A family of olfactory receptors uniquely expanded in marsupial and monotreme genomes are expressed by a T cell subset also unique to marsupials and monotremes.

Olfactory receptors (OR), expressed on olfactory neurons, mediate the sense of smell. Recently, OR have also been shown to be expressed in non-olfactory tissues, including cells of the immune system. An analysis of single-cell transcriptomes of splenocytes of the grey short-tailed opossum (Monodelphis domestica) found OR are expressed on a subset of T cells, the γμ T cells, that are unique to marsupials and monotremes. A majority of opossum γμ T cells transcriptomes contain OR family 14 transcripts, specifically, from the OR14C subfamily. Amongst the mammals, the OR14 gene family is expanded in the genomes of marsupials and monotremes, and rarer or absent in placental mammals. In summary, here we demonstrate the intriguing correlation that a family of OR genes, abundant in the genomes of marsupials and monotremes, are ectopically expressed in a particular subset of T cells unique to the marsupials and monotremes.