Platelet-bound lipopolysaccharide enhances Fc receptor-mediated phagocytosis of IgG-opsonized platelets.

Platelets express Toll-like receptor 4 (TLR4), and this has been shown to be responsible for the thrombocytopenia induced by lipopolysaccharide (LPS) administration in vivo. We studied the role of LPS in mediating platelet phagocytosis by THP-1 cells in vitro by flow cytometry. Opsonization of platelets with an IgG monoclonal (W6/32) antibody or with IgG autoantibody-positive sera from patients with autoimmune thrombocytopenia (AITP) significantly enhanced platelet phagocytosis (P < .001). In contrast, platelet phagocytosis did not occur if platelets were bound with only LPS. If, however, the LPS-bound platelets were also opsonized with either W6/32 or autoantibody-positive sera with titers greater than 4, there was a significant and synergistic increase in Fc-dependent platelet phagocytosis (P < .001, P = .003, P = .048, and P = .047). These results suggest that, in the presence of antiplatelet antibodies, bacterial products can significantly alter platelet phagocytosis, and this may have relevance to how Gram-negative infections enhance platelet destruction in some patients with AITP.