Near-infrared autofluorescence (NIRAF) in unstable atherosclerotic plaque has been suggested as a novel imaging technology for high-risk atherosclerosis. Intraplaque hemorrhage (IPH) and bilirubin, derived from the subsequent degradation of heme, have been proposed as the source of NIRAF, although their roles and the underlying mechanism responsible for NIRAF remain unclear. To test the proposed role of bilirubin as the source of NIRAF in high-risk atherosclerosis, Biliverdin reductase a gene and apolipoprotein E gene double-knockout (Bvra-/-Apoe-/-) mice were subjected to the Western diet and tandem stenosis (TS) surgery, as a model of both bilirubin deficiency and plaque instability. Human coronary arteries containing atherosclerotic plaques were obtained from heart transplant recipients. The NIRAF was determined by in vivo fluorescence emission computed tomography, and ex vivo infrared imaging. The cholesterol content was quantified by HPLC with UV detection. In Bvra+/+Apoe-/- TS mice, the NIRAF intensity was significantly higher in unstable plaque than in stable plaque, yet the NIRAF in unstable plaque was undistinguishable in Bvra+/+Apoe-/- and littermate Bvra-/-Apoe-/- TS mice. Moreover, the unstable plaque in TS mice exhibited a lower NIRAF compared with highly cellular plaque that lacked most of the features of unstable plaque. In human coronary arteries, the NIRAF associated with cholesterol-rich, calcified lesions, rather than just cholesterol-rich lesions. The NIRAF in atherosclerotic plaque can be dissociated from IPH and bilirubin, such that the compositional meaning of an elevated NIRAF remains obscure.
Keywords: atherosclerosis; bilirubin; intraplaque hemorrhage (IPH); near-infrared autofluorescence (NIRAF).