Targeting transmissible cancers in animals

Devil's facial tumor disease is a deadly transmissible cancer that has endangered the Tasmanian devil.

PHOTO: LONELY PLANET IMAGES / GETTY IMAGES MORE

For thousands of years, a non-life-threatening tumor has been transmitted between dogs during coitus. As the only known transmissible cancer, this canine transmissible venereal tumor (CTV) was considered an abnormality rather than an important biological phenomenon. However, in the 21st century, two unrelated transmissible cancers in Tasmanian devils (1, 2) and at least five transmissible neoplasias in aquatic molluscs have been discovered (3). The interest in these transmissible cancers is centered on their distinct characteristics. During emergence, communicable and noncommunicable cancers undergo similar genetic events and are indistinguishable until transmission occurs. This requires at least two additional conditions: a mechanism of cancer cell transfer and avoidance of the recipient's immune response. The simultaneous occurrence of both conditions in cancer is rare. Nevertheless, communicable cancers that affect vulnerable or commercially valuable species can have serious ecological and economic consequences. Preventive and therapeutic approaches will mitigate these impacts and prevent the emergence of new transmissible cancers.

The transmissible cancers that affect Tasmanian devils are called facial tumor dementia (DFTD) and present a reduced number of this marsupial currently at risk of about 80% since the first case reported in 1996. DFTD cancer cells originated from 39, a Schwann cell, the myelinating cell of the peripheral nerve (4). Cells are transmitted between demons by face-to-face bites (5), a characteristic behavior of Tasmanian devils during the mating season. This is an effective transfer mechanism since tumors appear on the face. When a healthy devil bites a devil with DFTD, some cancer cells lodge in the mouth of the devil aggressive. Minor injuries in the oral cavity, as a result of the demon attacking the sharp bones, cause rupture of the lining and allow the cancer cells to establish themselves. When a devil with DFTD inside the oral cavity bites a healthy devil, the DFTD cells on the aggressive devil's teeth are inoculated under the recipient's skin, providing another opportunity for research. A cancer.

A remarkable feature of transmission is that only tens to hundreds of cells are transferred. These few transmitted cells establish a cancerous mbad and often metastasize towards various internal organs. In most cases, the death of the host occurs in less than a year. The period between cancer establishment and death offers many opportunities for the sick devil to transmit cancer cells to healthy devils, thus perpetuating transmission.

For the establishment of a transmissible cancer, transmitted cells must acquire strategies to evade immune responses in genetically different hosts. Tasmanian devils have low genetic diversity that has probably facilitated the early spread of DFTD. However, devils can acutely reject independent devil skin grafts, suggesting that the immune response should recognize cancer cells from other devils (6). Pioneering research on CTVT cells has demonstrated a lack of expression on the surface of major clbad I histocompatibility complex (MHC I) antigens, which are important for activating immune responses against cells. Foreign. This down-regulation of MHC I surface molecules prevents immune rejection of CTVT cells and probably contributes to universal transmission and progressive growth of CTVT (7). These results seem to be relevant for DFTD transmission because DFTD cells do not express MHC I either (8). The absence of MHC I expression should make DFTD cells targets for NK (Natural Killer) cells and other innate lymphoid cells. There is circumstantial evidence of the presence of functional NK cells in Tasmanian devils (9). However, since DKD cells were not spontaneously killed by NK cells in vitro, their activation appears to be defective.

In 2014, a new transmissible cancer was discovered in Tasmanian devils and is believed to have originated from an independent ancestor of the original DFTD (DFT1) (2). This second cancer, called DFT2, has provided an additional opportunity to study transmissible cancers. Unlike DFT1 and CTVT cells, DFT2 cells express MHC I molecules (ten). However, these MHC I molecules are mainly non-polymorphic or unconventional, providing an intriguing picture of immune leakage in DFT2. The non-polymorphic MHC I molecules are genetically identical to the MHC I molecules of the infected host devil and, as such, do not elicit an allogenic immune response (rejection). Conversely, non-clbadical MHC I molecules inhibit immune responses. The combined expression of non-polymorphic and unconventional MHC I molecules allows the transmitted DFT2 cells to establish cancer in genetically similar hosts without inducing an immune response. DFT2 is currently located in a small, semi-isolated peninsula in Tasmania, where genetic diversity among demons is expected to be low. The lessons learned from DFT1 and the preliminary evidence from DFT2 indicate that, when DFT2 spreads in genetically different devil populations, the expression of MHC I could be progressively lost to avoid rejection of DFT2 cells by recognition. of the foreign MHC I (ten) (see figure).

Therapeutic control of DFTD should treat tumors and prevent their transmission. The treatment of cancer in humans is based on chemotherapy or radiotherapy, which aims nonspecifically dividing cells. None of these approaches are suitable for wild Tasmanian devils: multiple cycles of treatment, constant veterinary care, and the need to kill 100% of DFTD cancer cells are major obstacles to these nonspecific approaches. As with the treatment of human cancer, treatments can target specific cancer alterations caused by genetic defects. This approach requires knowledge of genetic defects in DFT1 and DFT2. Loss-of-function mutations have been identified in the WWC family member 3 genes (WWC3) in DFT1 and MPD2 in DFT2 (11). These genes encode components of the Hippo pathway that negatively regulate y-badociated protein (YAP1) and tafazzine (TAZ) to prevent proliferation (11). The pathways that interact with YAP1 and TAZ, including the proliferative pathway ERBB3 in DFT1 and the platelet derived growth factor receptor (PDGFR) pathway in DFT1 and DFT2, are also hyperactivated in DFTD due to the increased number of gene copies (11). The strong expression of ERBB3 in DFT1 over-reacts the downstream transcription factor and transcription activator signal transducer 3 (STAT3), which traps STAT1, blocking the transcriptional activation of the gene encoding β2-microglobulin, an essential component of the MHC I (12). Without production of β2-microglobulin, the MHC I molecules are not produced. In a mouse model of DFT1, chemotherapies specifically targeting these pathways reduced proliferation and favored the expression of MHC I in DFT1 (12). These chemotherapies could therefore provide therapeutic options in DFT1 by promoting immune rejection.

Since MHC I expression is downregulated in DFT1, immunotherapies that directly restore MHC I expression are an alternative to targeted chemotherapy. Immune responses to infection involve the production of cytokines, including interferon-γ (IFN-γ), which upregulate the expression of MHC I. DFT1 cells retain the ability to respond at IFN-γ (8), and recombinant IFN-γ treatment of the devil or drugs stimulating the immune response could therefore positively regulate MHC I and induce immune rejection. The up-regulation of MHC I was the basis of a DFTD vaccine and immunotherapy. As part of the restrictions imposed when working with endangered species and having limited access to demons, small-scale vaccination and immunotherapy projects were undertaken in captivity (13). Vaccines consisting of DFT1 cells treated ex vivo with IFN-γ to positively regulate MHC I and inactivated by sonication or serum antibody production induced by ionizing radiation, indicating the activation of An immune response. These serum antibodies have been shown to recognize normal DFT1 cells that do not express MHC I in vitro. This crucial discovery allowed the next step of the test: inoculation of live DFT1 cells to vaccinated devils to determine if immunization was protective. Of the six vaccinated devils tested, one did not develop a tumor and the others had a delayed onset.

Devils who developed tumors were then used to test IFN-γ immunotherapy injected directly into the tumor (13). The injection of IFN-γ was unsuccessful, probably because of its short half-life in vivo. However, the injection of live tumor cells treated ex vivo with IFN-γ to positively regulate tumor regression of MHC I probably caused allogeneic activation. The use of IFN-γ in the prevention and treatment of cancer is limited by its ability to positively regulate the cell surface inhibitory molecules (control points). This has been demonstrated with DFT1 cells treated with IFN-γ; these cells positively regulated the immune control point molecule programmed by cell death 1 ligand 1 (PD-L1), which prevents activation of T cells (14). To anticipate this possibility, and on the basis of the successful treatment of certain human cancers, monoclonal antibodies antagonizing the programmed cell death protein 1 (PD-1) -PD-L1 pathway in demons have been produced (14). These could be incorporated into an immunotherapy protocol, possibly with vaccination, using DFTD tumor cells expressing MHC I to treat or prevent DFTD.

The vaccination of wild Tasmanian devils provides an approach to prevent DFTD transmission. Unlike "vaccines" against human cancer, which are mainly therapeutic, a vaccine against DFTD could be preventative, as well as human vaccines that protect against infectious diseases. The Tasmanian devils were immunized with DFT1 cancer cells that had been pre-treated with IFN-γ to induce MHC I expression. The first cohort included 52 devils raised in captivity. These devils were immunized and released into the wild to help restore the devastated population of DFT1. Immune responses, determined by the presence of anti-DFT1 serum antibodies, were detected in 50 devils (15). However, preliminary follow-up studies indicated that protection could only be partial, as some devils had developed DFT1.

The long-term presence of CTVT in the canine population suggests a coevolution between the host and the tumor, leading to a transmissible cancer that is not usually fatal to dogs. Although the DFTD is currently deadly for most infected demons, a similar situation may occur over time, resulting in less aggressive DFTD cancers. In the meantime, interventions against DFT1 and DFT2 are needed to protect wild demons from these cancers. The challenges badociated with the administration of wild demon therapy are considerable, and a preventative vaccine that can be administered to trapped demons remains the most viable option of protection against DFTD. This vaccine must provide long-term immunity against DFTD in a single injection, which requires improvements over current vaccines. Research on CTVT and DFTD has shown that down-regulation of MHC I is essential, but not on its own, to the transmissibility of cancer. Further research will explore other mechanisms of immunodeficiency in the DFTD, in order to jointly target them to improve the efficacy of the DFTD vaccine. This research is already underway in studies of immune checkpoint molecules (14). However, it is possible that other aspects of the DFTD's survival will need to be elucidated for a robust vaccine against DFTD to be developed. Nevertheless, the strong DFTD vaccination responses that have been produced in the preliminary trials are promising and, given ongoing efforts to understand cancer transmission, a vaccination that promotes protective immunity against DFTD is a realistic expectation for the future.

Thanks: AP and GW are funded by the Australian Research Council and the University of Tasmania Foundation, Eric Guiler, Tasmanian Devil Research Grant, through funds raised by the Save the Call Tasmanian Devil. The authors thank A. Flies and B. Lyons for their helpful comments.