Binding of mouse monoclonal antibodies to human leukaemic cells via the Fc receptor: a possible source of 'false positive' reactions in specificity screening.

Mouse monoclonal antibodies (MoAbs) of different classes and subclasses directed against antigens not expected to be present on human cells have been screened by indirect immunofluorescence using flow cytometry for binding to human non-lymphocytic leukaemic cells and normal peripheral blood leucocytes. Antibodies of the IgG2a and IgG3 subclass, but not of the IgG1, IgG2b or IgM class bound to the blast cell and monocyte populations in a peripheral blood mononuclear cell preparation from a patient with acute myelomonocytic leukaemia. F(ab')2 fragments of an anti-salmonella antibody of IgG2a subclass failed to bind, indicating that the results were not due to cross-reactivity with antigens of the cell membrane, thus implicating the Fc region in binding. Furthermore, binding was partly blocked by inclusion of 10% heat-inactivated normal rabbit serum in the assays. IgG2a bound to varying degrees to the leukaemic cell populations in seven of the nine non-lymphocytic leukaemic specimens tested, but no binding to normal peripheral blood mononuclear cells or granulocytes was detected. The results emphasize the importance of including appropriate controls when screening MoAbs for binding to various types of human cells.