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.