Complement-dependent cytotoxicity of anti-human osteogenic sarcoma monoclonal antibodies

Monoclonal anti-human tumor antibodies of six isotypes in cytotoxic reactions with human and murine effector cells

Eighty-seven murine monoclonal antibodies (MAb) produced against human tumors of various origins and representing six different immunoglobulin classes were tested for antitumor reactivity in antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) assays. Mouse splenocytes, thioglycolate-elicited mouse peritoneal macrophages, freshly obtained nonadherent human peripheral blood lymphocytes, and human monocytes were used as effector cells, and human or rabbit serum as the source of complement. Of all four effector cell types tested, mouse macrophages showed the highest cytotoxic activity, based on net cytotoxicity, minimum requirement for Mab concentration, and effector cell number. Different immunoglobulin classes were associated with characteristic patterns of reactivity with the various effector cells or complement, independent of the target cell type used. MAb able to mediate ADCC were found among all IgG subclasses, with IgG2a and IgG3 MAb inducing lysis with all effector cell types. IgM and IgA MAb were nonreactive in the various ADCC assays, but IgM MAb were highly cytotoxic with complement.

Analysis of a human osteogenic sarcoma antigen and its expression on various human tumour cell lines

The murine monoclonal antibody 791T/36 cross-reacts with cells other than the immunizing osteogenic sarcoma cell line 791T, upon which the 791T/36-defined epitope is expressed on a protein of apparent molecular weight 72,000. An investigation was performed to determine whether the epitope occurred on similar molecules on other cell lines. Radiolabelled immunoprecipitates, prepared with the 791T/36 antibody, from three osteogenic sarcoma cell lines (2 OS, 788T and 278T), the prostate carcinoma EB33 and the colon carcinoma HcLo each contained a protein with a molecular weight of 72,000 as the major constituent, together with, in some cases, material of lower molecular weight. This heterogeneity was shown by neuraminidase treatment of the immune precipitates to be due to variations in sialic acid content of the antigens since, in five of the six cell lines tested, such treatment produced homogeneous material of apparent molecular weight 55,000. Chymotrypsin treatment of the immune precipitates produced in each instance a major polypeptide of molecular weight 47,000 which displayed no microheterogeneity. Immunoadsorbent-purified antigen from 791T cells was shown to bind strongly to Sepharose-wheat-germ agglutinin and less to Sepharose-concanavalin A, confirming the glycoprotein nature of this antigen. These studies demonstrate that molecules expressing the 791T/36-defined epitopes on different tumour cell lines are glycoproteins which display heterogeneity with respect to apparent molecular weight that is attributable to varying degrees of sialylation. No apparent differences were detected in the polypeptide "backbone" of these antigenic molecules.

Preparation of four daunomycin-monoclonal antibody 791T/36 conjugates with anti-tumour activity

As an approach to developing more specific anti-tumour therapeutic agents, daunomycin has been covalently linked to the human tumour localizing, murine monoclonal antibody 791T/36. Four procedures for coupling drug to antibody were investigated. The sugar amino group of daunomycin was modified by reaction with succinic anhydride or cis aconitic anhydride and these derivatives were linked to antibody, a carbodiimide reagent being used to produce stable peptide bonding. Alternatively, 14-bromo daunomycin was linked directly to antibody or antibody containing free thiol groups introduced by means of the heterobifunctional reagent SPDP [N-succinimidyl-3 (2-pyridyldithio) propionate] thus producing a thioether linkage. Each of the conjugates, with drug-antibody ratios of 3 to 4:1, retained a proportion of drug activity although the succinic anhydride derivative was the least cytotoxic. The three other conjugates specifically bound to tumour cells expressing the 791T/36 antibody defined antigen. In short-term assays in which tumour cells were briefly exposed to conjugates and then washed to remove non-bound conjugate, it was determined that the conjugate with the cis aconityl linkage displayed the greatest selective cytotoxicity against tumour cells reactive with the 791T/36 antibody. These studies illustrate the feasibility of preparing chemically defined drug-antibody conjugates retaining cytotoxicity and selectivity of action against tumour cells.

Immunomodulation: NK cells activated by interferon-conjugated monoclonal antibody against human osteosarcoma

A mouse monoclonal antibody raised against the human osteogenic sarcoma cell line 791T has been covalently coupled to purified human lymphoblastoid interferon alpha (IFN alpha). Conjugation does not interfere with antibody function, as the product binds to 791T cells and mediates complement-dependent tumour cell lysis to a degree equal to that of free antibody. The IFN activity, assessed by augmentation of natural killer (NK)-cell-mediated lysis, is reduced, but the conjugate does augment the killing of 791T and other tumour targets by peripheral blood NK cells. In admixture experiments the conjugate, when bound to unlabelled osteogenic sarcoma cells, also augments the killing of radiolabelled bystander cells. Neither free antibody nor the conjugate mediate antibody-dependent cellular cytotoxicity (ADCC), and augmented tumour cell lysis is a function of NK cell activation. This product provides for an alternative approach to cancer therapy via the activation of infiltrating hose effector cells using specifically targeted lymphokines.

Identification of an anti-human osteogenic sarcoma monoclonal-antibody-defined antigen on mitogen-stimulated peripheral blood mononuclear cells

An anti-human osteogenic sarcoma monoclonal antibody (mouse IgG2b) termed 791T/36 was found to exert complement-dependent cytotoxicity against phytohaemagglutinin (PHA)-stimulated peripheral blood mononuclear (PBMN) cells. This reaction was examined by flow cytofluorimetry using indirect membrane immunofluorescence to detect cell-bound antibody and by measurement of the binding of fluorescein-isothiocyanate-conjugated 791T/36 antibody to cells. The antibody reacted strongly with peripheral blood blast cells induced by PHA, and there was negligible reactivity with resting lymphocytes. Maximum binding was observed after 3 days' culture with PHA, coinciding with maximum DNA synthesis, and this represented of the order of 2 X 10(5) antibody molecules bound per cell. After cell surface radioiodination of PHA-stimulated PBMN cells, detergent lysis and immunoprecipitation of antigen with 791T/36 antibody and Sepharose-protein A, the apparent molecular weight of this antigen was determined to be 72,000 by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. This is identical to that of the 791T/36-defined antigen expressed on various osteogenic sarcoma cell lines [3, 17], and by this criterion the antigen is distinguishable from other cell surface markers of activated human lymphocytes.