Noncoding RNAs Involved in the Pathogenesis of Ankylosing Spondylitis.

Ankylosing spondylitis (AS) is a form of arthritis that can lead to fusion of vertebrae and sacroiliac joints following syndesmophyte formation. The etiology of this painful disease remains poorly defined due to its complex genetic background. There are no commonly accepted methods for early diagnosis of AS, nor are there any effective or efficient clinical treatments. Several noncoding RNAs (ncRNAs) have been linked to AS pathogenesis and inflammation via selective binding of their downstream targets. However, major gaps in knowledge remain to be filled before such findings can be translated into clinical treatments for AS. In this review, we outline recent findings that demonstrate essential roles of ncRNAs in AS mediated via multiple signaling pathways such as the Wnt, transforming growth factor-ß/bone morphogenetic protein, inflammatory, T-cell prosurvival, and nuclear factor-κB pathways. The summary of these findings provides insight into the molecular mechanisms by which ncRNAs can be targeted for AS diagnosis and the development of therapeutic drugs against a variety of autoimmune diseases.

Pannus inflammation in sacroiliitis following immune pathological injury and radiological structural damage: a study of 193 patients with spondyloarthritis.

BACKGROUND: The pathogenesis of sacroiliitis is unclear; therefore, we aimed to systematically study the immunopathology of sacroiliitis in patients with axial spondyloarthritis (axSpA), and explore the relationship between pannus formation, inflammation, and the structural damage caused by sacroiliitis. METHODS: Fine needle aspiration biopsy of the sacroiliac joint (SIJ) was performed in 193 patients with axSpA. Clinical, laboratory, and imaging data were collected at baseline and during the follow up. Immunohistochemistry analysis was performed to detect CD34+ microvessels, CD68+ osteoclasts/macrophages, vascular endothelial growth factor (VEGF), metalloproteinase-3 (MMP-3), tumor necrosis factor-α (TNF-α), and caspase-3. Autopsy subjects were used as controls. RESULTS: In early sacroiliitis (grade 0-1) all pathological features could be observed, with the most common being subchondral pannus formation. Among the 193 patients, 98 were followed up for 1-13 years (mean 3.6 years); 63.3% had radiological progression at the endpoint. Multiple regression analysis showed that cartilage pannus invasion (OR 2.99, P = 0.010) and endochondral ossification (OR 3.97, P = 0.049) at baseline were risk factors for radiological structural damage. Compared to SIJ controls, the subchondral microvessel density, number of CD68+ multinuclear osteoclasts, and the levels of VEGF, caspase-3, MMP-3, and TNF-α expressed at the interface of the bone and cartilage were significantly higher in patients with sacroiliitis. CONCLUSIONS: Subchondral fibrovascular tissue formation is the most important pathological feature in early sacroiliitis. The existence of cartilage pannus invasion or endochondral ossification at baseline can predict radiological structural damage during the follow up.

Clinical and genetic associations of radiographic sacroiliitis and its different patterns in psoriatic arthritis.

OBJECTIVES: We aimed to 1) identify clinical and genetic associations of sacroiliitis (SI) in patients with psoriatic arthritis (PsA), and 2) describe the different radiographic patterns of SI in PsA and their clinical and genetic associations. METHODS: 283 PsA patients, fulfilling CASPAR criteria, underwent detailed skin and rheumatologic assessments. In addition, HLA-B*27 and B*080101 status was recorded, which have been shown as the key genetic markers of radiographic SI in PsA. Grade 2 Unilateral or bilateral radiographic changes of SI were required for inclusion and involvement was further defined as asymmetrical or symmetrical. RESULTS: 70 patients (25%) had radiographic SI; all either with a present or past history of backache. Regression analysis demonstrated a significant association of SI with peripheral joint erosions (p=0.043), PASI maximum (p=0.041), younger age of PsA onset (p=<0.001), presence of HLA-B*0801 (p=0.002) and only marginal significance with HLA-B*2705 (p=0.059). Asymmetrical SI was noted in 51 patients (73%). In striking contrast to those patients with symmetrical SI, patients with asymmetrical SI were more likely to be female (p=0.04), have a trend towards more severe nail disease (p=0.08) and peripheral joint erosions (p=0.08), more osteolysis (p=0.01), more HLA-B*0801 positivity (p=0.001) and much less HLA-B*270502 positivity (p=<0.001). CONCLUSIONS: PsA developing at a younger age, severe skin disease, peripheral joint erosions, and HLA-B*0801 are significantly associated with SI, and there was only a marginal trend towards significance for HLA-B*2705. HLA-B*27 positive Axial-PsA patients resemble AS, while HLA-B*0801 positive Axial-PsA patients have asymmetrical and/or unilateral SI, which are typical of PsA.

Secukinumab: A Review in Ankylosing Spondylitis.

Secukinumab (Cosentyx(®)) is a fully human monoclonal antibody against the proinflammatory cytokine interleukin-17A. It is the first drug in its class to be approved for use in patients with active ankylosing spondylitis (AS). This article reviews the efficacy and tolerability of secukinumab in this indication and briefly summarizes its pharmacology. In ongoing phase III trials, 16 weeks' treatment with subcutaneous secukinumab 150 mg was effective in terms of improving the clinical signs and symptoms of disease and health-related quality of life in patients with AS, with these improvements maintained during longer-term (up to 2 years) treatment. In subgroup analyses, secukinumab was effective both in tumour necrosis factor (TNF) inhibitor-naïve patients and in patients intolerant of or refractory to TNF inhibitors. Secukinumab was generally well tolerated, with a tolerability profile consistent with that seen previously in patients with plaque psoriasis. In the absence of head-to-head trials, the position of secukinumab with respect to TNF inhibitors remains to be fully determined. Nevertheless, secukinumab is an effective and generally well tolerated treatment option for patients with AS.

Trafficking of antigens from gut to sacroiliac joints and spine in reactive arthritis and spondyloarthropathies: Mainly through lymphatics?

Bacterial trafficking from gut to mesenteric lymph nodes is physiologic only for a few commensal species, like Alcaligenes which produces antimicrobial-substances inhibiting growth of pathogenic bacteria. In reactive arthritis, some living bacteria transiently manage to travel from gut to joints/enthesis within dendritic cells and/or macrophages. Migration of dead or dormant bacteria outside the gut in spondyloarthropathies, including those associated with Crohn's disease, can occur either through blood or lymphatics. Migration through lymphatics instead of blood depends on the host, but also on the subset of pathogen, as shown for Salmonella. Retrograde trafficking within lymphatics of immune cells infected by dormant or dead bacteria, from mesenteric lymph nodes or thoracic duct to sacroiliac joint and spine, might contribute to axial involvement in some spondyloarthropathies and related disorders, since: 1- large influxes of pathogens can overwhelm lymph nodes, and Yersinia can even replicate within lymph nodes; 2- Whipple cells have been shown to circulate in thoracic duct lymph; 3- expansion of lymphatics is a prominent feature of gastro-intestinal inflammation, and obstruction of gut lymphatics a hallmark of Crohn's disease; 4- lymph reflux has been demonstrated in models of mesenteric lymph vessel obstruction; 5- reflux to sacroiliac has been observed in patients with chyluria undergoing lymphography; 6- lymphatics are present in the outer periosteum and paraspinal ligaments surrounding intervertebral discs and connected to thoracic duct. Accordingly, further studies on the trafficking of dendritic cells, macrophages and lymphocytes from gut to joints and spine in animal models of reactive arthritis or spondyloarthropathies should also focus on lymphatic routes.

Baseline sacroiliac joint magnetic resonance imaging abnormalities and male sex predict the development of radiographic sacroiliitis.

We evaluated the relationship between the baseline sacroiliac joint (SIJ) magnetic resonance imaging (MRI) findings and the development of radiographic sacroiliitis and tested their prognostic significance in cases of ankylosing spondylitis. Patients who had undergone an SIJ MRI at the rheumatology department were identified. Individuals for whom pelvic X-rays were available after at least 1 year of MRI were included in the analysis. All radiographs and MRI examinations were scored by two independent readers. Medical records of the patients were reviewed to obtain potentially relevant demographic and clinical data. We identified 1,069 SIJ MRIs, and 328 fulfilled our inclusion criteria. Reliability analysis revealed moderate to good inter- and intra-observer agreement. On presentation data, 14 cases were excluded because they had unequivocal radiographic sacroiliitis at baseline. After a mean of 34.8 months of follow-up, 24 patients developed radiographic sacroiliitis. The presence of active sacroiliitis (odds ratio (OR) 15.1) and structural lesions on MRI (OR 8.3), male sex (OR 4.7), fulfillment of Calin's inflammatory back pain criteria (P = 0.001), and total MRI activity score (P < 0.001) were found to be related to the development of radiographic sacroiliitis. By regression modeling, the presence of both active inflammatory and structural damage lesions on MRI and male sex were found to be predictive factors for the development of radiographic sacroiliitis. Our present results suggest that the occurrence of both active inflammatory and structural lesions in SIJs revealed by MRI is a significant risk factor for radiographic sacroiliitis, especially in male patients with early inflammatory back pain.

Imaging of the seronegative spondyloarthopathies.

The seronegative spondyloarthropathies are a diverse group of conditions most commonly affecting the axial spine, often presenting with back pain of an inflammatory nature. The primary unifying feature of these disorders is sacroiliitis. The distinction between subtypes of spondyloarthritis is based on genotype (HLA-B27 positivity as in ankylosing spondylitis), peripheral manifestations of disease (psoriatic and reactive arthritis), and factors such as age, gender, and comorbidity. Although radiography has long been used to diagnose the spondyloarthropathies, advanced imaging with magnetic resonance is better able to diagnose these disorders at their earliest stages and monitor disease-modifying therapies.

Clinical incidence of sacroiliac joint arthritis and pain after sacropelvic fixation for spinal deformity.

PURPOSE: Sacroiliac fixation using iliac screws for highly unstable lumbar spine has been reported with an improved fusion rate and clinical results. On the other hand, there is a potential for clinical problems related to iliac fixation, including late sacroiliac joint arthritis and pain. MATERIALS AND METHODS: Twenty patients were evaluated. Degenerative scoliosis was diagnosed in 7 patients, failed back syndrome in 6 patients, destructive spondyloarthropathy in 4 patients, and Charcot spine in 3 patients. All patients underwent posterolateral fusion surgery incorporating lumbar, S1 and iliac screws. We evaluated the pain scores, bone union, and degeneration of sacroiliac joints by X-ray imaging and computed tomography before and 3 years after surgery. For evaluation of low back and buttock pain from sacroiliac joints 3 years after surgery, lidocaine was administered in order to examine pain relief thereafter. RESULTS: Pain scores significantly improved after surgery. All patients showed bone union at final follow-up. Degeneration of sacroiliac joints was not seen in the 20 patients 3 years after surgery. Patients showed slight low back and buttock pain 3 years after surgery. However, not all patients showed relief of the low back and buttock pain after injection of lidocaine into the sacroiliac joint, indicating that their pain did not originate from sacroiliac joints. CONCLUSION: The fusion rate and clinical results were excellent. Also, degeneration and pain from sacroiliac joints were not seen within 3 years after surgery. We recommend sacroiliac fixation using iliac screws for highly unstable lumbar spine.

Clinical Incidence of Sacroiliac Joint Arthritis and Pain after Sacropelvic Fixation for Spinal Deformity

PURPOSE: Sacroiliac fixation using iliac screws for highly unstable lumbar spine has been reported with an improved fusion rate and clinical results. On the other hand, there is a potential for clinical problems related to iliac fixation, including late sacroiliac joint arthritis and pain. MATERIALS AND METHODS: Twenty patients were evaluated. Degenerative scoliosis was diagnosed in 7 patients, failed back syndrome in 6 patients, destructive spondyloarthropathy in 4 patients, and Charcot spine in 3 patients. All patients underwent posterolateral fusion surgery incorporating lumbar, S1 and iliac screws. We evaluated the pain scores, bone union, and degeneration of sacroiliac joints by X-ray imaging and computed tomography before and 3 years after surgery. For evaluation of low back and buttock pain from sacroiliac joints 3 years after surgery, lidocaine was administered in order to examine pain relief thereafter. RESULTS: Pain scores significantly improved after surgery. All patients showed bone union at final follow-up. Degeneration of sacroiliac joints was not seen in the 20 patients 3 years after surgery. Patients showed slight low back and buttock pain 3 years after surgery. However, not all patients showed relief of the low back and buttock pain after injection of lidocaine into the sacroiliac joint, indicating that their pain did not originate from sacroiliac joints. CONCLUSION: The fusion rate and clinical results were excellent. Also, degeneration and pain from sacroiliac joints were not seen within 3 years after surgery. We recommend sacroiliac fixation using iliac screws for highly unstable lumbar spine.

Interleukin 6 is not a crucial regulator in an animal model of tumour necrosis factor-mediated bilateral sacroiliitis.

OBJECTIVE: To evaluate the role of interleukin 6 (IL-6) in the pathogenesis of bilateral erosive sacroiliitis in human tumour necrosis factor transgenic (hTNFtg) mice, an animal model of ankylosing spondylitis (AS). METHODS: Histological sections of the sacroiliac joints from hTNFtg and IL-6(-/-)hTNFtg mice were evaluated, and wild type and IL-6(-/-)mice served as controls. mRNA levels of inflammation and tissue degradation related genes isolated from sacroiliac joints were also evaluated by quantitative PCR. RESULTS: Severe, erosive bilateral sacroiliitis in 14-week-old hTNFtg animals was accompanied by an upregulation of mRNAs related to tissue inflammation such as matrix metalloproteinase 3 (MMP3), MMP9 and MMP13 or osteoclast activation such as cathepsin K and tartrate-resistant acid phosphatase. In addition, IL-6 was increased in the sera and in the sacroiliac joints of hTNFtg animals. However, high expression of these marker genes in sacroiliac joints from IL-6(-/-)hTNFtg mice was also found. Moreover, absence of IL-6 in these animals did not alter bilateral, erosive sacroiliitis when compared to hTNFtg littermates. CONCLUSIONS: IL-6 is not a crucial regulator in an animal model of TNF-mediated bilateral, erosive sacroiliitis. This finding questions the potential of IL-6 blockade as a new treatment in patients with AS.

Inflammatory and degenerative sacroiliac joint disease in a primary back pain cohort.

OBJECTIVE: The prevalence of sacroiliac (SI) joint abnormalities in a primary low back pain population remains unresolved. The aims of our study were to define the prevalence of SI joint disease in this cohort, and to identify clinical features that might accurately predict radiographic changes in the SI joint and spine. METHODS: Lumbar spine and anteroposterior pelvis radiographs taken over a 3-year period for the evaluation of back pain at a major chiropractic college were scored for the presence of inflammatory or degenerative features. Data were subsequently extracted by means of a predetermined template from the clinical notes. The outcomes were correlated using Spearman's correlation coefficients. RESULTS: We identified 315 patients (173 men, 142 women), ages 18-60 years. Of these, 100 patients (31.7%) demonstrated SI joint abnormalities: 75 (23.8%) degenerative, 25 (7.9%) inflammatory. Sex was strongly associated with type of SI joint pathology; degenerative disease was predominantly found in women (68%), whereas inflammatory disease was predominantly found in men (63%). In women there was no correlation between degenerative SI joint abnormalities and degenerative changes in the lumbar spine. Of the clinical descriptors evaluated, none were associated with the radiographic findings with the exception of buttock pain, which was associated with inflammatory sacroiliitis. Neither being overweight nor pregnancy history was associated with degenerative changes in the SI joint. CONCLUSION: In a primary back pain cohort, degenerative SI joint disease may be an under-recognized clinical entity. It is strongly influenced by sex but is unrelated to degenerative changes in the lumbar spine. Currently proposed clinical discriminators performed poorly in correlating with radiographic changes in the SI joint.

Rheumatoid arthritis and ankylosing spondylitis - pathology of acute inflammation.

Histomorphological analysis of inflammatory lesions in rheumatoid arthritis (RA) and ankylosing spondylitis (AS) display similarities but also major differences. Ankylosing spondylitis is characterised by two key pathological findings: sacroiliac joint and spinal inflammation and new bone formation with the possible consequence of bone fusion, usually in the axial skeleton. In AS the primary site of inflammation is located at the enthesis or subchondral bone marrow with bone marrow oedema, lymphocytic infiltrates, increased osteoclast density and increased microvessel density are typical findings in acute inflammation. In RA joint inflammation has its origin in the synovial membrane of peripheral joints. Osteitis in the subchondral bone marrow reveals similar findings compared to AS and it is suggested to occur secondary to inflammation in the synovial membrane. Structural damage defines the outcome in both diseases. However, in AS it is defined by new bone formation and in RA by the destruction of cortical bone.

Clinical and imaging efficacy of infliximab in HLA-B27-Positive patients with magnetic resonance imaging-determined early sacroiliitis.

OBJECTIVE: To evaluate the efficacy of infliximab in HLA-B27-positive patients with magnetic resonance imaging (MRI)-determined early sacroiliitis, using both clinical and MRI assessments. METHODS: Forty patients with recent-onset inflammatory back pain, as assessed by the Calin criteria, HLA-B27 positivity, clinical disease activity as measured by the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), pain and morning stiffness, and magnetic resonance imaging (MRI)-determined sacroiliac joint bone edema were randomized in a double-blind manner to receive infliximab 5 mg/kg or placebo at 0, 2, 6, and 12 weeks. MRI scans were performed at baseline and 16 weeks and scored by 2 observers (blinded to both the order of the scans and to treatment group), using the Leeds scoring system. Clinical assessments included the BASDAI, the Bath Ankylosing Spondylitis Functional Index (BASFI), the Ankylosing Spondylitis Quality of Life (ASQoL) instrument, the ASsessment in Ankylosing Spondylitis International Working Group criteria (ASAS) for improvement, and markers of inflammation. RESULTS: The mean reduction in the total MRI score from week 0 to week 16 was significantly greater in infliximab-treated patients compared with placebo-treated patients (P = 0.033). On average, significantly more lesions resolved in the infliximab group (P < 0.001), while significantly more new lesions developed in the placebo group (P = 0.004). Significantly greater improvement in the infliximab group versus the placebo group was also observed for changes from week 0 to week 16 in the BASDAI (P = 0.002), BASFI (P = 0.004), and ASQoL (P = 0.007) scores. Responses according to the ASAS criteria for 40% improvement, the ASAS criteria for 20% improvement in 5 of 6 domains, and ASAS partial remission were achieved by 61%, 44%, and 56% of infliximab-treated patients, respectively. Infliximab was well tolerated, and no serious adverse events were observed. CONCLUSION: Infliximab was an effective therapy for early sacroiliitis, providing a reduction in disease activity by week 16. This study is the first to show that infliximab is effective for reducing clinical and imaging evidence of disease activity in patients with MRI-determined early axial spondylarthritis.

Efficacy of adalimumab in the treatment of axial spondylarthritis without radiographically defined sacroiliitis: results of a twelve-week randomized, double-blind, placebo-controlled trial followed by an open-label extension up to week fifty-two.

OBJECTIVE: To evaluate the efficacy and safety of the tumor necrosis factor (TNF) antagonist adalimumab in patients with axial spondylarthritis (SpA) without radiographically defined sacroiliitis refractory to conventional treatment. METHODS: Patients with active axial SpA (n = 46) were randomized to receive placebo or adalimumab at a dosage of 40 mg subcutaneously every other week for 12 weeks, followed by an open-label extension that continued up to week 52. The diagnosis of axial SpA required the presence of 3 of 6 diagnostic criteria, including 2 of the following 3 criteria: inflammatory back pain, HLA-B27 positivity, or acute inflammation of the spine or sacroiliac joints on magnetic resonance imaging, in the absence of radiographic evidence of sacroiliitis. The primary end point was a 40% response according to the improvement criteria of the Assessment of SpondyloArthritis international Society (ASAS40). RESULTS: All 46 patients (22 receiving adalimumab and 24 receiving placebo) completed the 12-week trial; 38 patients completed the extension period to week 52. At week 12, an ASAS40 response was achieved by 54.5% of the adalimumab-treated patients, as compared with 12.5% of the placebo-treated patients (P = 0.004). After switching to adalimumab, a similar degree of efficacy was also achieved by the patients who were initially treated with placebo. Efficacy was maintained in all patients until week 52. Young age at study entry and an elevated C-reactive protein concentration were the best predictors of achieving an ASAS40 response. Serious adverse events occurred in 5 patients, none of which was related to the study drug. CONCLUSION: Adalimumab is the first TNF antagonist to demonstrate good clinical efficacy and safety in patients with axial SpA without radiographically defined sacroiliitis.

Diagnosing early ankylosing spondylitis.

Diagnosis of ankylosing spondylitis is still delayed by many years. Several efforts have been made in the past few years to shorten this delay. A new set of criteria for inflammatory back pain has performed better than previous sets. MRI has evolved to become the standard imaging modality for the detection of sacroiliitis during early disease, and it clearly outperforms quantitative scintigraphy, which was the standard screening test for many years. Promising new developments such as whole body MRI and ultrasound (sonography) for the detection of enthesitis or sacroiliitis deserve further evaluation. Serum antibodies directed against a 28-kD Drosophila antigen may provide additional diagnostic information. A recently proposed diagnostic algorithm in patients with suspected early ankylosing spondylitis may help physicians confidently diagnose patients before definite radiographic sacroiliitis is detectable. Finally, referral strategies for patients seen by primary care physicians seem to work well and are currently under further valuation.

Innervation of the sacroiliac joint in rats by calcitonin gene-related peptide-immunoreactive nerve fibers and dorsal root ganglion neurons.

The sacroiliac joint (SIJ) can be a source of low back pain. Calcitonin gene-related peptide (CGRP) has been reported to play a significant role in nociceptive processing. However, the occurrence of CGRP-immunoreactive (CGRP-ir) sensory nerve fibers in the SIJ has not been fully defined. The present study investigated CGRP-ir nerve fibers supplying the SIJ. CGRP-ir nerve fibers in the vicinity of the SIJ cartilage and CGRP-ir neurons in the bilateral dorsal root ganglia (DRG) were examined immunohistochemically by administering anti-CGRP antiserum to rats. The SIJ was decalcified and cut into sections, and the CGRP-ir fibers around the SIJ cartilage were counted under microscopy. In another group, fluoro-gold (F-G), a neural tracer, was injected into the SIJ from the dorsal or ventral side with dorsal or ventral denervation. The number of F-G-labeled CGRP-ir neurons was counted in individual DRG. CGRP-ir fibers were observed more frequently in the tissues adjacent to the cranial part of the SIJ surface. In the case of dorsal denervation (ventral nerve supply), the CGRP-ir neurons composed 18.2% of the F-G-labeled neurons. In the case of ventral denervation (dorsal nerve supply), the CGRP-ir neurons composed 40.9% of the F-G-labeled neurons. There was a statistically significant difference in the number of CGRP-ir neurons between the ventral and dorsal nerve supplies to the SIJ. The cranial part of the dorsal side could be the part most associated with pain in the SIJ.

Combining information obtained from magnetic resonance imaging and conventional radiographs to detect sacroiliitis in patients with recent onset inflammatory back pain.

OBJECTIVE: To compare the contribution of changes on magnetic resonance imaging (MRI) and conventional radiography (CR) in the sacroiliac joints of patients with recent onset inflammatory back pain (IBP) in making an early diagnosis of spondyloarthritides. METHODS: The study involved 68 patients with IBP (38% male; mean (SD) age, 34.9 (10.3) years) with symptom duration less than two years. Coronal MRI of the sacroiliac joints was scored for inflammation and structural changes, and pelvic radiographs were scored by the modified New York (mNY) grading. Agreement between MRI and CR was analysed by cross tabulation per sacroiliac joint and per patient. RESULTS: A structural change was detected in 20 sacroiliac joints by MRI and in 37 by CR. Inflammation was detected in 36 sacroiliac joints by MRI, and 22 of these showed radiographic sacroiliitis. Fourteen patients fulfilled the mNY criteria based on CR. Classification according to the modified New York criteria would be justified for eight patients if it was based on MRI for structural changes only, for 14 if it was based on structural changes on CR, for 14 (partly) different patients if it was based on inflammation on MRI only, for 16 if it was based on inflammation and structural changes on MRI, for 19 if it was based on inflammation on CR combined with MRI, and for (the same) 19 if it was based on inflammation and structural damage on CR combined with MRI. CONCLUSIONS: CR can detect structural changes in SI joints with higher sensitivity than MRI. However, inflammation on MRI can be found in a substantial proportion of patients with IBP but normal radiographs. Assessment of structural changes by CR followed by assessment of inflammation on MRI in patients with negative findings gives the highest returns for detecting involvement of the SI joints by imaging in patients with recent onset IBP.

A longitudinal study on an autoimmune murine model of ankylosing spondylitis.

BACKGROUND: Proteoglycan aggrecan (PG)-induced arthritis (PGIA) is the only systemic autoimmune murine model which affects the axial skeleton, but no studies have been performed characterising the progression of spine involvement. OBJECTIVES: To follow pathological events in experimental spondylitis, and underline its clinical, radiographic, and histological similarities to human ankylosing spondylitis (AS); and to determine whether the spondyloarthropathy is a shared phenomenon with PGIA, or an "independent" disease. METHODS: Arthritis/spondylitis susceptible BALB/c and resistant DBA/2 mice, and their F1 and F2 hybrids were immunised with cartilage PG, and radiographic and histological studies were performed before onset and weekly during the progression of spondylitis. RESULTS: About 70% of the PG immunised BALB/c mice develop spondyloarthropathy (proteoglycan-induced spondylitis (PGISp), and the progression of the disease is very similar to human AS. It begins with inflammation in the sacroiliac joints and with enthesitis, and then progresses upwards, affecting multiple intervertebral disks. In F2 hybrids of arthritis/spondylitis susceptible BALB/c and resistant DBA/2 mice the incidence of arthritis was 43.5%, whereas the incidence of spondylitis was >60%. Some arthritic F2 hybrid mice had no spondylitis, whereas others developed spondylitis in the absence of peripheral arthritis. CONCLUSIONS: The PGISp model provides a valuable tool for studying autoimmune reactions in spondylitis, and identifying genetic loci associated with spondyloarthropathy.