How Low Oxygen Helps Neuroblastoma Turn Killer T Cells Off

Neuroblastoma is the most common solid tumor found outside the brain in children, arising from nerve crest cells during the development of the sympathetic nervous system and frequently taking root in the adrenal gland or sympathetic ganglia. It accounts for nearly 15 percent of childhood cancer deaths, and despite intensive multimodal treatment including surgery, chemotherapy, bone marrow ablation with stem cell support, and radiotherapy, the outlook for high-risk patients remains stubbornly poor. Now a new laboratory study from researchers at the 960th Hospital of PLA Joint Logistics Support Force in Jinan, China, published in Frontiers in Pediatrics, offers a detailed mechanistic explanation for one of the disease’s most frustrating clinical puzzles: why immune checkpoint inhibitors, which have transformed treatment of many adult cancers, have largely failed in neuroblastoma.

The research, led by Yujin Chao and Guili Zheng, focuses on the hypoxic tumor microenvironment, the oxygen-starved tissue that develops inside fast-growing solid tumors when abnormal blood vessel formation fails to keep pace with tumor expansion. Under normal oxygen conditions, the transcription factor HIF-1α is hydroxylated by prolyl hydroxylases and directed to proteasomal degradation, keeping its activity in check. When oxygen levels drop, however, HIF-1α is stabilized, translocates to the nucleus, and activates a broad program of downstream genes. Among the most consequential of these targets is PD-L1, the ligand that binds the PD-1 receptor on T cells and dampens their activation. Earlier work by Noman and colleagues established that the PD-L1 gene promoter contains a functional hypoxia response element, and that HIF-1α can bind directly to this element to drive PD-L1 transcription, a mechanism now documented across many solid tumors.

What remained unclear until now was whether this same axis operates in neuroblastoma and, critically, whether it is functionally responsible for driving CD8+ T-cell dysfunction in this disease. Prior studies had shown that hypoxia induces HIF-1α-dependent PD-L1 upregulation in neuroblastoma cell lines, but the downstream consequences for T-cell function had not been systematically tested. The new study set out to close that gap using a combination of gene silencing, pharmacological inhibition, chromatin immunoprecipitation, dual-luciferase reporter assays, and co-culture of tumor cells with primary human CD8+ T cells.

The team worked with three human neuroblastoma cell lines: SK-N-SH, SK-N-BE(2), which carries MYCN amplification, and SK-N-AS, which does not. Cells were exposed to 1 percent oxygen in a tri-gas incubator for periods ranging from 6 to 48 hours. Western blot analysis revealed that HIF-1α protein, virtually undetectable under normoxic conditions, rose rapidly under hypoxia, reaching 6.56 times baseline levels at 24 hours before declining slightly at 48 hours. PD-L1 protein climbed in parallel, increasing significantly from 12 hours onward and peaking at 9.44-fold above normoxic levels by 48 hours. Quantitative PCR confirmed that PD-L1 mRNA rose to 3.42-fold and 5.16-fold at 24 and 48 hours respectively, indicating transcriptional rather than merely post-translational regulation. Flow cytometry showed that the percentage of cells displaying PD-L1 on their surface jumped from 13.69 percent under normal oxygen to 49.74 percent after 48 hours of hypoxia.

To confirm that this was not an artifact of a single cell line or of prolonged culture, the researchers validated the effect in SK-N-BE(2) and SK-N-AS, observing PD-L1 mRNA increases of approximately 2.7-fold and 2.0-fold respectively, with corresponding protein increases. They also ruled out confounding from culture time or confluence by showing that PD-L1 remained stable in normoxic cells over the same period. Importantly, when hypoxic cells were returned to normal oxygen, PD-L1 levels gradually declined back toward baseline within 24 hours, demonstrating that the upregulation is reversible and directly tied to the hypoxic state rather than a permanent epigenetic change.

The mechanistic core of the study came from two complementary approaches. First, chromatin immunoprecipitation followed by quantitative PCR showed that HIF-1α physically occupies a predicted hypoxia response element, the sequence 5′-AGGTGC-3′, located 337 to 332 base pairs upstream of the PD-L1 transcription start site. Enrichment at this site increased roughly threefold under hypoxia compared with normoxia, while no enrichment was detected at a distal control region. Second, dual-luciferase reporter assays using a wild-type PD-L1 promoter showed strong hypoxia-induced activation, increasing nearly fourfold, whereas a mutant promoter in which the HRE was changed to 5′-AAAAAC-3′ showed no response. Together, these experiments establish that HIF-1α directly binds the PD-L1 promoter and transcriptionally activates it through this specific motif.

Having established the molecular mechanism, the researchers turned to the functional question: does hypoxia-primed neuroblastoma actually impair T-cell killing? They pretreated SK-N-SH cells for 48 hours under normoxia or hypoxia, then co-cultured them with activated primary human CD8+ T cells under normal oxygen for another 48 hours. The results were striking. The proportion of PD-1 and TIM-3 double-positive T cells, a hallmark of dysfunction or exhaustion-like phenotype, surged from 5.49 percent to 32.24 percent, roughly a sixfold increase. Apoptosis among the T cells rose from 10.61 percent to 26.89 percent, approximately a 2.5-fold increase. Meanwhile, the fraction of T cells producing interferon-gamma dropped by about 63 percent, Granzyme B-positive cells fell by roughly 55 percent, and CD107a degranulation, a marker of cytotoxic granule release, declined from 33.29 percent to 14.33 percent. Most dramatically, specific cytotoxicity, measured by a CFSE/PI assay that quantifies the percentage of tumor cells killed, plummeted from 79.94 percent to 38.40 percent, a reduction of more than half.

The rescue experiments provide the most clinically relevant findings. When the researchers knocked down HIF-1α using two independent siRNA sequences, achieving knockdown efficiencies above 70 percent, or treated cells with the pharmacological inhibitor YC-1, PD-L1 induction was substantially blunted. In co-culture, HIF-1α knockdown reduced the PD-1/TIM-3 double-positive fraction from 32.24 percent to 13.10 percent, and PD-L1 blockade with a neutralizing antibody brought it down to 11.94 percent. Specific cytotoxicity recovered to 70.81 percent after HIF-1α knockdown and 76.60 percent after PD-L1 blockade, values that were not statistically distinguishable from the normoxic control. In a separate rescue experiment using YC-1 at 25 micromolar, the PD-1/TIM-3 fraction fell to 10.45 percent and killing recovered to 70.41 percent. Importantly, the researchers confirmed that YC-1 at this concentration did not significantly affect T-cell viability or interferon-gamma production when applied directly to T cells, supporting the interpretation that its benefit is mediated through inhibition of HIF-1α within the tumor cells rather than through direct effects on the immune compartment.

The authors are careful to frame their findings appropriately. They note that the 48-hour co-culture window captures a dysfunction or exhaustion-like phenotype rather than bona fide chronic T-cell exhaustion, which requires sustained antigenic stimulation, transcriptomic profiling, and assessment of additional inhibitory receptors. They also acknowledge that their model preconditions tumor cells under hypoxia before co-culturing them with T cells under normoxia, which does not fully recapitulate a true hypoxic tumor microenvironment and does not assess direct hypoxic effects on T cells themselves. The T cells used were commercially sourced primary human CD8+ T cells, and because the vendor does not disclose the number of individual donors, inter-donor variability could not be assessed. All experiments were performed in vitro without animal model validation, and functional co-culture assays were conducted exclusively in the SK-N-SH line, although PD-L1 upregulation was confirmed in two additional lines with different MYCN status.

Despite these limitations, the study adds an important piece to the puzzle of why neuroblastoma remains an immunologically cold tumor with poor response to checkpoint monotherapy. It joins a growing body of evidence, including single-cell transcriptomic studies identifying the NECTIN2-TIGIT axis as a key checkpoint in neuroblastoma and reports that hypoxia drives terminal CD8+ T-cell dysfunction through chronic activation of the integrated stress response transcription factor ATF4, painting a picture of a tumor microenvironment that deploys multiple redundant mechanisms to suppress cytotoxic immunity. The demonstration that blocking either HIF-1α or PD-L1 can substantially, though not completely, restore T-cell killing in this model suggests that PD-L1 is one important node in a broader hypoxia-driven immunosuppressive network. The authors propose that combining HIF-1α inhibition with PD-L1 blockade, or with additional targets such as TIGIT, may yield additive or synergistic therapeutic benefits in preclinical neuroblastoma models, offering a rational path forward for a disease that has thus far resisted the immunotherapy revolution.

Subject of Research: Hypoxia-driven HIF-1α/PD-L1 signaling and CD8+ T-cell dysfunction in neuroblastoma

Article Title: Hypoxic conditioning of neuroblastoma cells promotes CD8+ T-cell dysfunction through the HIF-1α/PD-L1 axis

Article References: Hypoxic conditioning of neuroblastoma cells promotes CD8+ T-cell dysfunction through the HIF-1α/PD-L1 axis. (n.d.). https://doi.org/10.3389/fped.2026.1923893

Image Credits: AI Generated

DOI: 10.3389/fped.2026.1923893

Keywords: neuroblastoma, HIF-1α, PD-L1, CD8+ T cells, hypoxia, immune checkpoint, T-cell dysfunction, pediatric oncology, immunotherapy resistance, tumor microenvironment, YC-1, PD-1