Interactions between fresh human blood monocytes and tumor targets at the single cell level: binding and morphological characteristics.

Highly purified (97-99%), human peripheral blood monocytes, isolated by an EDTA-reversible adherence procedure, exhibit spontaneous killing in a rapid, 51Cr-release assay. Using the single cell conjugate/dextran assay, we have quantitatively determined select requirements for the adhesion of monocytes to neoplastic target cells. Monocytes expressed a broad binding capacity to a wide spectrum of nonadherent tumor cell lines, while binding at minimal levels to nontumor target cells. Binding of monocytes to K562 tumor cells is an extremely rapid event, reaching saturation levels within 5 min after cosedimentation. The binding of monocytes to K562 targets was reduced by low temperatures and pretreatment with metabolic inhibitors. Pretreatment of monocytes with cytoskeletal-disruptive drugs significantly reduced binding. Monocytes were shown to exhibit a specific, Mg2+-dependent, Ca2+-independent mechanism in their binding to K562 cells. Subsequent to binding, Ca2+ was required for monocyte-mediated target cell lysis to proceed. The binding of tumor cells by monocytes was studied morphologically using transmission (TEM) and scanning electron microscopic (SEM) techniques. It was observed that binding was achieved initially through complex, surface membrane interdigitations from apposing effector--target cells and progressed to stable conjugates, which were characterized by contact regions consisting of large expanses of closely apposed surface membranes. The application of the single cell conjugate/dextran assay to assess monocyte-binding requirements and the morphological characterization of the monocyte--target cell conjugation process may help in clarifying the mechanisms of monocyte-mediated tumor cell lysis.