Origin and evolution of the chicken leukocyte receptor complex

Abstract

In mammals, the cell surface receptors encoded by the leukocyte receptor complex (LRC) regulate the activity of T lymphocytes and B lymphocytes, as well as that of natural killer cells, and thus provide protection against pathogens and parasites. The chicken genome encodes many Ig-like receptors that are homologous to the LRC receptors. The chicken Ig-like receptor (CHIR) genes are members of a large monophyletic gene family and are organized into genomic clusters, which are in conserved synteny with the mammalian LRC. One-third of CHIR genes encode polypeptide molecules that contain both activating and inhibitory motifs. These genes are present in different phylogenetic groups, suggesting that the primordial CHIR gene could have encoded both types of motifs in a single molecule. In contrast to the mammalian LRC genes, the CHIR genes with similar function (inhibition or activation) are evolutionarily closely related. We propose that, in addition to recombination, single nucleotide substitutions played an important role in the generation of receptors with different functions. Structural models and amino acid analyses of the CHIR proteins reveal the presence of different types of Ig-like domains in the same phylogenetic groups, as well as sharing of conserved residues and conserved changes of residues between different CHIR groups and between CHIRs and LRCs. Our data support the notion that the CHIR gene clusters are regions homologous to the mammalian LRC gene cluster and favor a model of evolution by repeated processes of birth and death (expansion-contraction) of the Ig-like receptor genes.birth and death ͉ conserved synteny ͉ genomic organization ͉ Ig-like receptors N atural killer (NK) cells are a subpopulation of lymphocytes that function in innate immunity by recognizing and destroying virally infected and cancerous cells (1). NK cells are regulated by the interaction of inhibitory and activating signals emitted by cell surface receptors. The inhibitory receptors contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in their cytoplasmic (CYT) region, whereas the activating receptors contain a positively charged residue in their transmembrane (TM) region (2). The mammalian NK cell receptors fall into two categories, one belonging to the Ig superfamily and the other to the C-type lectin superfamily. The Ig-like receptors are encoded by a genomic region called the leukocyte receptor complex (LRC) (3). The LRC contains several gene families [e.g., the killer cell Ig-like receptors (KIRs), the leukocyte Ig-like receptors (LILRs), and the paired Ig-like receptors] and singleton genes. The presence of the LRC in all mammals studied thus far and the overall structural similarities of the LRC genes suggest that all of these genes have evolved from a common ancestral sequence and that the LRC region was formed before the mammalian radiation.The common origin of the LRC genes is also supported by the existence of several homologous sequences in the chicken (4-7). It has been proposed that the Ig-like domains...

Chicken genome: Current status and future opportunities

“…TNF is also absent, but its receptor, TNFRSF1A (ENSGALG00000014890) is present, suggesting that further sequencing will reveal this gene in the chicken. Similar analyses have been performed on the leukocyte receptor complex (Nikolaidis et al 2005) that regulates the activity of Tand B-lymphocytes and NK cells. A model of evolution by repeated birth and death of these Ig-like receptors' genes was proposed.…”

Concerted and Birth-and-Death Evolution of Multigene Families

“…In addition to the above multigene families, the gene families concerned with innate immunity have also been shown to undergo birth-and-death evolution (42,43,107,108). The evolutionary pattern of these genes is more complex than those of the adaptive immune system.…”

Ancient divergence of a complex family of immune-type receptor genes

“…Evolutionary inferences regarding the large families of activating/inhibitory leukocyte regulatory receptors have been limited mainly to humans and mice; however, recent work has extended to address relationships of the chicken CHIR gene complex to genes of the mammalian leukocyte receptor complex (Nikolaidis et al 2005;Viertlboeck et al 2005). A family of activating/inhibitory IgSF members also has been reported in a jawless vertebrate; however, similarity of these proteins to potential orthologs in jawed vertebrates is remote (Suzuki et al 2005).…”

“…Activating/inhibitory immune-type proteins constitute the single most diverse group of the mammalian IgSF receptors; however, their genetic regulation, developmental control, functional interactions, and evolutionary origins are understood only in part. One line of investigation that eventually could shed light on several of these issues is to characterize counterparts of these genes in species that occupy representative positions in vertebrate phylogeny (Nikolaidis et al 2005;Viertlboeck et al 2005). …”