Introduction

Although organ transplantation is an effective treatment for end-stage organ failure, the shortage of organs remains the primary challenge in the field of global clinical organ transplantation.1,2 The Clinical Trials of Organ Transplantation—or CTOT—is a group that provides money for research on solid organ transplants in adult patients. They fund research at different centers, and they do both clinical and translational research. Donor families express a desire for a greater understanding of organ donation and transplantation. However, long-term recipient survival is still limited by allogeneic immune rejection. Existing immunosuppressants (eg, tacrolimus and rapamycin) significantly reduce the risk of acute rejection. However, their non-specific immunosuppressive properties can cause severe side effects, including opportunistic infections, malignancies, and metabolic disorders. For instance medications like calcium-phosphorus regulators increase the risk of skin cancer.3,4 Current therapies primarily rely on systemic immunosuppression. This approach fails to precisely regulate the local immune microenvironment and inadequately suppresses the abnormal activation of antigen-presenting cells (APCs), such as dendritic cells (DCs) and macrophages. Thus, there is an urgent need for the development of highly targeted and safe immunomodulatory strategies.5,6

Nano-drug delivery systems provide a new approach to overcoming the limitations of traditional therapies. These systems leverage their tunable physicochemical properties, including size, charge, and surface functionalization.7 Size is often closely related to aggregation and is a key determining factor. Conjugates with disparate sizes assemble into aggregates of distinct morphologies, and their dimensions exert a profound impact on internalization by non-phagocytic cells. Once internalized by cells, these conjugates are entrapped within endosomal vesicles, with only a small fraction likely reaching the cytoplasm to exert functional effects.8 Additionally, surface-engineered nanoparticles have enabled precise drug delivery. This precise targeting can be achieved through passive methods, such as the enhanced permeability and retention (EPR) effect, or active approaches, such as ligand modification. These methods enable the accumulation of nanoparticles within the local vascular endothelium of the graft, thereby enhancing drug delivery efficiency. Biodegradable PLGA nanoparticles, for example, are known for their stability and can encapsulate siRNA to silence the CD40 gene. This simultaneously inhibits DC maturation and the differentiation of hematopoietic stem cells into APCs.5 Additionally, surface engineering strategies, such as PEGylation and cell membrane mimicry, can reduce the effects of Regulon, extend its circulating half-life, and establish the basis for precise immune regulation.5,9 Autoimmune diseases, allergies, transplant rejection, and other immune disorders have a widespread impact. Conventional systemic immunosuppressive therapies present challenges regarding efficacy and safety. Tolerogenic vaccines can induce antigen-specific tolerance by generating regulatory T cells; however, they require precise modulation of the immune microenvironment. Engineered nanoparticles and microparticles with optimized designs can enhance targeted immune modulation and antigen-specific tolerance induction. Particle-based tolerance vaccines show promise for clinical translation in treating these disorders.5

This paper systematically explores strategies for suppressing transplant rejection through the synergistic inhibition of multiple mechanisms by nanocarrier platforms. This approach reveals the necessity of coordinating multiple targets and overcoming the limitations of single-pathway intervention. Examples include the targeted delivery of immunomodulators to APCs or T cell subsets, the regulation of intercellular communication via exosomes or biomimetic carriers, and the spatiotemporal control of drug release through responsive hydrogels, metal-organic frameworks, and other materials. We emphasize the unique efficacy of nanocarriers in suppressing immune responses and transplant rejection while addressing challenges of clinical translation, such as scalable manufacturing and long-term safety. The scope of this review encompasses two major scenarios: solid organ transplantation (SOT) and hematopoietic stem cell transplantation (HSCT). For SOT, the focus is directed toward core issues including ischemia-reperfusion injury (IRI) and the dysregulation of the local immune microenvironment. For HSCT, priority is given to the modulation of graft-versus-host disease (GVHD) and the maintenance of a balanced graft-versus-leukemia (GVL) effect. By systematically contrasting the immunopathological features of these two transplantation modalities, this review highlights the universality and specificity of nanotechnology, thereby providing a unified yet differentiated therapeutic strategy for immune regulation across distinct transplantation types. Our goal is to lay the theoretical groundwork for developing next-generation smart anti-rejection nano therapies and drive paradigm shifts in transplant immunology.

Molecular Mechanisms of Immune RejectionTransplant Immune Microenvironment

Notably, the long-term success of transplanted organs fundamentally depends on precisely regulating the immune microenvironment at the transplant site.10,11 The transplant immune microenvironment is a highly complex, dynamic regulatory hub formed by the parenchymal cells of the transplanted organ, adjacent tissues, infiltrating immune cells, the extracellular matrix (ECM), and numerous soluble signaling molecules, particularly cytokines.10–12 This unique local niche is not a passive bystander in immune responses; rather, it is a core determinant of immune outcomes. Its composition and functional state directly influence whether the immune system induces tolerance toward the transplant or initiates robust rejection.10,11 Changes in the microenvironment directly drive the rejection cascade. The intricate interactions and signaling exchanges among its various components regulate the initiation, amplification, and effector phases of the immune response in a dynamic manner.12 Immune cells in the transplantation milieu modulate allograft rejection via synergistic or antagonistic crosstalk. Emerging evidence shows donor-derived regulatory dendritic cell (DCreg) adoptive transfer orchestrates allogeneic immune responses in living donor liver transplantation (LDLT) recipients multi-facetedly. At 12 months post-transplant, DCregs reduce peripheral effector T/NK cell proportions, enrich tolerogenic DC subsets, suppress pro-inflammatory factor release and donor-reactive T cell proliferation, and inhibit intra-graft effector cell infiltration and TH1 pathway activation. Collectively, DCregs mitigate rejection by remodeling immune microenvironments, providing critical support for personalized immunosuppressive regimen optimization in clinical liver transplantation.13

Cytokines are among the numerous soluble factors that constitute the immune microenvironment. They function as core regulatory elements and have garnered extensive attention and undergone in-depth investigation.14 Clinical evidence indicates that characteristic alterations in the immune microenvironment are significantly correlated with postoperative survival in kidney transplant patients, highlighting the pivotal role of this field in clinical translation.15 Remarkably, the functions of specific key factors exhibit significant dependence on the signaling context and are targeted to specific cells, demonstrating multifaceted and even contradictory functional properties. For instance, the cytokine TGF-β can have opposing functions when acting on different target cells, such as T cells, macrophages, or fibroblasts, depending on the microenvironmental signaling context. Tolerogenic/anti-inflammatory effects: Induces regulatory T cell (Treg) differentiation and suppresses effector T cell function. Fibrogenic/pro-inflammatory effects: Activates fibroblasts to promote extracellular matrix (ECM) deposition or, under specific conditions, promotes pro-inflammatory polarization of macrophages. These effects confer a complex and profound role in maintaining tissue homeostasis and mediating injury responses.14,16,17 Therefore, conducting a precise analysis of the composition and dynamic changes within the immune microenvironment at the transplant site, along with its core regulatory factors (such as TGF-β), is essential for developing effective strategies to suppress rejection and enhance long-term graft survival rates. A deeper understanding of its regulatory networks, including the multifaceted mechanisms of TGF-β and its determining factors, will help answer the fundamental question of how TGF-β dictates the trajectory of immune responses.12,14,16,17 This will establish the fundamental theoretical basis and research direction for designing next-generation precision intervention strategies that target microenvironment regulation and achieve graft-specific immune tolerance.

Biological Characteristics of T Cells and Rejection Mechanisms

T cells are the core effector cells of adaptive immunity and play a pivotal role in transplant rejection. They recognize allogeneic antigens via the T cell receptor (TCR), and under the mediation of co-stimulatory signals, they undergo activation, proliferation, and differentiation. Ultimately, the T cells migrate to the graft, where they mediate cytotoxic effects or recruit inflammatory cells, creating a chain reaction that leads to graft damage.18 Based on their functional phenotypes, T cells are primarily classified into two categories: CD4+ helper T cells and CD8+ cytotoxic T cells. Allogeneic reactive T cells have been identified as the core effector cells in acute cellular rejection.18 Recent research has primarily focused on precise interventions in T cell immune responses using the following approaches: In recent years, studies on precise interventions targeting T cell immune responses have focused on three main areas: First, enhancing the function of regulatory T cells (Tregs). Second, precisely screening T cell subsets prior to transplantation. Third, targeting interventions at tissue-resident memory T cells (TRMs).

In inflammatory diseases, particularly graft-versus-host disease (GVHD), a deficiency or dysfunction of regulatory T cells (Tregs) is a key pathological mechanism. John Koreth’s team conducted research on patients with steroid-refractory chronic GVHD and demonstrated that low-dose interleukin-2 (IL-2) therapy can induce Treg expansion to 20 times baseline levels within two weeks while maintaining long-term homeostasis. This expansion significantly suppresses the overactive immune system and has been clinically proven to effectively alleviate GVHD symptoms (P < 0.01).19 The key to transplant tolerance is the functional activation of antigen-specific regulatory T cells (Tregs) rather than their numerical expansion. Resting thymic Tregs (tTregs) transform into activated Tregs when exposed to effector cytokines. These activated Tregs enhance immune regulation through the high expression of inhibitory molecules (eg, CTLA-4/CD25) and chemokine receptors. Monitoring activated Treg subsets (CD45RO⁺/Foxp3⁺⁺) precisely enables a dynamic assessment of tolerance status. This assessment guides personalized immunosuppression regimens, reducing drug toxicity and improving long-term graft survival.20 FOXP3⁺ Treg cells maintain immune homeostasis by suppressing T cell activation and regulating antigen presentation. They also mediate tissue repair. Defects in their function can trigger autoimmune and kidney diseases. Transplant tolerance and targeted immunosuppression can be established through the in vivo expansion of natural Tregs using IL-2 or via adoptive transfer therapy, which involves the ex vivo expansion or conversion of antigen-specific Tconv cells.21 In the field of hematopoietic stem cell transplantation, a “precision trimming” strategy has been adopted. Since naïve T cells in the donor graft primarily drive chronic graft-versus-host disease (GVHD), while memory T cells mediate the graft-versus-leukemia (GVL) effect, researchers have developed techniques to eliminate naïve T cells while preserving memory T cells prior to transplantation. Compared to untreated groups, in which chronic GVHD incidence ranges from 30% to 70%, this technique reduced chronic GVHD incidence to 7% (P < 0.001), significantly alleviating symptoms. It also maintained the GVL effect, achieving a 77% three-year survival rate.22 Following lung transplantation, single-cell sequencing reveals the oligoclonal expansion of recipient-derived CD8⁺ T cells that differentiate into tissue-resident memory T cells (TRMs) and persist in cases of acute rejection. Although glucocorticoid therapy downregulates cytotoxic genes, it fails to eliminate TRMs in the airways. This clonal expansion originates from in situ activation rather than circulating infiltration, suggesting that TRM accumulation is a core feature of lung graft immune surveillance. Its long-term clinical implications are still unclear.23

Tissue-resident memory T cells (TRMs) at prior infection sites provide protection against reinfection at those sites. However, their role is unclear when homologous antigens persist in organ transplants, which is a critical issue in transplantation. Using a kidney transplant mouse model, we discovered that the recipient’s antigen-specific and polyclonal effector T cells differentiate into TRMs within the graft. These TRMs reside persistently in the new kidney, recognizing it as an “invader” and initiating and maintaining allogeneic graft rejection. These TRMs exhibit characteristics including non-recirculation, local proliferation, re-stimulation-induced IFN-γ production, non-exhaustion, and self-renewal capacity. In vivo depletion of TRMs attenuates rejection. The study also elucidates tissue-residency-associated transcriptional programs. Khodor I. Abou-Daya’s team reported that blocking TRM survival signals, targeting TRM-specific markers, and employing localized interventions instead of systemic immunosuppression are long-term therapeutic options for improving kidney transplant outcomes.24

T cells are at the core of adaptive immunity and play a crucial regulatory role in transplant rejection and chronic graft-versus-host disease (GVHD) (Figure 1).

Figure 1 Injuries, infections, drugs, or abnormalities in organs (heart, kidney, lung, skin) release damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs). Antigen-presenting cells (APCs) recognize these patterns, then present antigens via MHC I to naive CD8⁺ T cells (differentiating into cytotoxic T lymphocytes, CTLs) and via MHC II to naive CD4⁺ T cells (differentiating into effector T cells, Teffs). Cytokines (IL-1, IL-6, TNF-α, INF-γ, IL-12/23) modulate this process. Regulatory T cells (Tregs), affected by immunosuppressants, regulate Teffs. Besides, natural killer (NK) cells and CTLs target graft⁺ cells, and Tregs also interact with APCs to modulate immune responses.

Allogeneic reactive T cells are the primary effector cells in acute cellular rejection. These cells recognize donor antigens via the T cell receptor (TCR) and undergo activation and proliferation through co-stimulatory signals. Then, they migrate to the graft and exert cytotoxic effects or recruit inflammatory cells, causing tissue damage. Chronic GVHD arises from insufficient numbers or dysfunction of regulatory T cells (Tregs). Although tissue-resident memory T cells (TRMs) at prior infection sites provide local protection against reinfection, their role in organ transplantation is unclear. Studies in kidney transplant mouse models demonstrate that recipient effector T cells can differentiate into TRMs within the graft. These TRMs exhibit characteristics of non-recirculation, local proliferation, re-stimulation to produce interferon-γ, and an absence of exhaustion markers. These characteristics initiate and sustain graft rejection. Future research must address three critical issues:

1) In vivo tracing technology for TRM-specific targets.

2) Optimization of nanoparticle drug platform delivery efficiency in modulating the local immune microenvironment.

3) Establishing multi-target synergistic intervention strategies.18,19,22,24 The introduction of nanotechnology offers a new paradigm for overcoming the systemic toxicity limitations of traditional immunosuppressants and enhancing long-term graft survival.

Macrophage Functions and Regulation in Transplantation

As core effector cells of the innate immune system and key regulators of tissue homeostasis, macrophages occupy a pivotal position in the early decision-making process that governs immune rejection versus tolerance following organ transplantation. This section categorizes macrophage functions into two classical subtypes: pro-inflammatory M1 and anti-inflammatory/repair-promoting M2. Comparative analysis highlights the opposing functional roles of these subpopulations (Figure 2).

Figure 2 Schematic illustration of monocyte differentiation into M1 and M2 macrophages and their distinct functions. Monocytes, upon recognition of pathogen - associated molecular patterns (PAMPs) via Toll - like receptors (TLR - 2, TLR - 4, TLR - 5), differentiate into M1 macrophages under stimulation by cytokines such as IFN - γ, TNF - α, IL - 32, IL - 23, and LPS. M1 macrophages exert anti - pathogenic effects through releasing reactive oxygen species (ROS), nitric oxide (NO), lysosomal enzymes, and pro - inflammatory cytokines (MCP - 1, MIP - 1α, IL - 8, IL - 1β, IL - 6, TNF - α), and promote inflammation. In contrast, under IL - 4, IL - 13, and IL - 10 stimulation, monocytes differentiate into M2 macrophages, which secrete cytokines like TGF - β, IL - 10, PDGF, and FGF to inhibit inflammation and facilitate tissue repair and fibrosis.

The transplantation event triggers a sequential mechanism of macrophage polarization, beginning with the effects of ischemia-reperfusion injury (IRI). Lung transplantation studies indicate that following IRI in donor lungs, endogenous pulmonary monocytes rapidly activate and initiate a nonspecific inflammatory response.25 Takata et al discovered that IRI induces early M1-like polarization by upregulating CD86, an M1 co-stimulatory molecule, and TREM-1, a pro-inflammatory receptor. This confirms IRI as the core trigger of the rejection cascade.25 Subsequently, the secondary mechanism of molecular pathway regulation in polarization was explored. Using a liver transplantation rejection model, Cao et al revealed that downregulation of miR-449a expression resulted in the de-inhibition of its target gene, PLOD1. This activated the non-canonical NF-κB pathway and drove the transformation of macrophages toward the M1 phenotype.26 Additionally, comparative studies indicate that soluble CD83 (sCD83) in corneal transplantation effectively suppresses rejection by inducing a tolerogenic phenotype in macrophages, locally establishing an immunosuppressive microenvironment, and amplifying Treg cells.27 Knockout of the S100A9 gene significantly reduced the infiltration of hepatic monocytes and macrophages (p<0.05), as well as neutrophils, and downregulated proinflammatory factors. It also inhibited M1 polarization. In vitro studies confirmed that S100A9-TLR4 binding activates NF-κB, and that recombinant protein treatment directly induces the M1 phenotype in macrophages (p < 0.05). These findings establish the S100A9-TLR4/NF-κB-M1 pathway as a core mechanism of injury (IR) during fatty liver transplantation. Consequently, upstream pathways that regulate M1 macrophages often influence transplantation outcomes.28

The CD226/TIGIT-PVR signaling axis plays a key role in regulating macrophage polarization and determining transplant outcomes. TIGIT activation promotes the IL-10⁺/Arg1⁺ M2 phenotype by driving the ERK1/2-MSK1-CREB pathway, which suppresses proinflammatory factors and expands Treg cells, thereby establishing a tolerant microenvironment. However, CD226 antagonizes this effect by amplifying Th1/Th17-mediated rejection. Targeting this pathway synergistically suppresses in situ inflammation and adaptive immune responses, significantly prolonging graft survival.29 ADAR1 downregulates mature miR-21 by binding to its precursor (RIP validation), thereby releasing its inhibition on Foxo1, driving IL-10 secretion, and promoting macrophage M2 polarization. Overexpression of ADAR1 suppresses LPS-induced M1 conversion and significantly enhances graft survival in transplantation models (increased spleen macrophage M2 expression/reduced infiltration). This ADAR1-miR-21-Foxo1-IL-10 axis provides a novel strategy for targeting RNA editing to regulate transplant immunity.30 The immunoregulatory function of M2 macrophages does not exist in isolation; their interaction with regulatory T cells (Tregs) is a core mechanism for maintaining transplant tolerance. In allografts experiencing acute T cell-mediated rejection (TCMR), IL-34 expression is significantly downregulated. Overexpression of IL-34 can reduce the degree of rejection and directly promote M2 macrophage polarization, but it cannot directly induce naïve T cells to differentiate into Tregs—indicating that its regulation of Tregs depends on an indirect pathway mediated by anti-inflammatory factors secreted by M2. This process coordinately increases the ratio of Tregs to M2 in the recipient spleen and the graft, downregulates pro-inflammatory cytokines such as IFN-γ, IL-17, and TNF-α, and establishes an anti-inflammatory immune microenvironment.31 Overall, IL-34 plays a central hub role through M2 polarization, mediating Treg-M2 interactions to suppress the progression of TCMR and providing potential targets for clinical immunotherapy.

The available evidence suggests that the polarization state of macrophages (M1/M2) is an early factor that determines transplant outcomes. The regulatory mechanisms involved include IRI triggering, epigenetic regulation (such as microRNA), and soluble molecular cascades (like sCD83). Current prevailing views include the following: 1) M1 polarization is a common driving mechanism of rejection,26,32–34 and the core pathway of tolerance is M2 induction and Treg expansion.27 In the future, developing strategies to regulate specific polarization signaling networks and maintain M2 phenotype homeostasis will be essential for extending graft survival.

Oxidative Stress Mechanisms in Transplant IRI

Ischemia-reperfusion injury (IRI) is a critical clinical challenge in organ transplantation. The pathological cascade begins during the ischemic-hypoxic phase following organ procurement and leads to the collapse of cellular energy metabolism and ATP depletion. When blood flow is restored during reperfusion, dysfunction in the damaged mitochondrial electron transport chain triggers the production of an excessive amount of reactive oxygen species (ROS), which completely disrupts oxidative-antioxidative homeostasis.35,36 Excessive reactive oxygen species (ROS) exacerbate graft injury through a dual-damage mechanism. The pathological process of IRI can be divided into four interconnected stages. First is the initial “latency phase” during ischemia, which is characterized by metabolic disruption and redox imbalance. Next is an immediate post-reperfusion burst of ROS. This is followed by direct cellular damage from oxidative stress and intensified inflammatory responses. Ultimately, cell death pathways are activated, causing complete disruption of the endogenous antioxidant system. Throughout the process, these stages interact with other injury mechanisms.

In liver transplantation, IRI induces a collapse of the mitochondrial membrane potential, generating superoxide anions (O2−) and hydroxyl radicals (·OH). These radicals directly damage the phospholipid bilayer of hepatocyte membranes. This process activates Kupffer cells, which release proinflammatory factors such as TNF-α and IL-1β. It also induces ferroptosis through the Fenton reaction.37 In the early stages of ischemia-reperfusion injury, Kupffer cells (KCs), as resident macrophages of the liver, undergo a phenotypic switch, transforming from an anti-inflammatory M2 phenotype to a pro-inflammatory M1 phenotype.38 Current preclinical studies primarily use mitochondrial-targeted antioxidants, such as MitoQ, and iron chelators, such as deferoxamine, to block free radical chain reactions in this mechanism.39,40 In kidney transplantation, IRI leads to the detachment of the brush border of renal tubular epithelial cells, forming casts that cause obstruction. Meanwhile, the abnormal activation of the TLR4/MyD88/NF-κB signaling pathway promotes the release of inflammatory mediators, such as IL-6, COX-2, and iNOS. This activates the caspase-9/3-dependent apoptotic pathway, inducing acute tubular necrosis.41 Therapeutic strategies that target this mechanism focus on developing and applying TLR4 antagonists and caspase inhibitors.42 In renal ischemia/reperfusion (I/R) injury, AMPKα activity in renal tubular epithelial cells is dephosphorylated at the Thr172 site by ceramide-activated PP2A phosphatase. This process inhibits ULK1-mediated mitochondrial autophagy, resulting in the accumulation of dysfunctional mitochondria, impaired fatty acid oxidation, lipid deposition, and exacerbated apoptosis.43

Traditional systemic antioxidant therapies, such as N-acetylcysteine, fail to effectively block localized reactive oxygen species (ROS) surges in organs. This is due to their inability to penetrate cell membranes, low targeting efficiency, and short half-lives. Recent therapeutic research has focused on three areas: mitochondrial-targeted antioxidants, precise modulation of inflammatory pathways, and cross-regulation of programmed cell death. Meanwhile, advancements in cryopreservation and perfusion machines aim to reduce ischemic time. Antioxidants (such as N-acetylcysteine) and targeted drugs have also emerged as key areas of research.

Immunosuppressive Therapies of the Traditional Kind and Intervention Strategies That are EmergingCurrent Status of Traditional Immunosuppressants

Traditional immunosuppressants inhibit rejection by interfering with the activation of immune cells and inflammatory pathways within the immune microenvironment. This class of drugs includes glucocorticoids; calcineurin inhibitors, such as tacrolimus; antiproliferative agents, such as mycophenolate mofetil; mTOR inhibitors, such as sirolimus; biologics, such as anti-CD25 antibodies; and JAK inhibitors.44 The immune microenvironment is a dynamic network of local immune cells, stromal cells, the extracellular matrix, and soluble factors within the transplanted organ that directly determine the fate of graft tolerance or rejection.10–12 These drugs have systemic immunosuppressive effects, which they achieve by blocking T-cell activation, cytokine signaling, and lymphocyte proliferation pathways. Table 1 summarizes the mechanisms of action, clinical applications, and limitations of traditional immunosuppressants. The significant side effects of traditional treatments, particularly long-term systemic immunosuppression, have limited their further clinical application. Studies have confirmed that corticosteroids, mTOR inhibitors, and calcineurin inhibitors significantly increase the risk of skin cancer, raising safety concerns.3 Analysis of bronchoalveolar lavage fluid from lung transplant rejection patients using single-cell RNA-seq and TCR sequencing revealed oligoclonal expansion of recipient-derived cytotoxic CD8⁺ T cells during rejection. Although high-dose glucocorticoid therapy reduced the expression of these cytotoxic mediators, the clonal cells were not eliminated. Instead, the cells transformed into tissue-resident memory T cells (TRMs) within the allograft and persisted in the peribronchial space.23 Calcineurin inhibitors (CNIs), such as cyclosporine A (CsA) and tacrolimus (Tac), are key immunosuppressive drugs used to prevent rejection after solid organ transplantation. However, they can impair transplanted kidneys due to their significant nephrotoxic side effects.45–47 Mycophenolate mofetil, a highly effective prescription medication, is an antiproliferative immunosuppressant that frequently causes gastrointestinal side effects of unknown origin.48–50 mTOR inhibitors (sirolimus and everolimus) may cause drug-induced pneumonia (DIP), which is a toxicity related to the class of drugs. Sirolimus and everolimus are also representative drugs among them.51,52 The core mechanism involves abnormal immune activation due to inhibition of the mTOR pathway, which leads to lymphocytic alveolitis and interstitial infiltration. Computed tomography (CT) features include diffuse exudative/ground-glass opacities. Clinical data indicate: Incidence: - Radinol: 11% (13/122)- Sirolimus: 5% (4/77).53 In addition, biologics (such as anti-CD25 antibodies) and JAK inhibitors (such as tofacitinib) play an important role in combating immune rejection during organ transplantation, but their side effects still need to be taken seriously. This is introduced in Table 1.54,55 Hricik, DEet al, conducted a randomized, double-blind, controlled trial using infliximab in kidney transplantation research. Although infliximab is commonly used to treat autoimmune diseases, the drug was found to double the risk of infection in kidney transplant studies due to its ability to weaken the immune system’s defenses. This finding underscores the need for caution when using drugs for different conditions.56 A Phase II study examined a new combination therapy for preventing bone marrow transplant rejection. The new regimen, which includes cyclophosphamide, tacrolimus, and mycophenolate mofetil, demonstrated significantly greater efficacy in preventing graft-versus-host disease (GVHD) than the traditional baseline regimen of tacrolimus and methotrexate. During the critical first year after transplant, patients receiving the new regimen experienced fewer and milder episodes of GVHD and were able to stop taking anti-rejection medications earlier. However, there was no significant change in overall survival or cancer recurrence rates.57

Table 1 Mechanism of Action, Clinical Applications, and Limitations of Traditional Immunosuppressants

Traditional immunosuppressants are fundamental to graft survival because they broadly inhibit key pathways in the immune microenvironment. However, their nonspecific mechanisms of action result in severe side effects and therapeutic limitations. These limitations underscore the urgent need for novel intervention strategies with high targeting specificity and controllable side effects. Is there a better option?

Emerging Intervention Strategies

Emerging strategies that focus on reshaping the transplant immune microenvironment and targeting key immune regulatory nodes aim to overcome the limitations of traditional immunosuppression and achieve more precise, safe, and durable transplant tolerance. These strategies include microbiome modulation, novel biologic agents, and nanotechnology interventions, which are innovative directions in current transplant immunology research.

The regulation of the microbiome before and after transplantation is an area that requires further exploration. The microbiome profoundly influences organ transplant recipients. The dynamically evolving microbial community continuously engages in a “dialogue” with the host’s immune system, directly determining whether the graft and host can achieve “peaceful coexistence”.58 After a lung transplant, the lung microbiome shows signs of dysbiosis, including higher microbial abundance and altered composition.59 Lei and his team discovered that normal mice undergoing skin and heart transplants exhibited the most intense and rapid rejection responses to the new organs. Meanwhile, germ-free mice and those injected with antibiotics showed slower and weaker responses. Therefore, reducing the microbial community temporarily prior to transplantation can increase the chances of success.60 Emerging strategies, such as microbiome interventions, offer novel approaches to transplant tolerance by reshaping the environment of the immune dialogue. However, challenges remain regarding their clinical application, including the precision of microbial targeting and the development of personalized regulatory protocols.

Biologics are a class of drugs produced through biotechnology that specifically target key molecules or cells within the immune system. Below are the primary biologics and their mechanisms of action. In kidney transplantation, anti-T cell antibodies (such as rabbit anti-thymocyte globulin, rATG) are widely used to treat acute rejection resistant to glucocorticoids. Studies indicate that early administration of rATG significantly improves long-term graft function, with efficacy unaffected by Banff classification or rejection severity. Additionally, anti-T cell antibodies reduce acute rejection incidence by decreasing T cell receptor (TCR) turnover.61,62 Anti-B cell antibodies, such as rituximab, reduce antibody-mediated rejection by eliminating B cells. Although rituximab has demonstrated efficacy in treating chronic antibody-mediated rejection, its long-term effects require further investigation.63 Patients who receive solid organ transplants usually need long-term immunosuppressive therapy to prevent rejection of the transplanted organ. However, this immunosuppressed state increases their risk of developing cancer. The use of ICIs may disrupt this immune balance, potentially triggering rejection of the transplanted organ. Studies indicate that ICIs are associated with graft rejection rates as high as 40% in transplant recipients.64 Cytokines, such as IL-10, play a crucial role in immune tolerance. Genetically engineered anti-CD25/IL-10/CXCR3 fusion proteins can stimulate the production of regulatory T cells (Tregs) and inhibit the differentiation of Th1 and Th17 cells. This mitigates rejection reactions.65 Although immune checkpoint inhibitors have revolutionized cancer treatment, they carry a significant risk of transplant rejection in recipients of solid organ transplants.

Cell therapy involves extracting living cells from the body, modifying their functions, and/or expanding them in vitro. The cells are then reinfused into the patient to treat specific diseases. This technique has been used to treat hematological malignancies and immunological disorders. FOXP3+ regulatory T cells (Tregs) play a crucial role in maintaining immune tolerance and tissue homeostasis. These cells control immune responses by suppressing T cell activation and regulating antigen-presenting cells. They also promote tissue repair. Deficiencies in Treg function or numbers can predispose individuals to autoimmune and inflammatory diseases, including kidney disorders. Therapeutic approaches include expanding endogenous Tregs (eg, with IL-2) or using adoptive cell therapy involving ex vivo modification and expansion, such as generating antigen-specific or CAR-Treg cells, to treat immune disorders and induce transplant tolerance.21 Chimeric antigen receptor (CAR) technology has revolutionized adoptive cell therapy (ACT) by reprogramming cellular targeting and significantly enhancing precision while reducing off-target effects observed in tumor CAR-T therapies. CARs applied to regulatory T cells (Tregs) are called CAR-Tregs. The goal is to harness their immunosuppressive functions to treat autoimmune diseases, graft-versus-host disease (GVHD), and organ transplant rejection. CAR-Tregs show significant potential in inducing transplant tolerance. However, compared to CAR-T, research on CAR-Tregs is in its infancy, and their design and application are shrouded in uncertainty.66

Against this backdrop, nanotechnology emerges as a pivotal force in overcoming existing limitations thanks to its precise delivery, intelligent response, and ability to regulate multiple targets. We will further explore its innovative applications in suppressing immune rejection.

Therapeutic Approaches Corresponding to Nanoscale and Molecular MechanismsNano-Drug Delivery for Immune Microenvironment Modulation

Immune rejection remains the primary obstacle to successful allogeneic organ transplantation. Traditional strategies for modulating the immune microenvironment have significant limitations. One limitation is poor local targeting. Systemic administration has difficulty acting precisely on the transplant site, resulting in inadequate efficacy and pronounced systemic side effects. (2) Limited regulatory approaches: There is difficulty addressing the complex, networked interactions formed by multiple cell types (eg, T cells, dendritic cells, and macrophages) and multifactorial elements (eg, pro-inflammatory and anti-inflammatory cytokines) within the immune microenvironment. (3) Short duration of action: Exogenous immunomodulators are rapidly cleared or metabolized by the body, making it difficult to sustain long-term immune homeostasis. Nano-drug delivery platforms offer revolutionary solutions that overcome these limitations by providing unique advantages, such as localized high-concentration delivery, multifunctional target modification, and intelligent responsiveness.

Of all allogeneic organs, the liver exhibits the strongest relative immune tolerance. Researchers explored the mechanisms underlying this tolerance by comparing allogeneic transplantation mouse models of the liver, heart, and kidney. Transcriptomic analysis revealed that the liver graft microenvironment contains unique immune regulatory cell subsets. For example, CD11b+ dendritic cells within liver grafts were shown to suppress the activation of cytotoxic T cells by secreting anti-inflammatory cytokines, such as IL-10. This promotes the development of a locally tolerant microenvironment.67 To address the increased tendency for rejection in non-liver organ transplants, researchers have focused on using nanotechnology to create protective microenvironments. One innovative strategy is to engineer mesenchymal stem cell membrane-derived vesicles (MMVs) to specifically increase the expression of immune checkpoint molecules, such as Fas ligand (FasL) and programmed death-ligand 1 (PD-L1), on their surfaces. The functionalized MMVs are then cross-linked and immobilized within a hydrogel network to form MMV-gel complexes. This platform enables the MMVs to be retained at the transplantation site for a prolonged period. The sustained presentation of FasL and PD-L1 locally induces apoptosis in infiltrating allogeneic reactive T effector cells (Teff) and promotes the generation of regulatory T cells (Treg), effectively constructing an immune-protective microenvironment. Consequently, immune rejection is significantly suppressed and the survival of allogeneic transplants is protected.68 (Figure 3). The success of islet transplantation depends heavily on the precise regulation of the local immune microenvironment. Following implantation, islet grafts undergo an intense immediate blood-mediated inflammatory response (IBMIR) and are subsequently attacked by immune cells (eg, macrophages and T cells) and pro-inflammatory cytokines (eg, IL-1β, TNF-α, and IFN-γ). The release of these factors triggers a cascade amplification effect that further activates immune cells, ultimately leading to β-cell damage in the pancreas and transplant failure. Therefore, precisely intervening in the immune microenvironment surrounding the islet transplant to suppress harmful inflammatory cascades and immune activation is a core strategy for improving survival rates.69

Figure 3 (a). Construction of engineered mesenchymal stem cell membrane-derived vesicles (MMVs): Gene editing up-regulates Fas ligand (FasL) and programmed death ligand 1 (PD-L1) on MMVs’ surface. MMVs are then modified with thiolated PEGylated phosphatidylethanolamine (DSPE-PEG-SH), enabling crosslinking with acrylated hyaluronic acid (a-HA) via thiol-acrylate Michael addition reaction. (b). MMV-Gel-mediated graft immune protection mechanism: Grafts encapsulated in MMV-Gel form a local immuno-protective microenvironment after transplantation. When alloreactive T cells infiltrate, MMVs’ FasL binds to T cells’ Fas receptor to induce apoptosis, and PD-L1 binds to T cells’ PD-1 receptor to promote T cell exhaustion. These dual effects inhibit immune rejection. (Reproduced from Wang et al, Nat Commun, 2024, 15:5176 with permission).

In summary, nanomaterials can overcome the limitations of traditional methods through biomimetic design and intelligent response strategies. These nanomaterials enable the specific accumulation of drugs or modulators at the lesion site, achieve multi-target, synergistic regulation of immune cell and factor networks, and maintain long-lasting immune balance. These breakthroughs demonstrate that nanocarrier platforms have emerged as a pivotal strategy for precisely regulating the transplant immune microenvironment and have immense potential for clinical translation. Future research should focus on the long-term safety and efficacy of these platforms in large animal models and complex clinical settings. It should also focus on optimizing response strategies to accommodate individual variations and advancing the development of multimodal, synergistic therapeutic platforms.

Nanoparticle-Mediated T Cell Targeting for Rejection Suppression

T cells play a key role in adaptive immune responses, and precisely regulating their activation and function is essential for maintaining immune homeostasis. T cell activation depends on the specific recognition of antigen peptides bound to major histocompatibility complex (MHC) molecules by the T cell receptor (TCR). It also depends on secondary signaling via co-stimulatory molecules, such as CD28/B7, and the subsequent intracellular signaling cascades triggered by these signals. Meanwhile, immune checkpoint molecules, such as programmed cell death protein 1 (PD-1) and cytotoxic T lymphocyte-associated protein 4 (CTLA-4), suppress excessive T cell activation through negative signaling regulation. This ensures that immune responses are appropriate.

Regulatory T cells (Tregs) are key inducers of immune tolerance to transplants. Research indicates that Tregs effectively suppress allogeneic immune rejection and promote tissue repair and regeneration by inducing M2 macrophage polarization through the secretion of specific cytokines, such as TGF-β and IL-10.70,71 A traditional approach to preventing transplant rejection is to extend graft survival by suppressing or eliminating immune cells non-specifically. However, nanocarrier-mediated targeted immunosuppression strategies demonstrate superior potential. For example, poly(lactic-co-glycolic acid) (PLGA)-based nanoparticle delivery systems enable the selective depletion of specific immune cell subsets, which significantly prolongs graft survival in allogeneic skin transplant models.72 On the other hand, traditional approaches based on therapeutic antibodies to eliminate B or plasma cells often result in poor patient outcomes for antibody-mediated rejection (ABMR). Targeting specific T cell subsets is an effective strategy for suppressing rejection. Follicular helper T cells (Tfh) play a crucial role in promoting antibody production and chronic rejection and are a key intervention target. Functionalized mesoporous nanoparticles can deliver the immunosuppressant tacrolimus (Tac) efficiently to helper T (CD4+) cells, which significantly inhibits Tfh cell differentiation following allogeneic antigen exposure. This strategy reliably prevented rejection in a mouse kidney transplantation model.73 Exosome-like nanovesicles (NVs) have been found to exhibit dysregulation of splenic immune checkpoint molecules, such as PD-L1/PD-1 and CTLA-4/CD80, in cardiac and skin transplantation models. Based on these findings, researchers developed bioengineered PD-L1/CTLA-4 dual-targeted NVs that bind specifically to PD-1 on T cell surfaces and to CD80 on dendritic cell surfaces. Studies have demonstrated that these dual-targeted NVs effectively suppress CD8+ T cell density and cytokine secretion, enrich Treg cell populations, and significantly prolong transplanted organ survival. This is due to the synergistic enhancement of two critical immunosuppressive pathways: PD-L1/PD-1 and CTLA-4/CD80. These pathways have been shown to markedly reduce T cell activation and proliferation in both in vitro and in vivo studies.74

The nano-platform-induced and amplified regulatory T cells (Tregs) represent a core strategy for establishing tolerance. In vivo tracking studies reveal the role of innate immune cells in the homing of polyclonal human Tregs.75 Preclinical studies have confirmed that the adoptive transfer of regulatory T cells (Tregs) effectively prevents graft rejection, and early clinical trials have demonstrated its safety. Consequently, using nanotechnology to precisely modulate Treg function and numbers has become a promising area of research. One such approach is the use of tolerogenic artificial antigen-presenting cells (aAPCs or TolAPCs) based on poly(lactic-co-glycolic acid)/poly(β-amino ester) (PLGA/PBAE) loaded with transforming growth factor-β (TGF-β). These cells effectively promote the in vivo phenotypic polarization of regulatory T cells and enhance their immunosuppressive function76 (Figure 4). The curcumin-based nanocarrier platform significantly enhanced FoxP3 gene expression and Treg activity in patients with ankylosing spondylitis (AS), suggesting its potential application in the field of transplant immunology.77 PLGA nanoparticles loaded with the house dust mite allergen Der p 1 and the insulin-like growth factor 1 (IGF-1) (Der p 1/IGF-1 NPs) have been shown to effectively promote Treg cell differentiation in vitro, offering a novel approach to restoring immune tolerance function.78 Furthermore, nanoparticles (NPs) loaded with IL-2 and TGF-β and targeting T cells successfully induced the generation of polyclonal Treg cells, protecting mouse models from graft-versus-host disease (GvHD). Mechanistic studies revealed that reduced allograft reactivity coincided with a several-fold expansion of CD4+ and CD8+ Treg cells in NP-treated mice, alongside the acquisition of a tolerant phenotype by recipient dendritic cells.79

Figure 4 Schematic illustration of the mechanism for regulating the function of regulatory T cells (Treg) based on nanotechnology. Preclinical models have demonstrated that adoptive transfer of regulatory T cells (Treg) can effectively prevent graft rejection, and its safety has been supported in preliminary clinical trials. Thus, using nanotechnology to precisely regulate the function or quantity of Treg has become a highly promising direction. As shown, poly(lactic - co - glycolic acid)/poly(β - amino ester) (PLGA/PBAE) - based artificial antigen - presenting cells (aAPCs, TolAPC) loaded with transforming growth factor - β (TGF - β) can effectively promote the phenotypic polarization of Treg cells in vivo and enhance immunosuppressive function through the synergistic effect of surface signals.1 Adapted from the article “Biomimetic tolerogenic artificial antigen presenting cells for regulatory T cell induction” by Rhodes K R, Meyer R A, Wang J, Tzeng S Y, Green J J, published in Acta Biomaterialia (Volume 112, August 2020, Pages 136–148), used with permission.

In summary, the targeted modulation of T cell function via nanotechnology, particularly through the depletion of pathogenic subsets, the induction or expansion of immunosuppressive regulatory T (Treg) cells, and the enhancement of immune checkpoint inhibition pathways to suppress transplant rejection, has emerged as a key research focus in transplant immunology. Compared to traditional non-specific immunosuppression, these strategies demonstrate higher selectivity and potentially lower toxicity. In conclusion, multi-targeted, combined strategies represent the future direction. Nanotechnology-based research on immune tolerance induction is rapidly emerging. One frontier of this research is the development of “tolerogenic vaccines.” Rather than enhancing immune responses, the objective of these vaccines is to use engineered nanoparticle platforms to deliver specific antigens or immunomodulatory molecules to actively induce antigen-specific immune tolerance.5,80,81 For instance, the modification of therapeutic antigen peptides with polyethylene glycol significantly reduces the inflammatory response induced by peptide-based tolerance vaccines. This provides a theoretical foundation for the safe and effective application of these vaccines in the treatment of autoimmune diseases, allergies, and transplant rejection.82 In summary, strategies for the targeted depletion of specific pathogenic T cell subsets and the effective induction and expansion of Tregs using nano-platforms are relatively mature and reliable. Dual/multi-target immune checkpoint synergistic regulation strategies show tremendous potential. Engineered “tolerogenic vaccines” represent a new paradigm for inducing antigen-specific tolerance. Future research presents both challenges and opportunities and requires focused efforts to enhance the potential for clinical translation. Key priorities include improving biomaterial safety, optimizing carrier targeting, and achieving scalable production. Further exploration is needed into the mechanisms of nanoplatform-mediated precise T cell reprogramming and expanding synergistic therapeutic approaches with other tolerance-inducing pathways, such as non-T cell pathways, which are discussed below.

Nanoscale Effects on Macrophage Function

As core effector cells of the innate immune system, macrophages serve as key regulators that mediate early post-transplant inflammation and immune rejection.83 As research has progressed, the molecular mechanisms by which graft-infiltrating myeloid cells mediate rejection have gradually become clearer. Upon exposure to allogeneic grafts, recipient monocytes rapidly differentiate into functionally heterogeneous macrophage subpopulations, where polarization toward Ly6C⁺ regulatory macrophages (Mregs) has been shown to significantly attenuate rejection by suppressing effector T cell activation and promoting regulatory T cell (Treg) expansion. Conversely, polarization toward M2 macrophages has been demonstrated to markedly inhibit rejection.84–87 (Figure 5).

Figure 5 Schematic of myeloid cell regulatory mechanisms in graft immunity. Combining mTORi-HDL therapy with CD40-TRAF6-specific nanobiota (TRAF6i-HDL) revealed that short-term mTOR-specific high-density lipoprotein (HDL) nanobiota therapy (mTORi-HDL) can prevent aerobic glycolysis of macrophages and epigenetic modifications resulting in inflammatory cytokine production. The resulting regulatory macrophages can prevent allogeneic reactive CD8+ T cell-mediated immune responses and promote the expansion of tolerant CD4+ regulatory T cells (Tregs). Reproduced from (Braza et al, 2018, Immunity 49: 819–828) with permission from Immunity.

The introduction of nanomaterials offers novel strategies for precisely regulating macrophage polarization. Current research achieves immune regulation through three major pathways. The first approach is the polarization-suppressive pathway. In a liver transplantation-associated ischemia-reperfusion injury (IRI) model, platinum nanozymes (Pt-iNOS@ZIF) exert the following pleiotropic effects: they not only efficiently scavenge reactive oxygen species (ROS) generated by neutrophil infiltration via peroxidase-mimetic activity, but also sustainably release nitric oxide (NO) at physiologically relevant concentrations. Ultimately, while downregulating the expression of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), Pt-iNOS@ZIF simultaneously blocks the signal transducer and activator of transcription 1/interferon regulatory factor 5 (STAT1/IRF5) signaling axis. This strategy enables precise modulation of the local microenvironment, thereby significantly reducing the incidence of acute immune rejection.88 The second strategy entails M1 phenotype reprogramming. He et al engineered RBCM@CeO2/TAK-242 nanoparticles with a biomimetic delivery system, which circumvents clearance by the mononuclear phagocyte system (MPS) via specific recognition of the CD47 molecule on the red blood cell (RBC) membrane. Specifically, TAK-242 blocks toll-like receptor 4 (TLR4) receptors, thereby potently inhibiting TLR4 dimerization. In parallel, CeO2 nanozymes scavenge reactive oxygen species (ROS) in the nuclear factor kappa-B (NF-κB) signaling pathway, which in turn prevents inhibitor of nuclear factor kappa-B alpha (IκBα) degradation and p65 nuclear translocation. These synergistic actions markedly reduce the positivity rate of M1 phenotypic markers (CD86⁺/HLA-DR⁺), while concomitantly boosting the secretion of the anti-inflammatory cytokine interleukin-10 (IL-10). This nanoplatform effectively overcomes the limitations of free TAK-242, such as a short half-life and off-target toxicity.89 The third approach focuses on M2 polarization induction. Graphene oxide nanomaterials exert dual physical and biochemical regulatory effects: structurally, they inhibit matrix metalloproteinase (MMP) activity, mimic the natural mechanical microenvironment of liver tissue, enhance elastic modulus, and improve scaffold stability; biochemically, they activate the integrin β1-FAK axis and trigger the PI3K/Akt pathway, which drives signal transducer and activator of transcription 6 (STAT6) phosphorylation. This phosphorylation induces the expression of M2 phenotypic markers (Arg-1⁺/CD206⁺) and promotes M2 polarization, allowing interleukin-10 (IL-10)-loaded engineered liver grafts to effectively suppress inflammation and facilitate tissue repair.90

Notably, this polarization-based regulatory strategy is equally effective in autoimmune diseases. The LDH-CeO2 nanocatalytic platform efficiently transforms M1 to M2 in rheumatoid arthritis models through acid neutralization and metal ion bioactivity, providing a cross-disease-validated theoretical foundation for extending autoimmune disease regulation approaches to transplantation.91 Current research confirms the significant potential of nanoparticle drug delivery systems in reducing transplant rejection, preserving graft function, and promoting tissue regeneration through the polarization of macrophages. However, challenges remain in achieving spatiotemporal precision in polarization regulation, assessing the long-term biosafety of nanoparticle carriers, and validating their effectiveness across organ transplantation systems. Future research should focus on developing organ-specific delivery systems, identifying new targets such as epigenetic regulation, and translating laboratory findings into clinical applications.

Nanoparticles: Antioxidation, IRI Mitigation and Tissue Repair

During organ transplantation, IRI is a major unavoidable challenge. Its core mechanism is destructive oxidative stress. Ischemia during organ procurement leads to hypoxia and the collapse of energy metabolism. When blood flow is restored during reperfusion, impaired mitochondrial function triggers the explosive production of reactive oxygen species (ROS), which disrupts the balance between oxidation and antioxidants. Common approaches to counteract this phenomenon include antioxidant therapy for donors and recipients, antioxidant treatment of the graft, and enhancing ischemic tolerance to protect the transplant process.92 Traditional antioxidants, such as edaravone, have short half-lives and difficulty penetrating the blood-brain barrier. However, nanotechnology offers three actions: scavenging free radicals, regulating endogenous antioxidant enzymes, and suppressing inflammation. The advantages of nanotechnology—precision delivery, smart responsiveness, efficient drug loading, and multifunctional integration—provide innovative solutions to these challenges through two primary mechanisms.

Nanoparticle Regulation of O