FIG. 5.
Synergistic inhibition of cell-cell fusion by PA12 and 2D7. Dose-response curves were obtained for the MAbs used individually and in combination. A total of 0 to 50 μg of PA12 per ml, 0 to 25 μg of 2D7 per ml, or a combination of the two in a 2:1 ratio was added to a mix of HeLa-EnvJR-FL+ and PM1 cells, labeled with R18 and F18, respectively. Fluorescence RET was measured after 4 h of incubation. Results are expressed as percent inhibition of fusion and are the means of values from three independent experiments. Data were analyzed with the median effect principle, which can be written
where f is the fraction affected-inhibited, c is concentration, K is the concentration of agent required to produce the median effect, and m is an empirical coefficient describing the shape of the dose-response curve. Equation 1 is a generalized form of the equations describing Michaelis-Menten enzyme kinetics, Langmuir adsorption isotherms, and Henderson-Hasselbalch ionization equilibria, for which m = 1. In the present case, K is equal to the IC50. K and m were determined by curve-fitting the dose-response curves, and equation 1 was rearranged to allow calculation of c for a given f. The best-fit parameters for K and c are 8.8 μg/ml and 0.54, respectively, for PA12, 0.36 μg/ml and 0.68, respectively, for 2D7, and 0.11 μg/ml and 1.1, respectively, for their combination. These curves are plotted and indicate a reasonable goodness-of-fit between experiment and theory.
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