A new T cell receptor discovered in marsupials.
Parra, Z. E. Baker, M. L., Schwarz, R. S., Deakin, J. E., Lindblad-Toh, K., and Miller, R. D.
Abstract:
T cells recognize antigens using receptors (TCR) encoded by gene segments, called variable (V), diversity (D) and joining (J) that undergo somatic recombination to create diverse binding specificities. Four TCR chains (TCRα, β, γ, and δ) have been identified so far and as T cells develop in the thymus they express exclusively either an αβ or a γδTCR heterodimer. Here we show that marsupials have an additional TCR (TCRμ) that has V, D, and J that are either somatically recombined, as in conventional TCR, or are already pre-joined in the germ-line DNA in a manner consistent with their creation by retrotransposition. TCRμ does not have a known homolog in eutherian mammals but has features analogous to a recently described TCRδ isoform in sharks. TCRμ is expressed in at least two mRNA isoforms that appear capable of encoding a full-length protein, both of which are transcribed in the thymus and spleen. One contains two variable domains: a somatically recombined V and a pre-joined V. This appears to be the dominant isoform in peripheral lymphoid tissue. The other isoform contains only the pre-joined V and is structurally more similar to conventional TCR chains, however invariant. TCRμ is the first TCR described to utilize pre-joined gene segments and is likely present in all marsupials. Its similarity to a TCR isoform in sharks suggests that it, or something similar, may be present in other vertebrate lineages and therefore represent an ancient receptor system.