Mitchell W. SheltonVirginia Polytechnic Institute and State University

Introduction

The HLA Gene—Ankylosing spondylitis (AS) is a chronic, rheumatic disease characterized by uncontrolled osteogenesis and eventual immobility of the spinal and sacroiliac joints. The initial symptom is low back that radiates upwards over time to include the entire spine. There are two prominent genes, in addition to the environment, that are believed to lead a significant part in the pathogenesis of the disease: HLA-B27 and IL-23. Human Leukocyte Antigen (HLA), is a gene that codes for receptors that play a crucial role in the immune system, and as such are on every nucleated cell in the body. It is no surprise that almost 90% of cases of AS involve the mutated form of HLA, termed HLA-B27. The HLA genes code for the Major Histocompatibility Complex (MHC) protein group, which oversee two fundamental immune responses: the recognition of the body’s own cells and the presentation of antigens to the immune cells. One of the two types of MHC, MHC I, only attract cytotoxic T cells as the complex only presents antigens of a microbial invader that has infected the cell, and it uses the MHC I complex as its “apoptosis trigger”. The MHC II complex is only present on various immune cells: macrophages, B cells, and dendritic cells, collectively called the antigen presenting cells. The above two mechanisms form a coordinated immune response, which is a subset of adaptive immunity, a slow process that produces unique immune cells that will not only remember the antigen, but also produce antibodies to it and provide a quick response if the body encounters it again.

The pathogenesis of AS is easily explained by the involvement of the HLA genes. The HLA genes are polymorphic, meaning that there are several variations of the genes in not only the zygote, but in the population as well; that is, no two people will have identical HLA genes, other than identical twins. HLA-B27, after being transcribed and translated, will be placed on the cellular membrane as an active MHC complex, usually as MHC I. MHC I, being the receptor on all nucleated cells as well as the receptor that binds cytotoxic T cells, is now dysfunctional and begins to cause multiple problems in the body, and for reasons not totally described, specifically causing damage to the spinal and sacroiliac joints. A possible etiologic factor of AS in relation to HLA-B27 considers the dynamic world of the microbiome (see figure 4). The microbiome of our gut is the first line of defense for pathogens that enter the body through the GI or mucosal tracts, as such, MHC I complexes are very abundant here. 70% of patients that are positive for AS have lumen inflammation as well, induced by HLA-B27 presence. Further, 54% of people who have enteric diseases that destroy the gut mucosa, such as Crohn’s disease, have the HLA-B27 gene and get AS. The link between gut microbiota and the development of AS is a well researched topic, and is the reason that HLA-B27 is believed to be an important determinant of getting AS disease.

The Pathogenesis and Etiology of Ankylosing Spondylitis

Ankylosing Spondylitis (AS) is a rheumatic disease that affects the axial skeleton, specifically, the joints of the spine and the sacroiliac joints. AS is a chronic inflammatory disease with unknown causation, whereas the vertebral joints become inflamed, and bony protuberances are formed and subsequently fused, locking the intervertebral disc and the adjacent vertebra together. The result is pain with flexion, extension, and axial loading. The sacroiliac joints, when effected, cause intense lower back pain, and make it difficult to do the previous tasks, in addition to sitting, standing, and resting. The disease becomes prevalent around age 25, beginning with chronic low back pain that is caused by inflammation of the sacroiliac joints, or sacroiliitis. The pain will eventually diffuse up the spinal column, and mobility will decrease as the disease progresses. The most commonly used form of treatment is the use of exercise, joint range of motion training, and most importantly, anti-inflammatory drugs known as nonsteroidal anti-inflammatory drugs (NSAID)s. Pathogenicity and etiology of the two most prominently-researched genes in AS will be discussed, HLA-B27 and IL-23. Current research has identified several potential etiologic genes, one of which is crucial to the immune system, the Human Leukocyte Antigen (HLA); the majority of patients effected by AS have a mutation in the HLA gene sequence. The HLA genes code for a cell surface receptor, the Major Histocompatibility Complex (MHC), which allows the body’s immune cells to recognize the body’s cells as “self” and not produce antibodies for our own cells or target them for destruction by T cells. Additionally, the MHC complex coordinates antigen presentation to macrophages, B cells, and dendritic cells, which allows for controlled destruction and eventual antibody production. The HLA mutant gene form that is common in almost AS patients is HLA-B27. The other gene that is being researched is Interleukin-23 (IL-23), which is a pro-inflammatory mediator that recruits TH17 cells, which are a subset of T helper cells that promote inflammation, and subsequently, osteogenesis. Whether it be additive or singular, the genes discussed all form a role in the pathogenesis, and therefore the etiology of ankylosing spondylitis.

Anatomy of the Spine and Sacroiliac Joints—The spine is a collection of 33 vertebral bones that have the principal role of protecting the spinal cord and the multiple spinal nerves that exit the column. The vertebral column is separated into five different regions, denoted superior to inferior: cervical, thoracic, lumbar, sacral, and coccygeal. The vertebrae are all facing the same direction; the lamina always faces posterior to the spinal cord, while the body always faces anterior to the spinal cord. The body of the vertebra is the largest and gives the ability for weight bearing. The vertebrae are held together by ligaments and tendons at specialized joints called the facet joints, and they allow for flexion, extension, and rotation. The vertebral bodies are separated by an intervertebral disc, which is a section of semi-tough but elastic cartilage, allowing for compression and elasticity when axial loading is performed. The bottom of the spine is connected to the ilium, or pelvis, and forms the two sacroiliac (SI) joints. The SI joints serve as the force distributor from the axial body to the pelvis and lower extremities. See figure 3a & 3b for a complete description of sacroiliac and vertebral anatomy.

Ankylosing spondylitis (AS) is a chronic, debilitating inflammatory disease that increases the morbidity and decreases the quality of life in those diagnosed. Current research has identified a link between AS and HLA-B27 & IL-23 and how possessing these genes increases the chance of acquiring the autoimmune disease. HLA-B27, the mutated form of the coding gene for the MHC complex, is shown to be the most important link in AS etiology, as 90% of AS cases involve the mutated form of the gene. In addition, HLA-B27, coupled with interactions with the gut microbiota, show connections in enteric disease and AS disease progression. Also, IL-23 cytokines show a link in osteogenesis of the spinal regions, showing the pathogenetic process of AS. A highly probable connection exists between the two genes: HLA-B27 promotes cytotoxic T cell proliferation, specifically in the gut, as the MHC I complexes, which protect the body from infections, are present in abundance due to this being the first point of contact to the outside world. Cytotoxic T cells, as well as other immune cells, can secrete pro-inflammatory cytokines, notably IL-23, which is a prominent cytokine in cell proliferation and differentiation, showing pathogenesis of spinal fusion in the later stages of AS.

Future endeavors in research to decrease the morbidity of AS should focus on genetic control of HLA-B27 and IL-23, the two main genes in the etiology and pathogenesis of AS. Possible solutions include genetic testing early in vitro, coupled with miRNA control and knock-out methods. Justifiably, more biotechnological advancements must be made for such a genomic control of an autoimmune disease. Future treatments are promising, but will require time that is not available to patients currently suffering with AS. Treating symptoms with NSAIDs and physical therapy will remain the best form of treatment until new world medical advancements come into fruition.

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Figure citations (in order): http://img.webmd.com/dtmcms/live/webmd/consumer_assets/site_images/media/medical/hw/h9991602_001.jpghttps://s-media-cache-ak0.pinimg.com/originals/5e/87/ac/5e87ac47b9fed7dfd905ec4dccfb253d.jpghttp://www.physio-pedia.com/images/thumb/c/c6/Sacroiliac_joint.png/300px-Sacroiliac_joint.pnghttp://www.uscspine.com/images/spine2.gifhttps://ghr.nlm.nih.gov/art/large/hla.jpeghttp://www.nature.com/nrrheum/journal/vaop/ncurrent/images/nrrheum.2015.133-f1.jpghttp://www.rcsb.org/pdb/images/1HSA_bio_r_500.jpg?bioNum=1https://t4.ftcdn.net/jpg/01/01/37/09/500_F_101370922_ixQXt19EJntkhfha8uYWuot3hVpBQaWy.jpghttps://openi.nlm.nih.gov/imgs/512/195/3131604/PMC3131604_ijms-12-03998f1.png

Evidence

Conclusion & Discussion

Figure 3a—Sacroiliac anatomy

Figure 6—Gut microbiota effects

The IL-23 Gene—The IL-23 gene codes for a cytokine, which is a soluble signaling protein that is released by the body’s cells. Specifically, the IL-23 cytokine, which is released by two of the APCs, macrophages and dendritic cells, induces the differentiation of T-Helper 17 cells (TH17) cells. TH17 are pro-inflammatory cells, and are a central part of the immune system. The IL-23 cytokine, when it binds to cells, such as TH17 cells, begins the JAK-STAT pathway, which is the intracellular signaling pathway promoting cell differentiation and expansion; however, overexpression of IL-23 can lead to excessive inflammation and oncogenesis, and therefore excessive tissue growth. It is believed that a metabolic imbalance of IL-23 contributes to the pathogenicity of AS, as overexpressed IL-23 cytokines are seen in 30% of AS cases. It is believed that overexpression of IL-23 induces a massive TH17 response, most commonly congregated to the spinal and sacroiliac joints, leading to cell expansion, specifically of the osteocytes of the vertebral bodies. It is unknown whether a protein misfolding of IL-23 increases the chance for osteogenesis, but overexpression has been found and noted in several research projects. Additionally, it is believed by many researchers that IL-23 effects the microbiome, just as the HLA-B27 genes do; this has made it difficult for quantification of the immunologic components of the microbiome. Nevertheless, IL-23 clearly plays a role in AS, and understanding how it effects the spinal and sacroiliac joints and the microbiome will be crucial for the advancement in the study of AS.

Figure 1—Disease Progression Figure 2—Radiograph Comparison

References

Figure 3b—Vertebra anatomy

Figure 7—HLA-B27 3D protein form

Figure 4—Autopsy of patient with highly progressed AS

Figure 5—HLA genes of chromosome 6

Figure 8—IL-23 3D protein form

Figure 9—Ibuprofen, an NSAID structure