Collagen-binding integrin alpha11beta1 contributes to joint destruction in arthritic hTNFtg mice

Background: In rheumatoid arthritis (RA), fibroblast-like synoviocytes (FLS) undergo a tumor-like transformation, wherein they develop an aggressive phenotype that is characterized by increased adhesion to components of cartilage extracellular matrix (ECM) and that contributes extensively to joint destruction. The collagen-binding integrin alpha11beta1 was previously shown to be involved in similar processes in cancer-associated fibroblasts mediating tumorigenicity and metastasis in certain tumors. Therefore, this study aimed to study the role of integrin alpha11beta1 in RA and to characterize the effects of alpha11beta1 deficiency on the disease course and severity in arthritic hTNFtg mice. Methods: The expression levels of integrin alpha11beta1 were analyzed by immunohistochemistry, immunofluorescence, and western blot analysis in synovial samples and FLS of patients with RA and osteoarthritis (OA) as well as in samples from wild type (wt) and arthritic hTNFtg mice. Furthermore, the subcellular expression of integrin alpha11beta1 was investigated in co-culture experiments with cartilage explants and analyzed by transmission electron microscopy. To investigate the effects of integrin alpha11beta1 deficiency, itga11-/- mice were interbred with hTNFtg mice and disease severity was assessed by clinical scoring of grip strength and paw swelling over the disease course. Hind paws of 12-weeks-old mice of all genotypes were analyzed by uCT imaging followed by stainings of paraffin-embedded tissue sections with Toluidine-blue and tartrate-resistant acid phosphatase (TRAP) to evaluate established parameters of joint destruction such as inflammation area, cartilage destaining, FLS attachment to the cartilage surface, and bone damage. Results: Expression levels of integrin alpha11beta1 were clearly elevated in synovial tissues and FLS from RA patients and hTNFtg mice, compared to the controls derived from OA patients and wt mice. Interestingly, this expression was shown to be particularly localized in focal adhesions of the FLS. As revealed by transmission electron microscopy, integrin alpha11beta1 expression was particularly evident in areas of direct cellular contact with the ECM of cartilage. Evaluations of clinical scorings and histomorphological analyses demonstrated that itga11-/-hTNFtg displayed alleviated clinical symptoms, higher bone volume, less cartilage destruction, and reduced FLS attachment to the cartilage in comparison to hTNFtg mice. Conclusions: The collagen-binding integrin alpha11beta1 is upregulated in the context of RA and its deficiency in mice with an inflammatory hTNFtg background leads to a significant reduction in the arthritic phenotype which makes integrin alpha11beta1 an interesting target for therapeutical intervention. ### Competing Interest Statement The authors have declared no competing interest.