Binding and internalization of anti-sarcoma IgG and IgM antibodies

A variety of monoclonal antibodies directed against tumor cell surface antigens have been employed for tumor detection and delivery of toxins and radioisotopes to tumors in situ. The sequence of events following antibody binding to tumor cells may be critical to the success or failure of tumor imaging or therapy. Using indirect immunofluorescence, we have examined the binding and internalization of two different monoclonal antibodies (19-24 and MFH 4.10) by HT-1080, a cultured human sarcoma cell line. MAb 19-24 (IgG₁) and MAb MFH 4.10 (IgM) are directed against surface antigens on cells from human malignant fibrous histiocytoma (MFH). For both of these antibodies, the initial binding on the surface of HT-1080 cells at 4°C was uniform. After binding secondary fluorescent antibody, monoclonal antibodies underwent rapid internalization at 37°C. Surface-bound antigen-antibody complexes were completely cleared from the cell surface in 30-60 min. In the absence of secondary fluorescent antibody, MAb 19-24 remained bound to the cell surface at 37°C for up to 20 hr without being released or internalized. However, under these same conditions MAb MFH 4.10 was internalized completely within 30 min by HT-1080 cells and did not subsequently return to the cell surface. Such differences in the mechanisms of binding and uptake for these two antibodies may have significant implications for their future clinical or therapeutic uses.