The TEPC-15, HOPC-8, and S107 mouse plasmacytoma cells secrete IgA molecules which bind specifically to phosphorylcholine (PC). Plasmacytoma cells were cultured for 3 days in RPMI 1640 supplemented with 10% fetal calf serum. The secretion of myeloma proteins was determined by indirect plaque formation using either PC- or TNP-coated sheep red blood cells. The viability of the tumor cells was assessed by dye-exclusion. When the tumor cells were treated with PC-antigen (5 × 107 R36A pneumococci) secretion of PC-binding myeloma proteins was inhibited by 83% for TEPC-15 and 33% for HOPC-8. However, secretion of DNP-binding myeloma protein of MOPC-315 was not inhibited significantly. The percentage inhibition of plaque formation by the antigen paralleled the percentage reduction in viability of the tumor cells. Indirect fluorescent microscopy indicated that specific antigen caused a rearrangement of membrane-bound myeloma protein. These results suggested that a common mechanism may be operating for inhibition of tumor cells by both specific antigen and anti-idiotypic antibodies.
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