Sea bass (Dicentrarchus labrax) invariant chain and class II major histocompatibility complex: sequencing and structural analysis using 3D homology modelling

Antigen presentation in vertebrates: Structural and functional aspects

Antigen presentation is a key process of the immune system and is responsible for the activation of T cells. The main characters are the major histocompatibility complex class I (MHC-I) and class II (MHC-II) molecules, and accessory proteins that act as chaperones for these glycoproteins. Current knowledge of this process and also the elucidation of the structural features of these proteins, has been extensively reviewed in humans. Unfortunately, this is not the case for non-human species, wherein the function and structural characteristic of the antigen presentation proteins is far from being understood. The majority of previous studies in non-human species, especially in teleost fish and lower vertebrates, are limited to the transcriptomic level, which leads to gaps in the knowledge about the functional process of antigen presentation in these species. This review summarizes what is known so far about antigen presentation pathways in vertebrates from a structural and functional perspective. The focus is not only on the MHC receptors, but also, on the forgotten characters of these pathways such as the proteins of the peptide loading complex, and the MHC-II chaperone invariant chain.

Characterization of cDNA clones encoding major histocompatibility class II receptors from walleye (Sander vitreus)

The teleost major histocompatibility (MH) class II receptor presents peptides from exogenous sources to CD4⁺ T cells, leading to the initiation of the adaptive immune response. The genes encoding MH class II have been identified in a number of teleost species, but not in walleye, an important recreational fish and commercial fishery in North America. In this study, we cloned and characterized the sequences encoding walleye MH class II α and β chains. These sequences contained all of the domains typical for functional MH class II α and β chain proteins, and aligned with other teleost sequences of MH class II. The walleye MH class II α amino acid sequence, along with other members of the Supraorder Percomorpharia, contains a high concentration of methionine residues in the beginning of the leader peptide. Southern blotting indicated that there is more than one gene copy for both MH class II α and β, while northern blotting analysis of both genes showed that expression of these genes is greatest in lymphoid tissues and at potential entry points for pathogens. These results help to further the understanding of MH class II receptors in teleosts, and could prove useful in the study of disease issues in walleye such as dermal sarcoma virus.

MHC class II alpha, beta and MHC class II-associated invariant chains from Chinese sturgeon (Acipenser sinensis) and their response to immune stimulation

The major histocompatibility complex class II (MHC II) molecules play a vital role in adaptive immune response through presenting antigenic peptides to CD4⁺ T lymphocytes. To accomplish this physiologic function, the MHC class II-associated invariant chain interacts with the MHC II α/β subunits and promotes their correct assembly and efficient traffic. Here, we isolated the cDNAs of MHC II α, β and MHC II-associated invariant chains (designated as CsMHC II α, CsMHC II β, and CsMHC II γ) from Chinese sturgeon (Acipenser sinensis). The CsMHC II α, β, and γ mRNAs were widely expressed in Chinese sturgeon, and the highest expression was found in spleen for CsMHC II α and β chains, while in head kidney for CsMHC II γ chain. Stimulation to Chinese sturgeon with inactivated trivalent bacterial vaccine or polyinosinic polycytidylic acid (poly(I:C)) up-regulated the expressions of CsMHC II α, and β mRNAs, and their transcripts were overall more quickly up-regulated by poly(I:C) than by bacterial vaccine. Poly(I:C) induced higher CsMHC II γ expression than bacterial vaccine in intestine and spleen, while lower than bacterial vaccine in head kidney and liver. When co-expressed in mouse dendritic cells, the CsMHC II γ chain bound to both the MHC II α and β chains. Furthermore, the over-expressed CsMHC II γ chain, not CsMHC II α or CsMHC II β chain, activated NF-κB and STAT3 in mouse dendritic cells, and induced TNF-α and IL-6 expressions as well. This activity was nearly abolished by mutation of the Ser29/Ser34 to Ala29/Ala34 in CsMHC II γ. These results suggested that CsMHC II α, β, and γ chains might play important role in immune response to pathogen microbial infection of Chinese sturgeon possibly via a conserved functional mechanism throughout vertebrate evolution, which might contribute to our understanding the immune biology of sturgeons.

Grass carp (Ctenopharyngodon idellus) invariant chain of the MHC class II chaperone protein associates with the class I molecule

The invariant chain (Ii) is an important immune molecule, as it assists major histocompatibility complex (MHC) class II molecules to present antigenic peptides. The relationship between the Ii and MHC molecules in teleosts remains poorly understood. This study focused on the molecular structure of grass carp Ii (gIi), its organ distribution, correlations with gene transcription, and the association with MHC. gIi cDNA was cloned using designed degenerate primers and the rapid amplification of cDNA ends method (RACE). The gIi sequence was 92%-96% similar to that of other teleosts, but only 52%-67% similar to that of mammals, respectively. The gIi gene was distributed in all 12 organs examined by PCR. The gIi gene transcription levels were markedly higher in organs enriched with immune cells than in other organs (P < 0.01). Moreover, positive correlations were detected between transcription levels of the gIi and gMhcI or II genes in different organs (r = 8.415-8.523, P = 0.001). The gIi co-localized on endomembrane systems with either class I or II molecules in co-transfected cells observed by a laser confocal. Further testing confirmed that the gIi bound gMHCI and II molecules. Taken together, these results indicate that the gIi is associated with MHC class I and II molecules, suggesting homology of both MHC molecules.

Genomic structure and expression pattern of MHC IIα and IIβ genes reveal an unusual immune trait in lined seahorse Hippocampus erectus

The major histocompatibility complex (MHC) genes are crucial in the adaptive immune system, and the gene duplication of MHC in animals can generally result in immune flexibility. In this study, we found that the lined seahorse (Hippocampus erectus) has only one gene copy number (GCN) of MHC IIα and IIβ, which is different from that in other teleosts. Together with the lack of spleen and gut-associated lymphatic tissue (GALT), the seahorse may be referred to as having a partial but natural "immunodeficiency". Highly variable amino acid residues were found in the IIα and IIβ domains, especially in the α1 and β1 domains with 9.62% and 8.43% allelic variation, respectively. Site models revealed seven and ten positively selected positions in the α1 and β1 domains, respectively. Real-time PCR experiments showed high expression levels of the MHC II genes in intestine (In), gill (Gi) and trunk kidney (TK) and medium in muscle (Mu) and brood pouch (BP), and the expression levels were significantly up-regulated after bacterial infection. Specially, relative higher expression level of both MHC IIα and IIβ was found in Mu and BP when compared with other fish species, in which MHC II is expressed negligibly in Mu. These results indicate that apart from TK, Gi and In, MU and BP play an important role in the immune response against pathogens in the seahorse. In conclusion, high allelic variation and strong positive selection in PBR and relative higher expression in MU and BP are speculated to partly compensate for the immunodeficiency.

Unraveling the evolution of the Atlantic cod's (Gadus morhua L.) alternative immune strategy

Genes encoding the major histocompatibility complex (MHC) have been thought to play a vital role in the adaptive immune system in all vertebrates. The discovery that Atlantic cod (Gadus morhua) has lost important components of the MHC II pathway, accompanied by an unusually high number of MHC I genes, shed new light on the evolution and plasticity of the immune system of teleosts as well as in higher vertebrates. The overall aim of this study was to further investigate the highly expanded repertoire of MHC I genes using a cDNA approach to obtain sequence information of both the binding domains and the sorting signaling potential in the cytoplasmic tail. Here we report a novel combination of two endosomal sorting motifs, one tyrosine-based associated with exogenous peptide presentation by cross-presenting MHCI molecules, and one dileucine-based associated with normal MHC II functionality. The two signal motifs were identified in the cytoplasmic tail in a subset of the genes. This indicates that these genes have evolved MHC II-like functionality, allowing a more versatile use of MHC I through cross-presentation. Such an alternative immune strategy may have arisen through adaptive radiation and acquisition of new gene function as a response to changes in the habitat of its ancestral lineage.

Shark class II invariant chain reveals ancient conserved relationships with cathepsins and MHC class II

The invariant chain (Ii) is the critical third chain required for the MHC class II heterodimer to be properly guided through the cell, loaded with peptide, and expressed on the surface of antigen presenting cells. Here, we report the isolation of the nurse shark Ii gene, and the comparative analysis of Ii splice variants, expression, genomic organization, predicted structure, and function throughout vertebrate evolution. Alternative splicing to yield Ii with and without the putative protease-protective, thyroglobulin-like domain is as ancient as the MHC-based adaptive immune system, as our analyses in shark and lizard further show conservation of this mechanism in all vertebrate classes except bony fish. Remarkable coordinate expression of Ii and class II was found in shark tissues. Conserved Ii residues and cathepsin L orthologs suggest their long co-evolution in the antigen presentation pathway, and genomic analyses suggest 450 million years of conserved Ii exon/intron structure. Other than an extended linker preceding the thyroglobulin-like domain in cartilaginous fish, the Ii gene and protein are predicted to have largely similar physiology from shark to man. Duplicated Ii genes found only in teleosts appear to have become sub-functionalized, as one form is predicted to play the same role as that mediated by Ii mRNA alternative splicing in all other vertebrate classes. No Ii homologs or potential ancestors of any of the functional Ii domains were found in the jawless fish or lower chordates.

Molecular characterization and expression analysis of MHC class II alpha and beta genes in large yellow croaker (Pseudosciaena crocea)

MHC class II molecules play an important role in the activation of CD4(+) T cells, which are the central orchestrating cells of an immune response. Here, we report the cloning of MHC class II alpha and beta cDNAs from large yellow croaker (Pscr-DAAs and Pscr-DAB) by expressed sequence tags analysis and RACE-PCR techniques. Three different class II alpha and two class II beta sequences were obtained from spleens of two individual fish. Each of the three class II alpha sequences encodes a polypeptide of 239 amino acids while the two class II beta cDNA sequences encode for a protein of 249 aa. All the characteristic features of MHC class II chain structure could be identified in the deduced proteins of three class II alpha and two class II beta sequences, including the leader peptide, alpha1/beta1 and alpha2/beta2 domains, connecting peptide and transmembrane and cytoplasmic regions, as well as conserved cysteines and N-glycosylation site. RT-PCR analysis showed that MHC class II alpha and beta mRNAs were broadly expressed in various tissues examined, although at different levels. Upon stimulation with inactivated trivalent bacterial vaccine or polyinosinic polycytidylic acid (poly(I:C)), the expression levels of both alpha and beta genes were obviously up-regulated in intestine, kidney and spleen. Real-time PCR analysis demonstrated that the expression levels of class II alpha and beta were quickly up-regulated in spleen, kidney, and intestine at 12 h after induction with poly(I:C), while their expression levels significantly increased at 48 h upon immunization with bacterial vaccine, indicating that the up-regulation of both class II alpha and beta expression was induced by bacterial vaccine or poly(I:C) at the early phase of induction, and that class II alpha and beta transcripts were quicker up-regulated by poly I:C than by bacterial vaccine.