Natural Killer Cells in Kidney Transplantation: Dual Role in Allograft Injury and Immune Adaptation

1. Introduction

Kidney transplantation remains the treatment of choice for patients with end-stage renal disease. Thanks to advances in surgical techniques and modern immunosuppressive thera-pies, most recipients achieve not only a significant prolonga-tion of life but also a marked improvement in quality of life (Babich et al. 2026). Current 1-year post-transplant survival rates reach approximately 95% (Meier-Kriesche et al. 2001; Babich et al. 2026). Nevertheless, a substantial proportion of patients still do not achieve such favorable long-term out-comes. Complications result from both recipient-related fac-tors (comorbidities, age, and adherence) and donor-related factors. The latter, including incompletely understood mech-anisms of the alloimmune response, can lead to acute or chronic graft rejection and underscore the need for further intensive research into the pathogenesis of graft dysfunction (Pontrelli et al. 2020; Poggio et al. 2021; Ruan et al. 2024). The traditional theory of allograft rejection has focused primarily on the role of T and B lymphocytes and anti-HLA antibodies (Halloran 2010; Clatworthy 2011). However, this approach fails to explain all cases of allograft injury, par-ticularly those without detectable donor-specific antibodies (DSA) or classic lymphocytic infiltrates (Delville et al. 2016). Consequently, in recent years, increasing attention has been directed toward the innate immune system, especially natu-ral killer (NK) cells. Recent studies demonstrate that NK cells serve as important mediators of both graft injury and immu-nological adaptation to the transplanted organ (Pontrelli et al. 2020; Ruan et al. 2024).

NK cells are present in various tissues, including the skin, liver, lungs, and kidneys. They are particularly enriched in the liver, where they constitute approximately 25%–30% of intrahe-patic lymphocytes (Paul and Lal 2017). Unlike T lymphocytes, NK-cell activation does not require prior antigen presentation by antigen-presenting cells (APCs). This allows them to mount a rapid, innate immune response. For many years, NK cells were regarded primarily as cytotoxic effector cells. However, recent research has highlighted their remarkable phenotypic and functional plasticity (Ruan et al. 2024). In peripheral blood, two main subpopulations predominate: CD56^bright (approximately 5%–10%) and CD56^dim (approximately 90%) (Van Acker et al. 2017). Furthermore, under conditions of chronic antigenic stimu-lation—particularly cytomegalovirus (CMV) infection—adaptive NK cells with the NKG2C⁺ and CD57⁺ phenotype can emerge. These cells exhibit memory-like properties, representing a form of immunological memory within the innate immune system (Gumá et al. 2004; Ruan et al. 2024).

NK cells exhibit a clear functional duality in the context of kid-ney transplantation. On the one hand, they can contribute to allograft injury through direct cytotoxicity as well as amplification of the adaptive immune response. On the other hand, they may promote immunological tolerance, among others, by eliminat-ing activated T lymphocytes or donor-derived dendritic cells and through interactions with regulatory cells (Pontrelli et al. 2020). Of particular importance are data indicating the involvement of specific NK cell subsets in graft dysfunction independently of the presence of DSA (Ruan et al. 2024).

The dynamics of changes in the phenotype and function of NK cells following kidney transplantation remain poorly understood; however, a comprehensive assessment using flow cytometry may yield new biomarkers of immune adapta-tion (Ruan et al. 2024).

2. Aim

The aim of this narrative review is to summarize current knowl-edge on the dual role of NK cells in kidney transplantation, with emphasis on antibody-independent mechanisms, adaptive sub-sets, and their potential as biomarkers of graft outcome.

3. Search Strategy

Literature for this narrative review was identified through sys-tematic searches in PubMed, Scopus, and Web of Science up to May 2026 using combinations of terms, such as “NK cells,” “natural killer cells,” “KIR,” “NKG2C,” “kidney transplantation,” and “allograft rejection.” Relevant articles were selected, and additional studies were identified through snowballing (man-ual screening of reference lists of key papers and reviews).

4. The Heterogeneity and Plasticity of NK-Cell Subpopulations

NK cells constitute a highly heterogeneous population of lym-phocytes, whose phenotypic and functional diversity signifi-cantly influences the alloimmune response following kidney transplantation (Pontrelli et al. 2020; Ruan et al. 2024).

Two major subpopulations predominate in peripheral blood. The CD56^bright subpopulation accounts for approximately 5%–10% of circulating NK cells and is characterized by high expression of the inhibitory receptor NKG2A, low expression of CD16, and potent cytokine-producing capacity, particularly interferon (IFN)-γ (Van Acker et al. 2017). These cells primar-ily exert regulatory and immunomodulatory functions, includ-ing interactions with dendritic cells (Ruan et al. 2024). In kidney transplantation, an increased proportion of CD56^bright cells, as well as a high CD56^bright/CD56^dim ratio, has been associated with acute cellular rejection (Xu et al. 2019).

Although CD56^bright NK cells express high levels of the inhibitory receptor NKG2A, their potent Interferon-gamma (IFN-γ) secretion can predominate in the transplant setting, promoting T-cell recruitment and inflammation and thereby associating circulating CD56^bright cells with increased risk of cellular rejection (Kildey et al. 2019; Xu et al. 2019). The net functional outcome of this dual phenotype remains context-dependent and is still incompletely understood.

In contrast, the CD56^dim CD16⁺ subpopulation, which comprises approximately 90% of NK cells in peripheral blood, displays high cytotoxic activity, including strong antibody-dependent cel-lular cytotoxicity (ADCC) (Le Garff-Tavernier et al. 2011; Van Acker et al. 2017). The alloreactivity of these cells against the allograft depends largely on the expression of killer immuno-globulin-like receptors (KIRs) and the process of “licensing” (education) (Poli et al. 2009; Foley et al. 2011; Le Garff-Tavernier et al. 2011).

In addition, CMV infection—the most common viral complica-tion after kidney transplantation—plays a key role in driving the expansion of adaptive NK cells. Under chronic antigenic stimulation by CMV, there is a significant expansion of the NKG2C⁺CD57⁺ NK-cell population, which acquires features of immunological memory – a phenomenon considered unique to the innate immune system (Hadaya et al. 2008; Ruan et al. 2024).

The recipient's KIR/HLA genotype has been shown to influence the risk of CMV infection. In patients with active infection, NK cells exhibit a characteristic expansion with increased expres-sion of the activating receptors NKG2C and NKG2D, accom-panied by a transient reduction in IFN-γ production during the acute phase. Consequently, the increased presence of adaptive NK cells in the late post-transplant period primarily reflects long-term remodeling of the immune response aimed at effective antiviral defense, rather than the induction of tolerance toward the allograft (Hadaya et al. 2008).

In addition, the activity of NK cells is tightly regulated by inhibitory receptors, among which NKG2A/CD94 plays a particularly important role as an immune checkpoint mole-cule (Kaulfuss et al. 2023). NKG2A controls NK-cell expan-sion capacity by limiting both excessive proliferation and activation-induced cell death, thereby maintaining the bal-ance between effective antiviral response and prevention of overactivation.

NKG2A is a key inhibitory receptor that recognizes human leukocyte antigen E (HLA-E) on graft cells and suppresses NK-cell cytotoxicity (Kaulfuss et al. 2023). HLA-E exhibits limited polymorphism, primarily represented by the alleles HLA-E*01:01 and HLA-E*01:03, which differ in cell-surface expression levels and peptide-binding affinity. These differ-ences influence the strength of inhibitory signaling through NKG2A on both NK cells and CD8⁺ T cells. In the setting of allogeneic hematopoietic stem cell transplantation, HLA-E*01:03 homozygosity has been associated with a stronger graft versus leukemia effect mediated by NK cells and reduced relapse rates (Hosseini et al. 2013). Similar differences in HLA-E-NKG2A affinity, previously observed to modulate inhibitory signaling in both NK cells and CD8⁺ T cells in the context of COVID-19 (Hosseini et al. 2023), may also influence the balance between alloreactivity and tolerance in solid organ transplantation. Early after trans-plantation, an increased proportion of CD56^bright NK cells with high NKG2A expression and low KIR levels is often observed, resulting in impaired cytotoxic function. These abnormalities typically normalize within 1 year as CD56^bright cells differentiate into the CD56^dim subset, accompanied by decreased NKG2A expression and recovery of NK-cell cytotoxicity (Ghasemzadeh et al. 2016).

NK cells also mediate the “missing-self” response by recog-nizing and eliminating cells with low or absent HLA class I expression. To counteract this, novel therapeutic approaches using selective agonists of inhibitory receptors, that is, NKG2A and KIRs, are being developed. These strategies effectively suppress NK-cell-mediated rejection, particularly in the context of HLA-deficient induced pluripotent stem cell-derived grafts (Hiura et al. 2025).

Circulating and graft-infiltrating NK-cell phenotypes fre-quently show an inverse relationship, whereby a decrease in specific subsets in peripheral blood correlates with their enrichment within the allograft (Kildey et al. 2019; Ruan et al. 2024). Beyond the well-studied KIR and NKG2 families, the balance between activating natural cytotoxicity receptors (NCRs: NKp30, NKp44, NKp46) and inhibitory receptors fur-ther shapes NK-cell reactivity during transplantation, tipping the scales toward either graft injury or regulatory adaptation (Pontrelli et al. 2020; Ruan et al. 2024).

The main phenotypic markers, functional properties, and clinical relevance of the major NK-cell subpopulations in the context of kidney transplantation are summarized in Table 1.

5. Role of NK Cells in Ischemia-Reperfusion Injury

Ischemia-reperfusion injury (IRI) is an inevitable early event in kidney transplantation that triggers a sterile inflamma-tory response and significantly contributes to delayed graft function and subsequent alloimmune activation (Ma et al. 2023; Troise et al. 2025; Horowitz and Heeger 2026). NK cells are rapidly recruited to the injured graft within hours after reperfusion. They recognize stress-induced ligands on damaged tubular epithelial cells (e.g., via the NKG2D recep-tor) and exert direct cytotoxicity, thereby amplifying initial tis-sue damage (Ma et al. 2023; Fuhrmann et al. 2024).

Recent evidence indicates that NK cells play a pathogenic role in IRI through both contact-dependent killing of tubular epithelial cells and secretion of pro-inflammatory mediators that promote neutrophil recruitment and activation. NK cells facilitate neutrophil infiltration into the injured graft, thereby amplifying microvascular dysfunction. Neutrophils subse-quently generate reactive oxygen species (ROS), a central effector mechanism of reperfusion injury that causes oxida-tive stress and tubular cell death. Thus, the NK-cell-neutrophil axis and downstream ROS production constitute an impor-tant early innate pathway linking ischemia-reperfusion to subsequent alloimmune activation. Strategies targeting NK-cell activation pathways (such as dual mechanistic target of rapamycin (mTOR) and calcineurin inhibition) have shown promise in mitigating these effects in preclinical models (Ma et al. 2023; Tsao et al. 2024; Troise et al. 2025; Horowitz and Heeger 2026).

Ultimately, NK-cell activation during IRI represents an impor-tant early mechanism of graft injury that is largely indepen-dent of adaptive immunity. A better understanding of these processes, as reviewed by Horowitz and Heeger (2026), may open new therapeutic avenues to limit early graft damage and improve long-term outcomes.

6. The Role of NK Cells in Allograft Rejection

NK cells play a significant role in the pathogenesis of kid-ney allograft injury and rejection (Pontrelli et al. 2020). They contribute through both antibody-dependent mechanisms, primarily ADCC, and antibody-independent pathways. This involvement is supported by multiple lines of evidence from immunohistochemical, transcriptomic, and functional studies. DSA contribute to allograft rejection primarily through anti-body-mediated mechanisms. In antibody-mediated rejection (AMR), DSA bind to HLA antigens on graft endothelial cells, triggering complement activation and, importantly, ADCC. In ADCC, the Fc portion of DSA engages the CD16 (FcγRIII) receptor on CD56dim NK cells, leading to their activation, degranulation, and direct killing of antibody-coated target cells (Kildey et al. 2019; Pontrelli et al. 2020). This DSA-NK-cell interaction is a key effector pathway in microvascular inflammation (MVI) and chronic graft injury, even when com-plement deposition is limited.

Kidney allograft rejection is traditionally classified into two main types: T cell-mediated rejection (TCMR) and AMR (Pontrelli et al. 2020). In TCMR, an increased proportion of CD56^bright NK cells is commonly observed in peripheral blood. In contrast, AMR is associated with the accumulation of both CD56^bright and CD56^dim NK-cell subsets within the graft tissue (Kildey et al. 2019). Notably, CD56^bright NK cells can actively promote the pathological immune response in TCMR through the production of pro-inflammatory cytokines, such as tumor necrosis factor-α and IFN-γ (Kildey et al. 2019).

In chronic active antibody-mediated rejection (cAMR), MVI plays a central role and is characterized by inflammatory cell infiltration in glomerular and peritubular capillaries (Diebold et al. 2025). Typical histological features include glomerular basement membrane double contours (cg) and multilayering of peritubular capillary basement membranes. According to the Banff 2022 classification, a diagnosis of cAMR requires the presence of DSA and C4d deposition. However, MVI can also occur in DSA-negative and C4d-negative cases. These findings highlight the limitations of purely morphological assessment, which is why molecular diagnostics are increas-ingly used to detect even subclinical forms of antibody-medi-ated injury.

Importantly, recent high-dimensional analyzes have revealed a distinct pathway of antibody-independent allograft injury mediated by NK cells. Ruan et al. (2024) demonstrated that specific NK-cell subsets (particularly those expressing defined combinations of KIRs and NKG2A) exhibit height-ened alloreactivity that persists despite immunosuppression. Pretransplant release of Ksp37 by these cells was associ-ated with worse long-term graft function, and its intragraft expression predicted graft loss even in biopsies without overt histological rejection (Ruan et al. 2024). These findings strongly support the existence of a clinically relevant, DSA-independent mechanism of NK-mediated graft injury.

Beyond antibody-dependent mechanisms, HLA class I dis-parity between donor and recipient can directly activate recipient NK cells through the “missing-self” recognition path-way. Inhibitory KIRs on NK cells normally recognize self-HLA class I ligands. When donor endothelial cells lack the appropriate HLA ligands for the recipient's inhibitory KIRs, NK cells per-ceive this as “missing self” and become activated, leading to endothelial injury. In a study of kidney transplant recipients with MVI but no detectable DSA, 65.1% exhibited a genetically predicted mismatch between donor HLA class I and recipient inhibitory KIRs (Lebraud et al. 2022). This KIR-HLA ligand incompatibility enabled NK-cell-mediated allorecognition and damage to donor endothelium independently of antibodies. These findings highlight KIR-HLA mismatch as an impor-tant, DSA-independent risk factor for NK-cell-driven allograft injury.

From a prognostic perspective, serial monitoring of circulat-ing NK-cell phenotypes may provide early signals of graft dysfunction. However, because peripheral blood levels of certain subsets are often inversely related to their accumula-tion in the allograft, combined assessment of circulating and graft-infiltrating NK profiles is likely to offer superior predictive value for long-term outcomes, including MVI and graft loss independent of DSA (Kildey et al. 2019; Ruan et al. 2024; Horowitz and Heeger 2026).

Experimental evidence indicates that NK-cell depletion can improve allograft outcomes. In preclinical models, antibody-mediated NK-cell depletion combined with cyclosporine A significantly prolonged kidney graft survival and improved function (Ashraf et al. 2019). These observations suggest that targeted NK-cell depletion or functional inhibition may represent a promising strategy in selected high-risk trans-plant settings.

In summary, NK cells contribute to kidney allograft rejection through both antibody-dependent (ADCC, DSA/C4d-positive cAMR) and antibody-independent (KIR-mediated, Ksp37-associated) pathways. Their activity is incompletely sup-pressed by current immunosuppression and correlates with long-term graft loss. Recognizing NK cells as independent effectors of chronic rejection may open new avenues for bio-marker development and targeted therapy.

7. The Role of NK Cells in Promoting Immune Tolerance and Adaptation

While NK cells are widely recognized for their contribution to allograft injury, accumulating evidence highlights their func-tional duality and potential role in promoting immune toler-ance and long-term adaptation to the transplanted kidney. Far from being solely cytotoxic effectors, NK cells can exert immunoregulatory effects that may facilitate graft acceptance. Key mechanisms include the selective elimination of donor-derived APCs (particularly dendritic cells), which limits direct allorecognition by recipient T cells, as well as indirect modu-lation of the adaptive immune response through interactions with regulatory T cells (Tregs). These tolerogenic proper-ties position NK cells as active participants in the complex process of immune adaptation following transplantation. Understanding this regulatory arm of NK-cell biology is essential for identifying biomarkers of operational tolerance and for developing novel strategies aimed at achieving long-term graft survival with reduced immunosuppression.

One of the principal mechanisms by which NK cells may pro-mote immune tolerance is the selective elimination of donor-derived APCs, particularly dendritic cells, and activated alloreactive T lymphocytes. By killing donor dendritic cells, NK cells effectively limit direct allorecognition and dampen the initiation and amplification of the adaptive alloimmune response (Yu et al. 2006; Pontrelli et al. 2020).

Additionally, the CD56^bright NK-cell subset, which predomi-nates in secondary lymphoid organs and expresses high levels of the inhibitory receptor NKG2A, possesses signifi-cant immunoregulatory capacity. These cells can secrete anti-inflammatory cytokines such as IL-10 and transform-ing growth factor-β, thereby suppressing the proliferation and effector functions of conventional T cells (Pontrelli et al. 2020). Interestingly, in some transplant settings (notably liver transplantation), donor-derived CD56^bright NK cells con-stitute the dominant population infiltrating the graft early after transplantation, suggesting a potential role in local tolerance induction (Moroso et al. 2010).

A particularly important aspect of immune tolerance is the bidi-rectional crosstalk between NK cells and Tregs. Experimental models have demonstrated that adoptively transferred Tregs can prevent bone marrow rejection by suppressing alloreactive NK cells and modulating the expression of activating receptors on NK cells recognizing donor major histocompatibility com-plex (MHC) class I molecules (Mahr et al. 2017). Conversely, NK cells can influence Treg homeostasis and function through IFN-γ secretion and direct cell-cell interactions (Zagorulya et al. 2023; Ruan et al. 2024). This mutual regulation helps main-tain the balance between effector responses and immune suppression.

Such bidirectional communication between innate and adap-tive immune components constitutes an important link in the process of immune adaptation to the transplanted organ.

A critical process determining the functional competence of NK cells toward donor tissues is NK-cell education (also known as licensing). Mature NK cells acquire the ability to respond to cells lacking self-HLA class I molecules through the interaction of inhibitory KIR receptors with their cog-nate HLA ligands. In the transplant setting, reeducation of NK cells can occur in the presence of donor cells, lead-ing to a gradual reduction in alloreactivity. NK cells that become tolerant to donor antigens display decreased cyto-toxic responses and reduced production of pro-inflamma-tory cytokines.

However, not all NK-cell subsets undergo effective reedu-cation. Recent studies have shown that the NKG2A⁺KIR⁺ subpopulation exhibits particularly strong alloreactivity that persists after transplantation despite immunosuppression (Ruan et al. 2024). This suggests that while some NK-cell subsets successfully adapt and contribute to immune toler-ance, others remain resistant to reeducation and may serve as a source of chronic allograft injury.

Chronic CMV infection, the most common viral complication after kidney transplantation, drives the expansion of adaptive (memory-like) NK cells characterized by the NKG2C⁺CD57⁺ phenotype. These cells acquire features of immunological memory—a unique property within the innate immune system (Schlums et al. 2015). Adaptive NK cells play a crucial role in controlling CMV replication, and their higher frequency prior to transplantation is associated with a reduced incidence of CMV viremia post-transplant (Ataya et al. 2020). In the late post-transplant period, their accumulation primarily reflects long-term immune remodeling directed toward effective anti-viral defense (Hassan et al. 2022).

While their contribution to allograft tolerance remains contro-versial, some evidence suggests that CMV-driven expansion of adaptive NK cells may also promote the development of alloreactive clones capable of mediating chronic graft injury (Alari-Pahissa et al. 2025). Conversely, other studies indicate that these cells may support tolerance by eliminating acti-vated autologous T cells (Bielekova et al. 2006).

The shift observed in the late post-transplant period from a highly cytotoxic phenotype toward a more regulatory and adaptive phenotype may reflect the process of immune adap-tation to the transplanted kidney (Ruan et al. 2024). In the early phase after transplantation, highly cytotoxic NK cells predominate, favoring graft injury. Over time, under the influ-ence of chronic donor antigen exposure and immunosup-pressive therapy, NK cells undergo gradual reeducation, accompanied by increased expression of inhibitory receptors (NKG2A, T-cell immunoreceptor with Ig and ITIM domains (TIGIT)) and decreased cytotoxic potential (Qin et al. 2023). In conclusion, NK cells exhibit a dual role in kidney trans-plantation. On the one hand, they act as potent mediators of allograft injury through ADCC, missing-self recognition, and pro-inflammatory cytokine release. On the other hand, they can promote immune tolerance and adaptation by eliminat-ing donor dendritic cells, interacting with Tregs, and undergo-ing phenotypic reprograming (Yu et al. 2006; Pontrelli et al. 2020). The processes of NK-cell education, reeducation, and bidirectional crosstalk with adaptive immune components ultimately determine the balance between rejection and tolerance. Identification of NK-cell phenotypes associated with stable graft function may provide valuable biomarkers of immune adaptation and potential targets for therapeutic intervention (Ruan et al. 2024).

8. Conclusions and Future Perspectives

In conclusion, NK cells represent a heterogeneous and highly plastic population that plays a dual role in kidney trans-plantation. They contribute to allograft injury through ADCC, missing-self recognition via KIR-HLA mismatch, and secre-tion of cytotoxic mediators such as Ksp37. Simultaneously, they can promote immune tolerance by eliminating donor dendritic cells, interacting with Tregs, and undergoing pheno-typic reprograming toward regulatory and adaptive subsets. The balance between these opposing functions is shaped by NK-cell education, reeducation, CMV-driven adaptation, and the degree of immunosuppression. Recent high-dimensional studies have significantly advanced our understanding of NK-cell biology in transplantation and highlighted specific subsets as potential biomarkers.

Future research should focus on prospective validation of NK-cell signatures for early prediction of graft dysfunction, development of NK-targeted therapies, integration of NK-cell monitoring into routine post-transplant care, and systematic comparison of circulating vs. tissue-resident NK phenotypes together with the full spectrum of activating NCR, NKG2C/D and inhibitory receptors to refine biomarker panels and therapeutic strategies. Ultimately, a deeper understanding of NK-cell plastic-ity may pave the way for personalized immunosuppression and improved long-term kidney allograft survival.

9. Limitations

This narrative review has several limitations. Most data come from observational studies and experimental models with rela-tively small sample sizes. There is significant heterogeneity in immunosuppressive regimens and patient populations across studies, which limits direct comparisons. High-dimensional ana-lyzes are still performed in few centers, and prospective valida-tion of NK-cell-based biomarkers is lacking. Finally, the exact contribution of NK cells to operational tolerance in kidney trans-plantation remains incompletely understood due to the scarcity of tolerant patient cohorts.