New monoclonal antibodies SN3, SN3a, and SN3b directed to sialic acid of glycoprotein on human non-T leukemia cells.

We have generated and characterized three new monoclonal antibodies (mAbs), termed SN3, SN3a, and SN3b, which are directed to sialic acid of a glycoprotein(s) on human non-T leukemia cells. These mAbs were generated by immunizing mice with an antigen preparation isolated from cell-membrane glycoconjugates of NALM-1, a pre-B leukemia cell line. The initial characterization of the mAbs consisted of a sensitive cellular radioimmunoassay against various cultured human leukemia-lymphoma (HLL) and nonmalignant cell lines. They strongly reacted with all four (all three in the case of SN3a) non-T/non-B HLL cell lines tested and both pre-B HLL cell lines tested. However, they reacted with only one of three B HLL cell lines tested. In addition, these mAbs did not react with other cell lines, which include T- and myelomonocytic HLL cell lines and nonmalignant B-cell lines. Normal peripheral blood cells were also tested; the mAbs reacted with B cells and granulocytes but not with T cells, monocytes, erythrocytes, or platelets. In a test using SN3 and SN3b with uncultured cell specimens derived from various cancer patients, the mAbs primarily reacted with non-T/non-B and B HLL specimens, as well as with chronic myelocytic leukemia specimens. The biochemical nature of antigenic determinants defined by the three mAbs was studied by treating the non-T leukemia cells with sialidase and proteases. The results show that the antigenic determinants defined by these mAbs all contain a sialic acid residue(s) that is attached to the cells via a protein backbone(s). Competitive binding experiments show that binding of SN3 to the leukemia cells was blocked almost completely by SN3a and SN3b, as well as by BA-1. Both SN3 and SN3a are IgG1 antibodies, whereas SN3b is an IgM antibody; SN3b showed a strong complement-mediated cytotoxic activity against non-T leukemia cells.