Survey of radioactive agents for in vitro labeling of phagocytic leukocytes. II. Particles.

When various radioactive particles are incubated and tumbled in concentrated suspensions of blood phagocytes at body temperature for 1 hr, they bind to the phagocytic cells with a labeling yield of 30-40%. In vitro experiments show that, for some radioactive colloids, a sizeable fraction of the total cellular binding results from nonspecific surface adsorption to other cells and from reversible surface adsorption to phagocytes without engulfment. No completely satisfactory in vitro methods have been found for separating leukocytes with completely engulfed particles from those with surface-adherent particles; nonetheless, surface adherence can be partially reversed by 20% acid citrate dextrose (ACD) solution or by an excess of nonradioactive colloid. Gelatinization of colloidal particles tends to increase their binding to phagocytic cells but also increases the degree of nonspecific adherence to other cells. Technetium-99m-millimicrospheres, 0.5-2 mum in diameter, are optimal in size for phagocytosis by neutrophils, and their non-specific adherence to other cells is minimal. Because of the microspheres' poor stability in aqueous suspension, however, it is technically difficult to separate free from phagocytosed radioactivity after cell incubation. The highly stable small-particle colloids (less than 0.1 mum), such as 198Au-colloid or 111In-colloid without iron carrier, are phagocytosed poorly or not at all by neutrophils, although they are engulfed by mononuclear cells.