Abstract. Rheumatoid arthritis (RA) is a debilitating chronic joint disease, causing functional disability, severe pain, joint deterioration, and substantial socioeconomic burdens. Presently, there is no cure for RA. While some patients manage RA with methotrexate and other disease-modifying anti-rheumatic drugs (DMARDs), around 40% do not respond. Even with biologic treatments, 6-21% RA patients are unresponsive. Thus, a deeper understanding of the mechanisms of RA pathogenesis is crucial for developing effective treatments for RA patients. In RA, synovial macrophages (MΦs) are abundant and major sources of pro-inflammatory cytokines like TNFα, IL1, and IL6, contributing to RA pathogenesis. By analyzing single-cell RNA-seq databases of human synovial tissues from RA patients and mouse synovial MΦs in CAIA model, we discovered a significant reduction in INPP5E expression. INPP5E, a 5-phosphatase, dephosphorylates PI(4,5)P2, PI(3,5)P2, PI(3,4,5)P3, known for regulating the immune response and MΦ phagocytosis via PIP3/PI3K/AKT signaling. However, the role of INPP5E in regulating MΦs, RA pathogenesis, and its potential as a drug candidate remain poorly understood. Our preliminary data showed INPP5E deletion in MΦs worsened inflammation and bone destruction in a mouse CAIA model, while overexpression of INPP5E inhibited inflammation in IL1 induced MΦs. Moreover, we found that INPP5E associates with Aurora kinase A (AURKA), and overexpression of INPP5E suppressed NF-κB and DNA damage markers like H2AX and ATM/ATR phosphorylation. Proteomics results indicated INPP5E affects oxidative reduction, inflammation, NF-κB signaling, and DNA repair, suggesting anti-RA potential. Moreover, to overexpress INPP5E in RA MΦs, we developed novel F4/80 Ab Fab'-functionalized LNPs (Ab-LNPs) for INPP5E mRNA delivery. Our preliminary data showed that these LNPs can accumulate and target synovial MΦs and can effectively express GFP and INPP5E mRNA. Our preliminary data support two key hypotheses: 1) Macrophagic INPP5E inhibits RA pathogenesis by regulating AURKA/AKT/NF-κB and DNA damage/ATM/ATR signaling pathways; 2) Introduction of INPP5E through a novel targeted LNPs carrying INPP5E mRNA mitigates RA pathogenesis. To test hypothesis, in Aim 1, we will test if deletion or overexpression of INPP5E in MΦs significantly alter inflammation and bone erosion in CAIA model. We will explore pathomechanisms by characterizing AURKA/AKT/NF-κB, DNA damage/ATM/ATR and other INPP5E downstream signaling pathways. We will further characterize INPP5E interacting proteins and their function by performing mass spectrometry and RNA interference analysis. In Aim 2, we will demonstrate that targeted delivery of INPP5E mRNA to MΦ using a novel F4/80 Ab conjugated mRNA LNPs platform can effectively attenuate RA in acute and chronic RA mouse models. The completion of the proposed project is expected to reveal the novel role and mechanism of INPP5E in RA pathogenesis and provide a novel targeted ...