S100A8/A9 is not essential for the development of inflammation and joint pathology in interleukin-1 receptor antagonist knockout mice

Background: Excessive osteoclast activity, which is strongly stimulated by pro-inflammatory mediators, results in bone and cartilage degeneration as central features of many arthritides. Levels of the alarmin S100A8/A9 and interleukin (IL)-1β are both increased in arthritis patients and correlate with disease activity and progression of tissue erosion. We previously presented S100A8/A9 as a good biomarker for joint inflammation and arthritis pathology under circumstances of high IL-1 signaling in mice that lack the gene encoding IL-1 receptor antagonist (Il1rn ^−/− mice). Here, we investigated whether S100A8/A9 is also actively involved in the development of joint inflammation and both cartilage and bone pathology under these conditions by comparing Il1rn ^−/− mice with mice that have an additional deficiency for S100a9 (Il1rn ^−/− XS100a9 ^−/− ).

Methods: Il1rn ^−/− XS100a9 ^−/− on a BALB/c background were obtained by crossing S100a9 ^−/− mice and Il1rn ^−/− mice. Arthritis incidence and severity were macroscopically scored. Myeloid cell populations in the bone marrow and spleen were determined using flow cytometry. In vitro osteoclastogenesis of bone marrow cells was evaluated with TRAP staining. Microscopic joint inflammation, cartilage degeneration, and bone destruction were evaluated using histology of ankle joints of 12- and 20-week-old mice.

Results: Macroscopically scored arthritis severity was comparable between Il1rn ^−/− and Il1rn ^−/− XS100a9 ^−/− mice. Inflammation, cartilage erosion, and bone erosion were clearly present in 12-week-old mice of both strains lacking Il1rn ^−/− , but not significantly different between Il1rn ^−/− XS100a9 ^−/− and Il1rn ^−/− . Moreover, we observed that the numbers of neutrophils and monocytes were increased by the absence of Il1rn, which was affected by the absence of S100a9 only in the spleen but not in the bone marrow. In line with our other findings, the absence of S100a9 did not affect the osteoclastogenic potential of osteoclast precursors in the absence of Il1rn. Finally, in agreement with the findings in early arthritis development in 12-week-old mice, cartilage and bone erosion in 20-week-old mice was significantly higher in both Il1rn ^−/− strains, but the additional absence of S100a9 did not further affect tissue pathology.

Conclusion: S100A8/A9 deficiency does not significantly affect inflammation and joint destruction in mice with high IL1β signaling suggesting that S100A8/A9 is not essential for the development of arthritis under these conditions.