Immune recognition of devil facial tumour disease by the Tasmanian devil

The successful transmission of devil facial tumour disease (DFTD) as an allograft between Tasmanian devils raises many immunological questions about the disease, the devil's immune system and their interplay. Research on DFTD and devil immunology feed into an overarching goal of developing a protective vaccine against DFTD. The effects of DFTD on haematology and serum biochemistry parameters in the Tasmanian devil have been published. DFTD's effects on the immunological components of the devil's peripheral blood however, continue to be explored. T lymphocyte subsets and immunoglobulins provide useful indicators of immune competence, and have been assessed in the lymphoid organs of healthy devils and those with DFTD. To examine the peripheral blood T lymphocyte subsets of a wild population of Tasmanian devils, a novel method using immunohistochemistry on formalin fixed blood clots was developed. This overcame the limitations of available reagents and the remote field location where samples were collected. An ELISA was developed to measure the relative levels of IgM and IgG in serum from healthy and diseased devils. Devils with DFTD had reduced percentages of CD4+ and CD8+ lymphocytes, and increased IgM and IgG serum levels compared to healthy devils. The effect of age, sex and season on these components was also evaluated in healthy devils. Significant differences between age groups (juveniles and adults) were found for both T lymphocytes and immunoglobulins. Seasonal effects were noted for CD4+ lymphocytes and the CD4:CD8 ratio. There was no detectable sex effect on any of the components. DFTD is an aggressive cancer with most devils dying within 6 to 12 months of clinical signs first appearing. The long-standing assumption that DFTD always escapes the devil's immune response was addressed by looking for the presence of serum IgG antibodies against DFTD in a population of wild devils. Likewise, tumour biopsies were examined for the presence of tumour infiltrating lymphocytes as an indication of cell mediated immune responses. Approximately 10% of wild devils were capable of mounting an immune response against the disease. This correlated with tumour regression in four out of the six devils with a demonstrated immune response. The epigenetic down-regulation of the major histocompatibility complex class I molecule (MHC-I) is considered a principle mechanism by which the DFTD cells escape the devil's immune response. This down-regulation is reversible and DFTD cells incubated with the cytokine interferon gamma (IFN-g) express surface MHC-I. These cells are expected to be immunogeneic and thus formed the basis for two pilot immunisation trials on a total of six devils, including one adjuvant-only control, and one non-immunised control for live DFTD cell challenge. The immune responses induced by these immunisations were measured. A subsequent challenge with live DFTD cells was given to three of the immunised devils. Tumours developed in two of the devils but subsequent immunotherapy comprising a single subcutaneous injection of live DFTD cells expressing surface MHC-I, resulted in tumour regression in both devils. Serial biopsies of the regressing tumours demonstrated their immune mediated rejection. The State government's Save the Tasmanian Devil Program's wild devil recovery‚ÄövÑvp project allowed for the immunisation protocol used in one of the above trials to be carried out on 19 captive held devils prior to their wild release. The relatively large sample size allowed for a more robust assessment of the immune responses measured. There were 15 out of the 19 devils that developed anti-DFTD IgG antibodies in response to the immunisations prior to their release. The effects age and sex had on the responses were also considered. Juvenile (one year old) devils had significantly higher antibody responses than adults, and female devils showed higher antibody responses than males. Inevitable variations in the protocol administered, due to variable trapping success, meant some comparison between the number of immunisations each devil received could also be made. Post release monitoring trips showed serum antibody levels reduced over time in the small number of devils that were retrapped during these trips. While this thesis was underway, a second transmissible cancer affecting Tasmanian devils was discovered in 2014. This was named DFT2 and bears similar morphological features to DFT1 (the first DFTD). Genetic analyses confirmed that DFT2 arose independently to DFT1. The presence of immune cross-recognition of DFT1 and DFT2 in devils was explored. Both immunised and wild devils with serum IgG antibody responses against DFT1 showed similar responses against DFT2. This suggests that DFT1 and DFT2 have common antigens and a single vaccine could protect against both tumours.