Sialoglycoproteins adsorbed by Pseudomonas aeruginosa facilitate their survival by impeding neutrophil extracellular trap through siglec-9

PA is an opportunistic pathogen that is commonly associated with severe infection in immunocompromised hosts. Siglec-9 binds with Sias by cis interaction on the neutrophil surface, thereby reducing immunological activity. However, neutrophils bind with pathogens through trans interactions of siglec-9 with Sias. Neutrophils kill invading pathogens by NETs, along with extracellular phagocytosis. Here, we report the mode of the adsorption of Sias by PA from host serum, the interaction of PA+Sias with human neutrophils, and the resulting neutrophil immunological activity. The α2-3-linked sialoglycoproteins adsorbed by PA exhibited potent binding with the soluble siglec-9-Fc chimeras, CHO-siglec-9 and siglec-9 on neutrophils. The binding between PA+Sias and neutrophils was blocked by the synthetic sialoglycan Neu5Acα2-3Galβ1-4GlcNAc, confirming the linkage-specific, Sias-siglec-9 interaction. The PA+Sias and siglec-9 interaction on neutrophils reduced the level of ROS and the release of elastase, resulting in a reduction of NETs formation, demonstrating the role of the sialoglycoproteins adsorbed by PA in the weakening of neutrophil activity. The resistance of PA+Sias to NETs was made evident by the increased survival of PA+Sias. Moreover, the decrease in PA-Sias survival demonstrated the involvement of NETs formation in the absence of the Sias-Nsiglec-9 interaction. N-actylcysteine or sivelestat-pretreated neutrophils enhanced the survival of PA-Sias. DNAse-pretreated neutrophils did not exhibit any NETs formation, resulting in the enhanced escape of PA-Sias. Taken together, one of the survival mechanisms of PA+Sias is the diminution of innate immunity via its adsorption of sialoglycoproteins by its engagement of the inhibitory molecule siglec-9. This is possibly a general mechanism for pathogens that cannot synthesize Sias to subvert immunity