[The immunomodulating capacity of Shigella flexneri 2A bacteria that have lost their invasiveness plasmid]

An immunosuppressive effect produced by live microbial cells of S. flexneri virulent plasmid-carrying strain 2a 516 (pSF 140-) was shown in relation to the formation of splenic cells producing antibodies to heterologous antigen (sheep red blood cells). This effect was most pronounced at an early period (days 4-7) after the infection of mice. On the contrary, avirulent plasmid-free variant (pSF 140-), genetically related to S. flexneri strain 2a 516, produced an immunostimulating effect at this period of observation. Such immunostimulating action of S. flexneri plasmid-free variant of the smooth type on immunocompetent effector T and B lymphocytes with the preservation of active hematopoiesis is indicative of the possibility of using these variants of shigellae as vector strains in constructing gene engineering (vector) vaccines.