Mechanisms of humoral cytotoxicity testing: the role of rabbit serum, xenoantibody, and human IgM.
The reproducibility and sensitivity of a humoral microcytotoxicity assay which detects anti-astrocytoma antibodies depends on a consistent, effective complement preparation. Rabbit serum mixed 1/1 with human cord serum proved to be an ideal preparation with full complementarity and low nonspecific cytotoxicity. Human cord serum provided consistent small amounts of human IgM and did not contribute additional complement activity. The inhibition of xenoantibody by human IgM is the critical mechanism which determines the efficacy of the complement. Human IgM in a final concentration of 10 mg/dl in the rabbit serum produced a preparation with 10 to 15% nonspecific cytotoxicity against astrocytoma target cells. This residual nonspecific cytotoxicity serves a catalytic function and is necessary for the detection of weak specific antibodies.