Aspects of this experiment of cardiac cell therapy include (1) a transmural (non-reperfused) infarct was used as a remodeling stimulus in a clinically relevant large animal model; (2) allogeneic cells were used, simulating an off-the-self, early after MI, preventative approach to cardiac cell therapy; (3) the effect of a wide range of cell dosages was assessed; (4) cells were delivered into the nonischemic borderzone myocardium rather than the infarct region, and (5) a novel new subset of bone marrow stromal stem cell was used.
A moderately sized transmural anteroapical MI was induced in the animals in this study. This is a strong stimulus for ventricular remodeling, which resulted in immediate expansion (stretching) of the infarct region, global LV dilatation, and impaired global LV function. This model represents a more profound remodeling stimulus than the reperfused infarct models that have been used to assess other bone marrow stromal stem cells [14
]. Although all animals in this study experienced LV remodeling, the MPC injection significantly attenuated global LV dilatation and EF reduction.
Interestingly, the therapeutic effect of MPCs was most pronounced at the lower cell doses of 25 and 75 million cells. This phenomenon was observed at most end points for which significant differences between therapy and control groups could be demonstrated. Most cardiac cellular treatment strategies have generally assumed that high doses of cells are required to maximize efficacy; a few small-animal studies have supported this hypothesis [16
], but large-animal and early clinical studies have not assessed wide ranges in cell dose [2
]. The widely held belief that efficacy is directly related to the number of cells delivered has been further reinforced by studies that have documented rapid cell loss in the hours and days immediately after their delivery into the myocardium [20
Our data strongly suggest that high cell dosages limit efficacy. This phenomenon has several possible explanations, including:
- At high cell doses, competition for limited nutrient resources in the highly stressed environments of the infarct and borderzone regions may become intense and limit cell survival or function, or both;
- Large numbers of injected cells may elicit a more profound inflammatory or immunologic response, or both, that accelerates cell clearance or deactivation.
Although further experiments will be required to more fully elucidate this seemingly paradoxic observation, the implications for future clinical studies of cardiac stem cell therapy are important. These data may also help to explain the variable results that have been reported in recent clinical trials of stem cell therapy in the post-MI period in which the effect of cell dosing was not studied [3
The method of delivery (ie, intracoronary, transendocardial, epicardial) and the timing of delivery relative to the infarction could influence the dosage required to achieve an efficacious result. Multiple small doses delivered over an extended time period (eg, days to weeks) may represent a technique for optimizing the efficacy of MPC therapy for acute MI.
The beneficial effects on LV remodeling demonstrated in this experiment could not be attributed to regeneration of functioning myocardium within the infarct region. A detailed microscopic examination of the infarct specimens demonstrated isolated nests of myofibrillarlytic myocytes [21
] within the infarct region of all animals; however, their distribution and concentration was highly variable and independent of treatment status ().
Fig 6 (A) Masson trichrome stain at original magnification ×10 and (B)×40 of sheep infarct tissue 8 weeks after infarction demonstrates an island of myofibrillarlytic myocytes within the scar. These isolated nests of myocytes are commonly seen (more ...)
The salutary effects of MPC therapy also could not be attributed to the prolonged engraftment of large numbers cells. Although the long-term survival or differentiation of a small number of cells could not be absolutely ruled out, the number of surviving cells was quite low at 8 weeks after injection. These results suggest that the presence of MPCs in the early post-MI period inherently changes the long-term LV response to infarction.
Our results do provide some important mechanistic insight into how MPC therapy affects the remodeling process. The improved LV remodeling associated with cell treatment observed in this experiment was at least partly due to a change in infarct material properties that acted to reduce infarct expansion (stretching). We and others have identified infarct expansion as an important initiating and sustaining impetus for adverse LV remodeling [22
]. We have also demonstrated that mechanical infarct restraint can greatly limit adverse remodeling [23
]. The data presented here would suggest that MPC therapy, at optimized doses, is an effective biologic means for limiting infarct expansion and ameliorating the remodeling process.
The reason for the reduced infarct expansion in the therapy animals could not be definitively established, but our results support the hypothesis that MPC therapy improves blood supply within the borderzone regions that may alter infarct healing and tissue remodeling dynamics. Martens and colleagues [8
] and Kocher and colleagues [24
] have demonstrated a similar effect using bone marrow-derived stem cells in small-animal infarct models.
Interest in regenerative cell therapy to treat cardiac disease continues to grow. Although numerous cell types have been tested in preclinical and clinical studies, allogeneic STRO-3 positive MPCs possess properties that may make them uniquely suited for the early treatment of acute MI. The ease of isolation, ex vivo expansion potential, and allogeneic use of these cells combined with their biologic plasticity and tolerance for cryopreservation would allow for a readily available off-the-shelf therapy that would not be possible with other autologous cell strategies.
Most importantly, the results of this study would seem to mandate that future clinical trials of cardiac stem cell therapy include multiple dosage arms to establish efficacy and fully define potential risks.