HLA-B27 and ankylosing spondylitis geographic distribution versus malaria endemic: casual or causal liaison?

HLA-B Allele, Genotype, and Haplotype Frequencies in a Group of Healthy Individuals in Colombia

BACKGROUND

The sequencing of alleles of the HLA-B, a human leukocyte antigen (HLA) class I gene, was established as the most polymorphic of chromosome 6 and of the entire human genome. In this locus, the HLA-B*27 allele is highly polymorphic and has clinical relevance. Literature about the subtypes and singular frequency of these alleles in Colombia's healthy population is scarce.

OBJECTIVE

The aim of this study was to establish the HLA-B allele, genotype, and haplotype frequencies in a healthy Colombian population and analyze their association with the sex and geographical distribution of the individuals studied.

METHODS

This is a nonexperimental and descriptive study. The data from whole-blood samples whose HLA genes were genotyped by protocol with the Luminex 100/200 xMAP technology were evaluated. HLA-B*27 positivity was confirmed by the new-generation sequencing technology. The associations between the HLA-B alleles and demographic variables were evaluated by χ2 and Fisher exact tests.

RESULTS

Twenty-seven HLA-B genotypes were identified in 255 individuals, with the highest frequencies for HLA-B*35 (44.7%), B*40 (19.6%), and B*44 (16.8%). Additionally, 89 HLA-B alleles were found; the most common were HLA-B*35:01 (6.7%) and B*40:02 (6.5%). Nine individuals tested positive for the HLA-B*27 allele with genotype and allele frequencies of 3.5% and 1.8%, respectively; the HLA-B*27:05:02 subtype predominated.

CONCLUSIONS

Here, we report the most common HLA-B allele, genotype, and haplotype frequencies in a healthy Colombian population group and analyzed their association with the sex and geographical distribution of the individuals studied. Results for the HLA-B*27 allele confirm racial mixing in Colombia with a high degree of Caucasian influence, as well as the repopulation of Colombia's central region, attributed to the migration phenomena. Results agree with data published in Colombia that was obtained from cord blood samples.

Genetics of Ankylosing Spondylitis-Focusing on the Ethnic Difference Between East Asia and Europe

Ankylosing spondylitis (AS) is a common, highly heritable inflammatory arthritis affecting the mainly axial joints in both East Asia and Europe. To date, the pathogenesis of AS is still unknown, although we know that genetics play a vital role in it. The HLA-B27 allele is found in over 85% of AS patients. However, strong evidence suggests that other major histocompatibility complex (MHC) and non-MHC genes are also involved in the pathogenesis. In addition, current data showed that there were significant differences in both genomics and metagenomics among the different ethnic populations. The investigation of the key role of the microbiome in AS pathogenesis also highlighted the host–microbiome genetic interactions. Here, we systematically review current AS genetic research data and further compare genetic differences, especially between East Asian and European groups, which may highlight the challenge in future genetic studies.

Ankylosing Spondylitis: A Trade Off of HLA-B27, ERAP, and Pathogen Interconnections? Focus on Sardinia

The frequency of HLA-B27 in patients with Ankylosing Spondylitis (AS) is over 85%. There are more than 170 recognized HLA-B27 alleles but the majority of them is not sufficiently represented for genetic association studies. So far only two alleles, the HLA-B^*2706 in Asia and the HLA-B^*2709 in Sardinia, have not been found to be associated with AS. The highly homogenous genetic structure of the Sardinian population has favored the search of relevant variants for disease-association studies. Moreover, malaria, once endemic in the island, has been shown to have contributed to shape the native population genome affecting the relative allele frequency of relevant genes. In Sardinia, the prevalence of HLA-B^*2709, which differs from the strongly AS-associated B^*2705 prototype for one amino acid (His/Asp116) in the F pocket of the peptide binding groove, is around 20% of all HLA-B27 alleles. We have previously hypothesized that malaria could have contributed to the establishment of this allele in Sardinia. Based on our recent findings, in this perspective article we speculate that the Endoplasmic Reticulum Amino Peptidases, ERAP1 and 2, associated with AS and involved in antigen presentation, underwent co-selection by malaria. These genes, besides shaping the immunopeptidome of HLA-class I molecules, have other biological functions that could also be involved in the immunosurveillance against malaria.

Role of Human Leukocyte Antigens (HLA) in Autoimmune Diseases

Since the discovery of HLA 60 years ago, it has contributed to the understanding of the immune system as well as of the pathogenesis of several diseases. Aside from its essential role in determining donor-recipient immune compatibility in organ transplantation, HLA genotyping is meanwhile performed routinely as part of the diagnostic work-up of certain autoimmune diseases. Considering the ability of HLA to influence thymic selection as well as peripheral anergy of T cells, its role in the pathogenesis of autoimmunity is understandable. The aim of this paper is to provide a brief overview of the role and current clinical relevance of HLA-B27 in spondyloarthritis and HLA-B51 in Behçet's disease as well as HLA-DQ2/DQ8 in celiac disease and HLA-DRB1 in rheumatoid arthritis and to discuss possible future implications.

Role of Human Leukocyte Antigens (HLA) in Autoimmune Diseases

The aim of this review is to provide a brief overview of the role and current clinical relevance of HLA-B27 in spondyloarthritis and HLA-B51 in Behcet's disease as well as HLA-DQ2/DQ8 in celiac disease and HLA-DRB1 in rheumatoid arthritis and to discuss possible future implications.

Killer-cell immunoglobulin-like receptors (KIR) and HLA-class I heavy chains in ankylosing spondylitis

HLA-B27 (B27) interactions with the killer-cell immunoglobulin-like receptors (KIR) have been implicated in the pathogenesis of ankylosing spondylitis (AS), with consistent differences among populations. KIR3DL1 and possibly KIR3DS1 interact with classical B27, whereas KIR3DL2 binds B27 heavy chain dimers. The aim of this review is to summarize data from recent studies performed in our laboratory and from the literature, which provide support for a possible role of KIR3DL2/B27 dimer interactions in the pathogenesis of AS. Recent studies in cells from AS patients and from health controls carrying the predisposing B*2705 and the nonpredisposing B*2709 haplotypes, have shown a higher percentage of positive cells and a higher surface expression of KIR3DL2 receptors on natural killer (NK) and CD4+ T cells in B*2705 AS patients compared with B*2705, B*2709 and B27-negative healthy controls. Increased expression of HC10-reactive molecules on AS monocytes was seen, supporting the possible role of the KIR3DL2/B272 pair in the pathogenesis of AS. These results underline the importance of NK cells and innate immunity, and of CD4+ T cells in the inflammatory pathogenesis of AS.

The epidemiology of back pain, axial spondyloarthritis and HLA-B27 in the United States

The concept of inflammatory back pain (IBP) evolved in the 1970s, coincident with the discovery of the HLA-B27 association with ankylosing spondylitis (AS), leading to the development of criteria to determine the presence of IBP. The concept of IBP and it relationship with AS and axial spondyloarthritis (AxSpA) has further evolved, and an instrument developed (the Spondylitis Association of America Back Pain Tool), which was further modified and field tested for use in the 2009 to 2010 National Health and Nutrition Examination Survey (NHANES). This has shown the frequency of chronic back pain to have risen to 19.4%, with nearly one third having IBP. The prevalence of AxSpA has been defined at 1.0% to 1.4% and AS at 0.52% to 0.55%. The national prevalence of HLA-B27 in the United States is 6.1%, and intriguing data from NHANES 2009 suggest a decreasing frequency with increasing age. From this arise new questions and a work agenda ahead.

The interplay between the geographic distribution of HLA-B27 alleles and their role in infectious and autoimmune diseases: a unifying hypothesis

Due to its strong association with Ankylosing Spondylitis and the other Spondyloarthropathies, the HLA-B27 family of alleles and, in particular, the ancestral HLA-B*2705, has been the object of numerous studies. More recently, some novel interesting features have emerged such as the ability of HLA-B27 to confer resistance to the progression of HIV infection and to promote a spontaneous CD8+ T cell-mediated viral clearance of HCV. The co-occurrence of these protective and pathogenic features suggests a common ground, i.e. to promote a more pronounced immune/inflammatory response leading to an effective clearance of some pathogens on one side and to autoimmunity on the other. This might be due to the antigen presenting properties and/or to the co-inheritance of gene variants that contribute to an altered homeostasis in case of microbial infections or tissue injury. The existence of conserved HLA haplotypes have since long been thought to result from a selective pressure by some pathogens that have edited the immune response genes. The peculiar distribution of the ancestor HLA-B*2705 along a latitude-dependent gradient and the opposite distribution of their variants have suggested a correlation with endemic malaria. In this respect, Sardinia, a small Mediterranean island plagued by malaria, represents an interesting laboratory since its population is enriched in conserved HLA haplotypes and several genetic studies have disclosed their correlation with infectious and autoimmune diseases.

A functional polymorphism of the vasoactive intestinal peptide receptor 1 gene correlates with the presence of HLA-B*2705 in Sardinia

The association of HLA-B27 with ankylosing spondylitis (AS) is the strongest among all inflammatory diseases. However, the exact role of these molecules in disease pathogenesis is still unknown. The existence of HLA-B27 variants rarely found in patients introduces a further level of complexity. It is now accepted that other genes of minor impact contribute to modify disease susceptibility and these genes might be diverse in different populations depending on the genetic background. We report here a study performed in Sardinia, an outlier population in which two major HLA-B27 subtypes are present, B (*)2705 strongly associated with AS and B (*)2709 which is not, and show the co-occurrence of the B (*)2705 allele with a single nucleotide polymorphism (SNP) mapping at 3'-UTR of the receptor 1 (VIPR1) for the vasoactive intestinal peptide (VIP), a neuropeptide with anti-inflammatory properties. This same SNP is associated with a different kinetics of down-modulation of the VIPR1 mRNA in monocytes after exposure to lipopolysaccharide (P=0.004). This particular setting, HLA-B (*)2705 and a functional polymorphism in VIPR1 gene, might be due to a founder effect or might be the result of a selective pressure. Irrespectively, the consequent downregulation of this receptor in the presence of a 'danger' signal might influence susceptibility to AS.

HLA-B27 and ankylosing spondylitis geographic distribution as the result of a genetic selection induced by malaria endemic? A review supporting the hypothesis

The geographic distribution of HLA-B27 shows a latitude-related gradient inverse to that of malaria endemic. An apparent exception occurs in New Guinea, a region where malaria is present, but where HLA-B27 frequency shows, however, an orographic gradient antithetic to that of malaria incidence. We therefore suggest that Plasmodium falciparum may have exerted a negative selection on this gene. This might be due to a higher susceptibility to severe forms of malaria, associated with HLA-B27 or other close gene(s). In addition, we suggest here that the same selective pressure that has contributed to reduce the HLA-B27 frequency in some regions has favoured the fixing of newly generated B27 subtypes included in more advantageous HLA haplotypes. In some cases, as for B*2709 in Sardinia and B*2706 in Southeast Asia, these haplotypes may harbour factors that protect from Ankylosing Spondylitis, an autoimmune disease strongly associated with HLA-B27, thus offering a novel, powerful tool to dissect disease pathogenesis, and to identify additional genetic factors of susceptibility.