An isogenic pair of virulent and avirulent Yersinia enterocolitica O9 strains was used to examine the influence of the virulence plasmid on the non-specific modification of the cellular immunity in BALB/c mice after experimental infection with yersiniae. The modification of contact hypersensitivity response to dinitrofluorobenzene, resistance to the syngeneic lymphoma LSTRA, and resistance to Listeria monocytogenes was heavily influenced by the presence of the virulence plasmid. As a general rule for the modification of cellular immunity by yersiniae, the plasmid-bearing strain induced a short-term suppression followed by a potentiation, whereas the isogenic plasmid-less derivative induced only a short-term potentiation. The Yersinia-mediated enhancement of cellular immunity resulted in protection against infection with Listeria and partial protection against LSTRA transplantation. Results of Concanavalin A-induced proliferation of splenocytes from Yersinia-infected mice suggested a role for cytokines as gamma-interferon in the Yersinia-mediated immunopotentiation.
