PDE4A and systemic lupus erythematosus: PDE4 activity levels in the kidneys of 18 week-old MRL/lpr lupus-prone mice are increased by 27% (P<0.01) and 30% (P<0.01) compared with the 18 week-old CBA/J control and with the 8 week-old MRL/lpr lupus-prone mice, respectively (Figure 2D).