Our findings identify a previously unrecognized role of Mincle as a regulator of autophagy, which mediates NET formation without affecting ROS generation. Our study addresses a major challenge in the field by positing this pathway to be targeted for modulation of NETs while preserving ROS production, an important innate immune defense.

C-type lectin receptors (CLRs), the carbohydrate recognizing molecules, orchestrate host immune response in homeostasis and in inflammation. In the present study we examined the function of macrophage galactose-type lectin-1 (MGL1), a mammalian CLR, in pneumonic sepsis, a deadly immune disorder frequently associated with a non-resolving hyperinflammation. In a murine model of pneumonic sepsis using pulmonary infection with Klebsiella pneumoniae (KPn), the expression of MGL1 was upregulated in the lungs of KPn infected mice and the deficiency of this CLR in MGL1−/− mice resulted in significantly increased mortality to infection than the MGL1-sufficient wild-type mice, despite a similar bacterial burden. The phagocytic cells from MGL1−/− mice did not exhibit any defects in bacterial uptake and intracellular killing and were fully competent in neutrophil extracellular trap formation, a recently identified extracellular killing modality of neutrophils. Instead, the increased susceptibility of MGL1−/− mice seemed to correlate with severe lung pathology, indicating that MGL1 is required for resolution of pulmonary inflammation. Indeed, the MGL1−/− mice exhibited a hyperinflammatory response, massive pulmonary neutrophilia and increase in neutrophil-associated immune mediators. Concomitantly, MGL1 deficient neutrophils exhibited an increased influx in pneumonic lungs of KPn infected mice. Together these results show a previously undetermined role of MGL1 in controlling neutrophilia during pneumonic infection thus playing an important role in resolution of inflammation. This is the first report depicting a protective function of MGL1 in an acute pneumonic bacterial infection.

Sepsis is a complex immune disorder that is characterized by systemic hyperinflammation. Alarmins, which are multifunctional endogenous factors, have been implicated in exacerbation of inflammation in many immune disorders including sepsis. Here we show that Galectin-9, a host endogenous β-galactoside binding lectin, functions as an alarmin capable of mediating inflammatory response during sepsis resulting from pulmonary infection with Francisella novicida, a Gram negative bacterial pathogen. Our results show that this galectin is upregulated and is likely released during tissue damage in the lungs of F. novicida infected septic mice. In vitro, purified recombinant galectin-9 exacerbated F. novicida-induced production of the inflammatory mediators by macrophages and neutrophils. Concomitantly, Galectin-9 deficient (Gal-9-/-) mice exhibited improved lung pathology, reduced cell death and reduced leukocyte infiltration, particularly neutrophils, in their lungs. This positively correlated with overall improved survival of F. novicida infected Gal-9-/- mice as compared to their wild-type counterparts. Collectively, these findings suggest that galectin-9 functions as a novel alarmin by augmenting the inflammatory response in sepsis development during pulmonary F. novicida infection.

Sepsis is a complex immune disorder that is characterized by systemic hyperinflammation. Alarmins, which are multifunctional endogenous factors, have been implicated in exacerbation of inflammation in many immune disorders including sepsis. Here we show that that Galectin-9, a host endogenous β-galactoside binding lectin, functions as an alarmin capable of mediating inflammatory response during sepsis resulting from pulmonary infection with Francisella novicida, a Gram negative bacterial pathogen. Our results show that this galectin is upregulated and released during tissue damage in the lungs of F. novicida infected septic mice. In-vitro, purified recombinant Galectin-9 exacerbated F. novicida-induced production of inflammatory mediators by macrophages and neutrophils. Concomitantly, Galectin-9 deficient (Gal-9-/-) mice exhibited improved lung pathology, reduced cell death and reduced leukocyte infiltration, particularly neutrophils, in their lungs. This positively correlated with overall improved survival of F. novicida infected Gal-9-/- mice as compared to their wild-type counterparts. Collectively, these findings suggest that exatracellular galectin-9 functions as a novel alarmin by augmenting the inflammatory response in sepsis development during pulmonary F. novicida infection.