Systemic Delivery of Secreted Protein by Grafts of Epidermal Keratinocytes: Prospects for Keratinocyte Gene Therapy

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Abstract
Grafts of autologous keratinocytes genetically altered to secrete a new gene product are a potential vehicle for gene therapy. To consider the feasibility of such an approach, we have examined the ability of keratinocytes to secrete and deliver apolipoprotein E (apoE) to the circulation of mice bearing grafts of human keratinocytes. The grafted keratinocytes secreted two forms of apoE, an endogenous apoE encoded in the genome and a recombinant apoE encoded in a transfected gene construct. In vitro studies showed that endogenous apoE was secreted from basal keratinocytes whereas recombinant apoE was secreted from basal as well as suprabasal cells. On the basis of amounts of recombinant apoE present in the serum of grafted mice, we estimate that a graft occupying 2% of the surface area of an adult human would deliver 6.5–8.3 mg of recombinant apoE protein per day. The ability of grafts of epidermal keratinocytes to secrete and systemically deliver a protein is a prerequisite for epidermal gene therapy. The authors use apolipoprotein E (apoE) secreted from human keratinocytes grafted onto athymic mice to examine this capacity. The cells secrete two forms of apoE, an endogenous apoE and a distinguishable recombinant apoE. Both are recovered in the circulation of the mice. The authors analyze the efficiency of uptake from the epidermis, the source of secreted protein within the epidermis, and the possible applications of epidermal keratinocytes for gene therapy.