Complexity of the T cell receptor Cβ isotypes in the Mexican axolotl: structure and diversity of the VDJCβ3 and VDJCβ4 chains

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Abstract
We have reported previously the presence of two T cell receptor β‐chain constant region (Cβ) isotypes in the Mexican axolotl. Specific Dβ and Jβ segments were present at the Vβ‐Cβ1 and Vβ‐Cβ2 junctions and nine Vβ families which associate with both isotypes were characterized. This report describes two new Cβ isotypes, Cβ3 and Cβ4. About 70 % of the amino acids in Cβ3 are identical to Cβ1 and Cβ2. A Dβ3 and a single Jβ3 were found at the Vβ‐Cβ3 junctions. The Dβ3 consensus core sequence (TACGTGGCTACGTGGG) differs to all the presently known Dβ and the CDR3β loops of the Vβ‐Cβ3 junctions (mean: 11.1 amino acids) contain a majority of aromatic, small hydrophobic and basic residues. The CDR3β loops of the other isotypes are shorter (mean: 8.5 amino acids), contain a majority of acidic residues and very few aromatic residues. The axolotl Cβ4 sequence has about 46 % similarity to Cβ1, Cβ2 and Cβ3. Dβ4 is identical to Dβ2 and six new Jβ segments are used at the Vβ‐Cβ4 junctions. Four new families of Vβ segments (Vβ10‐Vβ13) are preferentially associated to Cβ4. A strong selective pressure must operate in most vertebrates to preserve the structural stability of the extracellular part of the Cβ chain. The four axolotl Cβ seem to have evolved more freely, perhaps to favor the early emergence of a large diversity of T cell receptors in an amphibian species which is not fully immunocompetent before the 5th month of development.