Cloning and modeling of CD8 β in the amphibian ambystoma Mexicanum. Evolutionary conserved structures for interactions with major histocompatibility complex (MHC) class I molecules

Mammalian and avian T-cells exhibit a large number of well characterized surface molecules associated with their maturation degree. Very little is known in comparison with T-cell differentiation in ectothermic vertebrates. This is mainly due to the lack of probes to identify T-cell subsets. We cloned and sequenced the first ectothermic CD8 β DNA complementary to RNA from an amphibian species, the Mexican axolotl. The CD8 β chain was 30–36% identical with its avian and mammalian homologues. The extracellular V-like domain contained the two typically conserved cysteines and was followed by a J-like sequence containing the canonical Phe-Gly-X-Gly stretch. The connecting peptide was much longer than in other species and contained potential O-glycosylation sites. The axolotl CD8 β and major histocompatibility complex class I molecules were modeled using human HLA-A2/CD8 αα complex as template. The backbone conformation of axolotl CD8 β matched well with the CD8 α-2 subunit of the human complex but significant structural differences were located in the CDR1, CDR2 and DE loops. Both axolotl and human class I showed large negative surface potential. The interacting area of the human CD8 α chain and of the corresponding region of axolotl CD8 β had positive electrostatic potential compatible with complexation with the corresponding class I molecules. The presence of a CD8 β homologue in an amphibian species implies that it was already present in the Devonian ancestor of amphibians and mammals, i.e. more than 400 million years ago.