Functional role of galectin-9 in directing human innate immune reactions to Gram-negative bacteria and T cell apoptosis

•Galectin-9 is involved in the opsonisation of Gram-negative bacteria.•Galectin-9 promotes anti-bacterial immune defence.•Galectin-9 suppresses cytotoxic T cell function.•This effect can be achieved by “opsonisation” of T cells by galectin-9.•Affected apoptotic T cells can be phagocytosed by macrophages. Galectin-9 is a member of the galectin family of proteins, which were first identified to specifically bind to carbohydrates containing β-galactosides. Galectin-9 is conserved through evolution and recent evidence demonstrated its involvement in innate immune reactions to bacterial infections as well as the suppression of cytotoxic immune responses of T and natural killer cells. However, the molecular mechanisms underlying such differential immunological functions of galectin-9 remain largely unknown. In this work we confirmed that soluble galectin-9 derived from macrophages binds to Gram-negative bacteria by interacting with lipopolysaccharide (LPS), which forms their cell wall. This opsonisation effect most likely interferes with the mobility of bacteria leading to their phagocytosis by innate immune cells. Galectin-9-dependent opsonisation also promotes the innate immune reactions of macrophages to these bacteria and significantly enhances the production of pro-inflammatory cytokines – interleukin (IL) 6, IL-1β and tumour necrosis factor alpha (TNF-α). In contrast, galectin-9 did not bind peptidoglycan (PGN), which forms the cell wall of Gram-positive bacteria. Moreover, galectin-9 associated with cellular surfaces (studied in primary human embryonic cells) was not involved in the interaction with bacteria or bacterial colonisation. However, galectin-9 expressed on the surface of primary human embryonic cells, as well as soluble forms of galectin-9, were able to target T lymphocytes and caused apoptosis in T cells expressing granzyme B. Furthermore, “opsonisation” of T cells by galectin-9 led to the translocation of phosphatidylserine onto the cell surface and subsequent phagocytosis by macrophages through Tim-3, the receptor, which recognises both galectin-9 and phosphatidylserine as ligands.