Generation of diversity by somatic mutation in theCamelus dromedariusT‐cell receptor gamma variable domains

Abstract

In jawed vertebrates the V-(D)-J rearrangement is the main mechanism generating limitless variations of antigen-specific receptors, immunoglobulins (IGs), and T-cell receptors (TCRs) from few genes. Once the initial diversity is established in primary lymphoid organs, further diversification occurs in IGs by somatic hypermutation, a mechanism from which rearranged TCR genes were thought to be excluded. Here, we report the locus organization and expression of the T-cell receptor gamma (TCRG) genes in the Arabian camel (Camelus dromedarius). Expression data provide evidence that dromedary utilizes only two TCRG V-J genomic arrangements and, as expected, CDR3 contributes the major variability in the V domain. The data also suggest that diversity might be generated by mutation in the productively rearranged TCRGV genes. As for IG genes, the mutational target is biased toward G and C bases and (A/G/T)G(C/T)(A/T) motif (or DGYW). The replacement and synonymous substitutions (R/S) ratios in TCRGV regions are higher for CDR than for framework region, thus suggesting selection toward amino acid changes in CDR. Using the counterpart human TCR γδ receptor as a template, structural models computed adopting a comparative procedure show that nonconservative mutations contribute to diversity in CDR2 and at the γδ V domain interface. Keywords: Camelus dromedarius · Protein modeling · Somatic hypermutation · T-cell receptor gamma (TCRG) Supporting Information available onlineCorrespondence: Dr. Salvatrice Ciccarese e-mail: ciccarese@biologia.uniba.it IntroductionTo respond to the wide spectrum of antigenic determinants presented by an almost limitless variety of diverse and evolving pathogens, the metazoan immune system developed a striking C 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu Eur. J. Immunol. 2012. 42: 3416-3428 Molecular immunology 3417 variation of immune receptor molecules and diversification mechanisms [1]. In jawed vertebrates, the T-cell response is mediated by signaling from the T-cell receptors αβ and γδ, both encoded by complex multigene families. Genetic information for receptor chains is carried by a germline pool of variable (V), joining (J), and diversity (D) genes that undergo somatic DNA rearrangements to generate receptors with diverse-binding specificity [2]. The "innate-like" γδ T cells have unique features when compared with the more abundant αβ T cells, e.g. a preferential distribution in both epithelial and mucosal sites, an immunoglobulin (IG)-like antigen recognition mechanism in addition to the MHC-restricted one. Moreover, their percentage in peripheral blood cells, depending on age and species, differs strikingly from that of αβ T cells [3]. Artiodactyls are referred to as "γδ-high species" since they exhibit a higher frequency and a wider physiological distribution of γδ T cells with respect to other mammalian species, including humans and mice which are referred to as "γδ-low species" [4]. The locus organization and expression of TCRG and TCRD genes have been charac...

Sequence and polymorphism analysis of the camel (Camelus dromedarius) myostatin gene

“…On the contrary, they found a C→T transition responsible for an Alanine to Valine amino acidic substitution when a 422 bp region of the first exon of the MYF5 gene was screened in the same animals. Vaccarelli et al (2012) detected, at the T-cell receptor (TCR) gamma genes, SNPs with a frequency of one every 114 to 485 nucleotides, depending on the considered rearrangements, and somatic hypermutation was suggested on that locus as a mechanism to diversify the Camelus dromedarius TCR gene repertoire face to the wide spectrum of antigenic determinants presented by a variety of diverse and evolving pathogens. High nucleotide sequence diversity as a consequence of somatic hypermutation had been previously reported also for the T-cell receptor delta chains (Antonacci et al, 2011).…”

Genomic and expression analyses of Tursiops truncatus T cell receptor gamma (TRG) and alpha/delta (TRA/TRD) loci reveal a similar basic public γδ repertoire in dolphin and human

“…1 ). According to our results, the dolphin locus is the simplest of the mammalian TRG loci identified to date (Additional file 1 ) [ 9 , 10 , 21 – 23 ]. It spans only 48 kb and its genes are arranged in a pattern comprising 2 TRGV, 3 TRGJ genes and a single TRGC (Additional file 2 ) gene.…”

Chromosome‐level assembly of wild Bactrian camel genome reveals organization of immune gene loci

“…It was reported that somatic hypermutation contributed to the generation of diversity within the dromedary TRG and TRD variable domains (Antonacci et al, 2011;Vaccarelli et al, 2012). This represented another evolutionary innovation of the camelid immune system, as somatic hypermutation was primarily adopted in B cells to enhance antibody diversity but was rarely documented in T cells (Ciccarese et al, 2014).…”

“…With regard to the T-cell receptors (TCRs), it was found that although the germline repertoire of dromedaries appeared limited compared to that of other artiodactyls, extensive somatic hypermutation occurred on rearranged TRDV and TRGV genes to enhance their diversity (Antonacci et al, 2011;Ciccarese et al, 2014;Vaccarelli et al, 2012). This was quite unusual, as somatic hypermutation was only documented for IGs and not for TCRs in other mammals.…”