Effects of Fraction I from Yersinia pestis on Phagocytosis In Vitro

Fraction I, obtained from capsular material of Yersinia pestis (formerly Pasteurella pestis), was studied for its effect on in-vitro phagocytosis. No impairment in phagocytic systems employing leukocytes from men or rabbits plus several different test strains of (Staphylococcus, Streptoccocus, and Proteus) bacteria was noted if opsonic activity was primarily dependent on γG opsonin. However, distinct interference with heat-labile components of serum participating in opsonization of gram-negative organisms such as Escherichia coli was noted. Fraction I activated the complement system, particularly affecting C′4 and C′2, and, in concentrations of 200 μg/ml of fresh human serum, it produced consumption of 40% of the total hemolytic activity of complement. Blocking of heat-labile opsonins by Fraction I, particularly for such organisms as E. coli, seems to be associated with its interaction with and partial activation of the complement system.