A monoclonal antibody with specificity for murine mu heavy chain which inhibits the formation of antigen-specific direct IgM plaques.

A panel of monoclonal rat antibodies binding to mouse mu heavy chain were tested for their ability to inhibit the formation of antigen-specific plaques in the hemolytic plaque assay. Nine antibodies inhibited SRC-specific direct IgM plaques at high concentrations (greater than 20 micrograms/ml). In contrast to all others, however, one antibody inhibited these plaques at much lower concentrations (down to 0.4 microgram/ml) when added to the assay. This antibody also inhibited plaques formed by cells secreting antibodies against trinitrophenyl or phosphorylcholine determinants. IgG plaques with any of the above specificities were not inhibited. IgM secretion was unaffected by the monoclonal anti-mu antibody. Its inhibitory effect on plaque formation rather appears to be a consequence of its ability to inhibit complement dependent, IgM mediated lysis of erythrocytes. This monoclonal anti-IgM antibody therefore provides a convenient reagent to distinguish specific direct IgM plaques from indirect IgG plaques.