Transcriptional control of plasma cells by Ikaros and Aiolos

Antibody-secreting plasmablasts and plasma cells provide acute and long -term protection against infection through the generation of antibodies against a vast number of pathogens (foreign antigens). This enormous number of different antigen specificities in the polyclonal B cell population is accomplished by recombination of the immunoglobulin genes during early B cell development and is further diversified by affinity maturation, a process which introduces somatic mutations into the antigen-binding site upon antigen encounter. Competition for the uptake of antigen and subsequent presentation to T cells selects the best B cell clones to differentiate into an antibody-secreting effector cell. Activated B cells initially differentiate into short-lived, proliferating plasmablasts that subsequently migrated to survival niches in the bone marrow and develop into quiescent long-lived plasma cells, also referred to as memory plasma cells, which constitute the basis for successful vaccination strategy. Beside the changes in cell cycle and longevity, plasma cell differentiation involves further massive changes in gene expression. The most significant functional change is the switch from the membrane-bound immunoglobulin transcript totheplasmacell-specific secreted immunoglobulin transcript, encoding the antibody. The development to antibody -secreting cells is also accompanied by marked alterations of the cell morphology, which is caused by the upregulation of immunoglobulin gene transcription. The secretion of large amounts of immunoglobulin proteins requires an increase of the secretory machinery of the cell. These changes in cell identity, morphology and function are implemented by a massive reprogramming of the transcriptional network in plasma cells. To date, only a few transcription factors, including IRF4, Blimp1, XBP1, and the Ikaros transcription factor family, are known to control the terminal differentiation of B cells to plasma cells. IRF4 controls the initiation of differentiation and subsequent surviva l of plasma cells, Blimp1 determines the plasma cell fate and function, and XBP1 activates the transcriptional program which is essential for the massive increase of protein secretion. Finally, although two expressed members of the Ikaros transcription factor family, Ikaros and Aiolos, have been implicated in the control of plasma cells, their function is unknown. Using conditional gene deletion and acute protein degradation in antibody-producing plasma cells, we aim to define the transcriptional role of Ikaros and Aiolos in plasma cells.

Our immune system relies on B cells, specialized cells that recognize foreign pathogens and produce antibodies to protect us from infections. B cells develop in several stages: early "immature" cells first form in the bone marrow, then later mature into different types that can either patrol the body, respond rapidly to threats, or become long-lived plasma cells that provide lasting immunity. Two closely related proteins, called Ikaros and Aiolos, are known to be important for early B cell development, but until now, their role in later stages of B cell life was poorly understood. With support from the FWF grant, our research investigated how Ikaros and Aiolos influence the generation and stability of mature B cells and plasma cells. To do this, we used advanced genetic tools in mice, allowing us either to switch off the genes for Ikaros and Aiolos in specific B cell populations or to rapidly degrade the proteins and observe the immediate consequences. Our work showed that Ikaros and Aiolos are jointly required for the formation of two key B cell types: follicular B cells, which cooperate with T cells to mount strong antibody responses, and marginal zone B cells, which provide a first line of defense against certain pathogens. We also discovered that Ikaros and Aiolos are essential for the generation of antibody-secreting plasmablasts and for the survival of long-lived plasma cells, the very cells that are crucial for long-term protection after vaccination or infection. Without both proteins, these cells could not develop or persist. To understand how Ikaros and Aiolos exert this control, we studied which genes they directly regulate. We found that both proteins act mainly as repressors, keeping many genes switched off, or at moderate expression level, that would otherwise disturb the normal function of B cells. These "forbidden" genes encode proteins involved in processes such as signaling, cell structure, or protein stability, activities that, if turned on at the wrong time, may undermine B cell health. By preventing such inappropriate activity, Ikaros and Aiolos act as guardians of B cell stability and homeostasis. Finally, in plasma cells we observed that Ikaros and Aiolos cooperate with another key regulator called Blimp1, a protein well known for driving plasma cell differentiation. Together, these three proteins form a network of repressors that safeguard the identity and survival of antibody-producing cells. In summary, our study reveals that Ikaros and Aiolos are not only critical for early B cell development, as previously thought, they also play indispensable roles in shaping and maintaining the mature B cell system. This knowledge improves our understanding of immune memory and may, in the future, help inform therapies for immune disorders or B cell-related diseases.