Transmission of a fatal clonal tumor by biting occurs due to depleted MHC diversity in a threatened carnivorous marsupial

Abstract

A fatal transmissible tumor spread between individuals by biting has emerged in the Tasmanian devil (Sarcophilus harrisii), a carnivorous marsupial. Here we provide genetic evidence establishing that the tumor is clonal and therefore foreign to host devils. Thus, the disease is highly unusual because it is not just a tumor but also a tissue graft, passed between individuals without invoking an immune response. The MHC plays a key role in immune responses to both tumors and grafts. The most common mechanism of immune evasion by tumors is down-regulation of classical cell surface MHC molecules. Here we show that this mode of immune escape does not occur. However, because the tumor is a graft, it should still be recognized and rejected by the host's immune system due to foreign cell surface antigens. Mixed lymphocyte responses showed a lack of alloreactivity between lymphocytes of different individuals in the affected population, indicating a paucity of MHC diversity. This result was verified by genotyping, providing a conclusive link between a loss of MHC diversity and spread of a disease through a wild population. This novel disease arose as a direct result of loss of genetic diversity and the aggressive behavior of the host species. The neoplastic clone continues to spread although the population, and, without active disease control by removal of affected animals and the isolation of disease-free animals, the Tasmanian devil faces extinction.

Fig. 1.

RT-PCR showing expression of MHC class I (A) and class II (B) genes by matched tumor, liver, spleen, and kidney samples. T, tumor biopsy; S, spleen; L, liver; K, kidney; N, negative control; M, marker.

Fig. 2.

Mixed lymphocyte reaction and Con A stimulation of lymphocytes from 30 devils and 2 eastern quolls obtained from the same region of Tasmania. Due to the limited amount of blood that could be collected from the quolls, only a single (96 h) time point was possible. To confirm that the devil lymphocytes could proliferate, Con A was added at a final concentration of 50 mg/ml.

Fig. 3.

Amino acid alignments of class I α1 and α2 domains from devils. (A) Amino acid alignment of unique class I α1 domain sequences from devils. Orientation of the residues in the molecule is indicated below the sequence as follows; asterisks, residue points toward the peptide binding site; plus sign, residue is on the α-helix pointing upward toward the binding site; dot, residue is on the α-helix pointing away from the peptide binding site. (B) Amino acid alignment of unique class I α2 sequences from devils. Orientation of the residues in the molecule is indicated below the sequence as in A.

Fig. 4.

Graphical representation of polymorphism in the PBR of class I sequences from humans (HLA-A, HLA-B, and HLA-C) (19), wild Gir lions (multiple loci) (n = 25), and Tasmanian devils (multiple loci) (n = 25). The y axis shows the number of substitutions at polymorphic residues involved in peptide binding. The x axis shows the amino acid position according to Bjorkman and Parham (19).