Leukocyte Inflammatory Responses Provoked by Pneumococcal

ABSTRACT Cell surface expression of sialic acid has been reported to decrease during immune cell activation, but the significance and regulation of this phenomenon are still being investigated. The major human bacterial pathogen Streptococcus pneumoniae causes pneumonia, sepsis and meningitis, often accompanied by strong inflammatory responses. S. pneumoniae expresses a sialidase (NanA) that contributes to mucosal colonization, platelet clearance, and blood-brain barrier penetration. Using wild-type and isogenic NanA-deficient mutant strains, we showed that S. pneumoniaeNanA can desialylate the surface of human THP-1 monocytes, leading to increased ERK phosphorylation, NF-B activation, and proinflammatory cytokine release. S. pneumoniaeNanA expression also stimulates interleukin-8 release and extracellular trap formation from human neutrophils. A mechanistic contribution of unmasking of inhibitory Siglec-5 from cis sialic acid interactions to the proinflammatory effect of NanA is suggested by decreased SHP-2 recruitment to the Siglec-5 intracellular domain and RNA interference studies. Finally, NanA increased production of proinflammatory cytokines in a murine intranasal challenge model of S. pneumoniae pneumonia. IMPORTANCE Sialic acids decorate the surface of all mammalian cells and play important roles in physiology, development, and evolution. Siglecs are sialic acid-binding receptors on the surface of immune cells, many of which engage in cis interactions with host sialoglycan ligands and dampen inflammatory responses through transduction of inhibitory signals. Recently, certain bac-terial pathogens have been shown to suppress leukocyte innate immune responses by molecular mimicry of host sialic acid struc-tures and engagement of inhibitory Siglecs. Our present work shows that the converse can be true, i.e., that a microbial siali