CD161-lectin-like transcript 1 axis beyond programmed cell death protein 1 in hepatitis B virus-associated hepatocellular carcinoma

Abstract

Hepatitis B virus (HBV)-associated hepatocellular carcinoma develops in a chronically inflamed, tolerogenic liver microenvironment. In this setting, T-cell dysfunction limits the efficacy of programmed death-1 (PD-1) blockade. Emerging evidence from Liu et al published in the World Journal of Gastrointestinal Oncology suggests that CD161 may represent an additional inhibitory axis within exhausted CD8+ T cells that is not fully captured by the PD-1/programmed death ligand-1 pathway. In their study, CD161+CD8+ T cells were enriched in tumor tissue, and CD161 expression persisted even as PD-1+ compartments contracted after PD-1 inhibition, suggesting that CD161-mediated restraint may remain active under therapeutic pressure. Mechanistically, sustained engagement of the CD161-lectin-like transcript 1 interaction may impair T-cell effector function and contribute to adaptive immune resistance in the HBV-infected liver. These findings highlight the importance of protein-level assessment of CD161 and parallel evaluation with PD-1 to accurately characterize immune inhibition. Key knowledge gaps include the cellular source of lectin-like transcript 1 within the tumor microenvironment, the influence of ongoing HBV virological activity on CD161+ T-cell subsets, and the identification of clinically meaningful progression endpoints. In this editorial, we place these findings in mechanistic and translational context, highlighting the potential role of the CD161-lectin-like transcript 1 axis as a complementary pathway of immune resistance beyond PD-1 blockade. Collectively, the current evidence supports a cautious, hypothesis-driven translational strategy to evaluate combined modulation of the CD161 and PD-1 axes in carefully selected patients with HBV-associated hepatocellular carcinoma.

Key Words: CD161; Hepatocellular carcinoma; Hepatitis B virus; Immune resistance; T-cell exhaustion; Programmed death-1/programmed death ligand-1 resistance

Core Tip: In hepatitis B virus-associated hepatocellular carcinoma, CD161+CD8+ T cells are enriched in tumor tissue and may persist despite programmed death-1 blockade, suggesting that immune inhibition is not fully explained by the programmed death-1/programmed death ligand-1 axis alone. These observations raise the possibility that the CD161-lectin-like transcript 1 pathway represents a complementary mechanism of adaptive immune resistance. This editorial highlights the importance of protein-level CD161 assessment and discusses the translational relevance of this axis as a potential biomarker and therapeutic target for future investigation.

This editorial refers to “CD161+CD8+ T cells in patients with hepatitis B virus-associated hepatocellular carcinoma” by Liu et al, 2026; https://doi.org/10.4251/wjgo.v18.i4.114547.

---

INTRODUCTION

Hepatitis B virus (HBV)-associated hepatocellular carcinoma (HCC) often presents with persistent T cell dysfunction that limits responses to conventional immune-checkpoint blockade therapies[1]. Emerging evidence suggests that CD161 may represent a distinct inhibitory axis within exhausted CD8+ T cells that is not fully captured by the programmed cell death receptor 1 (PD-1)/programmed death ligand-1 (PD-L1) pathway. This observation characterizes immune dysregulation in HBV-associated HCC as a multilayered regulatory state rather than a single checkpoint defect[2]. These findings provide a rationale for hypothesis-driven studies evaluating CD161 alongside PD-1 in HBV-associated HCC[2].

The recognition of the CD161 axis has broadened the conventional PD-1/PD-L1-centered view of immune dysregulation in HBV-associated HCC[3,4]. A clinical paradox persists: Although PD-1/PD-L1 inhibition is a mainstay of systemic therapy, objective response rates are only 15%-20%[5]. This suggests that a PD-1-centric approach oversimplifies the complex and tolerogenic hepatic microenvironment. These observations highlight the need for more sophisticated biomarkers beyond PD-1.

A study by Liu et al[2] published in the World Journal of Gastrointestinal Oncology revealed a clinically interpretable immunologic landscape of CD161 in HBV-associated HCC. In this context, CD161+CD8+ T-cell populations become prominent, and CD161 protein levels are elevated in tumor tissue compared with those in peritumoral tissue and chronic hepatitis controls, supporting an association with the tumor immune microenvironment. Following PD-1 blockade therapy, circulating PD-1+ populations decline markedly, whereas CD161+ populations remain relatively stable, supporting the possibility that the inhibitory effect of CD161 is not fully reversed by PD-1 blockade.

Notably, T cells and natural killer cells with high PD-1 expression often show low Killer cell lectin-like receptor subfamily B member 1 (KLRB1) expression, the gene encoding CD161, consistent with an exhausted state[6]. In addition, the correlation between decreased KLRB1 mRNA expression and the persistence of CD161 protein expression suggests that CD161 expression is not regulated solely at the transcriptional level. This may be influenced by factors such as T-cell receptor (TCR) stimulation, microenvironmental signals (e.g., transforming growth factor beta 1), and the high stability of the CD161 protein[7]. This may partly explain the sustained CD161 expression observed after PD-1 blockade and further support its potential role as an additional immune regulatory axis.

In clinical settings where primary and secondary resistance to PD-1 inhibitors remains common, these findings delineate a specific role for CD161[2]. Rather than functioning merely as a secondary marker, CD161 may represent a persistent immune regulatory layer that could help explain why PD-1 blockade alone is often insufficient in HBV-associated HCC.

MECHANISTIC CONTEXT: HOW THE CD161-LECTIN-LIKE TRANSCRIPT 1 AXIS MIGHT SHAPE T-CELL FUNCTION

Understanding the clinical significance of CD161 in HBV-associated HCC requires clarification of its mechanistic role in the tumor microenvironment (TME). T cells mediate antitumor immunity via CD8+ cytotoxic and CD4+ helper subsets, which are coordinated through TCRs. However, their activity is strictly modulated by a hierarchy of surface molecules known as immune checkpoints. Among these molecules, CD161 (encoded by the KLRB1 gene) has emerged as a regulator of T-cell functional restraint. Originally identified as a marker of natural killer cells and Th17-like lineages, CD161 is being increasingly recognized as an inhibitory receptor when it is expressed on tumor-infiltrating lymphocytes (TILs). Its ligand, lectin-like transcript 1 (LLT1), has been reported on antigen-presenting cells and malignant cells within the HCC microenvironment, although its dominant cellular source in HBV-associated HCC remains insufficiently defined[8,9].

Sustained CD161-LLT1 engagement acts as a “molecular brake” on anti-tumor immunity. Mechanistically, this interaction triggers inhibitory signaling cascades that interfere with TCR-proximal events, leading to reduced cytotoxicity and a skewed effector profile. In HCC, CD161+ T-cell states are associated with reduced production of key antitumor cytokines such as interferon-gamma and tumor necrosis factor-alpha and, in some contexts, increased expression of proinflammatory mediators such as interleukin-17[10,11]. Unlike many activation-induced receptors, CD161 signaling may persist, reinforcing chronic exhaustion.

In the specific context of the HBV-infected liver, this axis has increased clinical importance. The liver is a unique immunological site characterized by a high chronic antigen load and a persistent inflammatory milieu. These conditions may favor sustained LLT1 expression on myeloid and tumor cells, thereby maintaining CD161-mediated restraint[12]. This pathway may provide inhibitory signaling that is not fully captured by the PD-1/PD-L1 axis. Consequently, even when classical checkpoints are therapeutically inhibited, the CD161-LLT1 axis may provide an additional layer of immune restraint that helps sustain T-cell dysfunction and limits the efficacy of current monotherapies[13].

BIOMARKER STRATEGY: MEASURE WHAT MATTERS, WHERE IT MATTERS

To render CD161 clinically informative, protein-level evaluation should emphasize functional inhibition rather than mere presence. The key requirement is to capture surface CD161 expression, where the receptor actively mediates immune restraint, and to interpret it in parallel with PD-1 signaling pressure. Transcriptional readouts such as KLRB1 are insufficient to represent this functional checkpoint layer[7]. Several practical considerations should guide biomarker development.

Target analyte and functional readouts

CD161 should be assessed primarily at the protein level, in tumor tissue by immunohistochemistry or multiplex immunofluorescence, and in peripheral blood by flow cytometry. This is not merely technical: KLRB1 transcripts reflect cellular states, whereas membrane CD161 denotes the operative inhibitory receptor. Across cancers, including HCC, CD161 expression is consistently associated with immune checkpoint programs and immune-infiltrated tumor phenotypes[13-15]. Recent multiplex platforms further allow CD161 and PD-1 to be quantified in the same tissue section, maintaining the spatial context within the hepatic microenvironment[16].

Compartmentalization and longitudinal assessment

Tumor tissue remains essential for determining the intratumoral CD161+ TIL density and distribution. These parameters describe where functional inhibition is established at baseline. Longitudinal blood sampling of CD161+CD8+ populations then enables monitoring of checkpoint persistence or immune recovery during therapy. Because CD161 dynamics may not mirror PD-1 trajectories, paired tissue and blood analysis can provide a more accurate picture of immune restraint[14,15].

Cutpoint definition and analytical validity

For early translational studies, enrichment using higher CD161 expression levels within TILs is biologically justified, whereas median-based stratification remains practical for circulating compartments[7,13]. Analytical validity depends on standardized preanalytical handling, antibody validation, staining protocols, and scoring systems, particularly in multicenter studies[13,16]. The increasing availability of spatially resolved multiplex platforms enables the simultaneous evaluation of CD161, PD-1, and T-cell lineage markers within the same tissue section, preserving spatial relationships within the tumor immune microenvironment[16]. Because tumor tissue is often limited and because HCC lesions are spatially heterogeneous, multiplex assays may also improve feasibility and reduce sampling bias in small biopsy specimens[14-16]. Complementary peripheral blood monitoring of CD161+CD8+ populations may provide a minimally invasive strategy for longitudinal immune assessment when repeated tumor sampling is not feasible[14,15].

Importantly, CD161 is now recognized as part of the core exhaustion program of tumor-infiltrating T cells. Single-cell analyses revealed co-expression of CD161 with PD-1 and other inhibitory receptors in dysfunctional T-cell states in HCC and across malignancies[14,15]. These CD161+PD-1+ cells display greater functional impairment than single-positive populations do, and CD161 expression often persists during PD-1 blockade, which is consistent with adaptive resistance[13,15]. Accordingly, PD-1 expression alone may incompletely reflect immune competence, supporting combined assessment with CD161. The proposed CD161-LLT1 inhibitory framework in HBV-associated HCC is summarized in Figure 1, which integrates its mechanistic role, relationship to PD-1 resistance, and potential biomarker applications.

Figure 1 Proposed inhibitory role of the CD161-lectin-like transcript 1 axis in hepatitis B virus-associated hepatocellular carcinoma. The CD161-lectin-like transcript 1 axis may maintain an inhibitory layer that persists despite programmed death-1/programmed death ligand-1 blockade and suppresses CD8+ T-cell effector function within the hepatitis B virus-associated hepatocellular carcinoma microenvironment. Tissue-based protein assessment and blood-based immune profiling may support the detection and longitudinal monitoring of immune signatures linked to this axis during treatment. Future studies should evaluate CD161-lectin-like transcript 1 as a biomarker and therapeutic target in carefully designed hypothesis-driven translational settings. HBV: Hepatitis B virus; HCC: Hepatocellular carcinoma; LLT1: Lectin-like transcript 1; PD-1: Programmed death-1; PD-L1: Programmed death ligand-1.

KNOWLEDGE GAPS AND OUTSTANDING QUESTIONS

Cellular sources and spatial organization of LLT1 in HBV-associated HCC

Chronic HBV infection causes profound restructuring of the liver’s transcriptional and inflammatory landscape through reprogramming of the host cell genome, thereby creating a TME conducive to immune evasion and resistance[17]. Within this complex ecosystem, the CD161-LLT1 axis may represent an additional mechanism of immune suppression beyond PD-1/PD-L1[2]. This raises the possibility that ligand availability, particularly LLT1 expression within the TME, may be a critical determinant of persistent immune restraint in HBV-associated HCC[8,18]. However, a significant knowledge gap persists, as we have yet to accurately identify the primary cellular source of LLT1 in the TME. Mapping the spatiotemporal landscape of LLT1 is crucial both for understanding compensatory immune evasion and for guiding prognostic biomarker development[9,13,19].

HBV virological activity and the functional heterogeneity of CD161+ T-cell subsets

A major unresolved question is whether CD161-associated immune programs in HBV-associated HCC are partly shaped by ongoing viral activity. CD161 is expressed on heterogeneous CD8+ T-cell subsets, including mucosal-associated invariant T (MAIT) and non-MAIT memory populations, with distinct transcriptional and functional properties rather than a uniform inhibitory lineage[18]. Therefore, interpreting CD161 expression as a stable exhaustion marker requires careful phenotypic resolution. In chronic HBV infection, expanded non-MAIT CD161+CD8+ T cells correlate with HBV DNA levels and exhibit a functional trade-off: Attenuated antiviral cytokine production alongside heightened inflammatory potential[2,18]. These findings suggest a testable hypothesis in HBV-associated HCC. Persistent antigen loading, active covalently closed circular DNA transcription, or hepatitis flares may not merely increase CD161 expression but instead reshape specific CD161+CD8+ subsets toward a persistent state with altered effector function[2,18]. Under this framework, viral activity may tune the functional balance of CD161+ compartments, particularly within the non-MAIT CD8+ pool, thereby modulating whether CD161 engagement acts as a residual regulatory axis even when PD-1 signaling is therapeutically targeted[8,20].

Clinically relevant endpoints for CD161-based stratification

CD161 levels should be directly linked to clinically meaningful progression endpoints in patients with HBV-associated HCC. It remains uncertain which signal is most informative: Intratumoral CD161 expression on CD8+ TILs, the frequency of circulating CD161+CD8+ T cells, or persistence under PD-1 pressure[2]. Particular attention should be given to outcomes such as microvascular invasion and early extrahepatic spread. Spatial assays may help determine whether CD161+ T cells cluster at invasive fronts or vascular niches, whereas serial blood sampling may identify a “noncontracting” CD161 compartment that precedes dissemination[21,22]. These ideas remain speculative, but such endpoints may help translate mechanistic observations into clinically meaningful risk stratification.

TRANSLATIONAL PERSPECTIVES AND FUTURE DIRECTIONS

Although definitive mechanistic evidence is lacking, the current data support a testable translational hypothesis. The critical question for dual CD161 and PD-1 checkpoint targeting is not whether it should be applied broadly but how it can be rationally tested in carefully selected HBV-associated HCC populations. On a hypothesis-generating basis, candidate patients could include individuals with HBV-associated HCC exhibiting high intratumoral CD161 protein expression, which is preferentially enriched in exhausted CD8+ T-cell phenotypes. This strategy may be most relevant in patients with evidence of T-cell exhaustion and, where supported by available data, invasive molecular features such as reduced E-cadherin expression and elevated matrix metalloproteinase levels[13,23].

While PD-1 blockade is a standard immunotherapy in HCC, its efficacy is often limited by immune resistance. The CD161-LLT1 axis represents a complementary inhibitory pathway that suppresses CD8+ T-cell function. Therefore, combining PD-1 inhibition with modulation of the CD161-LLT1 axis represents a biologically plausible strategy to test whether broader T-cell reinvigoration can be achieved[8,24]. Such combinations are best tested in treatment settings where T-cell reinvigoration is still biologically meaningful, for example, in earlier lines of systemic therapy, and only when background liver inflammation and HBV replication are adequately controlled[6].

In addition to standard response criteria, translational studies should incorporate immune monitoring, including ex vivo CD8+ functional assays, cytokine profiling, partial epithelial-mesenchymal transition markers, circulating invasive signatures, and early metastatic events[18,25]. Safety assessments should focus on hepatotoxicity, HBV reactivation, and immune-related adverse events associated with combination immunotherapy[9,26]. Overall, this framework positions CD161-PD-1 cotargeting as a rational translational hypothesis that merits careful, stepwise prospective evaluation rather than premature implementation in routine practice.

Recent translational progress further supports the use of the CD161-LLT1 axis as a druggable immune checkpoint. Beyond preclinical studies of CD161 blockade, the investigational antibody IMT-009 has entered early-phase clinical evaluation in patients with advanced solid tumors and lymphomas[27]. In parallel, ZM008, a first-in-class anti-LLT1 monoclonal antibody, has advanced to first-in-human clinical development, including assessment as a monotherapy and in combination with anti-PD-1 therapy[28]. These early-phase programs do not yet establish clinical efficacy in HBV-associated HCC, but they indicate that the CD161-LLT1 pathway has moved from a purely mechanistic concept toward a therapeutically actionable target. This direction is particularly relevant in HBV-associated HCC, where chronic antigen exposure and liver-specific immune tolerance may sustain residual inhibitory signaling. In this context, the early clinical development of CD161/LLT1-directed agents strengthens the rationale for hypothesis-driven combination studies with PD-1 blockade.

CONCLUSION

Evidence supports CD161-LLT1 as a functionally relevant inhibitory layer in HBV-associated HCC, likely contributing to incomplete T-cell reinvigoration under PD-1 monotherapy. Key uncertainties include LLT1 sources/spatial distribution, the heterogeneity of CD161+ subsets, and interactions with ongoing HBV virological activity. Future studies should incorporate patient enrichment, parallel immune monitoring, and clinically meaningful progression endpoints. Until mechanistic and prospective validation is available, CD161-PD-1 cotargeting should remain a hypothesis-driven strategy.