Response to Letter Regarding Article “Extracardiac Progenitor Cells Repopulate Most Major Cell Types in the Transplanted Human Heart”
We thank Drs Kaye and Esler for their comments on our recent article1 exploring recipient-derived cardiomyocytes, endothelial cells, smooth muscle cells, and Schwann cells in sex-mismatched heart transplant patients. Using the Y chromosome to track male cells in the female heart, we concluded that 11.2% of Schwann cells were derived from extracardiac sources, suggesting that recipient-derived circulating progenitor cells play a role in the reparative process of peripheral nerves in the transplanted heart.
The Schwann cell data came from autopsy specimens taken 9 to 50 months after transplantation (Table 1 of our original article). Schwann cells were identified by using an antibody against S100 calcium-binding protein, widely used in clinical pathology practice.2,3 Schwann cells were only found in large nerve fibers near the epicardium, not in the endocardial regions where biopsies are obtained. We are aware that S100 has various subsets and is present in other cell types, including skeletal and heart muscle (subsets S-100A1 and S-100A6) and dendritic cells.4 To obtain the most specific expression for Schwann cells, we used an antibody (Dako, Z0311) that has the highest affinity for the S-100B subset (found predominantly in Schwann cells, melanocytes, and adipocytes) and weakest affinity for the S-100A1 and S-100A6 subsets. Cells were only counted as positive if they were spindle-shaped and located in an obvious nerve bundle. Comparable frequencies were obtained by brightfield and confocal fluorescent microscopy (minimizing the risk of mistaking leukocytes for Schwann cells). For these reasons, we believe that our study1 truly identified Schwann cells derived from extracardiac sources.
The comments by Kaye and Esler raise the question of what structural criteria one would need to definitively identify reinnervation in a transplanted heart. When a peripheral nerve is transected, the distal axon undergoes Wallerian degeneration, whereas the proximal stump typically forms a growth cone and invades the old axonal tract, guided by the surviving Schwann cells.5 Our study did not address the source of these nerve fibers because of the difficulty of studying their cell bodies, which are located some distance away (in the paravertebral ganglia for sympathetic nerves, in the epicardial ganglia for parasympathetic nerves). Hence, we do not know whether these were regenerated distal axons (presumably host-derived) or residual postsynaptic parasympathetic axons (graft-derived). Our identification of Schwann cells derived from extracardiac sources might therefore be better classified as a novel component of peripheral nerve repair, rather than as true reinnervation.
We appreciate the opportunity to expand on and clarify the significance of our findings.
Drs Laflamme and Murry have received research grant support from Geron Corp. Dr Murry has served as a consultant to and/or on an advisory board for Guidant. The other authors report no conflicts.
Minami E, Laflamme MA, Saffitz JE, Murry CE. Extracardiac progenitor cells repopulate most major cell types in the transplanted human heart. Circulation. 2005; 112: 2951–2958.
Anthony DC, Frosch MP, DeGirolami U. Peripheral nerve and skeletal muscle. In: Kumar V, Abbas A, Fausto N, eds. Robbins and Coltran’s Pathologic Basis of Disease. Philadelphia, Pa: Elsevier/Saunders, 2005: 1325–1346.