Abbreviations
AKT: protein kinase B
ALDH1: aldehyde dehydrogenase 1
AML: acute myeloid leukemia
AMP: adenosine monophosphate
AMPK: AMP-activated protein kinase
ATP: adenosine-triphosphate
Bcl-2: B-cell lymphoma protein/gene
BCR: B-cell receptor
BCRP: breast cancer resistance pump
CAF: cancer-associated fibroblasts
CML: chronic myeloid leukemia
CNI: calcineurin inhibitor
COX: cyclo-oxygenase
CRC: colo-rectal cancer
CSC: cancer stem cell
EBV: Epstein-Barr virus
ECD: extended criteria donors
ECM: extracellular matrix
EMT: epithelia-mesenchymal transition
EVR: everolimus
HCC: hepatocellular carcinoma
iPAL2: Calcium-dependent phospholipase A2
IRI: ischemia reperfusion injury
KT: kidney transplantation
LT: liver transplantation
MDR: P-glycoprotein
MLS: mitotic life span
MN: multinuclear cells
mTOR: mammalian target of rapamycin
mTORi: mammalian target of rapamycin inhibitors
MRP: multidrug resistance protein
MSC: mesenchymal cell
NAD+: nicotinamide dinucleotide
NF-kβ: nuclear factor of kappa chains of immunoglobulin
NMC: neiosis mother cell
PD-L1: programmed death-1 ligand-1
PG: polyploid giant cells
PI3K: phosphoinositide-3 kinase
P110β, p110 isoform of PI3K
ROS: reactive oxygen species
SHH: sonic hedgehog
SOT: solid organ transplant
SRL: sirolimus
TAM: tumor-associated macrophages
TGF-β, transforming growth factor-beta
TIC: tumor initiating cell
TIL: tumor infiltrating lymphocytes
TME: tumor microenvironment
ULK1: Unc-51-like autophagy activating kinase 1
VEGF: vascular endothelial growth factor
INTRODUCTION
Managing malignancies is essential for successful outcomes in solid organ transplantation (SOT) 1-3. Many solid organ transplants are performed due to malignancies, and a significant number of patients face the risk of developing recurrent or new tumors afterward 4. Along with the expanding indications for oncology patients, which has led to the emergence of transplant oncology 5, the shortage of grafts has encouraged the use of donors with a history of malignancies or selected low-risk tumors at the time of organ procurement 6. All these factors together may increase the risk of post-transplant malignancy development and donor-to-recipient tumor transmission, emphasizing the need to refine immunosuppressive strategies to balance protection against rejection and tumor growth 7,8. Additionally, transplantation for malignant disease often forms part of a multimodal treatment plan that includes surgery, chemotherapy, immunotherapy, and interventional radiology 9. While these strategies help increase eligibility for transplantation in individual patients, they also stimulate, manipulate, and prime both innate and adaptive immune systems 10,11.
Understanding the mechanisms that promote post-transplant tumor recurrence and donor-to-recipient tumor transmission is therefore essential for transplant oncology patients, as it may help improve pre- and post-transplant management of this patient group. Recent advances in immuno-oncology have highlighted the roles of both innate and adaptive immune systems in the initiation, progression, and spread of cancer 12. Although tumor growth has long been considered solely a result of immune escape, recent findings emphasize the active role of cancer cells in shaping the tumor microenvironment (TME), engaging immune cells to modify it, and coordinating their spread to distant organs 13. Beyond the bidirectional communication between tumor and immune cells, mechanisms such as senescence and inflammation – like neosis and phoenix rising – contribute to tumor self-renewal and growth, playing a role in post-inflammatory and post-surgical cancer recurrence. In this paper, we provide a narrative review of the mechanisms that lead to post-transplant cancer recurrence of malignancies present before transplantation, the development of de novo cancer, and the transmission of tumors from donors to recipients. This information aims to raise awareness among transplant clinicians and support the development of effective management strategies in clinical practice.
CANCER RECURRENCE AND DONOR-TO-RECIPIENT TUMOR TRANSMISSION
In transplantation settings, cancer recurrence refers to the return of cancer in a transplant recipient. This can happen with 1) malignancies that led to the transplant, such as hepatocellular carcinoma (HCC) in liver transplant (LT) patients, or 2) tumors that existed and were treated before the transplant, like prostate cancer. In both cases, tumor cells avoid the effects of pre-transplant treatment, transplant surgery, or both. The timing of cancer relapse can vary from weeks and months to several years after transplantation or the initial tumor diagnosis, with post-transplant immunosuppression playing a central role.
Conversely, donor-to-recipient (or donor-related) tumor transmission involves transferring cancer cells from donors to recipients through transplantation. Depending on the donor and recipient types, this transfer may be expected for extended criteria donors (ECDs) with previous or current tumors, and whose organs are allocated to specific patient populations needing transplants. It can also be discovered unexpectedly during the post-transplant period due to latent lesions in the donor organ. Like cancer recurrence, tumor transmission is believed to be worsened by post-transplant immunosuppression in the graft recipient. Both cancer recurrence and donor-related tumor transmission can be either site-specific or nonspecific, depending on tumor biology and its original location. For example, HCC may recur in the recipient’s liver (site-specific) or in the lungs (site nonspecific).
MECHANISMS OF CANCER RECURRENCE
Cancer progression, recurrence, and metastasis are complex biological processes involving the proliferation of cancer stem cells (CSCs), neosis (an alternative form of cell division that occurs in response to stress, DNA damage, and senescence), and the “phoenix rising” metabolic pathway.
CANCER STEM CELLS
Characteristics
Not all tumor cells, whether circulating or resident, can promote tumor growth 14. A significant body of evidence supports the idea that cancer stem cells (CSCs) play a vital role in tumor recurrence (Tab. I) 14,15. In 1937, Furth et al. first measured the number of malignant cells capable of contributing to the maintenance and survival of blood tumors 16. They demonstrated that not all cells within a malignant neoplasm can form tumors and that this ability is limited to CSCs 16. During the 1960s and 1970s, the concept of CSCs gained increasing support 17-19. Recent studies have confirmed that CSCs, also known as tumor-initiating cells (TICs), are responsible for the early development, progression, recurrence, and drug resistance of tumors 20. Substantial evidence suggests that CSCs may originate from normal stem cells, cancer progenitor cells, or differentiated cells through the accumulation of genetic mutations and subsequent genomic instability 21.
All CSCs share two main traits: self-proliferation (or self-renewal) and the ability to switch to a slow-proliferative or quiescent state in response to changes in the tumor microenvironment (TME) phenotype. Like normal stem cells, CSCs can self-proliferate, but they differ significantly in that they exhibit uncontrolled self-proliferative activity 14. To date, CSCs have been identified in various cancers, including malignancies of the blood, breast, brain, ovary, kidney, and others 22,23. Inactivation is not common in cells; however, some tumor cells adopt an inactive state. For example, inactive leukemic stem cells have been observed in a mouse model of chronic myeloid leukemia (CML) 14. Additionally, researchers have shown in vivo that a large portion of CSCs in acute myeloid leukemia (AML) and colorectal cancer (CRC) remain inactive for extended periods, reducing therapeutic effectiveness 14. Therefore, reactivation of inactive CSCs may lead to tumor recurrence even decades after the disease has been fully treated 24,25.
Markers
Various markers differentiate CSCs from other tumor cells and normal stem cells. Besides transcription factors, CSCs upregulate specific surface markers that can be used to distinguish them from other cancer cells. Some markers are shared by multiple cell types (i.e., inclusive markers), including other stem cells (i.e., stemness markers), differentiated cells, and tissue-specific cancer cells. Others are more specific (i.e., exclusive markers) (Tab. I) 14. CSCs are primarily characterized by the expression of surface markers like CD133 (prominin-1), CD44 (a cell-adhesion molecule), and intracellular markers such as ALDH1 (aldehyde dehydrogenase-1 enzyme) 26,27. However, the expression of CSC-specific markers varies widely depending on the cancer type. It is important to note that not all CSC-specific markers have been fully identified, and not all CSCs express all these markers. Additionally, other cell types can also produce some of these markers. Therefore, while these markers can help identify cell populations enriched in CSCs, they cannot definitively distinguish CSCs from other cell types 28,29.
Treatment resistance
CSCs utilize various mechanisms to develop resistance to treatment. One key factor is the release of anti-cancer drugs from the cell via plasma membrane ATP-dependent pumps such as breast cancer resistance protein (BCRP/ABCG2), multidrug resistance-associated protein 1 (MRP1/ACC1), and P-glycoprotein 1 (MDR1/ABCB1) (Tab. I) 30. Additionally, increased levels of aldehyde dehydrogenase-1 (ALDH1) contribute to CSC drug resistance. This cytosolic enzyme oxidizes aldehyde compounds, promoting resistance to cyclophosphamide 14. Moreover, CSCs may become drug-resistant by producing anti-apoptotic molecules, such as Bcl-2, which support the surrounding TME with essential substances and factors for long-term survival and self-proliferation 14. TME is a critical factor in the resistance of most B-cell-related malignancies to treatment 24,31. CSCs can develop resistance to genotoxic therapies, such as ionizing radiation, through various mechanisms, including increased activity of DNA damage checkpoints and enhanced DNA repair capacity 29. The number of CD133+ CSCs has been shown to increase in established glioma cell lines and xenograft tumor-bearing mice following high-dose radiotherapy, leading to the development of more invasive tumors 14. However, the reasons why radiation stimulates the division of CD133+ cells remain unclear 32. Furthermore, activation of mesenchymal stem cell (MSC) transcription factors induces epithelial-mesenchymal transition (EMT) in CSCs through various signaling pathways, such as the sonic hedgehog (SHH) cascade. The induction of EMT results in drug resistance, increased invasiveness, improved tumor growth, and ultimately, disease relapse 33.
Recent studies have emphasized the critical role of autophagy in the survival and drug resistance of CSCs 34. Autophagy – a cellular process where the cell degrades and removes damaged or unnecessary components through specialized lysosomes (autophagolysosomes), recycling them for energy or building blocks – is a conserved mechanism that helps cells withstand stress conditions like starvation, hypoxia, and exposure to chemotherapy or radiation. It plays a dual role in tumor development 35, acting both as a tumor suppressor and promoter by preventing the buildup of damaged organelles and proteins while also aiding tumor cell survival during starvation and hypoxia. Cancer cells trigger autophagy in response to cellular stress or increased metabolic demands. Autophagy enhances stress tolerance, maintains energy production, and supports tumor growth and drug resistance. Preclinical studies indicate that inhibiting autophagy makes cancer cells more vulnerable to chemotherapy and increases cell death across various cancer types 34. However, prolonged use of autophagy-inhibiting agents may promote cancer progression by prompting tumor cells to adopt alternative nutrient pathways 34. Several signaling pathways control autophagy and adjust cellular autophagic activity based on the availability of nutrients and growth factors 36. Key pathways involved in initiating and regulating autophagy include the mammalian target of rapamycin (mTOR), AMP-activated protein kinase (AMPK), and Unc-51-like autophagy activating kinase 1 (ULK1) 36. Interestingly, mTOR functions as a negative regulator of autophagy, and its inhibition results in increased cellular degradation and autophagy, contributing to its antiproliferative effects in clinical settings.
Engagement of the immune system
Since the early stages of proliferation, CSCs engage both innate and adaptive immune system components, modulating immune responses to gain a survival advantage by evading immune surveillance, suppressing effector cell responses, and creating a protective microenvironment. The interaction between CSCs and the immune system is bidirectional (i.e., from CSCs to immune cells and vice versa) and includes opposing signaling (i.e., stimulatory and inhibitory). CSCs have developed various escape strategies, ranging from forming a protective niche by initially recruiting and eventually suppressing effector cell functions within the tumor (i.e., immune-active microenvironment) to excluding effector cells altogether (i.e., immune-exhausted microenvironment). These strategies vary across tumors and within a specific malignancy depending on external stimuli and treatment pressure 37. Table II Iillustrates the main mechanisms involved in the immune engagement produced by CSCs. The primary mechanism is the CSC-driven upregulation of programmed death-1 ligand 1 (PD-L1), which leads to the suppression of T-cell effector activation (CD8+) and the creation of a tumor-protective niche 37,38.
CSCs can also reduce their visibility to cytotoxic T cells by downregulating MHC-I expression 39 and modifying the expression of tumor suppressors and oncogenes through epigenetic reprogramming and DNA methylation 40. Additionally, CSCs can alter their secretome by releasing cytokines, chemokines, and exosomes (e.g., IL-1, IL-6, IL-8, S100A9) to attract inflammatory and immune cells, tumor-associated macrophages (TAM) of the M2 phenotype, myeloid-derived suppressor cells (MDSCs), and suppressor T cells 41. CSCs can also present cell surface oncofetoproteins, thereby mimicking embryonic cells and their immune privilege (Tab. II) 37.
Several CSC surface markers engage in bidirectional signaling between tumor cells and the immune system to promote the recruitment, accumulation, and phenotype adaptation of immune effector cells (Tabs. I-II). CD19 – a transmembrane glycoprotein found on the surface of B cells throughout their development – is often present on CSCs and other mature tumor cells of myelomonocytic, dendritic, and B-cell origins, and it plays a role in modulating B-cell receptor (BCR) stimulation 42. CD24 is a sialoglycoprotein involved in B-cell development during the pro- and pre-B cell stages, where it helps regulate cell survival 43. It also affects cell adhesion, migration, differentiation, and apoptosis 43. CD24 may help tumor cells evade the immune system by interacting with Siglec-10 on immune cells, which inhibits phagocytosis by macrophages and NK cell cytotoxicity 43,44.
CD38 is an almost universally expressed marker on immune cells (B cells, T cells, NK cells, and macrophages, among others), involved in cell-to-cell interactions, signal transduction pathways, and regulation of calcium levels within cells 45. CD38 also functions as an ecto-enzyme – a protein active outside the cell – breaking down cell metabolites like NAD+ into smaller molecules such as ADP-ribose, which may act as secondary cellular messengers 45. Therefore, CD38 plays a role in modulating cell survival by regulating metabolic processes. Previous studies have shown that pancreatic and prostate cancer cells with low CD38 expression have higher cellular NAD+ levels and longer tumor cell survival 45.
CD44 – a common marker of CSCs – is a cell surface glycoprotein expressed in various cell types, with roles in cell adhesion and activation 46. It has been linked to many cellular processes, including leukocyte activation, lymphocyte homing, tumor metastasis, and hematopoiesis 46. The expression of CD44 may reprogram lymphocyte homing within the tumor microenvironment (TME), promoting cell-to-cell interactions and cross-signaling, which lead to tumor infiltration by both innate and adaptive immune cell populations 46.
The immune system can support this process by encouraging the transformation of non-CSCs into CSCs through interferon-γ (IFN-γ) and activating the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/Notch signaling pathway (47). It also helps preserve the stemness phenotype via CD+ 17 T cells through IL-17 and the activation of the nuclear kβ (NF-kβ), p38 mitogen-activated protein kinase (MAPK), and signal transducer and activator of STAT3 pathways 48. Additionally, CD8+ T cells can further enhance the malignant invasion, progression, and metastasis of CSCs through the activation of the PI3K-AKT-mTOR pathway 49, and cancer-associated fibroblasts (CAFs) within the tumor environment can sustain stemness and chemoresistance through the INF, IL-6, and IL-8 signaling pathways 50.
NEOSIS
When normal and cancer cells experience DNA damage, the accumulation of mutations and increased genetic instability drive the cells toward senescence. This leads to telomere shortening and disruptions in mitotic division (Fig. 2; Tab. III) 51. Dysregulation of nuclear division results in the formation of multinuclear (MN) or polyploid giant (PG) cells. These MN/PG cells show high genomic instability, are unable to respond to growth signals, have a limited mitotic lifespan (MLS), and die after a certain number of divisions. Typically, MN/PG cells die through apoptosis (programmed cell death), or if apoptosis is impaired, they are eliminated via mitotic crisis or catastrophe 51-53. Mitotic catastrophe is recognized as a tumor-suppressive mechanism, during which the nuclear envelope is disrupted, and mitotic spindle checkpoints are activated. Cells with damaged DNA then enter mitosis but are unable to complete it, ultimately leading to cell death. However, some cells may escape mitotic crisis and produce a specific type of mononuclear cell with a longer MLS. This process, called neosis, results in the formation of mononuclear cells known as Raju cells. The onset of neosis requires the presence of senescent MN/PGs, referred to as neosis mother cells (NMCs) 51-53. Raju cells temporarily exhibit stem cell properties, undergo rapid symmetric mitotic division, and develop into mature tumor cells due to their DNA repair mechanisms. The neosis model of self-renewal in cancer growth provides an alternative to the CSC pathway. During neosis, cells choose this division mode to survive rather than undergo mitotic crisis. These cells have defective senescence-associated cell cycle checkpoints, inactivated tumor suppressor genes, and activated oncogenes. As a result, they can grow again and evade mitotic crisis by undergoing secondary or tertiary neosis. This supports the idea that tumor cells are not immortal; some may die from mitotic crisis or mutations, while others can divide multiple times through neosis 53. It remains unclear how mitotically active mononuclear Raju cells develop from non-viable polyploid cells, but it is believed that this process could be triggered by factors such as the accumulation of genetic and epigenetic changes, defective checkpoint functions related to senescence, and malfunction of tumor suppressor genes like p53/pRB/p53Ink4a 54.
Neosis is a process that contributes to cancer recurrence. Several studies have also shown that the progeny of Raju cells produced by neosis are significantly more resistant to treatment. These cells can withstand chemotherapy- and radiotherapy-induced mitotic damage. The connection between neosis and drug resistance becomes clearer when several senescent cells, MN cells, PGs, and small cells with large nuclei are observed among tumor cells remaining after treatment 51-54. Neosis is recognized as a distinct process not only for explaining the mechanism behind tumor formation but also for describing the different steps that lead to therapeutic resistance and cancer recurrence. In both cases, surviving cells regenerate and restart cancer; however, in neosis, abnormal division causes the tumor to recur 14
PHOENIX RISING
Phoenix rising is a process where dying cells send signals that promote growth and division, leading to the formation of new cells (Tab. IV) 55. When tissues are damaged, cells in the affected area multiply to replace the lost ones. Stem cells located at or near the injury site play a crucial role in wound healing 56. Factors released from the wound attract stem cells to the injury site, guiding their differentiation and proliferation 57. Injury to tissues triggers an inflammatory response, with neutrophils and macrophages being the primary cells involved in the healing process. However, later experiments with mice lacking neutrophils and macrophages showed that these cells are not essential for wound healing 58. Recent findings suggest that factors related to cell death may play a significant role in tissue recovery. Compounds that induce apoptosis have been shown to initiate a paracrine cascade that ultimately stimulates stem cell proliferation 55. Although it may seem counterintuitive, apoptotic cells can promote tissue repair through a process called compensatory proliferation. Caspase 3 and caspase 7, key enzymes involved in the final stages of apoptosis, play a vital role in determining cell division and death 55. Activation of these caspases triggers a cascade by cleaving and activating the phospholipase enzyme iPAL2, which in turn increases the levels of prostaglandin E2 (PGE2). PGE2 acts as a growth promoter and supports tissue repair by stimulating local stem cell growth 55.
Dying tumor cells exploit apoptosis to produce growth signals and regenerate tumors damaged by radiation exposure. This highlights that although phoenix rising is a rapid and effective process for repairing injury, it can also lead to therapy resistance and tumor recurrence. In fact, cells destroyed by radiation or chemotherapy emit signals that stimulate the growth and proliferation of a few surviving stem and progenitor cancer cells at the site. The expansion of these remaining cells creates a risk for tumor reformation and relapse. Similar to tissue repair, apoptotic tumor cells activate caspases 3 and 7, which then cleave calcium-dependent phospholipase A2 (iPAL2); the active iPLA2 causes the release of arachidonic acid into the cytoplasm. Enzymes such as cyclooxygenase (COX) process arachidonic acid to produce various eicosanoid derivatives, like PGE2, which promote the growth and proliferation of cancer stem and progenitor cells (Tab. IV). Although the precise mechanism remains unclear, the Wnt/β-catenin signaling pathway has been linked to some cases of compensatory proliferation 59. It is well established that the Wnt/β-Catenin pathway plays a significant role in activating genes involved in cellular survival and proliferation (c-myc, cyclin D1, COX-2), anti-apoptosis (Bcl-2), and angiogenesis (vascular endothelial growth factor, VEGF), all of which can contribute to cancer recurrence 60.
TRANSFERRING THE EVIDENCE INTO CLINICAL PRACTICE
Recurrence is a multistep process
The experimental and clinical data from non-transplant research suggest that cancer recurrence is a multi-step process involving circulating CSCs, as well as other mechanisms of tumor cell growth such as neosis and compensatory proliferation (i.e., phoenix rising) (Fig. 1) 14. Once CSCs colonize tissues, they need a local inflammatory environment where they can interact with innate and adaptive immune cells through both specific and non-specific interactions to create a tumor-protective niche (i.e., TME). Because of shared biomolecular mechanisms, TMEs are unique to each cancer, and within the same cancer, they are influenced by the tumor cell immune genotype 61,62. As HCC research has demonstrated, tumor cells can develop various immune defense strategies. These usually include actively suppressing tumor-infiltrating lymphocytes (TILs), exhausting the T cell response by the tumor, or preventing the cancer from being recognized and targeted by the immune system 61. Through interactions with the inflammatory environment and accumulation of genetic mutations, CSCs change their phenotype during subsequent cell divisions, resulting in a highly diverse population of tumor cells. Some of these cells acquire a mesenchymal phenotype (EMT), lose adhesion to the tumor mass, and shed into the bloodstream 14,37.
CSC plasticity
An additional characteristic of CSCs is their plasticity – that is, their capacity to switch between actively dividing and quiescent states depending on changes in the TME and local and systemic stimuli 37. The range of stimuli that can affect CSCs is broad, including alterations in the TME such as hypoxia, surgical trauma, inflammation, cytokines, chemokines, chemical and mechanical changes in the extracellular matrix (ECM), and drivers like therapy (such as chemo- and radiotherapy, immunotherapy, post-transplant immunosuppression), infections (e.g., Epstein-Barr virus (EBV)), chemicals (e.g., alcohol, UV radiation, asbestos), and metabolic rewiring (such as NAD+ concentration in the extracellular environment) 37. These factors influence CSCs through activation or inhibition of intracellular signaling pathways (i.e., Wnt/β-catenin/Notch) and by altering the expression of tumor suppressor or tumor-promoting transcription factors and genes (e.g., p53/pRB/p53lnk4a) 37. Transplant-related events, like ischemia/reperfusion injury (IRI) and post-transplant immunosuppression, can significantly increase these risks via direct effects (e.g., production of reactive oxygen species (ROS), suppression of T cell clonal expansion through NF-kβ inhibition) and indirect effects (e.g., increasing viral infection risk). Notably, some molecular pathways involved in cellular stemness are targeted by commonly used immunosuppressants. NF-kβ, a transcription factor essential for IL-2 synthesis, plays a role in the IL-17-induced stemness phenotype of tumor cells. Paradoxically, chronic blockade of NF-kβ activation by calcineurin inhibitors (CNI) and the reduction of IL-2 release and IL-2-induced clonal expansion of CD4 Th1 cells are associated with an increased risk of malignancies post-transplant. This paradox highlights the redundancy of molecular pathways influencing tumor cell stemness and proliferation and the complex interplay of extracellular and intracellular signals in the cross-talk between cancer cells and immune cells (Tabs. I-II).
Additional mechanisms of tumor cell proliferation
Although crucial, CSCs are not the only mechanisms responsible for cancer recurrence and treatment resistance 14. Other pathways include Raju cell formation (i.e., neosis) and compensatory proliferation (i.e., the phoenix phenomenon). Neosis explains how tumor cells escape from mitotic crisis caused by the buildup of senescence-induced DNA damage or genotoxins. Although mainly described in non-transplant populations, this mechanism has not yet garnered interest among transplant scholars. Still, it could very well contribute to long-term cancer recurrence or de novo tumor growth in elderly patients. Additionally, neosis underscores the cancerogenic effect of aging-related cell injury, which is magnified in transplant patients on chronic immunosuppression 63.
Compensatory proliferation, also known as the phoenix rising phenomenon, is a process involving apoptosis-induced and inflammatory cell growth. It is associated with tissue injury, activation of granulocytes and macrophages, and is also triggered by programmed cell death through the phospholipids/arachidonic acid pathway 59. Several examples of compensatory proliferation have been observed in surgery, where the idea that surgical trauma is a key factor in cancer progression and metastasis is well established 64. This phenomenon might have been implied in a case of HCC metastasis at a skull trauma site previously reported by our group 65 and might participate in cell seeding and cancer recurrence in injured surgical areas.
Treatment selective pressure
While immune and targeted therapies have demonstrated significant success in treating many cancers 67-69, the ability of CSCs to adapt under therapeutic pressure remains a major obstacle to long-term effectiveness 37. Under treatment pressure, the dynamic plasticity of CSCs is activated and likely contributes to resistance development. This plasticity, a fundamental characteristic of CSCs, and the resulting phenotypic diversity enable tumors to remain resilient during aggressive progression, therapy resistance, and recurrence 37,67,68. The selective pressure of pre-transplant treatments on CSC plasticity, along with the mechanism of CSC adaptation to therapy, must be considered in the transplantation process for oncologic patients undergoing pre-transplant treatment, as it may enhance CSC resilience, promote relapse, and lead to resistance against post-transplant adjuvant therapies (Tab. I) 37.
The paradox of TME and immunosuppression
A vital aspect of cancer initiation, progression, and recurrence is the development of a supportive immune-inflammatory environment by tumor cells. A tumor is not just a group of cancer cells but a diverse collection of infiltrating and resident host cells, cytokines, chemokines, and ECM 13. Tumor cells induce significant molecular, cellular, and physical changes in their host tissues to support tumor growth and advancement. The composition of the TME varies among tumor types, but key features include immune cells, stromal cells, blood vessels, and ECM. Early in tumor growth, a dynamic and reciprocal relationship forms between cancer cells and TME components, promoting cancer cell survival, local invasion, and metastatic spread. To counteract an initially hostile microenvironment, including hypoxia and oxidative species, the TME orchestrates a metabolic and inflammatory response. The metabolic response aims to promote angiogenesis to restore oxygen and nutrient delivery and eliminate metabolic waste, while the inflammatory response guides adaptive and innate functions to support tumor growth.
Chronic immunosuppression plays a crucial role in cancer relapse and progression, contributing to a pro-tumorigenic, immune-exhausted TME 13. Several mechanisms drive this process, mainly involving CNI-induced suppression of IL-2 production by CD4+ Th-1 cells through blocking NF-kβ activation. This leads to a series of downstream events that shift the TME from an IL-2, IL-6, INF-γ (anti-tumorigenic) phenotype to a low IL-2, IL-10, TGF-β (pro-tumorigenic) one 13. Blocking IL-2 release prevents effector T cell clonal expansion (CD8+), reduces the expansion of CD4+ Th-2 cells and B-cell activation, and increases Treg expansion. Lower IL-2 levels promote tumor-specific Treg expansion and help shift macrophages from an M1 (anti-tumorigenic) to an M2 (pro-tumorigenic) phenotype, with macrophages, activated fibroblasts, and Tregs producing TGF-β. In turn, TGF-β stimulates VEGF release by tumor cells, macrophages, fibroblasts, and platelets, promoting blood vessel formation and the supply of critical nutrients 13.
The mammalian target of rapamycin inhibitors (mTORi), everolimus (EVR) and sirolimus (SRL), which have been used in clinical practice for a decade, not only demonstrate immunosuppressive properties but may also play a key role in shifting the TME from a pro-tumorigenic to an anti-tumorigenic state 70. According to the type of molecule (mTOR complex-1 versus mTOR complex-2 inhibitors) and the targeted molecular pathway (PI3K, AKT, p110δ), mTOR inhibitors can variably increase CD8+ tumor infiltration, inhibit TAM recruitment, reduce tumor-specific Treg expansion, suppress endothelial cell VEGF-driven activation, and decrease cytokine release from fibroblasts, potentially contributing to the inhibition of tumor growth and progression 70.
Management of cancer recurrence
Table V outlines proposed initiatives for transplant patients at risk of cancer relapse or donor-related tumor transmission, based on the biomolecular and immunologic considerations discussed in this paper.
CONCLUSIONS
Post-transplant cancer recurrence and transmission are complex, multistep processes in which circulating CSCs play a crucial role. Circulating cells alone cannot cause cancer recurrence. Instead, post-transplant tumor relapse, de novo cancer, or donor-related tumor transmission result from a complex interaction among CSCs, the recipient’s innate and adaptive immune systems, and the tumor niche, which is intensified by inflammation and immunosuppression. Understanding these mechanisms and implementing management strategies early in the transplant process are essential to reducing the risk of cancer relapse. Pre-transplant chemo, radio-, and immunotherapies can influence cancer biology and exert selective pressure on CSCs’ plasticity, which must be balanced against the risk of relapse. Surgical trauma may also promote compensatory proliferation during tumor shedding, and the metabolic effects of IRI can contribute to shifting CSCs from a quiescent to an active state. Host senescence, graft injuries, and post-transplant immunosuppression may further increase pressure on CSCs, especially over the long term. Maintaining a vigilant approach throughout the transplant process in oncologic patients is vital to optimizing clinical outcomes.
Conflict of interest statement
The authors declare no conflict of interest.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Author contributions
PDS, DP, QL, DC: conceptualized the study; EFK, MG, AR, JD: conducted the literature search; PDS, DP, RD, AR: wrote the preliminary draft; PDS, QL: made critical revisions. All authors prepared the draft and approved the final version.
Ethical consideration
Not applicable.
History
Received: June 24, 2025
Accepted: July 29, 2025
Figures and tables
Figure 1. Cancer growth, progression, and relapse are multistep processes involving cancer stem cells (CSCs), mature tumor cells, vascular endothelial cells, the extracellular matrix (ECM), and various components of the innate and adaptive immune systems, collectively known as the tumor microenvironment (TME). Within the TME, the tumor interacts with inflammatory and immune cells through bidirectional signaling, creating an inflamed niche whose properties – such as being pro-tumorigenic or anti-tumorigenic – depend mainly on the genomic and phenotypic profiles of the tumor cells. One key feature of pro-tumorigenic tumor niches is epithelial-to-mesenchymal transition (EMT), which involves modifications to cell adhesion mechanisms and subsequent tumor shedding.
Figure 2. Neosis is an alternative mode of cell division for DNA-damaged cells, such as tumor or senescent cell populations. Due to repeated mitotic cycles, exposure to genotoxins, and senescence, tumor cells lose their ability to divide properly and die through inflammatory (i.e., necrosis), non-inflammatory (i.e., apoptosis) mechanisms, or mitotic crisis (also known as mitotic catastrophe), which is a failure to divide. However, for reasons not fully understood, some DNA-damaged cells develop polyploidy – forming multinuclear cells (MNs) or polyploid giant cells (PGs) – and release mononuclear cells (also known as Raju cells) through vesicular formation. Although temporary, Raju cells share features of cancer stem cells (CSCs), re-enter the cell cycle, activate mitotic division, and replenish the tumor cell population, acting as tumor initiating cells (TICs).
| CSC | |
|---|---|
| Functional characteristics | Self-proliferating cells which can: self-renew through asymmetrical cell division (i.e., 1 CSC divides into 1 CSC and 1 mature tumor cell) polarize a protective TME through release of stimulating factors and inhibition of programmed cell death switch to a slow-proliferative or inactive (dormant) state and reactivate upon changes in the TME microenvironment stimulate EMT, loss of cell polarity and cell-cell adhesion, and gaining of migratory properties develop resistance to chemotherapy or radiotherapy |
| Mechanisms | Enhanced DNA repair capacity Production of anti-apoptotic signals (Bcl-2) Autophagy inhibition Release of anti-cancer drug through cell membrane efflux pumps (BCRP2/ABCG2; MRP1/ACC1; MDR1/ABCB1) Detoxification of drugs through ALDH1 Activation of EMT through activation of the SHH intracellular signaling pathway Activation of the Notch and Wnt signaling pathways |
| Cell markers (those markers more specific for CSCs are indicated in bold) | Stemness transcription factors and proto-oncogenes shared by CSCs and non-cancer stem cells Yamanaka’s transcription factors: OCT4, MYC, KLF4, SOX2 NANOG SALL4 Genomic markers: these are genes/pseudogenes/proto-oncogenes (over)expressed in CSCs and that contribute to CSCs self-renewal and resistance to chemotherapy through their protein transcripts Bcl-2 BCRP2 Cytoplasmic markers: these are enzyme overexpressed in CSCs and that contribute to CSCs resistance to chemotherapy ALDH1 Cell surface markers: these are transmembrane glycoproteins variably expressed in non-cancer stem-cells, cancer progenitor cells of different origins (brain, blood, liver, epithelium) CD133 (promin-1) EpCAM Differentiated-cell surface markers: these are transmembrane glycoproteins expressed on the surface of CSCs and differentiated cancer and immune cells CD19 CD24 CD38 CD44 CD90 Exclusive CSC markers: these are transmembrane proteins that label CSCs of only one or two cancers CD45 (bladder cancer) FUT 4 (brain cancer) CD15 (gastric cancer and Hodgkin’s lymphoma) MSI2 (gastric cancer) LINGO-1 (Ewing sarcoma) AFP (hepatocellular carcinoma) ANPEP (esophageal and liver cancer) ITGA6 (liver cancer) CD61 (breast cancer) |
| ABCB1: MRD1 or P-glycoprotein; ACC1, MRP1; AFP, alpha-feto-protein; ALDH1, aldehyde dehydrogenase-1; ANPEP: alanyl-aminopeptidase transmembrane; Bcl-2: B-cell lymphoma gene/protein; BCRP2: breast cancer receptor protein 2; CD15: Lewis X or sLex antigen; CD19: transmembrane B-cell activation regulator; CD24: heat-stable antigen or small-cell lung carcinoma cluster 4 antigen; CD38: ectoenzyme residing on the surface of lymphocytes and plasma cells and involved in NAD+ metabolism; CD44: regulator of cell-cell matrix interaction; CD45: also known as leukocyte common antigen (LCA): is a marker of bladder cancer progression; CD61: integrin-beta 3; CD90: a Thy-1 receptor glycoprotein involved in cell adhesion; CD133: CSC: cancer stem cell; EMT: epithelial mesenchymal transition; EpCAM: epithelial cell adhesion molecule; FUT4: fucosyltransferase 4; ITAG6: integrin subunit alpha-6; KLF4: Krueppel-like factor 4; MDR1: multidrug resistance-1 transmembrane protein (P-glycoprotein); MRP1: multidrug resistance associated protein 1; MSI2: Musashi RNA-binding-protein 2; MYC: myelocytomatosis oncogene transcription factor; NANOG: nanog homebox transcription factors; OCT4: octamer binding transcription factor-4; SALL4: Sal-like protein 4; SHH: sonic hedgehog; SOX2: sex-determining-region-Y-box-2; TME: tumor microenvironment; Wnt: wingless-related integration site. | |
| Mechanism | Effect |
|---|---|
| From CSCs to immune cells | |
| Upregulation of PD-L1 | Suppression of T cell effector activation CD8+ exhaustion Creation of a protective tumor niche |
| MHC I expression downregulation | Reduction of visibility to cytotoxic T cells CD8+ activation dampening |
| Epigenetic reprogramming | Tumor suppressor gene expression alteration Oncogene overexpression Overexpression of anti-apoptotic genes and transcripts |
| Secretome modulation and release of signaling molecules (cytokines, chemokines, and exosomes): IL-1, IL-6, IL-8, S100A9 | Recruitment of inflammatory and immune cells TAM M2 (secretion of TGF-beta) MDSCs (enhance CSCs through IL6, Notch signaling and exosomal S100A9) T suppressor cells Creation of a protective tumor niche Facilitation of tumor cell tissue colonization, growth and metastasis formation |
| Overexpression of CTA and onco-fetoproteins | Embryonic cell mimicry Immune privilege mimicry |
| Overexpression of transmembrane signal transducers (CD133, EpCAM, CD19, CD24, CD38, CD44) | Cross talking between CSCs and immune system to facilitate recruitment, buildup and phenotype adaptation of immune effector cells |
| From immune cells to CSCs | |
| INF-γ | Conversion of non-CSCs into CSCs Maintenance of the stemness phenotype through the PI3K/AKT/Notch signaling pathway |
| IL-17 | Maintenance of the stemness phenotype through the NF-kβ, p38 MAPK, and STAT3 pathways |
| CD8+ interaction with tumor cells through leptin receptor | Enhancement of malignant invasion and metastasis through activation of the PI3K-AKT-mTOR pathway |
| CAF | Maintain stemness and chemoresistance through INF, IL-6, and IL-8 signaling |
| AKT: protein kinase B; CAF: cancer-associated fibroblasts; CTA: cancer testis antigens; M2, IL, interleukin; INF-γ: interferon gamma; M2-like macrophages; MDSC: myeloid-derived suppressor cells; PD-L1: programmed death-1 ligand-1; PI3K: phosphoinositide 3-kinase; S100A9, TAM: tumor-associated macrophages; TGF: tumor growth factor. | |
| Raju cells (TIC) | |
|---|---|
| Functional characteristics | Tumor cells with prolonged MLS and stem cell properties induced by accumulation of genetic and epigenetic changes due to senescence or dysfunction of tumor suppressor genes (p53/pRB/p53lnk4a) |
| Mechanisms | Upon accumulation of multiple DNA injuries, damaged cells transform into multinuclear (MN)/polyploid giant (PG) cells (also known as neosis mother cells (NMCs) MN/PGs escape the mitosis catastrophe and develop into mature treatment resistance tumor cells by Prevention of nuclear envelope disintegration Blockade of mitotic spindle checkpoint activation Telomerase reactivation |
| MLS: mitotic life span; MN: multinuclear cells; NMC: neosis mother cells; PG: polyploid giant cells; TIC: tumor initiating cell. | |
| Phoenix rising | |
|---|---|
| Functional characteristics | Promotion of stem cell proliferation by cells undergoing apoptosis or injury in damaged/inflamed areas (i.e., wound healing-like process or compensatory proliferation) |
| Mechanisms | Apoptotic cells in injured/inflamed areas activate caspase-dependent cell death Caspases 3 and 7 triggers activation of iPAL2, in turn increasing PGE2 PGE2 acts as growth promoter supporting tissue repair The Wnt/β-catenin signaling pathway is involved in the process activating genes involved in cell survival and proliferation (c-myc, cyclin D1, COX-2), anti-apoptosis (Bcl-2), and angiogenesis (VEGF) |
| Bcl-2: B-cell lymphoma-like protein 2; c-myc: cellular MYC; COX-2: cyclooxygenase-2; iPAL: phospholipase A2; MYC: myelocytomatosis; PGE2: prostaglandin E2; VEGF: vascular endothelial growth factor. | |
| Strategy | Rationale |
|---|---|
| Tumor molecular profiling from the early clinical phases (histology, liquid biopsy, surgical specimes) | The cancer prognosis is strictly dependent on its genotypic (i.e., burden of genetic mutations) and phenotypic (i.e., expression of intracellular molecular pathways) profile Targeted molecular therapies require cancer cell profiling to drive selection of the most appropriate treatment regimen Information on PI3K/mTOR/AKT activation status is pivotal to guide the choice of post-transplant immunosuppression Caveats Diverse cancers show a high degree of intra-tumor heterogeneity (i.e., pancreatic cancer) and identification of a pan-cancer treatment may be challenging Genomic mutations emerge and accumulate over time and the phenotypic profile may change with treatment (i.e., treatment selective pressure) |
| Careful evaluation of treatment appropriateness | Treatment selective pressure can contribute to CSCs plasticity, heterogeneity and activation, since resistance mechanisms share common features with those involved in cancer progression and metastatic dissemination Caveats Oncologic referral often takes place when patients are considered no longer fit for surgery or systemic therapies Retrospective information on treatment is not always available |
| Host immune profiling | Any cancer elicits the innate and adaptive immune system response, and information on the host immune profiling, priming, reactive state or exhaustion is relevant to the post-transplant outcome Basic information should be gained on CD4+/CD8+ activation, PD-L1/PD-1 expression in TME, and IL-2 levels. |
| Mitigation of IRI | Hypoxia, ROS, and accumulation of acidic metabolites can stimulate CSC activation and treatment resistance Combined with graft senescence, IRI can stimulate evasion of the mitotic crisis Cell injury and the consequent inflammatory signaling can stimulate compensatory proliferation in surgical areas, through release of anti-apoptic signaling (i.e., phoenix rising) |
| Prevention/control of additive risk factors for CSCs activation | Changes in TME may be driven by surgical trauma at transplantation, as well as infections (EBV), chemicals (alcohol, UV), drugs and inflammation in the post-transplant period Caveats Most of these risk factors rely on patients’ adherence to life-style recommendations |
| Immunosuppression | The main driver to a pro-tumorigenic TME is CNI-dependent IL-2 depression Policies of CNI (TAC) exposure reduction are favored Combined use of antimetabolites and/or mTORi is favored either to facilitate CNI reduced exposure and to build upon the anti-tumorigenic properties of mTORi Caveats The clinical efficacy of mTORi facilitated CNI exposure is variably dependent on the type of cancer, its genotypic and phenotypic profile, and on expression of the mTOR signaling pathway in cancer cells |
| Monitoring of CTCs | Measurement of CTCs at the time of transplantation for patients with a history of cancer, cancer as indication or receiving grafts from ECD with previous or current malignancies is highly favored Longitudinal assessment and on-demand testing (i.e. rejection treatment, cancer relapse, adjuvant treatment administration) should be favored |
| AKT: protein kinase B; CNI: calcineurin inhibitor; CSCs: cancer stem cells; CTCs: circulating tumor cells; EBV: Epstein-Barr virus; IL-2: interleukin-2; IRI: ischemia reperfusion injury; PD-1: programmed death-1; PD-L1: programmed death-1 ligand-1; ROS: reactive oxygen species; TAC: tacrolimus; TME: tumor microenvironment. | |
