IRF4T95R causes primary immunodeficiency and promotes to B1a cell development
Creators: name: Yu, Zhijia
Publisher: name: The Australian National University
Year: 2023

description: B2 and B1 cells are two subsets of B cells that have different cellular origins and functions and are found in different anatomical locations. Although many studies have been conducted on B2 and B1 cell development, function, and differentiation, the transcription factor network for them still requires further investigation. Specifically, in the case of B1 cells, additional studies are needed to understand the transcription factors that control their development and function. The transcription factor IRF4 has been shown to affect B1 cells. Previous studies have indicated that deletion of one copy of IRF4 leads to a slight increase in B1 cells. However, very few studies have focused on understanding the role of IRF4 specifically on B1 cells. In my study, I utilized a mouse strain with a point mutation in the DNA binding domain of IRF4 (IRF4T95R) to demonstrate that IRF4 influences the development of early B1 precursors and transitional B1 B cells in the spleen. This influence is achieved through its effects on lineage selection, proliferation, and cell survival. Additionally, I showed that the Irf4T95R/+mice (referring to the mouse strain with the IRF4 point mutation) exhibited an expansion of B1a cells in various locations, including the spleen, peritoneal cavity, lymph nodes, bone marrow, and blood. In contrast, IRF4 KO mice demonstrated a near complete absence of B1a B cells. The expansion of B1a cells was already observed in transitional B1a cells at an early stage of development, suggesting that IRF4 plays a key role in the commitment to B1a cells. Moreover, through bulk RNAseq analysis of splenic B1a cells, I identified a decreased expression of Cd22, a well-known IRF4 target and a crucial regulator of BCR signaling. This reduction in CD22 expression correlated with an increasedcalcium flux response to BCR stimulation in B1a cells from Irf4T95R/+mice, indicating a dysregulated BCR response. Over time, a clonal population of B1a B cells expanded, with a median age of 183 days. This expansion mimicked the long indolent phase observed in humans with chronic lymphocytic leukemia (CLL), gradually transforming into a more aggressive disease and resulting in reduced survival compared to WT littermates. This mouse model represents the first instance of linking a specific point mutation in IRF4 to the development of CLL, thereby providing a new model to study CLL biology and explore novel therapeutic approaches. In addition to its function in B1 cells, IRF4 has been shown to be essential for B2 cell development and differentiation. However, the role of IRF4 in early B2 cell development is only evident when IRF8 is also absent due to functional redundancy between these two related transcription factors. This thesis describes that, consistent with previous studies in Irf4 KO mice, Irf4T95R/+and Irf4T95R/T95Rmice exhibited normal early B2 cell development in the bone marrow. However, the development of follicular B cells and marginal zone B cells was affected in the spleen, resulting in a reduced number of both cell types and altered expression of surface markers, particularly in homozygous Irf4T95R/T95Rmice. Regarding the later stages of B2 cell development and differentiation, studies have revealed that IRF4 deficiency leads to a failure of germinal center (GC) B cell and plasma cell formation, resulting in decreased antibody production. It has been shown that B cells upregulate IRF4 upon exiting the GC to differentiate into plasma cells or memory B cells. However, the expression of IRF4 in GC B cells is very low, and thefunction of IRF4 within the GC has not been thoroughly studied due to the lack of GC formation in IRF4 KO mice.