Abstract
This study reports the presence of two distinct MHC class II β genes in the Antarctic icefish Chionodraco hamatus, belonging to the classical (ChhaDAB) and nonclassical (ChhaDBB) evolutionary lineages. By the application of targeted sequencing approach, a remarkable molecular diversity in the exon 2 sequence of the highly expressed gene ChhaDAB has been observed, resulting in an estimate of 92 different variants translated in 87 different peptides from 54 analysed icefish individuals. A highly conservative estimate, based on a 95% sequence identity threshold clustering, translate this variability in 41 different peptide clusters belonging to four different clades and showing the signature of different kinds of selection. In stark contrast, the poorly expressed ChhaDBB gene displayed a very low level of molecular diversity within exon 2, in agreement with expectations for a nonclassical MHC class II β gene.
Highlights
Due to their key role as components of the adaptive immune system, the major histocompatibility complex class I and II molecules (MHC I and MHC II) have been widely studied in vertebrates[1,2,3]
A typical classical MHC class II β sequence is characterized by high expression and polymorphism, and by the conservation of key residues involved in the formation of hydrogen bonds with the backbone of peptide ligands and in the binding of CD4
The two MHC class II β sequences identified from the Chionodraco hamatus gills transcriptome show about 41% amino acid identity and have been designated as Chha-DAB and Chha-DBB, respectively
Summary
Due to their key role as components of the adaptive immune system, the major histocompatibility complex class I and II molecules (MHC I and MHC II) have been widely studied in vertebrates[1,2,3]. These molecules are heterodimers formed by two subunits (α and β), each composed by two domains, which bind self and non-self-peptides and present them to T-cells. One of the major discoveries in the study of MHC class I and II in teleost species resides in the different structural organization of their genes compared to other vertebrate groups[6,7]. A third lineage (the E lineage), found in other teleosts (sea bream, fathead minnow, carp, Atlantic salmon and rainbow trout), shows significant differences compared to all other fish MHC class II β sequences and shares several features with classical and nonclassical tetrapod molecules[15]
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