CD56-mediated activation of human natural killer cells is triggered by Aspergillus fumigatus galactosaminogalactan

Invasive aspergillosis causes significant morbidity and mortality in immunocompromised patients. Natural killer (NK) cells are pivotal for antifungal defense. Thus far, CD56 is the only known pathogen recognition receptor on NK cells triggering potent antifungal activity against Aspergillus fumigatus. However, the underlying cellular mechanisms and the fungal ligand of CD56 have remained unknown. Using purified cell wall components, biochemical treatments, and ger mutants with altered cell wall composition, we herein found that CD56 interacts with the A. fumigatus cell wall carbohydrate galactosaminogalactan (GAG). This interaction induced NK-cell activation, degranulation, and secretion of immune-enhancing chemokines and cytotoxic effectors. Supernatants from GAG-stimulated NK cells elicited antifungal activity and enhanced antifungal effector responses of polymorphonuclear cells. In conclusion, we identified A. fumigatus GAG as a ligand of CD56 on human primary NK cells, stimulating potent antifungal effector responses and activating other immune cells.Author summary: The opportunistic mold pathogen Aspergillus (A.) fumigatus possesses significant health risk, particularly for immunocompromised individuals. This ubiquitous fungus found in soil and decaying organic matter, causes a range of partially life-threatening infections known as aspergillosis. Natural killer (NK) cells, part of the innate immunity, recognize A. fumigatus by pathogen-associated molecular patterns (PAMPs) via their pattern recognition receptors (PRRs), leading to the elimination of infected cells, including direct cytotoxicity and cytokine production. Previously, we identified the NK-cell receptor CD56 as a PRR that recognizes A. fumigatus and triggers potent antifungal activity. However, the A. fumigatus ligand of CD56 has remained unknown. Here, we used different approaches to identify the A. fumigatus ligand of CD56 on NK cells. We found that CD56 directly binds to cell wall galactosaminogalactan. Specifically, deacetylated galactosamine residues of GAG played a role in the interaction with CD56 and triggered strong NK-cell activation, along with potent release of cytotoxic effectors and immune-enhancing chemokines. We also found that GAG-primed NK-cell supernatants activate and enhance effector responses of polymorphonuclear cells. Our study provides new insights on the A. fumigatus-NK-cell interplay by identifying GAG as an activating ligand of CD56 on human NK cells, stimulating potent antifungal effector responses.