Resetting the problem of cell death following muscle-derived cell transplantation: detection, dynamics and mechanisms.

We conducted a study in mice to reevaluate and clarify many aspects of the early survival of muscle cells following transplantation. Male mouse muscle cells (primary-cultures and T-antigen-immortalized clones) labeled with [14C]thymidine and beta-galactosidase were injected into female muscles. Each label was detected in the muscles after different time periods. TUNEL, alizarin red, and immunodetection of active caspase-3 were done in muscle sections. The donor cell labels disappeared from the muscles following donor cell death, but this was not instantaneous and even if the donor cells were killed before transplantation, the first 6 hours were not enough to clear [14C]thymidine and Y chromosome. Using the cell pellet before injection as the 100% baseline for cells injected to evaluate cell death can lead to misinterpretations: the Y-chromosome band was 5-fold stronger than that of a muscle injected with cells, irrespective of whether the cells were previously killed or not. There was no evidence of an immediate massive donor cell death. Necrosis (detected by alizarin red) and apoptosis (detected by active caspase-3) were present among the donor myoblasts following transplantation. Necrosis seemed to be the most important mechanism during the first hours. T-antigen immortalized cells died earlier and more massively than primary-cultured cells, but the surviving cells proliferated more. Indeed, they seemed to exhibit more apoptosis and they triggered a more rapid CD8+ cell infiltration. As a result of our findings, many concepts concerning the early donor cell death following myoblast transplantation must be reconsidered.