S. sonnei virulent strain containing invasiveness plasmid pSS120 was found to suppress delayed hypersensitivity (DH) to nonbacterial antigens. In the filtrate of the culture of this strain extracellular lipopolysaccharide (LPS), capable of inducing immunosuppressing activity in nonvirulent bacteria, was detected. S. sonnei strain having plasmid pSS120 whose invasiveness genes were blocked by the penetration of transposon Tn5, as well as strains containing no invasiveness plasmid, proved to be nonvirulent and could not inhibit immunosuppressing action. Extracellular LPS of nonvirulent strains had no inductive immunosuppressing activity. But it could be activated by chemical treatment. The results of this study indicate that the immunosuppressing action of S. sonnei LPS were linked with some genes of invasiveness plasmid pSS120.
Animals DNA Transposable Elements