TCR gamma chain diversity in the spleen of the duckbill platypus (Ornithorhynchus anatinus)
Parra, Z.E., Arnold, T., Nowak, M.A., Hellman, L. Miller, R. D.
Abstract:
The diversity of TCR gamma (TRG) chains expressed in splenic γδ T cells was determined for an egg-laying mammal (or monotreme), the duckbill platypus. Three distinct V subgroups were found in the TRG chain rearrangements analyzed. These three subgroups are members of a clade not found so far in eutherian (“placental”) mammals or birds. Each subgroup contains approximately five V gene segments, and their overall divergence is much less than is found in eutherians and birds. This is consistent with their recent evolution from an ancestral V gene segment with the loss of more ancient diversity that would have been present in ancestral mammals. Evidence for three C region genes in the platypus TRG locus was found and their sequences were compared to that of a second monotreme species, the short beaked echidna, and also to that of marsupials and eutherians. Many of the residues involved in TCR function, such as interactions with CD3, are conserved across all three mammalian lineages. However, surprisingly all non-eutherian mammals (monotremes and marsupials) lacked the second cysteine residue necessary to form the intra-domain disulfide bond in the C region, a loss apparently due to independent mutations in marsupials and monotremes. Monotreme TRGC regions also had among the most variation in the length of the connecting peptide region described for any species due to repeated motifs.