Cowpea mosaic virus nanoparticles can induce the immune system to clear metastatic cancers.

To survive and proliferate within an organism, tumours must evade immune surveillance. And they do this by expressing ligands that interact with receptors found on the surface of lymphocyte T cells (a type of white blood cell that makes up the immune system), and activating an 'immune checkpoint' that stops the immune system from removing foreign substances such as tumour antigens1. The understanding of this immunomodulatory pathway has led to the development of strategies aimed at 're-educating' the immune system against cancerous cells. Writing in Nature Nanotechnology, Steven Fiering and colleagues at Dartmouth College and Case Western University now show a new way to induce the immune system to clear metastatic cancers by using cowpea mosaic virus nanoparticles2 — self-assembling protein nanoparticles derived from a plant virus.

Over the past few years, several strategies have been investigated to augment the immune system to kill tumours, but each of these approaches has strengths and weaknesses. Vaccination against specific tumour antigens has been developed for multiple cancers, including melanoma, breast, colorectal, liver and blood3. However, for vaccines to work, the target antigen within the tumour must be expressed continuously. While this approach clears certain populations of tumour cells, it also induces the formation of resistant clones, which do not express that particular antigen and are thus responsible for tumour relapse. Another strategy entailed the use of oncolytic viruses (viruses that preferentially infect and kill cancer cells) to deliver pro-immunogenic genes to tumours. Here, tumour destruction is achieved through the combined action of viral lysis and the cytotoxic effects of an activated immune system resulting from the delivery of the genes4.

Another approach that has gained momentum is the inhibition of immune checkpoints. Immune checkpoints — the many inhibitory pathways in the immune system — are usually activated when a specific receptor expressed by T lymphocytes interacts with its ligand expressed on cancer cells. This interaction blocks the activation of the immune system and develops tolerance towards the cancer cells5. Inhibitors that block these checkpoints are usually antibodies raised against either the receptor or the ligand, and they work by physically preventing this ligand–receptor interaction. Although immune checkpoint antibodies have undergone extensive validation in clinical trials and have been approved by the US Food and Drug Administration for several cancers, they are not universally effective in all patients. They generally only delay tumour progression, and have significant toxicity6. An alternative strategy in this quest involves the inhibition of tumour-associated lymphocytes (known as regulatory T cells) that are responsible for hampering the immune response against the tumour7. However, pharmacologic depletion of these regulatory T cells (for example, using low doses of cyclophosphamide) lacks specificity and durability, and thus far, has yielded poor results8.

While nanoparticles have largely been explored as a delivery agent for chemotherapeutics, Fiering and co-workers found that cowpea mosaic virus nanoparticles on their own could induce a potent, but localized and self-contained, activation of neutrophils (a type of immune cell that helps fight infections) when administered to mice either by injecting the particles directly into the tumours or allowing the animals to inhale them. This activation significantly delayed the growth of tumours and protected the animals from tumour regrowth for a second time. These responses were seen in a variety of tumour models, including melanoma, and breast and ovarian carcinoma. The results suggest that the nanoparticles activated both innate and adaptive immune responses against the tumour, and a combination of proteins from plant viruses can be possible vaccines against tumours in animal models.

An appealing aspect of the study is the simplicity of the structure and the production of these nanoparticles (Fig. 1a). The nanoparticles are 30-nm icosahedral structures that do not contain viral DNA and therefore would appear safe from an infectious and genomic standpoint. They can be produced at scale without endotoxin contamination through molecular farming in plants. Because of their scalability, they can be administered in vivo via multiple dosing regimens, in an off-the-shelf manner that is preferable for patient treatment in an outpatient setting. Such a dosing regimen may be a fundamental requirement to achieve a sustained effect over time. This approach is also 'antigen free', in that it is not based on a specific protein expressed by the tumour. Therefore, it seems to be exempt from the limitations that affect vaccination strategies as discussed above.

Figure 1: Production and use of cowpea mosaic nanoparticles as a cancer immunotherapy in animals. a, Schematic showing the production of empty viral-like nanoparticles. The DNA (blue circle) encoding for the viral coat (capsid) proteins is artificially introduced into plant cells (represented here by a leaf). The plant functions as a factory for producing these proteins. Once produced, these proteins self-assemble into nanoparticles that resemble the original virus, but lack the viral genome. They are therefore called virus-like particles. b, Illustration of the mechanism of immune-mediated tumour lysis triggered by virus-like nanoparticles. When nanoparticles are injected in vivo, they are intercepted by quiescent neutrophils within the tumour. On nanoparticle uptake, these quiescent neutrophils become activated and they secrete chemokines (signalling molecules) that recruit more neutrophils to the tumour. In the process, T lymphocytes are also activated and are recruited to the tumour for final destruction of the tumour cells. Full size image

This study does pose several questions. Because lymphocytes are typically first responders after a viral (or pseudo-viral) infection, it is unclear why neutrophils are the major players in the antitumour response seen (Fig. 1b). Furthermore, it is curious how such a profound and untargeted response can be so specific to the tumour while sparing normal tissues. Does it suggest that the tumour maintains a frail immunologic equilibrium with its host, and that once the equilibrium is perturbed (by an infection, for example), there is a preferential immune attack that is tumour-specific while the rest of the organism is unaffected? If this is true, one can speculate that possibly less immunogenic infections would work better than highly immunogenic challenges as they will still be able to initiate an antitumour response, yet minimizing the risks of immune-mediated toxicity. Additionally, it remains an open question whether the use of a plant virus increased the ability to induce an antitumour response when compared with the use of vertebrate immune-inducing pathogens. Even after the cowpea mosaic nanoparticle treatment, Fiering and co-workers observed that parental tumours eventually managed to grow and kill a significant number of treated mice. This suggests that in several cases the tumours eventually managed to escape the immune response; here, one ponders over the mechanisms used by the tumour to evade the activated immune system and survive clearance. Answering these questions will significantly advance our understanding of the interplay between the immune system and tumours, and will pave the way to novel approaches in cancer immunotherapy.

References 1. Gubin, M. M. et al. Nature 515, 577–581 (2014). 2. Lizotte, P. H. et al. Nature Nanotech. 11, 295–303 (2016). 3. Pol, J. et al. Oncoimmunology 4, e974411 (2015). 4. Andtbacka, R. H. et al. J. Immunother. Cancer 2(Suppl. 3), P263 (2014). 5. Pardoll, D. Nature Rev. Cancer 12, 252–264 (2012). 6. Larkin, J. et al. N. Engl. J. Med. 373, 23–34 (2015). 7. von Boehmer, H. & Daniel, C. Nature Rev. Drug Discov. 12, 51–63 (2013). 8. Byrne, W. L., Mills, K. H., Lederer, J. A. & O'Sullivan, G. C. Cancer Res. 71, 6915–6920 (2011). Download references

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