In vivo synthesis and secretion of erythropoietin by genetically modified primary human keratinocytes grafted onto immunocompromised mice

P>Background: The skin is an easily accessible tissue with a high blood flow facilitating the distribution of secreted peptides. These features make it a very intriguing target to serve as a biofactory releasing a systemically needed factor, such as erythropoietin (EPO). Methods: To evaluate the potential of human keratinocytes (KC) to systemically synthesize EPO, EPO-transduced KC were grafted onto immunocompromised mice and EPO secretion was followed by serum ELISA. Furthermore, we assessed if topical colchicine application would select for enriched percentages of KC expressing the multi-drug resistance (MDR) gene as a selectable gene connected to the EPO gene (measured by fluorescence-activated cell sorting (FACS)-analysis) and result in enhanced EPO production (determined by ELISA). Results: Transduced KC showed stable EPO production in vivo during a 6-month observation period, pointing to engraftment of EPO-secreting KC progenitor cells. When adding colchicines the number of EPO/MDR+ KC were significantly enriched, both in skin grafts (in vivo) and in skin equivalents (in vitro). Of note, this did not result in enhanced EPO production. Rather, while EPO secretion was substantially increased in transduced KC grown as monolayers and selected with colchicine, it was reduced by more than 50% in both colchicine-treated skin grafts and skin equivalents. Conclusion: Keratinocytes carry the potential to serve as a genetically modified biofactory synthesizing human EPO. In vivo gene selection does not allow to select for increased EPO secretion, most likely because of altered secretory activity of transduced KC in the stratified, differentiated epidermis. Thus, further studies are necessary to optimize the release of EPO by genetically modified KC.