HLA-B27

Human leukocyte antigen (HLA) B27 (subtypes B*2701-2759)[1] is a class I surface antigen encoded by the B locus in the major histocompatibility complex (MHC) on chromosome 6 and presents antigenic peptides (derived from self and non-self antigens) to T cells. HLA-B27 is strongly associated with ankylosing spondylitis (AS), and other associated inflammatory diseases referred to as "spondyloarthropathies". Diseases associated with the HLA-B27 subtype can be remembered with the mnemonic PAIR, and include Psoriasis, Ankylosing spondylitis, Inflammatory bowel disease, and Reactive arthritis.

HLA-B*2705-peptide (chain A shown in green cartoon, chain B shown in yellow cartoon) complexed to a fragment of the influenza nucleoprotein NP383-391 (orange, sticks). PDB ID 2BST
B*2705-β2MG with bound peptide 2bst​
major histocompatibility complex (human), class I, B27
Alleles B*2701, 2702, 2703, . . .
Structure (See HLA-B)Available
3D structures
EBI-HLA B*2701
B*2702
B*2703
B*2704
B*2705 2bsr​, 2bss​,
2bst​, 2a83​,
1w0v​, 1uxs​,
1ogt​, 1hsa​,
1jgd​, 1jge​
B*2706
B*2709 1w0w​, 1uxw​,
1of2​, 1k5n​

The prevalence of HLA-B27 varies markedly in the general population. For example, about 8% of Caucasians, 4% of North Africans, 2-9% of Chinese, and 0.1-0.5% of persons of Japanese descent possess the gene that codes for this antigen.[1] In northern Scandinavia (Lapland), 24% of people are HLA-B27 positive, while 1.8% have associated ankylosing spondylitis.

A small group (<0.5%) of people infected with HIV are able to remain symptom-free for many years without medication. These long-term nonprogressors appear to be significantly common among people who are HLA-B27 positive.[2]

Disease associations

The relationship between HLA-B27 and many diseases has not yet been fully elucidated. Though it is associated with a wide range of pathology, particularly seronegative spondyloarthropathy, it does not appear to be the sole mediator in development of disease. For example, while 90% of people with ankylosing spondylitis (AS) are HLA-B27 positive, only a small fraction of people with HLA-B27 ever develop AS. People who are HLA-B27 positive are more likely to experience early onset AS than HLA-B27 negative individuals.[3] There are additional genes being discovered that also predispose to AS and associated diseases.[4] Additionally there are potential environmental factors (triggers) that may also play a role in susceptible individuals.[1]

Pathological mechanism

Due to its strong association with spondyloarthropathies, HLA-B27 is the most studied HLA-B allele. It is not entirely clear how HLA-B27 influences disease, however there are some prevailing theories as to the mechanism. The theories can be divided between antigen-dependent and antigen-independent categories.[5]

Antigen-dependent theories

These theories consider a specific combination of antigen peptide sequence and the binding groove (B pocket) of HLA-B27 (which will have different properties from the other HLA-B alleles). The arthritogenic peptide hypothesis suggests that HLA-B27 has a unique ability to bind antigens from a microorganism that trigger a CD8 T-cell response that then cross-reacts with a HLA-B27/self-peptide pair. Furthermore, it has been shown that HLA-B27 can bind peptides at the cell surface.[6] The molecular mimicry hypothesis is similar, however it suggests that cross reactivity between some bacterial antigens and self peptide can break tolerance and lead to autoimmunity.[5]

Antigen-independent theories

These theories refer to the unusual biochemical properties that HLA-B27 has. The misfolding hypothesis suggests that slow folding during HLA-B27's tertiary structure folding and association with β2 microglobulin causes the protein to be misfolded, therefore initiating the unfolded protein response (UPR) - a pro-inflammatory endoplasmic reticulum (ER) stress response. However, although this mechanism has been demonstrated both in vitro and in animals, there is little evidence of its occurrence in human spondyloarthritis.[6] Also, the HLA-B27 heavy chain homodimer formation hypothesis suggests that B27 heavy chains tend to dimerise and accumulate in the ER, once again, initiating the UPR.[5] Alternatively, cell surface B27 heavy chains and dimers can bind to regulatory immune receptors such as members of the killer cell immunoglobulin-like receptor family, promoting the survival and differentiation of pro-inflammatory leukocytes in disease.

One more misfolding theory, published in 2004,[7] proposes that β2 microglobulin-free heavy chains of HLA-B27 undergo a facile conformational change in which the C-terminal end of domain 2 (consisting of a long helix) becomes subject to a helix-coil transition involving residues 169-181 of the heavy chain, owing to the conformational freedom newly experienced by domain 3 of the heavy chain when there is no longer any bound light chain (i.e., β2 microglobulin) and owing to the consequent rotation around the backbone dihedral angles of residues 167/168. The proposed conformational transition is thought to allow the newly-generated coiled region (incorporating residues 'RRYLENGKETLQR' which have also been found to be naturally bound to HLA-B27 as a 9-mer peptide) to bind to either the peptide-binding cleft of the same polypeptide chain (in an act of self-display) or to the cleft of another polypeptide chain (in an act of cross-display). Cross-display is proposed to lead to the formation of large, soluble, high molecular weight (HMW), degradation-resistant, long-surviving aggregates of the HLA-B27 heavy chain. Together with any homodimers formed either by cross-display or by a disulfide-linked homodimerization mechanism, it is proposed that such HMW aggregates survive on the cell surface without undergoing rapid degradation, and stimulate an immune response. Three previously noted features of HLA-B27, which distinguish it from other heavy chains, underlie the hypothesis : (1) HLA-B27 has been found to be bound to peptides longer than 9-mers, suggesting that the cleft can accommodate a longer polypeptide chain; (2) HLA-B27 has been found to itself contain a sequence that has also been actually discovered to be bound to HLA-B27, as an independent peptide; and (3) HLA-B27 heavy chains lacking β2 microglobulin have been seen on cell surfaces.

Associated pathology

In addition to its association with ankylosing spondylitis, HLA-B27 is implicated in other types of seronegative spondyloarthropathy[8] as well, such as reactive arthritis, certain eye disorders such as acute anterior uveitis and iritis, psoriatic arthritis and ulcerative colitis associated spondyloarthritis. The shared association with HLA-B27 leads to increased clustering of these diseases.[9]

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See also

References
  1. M. A. Khan (2010). "HLA and spondyloarthropathies". In Narinder K. Mehra (ed.). The HLA Complex in Biology and Medicine. New Delhi, India: Jayppee Brothers Medical Publishers. pp. 259–275. ISBN 978-81-8448-870-8.
  2. Deeks, Steven G.; Walker, Bruce D. (September 2007). "Human Immunodeficiency Virus Controllers: Mechanisms of Durable Virus Control in the Absence of Antiretroviral Therapy". Immunity. 27 (3): 406–416. doi:10.1016/j.immuni.2007.08.010. PMID 17892849.
  3. Feldtkeller, Ernst; Khan, Muhammad; van der Heijde, Désirée; van der Linden, Sjef; Braun, Jürgen (March 2003). "Age at disease onset and diagnosis delay in HLA-B27 negative vs. positive patients with ankylosing spondylitis". Rheumatology International. 23 (2): 61–66. doi:10.1007/s00296-002-0237-4. PMID 12634937.
  4. Thomas, Gethin P.; Brown, Matthew A. (January 2010). "Genetics and genomics of ankylosing spondylitis". Immunological Reviews. 233 (1): 162–180. doi:10.1111/j.0105-2896.2009.00852.x. PMID 20192999.
  5. Hacquard-Bouder, Cécile; Ittah, Marc; Breban, Maxime (March 2006). "Animal models of HLA-B27-associated diseases: new outcomes". Joint Bone Spine. 73 (2): 132–138. doi:10.1016/j.jbspin.2005.03.016. PMID 16377230.
  6. Bowness, Paul (21 March 2015). "HLA-B27". Annual Review of Immunology. 33 (1): 29–48. doi:10.1146/annurev-immunol-032414-112110. PMID 25861975.
  7. Luthra-Guptasarma, Manni; Singh, Balvinder (24 September 2004). "HLA-B27 lacking associated β2-microglobulin rearranges to auto-display or cross-display residues 169-181: a novel molecular mechanism for spondyloarthropathies". FEBS Letters. 575 (1–3): 1–8. doi:10.1016/j.febslet.2004.08.037. PMID 15388324.
  8. Elizabeth D Agabegi; Agabegi, Steven S. (2008). Step-Up to Medicine (Step-Up Series). Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 978-0-7817-7153-5.
  9. Kataria, RK; Brent LH (June 2004). "Spondyloarthropathies". American Family Physician. 69 (12): 2853–2860. PMID 15222650.
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