title: Toll-Like Receptor Signalling Is Not Involved in Platelet Response to Streptococcus pneumoniae In Vitro or In Vivo.
abstract: Streptococcus (S.) pneumoniae strains vary considerably in their ability to cause invasive disease in humans, which is at least in part determined by the capsular serotype. Platelets have been implicated as sentinel cells in the circulation for host defence. One of their utensils for this function is the expression of Toll-like receptors (TLRs). We here aimed to investigate platelet response to S. pneumoniae and a role for TLRs herein. Platelets were stimulated using four serotypes of S. pneumonia including an unencapsulated mutant strain. In vitro aggregation and flow cytometry assays were performed using blood of healthy volunteers, or blood of TLR knock out and WT mice. For in vivo pneumonia experiments, platelet specific Myd88 knockout (Plt-Myd88-/-) mice were used. We found that platelet aggregation was induced by unencapsulated S. pneumoniae only. Whole blood incubation with all S. pneumoniae serotypes tested resulted in platelet degranulation and platelet-leukocyte complex formation. Platelet activation was TLR independent, as responses were not inhibited by TLR blocking antibodies, not induced by TLR agonists and were equally induced in wild-type and Tlr2-/-, Tlr4-/-, Tlr2/4-/-, Tlr9-/- and Myd88-/- blood. Plt-Myd88-/- and control mice displayed no differences in bacterial clearance or immune response to pneumonia by unencapsulated S. pneumoniae. In conclusion, S. pneumoniae activates platelets through a TLR-independent mechanism that is impeded by the bacterial capsule. Additionally, platelet MyD88-dependent TLR signalling is not involved in host defence to unencapsulated S. pneumoniae in vivo.
authors.affiliation: Center for Infection and Immunity Amsterdam (CINIMA), University of Amsterdam, Amsterdam, the Netherlands.|Center for Experimental and Molecular Medicine (CEMM), University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Sacha F
authors.initials: SF
authors.lastname: de Stoppelaar
authors.affiliation: Center for Infection and Immunity Amsterdam (CINIMA), University of Amsterdam, Amsterdam, the Netherlands.|Center for Experimental and Molecular Medicine (CEMM), University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Theodora A M
authors.initials: TA
authors.lastname: Claushuis
authors.affiliation: Laboratory for Experimental and Clinical Chemistry (LEKC), University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Marianne C L
authors.initials: MC
authors.lastname: Schaap
authors.affiliation: Key Laboratory of Infection and Immunity, Institute of Biophysics, Chaoyang District, Beijing, China.
authors.forename: Baidong
authors.initials: B
authors.lastname: Hou
authors.affiliation: Center for Infection and Immunity Amsterdam (CINIMA), University of Amsterdam, Amsterdam, the Netherlands.|Center for Experimental and Molecular Medicine (CEMM), University of Amsterdam, Amsterdam, the Netherlands.|Division of Infectious Diseases, University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Tom
authors.initials: T
authors.lastname: van der Poll
authors.affiliation: Laboratory for Experimental and Clinical Chemistry (LEKC), University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Rienk
authors.initials: R
authors.lastname: Nieuwland
authors.affiliation: Center for Infection and Immunity Amsterdam (CINIMA), University of Amsterdam, Amsterdam, the Netherlands.|Center for Experimental and Molecular Medicine (CEMM), University of Amsterdam, Amsterdam, the Netherlands.
authors.forename: Cornelis
authors.initials: C
authors.lastname: van 't Veer
mesh_terms: D000818:Animals; D001792:Blood Platelets / Q000378:metabolism* / Q000382:microbiology* / Q000502:physiology; D015550:Cell Degranulation; D006801:Humans; D007249:Inflammation / Q000473:pathology; D007962:Leukocytes / Q000378:metabolism; D008810:Mice, Inbred C57BL; D053594:Myeloid Differentiation Factor 88 / Q000378:metabolism; D010974:Platelet Aggregation; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000378:metabolism; D017452:Receptors, IgG / Q000378:metabolism; D018179:Receptors, Thrombin / Q000378:metabolism; D015398:Signal Transduction; D013296:Streptococcus pneumoniae / Q000502:physiology*; D051193:Toll-Like Receptors / Q000378:metabolism*
chemical_list: D053594:Myeloid Differentiation Factor 88; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D017452:Receptors, IgG; D018179:Receptors, Thrombin; D051193:Toll-Like Receptors