Abstract 9029: Fate Mapping of Monocytes Shows Rapid Turnover Kinetics in Acute Myocardial Infarction

Abstract

While previous studies have addressed monocyte behavior in circulation, less is known about the kinetics of these cells once they enter inflamed tissue. It is commonly believed that monocytes are recruited shortly after ischemic injury, and remain in the infarct until inflammation resolves, although this assumption has not been tested experimentally. We therefore studied the turnover of monocytes in infarcted myocardium of mice after coronary ligation (MI). Methods and Results: Bromodeoxyuridine (BrdU) was injected 24 and 12 hrs prior to analysis into mice with MI for pulse-chase experiments. Cell cycle analysis showed no proliferation of monocytes in the infarct tissue. Flow cytometry of cells retrieved 3 days after MI revealed that 43 ± 4 % of monocytes in the infarct had incorprated BrdU into their DNA, and hence were produced and recruited after the first BrdU pulse (i.e. within the last 24 hrs). Monocyte subset analysis revealed faster kinetics for inflammatory Ly-6C-high monocytes when compared to the non-inflammatory Ly-6C-low monocyte subset. To evaluate these rapid cell fluxes with a second, independent method, we transplanted infarcted hearts between isogenic mice with diverging CD45 leukocyte surface markers on day 3 after MI. Transplantations of non-infarcted hearts served as controls. Flow cytometric analysis showed that 6 hrs after transplantation 20 ± 1 % of the monocytes in the infarct were newly recruited from the transplant recipient mouse. This number increased to 40 ± 3 % at 12 hrs, and after 24 hrs the majority (60 ± 0.5%) of monocytes in the infarct derived from the recipient. These data were fitted using a mathematical model of monocyte dynamics (least-squares fit R2 > 99%; N (t) = e-(t/20) ), which reported an average monocyte residence time of 20 hours. Conclusion: We observed surprisingly fast monocyte turn-over in acute infarcts with an average tissue residence time of 20 hours.