Lamprey lymphocyte-like cells express homologs of genes involved in immunologically relevant activities of mammalian lymphocytes

Abstract

To shed light on the origin of adaptive immunity, a cDNA library was prepared from purified lymphocyte-like cells of a jawless vertebrate, the sea lamprey (Petromyzon marinus). Randomly selected cDNA clones were sequenced, and their homologies to proteins in the databases were determined. Of the sequences homologous to proteins involved in immune responses, five were selected for further characterization. Their encoding genes corresponded to loci that in jawed vertebrates are essential for activities of lymphocytes. These activities include regulation of T and B cell stimulation and proliferation (CD45); stabilization of molecular complexes involved in lymphocyte activation, adhesion, migration, and differentiation (CD9/CD81); adaptor functions in signaling leading to the activation of B lymphocytes (BCAP) and T lymphocytes (CAST); and amino acid transport associated with cell activation (CD98). The presence of these genes in the lamprey genome and their expression in lymphocyte-like cells support the notion that these cells perform many of the functions of gnathostome lymphocytes. It reopens the question of the stage jawless fishes reached in the evolution of their immune system.

Fig 1.

Neighbor-joining tree of CD45 sequences and related PTPs. The tree is based on the amino acid residues 906–972 and 1,104–1,661 of the alignment in Fig. 6. Numbers on the branches indicate bootstrap values. The tree is rooted at midpoint. The sequences are designated by the species name followed by the GenBank accession number. The highest scoring non-CD45 PTPs in a blastp search were chosen as outgroup sequences. Zebrafish NP_571963, PTP, receptor type A; rat AAB28877, receptor PTP-σ; fruit fly AAF49892 and Anopheles EAA08408 are translated from genes obtained from the genome-sequencing projects.

Fig 2.

Neighbor-joining tree of BCAP protein sequences. The tree is based on the amino acid alignment in Fig. 7. Numbers on the branches indicate bootstrap values. The tree is rooted at midpoint. The sequences are designated by the species name followed by the GenBank accession number.

Fig 3.

Alignment of the human, mouse, and lamprey CAST protein sequences. Amino acid residues are designated in the International Union of Pure and Applied Chemistry single-letter code. Exon–intron borders are indicated by vertical lines. Asterisks indicate indels introduced to optimize the alignment. The CD3ɛ-binding region is enclosed in an open box, and the lysine/arginine-rich segments enclosed in filled boxes. The conserved Tyr-82 is shaded. The GRF-rich region is indicated by a bracket, and shared residues between human/mouse and lamprey are indicated by open triangles. Accession numbers: NP_036231, human CAST; NP_665821, mouse CAST.

Fig 4.

Neighbor-joining tree of CD9 and CD81 proteins. The tree is based on the amino acid alignment in Fig. 8. Numbers on the branches indicate bootstrap values. The tree is rooted at midpoint. The sequences are designated by the species name followed by the GenBank accession number. and are the human Tspan2 and CO-029 sequences, respectively. Both are tetraspanins related to CD9 and CD81, and they were used as outgroups in the phylogenetic analysis.

Fig 5.

Neighbor-joining tree of CD98 proteins from different species. The tree is based on positions 136–800 of the amino acid alignment in Fig. 9. Numbers on the branches indicate bootstrap values. The tree is rooted at midpoint. The sequences are designated by the species name followed by the GenBank accession number. E. coli, Escherichia coli; S. pombe, Schizosaccharomyces pombe; D. thermophilum, Dictyoglomus thermophilum; C. elegans, Caenorhabditis elegans.