Study On The Crystal Structures Of PMHC I Complexes Or β2m Molecules In Four Lower Vertebrates

The adaptive immune system(AIS)arose abruptly in jawed vertebrate-500 million years ago,and then evolved and perfected gradually along with the evolution of bony fish,amphibians,reptiles,birds and mammals.Now,AIS has been defined a vital weapon against various pathogenic microorganisms in vertebrate.The main content of comparative immunology is clarifying the similarities and differences between the key immune molecules in different species,and then exploring the evolution of the immune system.However,unlike higher mammals such as human and mice,very little research is carried out in lower vertebrates,which is seriously restricting the comparative immunology study.Major histocompatibility complex class Ⅰ molecule(MHC class Ⅰ)is one of the core molecules in AIS,playing role in the restrictive recognition and presentation of endogenous antigen peptides,activating CD8 + cytotoxic T lymphocytes(CTL)to induce the specific cellular immune response.So far,MHC I structures of many kinds of mammals and Chicken have already been resolved,while is still blank in lower vertebrates.In this study,using molecular immunology and structural biology techniques,the crystal structures of Ginglymostoma cirratum pMHC Ⅰ complex(PGici-UAA*01),Xenopus leavisβ2m(Xela-β2m),Anolis carolinensis β2m(Anca-β2m)and Anas platyrhynchos pMHC Ⅰ complex(pAnpl-UAA*01)were resolved,respectively.In terms of antigen presentation,pGici-UAA*01 and pAnpl-UAA*01 have already evolved the peptide binding groove(PBG)similar to mammals,but there are also some distinct features.First,in pGici-UAA*01 structure,the large side chains of Trp68 formed steric hindrance in PBG,blocking peptide sink into a typical "M" conformation,which may be beneficial to docking with TCR.While in pAnpl-UAA*01 structure,a unique disulfide bond formed by Cys95 and Cys112 at the bottom of PBG was identified.Moreover,the extensive IAV peptides binding identified here provides a new perspective for the study of Anpls as a natural host of influenza virus.Moreover,the interactions as well as the interface area between heavy and light chains of pGici-UAA*01 and pAnpl-UAA*01 are stronger than that of mammals,indicating the stronger stability of pMHC Ⅰ in lower vertebrates.In addition,the comparison of Xela-β2m and Anca-β2m with all the known β2m structures shows that the inter-sheets interaction decreased gradually along the evolution of species.Furthermore,the key binding sites of MHC Ⅰ and,β2m are conserved and co-evoluted from fish to mammalian.In summary,this study focused on the four major categories of non-mammals,fish,amphibians,reptiles and birds,to analyze the structures of pMHC Ⅰ complex and β2m in each representative species,as well as their structural co-evolution.These results provide structural data for the origin and evolution of adaptive immune system.Moreover,the peptide binding motif of pAnpl-UAA*01 identified here can provide ideas for anti-IAV vaccine development.