Life is plastic

To the editor:

Paolo Bianco's comment, “Life in plastic is fantastic,” criticizes our paper “Multipotent cells can be generated in vitro from several adult human organs (heart, liver, and bone marrow)” from many standpoints. Our article¹  describes a method to generate in vitro, from many human tissues, a population of cells characterized by: extensive proliferative capacity, high telomerase activity, and expression of both pluripotent state-specific transcription factors and pathways crucial for self-renewal. Furthermore, these cells, named human Multipotent Adult Stem Cells (hMASCs), displayed clonogenicity and multipotency.

In Bianco's opinion there are 4 major criticisms: (1) Cells are improperly defined as stem cells because self-renewal was not shown in vivo. (2) Clonogenicity and multipotency can be simply contingent on in vitro growth. (3) Only cells that acquire a differentiation phenotype in vivo are important, while those differentiated ex vivo and implanted are not of clinical value. (4) Considering these cells for clinical application “may be less than prudent.”

(1) Regarding in vivo self-renewal, if this assay is mandatory to define a cell as a stem cell, the only entities that would satisfy this criterion, apart from embryonal carcinoma cells, embryonic stem cells (ES), and embryonic germ cells, are hematopoietic stem cells, spermatogonial stem cells,²  and tumor initiating cells.³  This raises the semantic question on how to define cells showing either the in vitro properties of hMASCs or cells for which, in addition, it has been shown a robust in vivo engraftment (eg, mesenchymal stem cells [MSCs],⁴  cardiac stem cells,⁵  etc).

(2) Considering multipotency, first it is imperative to specify that we demonstrated it at both polyclonal and clonal levels, excluding its contingency on clonal growth. Moreover, differentiation was not only demonstrated by “the expression of divergent differentiation markers,” but documenting the acquisition of specific functional activities (ie, among the others, spontaneous contractile activity for myocytes, voltage dependent ionic currents for neural cells, albumin release for hepatic cells). Bearing in mind this elucidation, we would like to underline that our paper does not deal with the existence, in postnatal tissues, of cells with transgermal potential, as stated by Dr Bianco, but with the possibility to generate, in culture, from human adult tissues, cells characterized by a robustly proven ability to differentiate into multiple functionally competent cell types.

(3) To dismiss the importance of generating cells predifferentiated in vitro for regenerative purposes would be in contrast with many works that use ES-derived mature cells to treat diseases ranging from Parkinson to myocardial infarction or that use cells in engineered tissues.⁶  Moreover, we agree that, from a theoretical point of view, “a cell pushed to express hepatocyte features might not survive if transplanted in vivo,” but recent data on MSCs used for liver regeneration point in the opposite direction.⁷ 

(4) Finally, we agree with the statement that the immediate translation of hMASC use from the in vitro assays to clinical settings is, at present, less than prudent. Confidence on their safety and usefulness will require extensive in vivo animal studies, such as those that are ongoing in our laboratory. Therefore, before expressing a priori preconceptions, it will be better to wait for in vivo data.