Session Title: Innate Immunity and Rheumatic Disease: Signaling Mechanisms
Session Type: Abstract Submissions (ACR)
Rheumatoid arthritis (RA) is an inflammatory disease of unknown etiology. Toll-like receptor (TLR) 9 recognizes pathogen-derived DNA and even self DNA under certain circumstances. Interestingly, environmental factors and especially bacterial infections have been suggested to favor RA development. In addition, TLR9 might also recognize damage-associated molecular patterns (DAMP) in RA. Since the involvement of TLR9 in RA remains unclear, we have investigated whether TLR9 is necessary for disease development in a mouse model of RA and we have analyzed TLR9 expression in samples from patients and controls.
Disease development was followed in the collagen-induced arthritis (CIA) mouse model. The impact of TLR9 was evaluated by comparing wild-type (WT) mice and TLR9-knockout (KO) true littermates. Arthritis was followed by clinical score evaluation. Inflammation and bone destruction were estimated by histology. Anti-collagen antibody, C3a and blood cytokine levels were measured by ELISA and Luminex. B and T lymphocytes as well as neutrophils were analyzed by flow cytometry. Osteoclastogenesis was analyzed by culturing bone marrow cells with M-CSF and RANKL and then counting TRAP-positive multinucleated cells by microscopy. Moreover, TLR9 expression was analyzed ex vivo by flow cytometry on whole blood from healthy donors and RA patients.
We clearly show for the first time that TLR9 is not crucial in inflammatory arthritis. Indeed, TLR9 is not required for arthritis development in CIA as TLR9-KO mice strongly developed clinical arthritis. Accordingly, WT and KO mice produced similar levels of anti-collagen antibodies. As a control, we verified that WT and KO mice responded to complete Freund’s adjuvant. Moreover, inflammation and joint destruction were observed in both WT and TLR9-KO mice and at a similar level. In agreement with those observations, no statistically significant difference was noted between WT and KO mice regarding the percentage or the activation of lymphocytes and neutrophils in the blood, spleen and lymph nodes. Osteoclastogenesis and complement activation (the latter being estimated by C3a production) are similar in WT and TLR9-KO CIA mice. Importantly, TLR7 does not compensate TLR9 deficiency in vivo as there is no TLR7 over-expression in TLR9-deficeint mice. The impact of TLR9 on cytokine secretion in vivo is currently being analyzed. In human samples, we have shown that leukocytes from healthy donors and RA patients express endosomal and cell surface TLR9 at the same extent and accordingly TLR9 expression is not correlated with disease activity in patients.
This is the first demonstration that TLR9-KO mice clearly develop arthritis. Immune cells from RA patients do not over-express TLR9 in comparison to healthy donors. Our results thus suggest that TLR9 does not play a crucial role in inflammatory arthritis development in the CIA model and that there is no intrinsic abnormal TLR9 expression in RA patients. This also suggests that TLR9 is not strongly involved in the recognition of DAMP in RA.
M. C. Boissier,
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ACR Meeting Abstracts - https://acrabstracts.org/abstract/inflammation-develops-in-a-toll-like-receptor-9-independent-manner-in-experimental-arthritis-and-rheumatoid-arthritis/