, tolerogenic DCs can release or induce enzymes such as indoleamine 2,3-dioxygenase (IDO) or heme oxygenase-1 (HO-1). IDO promotes the degradation of tryptophan to N-formylkynurenin leading to reduced T cell proliferation, whereas HO- 1 catalyzes degradation of hemoglobin resulting in production of monoxide and lower DC immunogenicity. Besides that, tolerogenic DCs also may produce retinoic acid (RA), which induces Treg differentiation. [10] [11]
Human tolerogenic DCs may be induced by various immunosuppressive drugs or biomediators. Immunosuppressive drugs, e.g. corticosteroid dexamethasone, rapamycin, cyclosporine or acetylsalicylic acid, cause low expression of costimulatory molecules, reduced expression of MHC, higher expression of inhibitory molecules (e.g. PDL-1) or higher secretion of IL-10 or IDO. In addition, incubation with inhibitory cytokines IL-10 or TGF-β leads to generation of tolerogenic phenotype. Other mediators also affect generation of tolerogenic DC, e.g. vitamin D3, vitamin D2, [12] hepatocyte growth factor or vasoactive intestinal peptide. The oldest and mostly used cytokine cocktail for in vitro DC generation is GM-CSF/IL-4. [10] [5]
Tolerogenic DCs may be a potential candidate for specific immunotherapy and are studied for using them for treatment of inflammatory, autoimmune and allergic diseases and also in transplant medicine. Important and interesting feature of tolerogenic DCs is also the migratory capacity toward secondary lymph organs, leading to T-cell mediated immunosuppression. The first trial to transfer tolerogenic DCs to humans was undertaken by Ralph Steinman's group in 2001. Relating to the DC administration, various application have been used in humans in last years. Tolerogenic DCs have been injected e.g. intraperitoneally in patients with Crohn's disease, intradermally in diabetes and rheumatoid arthritis patients, subcutaneously in rheumatoid arthritis patients and via arthroscopic injections in joints of patient with rheumatoid and inflammatory arthritis. [13]
Therefore, it is necessary to test tolerogenic DCs for a stable phenotype to exclude a loss of the regulatory function and a switch to an immunostimulatory activity.
Despite tol-DCs not being lineage specific, they generally express more cell-surface immuno-suppressive molecules and factors in comparison with immunogenic co-stimulatory molecules. Higher expression of inhibitory molecules is associated with their tolerogenic abilities.
These molecules are: PD-L1, immunoglobulin like transcripts ILT (ILT3/4/5), B7-H1, SLAM, DEC-205. [14] [15] [16] [17] Tolerogenic effect has been demonstrated also by over-expression of Jagged-1 on DCs which in turn induced antigen specific T regulatory cells producing TGF-b. [18]
Tol-DCs promotes central and peripheral tolerance. These tolerogenic properties are executed by deletion of T cells, induction of Tregs and anergized T cells, then by expression of immunomodulatory molecules such as PD-L1 and PD-L2, heme oxygenase 1, HLA-G, CD95L, TNF-related apoptosis inducing ligands, galectin-1 and DC-SIGN and production of immunosuppressive molecules such as IL-10, TGF-b, indoleamine 2,3-dioxygenase (IDO), IL-27 and NO. [19] [20] [21] [22]
Tol-DCs can be induced by various stimuli. It has been shown that following molecules induce/promote/favour induction of tol-DCs: IL-10, IL-27, TGF-b1, hepatocyte growth factor, vasoactive intestinal peptide, retinoid acid, vitamin D3, corticosteroids, rapamycin, cyclosporine, tacrolism, aspirin and ligands of AhR. [23] [24]
Currently are characterized two subpopulations of human tolerogenic DCs: CD83highCCR7+ and CD83lowCCR7− IL-10DCs. CD83high IL-10DCs display a stable phenotype under inflammatory conditions and show higher migratory capacity, providing migration to secondary lymphoid organs. Therefore, CD83high IL-10DCs could be promising and great candidates for tolerance-inducing vaccination studies in vivo. [5]
In 2011, Giannoukakis et al. published results of randomized, double-blind phase I study of autologous DCs vaccination in type I diabetic patients. Treatment with these cells was safe and well tolerated. [25]
The whole pool of tolerogenic dendritic cells can be divided in two large groups - Naturally occurring tolerogenic DCs and induced tolerogenic DCs.
Natural tol-DCs are mostly present in site of tolerogenic environment. They are maintained in their tolerogenic state by anti-inflammatory cytokines presented in those environments, but they can be easily overridden by inflammatory signals into being immunogenic. [23] They can be found in intestinal, pulmonary, cutaneous, blood and hepatic tissues. It is yet expected they will be found even elsewhere. [3]
Their tolerogenic effect is mostly due to their lack of immunogenic co-stimulatory molecules despite their ability to present antigens. This phenomenon results in T cells anergy. [3] Repetitive stimulation of T cells by iDCs can convert them into Tregs [26] [27] Immature and semimature dendritic cells are tolerogenic under steady-state conditions and once exposed to pro-inflammatory milieu they can also become immunogenic. [28] [29]
Tol-DCs can be induced by chemicals, pathological conditions or molecular modifications.
Certain pathogens are capable of hijacking host immune tolerance and induce Tregs in their surroundings. [30] [31] [32]
Tumours also developed ways of inducing tol-DCs resulting in differentiation and accumulation of Tregs in their stroma and draining lymph node. [33] [34]
As already mentioned above many pharmacological substances are capable of inducing tol-DCs including corticosteroids, rapamycin, cyclosporine, tacrolism, aspirin,.
Genetic manipulations can used to confer tolerogenic properties on DCs such as gene knock down, knock-out, transgenic over expression of proteins and others. [35]
In immunology, autoimmunity is the system of immune responses of an organism against its own healthy cells, tissues and other normal body constituents. Any disease resulting from this type of immune response is termed an "autoimmune disease". Prominent examples include celiac disease, post-infectious IBS, diabetes mellitus type 1, Henoch–Schönlein purpura (HSP) sarcoidosis, systemic lupus erythematosus (SLE), Sjögren syndrome, eosinophilic granulomatosis with polyangiitis, Hashimoto's thyroiditis, Graves' disease, idiopathic thrombocytopenic purpura, Addison's disease, rheumatoid arthritis (RA), ankylosing spondylitis, polymyositis (PM), dermatomyositis (DM), and multiple sclerosis (MS). Autoimmune diseases are very often treated with steroids.
A cytotoxic T cell (also known as TC, cytotoxic T lymphocyte, CTL, T-killer cell, cytolytic T cell, CD8+ T-cell or killer T cell) is a T lymphocyte (a type of white blood cell) that kills cancer cells, cells that are infected by intracellular pathogens (such as viruses or bacteria), or cells that are damaged in other ways.
The T helper cells (Th cells), also known as CD4+ cells or CD4-positive cells, are a type of T cell that play an important role in the adaptive immune system. They aid the activity of other immune cells by releasing cytokines. They are considered essential in B cell antibody class switching, breaking cross-tolerance in dendritic cells, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages and neutrophils. CD4+ cells are mature Th cells that express the surface protein CD4. Genetic variation in regulatory elements expressed by CD4+ cells determines susceptibility to a broad class of autoimmune diseases.
Immunotherapy or biological therapy is the treatment of disease by activating or suppressing the immune system. Immunotherapies designed to elicit or amplify an immune response are classified as activation immunotherapies, while immunotherapies that reduce or suppress are classified as suppression immunotherapies. Immunotherapy is under preliminary research for its potential to treat various forms of cancer.
The regulatory T cells (Tregs or Treg cells), formerly known as suppressor T cells, are a subpopulation of T cells that modulate the immune system, maintain tolerance to self-antigens, and prevent autoimmune disease. Treg cells are immunosuppressive and generally suppress or downregulate induction and proliferation of effector T cells. Treg cells express the biomarkers CD4, FOXP3, and CD25 and are thought to be derived from the same lineage as naïve CD4+ cells. Because effector T cells also express CD4 and CD25, Treg cells are very difficult to effectively discern from effector CD4+, making them difficult to study. Research has found that the cytokine transforming growth factor beta (TGF-β) is essential for Treg cells to differentiate from naïve CD4+ cells and is important in maintaining Treg cell homeostasis.
Gut-associated lymphoid tissue (GALT) is a component of the mucosa-associated lymphoid tissue (MALT) which works in the immune system to protect the body from invasion in the gut.
Co-stimulation is a secondary signal which immune cells rely on to activate an immune response in the presence of an antigen-presenting cell. In the case of T cells, two stimuli are required to fully activate their immune response. During the activation of lymphocytes, co-stimulation is often crucial to the development of an effective immune response. Co-stimulation is required in addition to the antigen-specific signal from their antigen receptors.
Immune tolerance, or immunological tolerance, or immunotolerance, is a state of unresponsiveness of the immune system to substances or tissue that would otherwise have the capacity to elicit an immune response in a given organism. It is induced by prior exposure to that specific antigen and contrasts with conventional immune-mediated elimination of foreign antigens. Tolerance is classified into central tolerance or peripheral tolerance depending on where the state is originally induced—in the thymus and bone marrow (central) or in other tissues and lymph nodes (peripheral). The mechanisms by which these forms of tolerance are established are distinct, but the resulting effect is similar.
T helper 17 cells (Th17) are a subset of pro-inflammatory T helper cells defined by their production of interleukin 17 (IL-17). They are related to T regulatory cells and the signals that cause Th17s to actually inhibit Treg differentiation. However, Th17s are developmentally distinct from Th1 and Th2 lineages. Th17 cells play an important role in maintaining mucosal barriers and contributing to pathogen clearance at mucosal surfaces; such protective and non-pathogenic Th17 cells have been termed as Treg17 cells.
Certain sites of the mammalian body have immune privilege, meaning they are able to tolerate the introduction of antigens without eliciting an inflammatory immune response. Tissue grafts are normally recognised as foreign antigens by the body and attacked by the immune system. However, in immune privileged sites, tissue grafts can survive for extended periods of time without rejection occurring. Immunologically privileged sites include:
In immunology, peripheral tolerance is the second branch of immunological tolerance, after central tolerance. It takes place in the immune periphery. Its main purpose is to ensure that self-reactive T and B cells which escaped central tolerance do not cause autoimmune disease. Peripheral tolerance prevents immune response to harmless food antigens and allergens, too.
Cancer immunology (immuno-oncology) is an interdisciplinary branch of biology and a sub-discipline of immunology that is concerned with understanding the role of the immune system in the progression and development of cancer; the most well known application is cancer immunotherapy, which utilises the immune system as a treatment for cancer. Cancer immunosurveillance and immunoediting are based on protection against development of tumors in animal systems and (ii) identification of targets for immune recognition of human cancer.
Interleukin 35 (IL-35) is a recently discovered anti-inflammatory cytokine from the IL-12 family. Member of IL-12 family - IL-35 is produced by wide range of regulatory lymphocytes and plays a role in immune suppression. IL-35 can block the development of Th1 and Th17 cells by limiting early T cell proliferation.
Chronic systemic inflammation (SI) is the result of release of pro-inflammatory cytokines from immune-related cells and the chronic activation of the innate immune system. It can contribute to the development or progression of certain conditions such as cardiovascular disease, cancer, diabetes mellitus, chronic kidney disease, non-alcoholic fatty liver disease, autoimmune and neurodegenerative disorders, and coronary heart disease.
T helper 3 cells (Th3) are a subset of T lymphocytes with immunoregulary and immunosuppressive functions, that can be induced by administration of foreign oral antigen. Th3 cells act mainly through the secretion of anti-inflammatory cytokine transforming growth factor beta (TGF-β). Th3 have been described both in mice and human as CD4+FOXP3− regulatory T cells. Th3 cells were first described in research focusing on oral tolerance in the experimental autoimmune encephalitis (EAE) mouse model and later described as CD4+CD25−FOXP3−LAP+ cells, that can be induced in the gut by oral antigen through T cell receptor (TCR) signalling.
Mucosal immunology is the study of immune system responses that occur at mucosal membranes of the intestines, the urogenital tract, and the respiratory system. The mucous membranes are in constant contact with microorganisms, food, and inhaled antigens. In healthy states, the mucosal immune system protects the organism against infectious pathogens and maintains a tolerance towards non-harmful commensal microbes and benign environmental substances. Disruption of this balance between tolerance and deprivation of pathogens can lead to pathological conditions such as food allergies, irritable bowel syndrome, susceptibility to infections, and more.
Regulatory B cells (Bregs or Breg cells) represent a small population of B cells that participates in immunomodulation and in the suppression of immune responses. The population of Bregs can be further separated into different human or murine subsets such as B10 cells, marginal zone B cells, Br1 cells, GrB+B cells, CD9+ B cells, and even some plasmablasts or plasma cells. Bregs regulate the immune system by different mechanisms. One of the main mechanisms is the production of anti-inflammatory cytokines such as interleukin 10 (IL-10), IL-35, or transforming growth factor beta (TGF-β). Another known mechanism is the production of cytotoxic Granzyme B. Bregs also express various inhibitory surface markers such as programmed death-ligand 1 (PD-L1), CD39, CD73, and aryl hydrocarbon receptor. The regulatory effects of Bregs were described in various models of inflammation, autoimmune diseases, transplantation reactions, and in anti-tumor immunity.
Tolerogenic therapy aims to induce immune tolerance where there is pathological or undesirable activation of the normal immune response. This can occur, for example, when an allogeneic transplantation patient develops an immune reaction to donor antigens, or when the body responds inappropriately to self antigens implicated in autoimmune diseases. It must provide absence of specific antibodies for exactly that antigenes.
Type 1 regulatory cells or Tr1 (TR1) cells are a class of regulatory T cells participating in peripheral immunity as a subsets of CD4+ T cells. Tr1 cells regulate tolerance towards antigens of any origin. Tr1 cells are self or non-self antigen specific and their key role is to induce and maintain peripheral tolerance and suppress tissue inflammation in autoimmunity and graft vs. host disease.
Infectious tolerance is a term referring to a phenomenon where a tolerance-inducing state is transferred from one cell population to another. It can be induced in many ways; although it is often artificially induced, it is a natural in vivo process. A number of research deal with the development of a strategy utilizing this phenomenon in transplantation immunology. The goal is to achieve long-term tolerance of the transplant through short-term therapy.