Objective— Interleukin (IL)-1 plays an important role in atherosclerosis. IL-1 receptor antagonist (IL-1Ra) is an endogenous inhibitor of IL-1. However, the role of IL-1Ra in the development of atherosclerosis is poorly understood.

Methods and Results— Mice that lacked IL-1Ra (IL-1Ra−/−) were crossed with apolipoprotein E-deficient (E−/−) mice and formation of atherosclerotic lesions was analyzed after 16 weeks or 32 weeks consumption of a normal chow diet. This study focused on the comparison of atherosclerotic lesion between IL-1Ra+/+/apoE−/− (n=12) and IL-1Ra^+/−/apoE−/− mice (n=12), because of the significantly leaner phenotype in IL-1Ra−/−/apoE−/− mice compared with the others. Interestingly, atherosclerotic lesion size in IL-1Ra+/−/apoE−/− mice at age 16 weeks was significantly increased (30%) compared with IL-1Ra+/+/apoE−/− mice (P<0.05). At 32 weeks, the differences of lesion size between these mice failed to achieve statistical significance. However, immunostaining demonstrated an 86% (P<0.0001) increase in the MOMA-2–stained lesion area of IL-1Ra+/−/apoE−/− mice. In addition, α-actin staining in these lesions was significantly decreased (−15%) compared with those in IL-1Ra+/+/apoE−/− mice (P<0.05).

Conclusions— These results suggest an important role of IL-1Ra in the suppression of lesion development during early atherogenesis and furthermore indicate its role in the modulation of plaque composition.