The Role of Idiotype-Specific, CD4+ T Cells in Tumor Resistance against Major Histocompatibility Complex Class II Molecule Negative Plasmacytoma Cells

It is known that immunoglobulins can be processed and that idiotypic peptides are presented on MHC class II molecules to T cells. It has also been demonstrated that T cells can recognize a complex of an Id-peptide/MHC molecule as a tumor-specific antigen on B lymphoma cells. However, plasmacytomas, an important type of B cell malignancies, most often lack class II molecules and are thus expected to be poor targets for Id-specific, CD4+ T cells. Nevertheless, we now demonstrate that cloned, MHC class II restricted T cells, specific for a λ2315 idiotypic peptide, convey protection in vivo (Winn assay) against the class II molecule-negative MOPC315 (α, λ2315) plasmacytoma. T cells can also inhibit the growth of MOPC315 cells in vitro provided that MHC compatible (H-2d) splenocytes and extra λ2315 are added. Based on these data we suggest that the myeloma protein secreted by MOPC315 cells attains such a high local concentration in vivo that it is processed and presented by neighboring host APC to the Id-specific T cells. Such activated T cells secrete lymphokines which may directly affect the growth of MOPC315 cells in the vicinity. Alternatively, lymphokines from activated T cells stimulate local host cells, like macrophages, to become tumoricidal.