Compared with the MHC of typical mammals, the chicken MHC is smaller and simpler, with only two class I genes found in the B12 haplotype. We make five points to show that there is a singledominantly expressed class I molecule that can have a strong effect on MHC function. First, we find only one cDNA for two MHC haplotypes (B14 and B15) and cDNAs corresponding to two genes for the other six (B2, B4, B6, B12, B19, and B21). Second, we find, for the B4, B12, and B15 haplotypes, that one cDNA is at least 10-fold more abundant than the other. Third, we use 2D gel electrophoresis of class I molecules from pulse-labeled cells to show that there is only one heavy chain spot for the B4 and B15 haplotypes, and one major spot for the B12 haplotype. Fourth, we determine the peptide motifs for B4, B12, and B15 cells in detail, including pool sequences and individual peptides, and show that the motifs are consistent with the peptides binding to models of the class I molecule encoded by the abundant cDNA. Finally, having shown for three haplotypes that there is a single dominantly expressed class I molecule at the level of RNA, protein, and antigenic peptide, we show that the motifs can explain the striking MHC-determined resistance and susceptibility to Rous sarcoma virus. These results are consistent with the concept of a ''minimal essential MHC'' for chickens, in strong contrast to typical mammals.antigen presentation ͉ avian ͉ essential ͉ evolution ͉ minimal

Little is known about the structure of major histocompatibility complex (MHC) molecules outside of mammals. Only one class I molecule in the chicken MHC is highly expressed, leading to strong genetic associations with infectious pathogens. Here, we report two structures of the MHC class I molecule BF2*2101 from the B21 haplotype, which is known to confer resistance to Marek's disease caused by an oncogenic herpesvirus. The binding groove has an unusually large central cavity, which confers substantial conformational flexibility to the crucial residue Arg9, allowing remodeling of key peptide-binding sites. The coupled variation of anchor residues from the peptide, utilizing a charge-transfer system unprecedented in MHC molecules, allows peptides with conspicuously different sequences to be bound. This promiscuous binding extends our understanding of ways in which MHC class I molecules can present peptides to the immune system and might explain the resistance of the B21 haplotype to Marek's disease.

In most mammals, the MHC class I molecules are polymorphic and determine the specificity of peptide presentation, whereas the transporter associated with antigen presentation (TAP) heterodimers are functionally monomorphic. In chickens, there are two classical class I genes but only one is expressed at a high level, which can result in strong MHC associations with resistance to particular infectious pathogens. However, the basis for having a single dominantly expressed class I molecule has been unclear. Here we report TAP1 and TAP2 sequences from 16 chicken lines, and show that both genes have high allelic polymorphism and moderate sequence diversity, with variation in positions expected for peptide binding. We analyze peptide translocation in two MHC haplotypes, showing that chicken TAPs specify translocation at three peptide positions, matching the peptide motif of the single dominantly expressed class I molecule. These results show that coevolution between class I and TAP genes can explain the presence of a single dominantly expressed class I molecule in common chicken MHC haplotypes. Moreover, such coevolution in the primordial MHC may have been responsible for the appearance of the antigen presentation pathways at the birth of the adaptive immune system. avian | evolution | bird

We report the proteomes of four life cycle stages of the Apicomplexan parasite Eimeria tenella. A total of 1868 proteins were identified, with 630, 699, 845 and 1532 found in early oocysts (unsporulated), late oocysts (sporulated), sporozoites and second-generation merozoites, respectively. A MudPIT shotgun approach identified 812 sporozoite, 1528 merozoite and all of the oocyst proteins, whereas 2D gel proteomics identified 230 sporozoite and 98 merozoite proteins. Comparing the invasive stages, we find moving junction components RON2 in both, whilst AMA-1 and RON4 are found only in merozoites and AMA-2 and RON5 are only found in sporozoites, suggesting stage specific moving junction proteins. During early oocyst to sporozoite development, refractile body and most ‘glideosome’ proteins are found throughout, whilst microneme and most rhoptry proteins are only found after sporulation. Quantitative analysis indicates glycolysis and gluconeogenesis are the most abundant metabolic groups detected in all stages. The mannitol cycle ‘off shoot’ of glycolysis was not detected in merozoites but was well represented in the other stages. However, in merozoites we find more protein associated with oxidative phosphorylation, suggesting a metabolic shift mobilising greater energy production. We find a greater abundance of protein linked to transcription, protein synthesis and cell cycle in merozoites than in sporozoites, which may be residual protein from the preceding massive replication during schizogony.

No abstract

The transmissible spongiform encephalopathies are characterized by conversion of a host protein, PrP C (cellular prion protein), to a protease-resistant isoform, PrP Sc (prion protein scrapie isoform). The importance of the highly¯exible, N-terminal region of PrP has recently become more widely appreciated, particularly the biological activities associated with its metal ion-binding domain and its potential to form a poly(L-proline) II (PPII) helix. Circular dichroism spectroscopy of an N-terminal peptide, PrP 37±53 , showed that the PPII helix is formed in aqueous buffer; as it also contains an Xaa±Pro±Gly consensus sequence, it may act as a substrate for the collagenmodifying enzyme prolyl 4-hydroxylase. Direct evidence for this modi®cation was obtained by mass spectrometry and Edman sequencing in recombinant mouse PrP secreted from stably transfected Chinese hamster ovary cells. Almost complete conversion of proline to 4-hydroxyproline occurs speci®cally at residue Pro44 of this murine protein; the same hydroxylated residue was detected, at lower levels, in PrP Sc from the brains of scrapie-infected mice. Cation binding and/or post-translational hydroxylation of this region of PrP may regulate its role in the physiology and pathobiology of the cell.

T cell receptor (TCR) recognition of peptide-MHC class I (pMHC) complexes is a crucial event in the adaptive immune response to pathogens. Peptide epitopes often display a strong dominance hierarchy, resulting in focusing of the response on a limited number of the most dominant epitopes. Such T cell responses may be additionally restricted by particular MHC alleles in preference to others. We have studied this poorly understood phenomenon using Theileria parva, a protozoan parasite that causes an often fatal lymphoproliferative disease in cattle. Despite its antigenic complexity, CD8+ T cell responses induced by infection with the parasite show profound immunodominance, as exemplified by the Tp1214–224 epitope presented by the common and functionally important MHC class I allele N*01301. We present a high-resolution crystal structure of this pMHC complex, demonstrating that the peptide is presented in a distinctive raised conformation. Functional studies using CD8+ T cell clones show that this impacts significantly on TCR recognition. The unconventional structure is generated by a hydrophobic ridge within the MHC peptide binding groove, found in a set of cattle MHC alleles. Extremely rare in all other species, this feature is seen in a small group of mouse MHC class I molecules. The data generated in this analysis contribute to our understanding of the structural basis for T cell-dependent immune responses, providing insight into what determines a highly immunogenic p-MHC complex, and hence can be of value in prediction of antigenic epitopes and vaccine design.

The chicken major histocompatibility complex (MHC) has strong genetic associations with resistance and susceptibility to certain infectious pathogens. The cell surface expression level of MHC class I molecules varies as much as 10-fold between chicken haplotypes and is inversely correlated with diversity of peptide repertoire and with resistance to Marek's disease caused by an oncogenic herpesvirus. Here we show that the average thermostability of class I molecules isolated from cells also varies, being higher for high-expressing MHC haplotypes. However, we find roughly the same amount of class I protein synthesized by high-and low-expressing MHC haplotypes, with movement to the cell surface responsible for the difference in expression. Previous data show that chicken TAP genes have high allelic polymorphism, with peptide translocation specific for each MHC haplotype. Here we use assembly assays with peptide libraries to show that high-expressing B15 class I molecules can bind a much wider variety of peptides than are found on the cell surface, with the B15 TAPs restricting the peptides available. In contrast, the translocation specificity of TAPs from the low-expressing B21 haplotype is even more permissive than the promiscuous binding shown by the dominantly expressed class I molecule. B15/B21 heterozygote cells show much greater expression of B15 class I molecules than B15/B15 homozygote cells, presumably as a result of receiving additional peptides from the B21 TAPs. Thus, chicken MHC haplotypes vary in several correlated attributes, with the most obvious candidate linking all these properties being molecular interactions within the peptideloading complex (PLC).ABC transporter | restrictive | permissive | heterozygous advantage | overdominance