Ligation of CD40 onvascular smooth muscle cells mediates loss of interstitial collagen via matrix metalloproteinase activity.

Interstitial collagen constitutes the predominant structural component of the fibrous cap of atherosclerotic plaque. The balance between synthesis and degradation of this extracellular matrix protein probably determines plaque stability and hence the tendency for plaque rupture. The CD40/CD40L signaling dyad has been implicated as an important regulatory pathway of collagen-degrading activity in atherosclerosis via the induction of matrix metalloproteinases (MMPs). However, the role of CD40 signaling in the synthesis of interstitial collagen and thus in the overall rate of collagen turnover has remained unknown. We demonstrate here that CD40 ligation on cultured human vascular smooth muscle cells (SMCs) diminishes the detectable content of de novo synthesized interstitial procollagens. Notably, the loss of collagen is not accompanied by a reduction in collagen transcript expression but can be prevented by MMP inhibition. These data demonstrate that CD40 signaling in human vascular SMC shifts interstitial collagen turnover towards the loss of this extracellular matrix protein by accelerating its degradation without concomitantly diminishing its synthesis. Thus, CD40/CD40L interactions might play a key role in rendering atheromatous lesions prone to rupture.