Post release immune responses of Tasmanian devils vaccinated with an experimental devil facial tumour disease vaccine

Context: Disease is increasingly becoming a driver of wildlife population declines and extinction risk. Vaccines are one of the most successful health interventions in human history, but few have been tested for mitigating wildlife disease. The transmissible cancer, devil facial tumour disease (DFTD), triggered the Tasmanian devil’s (Sarcophilus harrisii) inclusion on the international endangered species list. In 2016, 33 devils from a DFTD-free insurance population were given an experimental DFTD vaccination prior to their wild release on the Tasmanian north coast.

Aim: To determine the efficacy of the vaccination protocol and the longevity of the induced responses.

Method: Six trapping trips took place over the 2.5 years following release, and both vaccinated and incumbent devils had blood samples and tumour biopsies collected.

Key results: Eight of the 33 vaccinated devils were re-trapped, and six of those developed DFTD within the monitoring period. Despite the lack of protection provided by the vaccine, we observed signs of immune activation not usually found in unvaccinated devils. Firstly, sera collected from the eight devils showed that anti-DFTD antibodies persisted for up to two years post vaccination. Secondly, tumour infiltrating lymphocytes were found in three out of four biopsies collected from vaccinated devils which contrasts with the ‘immune deserts’ typical of DFT’s; only one of the twenty incumbent devils with DFTD had a tumour biopsy exhibiting immune cell infiltrate. Thirdly, immunohistochemical analysis of the vaccinated devils’ tumour biopsies identified the functional immune molecules associated with antigen presenting cells (MHC-II) and T cells (CD3), and the immune checkpoint molecule PD-1, all associated with anti-tumour immunity in other species.

Conclusions: These results correlate with our previous study on captive devils in which a prophylactic vaccine primed the devil immune system and, following DFTD challenge and tumour growth, immunotherapy induced complete tumour regressions. The field trial results presented here provide further evidence that the devil immune system can be primed to recognise DFTD cells, but additional immune manipulation could be needed for complete protection or induction of tumour regressions.

Implications: A protective DFTD vaccine would provide a valuable management approach for conservation of the Tasmanian devil.