Role of Yops in inhibition of phagocytosis and killing of opsonized Yersinia enterocolitica by human granulocytes.

The virulence plasmid of Yersinia enterocolitica codes for the production of the outer membrane protein YadA and the secretion of several proteins, called Yops, which may play a role in the interaction between granulocytes and this bacterium. We investigated whether the expression of YadA or the secretion of Yops affected the phagocytosis and killing of opsonized Y. enterocolitica by human granulocytes. The rates of phagocytosis and killing of Y. enterocolitica by granulocytes in suspension in the presence of rabbit Yersinia antibodies and complement were determined by microbiological assays. In addition, noningested cell-adherent bacteria were differentiated from ingested yersiniae by immunofluorescence microscopy. Plasmid-bearing opsonized Y. enterocolitica was able to inhibit phagocytosis and killing by human granulocytes. The inhibition of phagocytosis was specific for the plasmid-bearing strain of Y. enterocolitica, since granulocytes were still able to phagocytose and kill Staphylococcus aureus in the presence of Y. enterocolitica. Plasmid-cured Y. enterocolitica was readily phagocytosed and killed by these cells. To investigate the role of YadA or Yops in the inhibition of phagocytosis by granulocytes, the phagocytosis of mutant strains unable to express YadA or to secrete Yops was studied. A Y. enterocolitica mutant unable to secrete Yops lost its ability to inhibit phagocytosis; a mutant expressing only YadA was readily ingested by granulocytes. These results indicate that after attachment of opsonized Y. enterocolitica to granulocytes, Yops play an important role in inhibiting the ingestion of Y. enterocolitica by human granulocytes.