Severe arterial injury heals with a complex clonal structure involving a large fraction of surviving smooth muscle cells.

BACKGROUND AND AIMS: Smooth muscle cell (SMC) lineage cells in atherosclerosis and flow cessation-induced neointima are oligoclonal, being recruited from a tiny fraction of medial SMCs that modulate and proliferate. The present study aimed to investigate the clonal structure of SMC lineage cells healing more severe arterial injury.

METHODS: Arterial injury (wire, stretch, and partial ligation) was inflicted on the right carotid artery in mice with homozygous, SMC-restricted, stochastically recombining reporter transgenes that produced mosaic expression of 10 distinguishable fluorescent phenotypes for clonal tracking. Healed arteries and contra-lateral controls were analyzed after 3 weeks. Additional analysis of cell death and proliferation after injury was performed in wildtype mice.

RESULTS: The total number of SMC lineage cells in healed arteries was comparable to normal arteries but comprised significantly fewer fluorescent phenotypes. The population had a complex, intermixed, clonal structure. By statistical analysis of expected versus observed fractions of fluorescent phenotypes and visual inspection of coherent groups of same-colored cells, we concluded that >98% of SMC lineage cells in healed arteries belonged to a detectable clone, indicating that nearly all surviving SMCs after severe injury at some point undergo proliferation. This was consistent with serial observations in the first week after injury, which showed severe loss of medial cells followed by widespread proliferation.

CONCLUSIONS: After severe arterial injury, many surviving SMCs proliferate to repair the media and form a neointima. This indicates that the fraction of medial SMCs that are mobilized to repair arteries increases with the level of injury.