Dynamical characterization of two differentially disease associated MHC class I proteins in complex with viral and self-peptides.

Major histocompatibility complex (MHC) class I proteins are expressed on the cell surface where they present foreign and self-peptides to effector cells of the immune system. While an understanding of the structural prerequisites for antigen presentation has already been achieved, insight into subtype- or peptide-dependent dynamical characteristics of a peptide-MHC antigen is so far largely obscure. We approached this problem by employing 400-ns molecular dynamics simulations with two human MHC class I subtypes as model systems: the ankylosing spondylitis-associated HLA-B∗27:05 and the non-ankylosing spondylitis-associated HLA-B∗27:09. Both proteins differ only by a micropolymorphism at the floor of the peptide binding groove (Asp116His). A viral (pLMP2) and three self-peptides (pVIPR, pGR, and TIS) were evaluated. The stability of the binding grooves was found to be both subtype dependent and peptide dependent. A detachment from the C- and/or N-terminal pockets was observed for all peptides except TIS, resulting in a stabilization of the α1-helix in both TIS-displaying subtypes. Estimates of the entropy associated with the bound peptides showed an increased entropy for pLMP2 presented by B∗27:05 as compared to B∗27:09, in contrast to the self-peptides. Additionally, the flexibility of the α1-helix that is probably important for receptor binding to the B27:peptide epitope is significantly enhanced for B∗27:05. These in silico results show that the dynamic properties of peptide-MHC complexes are affected both by the bound peptide and by micropolymorphisms of the heavy chain. Our findings suggest a role for the conformational flexibility of MHC class I molecules in the context of recognition by receptors on effector cells.

Etiology of and risk factors for transient and persistent aminotransferase elevation in a population of virus-free blood donors: a multicentre study.

AIM: We evaluated the etiology and risk factors for transient and persistently elevated aspartate and/or alanine aminotransferase levels in virus-free blood donors. INCLUSION CRITERIA: HBsAg/HBV-DNA and anti-HCV/HCV-RNA negative blood donors with elevated aspartate aminotransferase and/or alanine aminotransferase, observed in 5 blood transfusion centres in Italy from 2004 to 2005. Aspartate aminotransferase/alanine aminotransferase levels were measured at entry and every 2 months during a period of 6 months. RESULTS: 291 individuals were evaluated (144 with persistent and 147 with transient abnormal aminotransferases). High body mass index was the most frequent (75.5%) etiological factor and was more common in the persistent elevated levels group, compared to the transient elevated levels group (82.0% vs 65.3%; p<0.01). Excessive alcohol intake (>2 units/day) was reported in 23.6%, with no differences between the two groups. Instead, recent use of medication or paint exposure were most frequently associated with transient elevated levels than persistent elevated levels (61.6% vs 23.3% for drugs and 13.7% vs 4.3% for paint, p<0.001). Considering the participants with transient elevated levels as controls, the multivariate analysis showed that high body mass index was the only independent predictor of persistent elevated aminotransferase levels (OR=5.3; 95%CI=1.88-13.42 for those with body mass index>29.9). CONCLUSIONS: In virus-free blood donors, excessive body mass index is the most frequent etiological factor of abnormal aminotransferases and it is the sole risk factor associated with persistently elevated aminotransferases.

T-cell responses against viral and self-epitopes and HLA-B27 subtypes differentially associated with ankylosing spondylitis.

HLA-B27 family comprehends some alleles strongly associated with Ankylosing Spondylitis (AS) and some others that are not. A comparative analysis at genetic and functional level is likely to give a clue to the understanding of disease pathogenesis. Here, we summarize our recent studies on the functional differences between B*2705, the most frequent and worldwide AS-associated allele and B*2709, an allele found in Sardinia where it accounts for 20% of all B27 alleles and where it is not associated with AS. The two B27 alleles are distinguished by a single amino acid change, located in the peptide binding groove, that correlates with relevant structural and functional differences in presenting viral and self peptides to T-cells. In particular, B*2709 individuals lack in their T-cell repertoire of CD8+ T-cells specific for a self-epitope (pVIPR) derived from the vasoactive intestinal peptide Type 1 receptor (VPAC1). This peptide shares extensive homology with a viral epitope, pLMP2, derived from EBV, toward which, both B*2705 and B*2709 individuals mount a vigorous CTL response. A likely explanation to this finding, also supported by crystallographic data, is that the autoreactivity present in the disease-prone B*2705 individuals can be unleashed by a molecular mimicry mechanism which does not occur in the B*2709 individuals. The possible implications of the T-cell cross-reactivity between pLMP2, pVIPR and other related peptides in AS pathogenesis are discussed.