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Journal of Virology, February 2000, p. 1258-1266, Vol. 74, No. 3
Center for Engineering in Medicine and
Surgical Services, Massachusetts General Hospital, Harvard Medical
School, and Shriners Hospital for Children, Boston, Massachusetts 02114
Received 5 August 1999/Accepted 2 November 1999
In this paper, we present a mathematical model with experimental
support of how several key parameters govern the adsorption of active
retrovirus particles onto the surface of adherent cells. These
parameters, including time of adsorption, volume of virus, and the
number, size, and type of target cells, as well as the intrinsic
properties of the virus, diffusion coefficient, and half-life
(t1/2), have been incorporated into a
mathematical expression that describes the rate at which active virus
particles adsorb to the cell surface. From this expression, we have
obtained estimates of Cvo, the starting
concentration of active retrovirus particles. In contrast to titer,
Cvo is independent of the specific conditions of the assay. The relatively slow diffusion (D = 2 × 10
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Toward a More Accurate Quantitation of the Activity
of Recombinant Retroviruses: Alternatives to Titer and Multiplicity
of Infection

8 cm2/s) and rapid decay
(t1/2 = 6 to 7 h) of retrovirus
particles explain why Cvo values are
significantly higher than titer values. Values of
Cvo also indicate that the number of defective
particles in a retrovirus stock is much lower than previously thought,
which has implications especially for the use of retroviruses for in vivo gene therapy. With this expression, we have also computed AVC
(active viruses/cell), the number of active retrovirus particles that
would adsorb per cell during a given adsorption time. In contrast to
multiplicity of infection, which is based on titer and is subject to
the same inaccuracies, AVC is based on the physicochemical parameters
of the transduction assay and so is a more reliable alternative.
*
Corresponding author. Mailing address: Shriners
Hospital for Children, 51 Blossom St., Boston, MA 02114. Phone: (617)
371-4878. Fax: (617) 371-4950. E-mail: jmorgan{at}sbi.org.
Present address: Bioengineering Laboratory, Department of Chemical
Engineering, State University of New York at Buffalo, Amherst, NY 14260.
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