A strong and durable CD8+ T-cell response is a key goal in cancer immunotherapy and antiviral vaccine development. Dendritic cells are central to this process: they capture antigens and present them via MHC-I molecules, stimulating cytotoxic T-cell responses.
However, the situation is more complex with extracellular antigens—such as those from peptide or subunit vaccines. Dendritic cells use a special cross-presentation pathway to present such antigens on MHC-I. This pathway requires the antigen to be transported into the cytosol, then into the ER, where it is processed and loaded onto MHC-I molecules for presentation to CD8+ T cells. This route is less efficient than the direct presentation of endogenous proteins.
The ER is a branched system of membranous cisternae and tubules connected to the nuclear envelope. It participates in protein synthesis and modification, lipid metabolism, and regulation of intracellular calcium levels.
Most research has focused on improving antigen uptake and transport into the cytosol. However, the final stage—delivery to the ER—remains understudied. Some studies have explored ER targeting using adenovirus E3 signal sequences, but these viral peptides are too long and hydrophobic, limiting clinical applicability. Shorter peptides proved insufficiently effective.
Sun Yat-sen University (China) researchers created a small, stable molecule to direct antigens specifically to the ER. This approach may improve cross-presentation efficiency and enhance antitumor and antiviral immune responses.
The new class of molecules was named SABER. SABER binds to the STING protein, located on the outer membrane of the ER, which plays a role in innate antiviral immune responses. Unlike most STING agonists, SABER maintains STING in an open conformation, allowing it to serve as a platform for antigen attachment and targeted ER delivery.
SABER Enhances Cross-Presentation Efficiency
The researchers tested whether ABM5—one of the most active SABER compounds—could efficiently deliver an antigen to the ER and improve immune presentation. ABM5 was linked to ovalbumin, a model-long antigen similar to neoantigens used in cancer vaccines. Despite binding to a long peptide, ABM5 retained its ability to activate STING. The ABM5-OVA complex triggered slower but longer-lasting STING activation and was significantly more effective at initiating cross-presentation than ovalbumin combined with other STING agonists. ABM5 proved effective not only with potent antigens but also with weakly immunogenic peptides.
ABM5-OVA enhanced cross-presentation through precise ER targeting via STING. Unlike other agonists, ABM5 directed the antigen to STING-rich sites, ensuring delivery to the ER, where MHC-I loading occurs. This effect was lost when using unstable peptide-STING links or pre-saturating STING. Cross-presentation required antigen degradation by proteasomes in the cytosol, followed by processing in the ER. Interestingly, ABM5’s effectiveness depended on its interaction with STING rather than downstream signal activation. The ABM5-OVA complex showed superior cross-presentation enhancement in type 1 (cDC1) and type 2 (cDC2) dendritic cells.
SABER Promotes Microreactor Formation for Antigen Processing
ABM5-OVA, unlike other STING agonists, induced the formation of a compact structure in cells where key cross-presentation players—STING, proteasomes, and TAP transporters—were concentrated. Researchers called this a microreactor, which accelerates antigen degradation and ER transport.
SABER-antigen binding not only activates STING but also spatially organizes all components of cross-presentation. In contrast, another STING agonist—diABZI—triggered rapid STING activation and relocation to the Golgi, preventing microreactor formation. The researchers suggest that the slower STING activation seen with ABM5-OVA allows time for cells to build an effective intracellular processing platform.
SABER Boosts CD8+ T-Cell Activation
The researchers encapsulated the SABER-antigen complex in lipid nanoparticles (LNPs) to protect it from degradation and enhance cellular entry. This delivery method significantly boosted cross-presentation. The effect was confirmed not only in mouse cells but also in human monocyte-derived dendritic cells.
Subcutaneous injection of LNP-encapsulated ABM5-OVA in mice triggered a robust CD8+ T-cell response: after two injections, the number of antigen-specific T-cells increased ninefold, and after three injections—by 120 times, surpassing potent adjuvants. The decisive factor was not just STING activation but targeted ER delivery.
SABER significantly enhanced cross-presentation in cDC1 and cDC2, whereas diABZI+OVA failed to produce this effect despite dendritic cell activation.
ABM5-OVA administration caused no notable local reactions or persistent systemic side effects, as assessed by body weight, blood biochemistry, serum interferon-β, and TNF levels.
SABER Enhances Antitumor Immunity
The researchers conducted a mouse experiment: animals were first vaccinated with ABM5-OVA and later injected with B16 melanoma cells expressing ovalbumin (B16-OVA). Tumor growth was significantly suppressed in the ABM5-OVA group: no mice died within 5 weeks. In contrast, only ~10% survived in the diABZI+OVA group.
ABM5-OVA was also effective when vaccination occurred after tumor onset in a therapeutic setting. In this case, it halted tumor growth and saved ~75% of the mice. In comparison, diABZI+OVA had minimal impact on disease progression—all mice died within 5 weeks.
SABER also showed high efficacy in treating malignant T-cell lymphoma and nearly prevented lung metastasis in a B16 melanoma model.
SABER Enables Personalized Cancer Vaccines
The scientists tested SABER’s effectiveness with real tumor neoantigens—antigens arising from cancer-specific mutations.
They used Adpgk, an immunogenic colorectal tumor neoantigen in mice. An ABM5-Adpgk vaccine in LNPs triggered a CD8+ T-cell response seven times stronger than diABZI+Adpgk. Three doses completely eradicated tumors in all mice, with no recurrence for 90 days. Upon re-injection with tumor cells on day 90, the vaccine provided long-term protection, with CD8+ T cells persisting as central memory cells for at least 60 days.
SABER was also tested in a tougher model—B16F10 melanoma, resistant to immune checkpoint blockade. A modified compound, ABN2, was used instead of ABM5 due to a cysteine in the M27 epitope. Vaccination started on day 6 after tumor implantation, mimicking clinical scenarios. ABN2-M27 monotherapy suppressed tumor growth, and when combined with anti-PD-1 antibodies, its efficacy surpassed poly-I:C + M27 + anti-PD-1 combinations.
SABER enhances antiviral immunity
To assess SABER’s antiviral potential, the researchers tested it in a COVID-19 model. A vaccine was created based on the SNT peptide — a conserved nucleocapsid region of SARS-CoV-2 recognized by both human and murine T cells.
Immunization with ABM5-SNT produced 150 times more IFN-γ–expressing T cells than diABZI+SNT.
Effectiveness was confirmed in a transgenic model: mice vaccinated with the human ACE2 receptor with ABM5-SNT had 100 times lower viral loads in the lungs and brain after infection with the Omicron BA.5.2 strain.
In addition to T-cell responses, the SABER platform enhanced antibody production, including cross-neutralizing antibodies against viral variants.
SABER is compatible with peptide and protein vaccines and can enhance cellular and humoral immunity.
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Reference
STING agonist-based ER-targeting molecules boost antigen cross-presentation
