Host-vector system for expression of a transgene in a target cell or human/animal tissue comprises a eukaryotic cell line transfected with:(a) a recombinant psuedo-retroviral sequence in which all or part of the env gene has been deleted, either totally or partially, and replaced by the transgene as shown in figure 1a, and(b) a nucleic acid sequence which includes a sequence which codes for an envelope protein, is under the control of a promoter, is opt. associated with the transgene and is flanked on its 3' end by a polyadenylation site as shown in figure 1b; provided that(c) both (a) and (b) are capable of trans-complementation to permit the host cell to produce infectious env-deficient virus particles. Also claimed are:(1) a process for the expression of a transgene for gene therapy which comprises:(i) constructing a host-vector system as above;(ii) contacting the system with the cells in which the transgene is to be expressed, and(iii) opt. retransferring the env-contg. sequence into the target cells;(2) a process for the expression of a transgene for gene therapy which comprises the double transfection of target cells with:(i) env-deficient virus particles obtd. using the above host-vector system, and(ii) a nucleic acid sequence contg. a retroviral env gene; where(iii) trans co-expression of the 2 sequences in vivo results in infectious env-deficient recombinant virus particles;(3) a recombinant viral vector contg. a transgene useful in gene therapy, where:(i)a gene essential to the constitution of infectious virus particles is replaced by the transgene;(ii)the essential gene is present on the same or a different vector but is not under the control of viral promoters, and(iii) the prod. of the gene is trans-acting and permits reconstitution of defective virus particles;(4) a medication for use in gene therapy, contg. "vehicles" contg.:(i) a recombinant retroviral sequence in which the env gene is replaced by a trans gene, and(ii) a sequence contg. an env gene under the control of a promoter, where the env-contg. sequence is outside the zone of regulation of the expression of the retroviral sequences, and(5) a medication for use in gene therapy, contg. eukaryotic cells which have been subjected to double transfection and are "represented" by the above host-vector system.
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