Mechanisms, clinical translation, and future perspectives of liver-mediated immunoprotection in combined organ transplantation

Abstract

Owing to its unique tolerogenic immune microenvironment, the liver can confer substantial immunologic protection on concurrently transplanted allogeneic grafts in combined organ transplantation. This review summarizes the molecular and cellular mechanisms underlying liver-mediated immunoprotection, including antigen presentation by liver sinusoidal endothelial cells and tolerogenic dendritic cells, enrichment of regulatory T cells, clonal deletion of donor-reactive T cells, clearance of donor-specific antibodies, and the establishment of microchimerism. We further synthesize clinical outcome data from combined liver-intestinal transplantation and multivisceral transplantation, and evaluate the translational potential of optimized immunosuppressive strategies, cell-based therapies, and emerging technologies such as normothermic machine perfusion. Although current evidence supports the immunoprotective role of the liver, its precise molecular mechanisms remain incompletely defined and require further validation. Prospective multicenter studies are also warranted.

Key Words: Liver transplantation; Intestinal transplantation; Multivisceral transplantation; Immune tolerance; Immunoprotection; Combined transplantation

Core Tip: This review highlights the immunoprotective mechanisms of the liver in combined intestinal and multivisceral transplantation, including liver sinusoidal endothelial cell-mediated antigen presentation, regulatory T cell enrichment, clonal deletion, and donor-specific antibody clearance. Concurrent liver inclusion reduces rejection rates but increases risks of graft-vs-host disease and infection. Normothermic machine perfusion and regulatory T-cell therapy represent promising strategies for optimizing immunomodulation.

INTRODUCTION

Among solid organs, the liver is uniquely endowed with tolerogenic properties often described as an “immune privilege”[1,2]. This distinctive property arises from its dual anatomical and immunologic identity. This tolerogenic phenotype is shaped by the liver’s constant exposure to gut-derived dietary antigens and microbial products, which creates a milieu of low-grade physiologic inflammation associated with chronic, low-dose endotoxin stimulation. It is further reinforced by the distinctive composition of the hepatic immune compartment, which is enriched in regulatory T cells (Tregs), Kupffer cells, tolerogenic dendritic cells (DCs), and liver sinusoidal endothelial cells (LSECs). In the setting of transplantation, the liver is relatively well accommodated by the recipient’s immune system. Indeed, approximately 5%-20% of liver transplant recipients may ultimately achieve operational tolerance, defined as stable graft function after complete withdrawal of immunosuppressive therapy[3].

Intestinal transplantation remains the only curative treatment for irreversible intestinal failure. However, the intestine contains abundant gut-associated lymphoid tissue and is therefore highly immunogenic, with a substantially higher incidence of acute rejection than that observed after transplantation of other solid organs[4,5]. Since the first multivisceral transplantation (MVT) performed by Starzl and colleagues in 1987, clinical observations have shown that concurrent liver transplantation can markedly reduce the incidence of intestinal graft rejection and improve graft survival. This phenomenon is referred to as liver-mediated immunoprotection[6]. The same protective effect has subsequently been demonstrated in other combined transplant settings, including combined liver-kidney transplantation and combined heart-liver transplantation. Given the rapid accumulation of evidence in the field of combined transplantation, recent health-policy evidence reviews have incorporated this topic into standardized evaluation frameworks[7]. This review systematically summarizes the molecular mechanisms underlying liver-mediated immunoprotection, clinical outcomes, and therapeutic strategies in combined liver-intestinal and MVT, and future directions for research.

MOLECULAR AND CELLULAR MECHANISMS OF LIVER-MEDIATED IMMUNOPROTECTION

The unique hepatic immune microenvironment

The hepatic immune microenvironment is maintained by both innate and adaptive immune populations, among which Kupffer cells, LSECs, and DCs are particularly relevant to transplant tolerance. Kupffer cells constitute approximately 80%-90% of hepatic macrophages and are characterized by high expression of programmed death ligand 1 (PD-L1) and interleukin-10 (IL-10). They can induce apoptosis of effector T cells through Fas/FasL signaling, a mechanism that has been linked to the development of liver transplant tolerance[8].

LSECs contribute to this tolerogenic setting through both structure and function. The architectural fenestrations allow LSCEs to efficiently capture circulating antigens, while the downregulation of costimulatory molecules, such as CD80 and CD86, enables cross-presentation, which consequently encourages CD8+ T cells to remain tolerant instead of differentiating into effector cells. In addition, experimental studies have demonstrated that LSECs undergo functional alterations early during ischemia-reperfusion injury, thus emphasizing their role in bridging initial graft damage to subsequent immune activation[9].

A similar tolerance-promoting phenotype is observed in hepatic DCs, characterized by the reduction in costimulatory activity and the upregulation of PD-L1 expression. Within this environment, antigen presentation by DCs preferentially promotes the differentiation of Tregs rather than activating effector T cells[10]. Additionally, donor-derived passenger leukocytes, particularly DCs, may migrate to recipient lymphoid tissues post-transplantation, where they promote the elimination or anergy of alloreactive T cells. This pathway likely represents one of the mechanisms underlying the establishment of liver transplant tolerance[11]. Zimmer et al[12] further interpreted these findings within a broader immunologic context, highlighting that liver transplant tolerance is attributable to the integrated functions of multiple hepatic cell populations rather than the effect of any single dominant cell subset.

Collectively, the hepatic immune microenvironment should be viewed as an integrated regulatory system rather than a simple collection of tolerogenic cells. Within this regulatory system, LSECs capture antigens and initiate non-effector T-cell responses, whereas tolerogenic DCs further support the differentiation of Tregs, and Kupffer cells function as a “safety valve” by clearing activated T cells and immune complexes. The redundancy and complementarity of this network may explain why liver-mediated immunoprotection is both robust and durable. However, most existing studies have focused on individual cellular subsets, and the spatiotemporal coordination among these subsets in the setting of combined transplantation remains insufficiently characterized. Addressing this gap should be a priority for future mechanistic studies. Moreover, the molecular pathways discussed above [PD-L1, IL-10, transforming growth factor-β (TGF-β)] exhibit extensive crosstalk rather than linear actions. Recent single-cell RNA-sequencing studies have uncovered marked heterogeneity within hepatic immune populations and revealed that immunometabolic reprogramming - e.g., enhanced fatty acid oxidation in Tregs and LSECs - critically sustains their suppressive functions. These network-level observations suggest that targeting single molecules may be insufficient to achieve durable tolerance.

Treg enrichment and clonal deletion

In liver-containing combined transplantation, Treg expansion is observed in both the recipient peripheral circulation and the graft. These cells suppress effector T-cell activation and proliferation through inhibitory cytokines, including IL-10 and TGF-β. In simultaneous liver-kidney transplantation, the liver has been shown to promote peripheral Treg expansion and reduce the risk of kidney allograft rejection, suggesting that liver-derived immunoprotection may extend to a concurrently transplanted nonhepatic organ through Treg-dependent mechanisms[13]. In addition, donor-specific antibodies (DSAs) are closely associated with rejection after intestinal transplantation, whereas the presence of the liver can modulate DSA kinetics, thereby providing humoral immune protection for the intestinal graft[14].

Microchimerism formation

In liver-containing combined transplantation, the liver introduces a substantial load of donor-derived hematopoietic stem cells and passenger leukocytes, which can persist long term in the recipient bone marrow and thymus, giving rise to a state of microchimerism. Within the recipient thymus, donor-derived cells may participate in negative selection, deleting T-cell clones with high affinity for donor antigens and thereby promoting the establishment of central tolerance. Graft-vs-host disease (GVHD) is a particular concern in combined transplants that include the intestine. Although rejection may be attenuated when the liver is included, the accompanying transfer of large numbers of donor lymphocytes can predispose recipients to GVHD, especially in intestine-containing combined transplants[15]. The functional contribution of microchimerism to tolerance remains debated. Some studies suggest it is an epiphenomenon of graft persistence rather than a primary driver of central tolerance; therefore, its role should be interpreted cautiously alongside other mechanisms.

Taken together, these mechanisms suggest that the liver mediates immunoprotection through coordinated effects at three levels: The cellular level, involving LSECs, tolerogenic DCs, and Kupffer cells; the molecular level, involving IL-10, TGF-β, and PD-L1; and the cellular trafficking level, involving passenger leukocytes and microchimerism. Importantly, these layers do not operate in isolation, but instead appear to reinforce one another in a stepwise manner (Figure 1). The establishment of microchimerism depends on the maintenance of an initially immunosuppressive microenvironment; once established, microchimerism may further consolidate the clonal advantage of Tregs. Clarifying this positive feedback loop may provide a framework for designing more durable tolerance-induction strategies. Such strategies are unlikely to succeed through modulation of a single pathway alone, and may instead require coordinated targeting of multiple, mutually reinforcing mechanisms.

Figure 1 Immunomodulatory strategies in combined transplantation. A: Standard immunosuppression; B: Cellular therapy; C: Liver-intrinsic immune protection. MMF: Mycophenolate mofetil; DSA: Donor-specific antibody; dd-cfDNA: Donor-derived cell-free DNA; Treg: Regulatory T cell; IL-10: Interleukin-10; TGF: Transforming growth factor; DC: Dendritic cell; PD-L1: Programmed death ligand-1; LSEC: Liver sinusoidal endothelial cell.

CLINICAL OUTCOMES OF COMBINED LIVER-INTESTINAL AND MVT

Survival and rejection outcomes after combined liver-intestinal transplantation

Tzakis et al[16] reported long-term outcomes from a single-center series of 100 consecutive MVT procedures, demonstrating that liver-containing combined transplant strategies can significantly improve graft survival. According to the 2021 annual intestinal transplantation report from the Organ Procurement and Transplantation Network/Scientific Registry of Transplant Recipients, combined liver-intestinal transplantation was associated with more favorable waitlist mortality and posttransplant survival than isolated intestinal transplantation. Nevertheless, GVHD remains one of the most severe complications in this field[17]. Sykes[15] systematically reviewed the pathogenesis and management of GVHD after intestinal transplantation and noted that the risk of GVHD may be higher after MVT and liver-containing combined transplantation than after isolated intestinal transplantation. This finding suggests that the protective effect of the liver does not extend to all immune-mediated complications.

Spectrum of complications

After combined transplantation, infectious complications account for much of the early postoperative morbidity. In the review by Yap et al[18], intra-abdominal and bloodstream infections were reported in up to 60%-80% of recipients and were closely linked to both graft loss and recipient death. Epstein-Barr virus-associated post-transplant lymphoproliferative disorder is particularly important after intestinal transplantation, where its incidence exceeds that reported for other solid-organ transplants and it remains a major cause of mortality in children[19]. Chronic rejection is a delayed form of graft injury, and Bral et al[20] reported that nearly 15%-25% of intestinal transplant recipients develop this complication, making it a leading cause of late graft failure.

Vascular complications present additional technical difficulties and are associated with adverse prognostic implications. Gloviczki[21] identified that arterial thrombosis, anastomotic stenosis, and pseudoaneurysm constitute principal vascular events in intestinal transplantation and MVT, with an overall incidence of nearly 5%-10%. Portal vein thrombosis is especially relevant to liver-containing procedures; Sancio et al[22] noted that Yerdel grade 3-4 thrombosis often necessitates a combined transplant approach.

These observations point to a practical dilemma in liver-containing combined transplantation. Inclusion of the liver may protect the graft from rejection-related loss, but the more intensive immunosuppression and greater operative complexity associated with these procedures can shift the risk toward infection-related graft failure and recipient death. Post-transplant management therefore cannot be guided by rejection avoidance alone. A more clinically meaningful endpoint is the joint survival of the graft and the recipient, which depends on adjusting immunosuppression as the relative risks of rejection and infection change over time. At present, clinicians lack reliable real-time markers for determining where an individual patient lies on this spectrum. Monitoring strategies that combine measures of immune activation with markers of excessive immunosuppression may provide a more useful basis for individualized treatment. These benefits, however, incur tangible trade-offs: Consumption of a scarce donor liver, elevated infectious morbidity, and a potentially higher GVHD risk than isolated intestinal transplantation. Thus, liver inclusion should be justified by individualized risk-benefit assessment rather than by immunoprotection alone.

Historical experience and procedural variants in MVT

Early experience from Lauro et al[23] showed that adult intestinal transplantation and MVT could be performed in highly complex surgical candidates, establishing the technical feasibility of combined transplantation in this population. Modified MVT was later developed as a liver-sparing alternative: The recipient liver is retained, while the stomach, duodenum, and small intestine, with or without the pancreas, are transplanted. In their review of modified MVT, Lauro et al[24] described the indications, operative considerations, and clinical outcomes of this approach. The main advantage is conservation of the donor liver; the trade-off is loss of the immunoprotective effect associated with liver inclusion.

Subsequent reviews by Pletcher et al[25] and Shahi et al[26], which focused on intestinal transplantation in adult and pediatric populations, respectively, described liver-containing transplantation as a more selective strategy and emphasized its benefits in specific clinical contexts rather than as a uniform requirement. The 500-case single-center study by Abu-Elmagd et al[27] represents a key milestone in the field of hepatic transplantation and establishes benchmark outcomes for intestinal and MVT. In addition, modified MVT has been implemented in rare but challenging clinical settings. For instance, Cruz et al[28] described the integrated application of spleen-preserving pancreaticoduodenectomy and modified MVT for familial adenomatous polyposis, including Gardner syndrome, whereas Bulder et al[29] reported the application of MVT in individuals with desmoid tumors. Although retransplantation remains a potential strategy for patients with graft failure, it is typically reserved for highly selected cases. The single-center study by Verna et al[30] suggested the possibility of achieving acceptable survival outcomes in carefully selected candidates. Quality-of-life outcomes also constitute an important consideration beyond survival. For instance, the multicenter study by Benazzo et al[31] revealed that the success of intestinal and MVT was associated with substantial long-term improvements in the quality of life of patients.

The historical evolution of MVT reflects two related yet distinct surgical priorities. One of the approaches involves including the liver when its immunoprotective properties are expected to enhance graft outcomes. The other strategy involves the preservation of the recipient liver whenever possible, thereby reducing the demand for donor livers and helping expand the available donor pool. These approaches should not be treated as competing models. In practice, they represent different solutions to different clinical problems. Future procedural decision-making should incorporate a broader set of variables, including the recipient’s immunologic profile, DSA burden, sensitization status, donor-organ quality, infection risk, and expected long-term quality of life, rather than relying solely on the presence or absence of end-stage liver disease. Although some centers have begun to explore individualized procedural selection based on immunologic risk stratification, these approaches have not yet been standardized or incorporated into consensus guidelines. Further prospective validation is needed before they can be routinely integrated into surgical decision-making. The key distinctions between conventional liver-containing MVT and modified liver-sparing MVT are summarized in Table 1. To facilitate cross-study comparison, we summarize key outcome metrics from the major series of liver-containing combined intestinal and MVT (Table 2). Although direct comparisons are limited by heterogeneity in patient selection, immunosuppressive regimens, and follow-up duration, the compiled data illustrate the range of reported outcomes and highlight the need for standardized reporting in future studies.

Table 1 Comparison of conventional liver-containing multivisceral transplantation and modified liver-sparing multivisceral transplantation.

Feature	Conventional liver-containing MVT	Modified liver-sparing MVT
Immunoprotective effect	+++ (with liver-mediated immunoprotection)	- (absent)
Risk of rejection	Lower	Higher
Requirement for donor liver	Required	Not required
Main indications	Intestinal failure with end-stage liver disease	Intestinal failure with preserved liver function
Intensity of immunosuppression	May be moderately reduced	High-intensity immunosuppression generally required
Ref.	[27]	[24,28]

MVT: Multivisceral transplantation.

Table 2 Summary of key clinical outcomes in liver-containing combined intestinal/multivisceral transplantation.

Ref.	n	Follow-up	Patient survival	Graft survival	Acute rejection	GVHD
Abu-Elmagd et al[27]	500	Up to 15 years	85% (1-year), 61% (5-year), 42% (10-year)	80% (1-year), 50% (5-year), 33% (10-year)	NR	NR
Tzakis et al[16]	100	Median 5.2 years	72% (1-year), 58% (5-year)	68% (1-year), 52% (5-year)	17% (MVT)	13%
Zheng et al[8]	98	Median 37.5 months	65% (1-year), 49% (3-year), 49% (5-year)	63% (1-year), 47% (3-year), 47% (5-year)	31.6% (moderate-severe, era 3)	NR
Ruart et al[10]	100	Median 25 months	72% (1-year), 57% (3-year)	NR	17% (MVT), 50% (MMVT)	13%

GVHD: Graft-vs-host disease; NR: Not reported; MVT: Multivisceral transplantation; MMVT: Modified multivisceral transplantation; OPTN/SRTR: Organ Procurement and Transplantation Network/Scientific Registry of Transplant Recipients.

IMMUNOSUPPRESSIVE STRATEGIES AND EMERGING IMMUNOMODULATORY APPROACHES

Standard regimens and innovations in organ preservation

Immunosuppression in combined transplantation generally consists of induction and maintenance phases. Maintenance therapy is typically based on tacrolimus in combination with mycophenolate mofetil. Del Bello et al[32] systematically discussed the complexity of immunosuppressive management in simultaneous combined transplantation, noting that liver-mediated immunoprotection may provide a rationale for dose-reduction strategies, although such approaches require strict individualized adjustment.

In the field of organ preservation, normothermic machine perfusion (NMP) has advanced substantially in recent years. Segovia et al[33] systematically reviewed preclinical and clinical evidence on intestinal NMP and suggested that this technology may extend preservation time and improve graft quality. In a porcine model, van Leeuwen et al[34] successfully achieved ex vivo NMP of a multivisceral graft comprising the liver, kidneys, pancreas, spleen, and intestine for up to 8 hours, providing an important platform for future ex vivo assessment and intervention in MVT. By performing a comprehensive review of recent advances in liver NMP, Dietz and Sarrazin[35] highlighted its translational potential for marginal donor assessment and graft preconditioning.

Contemporary practice is shifting toward biomarker-guided immunosuppression. Monitoring of DSAs enables early detection of humoral rejection, particularly in intestinal grafts without hepatic co-transplantation. Donor-derived cell-free DNA offers a non-invasive tool for early allograft injury detection, though its validation in multivisceral settings remains preliminary. Immune cell profiling (e.g., Treg subsets, activation markers) adds further granularity. Integration of these modalities may enable personalized dose titration - reducing immunosuppression in low-risk profiles while intensifying it in high-risk patients - but requires prospective validation in combined transplantation.

Cell-based therapy and tolerance induction

Recent advancements in cell-based therapeutic strategies and organ bioengineering have expanded the immunomodulatory approaches available in transplantation medicine[36]. The review by Nam[37] identified adoptive Treg therapy as an emerging strategy for inducing tolerance and noted that early trials using ex vivo-expanded Tregs in renal and hepatic transplantation settings have primarily demonstrated safety and feasibility. The review on xenotransplantation by Davies[38] outlined the historical development and current state of research in the field, highlighting the potential of gene-edited pigs to help donor organ shortages while emphasizing that immune barriers and biosafety concerns continue to pose as significant obstacles to clinical application.

Treg adoptive therapy and regulatory DC (DCreg) infusion function via distinct immune mechanisms. Treg therapy introduces cells that directly suppress immune responses, whereas DCreg infusion aims at modifying antigen presentation and guiding T-cell activation toward enhanced immune tolerance. These approaches are particularly relevant in combined transplantation settings as they mirror the mechanistic processes already operative within the liver. The Treg infusion strategy strengthens the Treg compartment, whereas DCreg infusion mirrors the tolerogenic antigen-presenting function of hepatic DCs. Despite is biologically plausibility, the clinical application of an integrative approach necessitates careful balancing of the overall level of immunosuppression and the associated risk of infection.

Cell-based therapy remains to be systematically evaluated in combined transplantation, which likely require a therapeutic objective different from that of isolated organ transplantation, as the liver already provides an endogenous layer of immunoprotection. Cell-based therapeutics may be better suited to supporting a calibrated reduction in immunosuppression while conferring sustained protection against rejection, rather than aiming for complete tolerance. Clinical trials should therefore evaluate whether Treg- or DCreg-based approaches are capable of safely reducing the intensity of maintenance immunosuppression without exacerbating the risk of rejection, infection, or GVHD. The mechanisms of immunomodulatory strategies in combined transplantation shows in Figure 1.

INNOVATIVE TECHNOLOGIES AND FUTURE DIRECTIONS

Artificial intelligence and risk prediction

Emerging technologies address distinct challenges in applying liver-mediated immunoprotection: Prediction of individual benefit, dynamic immune monitoring, and expansion of the donor pool. The following sections examine these innovations specifically through their potential to complement or enhance hepatic immunoprotection rather than as isolated technical advances. Using multivariable modeling, Wehrle et al[39] analyzed the long-term outcomes in patients undergoing combined hepatic transplantation and cardiac surgery. Despite the differences between this setting and intestinal or MVT, the study illustrated the application of risk models in refining candidate selection and assessing postoperative risk in complex combined procedures. However, vascular complications require a different type of innovation. The case report by Moris et al[40] described the successful treatment of a ruptured aortic conduit following MVT using parallel stent grafts and coil embolization. The results suggest that advanced endovascular techniques may offer promising organ-salvage options in selected transplant recipients who would otherwise face prohibitive operative risk.

Xenotransplantation and expansion of the donor-organ pool

Gene-edited pigs are emerging as a promising source of donor organs to address the shortage of organs. Advances in CRISPR-Cas9-mediated gene editing technology have enabled researchers to deactivate porcine endogenous retroviruses through the pig genome and remove several major xenoantigens, thereby reducing the key triggers of hyperacute rejection[38]. However, xenogeneic MVT remains far from clinical application, as its successful clinical translation will require additional genetic modification of donor animals, as well as more reliable management of cross-species immune injury, risk of infection, and perioperative physiological instability. From an immunoprotection perspective, genetically engineered porcine livers expressing human regulatory molecules (e.g., PD-L1) might theoretically augment tolerogenicity, whereas the vigorous innate response to xenografts could overwhelm it. Therefore, xenogeneic MVT will require not only overcoming xenograft rejection but also preserving or recreating the tolerogenic hepatic microenvironment.

CONCLUSION

Liver-mediated immunoprotection represents a key concept in combined transplantation settings, as it bridges the distinctive immunobiological features of the liver with measurable clinical benefits. The inclusion of the liver in combined hepatic-intestinal and MVT settings correlates with a reduced risk of rejection in concurrently transplanted non-hepatic grafts, as well with improved graft survival[13,16,27]; however, this benefit comes at a cost. An elevated risk of GVHD and infection implies that post-transplantation management strategies must focus on balancing the prevention of rejection against the complications of excessive immunosuppression[15,18]. Future progress will likely depend on the ability to more precisely measure and adjust this balance. Ex vivo immune modulation during NMP, Treg-based cellular therapeutics, novel biologics, and artificial intelligence-assisted immune monitoring strategies target different aspects of the challenges associated with these processes, from graft conditioning to post-transplantation risk assessment[32,34,37,39]. The clinical adoption of these approaches in routine practice will necessitate collaborative multicenter registries and prospective studies evaluating strategies aimed at reducing immunosuppression against clinically relevant outcomes, such as rejection, infections, GVHD, graft survival, and patient survival. A balanced view acknowledges that hepatic immunoprotection is clinically valuable yet conditional. Its net benefit depends on patient selection and mitigating procedure-related hazards. Future work should prioritize preoperative predictors of response to identify those most likely to gain from liver inclusion.