RESEARCH ARTICLE
Comparison of bicistronic retroviral vectors
containing internal ribosome entry sites (IRES)
using expression of human interleukin-12 (IL-12)
as a readout
Mark Harries
1
Neil Phillipps
1
Robert Anderson
2
Grant Prentice
2
Mary Collins
1
*
1
Department of Immunology,
University College London,
The Windeyer Institute of Medical
Sciences, 46 Cleveland St,
London W1P 6DB, UK
2
Department of Haematology,
Royal Free Campus, Royal Free and
University College Medical School,
London, UK
*Correspondence to: M. Collins,
Department of Immunology,
University College London,
The Windyer Institute of Medical
Sciences, 46 Cleveland St,
London W1p 6DB, UK.
E-mail: mary.collins@ucl.ac.uk.
Received: 28 February 2000
Revised: 14 April 2000
Accepted: 20 April 2000
Published online: 27 April 2000
Abstract
Background Many gene therapy applications require the co-ordinated
delivery of more than one reading frame. We wished to systematically
compare IRES in the context of a retroviral vector to determine which was the
most effective for protein production and viral titre. To do this we monitored
expression of IL-12, as co-ordinated expression of both p35 and p40 subunits
is required for production of the active heterodimer.
Methods Retroviral vectors were constructed to express human IL-12 in
which an IRES initiates translation of the p40 subunit, with the IRES
optimally aligned to the initiation codon of p40. Vectors containing an IRES
from either polio virus (PV), encephalomyocarditis virus (EMCV), foot and
mouth disease virus (FMDV) or murine leukaemia virus (MLV) were
compared with a vector expressing IL-12 as a single protein (Flexi-12; in
which the two IL-12 subunits are linked by a peptide).
Results All vectors produced high titre virus and directed synthesis of IL-12
in target cells. The bicistronic vectors containing the IRES from EMCV and PV
were the most effective in infected 3T3 cells, producing up to 40 ng IL-12/10
6
cells/48 h, similar to the 50 ng IL-12/10
6
cells/48 h obtained with Flexi-12.
The IRES from PV was the most ef®cient in human melanoma cells.
Conclusions Bicistronic retroviral vectors have been constructed that
effectively transduce target cells and produce high levels of protein. Target
cell speci®city of IRES function was observed. The combination of Flexi-12
and the IRES from PV will be useful in the generation of vectors expressing IL-
12 with a second protein such as IL-2 for transduction of melanoma cells.
Copyright # 2000 John Wiley & Sons, Ltd.
Keywords IL-12; IRES; retroviral vector; gene therapy
Introduction
Effective gene therapy may often require delivery of more than one transgene.
This can be achieved by using combinations of vectors, each expressing a
single protein; an approach that has practical disadvantages and low
reproducibility between vector preparations. Retroviral vectors are the
most widely used gene delivery vehicles in current clinical gene therapy
protocols [1]. A number of strategies may be employed to deliver more than
one transgene per retroviral vector including the use of internal promoters,
splicing and fusion proteins (reviewed in Ref. [2]). More commonly, IRES
THE JOURNAL OF GENE MEDICINE
J Gene Med 2000; 2: 243±249.
Copyright # 2000 John Wiley & Sons, Ltd.