Devil facial tumour disease (DFTD) is a recently emerged fatal transmissible cancer decimating the wild population of Tasmanian devils (Sarcophilus harrisii). Biting transmits the cancer cells and the tumour develops in the new host as an allograft. The literature reports that immune escape mechanisms employed by DFTD inevitably result in host death. Here we present the first evidence that DFTD regression can occur and that wild devils can mount an immune response against the disease. Of the 52 devils tested, six had serum antibodies against DFTD cells and, in one case, prominent T lymphocyte infiltration in its tumour. Notably, four of the six devils with serum antibody had histories of DFTD regression. The novel demonstration of an immune response against DFTD in wild Tasmanian devils suggests that a proportion of wild devils can produce a protective immune response against naturally acquired DFTD. This has implications for tumour–host coevolution and vaccine development.

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AbstractTransmissible cancers are spread via the passage of malignant cells. The survival of the Tasmanian devil, the largest marsupial carnivore, is threatened by two independent transmissible cancers, devil facial tumour (DFT) 1 and devil facial tumour 2 (DFT2). To aid the development of a peptide vaccine and to interrogate how histocompatibility barriers can be overcome, we analysed the peptides bound to Major Histocompatibility Complex class I molecules from the Tasmanian devil and its transmissible tumours. Comparison of the peptidomes from DFT1+IFNγ, DFT2 and host fibroblast cells demonstrates a shared motif, despite differences in MHC-I allotypes between the cell lines. Importantly, DFT1+IFNγ and DFT2 share the presentation of peptides derived from neural proteins, reflecting a common cellular origin that should be exploited for vaccine design. These results suggest that some polymorphisms between tumours and host are ‘hidden’ by a common peptide motif, providing the potential for permissive passage of infectious cells.

IntroductionThe marsupial species Tasmanian devil harbours two contagious cancers, Devil Facial Tumour 1 and 2 (DFT1 and DFT2); both are passed between individuals as an allogeneic graft, initiating tumours around the neck and face. DFT1 emerged 20 years ago, causes 100% mortality and has decimated the devil population. In contrast, DFT2 emerged only recently and is still being characterised. As allografts, both tumours should be recognised by T cells interacting with non-self Major Histocompatibility Complex (MHC) molecules and associated peptides. We aim to isolate tumour-specific MHC-bound peptides in order to determine MHC/peptide complexes breaking or inducing tolerance and design a vaccine.Material and methodsWe have characterised the immunopeptidomes of DFT1-IFNγ, DFT2 and devil fibroblast cell lines. The DFT1 cell line was stimulated with devil interferon-gamma to up-regulate MHC class I expression. MHC class I molecule-peptide complexes were isolated by immunoaffinity purification using an antibody against Tasmanian devil β2-microblobulin. Eluted peptide/MHC complexes were fractionated by reversed phase HPLC and analysed by Liquid Chromatography tandem mass spectrometry (LC-MS/MS). All experiments were performed in triplicate. Peptides were identified by searching spectra against custom Tasmanian devil databases using the PEAKS software and data visualised in R studio and GraphPad Prism.Results and discussionsBetween 6373 and 2243 peptides were identified for each cell line, but only peptides found in all replicates for each cell line analysed further. 1806 peptides were identified in all three replicates of the fibroblast cell line, while 455 and 379 peptides were common to all three replicates for the DFT1-IFNγ and DFT2 cell lines respectively. Analysis of peptide length shows a preference for 8mers and 9mers in devil fibroblasts and DFT2 cells and a preference for 9mers in DFT1-IFNγ. We next searched for binding motifs for the 8mers and 9mers across all cell lines and found potential anchor residues at position 3 and position 8/9, where there was a preference for hydrophobic amino acids (in particular Leucine). We then identified 61 and 55 peptides unique to DFT1-IFNγ and DFT2 respectively.ConclusionThis is the first study to characterise the repertoire of peptides bound to MHC molecules in contagious cancers and represents a pivotal step for understanding the immunological features of transmissible cancers.