LABELING OF LEUKOCYTES WITH COLLOIDAL TECHNETIUM-99M-SNF2 - AN INVESTIGATION OF THE LABELING PROCESS BY AUTORADIOGRAPHY

Autoradiography of smears and frozen sections of labelled cell suspens ions was used to study the distribution of radioactivity in and among blood cells labelled in either whole blood or leucocyte-rich plasma (L RP) with technetium-99m-SnF2 colloid. The tracer proved selective for neutrophils: the labelling probability (relative to that for erythrocy tes) for each cell type in LRP (mean of five samples) was: neutrophils , 9.4; lymphocytes, 3.7; monocytes, 3.0; eosinophils 1.4 erythrocytes, 1.0. When labelling was carried out rn whole blood (five samples), 74 .5%+/-8.3% of the cell-bound radioactivity was bound to erythrocytes, 13.6%+/-6.5% to neutrophils, and 11.9%+/-2.1% to lymphocytes, whereas in LRP (in which the leucocytes were only slightly outnumbered by eryt hrocytes), 76.5%+/-14.9% of radioactivity was neutrophil bound. Labell ed cells in smear autoradiographs exhibited two distinct silver grain pat terns, ''diffuse'', consistent with an intracellular radioactive p article (in neutrophils), and ''focal'', consistent with a cell surfac e-adhering particle in direct contact with the emulsion (in other leuc ocyte types and erythrocytes). The phagocytic inhibitor cytochalasin B neither reduced the proportion of labelled neutrophils nor altered th e labelling pattern. Neutrophils were able to scavenge radioactivity f rom the surface of erythrocytes. It is concluded that neutrophils bind Tc-99m-SnF2 intracellularly by phagocytosis, with high affinity; othe r cells become labelled at the cell surface reversibly and with lower affinity. This selectivity is high enough to permit predominantly leuc ocyte labelling in LRP but not in whole blood.