Relationship Between Chicken MHC â…  Molecule Structure And Disease Resistance

Major histocompatibility complex (MHC) is closely linked by a group of highly polymorphic loci, they play a crucial role in the immune system, which induce the animal body transplant rejection, present antigen peptides to T cells for recognition, and trigger an appropriate immune response. Polymorphism of chicken MHCâ… gene (BF) is associated with resistance and susceptibility of many important infectious diseases, so the knowledge about characters of MHC polymorphism is basic for resistibility breeding. A number of works focusing on poultry MHCâ… gene have been reported; still it is not clear about the molecular mechanism of MHC inducing diversity in resistance to infectious diseases between animal breeds. MHC polymorphisms are subject to the animal's response to the environmental pressure. Livestock and poultry in the breeding process are more artificially selected and bred, especially under infectious pathogen pressure, which inhabit in different areas and exhibit various produce and form characters, whether their MHC gene has diversity in evolvement under the pressure. In order to understand polymorphism character of MHC evolution in domesticated poultry we selected three local breeds of chicken (Qingyuan, Wenchang and Huaibei) in different areas of China, and cloned and sequenced MHCâ… cDNA gene.First 80 sequences of MHCⅠαgene were cloned from 90 individuals by RT-PCR, in which Qingyuan, Wenchang and Huaibei were 30, 24 and 26 respectively. Comparison of sequences indicated thatαgene exhibited high polymorphism and had 73 different sequences and 6 groups having identical sequence, in which there were 3 groups, its members were distributed in different breeds. We also sequenced 5β_2m gene, they exhibited high conversed and less diversity. Further we analyzed the distribution of polymorphic amino acid residues in chicken MHCⅠαchain, in which an aligning all sequences of amino acid the residues. The results revealed that there were 130 of polymorphic sites, which were distributed in total MHCⅠαchain including five regions, namely leader peptide,α1,α2,α3 and TM / CY regions, but 83 of them were located in theα1 andα2 regions (9¹55aa).In the phylogenetic tree all alleles belonged to two clusters together with other different taxa, especially one individual as cluster member was dependent on its genotype rather breed. This suggest that no correlation of MHCⅠαwith the chicken breeds and they keep genetic characters of a common ancestor. It is presumed that the MHCâ… molecule of domesticated chickens would more influenced by the pressure of the pathogens rather than the environment in different areas.Secondly, analysis and comparison of diversity inα1 andα2 regions with other species were carried out by Wu-kabat index and structure simulation. We found that 16 amino acid residues with high polymorphism (score≥4) in PBRs of Qingyuan Partridge chicken MHCâ… . There were 9 of polymorphic sites inα1 region, 7 of those sites inα2 region. Four main peaks (score≥8) were located at the residues 9, 111, 113 and 153. A total of 23 polymorphic sites in Huaibei Partridge chicken MHCâ… were located in the regions, including 10 sites ofα1 region , 13 sites ofα2 region, four main peaks were also located at the residues 9, 111, 113 and 153. Wenchang chicken MHCâ… had 25 highly variable sites,α1 andα2 regions had 13 and 12 sites respectively, and the residues 9,69,111,113,149 and153 with a score were greater than or equal to 8. Moreover, all these residues 9, 69, 111, 113, 149 and153 were polymorphic in three breeds, and the sites 9, 111, 113 and153 were highly polymorphic (score≥8). Compared with duck, human and mouse, the distribution of amino acid residues inα1 andα2 regions ofα-chain shows some similar features, namely some variable polymorphic sites (score≥4) at 9, 111(114), 113 (116) and 153 (156), especially these polymorphic sites in peptide-binding regions (PBRs) and their spatial location, indicating that they had similarity in gene structure. All these results suggest that MHC gene is an ancestral sequences that existed before separation of specie and has common character. In the evolution process the MHC gene of breeds in same specie have keep more common characters because under pressure of the specific pathogen rather than different living environment.A presenting antigen peptide is dependent on polymerization between MHCαandβchain and forming specific stable structure. The PBR locates inαchain, butβ_2m is essential for MHCâ… molecule function. To explore mechanism of MHC molecule presenting antigen we constructed a recombined segment containing MHCⅠαandβchains and expressed in a prokaryotic system. We connectedαchain toβchain with a linker and inserted it into a vector, this recombinant plasmid was named as pET-MHCⅠβ_2m-α. Then it was transferred into E. coli BL21 and expressed. The results showed that the pET-MHCⅠβ_2m-αwere successfully expressed, This MHCⅠβ_2m-αfusion protein with certain immune activity would provide experimental materials for exploring mechanism of MHCâ… molecule improving immune function as epitope carrier.