Toward a more accurate quantitation of the activity of recombinant retroviruses: Alternatives to titer and multiplicity of infection

In this paper, we present a mathematical model with experimental support of how several key parameters govern the adsorption of active retro-virus par ticles onto the surface of adherent cells, These parameters, including time of adsorption, volume of virus, and the number, size, and type of target c ells, as well as the intrinsic properties of the virus, diffusion coefficie nt, and half-life (t(1/2)), have been incorporated into a mathematical expr ession that describes the rate at which active virus particles adsorb to th e cell sm face. From this expression, we have obtained estimates of C-vo. t he starting concentration of active retrovirus particles. In contrast to ti ter, C-vo is independent of the specific conditions of the assay. The relat ively slow diffusion (D = 2 x 10(-8) cm(2)/s) and rapid decay (t(1/2) = 6 t o 7 h) of retrovirus particles explain why C-vo values are significantly hi gher than titer values. Values of C-vo also indicate that the number of def ective particles in a retrovirus stock is much lower than previously though t, which has implications especially for the use of retroviruses for in viv o gene therapy. With this expression, we have also computed AVC (active vir uses/cell), the number of active retrovirus particles that would adsorb per cell during a gives adsorption time. In contrast to multiplicity of infect ion, which is based an titer and is subject to the same inaccuracies, AVC i s based on the physicochemical parameters of the transduction assay and so is a more reliable alternative.