Decreased osteoclasts activity precedes the onset of RA disease. (a) No difference in the number of osteoblasts/mm over 6.5 mm of tibial cortico-endosteal surface in Balb/c IL1ra-/- compared to WT age matched controls before disease onset (n = 8). (b) A representative example of a horizontal section of tibia after X-ray micro-CT scanning of a Balb/c IL1ra-/- and a C57BL6 IL1ra-/- mouse and respective age matched WT control. (c) X-ray micro CT scanning of tibia of Balb/c IL1ra-/- mice showed significant increase in bone volume in the cancellous bone area compared to age matched WT control (n = 9). In contrast, no significant difference was found in C57BL6 IL1ra-/- mouse when compared to their age matched WT control (n = 6). (d) No significant difference in osteoclast numbers over 6.5 mm of tibial cortico-endosteal surface was observed in C57BL/6 IL-1ra-/- mice and their age-matched WT controls (n = 6). (e) A representative histological section of tibia showing decreased number of osteoclasts at the cortico-endosteal surface in Balb/c IL1ra-/- mice compared to WT age matched control and a graphic representation of the data showing a significant decrease in osteoclast numbers over 6.5 mm of tibial cortico-endosteal surface in Balb/c IL-1ra-/- mice compared to age matched wild type control mice (n = 8). All data are presented as means ± SEM and analysed by unpaired t-test. *P < 0.05. WT, wild type mice. Black arrows indicate presence of TRAP+ osteoclasts.