Apigenin Upregulates Regulatory B Cells, Suppresses Class Switch Recombination, and Restores Immune Homeostasis to Treat Autoimmune Arthritis and Sepsis

Background: Restoring immune homeostasis is crucial for inflammation resolution, in which B cells play dual regulatory roles. A promising therapeutic strategy involves simultaneously suppressing proinflammatory B-cell activity while enhancing regulatory B cells (Bregs). Natural anti-inflammatory compounds, such as the flavonoid apigenin (API), may achieve this dual immunomodulation, yet the precise mechanisms of API on B cells remain incompletely understood. Purpose: This study aimed to elucidate how API modulates B cells to exert immunoregulatory and protective effects in inflammatory conditions. Design: In vitro B-cell functional and molecular assays were integrated with two mechanistically distinct in vivo models—collagen-induced arthritis (CIA) in DBA/1 mice and cecal ligation and puncture (CLP)–induced sepsis in C57BL/6 mice—to evaluate API’s dual immunomodulatory effects and therapeutic potential. Methods: B cells from mouse splenocytes (n=5 per group) or human PBMCs (n=5 healthy donors) were assessed for differentiation, cytokine production, class switch recombination (CSR), and signaling pathways. p38 MAPK involvement was examined using pathway-specific inhibitors. In vivo, disease severity, survival, immune phenotypes, cytokines, and histopathology were evaluated. Results: API increased IL-10⁺ Breg frequency by ~2-fold (P < 0.001) and reduced Aicda expression by ~60% (P < 0.001), while suppressing plasma cell differentiation via Prdm1 downregulation. These effects were p38 MAPK-dependent. In CIA mice, API reduced arthritis scores by ~45% (P < 0.01); in CLP-induced sepsis, 7-day survival improved from 0% to 70% (P < 0.05), with attenuated lung injury and pro-inflammatory cytokine levels. Collectively, API promotes immune homeostasis restoration in both chronic autoimmune and acute inflammatory settings. Conclusion: API restores immune homeostasis in both chronic autoimmune and acute inflammatory settings by coordinating Breg differentiation and suppressing pathogenic B-cell responses, highlighting B-cell-centric immune rebalancing as a promising therapeutic strategy for inflammatory diseases. Plain Language Summary: Excessive inflammation is a defining feature of both chronic autoimmune diseases and acute systemic inflammatory conditions. Restoring immune balance is critical for resolving inflammation—a process in which B cells play dual roles by both promoting and suppressing immune responses. A rational therapeutic strategy, therefore, is to simultaneously curb proinflammatory B cell activity while enhancing the function of immunosuppressive regulatory B cells (Bregs). Natural anti-inflammatory compounds derived from herbal medicine offer a promising source of agents capable of achieving this dual immunomodulation. Apigenin (API), a naturally occurring flavonoid, is known for its broad anti-inflammatory effects. However, whether and how it exerts dual regulatory actions on B cells remains unclear. In this study, we show that API promotes the differentiation of regulatory B cells (Bregs) via activation of the p38 MAPK signaling pathway, suppresses antibody class-switch recombination (a process that generates pathogenic antibodies) through downregulation of the gene Aicda, and inhibits plasma cell differentiation by reducing Prdm1 and IL-6 expression. These coordinated actions underlie its therapeutic efficacy in two distinct models of immune dysregulation. In a mouse model of collagen-induced arthritis—which mimics human rheumatoid arthritis—API alleviated joint inflammation and bone erosion. In a cecal ligation and puncture-induced sepsis model—which replicates severe systemic infection—API improved survival and attenuated acute lung injury. Together, these findings identify B-cell–centered immune rebalancing using natural anti-inflammatory compounds as a promising therapeutic approach for both chronic autoimmune and acute inflammatory conditions. The diagram illustrates the immune status in inflammatory diseases and the therapeutic effects of apigenin (API). On the left, the inflammatory state in diseases like rheumatoid arthritis and sepsis lung injury is depicted, where elevated pro-inflammatory cytokines (TNF-α, IL-6, IL-1β) and increased pro-inflammatory cells (macrophages, dendritic cells, T cells) lead to immune imbalance, with reduced regulatory B cells (Breg). On the right, apigenin (API) treatment restores immune balance in these diseases: it activates p38 MAPK signaling to expand Breg cells and increase IL-10 production, while suppressing antibody diversification, plasma cell differentiation, and pro-inflammatory responses. This reduces inflammation and promotes tissue homeostasis, as shown by the normalization of joint and lung conditions. The diagram includes labels for immune cells, antibodies, API, and cytokines, with arrows indicating increases or decreases in activity.Diagram comparing immune imbalance in inflammatory diseases (rheumatoid arthritis, sepsis lung injury) with apigenin (API)-mediated restoration of immune balance and anti-inflammatory effects.