Oscar Negrón

BACKGROUND: Obesity predisposes individuals to multiple pathologies, including metabolic dysfunction–associated steatotic liver disease and diabetes. Although it is known that accumulation of proinflammatory macrophages within adipose tissues drives adiposity and provokes obesity-linked sequelae, the molecular mechanisms that provoke macrophage dysfunction in obesity remain elusive. Macrophages express high levels of uPA (urokinase plasminogen activator), and uPA has been implicated in leukocyte migration. METHODS: Human adipose tissues from patients receiving bariatric surgery were collected and analyzed for uPA protein levels. To determine the impact of uPA in adipose tissue and subsequent high-fat diet (HFD)–induced weight gain and metabolic diseases, a novel mouse model with a conditional knockout of uPA ( Plau ^{fl /fl} ) was generated. Plau ^WT/WT , Plau ^KO/KO (global uPA deficiency), and Plau ^fl/fl /LysM Cre+ (conditional uPA deficiency in macrophages) mice were fed low-fat diet or HFD for up to 20 weeks. RESULTS: Protein levels of visceral adipose tissue uPA positively correlated with body mass index in patients with obesity, and uPA levels decreased in adipose tissue 2 years after bariatric surgery. The expression and activity of uPA also increased in the adipose tissue of HFD-fed control mice. Plau ^KO/KO mice displayed reduced weight gain and metabolic sequelae through 14 weeks on a HFD compared with Plau ^WT/WT mice, but not with prolonged HFD feeding. Interestingly, Plau ^fl/fl /LysM Cre+ mice developed HFD-induced metabolic pathologies equivalently to Plau ^WT/WT mice. CONCLUSIONS: Our findings suggest that global uPA deletion, but not selective deletion of uPA in LysM+ myeloid cells, attenuates the development of early-stage HFD-driven obesity and pathologies consistent with metabolic syndrome.

The blood-clotting protein fibrin(ogen) plays a critical role in host defense against invading pathogens, particularly against peritoneal infection by the Gram-positive microbe Staphylococcus aureus. Here, we tested the hypothesis that direct binding between fibrin(ogen) and S. aureus is a component of the primary host antimicrobial response mechanism and prevention of secondary microbe dissemination from the peritoneal cavity. To establish a model system, we showed that fibrinogen isolated from Fibγ^Δ5 mice, which express a mutant form lacking the final 5 amino acids of the fibrinogen γ chain (termed fibrinogenγ^Δ5), did not support S. aureus adherence when immobilized and clumping when in suspension. In contrast, purified wildtype fibrinogen supported robust adhesion and clumping that was largely dependent on S. aureus expression of the receptor clumping factor A (ClfA). Following peritoneal infection with S. aureus USA300, Fibγ^Δ5 mice displayed worse survival compared to WT mice coupled to reduced bacterial killing within the peritoneal cavity and increased dissemination of the microbes into circulation and distant organs. The failure of acute bacterial killing, but not enhanced dissemination, was partially recapitulated by mice infected with S. aureus USA300 lacking ClfA. Fibrin polymer formation and coagulation transglutaminase Factor XIII each contributed to killing of the microbes within the peritoneal cavity, but only elimination of polymer formation enhanced systemic dissemination. Host macrophage depletion or selective elimination of the fibrin(ogen) β2-integrin binding motif both compromised local bacterial killing and enhanced S. aureus systemic dissemination, suggesting fibrin polymer formation in and of itself was not sufficient to retain S. aureus within the peritoneal cavity. Collectively, these findings suggest that following peritoneal infection, the binding of S. aureus to stabilized fibrin matrices promotes a local, macrophage-mediated antimicrobial response essential for prevention of microbe dissemination and downstream host mortality.

Significance: A thrombin-fibrin pathway drives rapid and robust bacterial killing of Staphylococcus aureus following peritoneal infection. Elimination of coagulase activity results in S. aureus protection from antimicrobial activity and a corresponding reduction in host survival. These findings provide a molecular basis for the prevalence of coagulase negative staphylococci in peritonitis and highlight a caution when considering anticoagulant use in patients susceptible to peritoneal infection.

Purpose of review: Fibrin(ogen) is a multifunctional clotting protein that not only has critical roles in hemostasis but is also important in inflammatory processes that control bacterial infection. As a provisional extracellular matrix protein, fibrin(ogen) functions as a physical barrier, a scaffold for immune cell migration, or as a spatially-defined cue to drive inflammatory cell activation. These mechanisms contribute to overall host antimicrobial defense against infection. However, numerous bacterial species have evolved mechanisms to manipulate host fibrin(ogen) to promote microbial virulence and survival. Staphylococcal species, in particular, express numerous virulence factors capable of engaging fibrin(ogen), promoting fibrin formation, and driving the dissolution of fibrin matrices. Recent findings: Recent studies have highlighted both new insights into the molecular mechanisms involved in fibrin(ogen)-mediated host defense and pathogen-driven virulence. Of particular interest is the role of fibrin(ogen) in forming host protective biofilms versus pathogen protective barriers and biofilms as well as the role of fibrin(ogen) in mediating direct host antimicrobial responses. Summary: Current data suggest that the role of fibrin(ogen) in staphylococcal infection is highly context-dependent and that better defining the precise cellular and molecular pathways activated will provide unique opportunities of therapeutic intervention to better treat Staphylococcal disease.