MICA gene triplet repeat polymorphism in patients with HLA-B27 positive and negative ankylosing spondylitis from Sardinia

OBJECTIVE

We investigated the distribution of MICA triplet repeat polymorphism in a random population and in patients with seronegative spondyloarthritis from Sardinia compared to continental Italy.

METHODS

We analyzed the distribution of MICA triplet repeat polymorphism in HLA-B*2709 [not associated with ankylosing spondylitis (AS)] and B*2705 (associated with AS) haplotypes, to verify whether the strong association of MICA-A4 with HLA-B27 reported in other populations is maintained in Sardinia, and compared the distribution of MICA-A alleles in HLA-B27 negative versus HLA-B27+ patients with AS.

RESULTS

We found that the frequency of MICA-A4 triplet repeat allele in a random Sardinia population is higher (53.2%) than in other Caucasian populations (around 20%); this allele is strongly associated with both HLA-B*2709 and B*2705. No significant difference between HLA-B27+ patients with AS and healthy controls was found: the MICA-A4 allele was present in more than 90% of subjects. MICA-A4 was found in 16 out of 20 HLA-B27 negative Sardinian patients with AS, with a frequency (80%) more similar to that of the HLA-B27+ group of patients than that of controls.

CONCLUSION

The high frequency of MICA-A4 allele in HLA-B27 negative patients with AS from Sardinia, suggests the presence within the HLA region of a susceptibility factor other and certainly weaker than B27. This factor is likely to be more easily found by analyzing genetically homogeneous populations like the Sardinian, characterized by a small number of very frequent haplotypes.

HLA-DP molecules bind cobalt: a possible explanation for the genetic association with hard metal disease

Metal dust inhalation induces an interstitial lung disease which may progress to pulmonary fibrosis (hard metal disease, HMD). Cobalt is believed to be the pathogenic agent of HMD. A strong genetic association of HMD with some HLA-DP alleles has been reported although the role of these molecules in the occurrence of the fibrotic disorder remains unclear. A possible explanation of these findings is that HLA-DP but not other HLA class II molecules can bind cobalt. This could have as a consequence an HLA-DP-mediated specific activation of the immune system. To test this hypothesis, we have set up an in vitro binding assay using 57Co and purified HLA-DP and -DR molecules. The results indicate that HLA-DP but not HLA-DR molecules bind cobalt. Moreover, the presence of HLA-DP Glu beta69, which is associated with susceptibility to HMD, determines a higher metal uptake. Molecular modelling of HLA-DP2 molecules places the Glu beta69 residue in a position relevant in determining peptide specificity. The possibility that binding of cobalt by HLA-DP molecules can interfere with their antigen presenting functions is discussed.

The naturally occurring polymorphism Asp116-->His116, differentiating the ankylosing spondylitis-associated HLA-B*2705 from the non-associated HLA-B*2709 subtype, influences peptide-specific CD8 T cell recognition

HLA-B27 molecules are interesting because of their strong association with ankylosing spondylitis (AS) and reactive arthritis (ReA). A pathogenetic role for these molecules has been postulated in presenting a putative "arthritogenic" peptide to CD8 T cells. The HLA-B*2709 subtype, although differing by a single amino acid (His116-->Asp116) from the widespread and strongly AS-associated subtype HLA-B*2705, is not found in patients. Since residue 116 interacts with the C terminus of the peptide, it is possible that the two subtypes differ in their antigen-presenting features. We show here that CD8 T cells can distinguish the two HLA-B27 subtypes when presenting a same epitope derived from Epstein-Barr virus-latent membrane protein 2. Moreover, alanine scanning mutagenesis analysis revealed that the peptide residues relevant for such recognition are different depending on whether HLA-B*2705 or -B*2709 molecules present the epitope. These results give support to the belief that functional differences determined by subtype-specific polymorphisms can have a pathogenetic relevance and open up a new scenario where subtle modifications within the peptide/HLA ligand might be responsible for the differential association between HLA-B27 subtypes and spondyloarthropathies.

Interferon-gamma-treated renal tubular epithelial cells induce allospecific tolerance

Following organ transplantation, tissue parenchymal cells commonly express major histocompatibility complex (MHC) class II molecules as a result of local cytokine release, and thus acquire the capacity to present donor MHC alloantigens to alloreactive CD4+ T cells. The consequences of such a presentation are likely to be relevant in the induction of tolerance to the transplanted tissues, and this has been reported in animal models of transplantation and in humans. In this study, the consequences of antigen presentation by interferon-gamma (IFN-gamma)-treated human renal tubular epithelial cells (RTEC) to resting and activated CD4+ T cells were investigated. Allogeneic RTEC were unable to stimulate proliferation by peripheral blood CD45 RA+ or RO+ CD4+ T cells from three HLA-mismatched responders. The response to RTEC was partially reconstituted by the addition of murine L cell transfectants expressing human B7.1 (DAP.3-B7), suggesting that the failure of RTEC to stimulate a primary alloresponse was due, at least in part, to a lack of costimulation. T cell clones dependent on B7-mediated co-stimulation also did not respond to peptide presented by RTEC. Most importantly, this lack of reactivity was accompanied by the induction of nonresponsiveness. Incubation with allogeneic, DR-expressing RTEC induced allospecific hyporesponsiveness in both CD45RA+ and RO+ cells. Similarly, overnight incubation with antigen-pulsed RTEC induced nonresponsiveness in the B7-dependent T cell clones. These results suggest that MHC class II expression on RTEC may contribute to the induction of an allospecific nonresponsiveness following organ transplantation.

MICA exon 5 microsatellite typing by DNA heteroduplex analysis: a new polymorphism in the transmembrane region

MICA (MHC class I chain-related gene A) is localized 47 kb upstream from HLA-B on the short arm of chromosome 6. It has been postulated that MICA protein folds similarly to the class I chain and may have the capacity to bind short ligands. Short tandem repeats (STR) within the transmembrane (TM) region of this gene have been described and five alleles consisting of 4 to 9 GCT codons, each encoding an alanine residue have been defined. We have applied DNA heteroduplex analysis to type MICA trinucleotide repeats in order to develop a simple and reliable method for their identification. This approach allowed the characterization of all MICA alleles. Moreover, a new polymorphism within the TM region was identified.

Susceptibility to hard metal lung disease is strongly associated with the presence of glutamate 69 in HLA-DP beta chain

Clinical, epidemiological and experimental data indicate that inhaled metal dust containing cobalt may produce an interstitial lung disease termed "hard metal disease" (HMD). Some aspects of this pathology such as the lack of correlation with dose exposure, the low frequency of the disease and the presence of T cells in the inflammation site, all suggest the existence of a genetic susceptibility, possibly to an immunological response to cobalt or to self proteins modified by cobalt. Here we report that HMD is strongly associated with residue Glu-69 of the HLA-DP beta chain. All patients, except for one with a rare genotype, possessed this marker as compared to 17 out of 35 exposed unaffected individuals (p = 0.0014). These data allow us to genetically distinguish a subgroup of cobalt-exposed individuals at risk for HMD, independently from the more common allergic reaction.

HLA-DPB1 mismatch at position 69 is associated with high helper T lymphocyte precursor frequencies in unrelated bone marrow transplant pairs

HLA incompatibility between bone marrow recipient and unrelated donor pairs is often associated with severe acute graft-versus-host disease following bone marrow transplantation. Due to the extensive polymorphism of HLA genes, finding genotypically identical pairs is a difficult challenge. Therefore, it is crucial to single out the relevance of each HLA gene and, within each sequence, the polymorphic positions that induce a T-cell response. Among HLA class II genes, the relevance of HLA-DPB1 in inducing graft-versus-host disease is still controversial. In this study, we selected 37 bone marrow transplant pairs on the basis of HLA class I A and B identity as determined by isoelectric focusing and of class II identity as determined by serology and by low-resolution genomic typing. We analyzed them for the possible relationship between frequency of cytotoxic T lymphocyte and helper T lymphocyte precursors (CTLp and HTLp, respectively) and genomically determined class II mismatches. Seventeen pairs had high numbers of both CTLp and HTLp. They were not further considered because of the difficulty in determining whether the T-cell response was induced by class I or class II mismatches. Of the remaining pairs with low CTLp and high HTLp, six had disparities at HLA-DRB1 and HLA-DPB1 genes, and 14 differed only at the HLA-DPB1 locus. Among the latter pairs, we found a correlation between HLA-DPB1 mismatches and HTLp frequency, thus suggesting that disparity at this locus influences the alloreactive T-cell response. When the HTLp frequency was correlated with each single mismatch found in the 14 pairs, it appeared that the nature of the amino acid at position beta69 played a relevant role in inducing alloreactivity.

A new approach to HLA-DPB1 typing combining DNA heteroduplex analysis with allele-specific amplification and enzyme restriction

Allelic polymorphism of HLA-class II antigens plays a key role in the regulation of the immune response and in transplantation immunity. The allelic diversity of these antigens can now be analyzed at the DNA level after amplification by polymerase chain reaction. In this study we apply a simple technique based on the electrophoretic analysis of DNA heteroduplexes to the typing of HLA-DPB1 alleles. In order to increase its resolution, a group-specific amplification was used which subdivides the 19 HLA-DPB1 alleles in two non-overlapping families. A separate analysis was then performed within each group of alleles. This approach allowed an unequivocal one-step typing of the alleles belonging to group 1 which comprises few alleles of high frequency. Some group 2 alleles require, as a further step, the test with a restriction enzyme. The combination of more than one technique represents, in our opinion, the easiest way to solve the micropolymorphism of class II alleles. We conclude that this method, which is very simple, quick, and accurate and does not require probes, may become the method of choice for HLA-DPB1 typing.