Turning the flu virus Into a powerful tool to fight cancer

A recent Views & Comments article published in Engineering highlights advances in repurposing influenza viruses as flexible therapeutic platforms for infectious diseases and cancer, driven by progress in reverse genetics and viral vector engineering.

Traditionally regarded as a major human pathogen, influenza virus is now being engineered to carry foreign genes and reduce virulence, serving not only for next-generation influenza vaccines but also as delivery vectors for heterologous antigens against other infections and cancers, supported by its ability to trigger robust mucosal and systemic immune responses.

Conventional influenza vaccine platforms, including egg-based inactivated and live-attenuated formulations, face constraints such as long production cycles, limited immunogenicity in vulnerable populations, and reduced protection caused by strain mismatch, creating demand for platforms with better genetic stability, rapid programmability, and stronger immunogenicity.

To tackle these challenges, researchers are developing strategies to precisely regulate viral fitness and biosafety, among which incorporating non-canonical amino acids (ncAAs) into influenza viral proteins emerges as a promising approach. This method achieves site-specific replication attenuation without impairing antigen presentation by introducing premature termination codons (PTCs) in essential viral genes, generating so-called PTC viruses.

The system relies on an orthogonal tRNA/aminoacyl-tRNA synthetase pair that selectively inserts a designated ncAA at the PTC site without cross-reacting with the host's endogenous translation machinery, forming a strict genetic firewall that confines viral replication to the orthogonal system.

Tests in engineered mammalian XH 293 cells show that PTC virus replication is limited to these cells and depends on the presence of the matching ncAA, and the virus cannot replicate in unmodified mammalian cells even with ncAA supplementation, establishing a multi-layered biosafety mechanism.

In mice, ferrets, and guinea pigs, PTC viruses induce significantly stronger immune responses than a commercial inactivated influenza vaccine, and all immunized mice survive wild-type influenza challenge while unvaccinated controls do not.

Beyond infectious disease prevention, the controllable PTC virus is adapted as a cancer vaccine platform through the chimeric antigen peptide (CAP) Flu system, which combines tumor-associated antigens tethered to viral hemagglutinin via bioorthogonal click chemistry, a CpG-rich TLR9 agonist for dendritic cell activation, and an anti-PD-L1 nanobody gene inserted into the viral genome.

Intranasal administration of CAP Flu in a lung metastasis model enhances dendritic cell recruitment and activation in tumors and draining lymph nodes, inducing robust humoral and cellular immunity and suppressing tumor growth effectively.

Compared with conventional viral vectors like adenovirus and vesicular stomatitis virus (VSV), the PTC influenza system offers unique advantages, including an orthogonal and genetically stable attenuation mechanism, strong mucosal immunity rarely seen in other vectors, and consistent stoichiometric antigen display by physically linking antigens to viral proteins, avoiding the instability of codon-deoptimized or temperature-sensitive influenza strains.

Clinical translation of the PTC platform still faces hurdles, such as preexisting influenza immunity limiting vector spread, the need for biosafety evaluations of ncAAs, and optimization of tumor-targeting specificity for non-pulmonary tumors.

Nevertheless, the modular and plug-and-play design of the PTC influenza platform supports programmable antigen payloads, immunomodulator integration, and orthogonal replication control, making it a viable strategy for next‑generation vaccines and viral immunotherapies as synthetic biology continues to evolve.