Some plasmacytomas produce myeloma proteins with known antibody specificities and the secretion of these proteins by individual tumour cells can be determined using haemolytic plaque assay. After a 3 day culture of mouse plasmacytoma cells in medium containing 10% normal mouse serum, the number of plaques was reduced to less than 10% when compared to that of tumour cells incubated with either foetal calf serum or normal rabbit serum. However, tumour cells incubated with sera from mice bearing TEPC-15, McPC-603, or MOPC-315 plasmacytomas displayed control levels of plaques. The production of plaques paralleled the viability of tumour cells suggesting that the reduction of plaque formation is due to the decreased viable cell number. The tumour-inhibiting activity was recovered from the fraction of apparent molecular weight of 300,000-400,000 after a partial purification using an agarose (A 0.5 M) column. This fraction, however, did not suppress in vitro induction of antibody production. Kinetic experiments using sera obtained sequentially from individual mice receiving either TEPC-15 or MOPC-315 plasmacytomas further indicated that the tumour-inhibiting activity is severely reduced during a 2 week period after tumour inoculation. The inhibition of tumour cells did not appear to be specific since tumour cells of three plasmacytomas (TEPC-15, MOPC-167 and MOPC-315), a mastocytoma (P815) and a lymphoma (EL-4) displayed a similar susceptibility to normal serum.
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