Interaction of cationic colloids at the surface of J774 cells: A kinetic analysis

We have characterized the binding of multilamellar colloids to J774 cells. Cationic colloids were shown to bind much more efficiently than neutral one s. Particle uptake by cells was followed by flow cytometry and fluorescence microscopy. Analysis of the kinetics of uptake of cationic particles indic ated that binding on the cell surface occurred with two characteristic time s. Analysis of the dissociation properties allowed discriminating between s everal alternative models for adsorption and led us to propose a mechanism that involved two independent classes of binding sites on the cell surface. One class of sites appeared to be governed by a classic mass action law de scribing a binding equilibrium. The other sites were populated irreversibly by particles made of 10% cationic lipids. This was observed in the absence of endocytosis, under conditions where both the equilibrium and the irreve rsible binding occurred at the cell surface. We determined the rate constan ts for the different steps. We found that the reversible association occurr ed with a characteristic time of the order of tens of seconds, whereas the irreversible binding took a hundred times longer. The presence of serum pro teins in the incubation medium did not drastically affect the final uptake of the particles. In contrast, the capture of the particles by cells signif icantly dropped when the fraction of positively charged lipids contained in the colloids was decreased from 10% to 5%. Finally, the results will be di scussed within a comprehensive model where cationic particles find labile b inding sites in the volume of the pericellular network (glycocalyx and extr acellular matrix) and less-accessible irreversible binding sites at the cel l membrane itself.