Epstein-Barr Virus (EBV), a ubiquitous gamma herpesvirus, infects over 90% of the global population and establishes life-long latency, primarily in B cells, though it can also infect epithelial, T, and NK cells (1). In immunocompetent individuals, primary EBV infection is often asymptomatic or manifests as self-limiting infectious mononucleosis (IM), with EBV-induced B-cell proliferation effectively controlled by T and NK cells. However, in immunodeficient patients, impaired immune surveillance leads to uncontrolled EBV-driven lymphoproliferation, resulting in severe complications such as hemophagocytic lymphohistiocytosis (HLH), persistent viremia, and malignancies of B-, T-, or NK-cell origin (2, 3). Given the critical role of T and NK cells in controlling EBV infection and related diseases, molecules that enhance their function are of particular interest.

TNFRSF9 (CD137/4-1BB), a member of the tumor necrosis factor receptor (TNFR) superfamily, encodes a critical costimulatory molecule that promotes CD8+ T-cell proliferation, survival, and cytolytic activity (4). The human 4-1BB protein, a 255-amino acid type I transmembrane protein, consists of an N-terminal signal peptide, an extracellular domain with four cysteine-rich domains (CRDs), a transmembrane region, and a cytoplasmic signaling domain containing a TRAF-binding motif (5). The CRDs, characterized by intra-domain disulfide bonds, form elongated structures that facilitate ligand binding and receptor aggregation, with CRD2 and CRD3 forming the ligand-binding domain. TNFRSF9 is primarily expressed on activated T cells and its ligand, 4-1BBL (TNFSF9), a type II transmembrane protein of the TNF superfamily, is predominantly expressed on antigen-presenting cells (APCs) such as B cells, macrophages, and dendritic cells. Ligand binding induces receptor clustering and recruitment of TRAF adaptor proteins (TRAF1 and TRAF2), activating NF-κB, PI3K/AKT, and MAPK signaling pathways. Activation of TNFRSF9 enhances the production of IFN-γ and perforin, key mediators of CD8+ T cells cytolytic function (6, 7). The essential role of TNFRSF9 in CD8+ T-cell immunity is further underscored by studies in TNFRSF9 deficient mice, which exhibited impaired IFN-γ production and reduced cytolytic CD8+ T-cell effector function, highlighting its importance in immune responses (8).

Variants in TNFRSF9 cause immunodeficiency 109 with lymphoproliferation (IMD109, OMIM #620282), a rare autosomal recessive disorder characterized by immunodeficiency, recurrent infection, hypogammaglobulinemia, EBV viremia, and EBV-induced lymphoproliferative disorders or lymphoma (9). First described by Alosaimi, et al. in 2019, IMD109 is extremely rare, with only 9 reported patients to date (4, 9–11). Here, we present a case from a Chinese family featuring Burkitt lymphoma, recurrent respiratory infections, conjunctivitis, and EBV viremia, harboring a homozygous TNFRSF9 variant (c.359G>C, p.C120S). In vitro studies demonstrated that this variant impairs NF-κB and AKT signaling pathways, providing mechanistic insights into its pathogenic role. Our findings expand the genetic and clinical spectrum of TNFRSF9-related disease and include a comprehensive review of the clinical phenotypes and genotypes of all reported patients with TNFRSF9 variants.

EBV-specific immunity relies on robust cellular and humoral immune responses, with CD8+ cytotoxic T cells playing a critical role in controlling EBV infection. Patients with genetic variants affecting T-cell development and function are highly susceptible to chronic EBV infection (14). Growing evidence suggests that genetic defects impairing immune surveillance pathways increase susceptibility to EBV-associated lymphoproliferative disease (EBV⁺ LPD) (4). To date, over 20 monogenic disorders, including variants in CD27, CD70, SH2D1A, ITK, MAGT1, PRKCD, PIK3CD, CORO1A, RASGRP1 and CTPS1, have been linked to immune dysregulation and susceptibility to EBV-driven B cell lymphoproliferative disorders (15–17). These defects impair virus-specific T-cell responses, highlighting critical pathways required for immune control of EBV. Research on these patients has offered valuable insights into the mechanisms of immune surveillance against EBV and the pathogenesis of EBV-driven malignancies.

In this study, we report a case presenting with EBV viremia, recurrent respiratory infections, and Burkitt lymphoma. By next-generation sequencing, a novel homozygous missense variant (c.359G>C, p.C120S) in the TNFRSF9 gene was identified. Functional studies revealed that the p.C120S variant reduced TNFRSF9 expression at both mRNA and protein levels, impaired AKT and NF-κB signaling pathways. These findings underscore the critical role of TNFRSF9 in maintaining immune homeostasis and controlling EBV infection, and highlight the importance of molecular diagnostics in guiding targeted therapies for Inborn Errors of Immunity.

A review of the literature revealed only nine reported patients of TNFRSF9 deficiency, with significant clinical heterogeneity even among patients carrying the same variant ( Table 2 ). For example, Rodriguez et al. described two siblings from a consanguineous Pakistani family with a homozygous loss-of-function (LOF) variant c.170del in TNFRSF9 (11). The brother had severe symptoms, including persistent fever, hepatosplenomegaly, recurrent lymphadenopathies, and high EBV load, culminating in a fatal acute episode of HLH at age 14. In contrast, his young sister remained asymptomatic despite persistent EBV viremia, except for abnormalities in memory B cell proportions. These findings suggest incomplete clinical penetrance or delayed disease onset in some patients with TNFRSF9 variants. In our study, the patient had abdominal Burkitt’s lymphoma in at 4 years, followed by recurrent respiratory infections, ear infections, hepatosplenomegaly, lymphadenopathy, and chronic EBV infection. His elder brother had severer symptoms and succumbed to nasopharyngeal carcinoma (NPC), which has not been reported in other patients. Taken together, the clinical manifestation, severity and prognosis can be heterogeneous in patients with TNFRSF9 variants.

Studies in TNFRSF9-deficient mice have demonstrated impaired T-cell survival, proliferation, and cytotoxicity, as well as defective NK-cell and cytotoxic T-lymphocyte (CTL) function (8, 12, 18). TNFRSF9 is also essential for B-cell activation, proliferation, and class switch recombination (CSR) through its interaction with CD137L in germinal centers (12, 19, 20). In patients, EBV-specific CTL cytotoxicity was reduced, underscoring the role of TNFRSF9 in controlling EBV infection and preventing lymphomagenesis (12). CD8+ T cells from affected patients showed impaired activation and reduced IFN-γ production. IFN-γ is essential for macrophage and NK-cell activation, antiviral and antitumor immunity, B-cell antibody production, Th1 differentiation, and memory T-cell proliferation. Its reduction likely contributes to weakened immune responses, increasing susceptibility to infections and impaired tumor surveillance. Additionally, a significant decrease in peripheral blood CD27+ memory B cells suggests an immunodeficient phenotype. In our patient, peripheral blood analysis revealed a significant decrease in CD27+ memory B cells was observed, and diminished IFNγ secretion by CD8+ T cells. A literature review of reported cases confirmed that all patients exhibited EBV viremia, with seven developing EBV-associated lymphoproliferative disorders and four diagnosed with lymphoma (4, 9–11). In our study, numberous EBV+ B cells were present in PBMC, in addition, we also observed a small number of EBV+ T and NK cells, indicating that EBV also infected these cells, though contamination could not be completely ruled out during the sorting of T cells or NK cells. Unfortunately, we did not test EBV by PCR for our patient after lymphocyte sorting during this period (between Oct 2024-Jan 2025), and we are unable to do this anymore as the patient has undergone hematopoietic stem cell transplantation. However, it is more likely that T and NK cells were infected by EBV, particularly in immunocompromised individuals, which had previously been in a patient with TNFRSF9 deficiency (4). These observations highlight TNFRSF9 deficiency as a cause of immunodeficiency with a predisposition to lymphomagenesis. TNFRSF9 is associated with various tumors, and somatic variants have been identified in hematopoietic, lymphoid, lung, and colorectal cancers (21–23). The COSMIC database lists 409 somatic variants of TNFRSF9 (https://cancer.sanger.ac.uk/cosmic/gene/analysis?ln=TNFRSF9 ), included in the C120S variant found in non-small-cell lung cancer (NSCLC), underscoring its relevance in tumorigenesis (24).

CD8+ T-cell function requires signals for survival, cell cycle progression, biomass formation, and differentiation into effector and memory cells. TNFRSF9 is known to activate NF-κB signaling for cytokine induction and CD8+ T cell survival (25, 26). It triggers TRAF-dependent NF-κB activation to upregulate anti-apoptotic proteins, including Bcl-2 and Bcl-XL, while also promoting cell cycle progression via the PI3K and MEK-1/2 pathways (26). Additionally, NF-κB-dependent CD137 signaling regulates T-cell proliferation and differentiation (27). TNFRSF9 activation has been shown to enhance CD8+ T cell proliferation by the TCF1/β-catenin axis via the PI3K/AKT/ERK pathway (28). In this study, we observed significantly reduced phosphorylated AKT levels and impaired NF-κB signaling in vitro, suggesting that the p.C120S variant disrupts these pathways, leading to abnormal CD8+T cell function. The observed defects in AKT and NF-κB signaling provide mechanistic insights into TNFRSF9 deficiency, implicating these pathways in immune dysregulation, impaired antiviral responses, and increased lymphoma susceptibility.

TNFRSF9 is a promising target for immunotherapy in both autoimmune diseases and malignancies. It functions as an immune suppressor by enhancing Treg expansion and mitigating T_H17 -mediated autoimmunity, while also acting as a potent immune stimulator to enhance T-cell and NK-cell cytotoxicity and tumor infiltration (29, 30). Preclinical studies in mouse models have shown that anti-TNFRSF9 agonist antibodies synergized with other therapies (31). Clinical trials of CD137 agonistic monoclonal antibodies and bispecific constructs are underway, demonstrating promising safety and efficacy (31, 32). Four patients with lymphoma responded well to chemotherapy and rituximab, maintaining stable conditions under IVIG and antibiotic prophylaxis (9). However, our patient experienced recurrent infections post-chemotherapy, underscoring the need for targeted immunotherapies.

In conclusion, this study identified a novel homozygous TNFRSF9 variant (p.C120S) in a patient with EBV viremia, recurrent respiratory infections, and Burkitt lymphoma. Functional analyses revealed reduced TNFRSF9 expression, impaired AKT and NF-κB signaling, and diminished CD8+ T-cell and memory B-cell function, highlighting the critical role of TNFRSF9 in immune regulation and EBV control. These findings expand the genetic and clinical spectrum of TNFRSF9-related immunodeficiency and provide a foundation for developing targeted therapies. Further research is needed to elucidate the mechanisms underlying clinical heterogeneity and to optimize treatment strategies for affected patients.