Elucidating the Role of Lactate in Immune Dysfunction during Sepsis: A Focus on B Cell Modulation - Abstract Sepsis is a systemic inflammation triggered by infection that leads to organ dysfunction. With 750,000 cases and 250,000 deaths per year, it is the most common cause of in-hospital deaths in the U.S. Immune paralysis is a high risk for the secondary infections and mortality of survived sepsis patients. However, the cellular and molecular mechanisms of immune paralysis have not been elucidated. Therefore, our long-term goal is to determine the mechanisms of sepsis-associated immune paralysis. Lactate level is an independent biomarker for the prognosis, severity, and mortality of sepsis. Recent clinical studies have reported that sepsis patients with high lactate levels exhibit impaired immune responses. In this application, we will define the role of lactate in mediating B cell dysfunction, thereby contributing to immune paralysis in sepsis. B cell immunity plays a pivotal role in the immune response following sepsis. We discovered that lactate induces the numeric, phenotypic and functional changes of B cells following sepsis. Transcription factor enrichment analysis (TFEA) revealed that FoxO1 is the most significantly suppressed transcription factor by lactate in B cells following sepsis. FoxO1 is a master transcription factor governing the expression of genes associated with B cell development, proliferation and immune response. In addition, we made a novel finding that lactate drives the production of γ-amino-butyric acid (GABA) in B cells post-sepsis. GABA, traditionally known as a neurotransmitter in the brain, is reported to be an effectively immunosuppressive molecule. We demonstrated that GABA is a B cell-associated metabolite, which may impair immune responses following sepsis. Mechanistically, we found that lactate promotes the lactylation of FoxO1 and histone 3 in B cells, as demonstrated by immunoprecipitation assay and supported by a machine learning-based prediction tool. Importantly, we observed that the expression of AARS1, a newly identified lactate sensor and lactyl-transferase, is highly enriched in B cells when compared to other immune cells, suggesting that B cells are sensitive to elevated lactate levels during sepsis due to their abundance of lactylation machinery. Based on our preliminary studies, we hypothesize that lactate suppresses B cell immune function via FoxO1 lactylation and promotes B cell-derived GABA release, thus contributing to immune paralysis during sepsis. To test this hypothesis, we will integrate genetic and pharmacologic approaches, utilize FoxO1 mutant mouse models, perform multi-omics analyses, and apply machine learning-based tool. We propose the following specific aims: 1) Define the mechanisms of lactate-induced FoxO1 lactylation in B cell dysfunction in sepsis; 2) Determine the mechanisms of lactate-promoted GABA production in B cells and its contribution to sepsis-associated immune paralysis; 3) Elucidate whether lactate-impaired B cell immune function is mediated by lactate transport (MCTs) and lactate receptor (GPR81).
