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536 Chapter 10: Graft Manipulation<br />

positive/negative purging leads to a considerably improved purging efficiency<br />

compared with positive selection alone. Nevertheless, tumor cells might evade even<br />

this purging approach. Hence, it is desirable to design purging strategies that do not<br />

rely solely on the physical elimination of tumor cells, but aim at the additional<br />

functional impairment of tumor cells, thus preventing their tumorigenicity in vivo.<br />

Therefore, we have evaluated different strategies with this aim by using antibodymediated<br />

apoptosis via the CD20 or CD95 antigen in lymphoma cells or using<br />

adenovirus-mediated purging (Mapara et al., manuscript in preparation). 24<br />

Alternatively, residual tumor cells could be targeted by an immunotoxin approach as<br />

demonstrated by Spyridonidis et al. 28<br />

<strong>and</strong> Myklebust et al. 15<br />

The introduction of a suicide gene by means of gene delivery vectors offers<br />

highly interesting new options in graft manipulation. As described above, this<br />

technique is highly effective, provided a high gene transfer efficiency can be<br />

obtained by the vector. In the case of tumor cell purging, adenoviruses are excellent<br />

vectors for transducing breast cancer cells (as reported above) or myeloma cells. 29<br />

Provided a vector is chosen that leads to stable integration of vector DNA into the<br />

transduced cells, such an approach would offer the opportunity to treat the<br />

recurrent malignant cells carrying the suicide gene in vivo. Furthermore, such an<br />

approach could also be applied for the elimination of T cells from allografts in vitro<br />

for the prevention of GVHD. A similar approach using herpes simplex thymidine<br />

kinase (Hs-Tk) has been successfully applied for the treatment of GVHD in vivo. 30<br />

In conclusion, combined positive/negative purging leads to a considerably

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