Role of autophagy in rejection after solid organ transplantation: A systematic review of the literature

Abstract

Organ transplantation has long been recognized as an effective treatment for end-stage organ failure, metabolic diseases, and malignant tumors. However, graft rejection caused by major histocompatibility complex mismatch remains a significant challenge. While modern immunosuppressants have made significant strides in reducing the incidence and risk of rejection, they have not been able to eliminate it completely. The intricate mechanisms underlying transplant rejection have been the subject of intense investigation by transplant immunologists. Among these factors, autophagy has emerged as a key player. Autophagy is an evolutionarily conserved mechanism in eukaryotic cells that mediates autophagocytosis and cellular protection. This process is regulated by autophagy-related genes and their encoded protein families, which maintain the material and energetic balance within cells. Additionally, autophagy has been reported to play crucial roles in the development, maturation, differentiation, and responses of immune cells. In the complex immune environment following transplantation, the role and mechanisms of autophagy are gradually being revealed. In this review, we aim to explore the current understanding of the role of autophagy in solid organ rejection after transplantation. Furthermore, we delve into the therapeutic advancements achieved by targeting autophagy involved in the rejection process.

Key Words: Autophagy; Organ transplantation; Graft rejection; Immunity; Immune modulation

Core Tip: The intricate mechanisms underlying transplant rejection have been the subject of intense investigation by transplant immunologists. As an evolutionarily conserved self-regulatory protective mechanism in eukaryotic cells, autophagy also plays a crucial role in the development, maturation, differentiation and response of immune cells. Our article reviews the role and mechanisms of autophagy of different types of immune cells in the rejection after solid organ transplantation, so as to provide a theoretical basis for better understanding the profound connotation of graft rejection and potential treatment approaches by targeting autophagy in the future.

INTRODUCTION

Organ transplantation, a life-saving procedure, has been extensively utilized globally since its initial success in the 1950s[1-3]. Owing to continual advancements in medical technology, the success rates of organ transplants have notably increased, offering renewed hope to numerous patients suffering from end-stage organ failure. Nevertheless, immune rejection remains a significant hurdle that impacts long-term graft survival and patient quality of life, despite remarkable progress in transplantation techniques. Immune rejection occurs when the recipient’s immune system attacks the graft’s alloantigens, potentially leading to graft dysfunction, severe complications, and even life-threatening situations for the patient[4]. While clinically administered immunosuppressants can mitigate rejection to some extent, their long-term use is associated with numerous side effects and cannot fully prevent rejection[5]. Consequently, effective rejection inhibition and enhanced long-term graft survival rates remain pivotal and challenging aspects in the transplantation field.

In recent years, autophagy, an important cellular self-regulatory mechanism, has gradually been recognized for its role in the immune system. Autophagy is not only involved in the degradation and recycling of intracellular substances but also plays an important role in the development, differentiation and function of immune cells[6-8]. In particular, in organ transplant rejection, the regulatory effect of autophagy on immune cells may have a profound impact on graft survival and patient prognosis. In this review, we discuss the research progress on autophagy and its molecular mechanism in organ transplantation rejection and its potential therapeutic value to provide new ideas and methods for the clinical practice of organ transplantation.

BASIC CONCEPTS AND MECHANISMS OF AUTOPHAGY

Definition and classification of autophagy

Autophagy is a highly conserved biological process in eukaryotic cells that involves the degradation and recycling of senescent, damaged or redundant organelles and proteins through lysosomes to maintain cell homeostasis[9-11]. Autophagy can be classified into macroautophagy, microautophagy and chaperone-mediated autophagy according to the pathway of substrate entry into lysosomes. Among them, macroautophagy is the most widely studied, and autophagy is usually referred to as macroautophagy.

Processes and regulation of autophagy

The process of autophagy includes the formation of autophagic vesicles, the fusion of autophagosomes and lysosomes, and the degradation of substances in autophagosomes[12]. The regulatory mechanisms of autophagy are complex and diverse and involve a variety of signaling pathways and molecules. Under conditions such as nutrient deficiency, oxidative stress, and pathogen infection, the level of autophagy in cells significantly increases in response to external stimuli. Autophagy is regulated mainly by proteins encoded by autophagy-related genes (ATGs), which play key roles in the formation, fusion and degradation of autophagic vesicles[13]. The molecular regulatory mechanisms of autophagy involve multiple key proteins and signaling pathways that play important roles in the initiation, expansion, and completion of autophagy. The core regulatory proteins involved in autophagy include members of the ATG-related proteins family, such as ATG 5, ATG 7, ATG 12, and microtubule-associated protein 1 light chain 3 (LC3), which can form two types of ubiquitin-like proteins and link systems: (1) The ATG 12-ATG 5-ATG 16 system, which is activated by ATG 7 (E1-like kinase), and is responsible for the excitation and positioning of the LC3 link system by participating in the expansion of autophagosomes; and (2) The LC3 link system, in which the LC3 precursor is modified by ATG 4 to form LC3-I, and its lipid-binding site - lipid phosphatidylethanolamine (PE) is bound by ATG 7 and ATG 3 to form a lipid-soluble LC3-II-PE. Since LC3-II can bind to the autophagosome membrane, it often serves as a marker for autophagosome formation[14].

During the induction phase of autophagy, the ATG 1/ unc-51-like kinase 1 (ULK 1) kinase complex plays a major role. ATG 1 is a protein serine/threonine kinase, but its own kinase activity does not affect the process of autophagy. ATG 1 can bind to ATG 13 (highly phosphorylated), activate the phosphatase protein phosphatase 2A to dephosphorylate ATG 13, and increase the binding to ATG 1. The ATG 1-ATG 13 complex binds to ATG 17-29-31 to initiate autophagy. In mammals, ULK 1/2, as a homolog of ATG 1, forms a ULK 1/2 complex with ATG 13 and focal adhesion kinase family interacting protein of 200 kD, which is a mammalian target of rapamycin (mTOR). When stimulated by energy deprivation or various factors, mTOR activity is reduced, which results in the activation of adenosine phosphate-activated protein kinase, the activation and phosphorylation of the ULK 1/2 complex, and the induction of phagosome nucleation. The precursor nucleation phase is mainly mediated by the class III phosphoinositide 3-kinase complex, which includes vacuolar sorting protein 34, beclin1 (ATG 6 homolog) and ATG 14. This complex mediates phagocytic bubble nucleation while recruiting the ATG 12-ATG 5-ATG 16 complex as well as LC3 to play a ubiquitin-like role in promoting the elongation of the phagocytic bubble membrane. With the expansion and closure of autophagosomes, they gradually mature. Under the action of the cytoskeleton and related motor proteins, tethering factors and soluble N-ethyl maleimide sensitive factor attachment protein receptor-related proteins promote the fusion of lysosomes to form autophagosomes. Their inner membrane and contents are degraded by lipases, proteases and proteases in lysosomes into small molecules for cell utilization[15].

ORGAN TRANSPLANTATION REJECTION AND IMMUNE TOLERANCE

Types and mechanisms of organ transplant rejection

Organ transplant rejection can be categorized into hyperacute, acute, and chronic rejection[16]. Hyperacute rejection, which occurs within minutes to hours post-transplantation, is caused primarily by ABO blood group incompatibilities or preexisting antibodies[17]. Acute rejection, the most prevalent type, typically manifests within weeks to months following transplantation. This rejection is predominantly mediated by T lymphocytes responding to the recognition of allotypic antigens on the graft[18,19]. Chronic rejection, a delayed form, often arises within months or years after the procedure. It involves a complex interplay between immune and nonimmune cells, leading to a gradual decline in graft function[20,21].

Immunological mechanisms of graft rejection

The immune response underlying transplant rejection involves the activation and proliferation of various immune cells, including T lymphocytes, B lymphocytes, macrophages, and dendritic cells (DCs), as well as the secretion and regulatory actions of cytokines. T lymphocytes play a pivotal role in recognizing and attacking graft alloantigens. B lymphocytes contribute by producing antibodies, whereas macrophages and DCs, as antigen-presenting cells (APCs), initiate immune responses by capturing, processing, and presenting antigens to T lymphocytes. Cytokine secretion and regulation influence the type and intensity of the immune response.

Rejection is initiated by the recognition of graft antigens. Donor-derived APCs, particularly the DCs remaining in the graft, present the intact donor peptide-major histocompatibility complex (pMHC) without antigen processing to the recipient’s T lymphocytes via direct recognition[22]. Recipient APCs repeat this process, processing and presenting antigens to naive T lymphocytes through indirect recognition mechanisms[23]. Additionally, a semi-indirect recognition pathway exists in which recipient APCs present intact donor alloantigen pMHC complexes transferred from donor APC to recipient T cells by exosome or other means[24]. Among these pathways, direct recognition is considered the most critical. The characteristics of different recognition ways are shown in Figure 1.

Figure 1 The characteristics of different recognition ways in organ transplantation. Direct recognition: Recipient T cells directly recognize intact alloantigen peptide-major histocompatibility complex presented by the donor antigen presenting cell (APC) without antigen processing; Indirect recognition: Recipient T cells recognize the donor alloantigen-self major histocompatibility complex presented by recipient APC processing; Semi-direct recognition: Recipient T cells recognize intact donor alloantigen peptide-major histocompatibility complex on the surface of recipient APC that transferred from donor APC by exosome or other means. MHC: Major histocompatibility complex; APC: Antigen presenting cell; TCR: T-cell receptor; p-MHC: Peptide-major histocompatibility complex.

The recognition of transplantation antigens provides the initial signal for T cell activation, whereas the interaction of the costimulatory molecules CD80/86 and CD40 on APCs with the T cell surface receptor CD28 and CD40 L provides the second signal necessary for lymphocyte activation, enabling the full activation of reactive T lymphocytes. Besides, inflammation cytokine signals such as interleukin (IL)-2/IL-2R further promote the activation T cells. These signals are transmitted intracellularly via the calcium - calcineurin pathway, mTOR signaling, and nuclear factor of activated T cells (NF-AT) pathway, activating transcription factors that produce IL-2 and other proinflammatory cytokines and promote T cell proliferation. This stimulates lymphocyte proliferation and differentiation into effector T and B lymphocytes[25-27]. The signaling pathway of activation and proliferation of T cells after recognizing alloantigen is shown in Figure 2.

Figure 2 The signaling pathway of activation and proliferation of T cells after recognizing alloantigen. Alloantigen recognition provides the initial signal for T cell activation, then the interaction of the costimulatory molecules CD80/86 and CD40 on antigen presenting cell and B cell with the T cell surface receptor CD28 and CD40 L provides the second signal, enabling the full activation of reactive T lymphocytes. These signals then transmitted intracellularly activating the calcium-calcineurin pathway and then promoting dephosphorylation of nuclear factor of activated T cells and its nuclear translocation, resulting in activating transcription factors that produce interleukin (IL)-2 and other proinflammatory cytokines. Inflammation cytokine IL-2 engagement with IL-2R activating mammalian target of rapamycin complex 1 and then cyclin D1/CDK4 signaling, thus promote cell cycle from G1 to S phase and ultimately facilitate proliferation of T cells. APC: Antigen presenting cell; MHC: Major histocompatibility complex; NF-AT: Nuclear factor of activated T cells; TCR: T-cell receptor; mTOR: Mammalian target of rapamycin; IL: Interleukin; PRAS40: Proline-rich Akt substrate of 40 kDa; mLST80: Mammalian lethal with SEC13 protein 8.

Induction and maintenance of immune tolerance

Immune tolerance refers to the immune system’s lack of response, or production of a minimal immune reaction, to a specific antigen[28]. This serves to preserve the stability of the body’s internal environment. In the context of organ transplantation, inducing immune tolerance plays a crucial role in minimizing the need for immunosuppressive drugs, mitigating the chances of rejection, and enhancing graft survival. Nevertheless, the precise mechanism underlying immune tolerance induction remains elusive and warrants further exploration.

Role of autophagy in organ transplant rejection

Autophagy can regulate the activity and function of a variety of immune cells. For example, autophagy can regulate the maturation and function of DCs, affecting their antigen presentation and immune stimulation ability[29]. Autophagy can also regulate the metabolic state and signaling pathways of lymphocytes, affecting their proliferation, differentiation and survival[30,31]. In addition, autophagy can affect the activity and function of innate immune cells, such as macrophages and natural killer cells, and regulate their ability to phagocytose, kill and secrete cytokines[32]. Here, we compiled the autophagy status of different immune cells from literature on various organ transplantation models, as well as the impact of manipulating autophagy on immune cells and transplantation rejection outcomes (Table 1).

Table 1 Role of autophagy in different immune cells in organ transplantation rejection.

Cell type	Transplant model	Autophagy level in transplantation	Autophagy manipulating	Autophagy level after manipulating	Functional outcome after autophagy manipulating	Ref.
T cell	Heart, mice	N/A	BECN1 knock down	N/A	Increased alloreactivity of T cells; Enhanced graft rejection	Verghese et al[37], 2014
	Liver, rats	↑	3-methyladenine	↓	Inhibited survival and function of T cells; prolonged allograft survival	Chen et al[38], 2019
	Skin and heart, mice	N/A	Chloroquine	N/A	Reduced infiltration of T cells; prolonged allograft survival	Cui et al[40], 2020
	Liver, rats	↑	JNK inhibitor SP600125	↓	Inhibited survival and function of T cells; prolonged allograft survival	Wang et al[39], 2024
B cell	Heart, mice	↑	ATG7 knock down	↓	Inhibited secondary alloantibody responses, decreased alloreactive memory B cells	Fribourg et al[45], 2018
DC	Skin, mice	N/A	Sorafenib	↑	Inhibited viability, maturation of dendritic cells; prolonged allograft survival	Lin et al[58], 2013
Treg	Heart, mice	↓	The miR-146a knock down	↑	Enhanced inhibitory effects of Tregs; prolonged allograft survival	Lu et al[65], 2021
	Heart, mice	↓	lncRNA PVT1 overexpression	↑	Enhanced inhibitory effects of Tregs; prolonged allograft survival	Lu et al[66], 2021
MDSC	Heart, mice	N/A	mTOR conditional knockout	↑	Promoted differentiation, immunosuppressive function of MDSCs; prolonged allograft survival	Li et al[68], 2021
Macrophage	Corneal, rats	N/A	Kaempferol	↑	Reduced M1 polarization, inflammasome activation of macrophage; alleviated graft rejection	Tian et al[73], 2021

DC: Dendritic cell; MDSC: Myeloid-derived suppressor cell; mTOR: Mammalian target of rapamycin; ATG: Autophagy-related gene; Treg: Regulatory T cell; miR: MicroRNA; lncRNA: Long noncoding RNA; BECN1: Beclin 1; JNK: Jun N-terminal kinases; PVT1: Plasmacytoma variant translocation 1; MDSCs: Myeloid-derived suppressor cells; N/A: Not applicable.

Effects of autophagy on T lymphocytes

Autophagy is involved in the differentiation and functional development of T lymphocytes. Although bone marrow cells can differentiate in the absence of autophagy, severe bone marrow hyperplasia can occur, and the lack of autophagy in mouse hematopoietic progenitor cells severely impairs the differentiation of the lymphoid lineage[33]. Thymocyte development is more dependent on functional autophagy, and in the absence of autophagy to remove damaged mitochondria (mitophagy), the number of naïve T cells is greatly reduced[8]. Autophagy also plays a crucial role in the functional development of mature T cells. The survival of T cells during proliferation depends on autophagy, and a lack of autophagy leads to mitochondrial dysfunction and promotes the aging and apoptosis of T cells[34]. In addition, a lack of autophagy leads to impaired memory formation in CD8⁺ T cells, whereas the promotion of autophagy helps to restore T cell memory responses[35,36].

Graft antigen-reactive T cells are undoubtedly the key actors leading to organ rejection injury. Although the important role of autophagy in the biological function of T cells has been described above, the role of autophagy in T cells in the microenvironment of transplantation is still controversial. An earlier study in a mouse heart transplantation model suggested that autophagy is necessary to promote effector T cell death during transplantation tolerance induction, and autophagy-deficient beclin-1 heterozygous effector T cells adoptively transferred to allogeneic recipients exhibited increased proliferation, reduced cell death, and increased Interferon-γ production[37]. In the rat liver transplantation rejection model, we confirmed that autophagy in CD8⁺ T cells is a key factor mediating liver transplantation rejection[38]. The underlying mechanism is that upregulated jun N-terminal kinases signaling promotes beclin 1/ B-cell lymphoma-2 complex depolymerization and upregulates beclin 1 expression, which activates autophagy to promote the survival and proliferation of CD8⁺ T cells. Pharmacological inhibition of autophagy with the inhibitor 3-methyladenine (3-MA) or inhibition of jun N-terminal kinases signaling significantly prolonged receptor survival[39]. Similarly, a study involving mouse skin and heart transplant models revealed that the administration of chloroquine, another autophagy inhibitor, inhibited the activation and function of murine cardiac alloreactive T cells by increasing cytotoxic-T-lymphocyte-associated antigen 4 expression during T-cell activation in vitro and in vivo[40]. However, in graft-versus-host disease (GVHD), which has immunological mechanisms similar to those of rejection, the function of autophagy in T cells is again complicated. Oravecz-Wilson et al[41] reported that allogeneic stimulation can promote autophagy in T cells both in vitro and in vivo, but ATG5 deficiency in donor T cells attenuates the induction of autophagy, decreases proliferation, and promotes an increase in effector cytokines. This uncoupling of proliferation and the effector response ultimately alleviates GVHD. Although the reason for these differences is not clear, it may be partly due to the differences in the organ transplant models involved, as the immune microenvironment itself varies greatly from one transplant model to another. The difference in autophagy research methods is also one of the reasons. The detection of autophagic flow and autophagic flux is considered the gold standard for detecting the level of autophagy, but it is very difficult for immune cells to operate, and simply using flow cytometry, immunohistochemistry, and western blotting to detect the expression of autophagy-related proteins may mask true autophagy. The abovementioned factors ultimately limit the unity of the conclusions of different researchers on the role of T cell autophagy in organ transplantation rejection.

Effects of autophagy on B lymphocytes

In the past, B cells were considered participants in graft rejection rather than initiators of it. B cells have been known to participate in graft rejection by providing a second activation signal, secreting cytokines, and acting as APCs to activate T cells[42]. Recently, B cells have been shown to secrete antibodies to induce antibody-mediated rejection (AMR), which has attracted increasing attention in organ transplantation. AMR is mediated primarily by donor-specific antibodies that usually target both human leukocyte antigen (HLA) and non-HLA antigens in the donor organ, leading to complement activation, inflammatory responses, and tissue damage. The recipient’s preexisting anti-donor HLA antibodies or those produced by the recipient’s B cells activated by transplantation antigens after transplantation damage the graft by activating complement and antibody-dependent cellular cytotoxicity, resulting in rejection[43]. Notably, AMR is an important mechanism mediating rejection after kidney and heart transplantation, but it is relatively rare in liver transplantation[44].

In addition to its role in T cells, autophagy plays an important role in regulating B cell function and survival; however, other detailed reviews are lacking. We discuss the role and mechanism of B cell autophagy in the microenvironment of transplantation. In a mouse model of heart transplantation, Fribourg et al[45] reported that autophagy is required for memory B cell function and secondary alloantibody responses and that the inhibition of autophagy with the inhibitor 3-MA prevents memory donor-specific antigen responses to alloantigens. A renal transplant clinical cohort study revealed that the expression of ATGs was significantly associated with the development of AMR[46]. Zhang et al[47] reported that the levels of the autophagy-related protein ATG12 and autophagy were increased in the B cells of patients with AMR after kidney transplantation. Further application of miR-107 may regulate autophagy by targeting ATG12 to reduce the secretion of IgG and IgM antibodies by the B lymphocytes of transplant recipients. These studies suggest that autophagy in B cells promotes rejection by regulating function and memory formation.

Effects of autophagy on DCs

DCs are important APCs in the immune system that can activate T lymphocytes and trigger immune responses, thus playing an important role in organ transplantation rejection. Autophagy plays an important role in the maturation and function of DCs[48]. Autophagy can regulate the metabolic state and functional activity of DCs, thereby affecting their ability to take up, process and present antigens. During endogenous antigen presentation, autophagy contributes to the degradation of endogenous antigens into peptides. These peptides can then be transported to the endoplasmic reticulum by transporters associated with antigen processing, where they are further processed and bound to major histocompatibility complex (MHC) I molecules to form peptide-MHCI complexes and activate CD8⁺ T cells[49,50]. In addition, autophagy is required for MHC II molecules to present antigens in vivo, and dendritic cell-mediated LC3-PE-coupled autophagy can promote MHC II molecules to present foreign antigens to CD4⁺ T cells[51], which further expands the role of autophagy in antigen presentation. In contrast to its role in antigen presentation, most of the current literature suggests that autophagy has an inhibitory role in the immunogenic maturation of DCs[52-54]. Immature DCs, which have low expression of MHC II class molecules and CD40 (-), CD80 (-) and CD86 (-) stimulating molecules and a reduced ability to stimulate the T cell response, can induce antigen-specific T cell apoptosis or anergy[55,56]. In addition, autophagy is also thought to inhibit the migration of DCs to lymph nodes, which may be related to the regulatory effect of autophagy on cytoskeletal proteins[57].

The paradoxical role of autophagy in antigen presentation and immunogenic maturation in DCs undoubtedly complicates its role in organ transplant rejection. A study using a mouse skin graft model revealed that sorafenib induced autophagy in DCs and inhibited their maturation and survival, thereby reducing graft rejection injury and prolonging skin graft survival[58]. Unfortunately, very few studies have investigated the role of DC autophagy in transplant rejection. Some researchers have explored the role of dendritic cell autophagy in the graft-versus-host reaction in bone marrow hematopoietic stem cell transplantation, which may provide some insight. A study of DCs after hematopoietic cell transplantation revealed that complement C3aR/C5aR signaling promoted host DC survival and activation by inhibiting lethal mitophagy, whereas blocking C3aR/C5aR activation significantly improved GVHD outcomes after hematopoietic cell transplantation (HCT) by enhancing mitophagy in recipient DCs[59]. Undoubtedly, regardless of the mechanism, these studies suggest that autophagy may play a negative regulatory role in DCs and may be beneficial for the amelioration of rejection.

Effects of autophagy on regulatory cells

In the graft microenvironment, the major regulatory cell populations include regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), the latter of which are a heterogeneous group of bone marrow-derived cells that are precursors of DCs, macrophages, and/or granulocytes[60-62]. The regulatory T cell population can inhibit immune responses, not only through the cytotoxicity of natural killer cells and natural killer T cells, but also through the immune response mediated by CD4⁺ and CD8⁺ T cells. Therefore, Tregs are considered to negatively regulate rejection and promote the development of immune tolerance.

Tregs are a subset of T cells with immunosuppressive functions that play important roles in maintaining immune homeostasis and preventing autoimmune diseases. Autophagy plays an important role in the function and stability of Tregs. Autophagy is considered to maintain the metabolic homeostasis and functional activity of Tregs, thereby enhancing their immunosuppressive capacity. In addition, autophagy can promote the proliferation and differentiation of Tregs[63,64]. In the transplant microenvironment, the regulatory role of autophagy in Tregs was also revealed. A mouse heart transplantation model revealed that miR-146a knockdown enhanced Treg autophagy and function, and autophagy-enhanced Tregs exhibited significant immunosuppressive functions and therapeutic efficacy for graft rejection[65]. In another heart transplantation study, the mechanism of miR-146a-induced Treg autophagy was explored in depth. The lncRNA plasmacytoma variant translocation 1 was found to upregulate Treg autophagy through miR-146a/tumor necrosis factor receptor-associated factor 6 to enhance its immunosuppressive function, thereby alleviating rejection[66]. In addition, in GVHD after HCT, autophagy was found to be essential for the survival of tyrosine-based inhibitory motif domain + Tregs in the bone marrow, and a therapeutic dose of granulocyte-colony-stimulating factor mobilized Tregs into the periphery and induced autophagy to promote reconstruction after stem cell transplantation, which was required for the alleviation of GVHD. Knockout of the key autophagy gene ATG7 in Treg resulted in reduced Treg reconstitution, aggravated GVHD and reduced the survival rate after HCT[67].

MDSCs can be divided into granulocytic MDSCs (G-MDSCs and CD11b + Ly6G + Ly6C-low) and monocytic MDSCs [monocytic-MDSCs (M-MDSCs) and CD11b + Ly6G-Ly6C-high][62]. From the characterization of MDSCs, it is clear that they are studied mainly in tumors. In recent years, the role of MDSCs and their autophagy-related mechanism in transplantation has also attracted attention. A mouse heart transplantation study revealed that mTOR deficiency promotes M-MDSC differentiation and enhances intracellular autophagy, leading to increased immunosuppressive M-MDSC function. The infusion of MTOR-deficient M-MDSCs can prolong cardiac allograft survival and establish immune tolerance in recipient mice by inhibiting T cell activation and inducing Tregs[68].

Effects of autophagy on innate immune cells

Other innate immune cells, such as monocytes/macrophages, neutrophils and natural killer cells, also participate in immune rejection through their own mechanisms. The innate immune system recognizes foreign grafts through pattern recognition receptors, which recognize pathogen-associated molecular patterns and damage-associated molecular patterns[69]. For organ transplantation, cell surface components in the graft, such as polysaccharides and proteins, may be regarded as nonself components by innate immune cells, thereby triggering immune responses. Innate immune cells regulate the activation and differentiation of adaptive immune cells by secreting signaling molecules such as cytokines and chemokines. In addition, macrophages can phagocytose and process antigens in the graft, thereby further activating the adaptive immune response[70]. In addition, innate immune cells are capable of mediating inflammatory responses that lead to local tissue damage and dysfunction at the transplant site. This inflammatory reaction is an important part of graft rejection and one of the major causes of graft failure[71,72].

The role of autophagy in the regulation of innate immune cells cannot be ignored. A study of corneal transplantation in rats revealed that kaempferol could inhibit the activation of the nod-like receptor family pyrin domain containing 3 inflammasome by inducing autophagy, thereby inhibiting macrophage polarization and ultimately reducing corneal graft rejection[73]. In particular, a recent mouse heart transplantation study published in Science showed that monocytes/macrophages can mediate immune memory to alloantigens, which is dependent mainly on cell surface paired Ig-like receptors, and that blocking the binding of paired Ig-like receptors-A to donor MHC-I molecules blocked memory formation and attenuated rejection in kidney and heart grafts[74]. Given the important role of autophagy in immune cell memory formation, it is reasonable to speculate that autophagy is more or less involved in the above studies, which, of course, requires further investigation.

CLINICAL APPLICATION OF TARGETING AUTOPHAGY IN SOLID ORGAN TRANSPLANTATION

With the in-depth understanding of the mechanism of autophagy in immune rejection, there has been a growing interest in exploring its potential clinical applications in organ transplantation. Rapamycin, a classical autophagy inducer, has been widely used as an important immunosuppressant in organ transplantation, which can promote autophagy by inhibiting mTOR[75-77]. Rapamycin has previously been shown to inhibit mTOR by binding to its intracellular receptor FK506 binding protein 12, which inhibits the activation of cyclin D1/CDK4 signaling and thus arrests cell cycle from G1 to S phase and ultimately reducing post-transplant rejection[75,78]. Recent studies indicate that rapamycin may exert immunomodulatory effects by regulating autophagy[73,79,80], although the weight of autophagy in this process remains to be fully elucidated. Other drugs with autophagy-modulating properties have also garnered attention. Hydroxychloroquine (HCQ) is a traditional antimalarial drug and an autophagy inhibitor that blocks autophagosome-lysosome fusion[81,82]. HCQ can interfere with the degradation function of APC lysosomes and block the formation of peptide-MHC protein complexes to inhibit the activation of T cells, which is also considered to be significantly related to autophagy[83]. HCQ has been widely used in the treatment of autoimmune diseases such as systemic lupus erythematosus and scleroderma[84], and has gradually been preliminarily explored in the application of transplantation, especially stem cell transplantation. Two early phase II clinical trials found that HCQ was well tolerated and effective in the prevention and treatment of GVHD after allogeneic bone marrow transplantation[85,86]. However, a subsequent phase III clinical trial confirmed that HCQ had no significant effect on acute/chronic GVHD prevention and survival[87]. A recent retrospective propensity-matched study in kidney transplantation also found that HCQ combined with standard immunosuppressive therapy failed to prevent rejection among renal transplant recipients[88]. 3-MA is another widely used autophagy inhibitor that blocks autophagosome formation by inhibiting the phosphatidylinositide-3kinase-III complex[89-91]. However, due to its high toxicity, poor specificity, and solubility issues, its use is currently limited to experimental research. Additionally, some surgical techniques can modulate autophagy, with a few reported in organ transplantation. An early study on liver transplantation found that ischemic preconditioning can promote the expression of autophagy-related proteins and reduce the incidence of acute and chronic rejection in fatty liver grafts[92]. Overall, there’s a lack of clinical or preclinical studies specifically targeting autophagy in transplant immunology. Drugs like rapamycin which are already used in transplantation, possess multiple signal modulation capabilities[93,94], making it challenging to determine the specific role of autophagy in its immunomodulatory effects.

CONCLUSION

As an important intracellular biological process, autophagy plays an important role in organ transplantation rejection. By regulating the activity and function of immune cells, affecting the production and secretion of cytokines and chemokines, and reducing oxidative stress and the inflammatory response in the graft, the regulation of autophagy can induce immune tolerance, reduce graft rejection, and promote graft survival. However, because the effects of autophagy on different immune cell populations are not consistent, the mechanism of autophagy in organ transplantation is still not fully understood and is controversial, and the regulatory strategy of autophagy still needs to be further optimized and improved. Future research can explore the specific mechanism of autophagy in organ transplantation, develop more specific and effective autophagy regulation strategies, explore the therapeutic value of autophagy in transplantation and rejection after transplantation, and guide individualized immunotherapy to provide new ideas and methods for the clinical practice of organ transplantation.