Chimerism of the Transplanted Heart

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Abstract
Quaini and colleagues (Jan. 3 issue)1 claim that stem cells of host origin are found in transplanted hearts. The authors identified host-derived cells within the transplanted organs on the basis of the detection of Y chromosomes in male recipients of female hearts. To establish the stem-cell nature of these cells, they used immunohistochemical studies to evaluate the expression of three molecules (c-kit, MDR1, and Sca-1) that have previously been used to identify hematopoietic stem cells in mice (Sca-1 and c-kit) and humans (MDR1 and c-kit).2-4 As the authors indicate, none of these three molecules are unique to hematopoietic stem cells; at present, only functional studies in combination with multidimensional flow cytometric analysis to evaluate the presence or absence of at least four antigens can suggest the identity of these cells.5 Furthermore, the use of the Sca-1 reagent in the current study is particularly troublesome, since the antibody used by the investigators is specific for mouse cells and is not cross-reactive with human bone marrow cells (unpublished data). Thus, the authors have inadvertently provided an example of rather high background levels of nonspecific antibody staining. This not only negates their conclusion that Sca-1 cells are present but also calls into question all their immunohistochemical findings. Quaini et al. have used inappropriate methods to reach an inappropriate conclusion and, in so doing, have added confusion to an emerging area of research already mired in controversy.