Cancers currently remain the second leading cause of death in the world, despite the improvement in patient care.
The seriousness of these diseases lies in their great diversity. Although some cancers are effectively treated with surgery, chemotherapy and radiation therapy, others respond little or not at all to these treatments. Improving the management of these diseases is therefore a major challenge for our health system.
For several years, the development of immunotherapies, treatments that take advantage of various components of the immune system to fight tumors, has been a promising avenue for achieving this objective.
Some of these promising immunotherapies use modified viruses, which cause adverse effects in many patients. To overcome this problem, our team studied the possibility of using a non-pathogenic microorganism for humans instead, Neospora caninum. The first results, obtained in mice, are very encouraging.
First immunotherapies: a positive impact
Unlike chemotherapy and radiotherapy, which prevent the multiplication of tumor cells, but induce serious side effects (because these treatments also attack non-cancerous cells in the body), immunotherapy stimulates the immune system of the patient to fight more specifically against cancer.
This approach exploits different strategies, whether it is the use of antibodies preventing cancer cells from inactivating the immune system (called immune checkpoint inhibitors) or specifically targeting cancer cells, or the use of microorganisms living organisms that induce a strong immune response to destroy tumor cells.
These immunotherapeutic approaches have been used since 2001 to treat melanoma: the development of the first antibody to inhibit immune checkpoints, ipilimumab (trade name: Yervoy), enabled more than 53.6% of patients treated to survive 2 years. This antibody recognizes a protein (CTLA-4) which plays a role in the inactivation of T lymphocytes, immune cells which have in particular an antitumor activity. By binding to this protein, ipilimumab inactivates T lymphocytes, which can then proliferate.
In 2015, another advance in the field of melanoma care reduced tumors and increased survival for some patients affected by the disease. This strategy is based on the use of a herpes virus (herpesvirus type 1), modified to multiply in tumor cells and cause their death (trade name: Imlygic). This virus has also been modified to produce a human protein that stimulates the antitumor immune response.
Immunotherapies could be the key to treating cancers that are currently incurable, because they are refractory to existing antitumor therapies. This is particularly the case of glioblastoma, a serious brain cancer for which the average survival of patients is 15 months after diagnosis, or pancreatic cancer, associated with an average survival of 8 months.
However, using viruses as part of immunotherapies may not be trivial. Indeed, there is a particular risk that their genetic material will integrate into that of human cells (in the case of certain DNA viruses), causing unwanted mutations which could have deleterious consequences.
To circumvent this problem, we have developed with our collaborators an immunotherapy based on a microorganism named Neospora caninum (N.caninum).
Neospora caninum, a microorganism as a new therapeutic hope?
Identified in 1984 in dogs, Neospora caninum is a unicellular parasite. It is also obligate intracellular, meaning it infects other cells in which it replicates.
Responsible for severe neurological disorders and abortions in certain animals (cattle and canines), it is on the other hand totally harmless for humans and for most rodents, probably due to differences in immune responses. In contrast N.caninum is able to multiply in vitro in cells of human or mouse origin.
Like the viruses used in immunotherapy, N.caninum can destroy the cells it infects. It induces a strong cellular immune response, sought to fight cancer. These two characteristics therefore make it a relevant candidate for antitumor immunotherapy.
With this in mind, we decided to test its effectiveness in the context of an immunotherapy aimed at treating mice for cancer of the thymus (gland located in the upper part of the thorax, behind the sternum, between the lungs) called thymoma. Benign and slowly growing, this type of cancer is generally asymptomatic and treated mainly by surgery.
The interest of this model is to provide proof of the anticancer efficacy of N.caninum before testing it on models of cancers refractory to existing treatments.
Our results, published in the scientific journal Journal for ImmunoTherapy of Cancer demonstrate that, in mice, N.caninum is able to control the development of a tumor until complete regression, and this, in three different ways. These very positive results were obtained not only after the (unmodified) microorganisms were administered directly within the tumour, but also at a distance from it.
Three mechanisms controlling tumor development
In the first place, N.caninum has been shown to be able to directly destroy cancer cells. Four days after treatment, vacuoles (compartments located inside a cell) containing the microorganisms were observed in the tumor cells. Formed by N.caninum, they allow it to multiply in the host cell while being protected from any degradation. After such a multiplication step, the parasitized cell is destroyed.
The observation of such vacuoles in the tumor means that N.caninum is capable of multiplying in cancer cells and therefore, by extension, of destroying them. N.caninum was detected in other cells, but did not persist or cause damage.
The second way in which N.caninum controls tumor development through the stimulation of a cellular immune response. After treatment, a strong response from the mouse immune system was detected within the tumor. This reaction is characterized not only by high levels of inflammatory molecules, but also by the recruitment of immune cells specialized in the destruction of cancer cells, whether they are infected with N.caninum or not. These cells are the cytotoxic T lymphocytes and the cells Natural Killer (NK), whose particularity is to produce proteins that degrade cell membranes, leading to their destruction, and therefore those of the cells.
Finally, N.caninum affects tumor development via reprogramming of the tumor microenvironment. Tumors persist in the body because they are particularly capable of “putting to sleep” the immune system within them, by forming a so-called immunosuppressive micro-environment, which promotes their development.
In this particular micro-environment, several poor prognostic factors are expressed. This is the case, for example, of the growth factor VEGF (Vascular Endothelial Growth Factor), a protein involved in the creation of new blood vessels (which bring nutrients to the tumor), or PD-L1 (Programmed Death-Ligand 1), a protein that prevents the death of cells that express it strongly.
However, after treatment with N.caninum, these two molecules are produced at lower levels within the tumour. This decrease in concentration makes it possible to reprogram the tumor microenvironment so that it participates in the elimination of cancerous cells.
Promising preliminary results
Obtained in mice, these results are still preliminary, but very encouraging. They demonstrate that N.caninum could be a good candidate to enrich the arsenal of cancer immunotherapies.
Betting on using a micro-organism to treat cancer was risky, because of its ability to multiply in cells. However, in this model of thymic lymphoma (thymoma), N.caninum was no longer detectable at the end of the experiments. Although humans are not susceptible to infection with N.caninumits elimination by the immune system must be confirmed before considering a therapeutic use.
After having demonstrated its effectiveness in a benign cancer model, it now remains to study the anticancer properties of N.caninum in a difficult-to-treat cancer model, with the aim of one day using it to cure patients suffering from incurable cancers such as glioblastoma.