Protein-based nanoshuttle to deliver anticancer vaccines

US researchers are using a natural protein called albumin as a way to ‘shuttle’ cancer-fighting nanovaccines into the body. Published in the journal Nature Communications, their study marks a significant step towards the application of cancer nanovaccine immunotherapy in humans.

Nanovaccines that work to mount an immune response against a tumour basically consist of two components: the part that delivers the vaccine to the correct site, the lymph nodes, where immune system activation happens; and the part that activates the immune cells to expand and specifically target the tumour. But while they have shown significant promise in fighting cancer, clinical application has been hampered by complications in large-scale manufacturing, quality control and safety.

Now, biomedical engineers at the National Institute of Biomedical Imaging and Bioengineering (NIBIB) have developed a more simplified approach that enables nanovaccines to bind to the albumin protein, which is naturally present in the body and regularly filters through the lymph nodes. The protein then delivers these nanocomplexes to the lymph nodes, resulting in potent immune activation against multiple tumour types in mouse cancer models.

“The vaccine essentially hitches a ride with albumin to travel to the lymph nodes, eliminating the need to create a separate delivery vehicle,” explained Guizhi Zhu, first author on the study. “Given that large-scale manufacturing and long-term safety are the primary hurdles of current nanovaccine technology, our approach offers a detour to accelerate eventual use of nanomedicines in the clinic.”

Several nanovaccines were engineered by the researchers, each with a different antigen — the component that stimulates the immune cells to attack a specific tumour type. To each antigen, the engineers also added a small dye molecule called Evans blue (EB), which binds to the albumin in the body and has been used for nearly a century to study albumin-binding proteins. The vaccine-EB complex was named AlbiVax (albumin-binding vaccine) because it immediately binds to albumin. The nanovaccine also included a small segment of DNA bound to the EB; the DNA is a ‘danger signal’ to the immune system and so helps make the immune response more robust.

The complete formulation thus consists of an antigen and a DNA segment, both bound to EB. Upon injection, both components bind to albumin, which is why the vaccine is considered to be self-assembling. The albumin carrying the antigen and the DNA then delivers both to the lymph nodes where the antigen activates immune cells that specifically target the tumour, and the DNA enhances that activation, optimising the immune attack.

Schematic of self-assembly of the AlbiVax nanovaccine. The tumour-specific antigen and DNA segment were attached to Evans blue (purple arrows). Upon injection, albumin in the blood (green arrows) binds to the Evans blue and brings the complexes into the lymph system where the DNA and antigen interact with the immune cells (blue) in the lymph nodes. The interaction triggers a vigorous immune response against the target tumour. Image credit: Zhu, et al.

The vaccines were tested on several tumour types and in several ways. In one experiment, tumour-free mice were vaccinated against mouse thymus tumour cells three times, at two-week intervals. At day 70 a large dose of the tumour cells was injected into seven immunised mice as a challenge. Five of the seven mice survived for more than four months.

These five surviving mice were subsequently injected with another large dose of the tumour cells, and four of the five mice survived for more than six months. Blood tests showed that four months after the last immunisation, the mice still had circulating immune cells that were specifically killing the thymus tumour cells.

An AlbiVax nanocomplex vaccine was also engineered against a human colon cancer cell line. Human colon cancer cells were injected into the mice, where they establish tumours in various organs — mostly in the lung. The mice were given the vaccine six days after the lung tumours were established. In this experiment the mice were also treated with an antibody called anti-PD-1, to counter the effects of the protein PD-1 that appears on the surface of some tumours and acts to slow down the immune attack. The combination of the nanovaccine and anti-PD-1 resulted in the complete regression of lung tumours in six of the 10 mice for four months.

The team is particularly optimistic about the long-lasting immunity that they were able to induce with the AlbiVax system, as evidenced by continued robust antitumour activity for up to six months. This was the longest time point tested in these experiments and represents a significant part of the lifespan of a mouse, which is about two years.

“Albumin is an interesting protein and it has been studied for over 40 years for drug delivery using different technologies,” said Zhu. “Compared with other albumin-binding technologies, our proprietary technology has been developed using clinically safe EB, making it very promising for eventual clinical translation. By simply synthesising albumin-binding vaccines, our technology can be applied to virtually any molecular vaccine or molecular therapeutics.”

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