Prototypic T cell receptor and CD4-like coreceptor are expressed by lymphocytes in the agnathan sea lamprey

Abstract

All jawed vertebrates have highly diverse lymphocyte receptors, which allow discrimination between self and nonself antigens as well as the recognition of potential pathogens. Key elements of the anticipatory recombinatorial immune system in jawed vertebrates are the TCR, Ig, and MHC genes, but their ancestral genes have not been found in more basal vertebrates. In this study, we extended our analysis of the transcriptome of lymphocyte-like cells in the lamprey to identify the TCR-like and CD4-like genes. The structural features of these genes and their preferential expression in lymphocytes make them attractive candidates for ancestral TCR and CD4 genes. The TCR-like gene contains both V (variable) and J (joining) sequences in its first exon and exists as a single-copy gene that is invariant. Thus, the TCR-like gene cannot account for the receptor diversity that is required for the immune responses reported for lamprey, but it could have been easily modified to serve as an evolutionary precursor of modern TCR and Ig genes.

Fig. 1.

Multiple alignment of the lamprey TCR-like and CD4-like molecules with related vertebrate lymphocyte receptors. (A and C) Jawed vertebrate TCRα and CD4 molecules. Approximate location of Ig and transmembrane (TM) domains as well as the location of the J region in TCRs are indicated. Arrowheads indicate the positions of intron 4 in the lamprey TCR-like and intron 6 in the CD4-like. Cysteine residues are highlighted in red. Green, ≥80% identity and similarity; yellow, 60-79% identity/similarity. Amino acid similarity groups are as follows: acidic, D and E; aromatic, F, W, and Y; basic, H, K, and R; hydrophobic, A, I, L, M, and V; polar, N, Q, S, and T; and ungrouped, G and P. (B and D) Stick models of exons and introns in the TCR-like and CD4-like genes. The position of introns was determined by sequencing genomic clones and are indicated by solid lines. SP, signal peptide; CP, connecting peptide. GenBank accession nos. for the sequences shown in A are as follows: duck, AN3311A9; chicken, AAC98541; human, AAC14929; takifugu (Takifugu rubripes), AAF97794; zebrafish (Danio rerio), AAL23935; and lamprey TCR-like, AY686861. GenBank accession nos. for the sequences shown in C are as follows: human, NP_000607; white whale (Delphinapterus leucas), AAD23738; rabbit, P46630; mouse, NP_038516; rat, NP_036837; duck, AAK59279; chicken, NP_989980; and lamprey CD4-like, AY686862.

Fig. 2.

Phylogenetic and structural analysis of the lamprey TCR-like and CD4-like molecules. (A) Neighbor-joining trees of the two N-terminal Ig domains of the TCRα and TCRβ chains are shown for the following: takifugu, residues 1-234; tetraodon (Tetraodon nigroviridis), residues 1-232 (GenBank accession no. CAC86241); zebrafish, residues 1-226; damselfish (Stegastes partitus), residues 1-256 (GenBank accession no. AAG46047); Atlantic cod (Gadus morhua), residues 1-239 (GenBank accession no. CAB39549); Rainbow trout (Oncorhynchus mykiss), residures 1-238 (GenBank accession no. AAK63017); Bastard halibut (Paralichthys olivaceus), residues 1-254 (GenBank accession no. BAB82597); Horn shark (Heterodontus francisci), residues 1-230 (GenBank accession no. AAA61563); human, residues 1-228; chicken, residues 1-235; duck, residues 1-237; lamprey, residues 1-243; mouse CD166, residues 1-239 (GenBank accession no. Q61490). (B) Neighbor-joining trees of the two N-terminal Ig domains of the CD4 coreceptors are shown for the following: human, residues 1-201; whale, residues 1-201; rabbit, residues 1-206; mouse, residues 1-205; rat, residues 1-204; duck, residues 1-213; chicken, residues 1-217; and lamprey, residues 1-208. (C) Structural 3D model of the N-terminal IgV domains from the lamprey TCR-like and CD4-like molecules (color) threaded on the crystal-structure coordinates (black) of the mouse TCRVα (PDB ID code 1B88) and human CD4 N-terminal IgV (PDB ID code 1CDY) created by using the 3d-pssm program. (D) Sequence and structural alignments of the lamprey TCR-like and CD4-like Ig domains used for the 3D-structure modeling. SS, secondary structure (helix, red; strand, green; coil, blue); PSS, predicted structure; CORE, measure of burial and contacts (0, unburied).

Fig. 3.

Lamprey TCR-like and CD4-like expression patterns and genomic loci. (A) Virtual Northern blots using cDNA amplified from tissues and cell populations purified from hematopoietic organs and blood of unstimulated and immunostimulated larvae. GAPDH hybridization served as a control. (B) Genome blots of three lampreys. Genomic DNA was digested with the indicated restriction enzymes.