Low level laser arrests abdominal aortic aneurysm by collagen matrix reinforcement in apolipoprotein E-deficient mice

Recent in vitro studies by our group indicated that low level laser irradiation (LLLI) modifies cellular processes essential to the progression of abdominal aortic aneurysm (AAA). Using high-frequency ultrasonography (HF-u/s) in the angiotensin-II (Ang-II)-infused, apolipoprotein-E-deficient (Apo-E(-/-) ) mouse model of AAA, we found that LLLI markedly inhibited aneurysm formation and preserved arterial wall elasticity. We now report, using quantitative histopathology, the likely mechanism underlying the preventative effect of LLLI on aneurysm formation in this model.

This study was performed on 32 Apo-E(-/-) mice of which 10 were Ang-II-infused and LLL-irradiated (780 nm, 2 J/cm(2) , 9-minutes), 12 were Ang-II-infused but not irradiated, and 10 were saline infused. The aortas were excised at 28d, sectioned at 250 µm intervals, and stained with H + E, Movat-pentachrome and picrosirius-red for histomorphometry, and immunostained with Mac-2 and α-actin for detection of macrophages and SMCs, respectively.

Transmural disruptions of the aorta occurred with distinct predilection for branch orifices. In the LLLI-treated animals, the frequency of these disruptions was lower (#branches with break points: 17 of 40 vs. 32 of 48, P = 0.023 by Chi-squared), their size smaller (length [mm]: 0.48 ± 0.26 vs. 0.98 ± 1.42, P = 0.044 by ANOVA with FPLSD), and the number of Mac-2-positive macrophages in the intramural areas of these disruptions lower than in the non-treated control (#Macrophages/0.01 mm(2) at break points: 11.6 ± 7.2 vs. 26.0 ± 15.7, P = 0.016 by Kruskal-Wallis). The average size of the medial SMCs was larger reflecting a heightened synthetic state (SMC size [µm(2) ]: 463.9 ± 61.4 vs. 354.9 ± 71.7, P = 0.001 by ANOVA with FPLSD). Furthermore, at sites of transmural disruption, the %area occupied by collagen of the overall area of attempted repair (%Col/WO) was significantly greater in the LLLI-treated animals versus control (%Col/WO: 41 ± 13 vs. 32 ± 16, P = 0.009 by ANOVA with FPLSD).

Enhanced matrix reinforcement and modification of the inflammatory response at sites of transmural injury are prominent mechanisms by which LLLI reduces AAA progression in this model.