An In Vivo Model To Investigate the Nature of the Cell-of-Origin of Multiple Myeloma.

Multiple myeloma (MM) and related plasma cell dyscrasias are characterized by monoclonal expansion of terminally differentiated plasma cells. However, it is puzzling how the quiescent plasma cell can be the source of often aggressive and relapsing neoplasms. We and others have postulated that the myeloma clonogenic progenitor may reside in a more immature compartment with greater self-renewal capacity, most probably a maturing plasmablast precursor in the germinal center. To investigate the nature of cell-of-origin for these diseases and the genetic requirements for pathogenesis, we have created a novel flexible mouse model system that enables the delivery of stochastic, sequential, somatic mutations to precisely defined compartments of the germinal center in secondary lymphoid tissues. To this end, we have used BAC transgenic technology to express distinct types of avian leukosis virus (ALV) receptors, TVA and TVB, in the expanding centroblast of the dark zone and the committed plasmablast of the light zone, respectively. Mammalian tissues are refractory to transduction by retroviruses of the ALV family unless they ectopically express the cognate avian-derived receptors. Thus, the coding sequences for the TVA receptor, fused to a fluorescent protein tag were placed under the control of transcription factor A-myb, expressed in centroblasts of the dark zone. Similarly, sequences encoding a fluorescent-tagged TVB receptor were placed under the control of transcription factor Blimp1, expressed in the earliest committed plasmablasts as well as mature plasma cells. Analysis of the Blimp1: TVB mice showed that expression of the avian retroviral receptor in the hematopoietic system is limited to the light zone of germinal centers, extrafollicular collections of CD138+ cells in the spleen and lymph nodes as well as long-lived bone marrow plasma cells. Analysis of A-myb: TVA transgenic mice demonstrated expression of the fusion receptor to be restricted to B cells in the immunized spleen but not T cells. Both transgenic systems have been crossed into an Ink4a/Arf-deficient background. We have been transducing plasma cell precursors generated in the course of immune responses to T-dependent antigens with retroviral vectors carrying genes important in myelomagenesis, such as cyclin D1 or c-Myc. Animals are being monitored for development of plasma cell dyscrasias by periodical serum protein electrophoresis (SPEP) and other assays.