Complement-dependent cytotoxic antitumor antibody. II. Quantitative determination of cell-bound immunoglobulin M1

A method for the quantitative determination of rabbit IgM bound to cell surfaces has been developed. This method was based on the ability of goat IgG specific for rabbit IgM heavy chains to bind 125I-labeled protein A when bound to the antigen. With the use of this technique the production of specific IgM antitumor antibodies in New Zealand White rabbits after immunization with guinea pig hepatoma cells line-1 and line-10 was followed. Differences in the production of IgM were observed between the different bleedings from rabbits immunized with line-1. No significant IgM antibody was produced following immunization of rabbits with line-10 tumor cells. This indirect method for determining IgM on the cell surfaces was objective, easy to perform, and detected complement-fixing and noncomplement-fixing antibodies. In addition, this technique could be applied to quantify other components on the cell surface for which a suitable specific antibody was available.