Role of Galectin-3 in Macrophage Antibacterial Responses in a Listeria monocytogenes Infection Model

Galectin-3 is a β-galactoside-binding animal lectin expressed in epithelial cells and various immune cell types, with many proposed functions in development, differentiation, cancer and immune responses. Macrophages constitute an important first line of defense against infectious agents by recognizing, engulfing, and destroying pathogens, as well as releasing mediators that initiate inflammation. Abundantly expressed in macrophages, galectin-3 has been reported to regulate phagocytosis, LPS-mediated inflammation, and alternative macrophage activation. Here we have investigated the functions of galectin-3 in macrophage antibacterial responses using a Listeria monocytogenes (LM) infection model. First, our data showed that endogenous galectin-3 plays a negative role in macrophage listericidal activity through suppressing the production of NO. Compared to gal3+/+ bone marrow-derived macrophages (BMM), gal3-/- cells produced a higher amount of nitric oxide (NO) and contained fewer surviving intracellular bacteria; Inhibition of NO biosynthesis eliminated these differences between gal3+/+ and gal3-/- cells. We also unraveled an important function of galectin-3 in inflammasome activation. The inflammasome is a cytosolic multiprotein complex that mediates the activation of caspase-1, which promotes secretion of the proinflammatory cytokines IL-1β and IL-18. We demonstrated that galectin-3 deficiency in BMM resulted in diminished IL-1β production after LM infection or ATP treatment. In addition, the reduced IL-1β production in gal3-/- BMM was associated with higher amounts of unprocessed pro-IL-1β in the cytosol and lower caspase-1 enzymatic activity, suggesting that galectin-3 plays a critical role in caspase-1 activation. Finally, we showed that galectin-3 directly interacts with LM both in vitro and in the cytosol of infected macrophages. While galectin-3 was found to accumulate at the cytosolic face of phagosomes containing LM, lysis of phagosomal membrane and escape of LM to the cytosol rapidly recruited galectin-3 to the bacterium and formed galectin-3-LM complexes. We propose that these complexes might be involved in the assembly of inflammasome and activation of caspase-1. In conclusion, these data provided support for a role of galectin-3 in regulating macrophage antibacterial functions.