Cell-bound myeloma proteins on the surface of myeloma cells: potential targets for the immune system

Rosette formation with 2,4,6-trinitrophenylated (Tnp)-sheep erythrocytes was used to demonstrate cell-bound myeloma proteins on cells from each of the eleven mouse plasmacytomas tested. Tnp-erythrocytes were bound directly by cells from two tumors (MOPC-315 and MOPC-460) that formed myeloma proteins with antihapten combining sites; rosette formation with cells from the other tumors required hybrid antibody fragments with one site specific for Tnp and the other specific for an appropriate immunoglobulin on the tumor cell surface. The cell-bound immunoglobulin of MOPC-315 and of MOPC-460 had the same heavy and light chains, idiotypic determinants, and ligand-binding specificities as the respective myeloma proteins secreted by these tumors. A variant tumor (MOC-315 NR) was grown from the small proportion of MOPC-315 cells (5-10%) that did not bind Tnp-erythrocytes directly: this tumor secreted only the light chain of protein 315, carried this chain on the cell surface, and resembled the variant tumors that arose, probably by immunoselection, in BALB/c mice challenged with MOPC-315 cells while making an immune response to the idiotype of protein 315.