1. Introduction

Dendritic cells (DCs) are the most potent professional antigen-presenting cells (APCs) and are responsible for maintaining immune homeostasis ^[1][2]. In addition to inducing innate immune responses, DCs play a central role in inducing T cell-mediated immune responses. Immature DCs (imDCs) take up antigens and present them to naïve T cells, a process that induces DC maturation; by contrast, fully mature DCs (mDCs) promote adaptive immune responses by inducing effector T cells ^[3]. Active immunotherapy with mDCs affects adaptive immune responses to growing tumors, making mDCs an alternative to conventional cancer treatments ^[4]. In addition, the potency of DC-based immunotherapy can be increased by combining DCs with immune checkpoint inhibitors ^[5][6][7].

DCs are also responsible for establishing and maintaining peripheral immune tolerance. The intestinal immune system maintains a balance between responses to harmless bacteria and food antigens and immunity against pathogens ^[8][9][10]. Intestinal CD103⁺ DCs play a central role in regulating mucosal immunity via induction of CD4⁺CD25⁺Foxp3⁺ regulatory T (Treg) cells, which in turn inhibit cytokine production and reduce the functional activity of effector T cells ^[11][12].

Tolerogenic DCs (tDCs) show anti-inflammatory and immunosuppressive activity against various autoimmune diseases, including rheumatoid arthritis (RA) ^[13], experimental autoimmune myocarditis (EAM) ^[14], and acute myocardial infarction (AMI) ^[15]. Studies in animal models show that tDCs loaded with specific antigens ameliorate inflammation by activating Treg cells and/or by suppressing effector T cells ^[13][14][15]. Based on previous studies, various clinical trials have been conducted in patients with RA, type 1 diabetes, multiple sclerosis, and Crohn’s disease ^[16]. To date, however, few markers that distinguish tDCs from mDCs have been identified. There are many specific candidate markers of tDCs. Co-stimulatory molecules such as CD80 and CD86 are regarded as representative markers of tDCs. Indeed, expression of co-stimulatory molecules by tDCs and mDCs has been analyzed and compared; the results suggest that these molecules are not specific markers that can be evaluated independently. Moreover, cytokines produced by tDCs differ from those produced by mDCs. Anti-inflammatory cytokines, such as interleukin (IL)-4 and IL-10, are considered to be markers of tDCs, but these molecules are difficult to detect. Some biomolecules can be regarded as markers of tDCs. For example, high amounts of indoleamine 2,3-dioxygenase (IDO), an enzyme involved in tryptophan catabolism, are produced by CD103⁺ DCs in the mouse intestine ^[17]. Additionally, expression of complement subunit C1q may be a distinctive molecular marker of tDCs, although reports are contradictory. Thus, despite increasing knowledge about tDCs, a reliable marker remains elusive.

While attempting to identify potential markers of tDCs, we analyzed the gene expression profiles of different DC subsets ^[18]. Currently, we are attempting to correlate gene expression profiles with the function of these DC subsets. However, it is difficult to identify specific markers that regulate DC function. This review describes the distinct characteristics of tDCs and their roles in several diseases.

2. DCs Play a Central Role in Inducing Anti-Tumor Immune Responses

DCs play a key role in generating anti-tumor immune responses by presenting antigens to naïve T cells and inducing their differentiation into effector T cells (type 1 T helper (Th1) cells and cytotoxic T lymphocytes (CTL)). Presentation of antigen to T cells by DCs regulates anti-tumor immune responses through the immunological synapse by inducing different types of signals. DCs present antigens to naïve T cells expressing the major histocompatibility complex (MHC): T cell receptor complex (signal 1) and co-stimulatory molecules (signal 2). Activation of both signals induces activation and expansion of antigen-specific T cells ^[19][20][21]. DCs also produce additional signaling molecules (signal 3), which modify the different types of immune responses ^[22]. By signaling through the immunological synapse, DCs modulate differentiation of T cells that play a central role in adaptive immune responses. In addition, DCs are important activators of natural killer (NK) cells, which may play a critical role in eliminating virus-infected cells. DCs secrete IL-12, which activates NK cells; thus, DCs are critical for early anti-tumor immune responses.

Earlier studies report that mDCs produce high levels of pro-inflammatory cytokines, such as IL-12, IL-1β, IL-6, and tumor necrosis factor (TNF)-α, and that they express high levels of cell surface molecules such as MHC class II, CD40, and CD80/CD86. These cells play a crucial role in active immunotherapy by stimulating T cells. Despite the finding that previous use of immunotherapies, such as immune checkpoint inhibitors, and/or antibodies specific for anti-inflammatory molecules, provides objective survival benefits for patients, some of these treatments can have serious adverse effects related to excessive immune activation ^[23]. Combination immunotherapy with DC vaccines intensifies specific anti-tumor responses by increasing the CD4⁺ to CD8⁺ T cell ratio ^[5][6][7]. Although DC vaccines can induce effective anti-tumor responses, clinical results have not lived up to those observed in in vitro experiments and animal models. Dendritic cells also play a central role in embryo implantation by inducing maturation of uterine NK cells, tissue remodeling, and angiogenesis ^[24][25]. However, dysregulation of the balance between various DC subsets may disturb immune homeostasis, leading to infertility. During a normal pregnancy, maternal immunity maintains immune tolerance against the semi-allogeneic fetus. Expression of CD80/CD86, HLA-DR, and IL-12p70 by circulating DCs in pregnant women are lower than those in non-pregnant women ^[26]. During the early stages of pregnancy, Treg cells in draining lymph nodes migrate to the uterus and increase in number, resulting in induction of immunological tolerance. Thus, tDCs may reduce implantation failure rates by maintaining the balance among Th1, Th2, Th17, and Treg cells ^[27].

Although mDCs can induce antigen-specific immunogenic responses, excessive activation of the immune response leads to disorders of immune homeostasis. Other DC subtypes may be tolerogenic and therefore be responsible for establishing and maintaining immune tolerance. The activities of different DC subtypes may depend on their method of generation and/or on the presence of immunosuppressive factors.

3. Tolerogenicity of DCs

Immune cells, such as polymorphonuclear (PMN) leukocytes, monocytes, macrophages, and DCs, play key roles in protecting the host from aggressive inflammation triggered by pathogens or self-antigens ^[28]. Neutrophils, also known as PMN leukocytes, are the most abundant leukocyte in humans and are present in large numbers at sites of autoimmune damage. During inflammatory responses, neutrophils not only act as effector cells through phagocytosis and the production of lytic enzymes, reactive oxygen species and inflammatory mediators; but interact with other immune cells, such as NK cells, macrophages, DCs, T cells and B cells ^[29][30]. However, under low density conditions, neutrophils show immunosuppressive activity ^[31]. Likewise, new evidence has emerged indicating that circulating monocytes and tissue-resident macrophages have phenotypic and functional heterogeneity. Although CD14⁺CD16⁺ monocytes and Ly6C^high macrophages promote tissue damage and aggravate disease symptoms, in experimental models of inflammation and autoimmune disease, an increased frequency of CD14^lowCD16⁺ monocyte and Ly6C^low macrophages has been observed in models responding to therapy ^[32][33]. In addition, some tissue-resident conventional DCs or specialized subsets of DCs, such as Langerhans cells (CD207⁺) in the skin or CD103⁺ DCs in the mucosa, have critical roles in maintaining immune tolerance by increasing the Treg cell population and promoting the production of anti-inflammatory cytokines ^[34].

CD103⁺ DCs, which play a decisive role in maintaining mucosal immune homeostasis, possess immunosuppressive activity. Epithelial surfaces are continuously exposed to various antigens, including dietary materials, commensal bacteria, pathogenic viruses, and allergens. Goblet cells, which are specialized epithelial cells located in the small intestine, present antigens to CD103⁺ DCs, which then transport antigens to naïve T cells in Peyer’s patches ^[11]. Tolerogenicity of CD103⁺ DCs is essential for preventing immune responses against harmless materials, such as foods and/or commensal bacteria; otherwise, there is a real risk of a continuous and excessive immune response. Although CD103 is regarded as a marker of tDCs, accurate expression patterns and functions remain unclear. Furthermore, surface molecules CD80 and CD86 are regarded as markers of DC maturation. These co-stimulatory molecules bind to CD28 and cytotoxic T lymphocyte-associated protein-4 (CTLA-4) (CD152) on T lymphocytes, thereby regulating T cell activation ^[35]. Although both CD80 and C86 bind to CD28 and CTLA-4, the latter molecules trigger opposing immune responses. Binding of CD80 and CD86 to CD28 increases T cell activation by promoting differentiation of T cells into IFN-γ-producing type 1 T helper (Th1) cells and IL-4-producing type 2 helper (Th2) cells ^[36]. By contrast, binding of CD80 and CD86 to CTLA-4, which is expressed on Tregs, negatively regulates immune responses by removing CD80 and CD86 from DCs, a process that inhibits CD28-mediated stimulation of other effector T cells ^[37]. Moreover, DCs exposed to immunosuppressive agents that block DC maturation show lower expression of CD80 and CD86 than mDCs ^[13]. These findings suggest that CD80 and CD86 are potential markers that are specific for different DC subsets; however, they are not indicative of DC maturation status.

Studies in animal models of RA, EAM, and AMI, show that tDCs exert therapeutic effects ^[13][14][15]. Based on previous results, clinical trials have been conducted in patients with RA, type 1 diabetes, multiple sclerosis and Crohn’s disease ^[38][39][40]. Although tDC-based immunotherapy has potential, the mechanisms underlying its immunomodulatory activity and the specific markers expressed by DCs remain unclear. Further studies of gene and protein expression are needed to identify markers specific for tDCs and to fully understand their functions in immune suppression.