Abstract Spontaneous bacterial peritonitis (SBP) is a major complication of cirrhosis and associated with high mortality rate. Moreover, the effectiveness of antimicrobial therapy, the current standard of care, is deteriorating due to the global emergence of multidrug-resistant pathogens. Hence, it is essential to develop novel management strategies, for which furthering our understanding of the pathophysiology is imperative. Gut dysbiosis and impaired intestinal immunity in cirrhosis cooperatively predispose to the translocation of gut flora into the mesenteric lymphatics and, subsequently, to the peritoneum. Although this mechanism has been regarded as the primary pathophysiology, SBP develops only when the balance between peritoneal antibacterial immunity and bacterial virulence shifts in favor of the invading pathogen. Peritoneal macrophage (PM) is the major cell type in the peritoneum and is considered as the first line of antibacterial defense owing to the robust capacity in the engulfment and phagolysosomal digestion of pathogens. Moreover, PM sensing of microbe results in the production of cytokines/chemokines required for neutrophil recruitment and activation. Accordingly, PM is also essential for triggering the second wave of antibacterial response. Despite its perceived importance, the role of PM in SBP has been poorly understood. Furthermore, it remains largely elusive whether the development of cirrhosis alters the antibacterial properties of PM, and how it impacts the overall potency of peritoneal antibacterial immunity. Our single cell RNA sequencing analyses of peritoneal immune cells of rat with cirrhosis demonstrate a profound alteration of PM characteristics, including a substantial downregulation of the master regulator of PM, GATA6, a vitamin A (VA)-inducible transcription factor. In addition, PM of cirrhotic rats exhibit a markedly reduced capacity to produce inflammatory mediators in response to bacterial peritonitis, resulting in impaired neutrophil recruitment and activation as well as the insufficient bacterial clearance. Our studies also reveal that two common features of cirrhosis, VA deficiency and ascites accumulation, both independently contribute to the dysregulation of PM antibacterial function, with both processes involving GATA6 downregulation. Moreover, PM isolated from cirrhosis patients demonstrate a decreased number of GATA6-expressing PM, which correlates with the degree of antibacterial function impairment. These findings led to our hypothesis: “cirrhosis impairs the antibacterial properties of PM through the downregulation of GATA6 expression, which contributes to SBP development”. Accordingly, this proposal aims to close the knowledge gap regarding the role of PM in SBP through defining: (Aim 1) the impact of cirrhosis on PM antibacterial functions, (Aim 2) the influence of VA insufficiency on PM antibacterial properties, and (Aim 3) the effect of cirrhotic ascites on the antimicrobial activity of P...