Ankylosing spondylitis (AS) is a crippling autoimmune disorder that causes inflammation, fusion of the spinal vertebrae and osteoporosis. Over 40 years ago the strong association of AS to the HLA-B gene was discovered, but the pathogenesis of the disease is still completely unknown. More recent genetic studies have identified the association of another antigen presentation gene, endoplasmic reticulum aminopeptidase 1 (ERAP1) to AS. After HLA-B, ERAP1 has the strongest association, and highest odds ratio of any gene in AS and epistatic (non-additive) interactions between ERAP1 and HLA-B further increase risk for developing AS. Because both genes were known to contribute to antigen presentation, their epistatic interaction led to the hypothesis that antigen presentation may be responsible for the pathogenesis of the disease. However, the extent to which ERAP1 contributes to antigen presentation and how antigen presentation may be affecting bone homeostasis are not known. For that reason we investigated the role that ERAP1 plays in antigen presentation. Using ERAP1-/- mice, we demonstrated that ERAP1 is required to create and destroy immunogenic epitopes and could result in a complete inversion of the immunodominant T-cell response. We then demonstrated that similar changes could be mediated by polymorphic disease-associated human ERAP1 variants in cells and in transgenic mice. These studies furthered our understanding of how ERAP1 and AS-associated ERAP1 polymorphisms impacted antigen presentation, and supported the antigen-presentation hypothesis of AS. However, the central question in the field remained: can antigen presentation impact bone homeostasis? To investigate if ERAP1 and/or antigen presentation played a role in bone homeostasis, we performed a detailed skeletal analysis of mice deficient in ERAP1. Surprisingly, loss of ERAP1 was sufficient to cause spontaneous vertebral fusion, osteoporosis, and cellular infiltrate in the spine. This result demonstrated that ERAP1 was directly linked to bone homeostasis within the axial skeleton, but did not address if ERAP1 was mediating this effect via its role in antigen presentation or via a non-canonical function. To help discern the molecular mechanisms underlying this phenotype, we repeated the analysis in β-2-microglobulin knockout (β2m-/-) mice, that lack all cell-surface presentation of MHC-I. β2m-/- mice developed similar patterns of vertebral fusion and osteoporosis and appeared to have a later-onset of disease. Additionally, in the ERAP1 transgenic mice we uncovered multiple effects of ERAP1 outside of antigen presentation, such as modulation of NK-mediated cytotoxicity, modulation of NKG2D signaling on NK cells, and we determined that a variant of ERAP1 associated with a high risk for AS is a dominant lethal allele in male mice, independent of it's function in antigen presentation. These important findings define the role of ERAP1 in antigen presentation, directly connect ERAP1 to bone homeostasis and pathology in the axial skeleton, and suggest that antigen presentation plays a role in bone homeostasis. This work also provides the first simple genetic animal model of AS that spontaneously develops all of the key symptoms of AS and therefore will be valuable for the development and testing of therapeutics for the disease.
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