Macrophage galactose-type lectin-1 deficiency is associated with increased neutrophilia and hyperinflammation in gram-negative pneumonia

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 nonresolving hyperinflammation. In a murine model of pneumonic sepsis using pulmonary infection with Klebsiella pneumoniae, the expression of MGL1 was upregulated in the lungs of K. pneumoniae-infected mice, and the deficiency of this CLR in MGL1^-/- mice resulted in significantly increased mortality to infection than in 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 an increase in neutrophil-associated immune mediators. Concomitantly, MGL1-deficient neutrophils exhibited an increased influx in pneumonic lungs of K. pneumoniae-infected mice. Taken 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. To our knowledge, this is the first study depicting a protective function of MGL1 in an acute pneumonic bacterial infection.