Effect of concanavalin A on lymph node macrophages: stimulation of endocytic cisternae

Incubation of isolated lymph node macrophages with concanavalin A (Con A) resulted in a dense and continuous labeling of the plasmalemma and filopodia which were closely adherent to each other and the cell surface. Within a short time period (3-5 min), membranes of the closely apposed filopodia became invaginated into the cytoplasm to form numerous interconnecting cisternae. After 10 min the system of internalized membranes had migrated into the deeper cytoplasm and was closely associated with numerous actin filaments and other components of the cytoskeleton. The internalized plasmalemma remained in the cytoplasm up to 24 hr without fusing with lysosomes. Concomitant with plasmalemmal invagination and the formation of cisternae there were also changes in the Golgi apparatus. These appeared in the form of hypertrophied Golgi saccules and the accumulation of numerous vesicles around the Golgi. Treatment of isolated lymph node macrophages with either succinylated Con A, alpha-methyl-D-mannoside, or ferritin particles alone failed to produce the cisternal structures. The results suggested that the tetravalency of Con A may be responsible for the binding of adjacent Con A-labeled membranes to each other and for maintaining a crosslinking of membranes during invagination and internalization. It is suggested that this process of extensive membrane internalization represents a specialized form of endocytosis. At 24 hr after incubation with Con A, cisternal structures in close proximity to the Golgi vesicles showed signs of degradation. By 48 hr there was a breakdown of cisternal membranes with a release of Con A marker particles into large phagocytic vesicles, which also showed reaction product for acid phosphatase, suggesting a fusion with lysosomes.