The role of the pro-inflammatory cytokine Interleukin-18, its processing enzyme Caspase-1, and potential alternative IL-18-activating pathways in atherosclerosis
Atherosclerosis is the predominant underlying pathology of cardiovascular disease, the most common cause of premature death in the industrialized world. Recent research attributed inflammation a crucial role in atherosclerosis. Indeed, atherosclerotic lesions are characterized by abundance of immune cells and their effector molecules, accelerating atherogenesis and eventually leading to clinical symptoms such as unstable angina, myocardial infarction, or stroke. We have recently implicated the pro-inflammatory cytokine interleukin (IL)-18 as a novel mediator in this disease. The present work aims to evaluate the in vivo role of this cytokine and its activating protease Caspase-1 in atherosclerosis. Interestingly, IL-18-deficiency limited early lesion development in hyperlipidemic mice but did not affect atherogenesis after prolonged hyperlipidemia. These studies suggest a direct role for IL-18 in disease progression extending beyond the classical function of this cytokine, the induction of interferon gamma. Additional experiments employing chimeric mice, that lacked the IL-18R alpha on either the hematopoietic or the vascular cells, generated by bone-marrow transplantation, revealed that IL-18R alpha does not participate in the pro-atherogenic effects of IL-18 Surprisingly, deficiency of Caspase-1 did not diminish atherogenesis, thus suggesting alternative mechanisms of IL-18 activation during atherosclerosis. Subsequent experiments tested whether matrix metalloproteinases (MMPs), enzymes prominently expressed in atherosclerotic lesions mediate the maturation of the IL-18 precursor (proIL-18). Indeed, several recombinant MMPs proteolytically cleaved the precursor in vitro, and MMP-2- and MMP-8 processed proIL-18 exhibited IL-18 activity. Sequence analysis of processed proIL-18 demonstrated that cleavage sites of MMP-2 and MMP-8 differ from that of Caspase-1. Finally, the presence of mature/processed IL-18 in atherosclerotic tissue of Caspase-1-deficient mice highlighted the potential in vivo relevance of such an alternative, MMP-mediated IL-18 activation mechanism for this pro-inflammatory disease. In sum, this work directly demonstrates the pro-atherogenic role of IL-18 independent of IL-18R alpha or Caspase-1. These surprising observations provide a novel understanding of IL-18 biology and may foster re-thinking of current approaches for the therapeutic intervention of this pathway in prevalent inflammatory diseases
