MUM1/IRF4 expression in the circulating compartment of chronic lymphocytic leukemia

MUM1/IRF4 is normally expressed in late germinal center/post germinal center B-cells. Previous studies of chronic lymphocytic leukemia in bone marrow and lymph node have demonstrated variable expression of MUM1/IRF4 and conflicting prognostic significance. In this study we evaluated MUM1/IRF4 expression in peripheral blood CLL cells utilizing Histogel cell blocks. MUM1/IRF4 was absent in 4/36 (11%) specimens. The remaining cases demonstrated variable intensity and proportion of positive cells: <20% positive 16/36 (44%), 20 - 50% positive 12/36 (33%), >50% 4/36 (11%). No correlation was identified between MUM1/IRF4 and percent of CD38 positive cells, CD38 status (+/-), ZAP-70 status (+/-), and IgVH mutational status. The variability in MUM1/IRF4 staining suggests a level of biologic complexity that is not adequately reflected in the current binary models of CLL pathobiology. This heterogeneity may reflect the role of MUM1/IRF4 as an effector and integrator of several lymphocyte activation pathways including antigenic and environmental stimuli.