D-094. Soluble Fibrin Inhibits Monocyte Adherence and Phagocytosis of Staphylococcus aureus
Activation of blood coagulation, including elevated levels of soluble fibrin (sFn), are common in bacterial infections. Many bacteria express adhesins and hemagglutinins which also bind fibrinogen (Fg) and sFn, and reports suggest that such binding increases adherence and phagocytosis by monocytes and neutrophils. However, in a cancer model, we have shown that profound inhibition of immune cell adherence to tumor cells and endothelium occurs when both immune and target cells are pre-exposed to sFn (as is most likely in the physiological setting). It was thus hypothesized that pre-exposure of pathogenic bacteria possessing adhesins, and phagocytes (monocytes) with sFn would similarly inhibit immune cell adherence and phagocytosis. To test this, experiments were performed to demonstrate sFn (FITC-labeled) binding to monocytes and Staphylococcus aureus (SA) followed by microscopic observation and quantification. To investigate adherence, untreated or sFn-treated SA (SYTO 13 labeled: 1 x 106/ml) were perfused (1 h) across untreated or sFn pre-treated adherent monocytes in a microscope mounted perfusion chamber (370C) at physiological shear rates, washed, and imaged. Adherent bacteria were quantified using Image Pro plus software. Phagocytosis was assessed by incubation (1 h, 370C) of untreated or sFn treated, FITC labeled, SA (1 x 107/ml) with untreated or sFn treated monocytes (1 x 106/ml), followed by quenching of non-phagocytosed bacteria with crystal violet, and microscopic quantification of bacterial uptake.sFn bound strongly to both SA and monocytes, and pre-treatment of bacteria and monocytes with sFn inhibited adherence by 51+ 4% and phagocytosis by 22+3%. These results suggest that, in diseases such as sepsis, increased levels of circulating sFn may be immunosuppressive, leading to exacerbation of disease.