Human Monocyte Recognition of Complement-Coated Lymphoblastoid Cells

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Abstract
The macrophage system in man plays a significant role in the detection of foreign cells. The mechanisms by which macrophages recognize malignant cells, however, are not well understood. We used human monocytes and four lymphoblastoid cell (LC) lines derived from human acute lymphocytic leukemia to investigate the initial recognition of tumor cells by monocytes. IgM antibody mediated the binding of these cells to monocytes only in the presence of complement. The stepwise addition of complement components to IgM-coated LC indicated that C3 was necessary for monocyte binding. Similarly, monocyte recognition of IgM-coated LC was maximal in the presence of sera from patients with congenital C5 or C6 deficiency, but absent in the presence of sera deficient in C4 or from a patient with congenital C2 deficiency. Complement activation was associated with C3 consumption and the deposition of substantial amounts of C3 on to LC. Although 3H-C3 bound to LC appeared stable for 2 hours, approximately 4.0 ± 2 × 105 3H-C3 per LC was necessary for monocyte recognition, compared to approximately 2.7 ± 0.5 × 103 3H-C3 per RBC. The data indicate that LC can be recognized by monocytes through complement by mechanisms similar to nonmalignant target cells. However, substantial amounts of C3 are necessary to induce monocyte recognition of IgM-coated LC and, thus, such complement mediated recognition may be inefficient.