IRF-4/MUM-1 Expression Is a Critical Switch in the Generation of Plasma Cells Versus Memory B-Cells.

Most types of human B-cell lymphomas harbor somatically mutated Ig variable region genes, reflecting their origin from cells that have undergone the germinal center (GC) reaction of T-dependent immune responses. The lymphomas exhibit diverse phenotypes and clinical behaviors, likely as a consequence of differences both in the mechanisms of transformation and in the specific target cell. We have recently identified two distinct subsets of GC B-cells that may represent late stages of the GC-reaction and may be the precursors of plasma cells and memory B-cells. The corresponding subsets are characterized by downregulation of the GC-marker BCL6 and the alternative expression of IRF-4/MUM-1 or nuclear c-Rel. These two subsets seem to reflect distinct cellular programs which are altered during B-lymphomagenesis in various tumor subtypes which co-express BCL6, IRF-4 and nuclear c-Rel, an event never observed in normal B-cells. In order to gain insights into the physiologic role of IRF-4/MUM-1 and nuclear c-Rel in GC-development, we are ablating their expression specifically in mouse GC B-cells. Transgenic mice were generated that carry an IRF-4/MUM-1 null allele and a conditional IRF-4/MUM-1 allele which, following Cre-mediated deletion of the loxP-flanked promotor region and exons 1 and 2, expresses eGFP, thus allowing to track the development of the IRF-4/MUM-1 deficient cells at the single cell level. IRF-4/MUM-1^fl/- mice were crossed with transgenic mice that express the Cre-recombinase specifically in B-cells undergoing class-switch to IgG1, an event occurring in a large fraction of GC B-cells. Upon immunization with sheep red blood cells or nitrophenyl-(NP)-KLH, mice unable to express IRF-4/MUM-1 in late GC B-cells (IRF-4/MUM-1^fl/-/Cγ 1^Cre/+), in contrast to control mice (IRF-4/MUM-1^fl/+/Cγ 1^Cre/+), did not develop plasma cells (IgG1⁺CD138⁺) in the peripheral lymphoid organs, blood, and bone marrow. On the other hand, the generation of memory B-cells appears normal since antigen-specific B-cells were present in the spleen (eGFP⁺B220⁺PNA^-IgG1⁺) and blood (eGFP⁺B220⁺CD38⁺IgG1⁺). These results suggest that the IRF-4/MUM-1 gene product is required for the development of antigen-selected GC B-cells into plasma cells. We suggest that the expression of IRF-4/MUM-1 in a GC centrocyte is the critical event in the commitment of B-cells to differentiate into a plasma cell versus a memory B-cell, and are currently testing the role of nuclear c-Rel in the same process.