Xenografted adult human mesenchymal stem cells provide a platform for sustained biological pacemaker function in canine heart

Background - Biological pacemaking has been performed with viral vectors, human embryonic stem cells, and adult human mesenchymal stem cells ( hMSCs) as delivery systems. Only with human embryonic stem cells are data available regarding stability for > 2 to 3 weeks, and here, immunosuppression has been used to facilitate survival of xenografts. The purpose of the present study was to determine whether hMSCs provide stable impulse initiation over 6 weeks without the use of immunosuppression, the dose of hMSCs that ensures function over this period, and the catecholamine responsiveness of hMSC- packaged pacemakers. Methods and Results - A full- length mHCN2 cDNA subcloned in a pIRES2- EGFP vector was electroporated into hMSCs. Transfection efficiency was estimated by GFP expression. I-HCN2 was measured with patch clamp, and cells were administered into the left ventricular anterior wall of adult dogs in complete heart block and with backup electronic pacemakers. Studies encompassed 6 weeks. IHCN2 for all cells was 32.1 +/- 1.3 pA/ pF ( mean +/- SE) at - 150 mV. Pacemaker function in intact dogs required 10 to 12 days to fully stabilize and persisted consistently through day 42 in dogs receiving >= 700 000 hMSCs ( approximate to 40% of which carried current). Rhythms were catecholamine responsive. Tissues from animals killed at 42 days manifested neither apoptosis nor humoral or cellular rejection. Conclusions - hMSCs provide a means for administering catecholamine- responsive biological pacemakers that function stably for 6 weeks and manifest no cellular or humoral rejection at that time. Cell doses >= 700 000 are sufficient for pacemaking when administered to left ventricular myocardium.