Dual Blockade of TNF and IL-17A Inhibits Inflammation and Structural Damage in a Rat Model of Spondyloarthritis.

The tumor necrosis factor (TNF) and IL-23/IL-17 axes are the main therapeutic targets in spondyloarthritis. Despite the clinical efficacy of blocking either pathway, monotherapy does not induce remission in all patients and its effect on new bone formation remains unclear. We aimed to study the effect of TNF and IL-17A dual inhibition on clinical disease and structural damage using the HLA-B27/human ß2-microglobulin transgenic rat model of SpA. Immunized rats were randomized according to arthritis severity, 1 week after arthritis incidence reached 50%, to be treated twice weekly for a period of 5 weeks with either a dual blockade therapy of an anti-TNF antibody and an anti-IL-17A antibody, a single therapy of either antibody, or PBS as vehicle control. Treatment-blinded observers assessed inflammation and structural damage clinically, histologically and by micro-CT imaging. Both single therapies as well as TNF and IL-17A dual blockade therapy reduced clinical spondylitis and peripheral arthritis effectively and similarly. Clinical improvement was confirmed for all treatments by a reduction of histological inflammation and pannus formation (p < 0.05) at the caudal spine. All treatments showed an improvement of structural changes at the axial and peripheral joints on micro-CT imaging, with a significant decrease for roughness (p < 0.05), which reflects both erosion and new bone formation, at the level of the caudal spine. The effect of dual blockade therapy on new bone formation was more prominent at the axial than the peripheral level. Collectively, our study showed that dual blockade therapy significantly reduces inflammation and structural changes, including new bone formation. However, we could not confirm a more pronounced effect of dual inhibition compared to single inhibition.

Paradoxical Augmentation of Experimental Spondyloarthritis by RORC Inhibition in HLA-B27 Transgenic Rats.

Objective: IL-17A plays a major role in the pathogenesis of spondyloarthritis (SpA). Here we assessed the impact of inhibition of RAR related orphan receptor-γ (RORC), the key transcription factor controlling IL-17 production, on experimental SpA in HLA-B27 transgenic (tg) rats. Methods: Experimental SpA was induced by immunization of HLA-B27 tg rats with heat-inactivated Mycobacterium tuberculosis. Splenocytes obtained at day 7, 14 and 21 after immunization were restimulated ex vivo to assess the induction of pro-inflammatory cytokines. Rats were then prophylactically treated with a RORC inhibitor versus vehicle control. The biologic effect of RORC inhibition was assessed by pro-inflammatory cytokine expression in draining lymph nodes. Arthritis and spondylitis were monitored clinically, and the degree of peripheral and axial inflammation, destruction and new bone formation was confirmed by histology. Results: Ex vivo mRNA and protein analyses revealed the rapid and selective induction of IL-17A and IL-22 production by a variety of lymphocyte subsets upon disease induction in HLA-B27 tg rats. Prophylactic RORC inhibition in vivo suppressed the expression of IL-17A, IL17F, and IL-22 without affecting the expression of other T helper cell subset related genes. This biological effect did not translate into clinical efficacy as RORC inhibition significantly accelerated the onset of arthritis and spondylitis, and aggravated the clinical severity of arthritis. This worsening of experimental SpA was confirmed by histopathological demonstration of increased inflammation, destruction, and new bone formation. Conclusion: Despite a significant suppression of the IL-17 axis, RORC inhibitor treatment accelerates and aggravates experimental SpA in the HLA-B27 tg rat model.

Transmembrane TNF drives osteoproliferative joint inflammation reminiscent of human spondyloarthritis.

TNF plays a key role in immune-mediated inflammatory diseases including rheumatoid arthritis (RA) and spondyloarthritis (SpA). It remains incompletely understood how TNF can lead to different disease phenotypes such as destructive peripheral polysynovitis in RA versus axial and peripheral osteoproliferative inflammation in SpA. We observed a marked increase of transmembrane (tm) versus soluble (s) TNF in SpA versus RA together with a decrease in the enzymatic activity of ADAM17. In contrast with the destructive polysynovitis observed in classical TNF overexpression models, mice overexpressing tmTNF developed axial and peripheral joint disease with synovitis, enthesitis, and osteitis. Histological and radiological assessment evidenced marked endochondral new bone formation leading to joint ankylosis over time. SpA-like inflammation, but not osteoproliferation, was dependent on TNF-receptor I and mediated by stromal tmTNF overexpression. Collectively, these data indicate that TNF can drive distinct inflammatory pathologies. We propose that tmTNF is responsible for the key pathological features of SpA.

mTOR Blockade by Rapamycin in Spondyloarthritis: Impact on Inflammation and New Bone Formation in vitro and in vivo.

Introduction: Spondyloarthritis (SpA) is characterized by inflammation, articular bone erosions and pathologic new bone formation. Targeting TNFα or IL-17A with current available therapies reduces inflammation in SpA, however, treatment of the bone pathology in SpA remains an unmet clinical need. Activation of the mammalian target Of rapamycin (mTOR) promotes IL-17A expression and osteogenesis. Therefore, the inhibition of mTOR (with rapamycin) could be a promising therapeutic avenue in SpA. Objectives: To investigate the effect of blocking mTOR on inflammation, bone erosions and new bone formation in SpA. Methods: Peripheral blood mononuclear cells (PBMCs) from patients with SpA were stimulated with anti-CD3/CD28 in the presence or absence of rapamycin and the resulting cytokine expression was assessed. Fibroblast-like synoviocytes (FLS) from SpA patients were assessed for osteogenic differentiation potential in conditions with TNFα, IL-17A, or TNFα plus IL-17A, in the presence or absence of rapamycin. HLA-B27/Huß2m transgenic rats were immunized with low dose heat-inactivated Mycobacterium tuberculosis (M. tub), treated with 1.5 mg/kg rapamycin prophylactically or therapeutically and monitored for arthritis and spondylitis. Histology and mRNA analysis were performed after 5 weeks of treatment to assess inflammation and bone pathology. Results:In vitro TNFα and IL-17A protein production by SpA PBMCs was inhibited in the presence of rapamycin. Rapamycin also inhibited osteogenic differentiation of human SpA FLS. Ex vivo analysis of SpA synovial biopsies indicated activation of the mTOR pathway in the synovial tissue of SpA patients. In vivo, prophylactic treatment of HLA-B27/Huß2m transgenic rats with rapamycin significantly inhibited the development and severity of inflammation in peripheral joints and spine (arthritis and spondylitis), with histological evidence of reduced bone erosions and new bone formation around peripheral joints. In addition, therapeutic treatment with rapamycin significantly decreased severity of arthritis and spondylitis, with peripheral joint histology showing reduced inflammation, bone erosions and new bone formation. IL-17A mRNA expression was decreased in the metacarpophalangeal joints after rapamycin treatment. Conclusion: mTOR blockade inhibits IL-17A and TNFα production by PBMCs, and osteogenic differentiation of FLS from patients with SpA in vitro. In the HLA-B27 transgenic rat model of SpA, rapamycin inhibits arthritis and spondylitis development and severity, reduces articular bone erosions, decreases pathologic new bone formation and suppresses IL-17A expression. These results may support efforts to evaluate the efficacy of targeting the mTOR pathway in SpA patients.

Interleukin-17A Inhibition Diminishes Inflammation and New Bone Formation in Experimental Spondyloarthritis.

OBJECTIVE: It remains unclear if and how inflammation and new bone formation in spondyloarthritis (SpA) are coupled. We undertook this study to assess the hypothesis that interleukin-17A (IL-17A) is a pivotal driver of both processes. METHODS: The effect of tumor necrosis factor (TNF) and IL-17A on osteogenesis was tested in an osteoblastic differentiation assay using SpA fibroblast-like synoviocytes (FLS) differentiated with dexamethasone, ß-glycophosphatase, and ascorbic acid. IL-17A blockade was performed in HLA-B27/human ß2 -microglobulin (hß2 m)-transgenic rats, which served as a model for SpA in both prophylactic and therapeutic settings. Inflammation and new bone formation were evaluated by micro-computed tomography imaging, histologic analysis, and gene expression profiling. RESULTS: TNF and IL-17A significantly increased in vitro osteoblastic differentiation. In vivo, prophylactic blockade of IL-17A significantly delayed spondylitis and arthritis development and decreased arthritis severity. Anti-IL-17A treatment was also associated with prevention of bone loss and periosteal new bone formation. Therapeutic targeting of IL-17A after the initial inflammatory insult also significantly reduced axial and peripheral joint inflammation. This treatment was again associated with a marked reduction in spinal and peripheral structural damage, including new bone formation. RNA sequencing of target tissue confirmed that IL-17A is a key driver of the molecular signature of disease in this model and that therapeutic anti-IL-17A treatment reversed the inflammatory signature and the selected gene expression related to bone damage. CONCLUSION: Both prophylactic and therapeutic inhibition of IL-17A diminished inflammation and new bone formation in HLA-B27/hß2 m-transgenic rats. Taken together with the ability of IL-17A to promote osteoblastic differentiation of human SpA FLS, these data suggest a direct link between IL-17A-driven inflammation and pathologic new bone formation in SpA.

The Initiation, but Not the Persistence, of Experimental Spondyloarthritis Is Dependent on Interleukin-23 Signaling.

IL-17A is a central driver of spondyloarthritis (SpA), its production was originally proposed to be IL-23 dependent. Emerging preclinical and clinical evidence suggests, however, that IL-17A and IL-23 have a partially overlapping but distinct biology. We aimed to assess the extent to which IL-17A-driven pathology is IL-23 dependent in experimental SpA. Experimental SpA was induced in HLA-B27/Huß2m transgenic rats, followed by prophylactic or therapeutic treatment with an anti-IL23R antibody or vehicle control. Spondylitis and arthritis were scored clinically and hind limb swelling was measured. Draining lymph node cytokine expression levels were analyzed directly ex vivo, and IL-17A protein was measured upon restimulation with PMA/ionomycin. Prophylactic treatment with anti-IL23R completely protected against the development of both spondylitis and arthritis, while vehicle-treated controls did develop spondylitis and arthritis. In a therapeutic study, anti-IL23R treatment failed to reduce the incidence or decrease the severity of experimental SpA. Mechanistically, expression of downstream effector cytokines, including IL-17A and IL-22, was significantly suppressed in anti-IL23R versus vehicle-treated rats in the prophylactic experiments. Accordingly, the production of IL-17A upon restimulation was reduced. In contrast, there was no difference in IL-17A and IL-22 expression after therapeutic anti-IL23R treatment. Targeting the IL-23 axis during the initiation phase of experimental SpA-but not in established disease-inhibits IL-17A expression and suppresses disease, suggesting the existence of IL-23-independent IL-17A production. Whether IL-17A can be produced independent of IL-23 in human SpA remains to be established.

Innate Immune Activation Can Trigger Experimental Spondyloarthritis in HLA-B27/Huß2m Transgenic Rats.

Spondyloarthritis (SpA) does not display the typical features of auto-immune disease. Despite the strong association with MHC class I, CD8+ T cells are not required for disease induction in the HLA-B27/Huß2m transgenic rats. We used Lewis HLA-B27/Huß2m transgenic rats [21-3 × 283-2]F1, HLA-B7/Huß2m transgenic rats [120-4 × 283-2]F1, and wild-type rats to test our hypothesis that SpA may be primarily driven by the innate immune response. In vitro, splenocytes were stimulated with heat-inactivated Mycobacterium tuberculosis and cytokine expression and production was measured. In vivo, male and female rats were immunized with 30, 60, or 90 µg of heat-inactivated M. tuberculosis and clinically monitored for spondylitis and arthritis development. After validation of the model, we tested whether prophylactic and therapeutic TNF targeting affected spondylitis and arthritis. In vitro stimulation with heat-inactivated M. tuberculosis strongly induced gene expression of pro-inflammatory cytokines such as TNF, IL-6, IL-1α, and IL-1ß, in the HLA-B27 transgenic rats compared with controls. In vivo immunization induced an increased spondylitis and arthritis incidence and an accelerated and synchronized onset of spondylitis and arthritis in HLA-B27 transgenic males and females. Moreover, immunization overcame the protective effect of orchiectomy. Prophylactic TNF targeting resulted in delayed spondylitis and arthritis development and reduced arthritis severity, whereas therapeutic TNF blockade did not affect spondylitis and arthritis severity. Collectively, these data indicate that innate immune activation plays a role in the initiation of HLA-B27-associated disease and allowed to establish a useful in vivo model to study the cellular and molecular mechanisms of disease initiation and progression.

The Transcriptional Coactivator Bob1 Is Associated With Pathologic B Cell Responses in Autoimmune Tissue Inflammation.

OBJECTIVE: The molecular mechanisms steering abnormal B cell responses in autoimmune diseases remain poorly understood. We undertook this study to identify molecular switches controlling pathologic B cell responses in rheumatoid arthritis (RA). METHODS: Candidate molecules were identified by gene expression profiling of RA synovitis and validated by quantitative polymerase chain reaction and immunohistochemistry. B cell-specific expression was confirmed by immunofluorescence, immunoblotting, and flow cytometry. The role of Bob1 in pathologic B cell responses was assessed in collagen-induced arthritis (CIA). RESULTS: Transcriptional profiling of RA synovitis revealed a prominent B cell signature, with the transcriptional coactivator Bob1 and its putative target BCMA being among the most up-regulated genes. Further analysis confirmed the microarray data and demonstrated elevated levels of Bob1 in B cells in RA synovium. A functional study showed that Bob1-deficient mice failed to produce pathogenic anti-type II collagen (anti-CII) antibodies and were resistant to CIA. Adoptive transfer of cells from Bob1-deficient and Bob1-sufficient mice to recombination-activating gene 1-null mice demonstrated that Bob1 deficiency exclusively in B cells abrogated germinal center (GC) B cell formation, anti-CII antibody production, and CIA development. Consistent with data from animal studies, immunophenotyping of human B cell subsets revealed increased expression of Bob1, predominantly in centrocytes and centroblasts. Correspondingly, Bob1 expression in RA synovitis was strongly correlated with CD21L, a molecular marker of GCs. In addition, similar Bob1 overexpression and correlation with CD21L expression was evidenced in parotid salivary gland tissue from patients with primary Sjögren's syndrome. CONCLUSION: These expression and functional data identify the transcriptional coactivator Bob1 as a candidate molecular switch of pathogenic B cell responses in autoimmune diseases in humans.

Insulin-Like Growth Factor I Does Not Drive New Bone Formation in Experimental Arthritis.

INTRODUCTION: Insulin like growth factor (IGF)-I can act on a variety of cells involved in cartilage and bone repair, yet IGF-I has not been studied extensively in the context of inflammatory arthritis. The objective of this study was to investigate whether IGF-I overexpression in the osteoblast lineage could lead to increased reparative or pathological bone formation in rheumatoid arthritis and/or spondyloarthritis respectively. METHODS: Mice overexpressing IGF-I in the osteoblast lineage (Ob-IGF-I+/-) line 324-7 were studied during collagen induced arthritis and in the DBA/1 aging model for ankylosing enthesitis. Mice were scored clinically and peripheral joints were analysed histologically for the presence of hypertrophic chondrocytes and osteocalcin positive osteoblasts. RESULTS: 90-100% of the mice developed CIA with no differences between the Ob-IGF-I+/- and non-transgenic littermates. Histological analysis revealed similar levels of hypertrophic chondrocytes and osteocalcin positive osteoblasts in the ankle joints. In the DBA/1 aging model for ankylosing enthesitis 60% of the mice in both groups had a clinical score 1<. Severity was similar between both groups. Histological analysis revealed the presence of hypertrophic chondrocytes and osteocalcin positive osteoblasts in the toes in equal levels. CONCLUSION: Overexpression of IGF-I in the osteoblast lineage does not contribute to an increase in repair of erosions or syndesmophyte formation in mouse models for destructive and remodeling arthritis.

Data showing non-conventional HLA-B27 expression in axial joints and gut tissue from B27 transgenic rats, and in frozen and paraffin-fixed synovial SpA tissue.

Data is presented showing expression of non-conventional (NC) heavy chain forms of B27 in synovial tissues from SpA patients. Data is presented showing the expression patterns of NC-B27 in joint, gastrointestinal and lymphoid tissues from B27 transgenic (TG(1)) rats with M. tuberculosis-induced SpA. Expression of NC-B27 was determined by immunohistochemistry and flow cytometry using HC10 and HD6 antibodies. These data are the extension of the data presented and discussed in "Non-conventional forms of HLA-B27 are expressed in Spondyloarthritis joints and gut tissue" (O. Rysnik, K. McHugh, L. M. van Duivenvoorde, M. N. van Tok, G. Guggino, J. D. Taurog, S. Kollnberger, F. Ciccia, D. L. Baeten, P. Bowness, 2016) [1].

Non-conventional forms of HLA-B27 are expressed in spondyloarthritis joints and gut tissue.

OBJECTIVES: Human leukocyte antigen (HLA)-B27 (B27) is the strongest genetic factor associated with development of Ankylosing Spondylitis and other spondyloarthropathies (SpA), yet the role it plays in disease pathogenesis remains unclear. We investigated the expression of potentially pathogenic non-conventional heavy chain forms (NC) of B27 in synovial and intestinal tissues obtained from SpA patients. We also determined the presence of NC-B27 in joints, lymphoid and gastrointestinal tissue from B27 transgenic (TG(1)) rats with M.tuberculosis-induced SpA. METHODS: Expression of NC-B27 in human SpA joints and gut and in (21-3 × 283-2)F1 HLA-B27/Huß2m rat tissue was determined by immunohistochemistry, flow cytometry and confocal microscopy analysis using HC10 and HD6 antibodies. RESULTS: Both HC10- and HD6-reactive HLA molecules were present in synovial tissue from SpA patients. Both NC-B27 and KIR3DL2, a ligand for NC-B27, were expressed in inflamed terminal ileal tissues in patients with early SpA. Infiltrating cells in inflamed joint tissues isolated from B27 TG(1) rats expressed high levels of NC-B27. NC-B27 were also expressed in joint-resident cells from ankle and tail joints of B27 TG(1) rats prior to clinical arthritis. The expression of NC-B27 on B27 TG(1) rat CD11b/c(+), CD8α(+), cells from spleens and LNs increased with animal age and disease progression. CONCLUSIONS: Non-conventional HLA class 1 molecules are expressed on resident and infiltrating cells in both synovial and GI tissues in human SpA. NC-B27 expression in joints and lymphoid tissues from B27 TG(1) rats prior to the onset of arthritis is consistent with the hypothesis that they play a pathogenic role in SpA.

Interleukin-9 Overexpression and Th9 Polarization Characterize the Inflamed Gut, the Synovial Tissue, and the Peripheral Blood of Patients With Psoriatic Arthritis.

OBJECTIVE: To investigate the expression and tissue distribution of Th9-related cytokines in patients with psoriatic arthritis (PsA). METHODS: Quantitative gene expression analysis of Th1, Th17, and Th9 cytokines was performed in intestinal biopsy samples obtained from patients with PsA, HLA-B27-positive patients with ankylosing spondylitis (AS), patients with Crohn's disease (CD), and healthy controls. Expression and tissue distribution of interleukin-23 (IL-23), IL-17, IL-22, IL-9, and IL-9 receptor (IL-9R) were evaluated by immunohistochemistry and confocal microscopy. Flow cytometry was used to study the frequency of Th9 cells among peripheral blood, lamina propria, and synovial fluid mononuclear cells. The functional relevance of IL-9R expression on epithelial cells was assessed in functional in vitro studies. Th9 cells in synovial tissue from patients with PsA were also studied. RESULTS: Subclinical gut inflammation in PsA patients was characterized by a clear Th17 and Th22, but not Th1, polarized immune response. Unlike AS and CD, a strong and significant up-regulation of IL-9 was observed in PsA gut, especially among infiltrating mononuclear cells, high endothelial venules, and Paneth cells. IL-9-positive mononuclear cells were demonstrated to be in large part Th9 cells. IL-9 overexpression was accompanied by significant Paneth cell hyperplasia. Paneth cells strongly overexpressed IL-9R, and stimulation of epithelial cells, isolated from PsA patients, with IL-9 resulted in overexpression of α-defensin 5 and IL-23p19. Peripheral and synovial expansion of α4ß7+ Th9 cells was also observed in patients with PsA. Increased expression of IL-9 and IL-9R was also found in synovial tissue. CONCLUSION: Strong IL-9/Th9 polarization seems to be the predominant immunologic signature in patients in PsA.

The cartilage protein melanoma inhibitory activity contributes to inflammatory arthritis.

OBJECTIVE: Melanoma inhibitory activity (MIA) is a small chondrocyte-specific protein with unknown function. MIA knockout mice (MIA(-/-)) have a normal phenotype with minor microarchitectural alterations of cartilage. Our previous study demonstrated that immunodominant epitopes of MIA are actively presented in an HLA-DR4-restricted manner in the inflamed RA joint. The objective of this study was to investigate the potential role of MIA as an autoantigen. METHODS: Collagen-induced arthritis (CIA) and anti-collagen antibody-induced arthritis (CAIA) were induced in MIA(-/-) mice. Anti-type II collagen (anti-CII) antibodies were measured by ELISA. T cell proliferation and cytokine production were assessed by flow cytometry. RESULTS: MIA(-/-) mice had a markedly reduced incidence and severity of CIA and CAIA compared with wild-type (WT) mice. Attenuation of disease was not related to defective binding of anti-CII antibodies to cartilage in the absence of MIA. However, MIA(-/-) mice had significantly reduced anti-CII IgG1 and IgG2a antibody levels accompanied by an increase in FoxP3-expressing CD25(+)CD4(+) regulatory T cells. This was paralleled by a significant reduction in CII-specific IFN-γ production by T cells in MIA(-/-) but not WT animals, suggesting a qualitative impact of MIA on the collagen-induced Th1 response. Furthermore, Ag-specific proliferation of T cells after restimulation with MIA in WT but not MIA(-/-) mice indicated the existence of MIA-specific T cells in the context of CIA. CONCLUSION: These data support a role for MIA as an autoantigen during arthritis development. Whether MIA can influence the balance of pathogenic vs regulatory responses in human RA remains to be investigated.

Disease-specific and inflammation-independent stromal alterations in spondylarthritis synovitis.

OBJECTIVE: The molecular processes driving the distinct patterns of synovial inflammation and tissue remodeling in spondylarthritis (SpA) as compared to rheumatoid arthritis (RA) remain largely unknown. Therefore, we aimed to identify novel and unsuspected disease-specific pathways in SpA by a systematic and unbiased synovial gene expression analysis. METHODS: Differentially expressed genes were identified by pan-genomic microarray and confirmed by quantitative polymerase chain reaction and immunohistochemical analyses of synovial tissue biopsy samples from patients with SpA (n=63), RA (n=28), and gout (n=9). The effect of inflammation on gene expression was assessed by stimulating fibroblast-like synoviocytes (FLS) with synovial fluid and by analysis of synovial tissue samples at weeks 0 and 12 of etanercept treatment. RESULTS: Using very stringent statistical thresholds, microarray analysis identified 64 up-regulated transcripts in patients with SpA synovitis as compared to those with RA synovitis. Pathway analysis revealed a robust myogene signature in this gene set. The myogene signature was technically and biologically reproducible, was specific for SpA, and was independent of disease duration, treatment, and SpA subtype (nonpsoriatic versus psoriatic). Synovial tissue staining identified the myogene expressing cells as vimentin-positive, prolyl 4-hydroxylase ß-positive, CD90+, and CD146+ mesenchymal cells that were significantly overrepresented in the intimal lining layer and synovial sublining of inflamed SpA synovium. Neither in vitro exposure to synovial fluid from inflamed SpA joints nor in vivo blockade of tumor necrosis factor modulated the SpA-specific myogene signature. CONCLUSION: These data identify a novel and disease-specific myogene signature in SpA synovitis. The fact that this stromal alteration appeared not to be downstream of local inflammation warrants further analysis of its functional role in the pathogenesis of the disease.

Relationship between inflammation, bone destruction, and osteoproliferation in the HLA-B27/human ß2 -microglobulin-transgenic rat model of spondylarthritis.

OBJECTIVE: Inhibition of inflammation and destruction, but not of osteoproliferation, in patients with spondylarthritis (SpA) treated with anti-tumor necrosis factor raises the question of how these three processes are interrelated. This study was undertaken to analyze this relationship in a rat model of SpA. METHODS: Histologic spine and joint samples from HLA-B27/human ß(2) -microglobulin (hß(2) m)-transgenic rats were analyzed for signs of spondylitis and destructive arthritis and semiquantitatively scored as showing mild, moderate, or severe inflammation. RESULTS: In rats exhibiting spondylitis, mildly inflamed sections displayed lymphocyte infiltration in connective tissue adjacent to the junction of the anulus fibrosus and vertebral bone but not at the enthesis. Moderately inflamed tissue samples contained osteoclasts eroding bone outside the cartilage end plate. In sections from rats with severe inflammation, the cartilage end plate and underlying bone marrow were also affected. End-stage disease was characterized by complete destruction of the intervertebral disc and vertebrae, with ongoing infiltration. Osteoproliferation was not observed in samples from rats with no or mild inflammation, but was present at the edge of the vertebrae in sections with moderate inflammation and persisted during severe inflammation and end-stage destruction. Osteoproliferation occurred at the border of inflammation, at a distance from bone destruction. A strong correlation between the extent of inflammation, destruction, and osteoproliferation was observed. Sections from rats with arthritis displayed a similar pattern of synovial inflammation associated with bone destruction, and simultaneous but topographically distinct osteoproliferation starting from the periosteum. CONCLUSION: SpA in B27/hß(2) m-transgenic rats is characterized by destructive inflammatory pannus tissue rather than by enthesitis or osteitis. Destruction and osteoproliferation occur simultaneously but at distinct sites in joints with moderate to severe inflammation.