The marsupial CD8 gene locus: Molecular cloning and expression analysis of the alpha and beta sequences in the gray short-tailed opossum ( Monodelphis domestica) and the tammar wallaby ( Macropus eugenii)

Abstract

In eutherian mammals, CD8 is a key receptor of cytotoxic T cells and plays a pivotal role in the recognition and elimination of infected host cells by cell-mediated cytotoxicity. Here, we report the molecular cloning and expression analysis of CD8α and CD8β cDNAs in two marsupial species, the gray short-tailed opossum and the tammar wallaby. The opossum and tammar CD8 sequences share a high degree of amino acid identity of 63% (CD8α) and 57% (CD8β) to each other as well as 36–45% (CD8α) and 38–41% (CD8β) with their eutherian counterparts. In addition, many of the signature features of eutherian CD8α and CD8β are preserved in both marsupials including the two invariant cysteines that form the intra-chain disulphide bond in the extracellular IgSfV domain and the two hinge region cysteines involved in dimerisation between the two subunits. The p56 lck binding motif in the cytoplasmic tail of the CD8α subunit is also conserved. Interestingly, the opossum CD8α and the tammar CD8β sequences have a truncated cytoplasmic tail. RT-PCR analysis of CD8α and CD8β transcripts in the tissues of the adult opossum and tammar showed broad tissue expression with a high level of expression observed in the lymphoid tissues of both marsupials. Furthermore, RT-PCR analysis of CD8α and CD8β transcripts in the immune tissues of tammar young over the first 120 days of pouch life revealed a pattern of expression analogous to the maturation of the lymphoid tissues. This is the first report confirming the presence of CD8 in the tissues of a marsupial and will provide the tools to further analyse T cell subsets in this unique group of mammals.