Highlights of 2025: advancements in natural killer cell biology

Recent findings uncover new mechanisms governing NK cell activation, inhibition, metabolism, and memory-like responses, alongside a newly described role in systemic immune regulation via adipose tissue. Officially recognized as a distinct cell type in 1975,1, 2 natural killer (NK) cells are lymphocytic components of the innate immune system, providing a rapid elimination response against altered cells. Their cytotoxic activity is regulated through a sophisticated balance between activating and inhibitory receptors, conferring high specificity and preventing the destruction of healthy cells. Due in part to this key biological feature, emerging NK cell-based therapeutic strategies have been proposed over the past decades, aiming to provide effective treatment with potentially reduced side effects for diseases such as cancer and other conditions.3-5 However, the biological limitations of the optimal natural killer cell responses are still being investigated to inform the development of effective immunotherapeutics. Over the past decades, various ligand-receptor interactions have been described as essential for activating or inhibiting NK cell effector functions.6 Moreover, following activation, NK cells may act differently depending on the stimulated axis, either directly killing the target cell through perforin and granzyme release or orchestrating an inflammatory response through cytokine and chemokine secretion, thereby promoting the recruitment and activation of other immune cells such as dendritic and effector T cells.7 Therefore, depending on the ligand-receptor axis stimulated, these cells play a crucial role not only in immunosurveillance but also in immunomodulation. However, despite the well-established responses triggered by several cytokines, chemokines, and other proteins in this innate lymphocyte population, major molecular mechanisms underlying these processes remain unclear. Addressing this knowledge gap, recent studies have provided critical insights into NK cell biology, significantly advancing the field and opening new avenues for translational applications. In this context, recent studies identifying previously uncharacterized molecular features of NK cells are providing a foundation for strategies aimed at improving their effector function. In this article, we highlight five key findings in the field during 2025, focusing on NK cell responses to external stimuli and their potential applications in NK cell–based immunotherapies. Regarding known activating molecules, Interleukin-15 (IL-15) is a critical cytokine for NK cell development, survival, and activation. Its interaction with the IL-15 receptor (composed of IL-2Rβ (CD122) and common γc subunit (CD132)) triggers the acquisition of a cytotoxic phenotype through the JAK1/3 and STAT5A/B signaling pathway.8 In addition, intrinsic negative regulation of this pathway following activation represents an important limitation for NK cell harnessing. Cytokine-inducible SH2-containing protein (CIS, encoded by Cish) is a well-known negative regulator of the IL-15–IL-15R axis.5 However, as a scaffold protein, CIS does not directly regulate gene targets but instead contributes to the degradation of IL-15R.9 Nevertheless, the intracellular protein complexes associated with this checkpoint inhibitor remained unclear. In 2025, Nikolic and collaborators reported that CIS is primarily a substrate of the Cullin-RING ligase 5 (CRL5) complex, which is ultimately responsible for the downregulation of JAK proteins through ubiquitination. They identified two essential enzymes within the CRL5 complex, Ariadne RBR E3 Ubiquitin Protein Ligase 2 (ARIH2) and Ubiquitin-conjugating enzyme E2 F (UBE2F). Deletion of UBE2F was shown to enhance NK cell effector activity upon IL-15 stimulation in vivo,10 opening avenues for targeting these proteins to achieve more efficient NK cell responses. Similarly, IL-12 is another essential activating cytokine for NK cells, well known to promote their cytotoxicity and cytokine production via TYK2/JAK2 and STAT4 signaling upon binding to its receptor (IL-12Rβ1/IL-12Rβ2). In 2025, Li and collaborators reported an upregulation of the serine metabolic pathway in NK cells after IL-12 and IL-15 stimulation.11 This finding is consistent with increased proliferation and regulation of fatty acid pathways, as serine is crucial for membrane lipid synthesis. Comparing mouse and human NK cells, they observed that human NK cells exhibit low expression of de novo serine synthesis enzymes as well as reduced cytotoxicity when cultured in low-serine media, demonstrating the importance of this amino acid for triggering effector activity. Moreover, inhibition of the serine metabolic network reduced Interferon-gamma (IFN-γ) production, revealing a crucial role for this pathway in NK cell cytotoxic function. Accordingly, the study elucidated the role of serine as an important mediator of fatty acid metabolism, sustained proliferation, and cytokine production in cytokine-activated NK cells, reinforcing the tight regulation of effector function even after activation. Building on Li et al.'s findings on fatty acids, proliferation, and NK cell cytotoxicity, Krapić and colleagues further highlighted the role of NK cells in adipose tissue regulation in 2025.12 This study showed that during the peak of viral infections, NK cells are the most abundant immune cell type and are directly associated with increased IFN-γ levels and lipid loss in visceral adipose tissue. The study reported that production and release of this cytokine are mediated by activation of the NKG2D receptor on NK cells, which induces lipid release from adipocytes into the bloodstream. Finally, B and T cells cultured in the presence of these lipids exhibited upregulation of activating receptors,12 suggesting a mechanism of systemic immune activation promoted by NK cells through lipid regulation, highlighting this macromolecule as a relevant factor in controlling immune responses. Regarding well-established inhibitory pathways, canonical transforming growth factor-beta (TGF-β) signaling is a central regulatory axis in NK cell suppression. Upon agonist binding (e.g. TGF-β1), activation of the receptors TGFβRII and TGFβRI impairs NK cell cytotoxic activity through the downregulation of key receptors such as NKG2D and CD16, and effector molecules such as granzymes and cytokines.13 To mediate the TGF-β–induced response in NK cells, SMAD2, SMAD3 and SMAD4 are required to promote genetic alterations in these cells; however, the individual roles of each macromolecule remain unknown to date. In 2025, Rea and collaborators opened avenues in this field, reporting the crucial function of SMAD4 in the promotion of NK cell modification.14 The group demonstrated that NK cells from individuals with SMAD4 haploinsufficiency (SMAD4+/−) presented a less pronounced decrease in NKG2D and granzyme B levels, as well as in the number of activated NK cells, after 6 days of TGF-β exposure. At the same time, SMAD4+/− cells exposed to TGF-β presented higher cytotoxicity against K562 leukemic cells compared to their normal genetic counterparts (SMAD4+/+). Moreover, the same pattern observed in SMAD4+/− cells was preserved in SMAD4KO human NK cells, while their antitumour capacity was enhanced when tested in 3D in vitro models, as well as in vivo xenograft models of colorectal and breast cancers. Finally, the group evaluated the infiltration and migration capacity of these cells, showing that the knockout of SMAD4 promoted deeper infiltration and higher killing capacity in 3D in vitro tumor models in the presence of TGF-β. Therefore, the study determined that the SMAD4 protein complex can be highlighted as a promising target not only to decrease the inhibitory actions of this signaling pathway,13 but also to enhance the antitumour capacity of NK cells. Besides being classical components of the innate immune system, NK cells can exhibit adaptive features after specific activation, such as clonal expansion and antigen-memory generation, mainly in response to viral infections.15 However, the mechanisms behind the innate-to-adaptive features in NK cells are not completely understood. Addressing this gap, in 2025 Mujal and collaborators highlighted the role of the spleen in mediating crucial signaling pathways required for NK cell programming.16 The study showed that, after murine cytomegalovirus (mCMV) infection, the mouse spleen exhibited compartmentalized activation and expansion of virus-specific NK cells, which presented differential expression of receptors associated with cellular growth and higher proliferative rates. One key feature of the spleen microenvironment is the high presence of tumor necrosis factor (TNF), produced and released by splenic myeloid cells, which is required to trigger NK cell clonal expansion through tumor necrosis factor receptor 2 (TNFR2) signaling. Moreover, the mediation of NF-κB signaling after TNFR2 activation also appeared as an essential pathway to trigger not only expansion but also the cytotoxic phenotype of NK cells. In summary, these recent studies represent important advancements in the understanding of NK cell biology (Figure 1), providing insights that might be applied to the treatment of human diseases using NK cell–based therapies. They reinforced exclusive features of NK cells, uniquely positioning them between the innate and adaptive immune systems. They also highlight the importance of a deep understanding of network proteins that are required to provide responses to external stimuli, clarifying their roles not only in NK cell activation and inhibition, but also in determining the resulting effector function performed by this innate lymphocyte population in different disease settings. Funding for the Guimaraes's Laboratory was partially provided by the Medical Research Future Fund, Australia (with the support of the Queensland Children's Hospital Foundation, Microba Life Sciences, Richie's Rainbow Foundation, Translational Research Institute and The University of Queensland, Australia) under award number 2019485; and funding from the Cooper Rice-Brading Foundation, Australia, The Tie Dye Project, Bricks & Smiles, The Kids Cancer Project, Australia, Tour de Cure, the PA Research Foundation, the National Breast Cancer Foundation (award number 2023/IIRS0063). B.A.L. was supported by a UQ HDR and A*STAR ARAP Scholarship. The authors thank Jessica Grant for assistance with editing and grammar correction of this manuscript. The content is solely the responsibility of the authors and does not necessarily represent the official views of the organizations and funding agencies. Beatriz Aires-Lopes: Conceptualization; writing – original draft. Fernando Souza-Fonseca-Guimaraes: Conceptualization; writing – review and editing; supervision. F.S.F.G is a Board Member of Cure Cancer Australia Foundation and a member of the Scientific Advisory Committee of ANZSA. Sanofi TSH, Microba Life Sciences, and Cartherics sponsor research in the laboratory of F.S.F.G. Authors have no commercial, proprietary, or financial interest in this study.