Two distinct mechanisms of antitumor activity mediated by the combination of interleukin 2 and monoclonal antibodies.

Previously we described a human B-lymphoma xenotransplantation model in which the immunotherapeutic activity of monoclonal antibodies (mAbs), directed against human B-cell antigens, can be studied. An anti-CD19 mAb of IgG2a subclass was therapeutically active by itself in this model, and its efficacy was increased by simultaneous administration of recombinant interleukin 2 (rIL-2). Immunotherapy with IgG1 or IgG2b isotype variants of an anti-CD19 mAb was ineffective if given alone. We now show that the combination of these ineffective isotype variants with rIL-2 results in significant antitumor effects, although IgG2a anti-CD19 mAb in combination with rIL-2 was therapeutically more active. In vitro studies of the effector mechanisms possibly involved in these treatment modalities indicate that the antitumor activity of IgG1 or IgG2b anti-CD19 mAbs in combination with rIL-2 may be mediated by rIL-2-induced antibody-dependent cellular cytotoxic activity of lymphocytes. The antitumor effect of IgG2a anti-CD19 mAb in combination with rIL-2 may be mediated not only by rIL-2-induced antibody-dependent cellular cytotoxic activity of lymphocytes but also by IgG2a-restricted antitumor activity of monocytes/macrophages. These results may explain the greater in vivo efficacy of treatment with rIL-2 and IgG2a subclass mAb versus treatment with rIL-2 and IgG1 or IgG2b subclass mAbs.

Biochemical characterization and cellular localization of a formalin-resistant melanoma-associated antigen reacting with monoclonal antibody NKI/C-3.

A monoclonal antibody (MAb NKI/C-3) produced against a purified membrane preparation of human melanoma cells reacts preferentially with sections of formaldehyde-fixed and paraffin-embedded tissues of melanoma, nevocellular nevi, carcinoids and medullary carcinomas of the thyroid. NKI/C-3 did not react with basal-cell carcinoma, brain tissue or brain tumors, and in only 14/196 other tumors was a clear cross-reactivity observed, e.g. with prostate carcinomas and a minority of primary breast, ovarian, lung and clear-cell carcinomas. This antibody was used in an immuno-electron microscopic study for the cellular localization of the antigen. The antigen was dispersed in the cytoplasm of melanoma cells, and more concentrated inside vacuoles and sometimes also on the melanosomes. Occasionally, the antigen was seen on the cell surface. The nature of the antigen was determined in an enzyme immunoassay (EIA). It was found that the antigen is a glycoprotein with a disulfide-dependent configuration that is essential for recognition by the MAb. The antigen was distributed heterogeneously during gel filtration as well as during SDS-polyacrylamide gel electrophoresis in the region of 25-110 kd proteins. A purified antigen preparation that was obtained after affinity chromatography on a column of MAb NKI/C-3 linked to Sepharose 4B contained a carbohydrate:protein ratio of 1:3.5.